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We sail a Taswell 43 monohull sailboat, with a Yanmar 55hp engine turning a 20" diameter Autoprop. After getting the engine rebuilt, she now gives us full RPM (3600)on demand. But we have discovered a problem in the process. At normal cruising RPMs(1800-2400RPM), the stern "tucks" approx. 7" putting the engine exhaust port (transom mounted) just underwater, and coating the lower transom with a black sooty smudge that's difficult to get off. But...as we increase power, the stern tucks more and more... up to 22" from the still water line, putting the transom almost half way under water! And that coats the whole transom area with the smudge.
The smudge/blackened transom is one issue, but the excessive "tucking" is something else. We're stumped!
Autoprop says the prop is the proper size! I can't believe that a 22" tuck (under full power) is normal....but I don't know what the issue could be. We've unloaded and reloaded the aft end of the boat-without change. Any ideas????
Thanks
 

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One of None
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Pushing displacement hulls to hull speed will make the boat ride lower. "squat" trying to make the boat go faster is waste HP. Could even be why you needed a rebuild on the engine if that is your "normal" way of cruising.

the exhaust gases condense and also the air moves towards the transom when under way so you get soot, lots of it. Even my boat with only 16 hp, if I max rpm the engine the transom squats. Crusing speed is not always max rpms. Might be time to slow down and enjoy the ride :)
 
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Sounds like you're pushing too hard, alright.. try backing off to better trim, and see if you really 'lost' any speed.. certainly you'll be making smaller waves and likely saving fuel to boot!
 

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Could be the first sailboat I've ever seen to install stern planes. :)

Seriously, just stop channeling your inner stinkpotter and back the throttle down until you just begin to see speed (through the water, not SOG) drop below hull speed. Squatting is normal, we have about 10" at normal cruise.

Once that new motor starts losing compression and the prop gets fouled like the rest of us, you'll no longer have this problem! :)
 

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It's an Autoprop - what is the pitch? Your Tazwell 43 / 55 hp combo is good with the 22 diameter.

The soot says you are over-propped (unburned fuel), but you say you reach full RPM which makes no sense.

Either way, I think you need to lower the pitch and reduce the load on the engine to prevent the soot. You should be able to reach full hull speed (about 8 knts) at 2/3 RPM, any more is a waste and not efficient.

A waterline length of 35.33 (i.e. yours) isn't that much different from mine - 32.5, and I run a 16 x 15 auto prop on 44hp just fine to hull speed.
 

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Farr 11.6 (Farr 38)
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As many have said, you may be over reving your engine. Cruising boats are often equipped with engines that produce more power than the boat can use in flat water and calm winds. The extra horsepower means that the engine can operate at a lower rpm and will have some reserve thrust for motoring into a stiff wind and a steep chop. Assuming that you have a clean bottom (a dirty bottom can increase squat) then the next step is to go out and experiment a bit. Pick a windless or light wind day and take the boat out and start motoring. Set the throttle at a speed that you consider to be cruising speed, and then experiment with throttling up and down in small increments. maybe 50 rpm over 600 to a 1000 rpm range. At some point below cruising speed, the speed will seem to increase linearly, but as you start approaching displacement speeds, big increases in RPM will only result in very small increases in speed. After that point, all that adding RPM's is doing is burning more fuel and causing the transom to squat. Its not adding much speed. Once you hit a point of diminishing returns you can back down a few hundred RPM and save fuel and wear and tear on the engine. Pushing the engine over that limit also can result in inefficient combustion and so a blackened transom.

If you want to be scientific about it, do multiple rounds throttling up and down in the same small increments of 50 rpm over 600 to a 1000 rpm range. Let the boat settle down and then record the speeds at each rpm. Its pretty easy to plot these using an Xcell spreadsheet and produce a graph showing RPM to speed. Its not hard to see the approximate point at which the rpms rise quickly while the speed does not. Perhaps a 100 rpm beliow that point is your ideal cruising speed.

Now then, all bets are off if the boat has a dirty bottom or you are motoring into a headwind or steep seas, in which case there is no good answer since you will end up overworking the engine some no matter what you try to do, and so will have more squat than ideal.

Jeff
 

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Autoprop says the prop is the proper size!
Yes, they always say that but in general they over-prop you. That's why you have the smudge and they 'dig in' so well that's also why you squat.

I put an Auto-Prop on my previous C320 and loved the power especially when motor sailing but hated the black smudge.

My current boat has an Auto-Prop that even had an article published about it being sent back for being 'over propped' before I bought it. I still have some black smudging but since my boat is a motor sailor I still love the way it works with the sails - that's the Auto in Auto-Prop.
 

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Farr 11.6 (Farr 38)
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It's an Autoprop - what is the pitch? Your Tazwell 43 / 55 hp combo is good with the 22 diameter.

The soot says you are over-propped (unburned fuel), but you say you reach full RPM which makes no sense.

Either way, I think you need to lower the pitch and reduce the load on the engine to prevent the soot. You should be able to reach full hull speed (about 8 knts) at 2/3 RPM, any more is a waste and not efficient.

A waterline length of 35.33 (i.e. yours) isn't that much different from mine - 32.5, and I run a 16 x 15 auto prop on 44hp just fine to hull speed.
Chuck,

Autoprops are supposed to be self-pitching. The blades are supposed to rotate into the ideal pitch and automatically adjust as the conditions change. I know there are a lot of people who love them, but I also have spoken to people who have talked about issues with Autoprops in a range of conditions where they felt that the prop did not self adjust properly, one being the over reving the engine for the boat speed. But their main gripe seemed to be in choppy conditions where the prop seemed to be changing pitch and altering the behavior of the engine so that the engine would seem to cycle from bog to race and back.

The other comment is that although the Taswell has a similar waterline length that is not all that much longer than your boat, it is a wildly heavier boat with a D/L around 324, vs your boat with a D/L is somewhere in the mid-240 range.

Off the topic and FWIW, I have never understood why the Taswell 43's are so absurdly heavy. Its predicessor, the Norseman 443 has two feet more waterline, has roughly the same tankage and a lot more ballast, and weights 4-5,000 lbs less. I have never understood why so few Norseman 443's were built or why Ta Shing replaced them since the 443's were a nicer boat all around.

Also off topic, I have been thinking about your chainplate redesign. I have a few ideas, but I keep coming back to the idea of reinforcing the hull in that area, adding athwartships knees tied at the top and bottom by a longitudinal stringer, and bolting straight chainplates to that knee.

Jeff
 

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Hey Christy/Stan.... where you been?? Good to 'see' you here again!
 

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You would think that the downward angle of the shaft and prop at a the same pitch would lift the stern. And that is not the case I can see. With my prop berried between my keel and rudder in it's hole, I am lucky to get to hull speed. It is considered an auxiliary engine. Some where over the years some one said it would be nice to be able to motor along and changed from an auxiliary setup to a primary means of propulsion. I have seen some sail/motor boats get up on plane and move fast. Looks very sill. And I have also seen sail boats motoring along when they could be sailing at the same speed. Never will understand it. Do you need to be somewhere on a schedule? Does not sound like sailing to me.

Not sure anyone has said that you can not move the boat any faster than hull speed, no matter how much HP and prop you have. And there is no doubt you are overpowering. But in big seas and wind the setup you have would be desirable, as long as it is not a following sea, you will get swamped. Is your boat a double ended? I could see with a double ended boat the issue would be much worse then with the big flat, with diving platform sterns. And with a 40'+ boat and a high free board of newer boats, I can't see that 22" of squat is an issue. The exhaust outlet being underwater is not an issue, exhaust will work fine, And the black stains are normal for all boats, even my gas engine. A marine polish like Star Bright that has kerosene in it will clean that up easy.
 

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After pondering this issue...I can see where a square stern boat would squat. What is happening is the boat when cutting through the water is leaving a void in the water behind the stern, a hole so to speak, faster you go the deeper the hole is created. Then the water will eddy around the stern, pushing down more. And if a step or swim platform gets underwater the eddy will push down on that. A double ended boat will not have this issue...like sails, clean air in, clean air out...smooth.

Is that an appropriate hypothesis?
 

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After pondering this issue...I can see where a square stern boat would squat. What is happening is the boat when cutting through the water is leaving a void in the water behind the stern, a hole so to speak, faster you go the deeper the hole is created. Then the water will eddy around the stern, pushing down more. And if a step or swim platform gets underwater the eddy will push down on that. A double ended boat will not have this issue...like sails, clean air in, clean air out...smooth.

Is that an appropriate hypothesis?
Close... but think of a bulldozer.. pushing a bigger and bigger hill of dirt. as the hill gets larger the more power it takes to go slower.

Planing hulls will plane. like Mac 26 motor sailers. displacement keel boats look like they are on plane when surfing the top of big swells but it's not really the same. Turbulence in the water (bubbles) creates less buoyancy also.
 
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Hey Christy/Stan.... where you been?? Good to 'see' you here again!
Oh... I've been here but mostly just lurking. The past few years it seems that too many times I've posted from my non-existent memory and been appropriately corrected and slinked away for my stupid mistakes.

One more comment on the "tuck" though. Nauticat 33's and 331's and probably the others too being designed as Motor sailors have a little squared off section of hard(ish) chine well below the waterline about 3/4 of the way back that I think reduces squat and translates more power into climbing over the bow wave. I say that because I have motored (just a brief test with either a newly painted bottom or freshly cleaned) at up to 8.2 with my 'hull speed' being only 7.1...... and yes I did the slack tide two directions to make suere, GPS and paddle wheel verification etc... many times over the years pushing 18,000lbs with 75hp (90 on the old 33's). Out on the Nauticat lists over hull speed is commonly spoke of. It doesn't make it like my friends Nordic Tug with multiple chines to make it a 'semi-displacement' that does 1.5 times hull speed, but from what I can see it helps. Just a comment on sailboat hull speed because I'm wondering if other manufactures have added a little hull modification to try and 'break the law' ;)
 

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As everyone above said.. you are over propped/powered. Once you hit hull speed, all more power is going to do is dig a deeper hole. Slow down, enjoy the scenery, better mileage, and less soot on the stern
 

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We sail a Taswell 43 monohull sailboat, with a Yanmar 55hp engine turning a 20" diameter Autoprop. After getting the engine rebuilt, she now gives us full RPM (3600)on demand. But we have discovered a problem in the process. At normal cruising RPMs(1800-2400RPM), the stern "tucks" approx. 7" putting the engine exhaust port (transom mounted) just underwater, and coating the lower transom with a black sooty smudge that's difficult to get off. But...as we increase power, the stern tucks more and more... up to 22" from the still water line, putting the transom almost half way under water! And that coats the whole transom area with the smudge.
The smudge/blackened transom is one issue, but the excessive "tucking" is something else. We're stumped!
Autoprop says the prop is the proper size! I can't believe that a 22" tuck (under full power) is normal....but I don't know what the issue could be. We've unloaded and reloaded the aft end of the boat-without change. Any ideas????
Thanks
The maximum speed of a full displacement hull is determined by the length of the wave created as the hull passes through the water, with a crest at the bow, another at the stern and the trough midway between them. The yacht is essentially "pushing" this wave across the surface of the water. In relatively deep water where the effect of depth can be ignored, the maximum speed that a wave can travel is a function of its length, the distance between crests and can be calculated as v = (gL/2Pi)^.5. (g= the acceleration of gravity and L=wave length)

This formula roughly translates into the familiar equation for maximum boat speed of 1.34 x (Square Root of the Water Line Length). The water line wave simply cannot move any faster, regardless of the amount of energy expended. What the energy in excess of the amount used to reach the maximum wave speed can do is increase the wave amplitude, the height between the bottom of the trough and the top of a crest. About the maximum wave amplitude that can be supported is about 1/7th of the wave length or a peak angle of about 120º from face to face of the wave.

On your boat, the waterline length is reportedly 35.33'. The maximum speed a wave of that length can make is about 7.98 knots. Beyond the energy needed to get there, you just increase wave amplitude as noted about. And, at 1/7th the wave length, the maximum wave amplitude that can be supported is about 5.05 feet or about 30 inches above and below the mid-point between the trough and the crest or, in general, the level of your water line. Push the boat a little harder and, depending upon the shape of the forefoot, the boat might "climb" up the back of the bow wave somewhat, dropping the stern down the face of the stern wave, or squatting, by an equal amount, giving the yacht a bow up attitude. Pumping more energy into the sea will just exacerbate the situation but not increase your speed unless you have so much energy that the boat "planes" up and over the bow wave (which a Taswell 43 isn't likely to do) in which case wave length ceases to be a determining factor to speed.

Others have given you a means of determining the amount of energy necessary to move the yacht through still air and water-RPM verses speed. With RPM and propeller characteristics, one can determine the amount of thrust the engine is developing but the speed the yacht can make under such conditions is not the determinant of how much thrust you might need or be wise to have in your back pocket as sea and wind conditions may have to be overcome by the thrust of your power train before you even begin to think about limiting speeds.

In short, based upon the foregoing, it doesn't seem like you have a problem...eh?
 
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