Ok all you "old" salts. I have a question about hull speed. My Hunter 28.5 has a hull speed rating of 6.5 knots. I have read that running under sail at max. hull speed is a bad idea. This past weekend I had the speed up as high as 6.2 knots. The boat did not seem over-powered, and responded well to rudder changes, though there was a vibration sensation comming through the hull. The seas at the time were about 1 foot. The boat was healed over, but not wildly so. My Datamarine wind meter is broken, so I don't know how strong the wind was that day, but still, like I said, the boat behaved well and I didn't feel like we were pushing the envelope in any way. But was I, even if I didn't realize it at the time? So, what is the danger here? How long is it safe to run at or near hull speed? Is it a rigging failure issue? Or will we tear off the rudder or keel or something? Or what?
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Skipper Stress
Sailing at and above "hull speed" is a real kick. Racers often exceed hull speed with no structural problems that I know of. I've never seen anything in writing about problems with exceeding hull speed, either. One can blow out the sails, that is, ruin their shape, if they are over stressed but just add money and they can be replaced or recut. The only other problem I can see is skipper stress from a white knuckle ride! Once you get used to it there should be less stress. When the boat exceeds hull speed then the wake becomes increasingly large and the boat sinks in the water more. Note on your transom how the back water rises up. Have fun and enjoy it while it lasts.
Sailing at and above "hull speed" is a real kick. Racers often exceed hull speed with no structural problems that I know of. I've never seen anything in writing about problems with exceeding hull speed, either. One can blow out the sails, that is, ruin their shape, if they are over stressed but just add money and they can be replaced or recut. The only other problem I can see is skipper stress from a white knuckle ride! Once you get used to it there should be less stress. When the boat exceeds hull speed then the wake becomes increasingly large and the boat sinks in the water more. Note on your transom how the back water rises up. Have fun and enjoy it while it lasts.
Hull speed is only theoretical.
Mike: Hull speed is only a theoretical speed of the boat. May times your boat will excees this speed (especially under sail). I have also NEVER heard anything about not exceeding the hull speed. As John pointed out if you are overpowered you may be stressing your equipment, but that did not seem to be the case as you pointed out. When I returned from Hawaii on a Santa Cruz 52 a couple of years ago, we where running this boat at 12-15 kts and on one occasion hit 18+ kts. I would not worry about it. When you get overpowered you need to reef. You will find that these boats will go FASTER in heavy air with a reef than without the reef. Keeping the boat within about 15 deg of heel and they perform well.
Mike: Hull speed is only a theoretical speed of the boat. May times your boat will excees this speed (especially under sail). I have also NEVER heard anything about not exceeding the hull speed. As John pointed out if you are overpowered you may be stressing your equipment, but that did not seem to be the case as you pointed out. When I returned from Hawaii on a Santa Cruz 52 a couple of years ago, we where running this boat at 12-15 kts and on one occasion hit 18+ kts. I would not worry about it. When you get overpowered you need to reef. You will find that these boats will go FASTER in heavy air with a reef than without the reef. Keeping the boat within about 15 deg of heel and they perform well.
hull speed is for real
Hull speed is based on wave physics, so I guess it's theoretical in that sense. However, your boat will only exceed it if it can plane, or if the hull is very, very thin to the point that it can pierce the bow wave, a la catamarans. They only time displacement boats plane--that I know of--is when they surf down big waves. And that's the only time they exceed hull speed. Hull speed is a fact of life for every displacement boat, from Solings to Aircraft Carriers. The only reason that sailing at hull speed might be a problem is the fact that the boat's rigging (or heeling) is the only thing left to absorb additional wind energy. You can't go any faster. Me, I work really hard to get there. Enjoy it when it happens. p.s. The vibration you felt was probably the prop free-wheeling. Try putting your engine in (reverse?) gear.
Hull speed is based on wave physics, so I guess it's theoretical in that sense. However, your boat will only exceed it if it can plane, or if the hull is very, very thin to the point that it can pierce the bow wave, a la catamarans. They only time displacement boats plane--that I know of--is when they surf down big waves. And that's the only time they exceed hull speed. Hull speed is a fact of life for every displacement boat, from Solings to Aircraft Carriers. The only reason that sailing at hull speed might be a problem is the fact that the boat's rigging (or heeling) is the only thing left to absorb additional wind energy. You can't go any faster. Me, I work really hard to get there. Enjoy it when it happens. p.s. The vibration you felt was probably the prop free-wheeling. Try putting your engine in (reverse?) gear.
It is a Curve
With enough power any boat can excede it's hull speed. The power requirements to gain any additional speed climb right up through the roof as you aproach that point of the curve thought of as hull speed. Even if there is enough power available the boats handling characteristics may turn to poop. I remember in my youth sailing dinghys in 20 knot winds we often would get them on a plain and then the whole boat would vibrate as if someone had dropped a couple of quarters in the timer by the headboard of a pay by the hour motel.
With enough power any boat can excede it's hull speed. The power requirements to gain any additional speed climb right up through the roof as you aproach that point of the curve thought of as hull speed. Even if there is enough power available the boats handling characteristics may turn to poop. I remember in my youth sailing dinghys in 20 knot winds we often would get them on a plain and then the whole boat would vibrate as if someone had dropped a couple of quarters in the timer by the headboard of a pay by the hour motel.
Hull speed formula
I believe the formula for displacement hulls is 1.3 X the square root of the waterline lenght. There's a bunch of modifiers (hull shape is a big one - barges don't do well at hull speed), but it will be pretty close. As you reach it, your bow wave becomes larger and the boat tends to sink into the trough it leaves which takes more power to climb over which makes a bigger bow wave etc, etc. Enjoy the edge of the envelope.
I believe the formula for displacement hulls is 1.3 X the square root of the waterline lenght. There's a bunch of modifiers (hull shape is a big one - barges don't do well at hull speed), but it will be pretty close. As you reach it, your bow wave becomes larger and the boat tends to sink into the trough it leaves which takes more power to climb over which makes a bigger bow wave etc, etc. Enjoy the edge of the envelope.
1.34 times sq. root of water length
That is the formula for calculating the "estimated" hull speed of a displacement boat. Other factors may come into play, but theoritically the boat's maximum speed through the water will be pretty close to the formula. When you start planing, for that brief period, it is not acting like a displacement hull. Exhilarating ! My previous boat was a 28.5 (LWL 23'9" : Sq. root of 23.75 = 4.8734 X 1.34 = 6.5 knots) and there are many a time when I planed at over 7 knots. I occasionally broke the 8 knot barrier and it usually took several days to wipe out the silly grin off my face. Enjoy it. That sure is a lot more fun than some of the drifting contest the doldrums of summer put us through.
That is the formula for calculating the "estimated" hull speed of a displacement boat. Other factors may come into play, but theoritically the boat's maximum speed through the water will be pretty close to the formula. When you start planing, for that brief period, it is not acting like a displacement hull. Exhilarating ! My previous boat was a 28.5 (LWL 23'9" : Sq. root of 23.75 = 4.8734 X 1.34 = 6.5 knots) and there are many a time when I planed at over 7 knots. I occasionally broke the 8 knot barrier and it usually took several days to wipe out the silly grin off my face. Enjoy it. That sure is a lot more fun than some of the drifting contest the doldrums of summer put us through.
I'VE BEEN TOLD BY OLD SALTS THAT...
once you begin to exceed the hull speed that you and your boat enter another dimension. Sort of like sailing through a fog bank and coming out the other side in the clear (heh, heh). I've had our previous H28 at 9.5 knots in strong following wind and seas, according to my hand held GPS and boat speed meter. Both the skipper and boat were in seventh heaven (ah that other dimension). Terry
once you begin to exceed the hull speed that you and your boat enter another dimension. Sort of like sailing through a fog bank and coming out the other side in the clear (heh, heh). I've had our previous H28 at 9.5 knots in strong following wind and seas, according to my hand held GPS and boat speed meter. Both the skipper and boat were in seventh heaven (ah that other dimension). Terry
Saw 8.6 kts earlier today on an '81 Hunter37 while on a broad reach during the annual Tri-Service (Army-Navy-Air Force) regatta on the lower Chesapeake Bay. Surface winds were about 25 kts, seas 4-6 feet. Don't know how we placed but who cares; we had a ball.
hull speed is useful
I use the hull speed on my 336 to calibrate the speedo. In flat calm water, I run the engine up to nearly 4000 rpm, then I let the speedo stabilize for 30 seconds or so, then record the number. Since I know the boat cannot go any faster than its hull speed (about 7.2 knots for the 336), this gives me the calibration factor to enter into the instrument. For my speedo, the correction factor is calcalated by recorded speed divided by hull speed. I think this would be a good thing for some of you to do -- based on your reported speeds under sail
I use the hull speed on my 336 to calibrate the speedo. In flat calm water, I run the engine up to nearly 4000 rpm, then I let the speedo stabilize for 30 seconds or so, then record the number. Since I know the boat cannot go any faster than its hull speed (about 7.2 knots for the 336), this gives me the calibration factor to enter into the instrument. For my speedo, the correction factor is calcalated by recorded speed divided by hull speed. I think this would be a good thing for some of you to do -- based on your reported speeds under sail
Calibrating Speed
Not a reliable way to calibrate speed, Paul.. There are far too many variables involved to use theoretical hull speed as a calibration point. The way to calibrate your speed is to clock yourself between two known markers, and then do a reverse run. You can zero your speed gauge so that it will do all the integration for you if your speed varies (it records trip distance and average speed). Your instrument manual, Chapman's, and many other references describe the technique, but it just comes down to velocity X time = distance. Just remember to amke a reverse run to cancel out any current effects.
Not a reliable way to calibrate speed, Paul.. There are far too many variables involved to use theoretical hull speed as a calibration point. The way to calibrate your speed is to clock yourself between two known markers, and then do a reverse run. You can zero your speed gauge so that it will do all the integration for you if your speed varies (it records trip distance and average speed). Your instrument manual, Chapman's, and many other references describe the technique, but it just comes down to velocity X time = distance. Just remember to amke a reverse run to cancel out any current effects.
9 knots
Last year when hurricane Michelle was coming, my Hunter 30 topped 9 knots, by the GPS, against the Gulf Stream with 30 mph following winds. It was going down a nice sized wave and the boat was feeling like driving a car over a wash board road at a high rate of speed. It was great! Made record time from Key Largo to Key West. But I had to tighten a lot of loose nuts over the next few weeks.
Last year when hurricane Michelle was coming, my Hunter 30 topped 9 knots, by the GPS, against the Gulf Stream with 30 mph following winds. It was going down a nice sized wave and the boat was feeling like driving a car over a wash board road at a high rate of speed. It was great! Made record time from Key Largo to Key West. But I had to tighten a lot of loose nuts over the next few weeks.
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