My 35 O'Day with a Universal 25M (21 HP) has a fixed twin bladed prop. I find I can only comfortably cruise at 5.3 knots under power (for those totally windless days) although the hull speed is much higher. The engine will rev higher but to gain more speed it sounds like I am pushing the engine too hard and it is uncomfortable. Maxing out the engine I can only get about 6 knots and obviously this was just done as a test for a very short period to avoid engine damage. I don't have a tach on my insturment panel. If I assume holding the rpm's of the engine at the speed I currently use for 5.3 knots can i gain hull velocity by changing the pitch or size of the prop or number of blades on the prop? what negative consequences will result?
- Status
Props again
Jan.3, 2000 Dave, buddy, didn't we just talk about dinghies? A three bladed prop will definitely add to your boat's power and performance, but not perhaps as much as you might think. Assuming the pitch and diameter remained about the same on the 3 bladed prop as that on your 2 blade, it will require a greater amount of energy to drive three blades than 2. This means that your boat engine will be working harder to obtain the same RPM that currently gives you 5.3 knots, and using a bit more fuel to do it, but not much. There are some other issues to consider as well. Two things : Absolutely positively get yourself a tach. You simply can't accurately judge engine preformance by ear. The reason this is important is to prevent excess wear on your engine caused by over revving, and to accurately gauge engine performance. Remember that last theoretical knot of hull speed you are looking for ( hull speed= water line length x 1.36) requires a disproportionately large amount of energy and creates a great deal more wear to achieve because resistance rises disproportionately to speed. One of our physicist readers can give you the actual number but I believe resistance rises by the cube of the speed. Thus your boat will cruise alot easier at 5.5 knots then at 6.5 knots and use a lot less fuel and make a lot less commotion doing it. With a tach you will know rather than guess how hard you are running the motor. How high should you rev the engine to keep wear under control? On a new motor not more than 80% of maximun RPM under load. On an older motor keep it down closer to 70%. To determine your maximum RPM take the boat out and while under way under engine power alone increase throttle to maximum and watch the tach. When RPM stops increasing hold it for about 1 minute to make sure you have maxed out the load and note the RPM, and immediately back off the throttle. Never exceed 70 to 80 % of that RPM number except in an emergency. Second, check the archives under "props" where you will find a number of useful discussions on the subject. Personally I think if you are going to reprop you should do the job right and get a feathering 3 blade prop which will give you extra tourque power for punching into head seas and less resistance when sailing. A feathering prop will give you a 1/2 /to 3/4 knot advantage under sail compared to a fixed 2 or 3 blade prop. Get a MaxProp made by PYI or a similar product, which gives as much power in reverese as forward, is smooth running under power and the best performance improvement you can make on the boat short of adding sail area. On actual wheel and pitch size I defer to the experts. PYI or your local prop shop can tell you the correct wheel and pitch size for your boat and motor. Remember, it is a serious mistake to add too much pitch or diameter to a boat prop. Oversizing decreases engine performance, while adding to wear and fuel consumption. One other tip: your propellor blades need a minimum of 2 inches of clearance from the hull to prevent additional resistance and vibration from occurring, so make sure you have at least that on your new wheel. Fair Winds, Brian Pickton of BeneteauOwners.net Aboard the Legend, Rodney Bay, St. Lucia
Jan.3, 2000 Dave, buddy, didn't we just talk about dinghies? A three bladed prop will definitely add to your boat's power and performance, but not perhaps as much as you might think. Assuming the pitch and diameter remained about the same on the 3 bladed prop as that on your 2 blade, it will require a greater amount of energy to drive three blades than 2. This means that your boat engine will be working harder to obtain the same RPM that currently gives you 5.3 knots, and using a bit more fuel to do it, but not much. There are some other issues to consider as well. Two things : Absolutely positively get yourself a tach. You simply can't accurately judge engine preformance by ear. The reason this is important is to prevent excess wear on your engine caused by over revving, and to accurately gauge engine performance. Remember that last theoretical knot of hull speed you are looking for ( hull speed= water line length x 1.36) requires a disproportionately large amount of energy and creates a great deal more wear to achieve because resistance rises disproportionately to speed. One of our physicist readers can give you the actual number but I believe resistance rises by the cube of the speed. Thus your boat will cruise alot easier at 5.5 knots then at 6.5 knots and use a lot less fuel and make a lot less commotion doing it. With a tach you will know rather than guess how hard you are running the motor. How high should you rev the engine to keep wear under control? On a new motor not more than 80% of maximun RPM under load. On an older motor keep it down closer to 70%. To determine your maximum RPM take the boat out and while under way under engine power alone increase throttle to maximum and watch the tach. When RPM stops increasing hold it for about 1 minute to make sure you have maxed out the load and note the RPM, and immediately back off the throttle. Never exceed 70 to 80 % of that RPM number except in an emergency. Second, check the archives under "props" where you will find a number of useful discussions on the subject. Personally I think if you are going to reprop you should do the job right and get a feathering 3 blade prop which will give you extra tourque power for punching into head seas and less resistance when sailing. A feathering prop will give you a 1/2 /to 3/4 knot advantage under sail compared to a fixed 2 or 3 blade prop. Get a MaxProp made by PYI or a similar product, which gives as much power in reverese as forward, is smooth running under power and the best performance improvement you can make on the boat short of adding sail area. On actual wheel and pitch size I defer to the experts. PYI or your local prop shop can tell you the correct wheel and pitch size for your boat and motor. Remember, it is a serious mistake to add too much pitch or diameter to a boat prop. Oversizing decreases engine performance, while adding to wear and fuel consumption. One other tip: your propellor blades need a minimum of 2 inches of clearance from the hull to prevent additional resistance and vibration from occurring, so make sure you have at least that on your new wheel. Fair Winds, Brian Pickton of BeneteauOwners.net Aboard the Legend, Rodney Bay, St. Lucia
Brian, I believe it is the square root of the LWL
times 1.34 to obtain boat speed. Good article...don't mean to be picky. Tom
times 1.34 to obtain boat speed. Good article...don't mean to be picky. Tom
OOPS....
Jan.4,2000 Dear Tom, Thanks for catching the mistake. Even though I review what I write that kind of error slips through from time to time and I appreciate the help in keeping things straight. Your right of course, it is the square root of the water line length. The multiplier varies and as boats have gotten slicker the number has gone up. The first time I saw it it back in the 70's it was at 1.34 and since has climbed. The most recent generally accepted multiplier appears to be about 1.36 or 1.37, although racing boats like the IACC class rate higher. Brian Pickton, of BeneteauOwners.net Aboard The Legend, Rodney Bay, St. Lucia
Jan.4,2000 Dear Tom, Thanks for catching the mistake. Even though I review what I write that kind of error slips through from time to time and I appreciate the help in keeping things straight. Your right of course, it is the square root of the water line length. The multiplier varies and as boats have gotten slicker the number has gone up. The first time I saw it it back in the 70's it was at 1.34 and since has climbed. The most recent generally accepted multiplier appears to be about 1.36 or 1.37, although racing boats like the IACC class rate higher. Brian Pickton, of BeneteauOwners.net Aboard The Legend, Rodney Bay, St. Lucia
Hull speed
Brian, The hull speed has nothing to do with the slickness of the hull. The hull speed is a function of the water. A wave moves at 1.34 times the square root of the distance between crests.Thus a 20 foot boat the crests will be 20 feet apart. A 40 foot boat the crests will be 40 feet apart. The boat will not climb it's bow wave thus the fastest the boat will go is crest to crest. Thus the formula Hull speed = 1.34 x the square root of the waterline lenght. Brian, not trying to be smart. Just thought it a good time to explain this. r.w.landau
Brian, The hull speed has nothing to do with the slickness of the hull. The hull speed is a function of the water. A wave moves at 1.34 times the square root of the distance between crests.Thus a 20 foot boat the crests will be 20 feet apart. A 40 foot boat the crests will be 40 feet apart. The boat will not climb it's bow wave thus the fastest the boat will go is crest to crest. Thus the formula Hull speed = 1.34 x the square root of the waterline lenght. Brian, not trying to be smart. Just thought it a good time to explain this. r.w.landau
Agree with Brian
You need to install a tach. This will tell a big story. It will also help determine your alternatives should you change prop size. r.w.landau
You need to install a tach. This will tell a big story. It will also help determine your alternatives should you change prop size. r.w.landau
I mislead you R.W.
Jan. 5, 2000 Dear R.W. Landau When I was speaking of boats being "slicker" I was thinking of the over all improvements of design that have lead boats to carry ever more sail area and to sail more efficently, and not the actual quality of smoothness of the hull. Sorry, I didn't mean to be taken literally, but I do appreciate your clearing up any confusion I might have caused. And while were on the subject of confusion there is something I hope you can help clear up for me. Since your plainly better informed on this than I am, I for one would be grateful R.W. if you would wade in on the topic of the multiplier once again visa vis. the increase in water line length as the boat heels and how that effects the multiplier used. My understanding is that at 1.34 this is a static estimate of boat speed when the boat is upright and does not take into consideration the increase of water line length caused by heeling. Is that right? My further understanding is that later users of the formula have attempted to correct for this by expanding the multiplier to consider this factor of boat heel and the fact that modern boats, the Legend included, can be induced to surf down waves at speeds greater than the static upright number considers. Is this correct or have I been mislead? In our case our water line length is 39'10" and results in a theoretical maximum speed using the formula multiplier you propose of 1.34 of 8.379 knots. Our real world experience has been that we routinely sail at 8.5 to 9.5 knots allowing for current to be deleted from the calculation, and occasionally faster when surfing. For example, in complete defiance of the supposed laws of physics, we once had The Legend doing 18 knots going down wind in hurricane force winds.( Don't try this at home.) I noted at the time that this was because the seas were small and the length between crests corresponded to The Legend's water line length. We were actually moving at the speed of the wave train which effectively replaced our own bow and stern waves. If the wave train had moved slower, we would have too. Now I appreciate that I can make mistakes about calculating for current in local conditions but no amount of error I could make would result in 18 knots of speed. We made a passage that ordinarily would take us around 4 hours in an hour and 15 minutes. You should have seen the rooster tail, I remember worrying about the rudder coming off, but I digress. I understood that it was because of actual experiences like this that we see multipliers like 1.36 and 1.37 and even higher in some cases in an attempt to reflect what might be the real world performance of these boats verses the more academic approach. Is that your understanding as well, and if not, can you please tell me where these higher numbers originated? Do you think the formula should reflect this more dynamic reality, or in your opinion, does it already do so? The reason I ask is that the numbers just don't reflect our actual experience on our best sailing days even when current has been accounted for. I for one would appreciate any information you can give on the topic to help clarify my confusion, R.W. I look forward to hearing more from you. Fair Winds, Brian Pickton of Beneteau Owners.net Aboard the Legend, Rodney Bay St. Lucia
Jan. 5, 2000 Dear R.W. Landau When I was speaking of boats being "slicker" I was thinking of the over all improvements of design that have lead boats to carry ever more sail area and to sail more efficently, and not the actual quality of smoothness of the hull. Sorry, I didn't mean to be taken literally, but I do appreciate your clearing up any confusion I might have caused. And while were on the subject of confusion there is something I hope you can help clear up for me. Since your plainly better informed on this than I am, I for one would be grateful R.W. if you would wade in on the topic of the multiplier once again visa vis. the increase in water line length as the boat heels and how that effects the multiplier used. My understanding is that at 1.34 this is a static estimate of boat speed when the boat is upright and does not take into consideration the increase of water line length caused by heeling. Is that right? My further understanding is that later users of the formula have attempted to correct for this by expanding the multiplier to consider this factor of boat heel and the fact that modern boats, the Legend included, can be induced to surf down waves at speeds greater than the static upright number considers. Is this correct or have I been mislead? In our case our water line length is 39'10" and results in a theoretical maximum speed using the formula multiplier you propose of 1.34 of 8.379 knots. Our real world experience has been that we routinely sail at 8.5 to 9.5 knots allowing for current to be deleted from the calculation, and occasionally faster when surfing. For example, in complete defiance of the supposed laws of physics, we once had The Legend doing 18 knots going down wind in hurricane force winds.( Don't try this at home.) I noted at the time that this was because the seas were small and the length between crests corresponded to The Legend's water line length. We were actually moving at the speed of the wave train which effectively replaced our own bow and stern waves. If the wave train had moved slower, we would have too. Now I appreciate that I can make mistakes about calculating for current in local conditions but no amount of error I could make would result in 18 knots of speed. We made a passage that ordinarily would take us around 4 hours in an hour and 15 minutes. You should have seen the rooster tail, I remember worrying about the rudder coming off, but I digress. I understood that it was because of actual experiences like this that we see multipliers like 1.36 and 1.37 and even higher in some cases in an attempt to reflect what might be the real world performance of these boats verses the more academic approach. Is that your understanding as well, and if not, can you please tell me where these higher numbers originated? Do you think the formula should reflect this more dynamic reality, or in your opinion, does it already do so? The reason I ask is that the numbers just don't reflect our actual experience on our best sailing days even when current has been accounted for. I for one would appreciate any information you can give on the topic to help clarify my confusion, R.W. I look forward to hearing more from you. Fair Winds, Brian Pickton of Beneteau Owners.net Aboard the Legend, Rodney Bay St. Lucia
Multiplying factor of 1.34
In all articles on the subject I have read 1.34 has been the number. I have never seen a higher factor used. As far as heeling of the boat increasing the waterline length and thus a higher factor you seem to be confusing the formulas elements. The factor is a constant and the LWL is a variable. Upright at rest the LWL is the design number for the boat assuming the design displacement. A loaded boat will cause the boat to sink a bit deeper and thus a longer LWL unless you have a plumb bow and stern. On most boats as the boat heels the LWL increases and thus the hull speed increases as the distance between the bow and stern waves increases. I don't doubt if hydrodynamacists at MIT might be tweaking the 1.34 factor but who really cares? It is just a number for comparison purposes and not an absolute. Conditions are always changing and actual results are compared to this (you could just as easily compare how you actually do to a constant like 5 knots set for all boats).
In all articles on the subject I have read 1.34 has been the number. I have never seen a higher factor used. As far as heeling of the boat increasing the waterline length and thus a higher factor you seem to be confusing the formulas elements. The factor is a constant and the LWL is a variable. Upright at rest the LWL is the design number for the boat assuming the design displacement. A loaded boat will cause the boat to sink a bit deeper and thus a longer LWL unless you have a plumb bow and stern. On most boats as the boat heels the LWL increases and thus the hull speed increases as the distance between the bow and stern waves increases. I don't doubt if hydrodynamacists at MIT might be tweaking the 1.34 factor but who really cares? It is just a number for comparison purposes and not an absolute. Conditions are always changing and actual results are compared to this (you could just as easily compare how you actually do to a constant like 5 knots set for all boats).
Hull speed is not a maximum speed
There is no law of physics that limits displacement boat speed to hull speed. All it takes is energy to push through the wave, and hull shape that makes this easier. Catamarans, with their slender hulls and large SA/D, routinely sail beyond hull speed.
There is no law of physics that limits displacement boat speed to hull speed. All it takes is energy to push through the wave, and hull shape that makes this easier. Catamarans, with their slender hulls and large SA/D, routinely sail beyond hull speed.
Hull speed is not an absolute limit
A few things about hull speed - Firstly, it is not a limit. Any boat can be induced to go faster than its hull speed if enough power is employed. (I find my boat to be fasted when the power employed comes from being hauled down the highway
). As a poster pointed out recently in a thread about prop size, the increase in energy required to go a certain speed is not linear with respect to the increase in speed itself. I believe the relationship is actually a log function, so the increase is even greater than a geometric function. [For those who care: Linear relationship look like lines on a graph. Geometric relationships curve with a continuous radius on a graph. Logs curve with a decreasing radius.] What this means is that if it takes one random energy unit to go one knot, it might take four to go two knots. If the equation were geometric it would then take 16 units to go three knots, but because of the log function it might actually take 24 units to go three knots. At some point the graph gets extrememly steep, and you might have to add an tremendous amount of power to get to the next point on the graph. On boats, this is the hull speed, and occurs because of the inability of a hull to climb over its bow wave. Obviously, there is a solution to this problem or Jetskis that have a theoretical hull speed of only a couple of knots couldn't run into things at high speeds. Planning hulls are the easy solution. The other solution is to simply apply the necessary power. The power can come from loads of sail and wind, lots of engine, gravity in the case of small things (boards, Lasers) in the surf (learn from my mistakel; don't sail your Laser in the surf), or from the energy of the wave front from behind pushing the hull forward. Any of these will result in surpassing hull speed. What the boat does at that point depends on the hull. My O'day and my Catalina both try to plane. The O'day suceeds, the Cat does not. My Zodia has a non-planing hull, but goose it does plane, but doesn't turn. My Walker Bay dinghy never even trys to plane, it just packs water up against the bow. Push it hard enough and it will swamp itself with its bow wave. The formula for determining hull speed is more complicated than (wl*1/x) * 1.34. The full formula takes into account beam, where its carried, etc. The narrower the boat, the higher the hull speed, everything else remaining equal. Multis do, in fact, have hull speeds that even they would have a hard time reaching, but they are very high because of the extreme length to beam ratio for each ama. You can also play tricks by disrupting the bow wave. Tankers and big ass container ships do this with they bow bulb. This does more than just lengthen the hull. If designed correctly, they can create a wave inverted relative to the wave when the hull parts the water, thus effectively canceling it out. This works best only at one speed, but works to some degree over a wide range. If you have ever seen what it looks like when a sub is traveling across the surface, you can see this at work. Justin - O'day Owners' Web
A few things about hull speed - Firstly, it is not a limit. Any boat can be induced to go faster than its hull speed if enough power is employed. (I find my boat to be fasted when the power employed comes from being hauled down the highway
). As a poster pointed out recently in a thread about prop size, the increase in energy required to go a certain speed is not linear with respect to the increase in speed itself. I believe the relationship is actually a log function, so the increase is even greater than a geometric function. [For those who care: Linear relationship look like lines on a graph. Geometric relationships curve with a continuous radius on a graph. Logs curve with a decreasing radius.] What this means is that if it takes one random energy unit to go one knot, it might take four to go two knots. If the equation were geometric it would then take 16 units to go three knots, but because of the log function it might actually take 24 units to go three knots. At some point the graph gets extrememly steep, and you might have to add an tremendous amount of power to get to the next point on the graph. On boats, this is the hull speed, and occurs because of the inability of a hull to climb over its bow wave. Obviously, there is a solution to this problem or Jetskis that have a theoretical hull speed of only a couple of knots couldn't run into things at high speeds. Planning hulls are the easy solution. The other solution is to simply apply the necessary power. The power can come from loads of sail and wind, lots of engine, gravity in the case of small things (boards, Lasers) in the surf (learn from my mistakel; don't sail your Laser in the surf), or from the energy of the wave front from behind pushing the hull forward. Any of these will result in surpassing hull speed. What the boat does at that point depends on the hull. My O'day and my Catalina both try to plane. The O'day suceeds, the Cat does not. My Zodia has a non-planing hull, but goose it does plane, but doesn't turn. My Walker Bay dinghy never even trys to plane, it just packs water up against the bow. Push it hard enough and it will swamp itself with its bow wave. The formula for determining hull speed is more complicated than (wl*1/x) * 1.34. The full formula takes into account beam, where its carried, etc. The narrower the boat, the higher the hull speed, everything else remaining equal. Multis do, in fact, have hull speeds that even they would have a hard time reaching, but they are very high because of the extreme length to beam ratio for each ama. You can also play tricks by disrupting the bow wave. Tankers and big ass container ships do this with they bow bulb. This does more than just lengthen the hull. If designed correctly, they can create a wave inverted relative to the wave when the hull parts the water, thus effectively canceling it out. This works best only at one speed, but works to some degree over a wide range. If you have ever seen what it looks like when a sub is traveling across the surface, you can see this at work. Justin - O'day Owners' Webtheoretical hull speed
Brian, I was talking of theoretical hull speed. David, Been There, and Justin are all correct. The beam plays a part in that it increases water line length, not the formula. Some boats have design that is conducive to surfing. The definition of hull speed is the predictable speed of a displacement hull. Many factors can play into the small surges of extra speed, waves in the body of water the boat is in, currents, tides, etc.,but unless the boat has a planning hull the speed will return to the theoretical hull speed. I am not a designer, nor a naval architect. I have an engineering degree and alot of boat reading and experience. This theory has been around for a while. A simple read for this, (I have learned that backing up my suggestions helps) is "The Annapolis book of Seamanship" page 24. Justin's comment about the bulb is part of submarine design and is very effective. I think this was one of the thoughts when the bulb keels were designed. A sub can go faster under water than it can on top. That is all I know on that subject. I could connect someone with a Westinghouse engineer from their submarine program, but I don't know how much they would tell you. David, I am sorry this thread got a bit off track. So is it a tach or what? r.w.landau
Brian, I was talking of theoretical hull speed. David, Been There, and Justin are all correct. The beam plays a part in that it increases water line length, not the formula. Some boats have design that is conducive to surfing. The definition of hull speed is the predictable speed of a displacement hull. Many factors can play into the small surges of extra speed, waves in the body of water the boat is in, currents, tides, etc.,but unless the boat has a planning hull the speed will return to the theoretical hull speed. I am not a designer, nor a naval architect. I have an engineering degree and alot of boat reading and experience. This theory has been around for a while. A simple read for this, (I have learned that backing up my suggestions helps) is "The Annapolis book of Seamanship" page 24. Justin's comment about the bulb is part of submarine design and is very effective. I think this was one of the thoughts when the bulb keels were designed. A sub can go faster under water than it can on top. That is all I know on that subject. I could connect someone with a Westinghouse engineer from their submarine program, but I don't know how much they would tell you. David, I am sorry this thread got a bit off track. So is it a tach or what? r.w.landau
That doesn't make sense, RW
"The definition of hull speed is the predictable speed of a displacement hull. .. unless the boat has a planning hull the speed will return to the theoretical hull speed." Both of these statements are wrong. The "predictable speed of a displacement hull" continues to go up as force is increased. The amount of force required grows rapidly, but I doubt there is an asymptote. Given enough force, the speed limit is when the hull's structure fails from pushing the water aside. Hull speed is defined as the speed of a gravity wave whose length equals the hulls waterline length. Nothing more, and nothing less. There are lots of displacement boats that will sail for hours doing significantly more than hull speed. They return to theoretical hull speed only when the wind no longer favors them.
"The definition of hull speed is the predictable speed of a displacement hull. .. unless the boat has a planning hull the speed will return to the theoretical hull speed." Both of these statements are wrong. The "predictable speed of a displacement hull" continues to go up as force is increased. The amount of force required grows rapidly, but I doubt there is an asymptote. Given enough force, the speed limit is when the hull's structure fails from pushing the water aside. Hull speed is defined as the speed of a gravity wave whose length equals the hulls waterline length. Nothing more, and nothing less. There are lots of displacement boats that will sail for hours doing significantly more than hull speed. They return to theoretical hull speed only when the wind no longer favors them.
Submarines
I have had considerable experience sailing in the vacinity of submarines under way. I sail near New London and the subs are often entering and leaving the sub base. I have also been surprised by a few on the Southeast side of the Race popping to the surface near me, I hope their sophisticated instrumentts let them know I'm there? My experience shows that on the surface they create a huge wake and bow wave, but think about it. They are designed for speed under water and stealth. Under water the concern is laminar flow over a surface and mixing of the streams on the downstream side of the flow. In engineering school we did considerable research in water tunnels for determining how different shapes effect this mixing with the intent of minimizing the noise created by the vortices and turbulence in the mixed stream. Stealth is quite critical for successful submarine operations so noise is the key factor. A sub under high speed has already given itself away, stealth is critical at speeds of say 5 knots. Other items of interest are noise from the prop., etc. But back to the sub shape on the surface. In operation the sub bow on the surface is submurged so the water starts to flow up the top surface and then flows off to either side of the hull. This causes a lot of turbulence and a big bow wave. A submarine underwater is more like an airplane and the hull shapes are similar but an airplane fuselage is more like an airfoil to give lift while a sub hull is more interested in neutral buoyancy, the sub is essentially "flying" in a liquid versus a gas medium. A sub is operating in three dimensional space versus all boats and ships are in two dimensions (ignoring the slight vertical displacements of wave effects). Thus the helmsman on a sub has to worry about navigating in spherical space versus a 360 degree compass card. One last note, a los angeles class sub going by within 50 yards is an awesome sight and makes you quite proud to be an american.
I have had considerable experience sailing in the vacinity of submarines under way. I sail near New London and the subs are often entering and leaving the sub base. I have also been surprised by a few on the Southeast side of the Race popping to the surface near me, I hope their sophisticated instrumentts let them know I'm there? My experience shows that on the surface they create a huge wake and bow wave, but think about it. They are designed for speed under water and stealth. Under water the concern is laminar flow over a surface and mixing of the streams on the downstream side of the flow. In engineering school we did considerable research in water tunnels for determining how different shapes effect this mixing with the intent of minimizing the noise created by the vortices and turbulence in the mixed stream. Stealth is quite critical for successful submarine operations so noise is the key factor. A sub under high speed has already given itself away, stealth is critical at speeds of say 5 knots. Other items of interest are noise from the prop., etc. But back to the sub shape on the surface. In operation the sub bow on the surface is submurged so the water starts to flow up the top surface and then flows off to either side of the hull. This causes a lot of turbulence and a big bow wave. A submarine underwater is more like an airplane and the hull shapes are similar but an airplane fuselage is more like an airfoil to give lift while a sub hull is more interested in neutral buoyancy, the sub is essentially "flying" in a liquid versus a gas medium. A sub is operating in three dimensional space versus all boats and ships are in two dimensions (ignoring the slight vertical displacements of wave effects). Thus the helmsman on a sub has to worry about navigating in spherical space versus a 360 degree compass card. One last note, a los angeles class sub going by within 50 yards is an awesome sight and makes you quite proud to be an american.
Been There
I should have said the maximun predictable speed of a displacement hull. What are you powering this displacement hull with? What displacement hulls plane for hours faster than hull speed? What is a gravity wave? r.w.landau
I should have said the maximun predictable speed of a displacement hull. What are you powering this displacement hull with? What displacement hulls plane for hours faster than hull speed? What is a gravity wave? r.w.landau
R.w., Justin, Thanks Guys
Jan.6, 2000 As the man said, I may be no smarter but I am certainly better informed. Thanks for the information. Now how does this relate to feathering and folding props? Do subs use them? Is that boomer's metal "sail" really effective? It's been an interesting discussion but I owe you an apology Dave for getting your inquiry going in a different direction, sorry about that. Brian Pickton of BeneteauOwners.net Aboard the Legend, Rodney Bay St. Lucia
Jan.6, 2000 As the man said, I may be no smarter but I am certainly better informed. Thanks for the information. Now how does this relate to feathering and folding props? Do subs use them? Is that boomer's metal "sail" really effective? It's been an interesting discussion but I owe you an apology Dave for getting your inquiry going in a different direction, sorry about that. Brian Pickton of BeneteauOwners.net Aboard the Legend, Rodney Bay St. Lucia
Gravity waves, catamarans
A gravity wave is just your typical water wave that propagates along for 100s or 1000s of miles, influenced only by the force of gravity. Catamarans and trimarans powered by sail routinely travel 20% to 40% beyond hull speed, as long as they have a favorable wind. THEY DO NOT PLANE. They continue to displace water equal to their weight. Their narrow hulls are designed to slice through the water, not get up on top of it.
A gravity wave is just your typical water wave that propagates along for 100s or 1000s of miles, influenced only by the force of gravity. Catamarans and trimarans powered by sail routinely travel 20% to 40% beyond hull speed, as long as they have a favorable wind. THEY DO NOT PLANE. They continue to displace water equal to their weight. Their narrow hulls are designed to slice through the water, not get up on top of it.
Displacement Hull Speed
Just to clarify some of the previous comments, the formula of 1.34 x SQROOT of Waterline lenght is, like many, an approximation. Displacement hull boats CAN go faster, provided something is providing a significantly greater amount of thrust energy. Power boats of equivalent waterline length usually have an engine horsepower rating many times greater than a sailboat in order to achieve planing speeds. A gale force wind could conceivably drive a sailboat past the displacement hull speed, if the sails, rigging, and spars hold up. --Ron
Just to clarify some of the previous comments, the formula of 1.34 x SQROOT of Waterline lenght is, like many, an approximation. Displacement hull boats CAN go faster, provided something is providing a significantly greater amount of thrust energy. Power boats of equivalent waterline length usually have an engine horsepower rating many times greater than a sailboat in order to achieve planing speeds. A gale force wind could conceivably drive a sailboat past the displacement hull speed, if the sails, rigging, and spars hold up. --Ron
1. Hull speed calc. 2. Multihulls.
1. Hull speed. The oft-repeated till ad nauseum formula of hull speed = square root of waterline length x 1.36 is an APPROXIMATION for middle-of-the-road DISPLACEMENT hulls. This is intended to take into account the beam, length-to-width, appropriate sail-area-to-weight ratios, etc. For anyone sailing a normal Beneteau, Catalina, O'Day, or Hunter this should suffice and be accepted without so much debate. Brian (I think it was) is correct though that there is MUCH more to the precise formulae, but this easy method works well even for bona-fide designers who don't even need to check it the long way(s). There are other coefficients for others types of hulls but 1.34-1.36 is effectively infallible for normal cruising boats. Also, it is correct that increases of input power can propel a boat past hull speed, but at what cost, and for what reason? I had a friend with a Tartan 33 which powered at 5-1/2 knots and used 1/3-gallon/hour. To get it to 6 kts he used 2/3-gallon (TWICE the fuel) and created a 5-ft bow wave and a harbour full of black smoke. Sounds like David's problem. I mean what's the point? Increasing prop pitch will be like gearing up or down in a car. You might get up to speed faster but your top speed will be lower (lower pitch is like a high-RPM Chevy small-block) or else you will take longer getting there but have a theoretical higher speed for the given RPM (higher pitch is like torque-monsters Buick or Cadillac). I hate to say it but whatever came stock on the boat is probably close to optimum. Sailing off the wind, given appropriate weather conditions, most boats can surprise you with blinding speed– we had a 35-ft C&C at 10 knts off Branford Cove once. But it only tells you the power of the weather you have mamanged to harness and does NOT dispel the theoretical hull speed. The boat, believe it or not, is working VERY inefficiently at such overdriven conditions and you'd just better be glad it knows enough to run with the weather rather than lie down and die under it. 2. Multihulls. Hobie-Cats slice through the water. They are too underpowered and laden with human cargo to get up on plane. This was intentional for safety reasons by the designer of the first widely-marketed, affordable, user-friendly catamaran. Eric Tabarly's 73-ft Pen Duick trimaran planed (at like 18 kts!). A proa will plane. Racing multihull sailboats plane. The 80-ft New Zealand Endeavour sailed the 1994 Whitbread at like 17-20 kts– probably well over the 1.36 rule, and I would call that a near-planing hull (like a J-27, almost). Planing boats, especially multihulls, behave so ENTIRELY DIFFERENTLY than displacement boats that the comparsion is like 4-wheel cars to motorcycles– you almost have to forget what you know about one to comprehend the other. In any case the 1.36 'easy' rule will NOT apply to them– it gets higher as the coefficients which bear on displacement go down. I refer the doubting to several books, starting with the classic 'Skene's Elements of Yacht Design' and also the 1974-ish 'Developments in Yacht Design' which Warren Luhrs and my father consulted whilst designing the first Hunter 54, Tuesday's Child. J Cherubini II Cherubini Art & Nautical Design Org. JComet@aol.com
1. Hull speed. The oft-repeated till ad nauseum formula of hull speed = square root of waterline length x 1.36 is an APPROXIMATION for middle-of-the-road DISPLACEMENT hulls. This is intended to take into account the beam, length-to-width, appropriate sail-area-to-weight ratios, etc. For anyone sailing a normal Beneteau, Catalina, O'Day, or Hunter this should suffice and be accepted without so much debate. Brian (I think it was) is correct though that there is MUCH more to the precise formulae, but this easy method works well even for bona-fide designers who don't even need to check it the long way(s). There are other coefficients for others types of hulls but 1.34-1.36 is effectively infallible for normal cruising boats. Also, it is correct that increases of input power can propel a boat past hull speed, but at what cost, and for what reason? I had a friend with a Tartan 33 which powered at 5-1/2 knots and used 1/3-gallon/hour. To get it to 6 kts he used 2/3-gallon (TWICE the fuel) and created a 5-ft bow wave and a harbour full of black smoke. Sounds like David's problem. I mean what's the point? Increasing prop pitch will be like gearing up or down in a car. You might get up to speed faster but your top speed will be lower (lower pitch is like a high-RPM Chevy small-block) or else you will take longer getting there but have a theoretical higher speed for the given RPM (higher pitch is like torque-monsters Buick or Cadillac). I hate to say it but whatever came stock on the boat is probably close to optimum. Sailing off the wind, given appropriate weather conditions, most boats can surprise you with blinding speed– we had a 35-ft C&C at 10 knts off Branford Cove once. But it only tells you the power of the weather you have mamanged to harness and does NOT dispel the theoretical hull speed. The boat, believe it or not, is working VERY inefficiently at such overdriven conditions and you'd just better be glad it knows enough to run with the weather rather than lie down and die under it. 2. Multihulls. Hobie-Cats slice through the water. They are too underpowered and laden with human cargo to get up on plane. This was intentional for safety reasons by the designer of the first widely-marketed, affordable, user-friendly catamaran. Eric Tabarly's 73-ft Pen Duick trimaran planed (at like 18 kts!). A proa will plane. Racing multihull sailboats plane. The 80-ft New Zealand Endeavour sailed the 1994 Whitbread at like 17-20 kts– probably well over the 1.36 rule, and I would call that a near-planing hull (like a J-27, almost). Planing boats, especially multihulls, behave so ENTIRELY DIFFERENTLY than displacement boats that the comparsion is like 4-wheel cars to motorcycles– you almost have to forget what you know about one to comprehend the other. In any case the 1.36 'easy' rule will NOT apply to them– it gets higher as the coefficients which bear on displacement go down. I refer the doubting to several books, starting with the classic 'Skene's Elements of Yacht Design' and also the 1974-ish 'Developments in Yacht Design' which Warren Luhrs and my father consulted whilst designing the first Hunter 54, Tuesday's Child. J Cherubini II Cherubini Art & Nautical Design Org. JComet@aol.com
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