Don't sound like you have a fouled bottom nor a fouled prop. Certify the accuracy of the tachometer and if you are still missing 700 RPM then get a proper pitch prop. You should be able to attain full rated RPMs for proper engine operation and useful life.
Engine RPMs vs. Hull Speed
- Thread starter garylinger
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yes and thank you......
just wondered about the 2 blade three blade thing as back in power boat days i always had three blade clevers and i know there is a more stable response in a three blade as to a two blade ...this low hp thing is something i will have to go through a learning curve i am sure...but then maybe not if i always have good wind lol
regrads
woody
No. "Slip" is a term that I have almost never heard dealing with props among professionals although yachting books use it a lot. A prop blade is just a rotating foil and therefore has to act at an angle of attack to produce any lift (thrust in this case). If there was zero "slip" there would be zero propulsion.
"Slip" is not wasted energy and low slip does not indicate efficiency. Thrust is a function of the amount of water moved backward from its previous position and how fast it is set into motion. This is easier to visualize if you think of the boat going by you and the previously motionless water moving. Look closely at the rooster tail left by a fast boat and you'll see that the water is moving against the far shoreline in the direction opposite the boat.
There is a direct relationship and linkage between the thrust as represented by the mass and acceleration of the water and the difference in pressure on each side of the prop blades. The overall pressure difference will equal the resistance of the hull. The less blade area, the higher the pressures will need to be.
Two blade props are actually inherently more efficient than 3 blade props which are more efficient than 4 blade wheels. The conventional wisdom / urban myth / forum talk differences between them generally apply when props of the same diameter are being compared. The 3 blade wheel can have more blade area for a given diameter so often give better results when diameter is restricted as it is in many sailboats. If you have enough prop clearance to get the necessary area in a two blades, it can work just fine; even in bucking conditions. My boat has a two blade wheel and is just as competent under power as if she were a small trawler yacht.
Three are so many variables here that you have to have calculations done to get it right. The computer programs most prop shops have now make that a lot more accessible.
In this particular case, I agree that the tach is one of the first things to check. If the prop load is holding the engine at less than its maximum RPM and you keep adding fuel, it isn’t going to fully burn and there will be clouds of black smoke coming out the transom. It doesn’t sound like that is happening in this case.
Back before computers and when much less instrumentation was used on sea trials, we liked to see just a hint of black smoke at full throttle that would disappear after backing off about 100 RPM. That’s a good indication that the engine prop match is about right. Now that things are more sophisticated, they can set up the engines so the black smoke would appear just past WOT.
Maine Sail is right. If your prop is holding your engine back, you may have problems with your engine down the line. Even at lower throttle settings, there will be a mismatch between optimum engine speed and torque. If impatience leads you to operate a lot at WOT, you’ll be compounding the problem but building up lots of unburned fuel around the valves and in the exhaust system. You also paid for that fuel and are turning it into maintenance costs instead of propulsion.
I took the suggestions and have ordered an inexpensive digital photo laser tach. When it arrives I will do some checking. In the meantime I asked Stanley at Beta-Marine for his advice and this is his response to both his advice and also recommendation for proper sized prop -
"I suggest that you have your prop re-pitched as you are overloading the engine with this propeller. The engine needs to reach 3,600 RPM (3,500 is ok) in gear.
13'x8" three blade RH with 50% BAR.
If you insist on a 2 blade, use 14"x10"."
My current prop is 13 inches but I don't know what the pitch is. I would think that the smaller diameter prop would give just the opposite results of what I have. Bob has a much greater diameter prop on his S2. Woody, what prop do you have (I'm sure you have said in the past but I don't remember)?
"I suggest that you have your prop re-pitched as you are overloading the engine with this propeller. The engine needs to reach 3,600 RPM (3,500 is ok) in gear.
13'x8" three blade RH with 50% BAR.
If you insist on a 2 blade, use 14"x10"."
My current prop is 13 inches but I don't know what the pitch is. I would think that the smaller diameter prop would give just the opposite results of what I have. Bob has a much greater diameter prop on his S2. Woody, what prop do you have (I'm sure you have said in the past but I don't remember)?
Gary:
One thing that is important when sizing a prop is the tip clearance (the distance between the hull & the prop tip). If this distance is too close you will have very severe prop walk.
I suggest that you get the specs of your engine/transmission and contact a Prop Shop to have them size a prop for your vessel.
One thing that is important when sizing a prop is the tip clearance (the distance between the hull & the prop tip). If this distance is too close you will have very severe prop walk.
I suggest that you get the specs of your engine/transmission and contact a Prop Shop to have them size a prop for your vessel.
I got a chuckle out of Roger's information... not at his expense. 
Way back when the world was younger I used to car pool up to the annual PIYA meetings with some other local yacht club worthies to their all-day Saturday meetings. About a six hour drive. One of the regulars was the late N.A. Robert Smith (designer of the Cascade sailboats, among others, and a S&S veteran in his younger days). I recall his discussing props one memorable trip. He was adamant that the very best (theoretical prop for a sailboat was a one-blade! Due to weight and thrust problems he admitted that it was not practical... but the take-away was that fewer blades were actually more efficient, given room to swing the diameter needed.
Of course, with little room to swing a very large diameter prop under our hull, all the theory is just... theory. Over the last 16 years we have gone from a factory two-blade fixed (which was way over pitched) to a two-blade featherer for a decade, to a year with a fixed three-blade. The current "cruising" prop does truly have a lot of thrust compared to the prior two's. When our $$-flow improves some day we plan to move to a three blade featherer.
Props are really quite an interesting mix of engineering and magic.
L
Way back when the world was younger I used to car pool up to the annual PIYA meetings with some other local yacht club worthies to their all-day Saturday meetings. About a six hour drive. One of the regulars was the late N.A. Robert Smith (designer of the Cascade sailboats, among others, and a S&S veteran in his younger days). I recall his discussing props one memorable trip. He was adamant that the very best (theoretical prop for a sailboat was a one-blade! Due to weight and thrust problems he admitted that it was not practical... but the take-away was that fewer blades were actually more efficient, given room to swing the diameter needed.
Of course, with little room to swing a very large diameter prop under our hull, all the theory is just... theory. Over the last 16 years we have gone from a factory two-blade fixed (which was way over pitched) to a two-blade featherer for a decade, to a year with a fixed three-blade. The current "cruising" prop does truly have a lot of thrust compared to the prior two's. When our $$-flow improves some day we plan to move to a three blade featherer.
Props are really quite an interesting mix of engineering and magic.
L
OK, I was being simplistic! "Slip" is a VERY important term if you're doing prop calcs. As you imply, since water is incompressible the prop turning through the water WILL move the water according to the pitch calc (pitch x shaft RPM = inches per minute). (Except for a cupped blade like the Campbell Sailor, but we won't go into that!) But as you noted, you will see water going out the back of the boat faster than the boat is going forward - this is "slip". So if the pitch calc says, say, 10 knots, yes the water is going backwards at 10 knots, but the boat may only be going forwards at 7 knots. (30% slip) If you tied the boat to the dock, you have 100% slip.
THEN we get into the actual prop: is it REALLY moving the water at 10 knots? If it was a full screw in a pipe, yes. But there are "inefficiencies" (I'm being sloppy here - this refers to the speed of the water being pushed backward, not energy). As anyone who's backed a sailboat knows, some of the water is displaced sideways. Also, if you go to the "limit" and have two blades 1/8" wide, you can see that although they may be pushing the water right at the blades back at 10knots, the whole column of water described by the prop diameter will NOT be moving back at 10 knots.
So: bigger diameter prop, less slip. More surface area of the blades, less slip. More "lift" on the blades (ie Campbell Sailor), less slip (although is is a bit different since the result is the blades move the water at MORE than 10 knots, but the effect is the same. I've been arguing with CS about what they call the pitch for some time...)
Of all the prop-calc programs I've seen, I have ONE that tries to calculate slip (often the user has to enter a guess). It doesn't do too bad a job, but it's not 100% accurate by any means. It simply gets you in the ballpark.
druid
Oh, and one more thing about 2-blade vs 3-blade (told to me by the CS crew...): the keel of a sailboat creates a vertical column of "dirty" water. For a 2-blade prop, both blades are in the dirty water at the same time, whereas a 3-blade prop only has one blade in the dirty water at once. According to CS, this creates less vibration.
druid
druid
For anyone following this good thread and wondering if their alternator based tachometer on the engine instrument panel is accurate (as I now am), I've seen on eBay laser tachometers for as low as $19.00 including shipping. Link below. Of course I don't know if they are any good, but for the occassional use, odds are the device will be worth the $19 at risk. Although my prop/hull/engine seem to be matched well with the instrument cluster tack showing 2550 rpm against the engine rated 2650 rpm, I am now tempted to investigate the real deal.
http://cgi.ebay.com/Digital-Photo-L...emQQptZLH_DefaultDomain_0?hash=item3a5583ffa2
http://cgi.ebay.com/Digital-Photo-L...emQQptZLH_DefaultDomain_0?hash=item3a5583ffa2
Tech setting
Did you check the setting on the back of the tech? Tech counts pre-rectifiered AC sinewave to determine the displacement of the needle. The needle points to the face plate which tells you the RPM. Depending on the number of poles inside the alternator and the pulley ratio, the tech has to be set accordingly. Secondly, if there is belt slip you'll get wrong reading as the tech only knows how fast the alternator is turning.
Did you check the setting on the back of the tech? Tech counts pre-rectifiered AC sinewave to determine the displacement of the needle. The needle points to the face plate which tells you the RPM. Depending on the number of poles inside the alternator and the pulley ratio, the tech has to be set accordingly. Secondly, if there is belt slip you'll get wrong reading as the tech only knows how fast the alternator is turning.
Re: Tech setting
Dear Alexso38:
Thanks your points to consider. My alternator belt is tensioned firm but not too tight ... so probably no slippage. But as to the # of poles on the altenator and the pully ratio, I've always just took it as faith that Yanmar got it right for the instrument needle tach setting. Also that the 30 year old needle tach itself is still working as designed. Hence my thought to get a laser rpm meter to verify ... or then I'll have the info to adjust the needle tach if it can be done.
thanks/regards,
Dear Alexso38:
Thanks your points to consider. My alternator belt is tensioned firm but not too tight ... so probably no slippage. But as to the # of poles on the altenator and the pully ratio, I've always just took it as faith that Yanmar got it right for the instrument needle tach setting. Also that the 30 year old needle tach itself is still working as designed. Hence my thought to get a laser rpm meter to verify ... or then I'll have the info to adjust the needle tach if it can be done.
thanks/regards,
Then you haven't seen any real ones and the one that asked you to enter a guess was probably the closest to being what professionals use.
Every real propeller program is basically this:
(Very precise value) x (very precise value) / (very precise value) x (Big Guess) = Propeller
The big guess is usually called Va for "Speed (Velocity) of Advance". When a boat moves through the water, even if being towed, it drags water with it. At the prop location the water may be moving only about 7 knots if the boat is going 10. The fatter the boat and the steeper the water flow lines just ahead of the prop, the greater the difference. Even on fairly clean planning hulls, there may still be a 10% difference. This throws everything off.
Prop calculations would be easy if Va could be easily predicted. Hull shapes and all the complex factors that influence flow just make a few rules of thumb and educated guess the state of the art. To get an exact match without data from a sister hull, you usually have to put a prop on and carefully compare sea trial performance with predictions. You then use the difference to fine tune the Va guess and re-pitch the prop or change it.
An exact match on sailboats isn't so important that the prop isn't considered "right" the first time. Va data is also more available for production boats than for new designs. For a working power vessel running thousands of hours a year, the effort of a perfect match can pay off significantly.
For a heavy, traditional boat, Va may be as low as 70% of speed through the water.
Prop slip is more useful to power boats. It primarily has to do with hull efficiency and not prop efficiency. What the prop slip programs do is to assume a given hull with have some prop slip number. Given a target boat speed based on HP, they will use these numbers to calculate a pitch number. Its a good method to select a ball park pitch for a given planning boat hull and HP.
But for a displacement hull sailboat - prop slip is a fairly worthless number. At low speeds where hull drag is low, you will have very low prop slip since you don't need to accelerate much water to drive the hull. If you have max HP and are pushing the boat into the hull speed range, you will have high prop slip. Doesn't really tell you much.
I think Roger is correct..
But for a displacement hull sailboat - prop slip is a fairly worthless number. At low speeds where hull drag is low, you will have very low prop slip since you don't need to accelerate much water to drive the hull. If you have max HP and are pushing the boat into the hull speed range, you will have high prop slip. Doesn't really tell you much.
I think Roger is correct..
Fine. Take your hull speed, multiply by 1202 to get in/min, divide by shaft RPM and that will give you prop pitch. Buy whatever diameter or number of blades you like.
good luck
druid
It sounds to me like if you are actually only turning 2900 you are lugging the engine, which is not good for it. Think about trying to run into a nice sloppy head sea and what that will do the the engine. I actually have my boat a bit underproped so that I can run into a sloppy head sea and have the engine (Yan 3GM30F) just pure right along. It is hard to hurt a simply diesel, but lugging is one way to do it.
Kevin
Kevin
But that is where I am REALLY confused - if anything I would think that I am underpropped with the 13 inch, 2 bladed prop especially considering that others with the same hull have a much larger diameter prop...
What?
Let's see 5280 ft/mile x 12inch/ft / 60 min/hr = 1056 * 1.15 mph/knot = 1214 so it seems 1202 is ~correct.
What's wrong here?
6.5 knots x 1202 = 7813 / 3600 rpm = 2.17 pitch?
Let's see 5280 ft/mile x 12inch/ft / 60 min/hr = 1056 * 1.15 mph/knot = 1214 so it seems 1202 is ~correct.
What's wrong here?
Free prop calculator
Just to make sure we aren't going crazy. I found a freeware prop calculator and ran it just for giggles and it came up with a 14 inch 9 pitch.
Gary,
Let's not let this get to complicated with lots of prop theory and lets assume for now that your tach is correct, even though it may not be. What do we know?
1) Beta wants you within 100 RPM of 3600. This is a brand new engine, engines are expensive, I would take their advice on this.
2) You have a 13" prop that is only allowing 2900 RPM at full throttle.
3) Pitch can solve your issue.
Let's look at some hypothetical numbers seeing as you don't know your actual pitch.
13X13 = 2900
13X12 = 3100
13X11 = 3300
13X10 = 3500
13X9 = 3700
You can see the effect of pitch..
The above numbers assume 200 RPM per inch of pitch. 200 per 1" is what Michigan Wheel, Norm at Campbell Sailor and others use as a rough guideline on sailboat props. Many bronze props may be limited to about two inches of pitch change. A good prop shop should be able to tell if it has already been re-pitched or not. It is likely that you'll need a new prop if the 2900 is accurate as you'd need nearly 3-4" of pitch change @200 per inch to hit 3500 RPM and be within Beta's range. I think it would be very tough to find a shop willing to re-pitch a prop by 3-4"..
Bottom line, let a professional prop shop do your calculations and size your prop, especially if the one you have is outside the limits of a re-pitch. Norm at North by West is excellent but do be aware that he has over pitched many a prop, including mine. He did however re-pitch it at no cost to me. I went from a Michigan Wheel three blade 16" X 12"P to a three blade 16"X10P Sailor and was off by 300 RPM. The CS prop is very efficient.
Campbell Sailor Prop Review (LINK)
Norm then reduced the pitch another inch, to 16"X9P for about 200 RPM and then took out some cupping for another 50+/- RPM. What ever shop you buy from get a guarantee that they will get your prop within 100 RPM of 3600, and that if they don't, you won't be paying for a re-pitch.
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