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Old 12-07-2009, 07:01   #1
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Sailboat Pulls to Starboard - Full Power - Normal?

In my continuing quest to acquire a new yacht, I sea-trialled another boat on Friday.

The boat is a 55-foot cruising sailboat with a 100-horse Yanmar engine and a Bruton Varifold propellor, displacing 44,000 pounds. The prop has just been serviced in the Bruton factory.

The propulsion system seemed very strong, and was able to drive the boat past hull speed. I got about 10 knots in calm water under full throttle at nearly 4,000RPM. The Yanmar could sustain that kind of load for extended periods without heating up or complaining -- very impressive.

Only one problem: with a lot of throttle, full throttle or anything over about 3/4 throttle, the boat pulled to starboard. Not so much that I couldn't hold it in a straight line, but I would think it would make an autopilot work pretty hard. I have never experienced this; on the other hand, I don't have much experience in boats that big. The engineer who was with me told me that this is normal behavior.

Can it really be so, that this is normal? Maybe it's the prop? I am a little leery about folding props in general (especially after having the blades fall off of one in the Caribbean a couple years ago). A recent review in Yachting Monthly said that this particular prop gives extremely high thrust forward (the most of any propellor under test), but that it is weak in reverse. I can confirm the last thing -- the boat was very reluctant to stop and accelerated very weakly in reverse. But again, maybe that's a function of the size of the boat.

Anyone have any comments?
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Old 12-07-2009, 07:26   #2
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I don't think that a 100HP engine can make a 44 ton sailboat plane (exceed hull speed). If you were reading more than 10knots then you should add "fix speed paddles" to the list of to-do items.

You also shouldn't be able to reach 4000RPM on that Yanmar, assuming that it is a 4JH3-THE it should reach maximum RPM at 3800RPM and your drive/prop should be geared to reach hull speed at 85% of that. So if you can get the engine to go over 3200RPM then the prop needs re-pitching.

I just did a bit of Googling for some numbers and found this great thread.

I've noticed a pull to starboard on my boat as well, but it is only 20 tons and a fin keel, so the prop walk can be explained away, but your boat is much heavier. Is it a fin keeled one as well?
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Old 12-07-2009, 08:01   #3
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Thanks for that!

Yes -- fin keel, semi-balanced rudder on partial skeg, direct drive with p-bracket.

10 knots on the GPS corrected for current (average of a two-way run on a reciprocal course). Boat has a 46 foot waterline, so I reckon that's a little above hull speed which I calculate at 9.1 knots. I thought a boat could crash through hull speed a little ways, without planing, if it has enough power. Obviously running this way will be very inefficient.

I'm sorry your boat pulls to starboard as well. I would think it is rather annoying?

Yes -- the engine is Yanmar 4JH3HTE -- only 2000 cc's and turbocharged and intercooled to bejeezus. I agree that it should be governed at 3800. I'm not sure if the governor is a little more liberal than that, or if the tach was overreading. May well be that the prop is not correctly pitched, but shouldn't it be able to reach the speed at which it has maximum output?
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Old 12-07-2009, 08:42   #4
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I think that the pitch of the propellor should be set so that hull speed would be reached at the recommended continuous RPM speed of the engine as recommended by the manufacturer. I found This 4JH3_HTE pdf sheet on the web which would indicate redline at 3800 and maximum continuous RPM of 3700 - I would have guessed a lot lower and I have the same engine and don't run over 3200.

I only notice the pull under power when hand-steer and the pressure is just enough to be noticeable but not very strong.
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Old 12-07-2009, 09:47   #5
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Quote:
Originally Posted by Dockhead View Post
Only one problem: with a lot of throttle, full throttle or anything over about 3/4 throttle, the boat pulled to starboard.
At hull speed there is a wave right at the stern of the boat. If you look at boats doing hull speed there is one wave at the bow, the next wave is at the stern. The water level on the stern rises 6’’ or more and drop 6’’ or more amidships. In a wave there is a difference in water speed near the surface and at depth. Maybe just maybe due to the size and where your prop is it sees this small differential. The blade moving to the port side at the top moves through slower moving water(less thrust) than the blade at the bottom which is moving to starboard(more thrust). Since the blades are angled in the water they also push sideways hence the “prop walk”.
The crest of the wave a the bow and stern is what prevents a displacement hull to exceed hull speed. As you try to outdistance the wave at the stern you begin to try to climb a hill of water. The level of water at the bow is higher than at the stern. This takes a lot of power. It is similar as to when you get stopped almost dead in the water in square waves.
The only way to get over this hill of water is to prevent the stern from squatting. To do this you need a wide flat stern which extends from further forward than amidships as is seen in powerboats, they plane. A lot of modern fin keeled sailboats have a wide slightly flattened stern to be able to slightly exceed hull speed. If you look at the bottom of them aft of the keel they a very different that a traditional hull. It also makes them more susceptible to broaching in following seas.
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Old 12-07-2009, 09:58   #6
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I have been able to push full displacement boats a knot and sometimes two knots above theoretical hull speed.

I agree with the cause of the propeller walking.
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Old 12-07-2009, 11:00   #7
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Tachometers are commonly off. for a potential buy, your concern should be the hard pull to starboard. I have not experienced a strong pull in anything but a planing powerboat. I have experienced a little pull, but it sounds pretty heavy. I dont see how it could be a prop problem at all. Maybe the rudder is not correct? Are you really saying it pulls or you that you have to steer it to one side to go in a straight line?
Re-reading your post, are you sure it is more severe than it should be? Is it like a bad case of weather helm sailing? or does it just start slowly to one side if you let go of the wheel? Or does it immediately take a sharp right turn when you let go? Is the prop right in front of the rudder? In an aperture? or a ways forward? Sometimes prop wash directly on the rudder especially in an aperture can cause issues like that.
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Old 12-07-2009, 11:21   #8
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Quote:
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I have been able to push full displacement boats a knot and sometimes two knots above theoretical hull speed.

I agree with the cause of the propeller walking.
Theoretical is just that theoretical. It has a lot to do with the form factors. A better way of defining it would be where the ratio of power to speed starts to rapidly increase.
From my point of view it is desirable to be able to exceed hull speed as it indicates you have plenty of reserve power and a properly sized prop to push you against square seas and/or a very strong wind.
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Old 12-07-2009, 12:02   #9
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Quote:
Originally Posted by Cheechako View Post
Tachometers are commonly off. for a potential buy, your concern should be the hard pull to starboard. I have not experienced a strong pull in anything but a planing powerboat. I have experienced a little pull, but it sounds pretty heavy. I dont see how it could be a prop problem at all. Maybe the rudder is not correct? Are you really saying it pulls or you that you have to steer it to one side to go in a straight line?
Re-reading your post, are you sure it is more severe than it should be? Is it like a bad case of weather helm sailing? or does it just start slowly to one side if you let go of the wheel? Or does it immediately take a sharp right turn when you let go? Is the prop right in front of the rudder? In an aperture? or a ways forward? Sometimes prop wash directly on the rudder especially in an aperture can cause issues like that.

Yes, it starts to want to wander off to starboard, and the wheel wants to turn to starboard, so I have to apply pressure on the wheel. There's not a noticeable steering angle but the pressure is like weather helm. It's not so much that you can't hold it (but you do need two hands), and it's not so much that the autopilot couldn't hold it, I think, but it is unpleasant.

I was not smart enough to try it at more normal cruising speed and power setting to see whether I can detect any prop walk. Maybe it tracks straight then. Damn, that was stupid. I was more concerned to see whether or not the engine behaved itself at high power settings and just didn't think of it.

Yes, the prop is located between the fin keel and the rudder, like most bigger sailboats. It will be washing on the rudder for sure. But it's a right hand propellor -- wouldn't prop walk going forwards with a right hand propellor ordinarily pull the boat to port?
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Old 12-07-2009, 12:32   #10
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(but you do need two hands)...
Yea, it sounds pretty unpleasant ... if it bothered you the first time out, it will drive you crazy if you own it! Although..... you say "wander"..... one phenomenon of skeg hung rudders is they take a lot of pressure to turn, I guess because they usually are not balanced at all... but didnt you mention it was a partially balanced rudder? re prop location: I was just wondering if the prop was right in front of the rudder a few inches... one boat I had I could feel the prop wash in the rudder... kind of like feeling each blade as it passed the rudder... made the tiller wobble a little at certain rpms. yea, it's easy to forget to do things on a trial sail..
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Old 12-07-2009, 17:38   #11
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My boat is only 30 ft, fin keel skeg hung rudder. I had the prop cut down and depitched this year, so I could rev the engine to a higher / more acceptable cruising rpm. Last year i could only get about 6 kts at 2200 rpm. Now, with the smaller prop, I can get 6.7 kts, at 3200 rpm.

However, I now get a fairly strong pull to port, while motoring, and it is more pronounced at higher speed / higher rpms. I know the rudder / steering are OK because it does not do it while sailing.

I assumed that it was just the right hand propeller, causing the bow to go to port, from the torque / prop walk.

I also assumed it was worse this year, as my engine would be putting out more HP at 3200 rpms, that at 2200 rpms, last year (where I did not notice it).

Is this prop walk ( in forward) not common????

To the OP: Is your prop left hand?
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Old 13-07-2009, 02:19   #12
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Quote:
Originally Posted by Northeaster View Post
My boat is only 30 ft, fin keel skeg hung rudder. I had the prop cut down and depitched this year, so I could rev the engine to a higher / more acceptable cruising rpm. Last year i could only get about 6 kts at 2200 rpm. Now, with the smaller prop, I can get 6.7 kts, at 3200 rpm.

However, I now get a fairly strong pull to port, while motoring, and it is more pronounced at higher speed / higher rpms. I know the rudder / steering are OK because it does not do it while sailing.

I assumed that it was just the right hand propeller, causing the bow to go to port, from the torque / prop walk.

I also assumed it was worse this year, as my engine would be putting out more HP at 3200 rpms, that at 2200 rpms, last year (where I did not notice it).

Is this prop walk ( in forward) not common????

To the OP: Is your prop left hand?

According to the survey, it's a right-hand prop, which surprised me, since the pull is to starboard.

Maybe this occurs when the propellor is near cavitation, and the boat is near hull speed. If you throttle back a little, say to your previous cruising speed of 6 knots, does your boat still pull to port?
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Old 13-07-2009, 04:29   #13
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Couple things. If the boat is pulling to starboard it is not the prop it is the wash over the rudder. The rudder may not be symetric or the alignment is not quite right and that is why you see it pulling to starboard at higher revs. (more flow)

Sounds like the engine is pitched properly if you can get to WOT. I would not mess with that.

10 knots for that wl is doable, it is not hard to drive the boat over hull speed.

We have 55 foot of water line and tick along at 9 and with revs of 2400, we tend to run around that number but do run at WOT a couple times a year to blow out the carbon. 32 tons, W108C6, 24" 3 blade Maxprop, WOT is 3600.
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Old 13-07-2009, 09:17   #14
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I have a thought on your issue of "wandering" to stbd while under full ahead power:
Since you have a right-hand prop (in forward), you have a LEFT hand prop in reverse. Left hand reverse props will walk to port when reversing, so often the prop shaft is angled to stbd to counter-act this (caveat...I should say, IF the manufacturer needed to angle the prop shaft so it could clear the rudder, they will usually angle it so it minimizes, rather than accentuates, prop walk in reverse).
So, you may have a shaft that is angled to stbd. Under full revs in forward, that thrust of water is unevenly washing over the skeg/ rudder.
Perhaps, because of this uneven distribution of force, or maybe even Bernoulli's equation comes into play here, your boat (and others with a similar set-up, at full revs), wanders to stbd.
Just a thought...
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Old 13-07-2009, 10:04   #15
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Joil and John - I know I am not the OP, but having similar issue, I believe you guys have hit the nail on the head, with the idea of prop wash and/or propeller shaft alignment. My prop shaft is aligned to pass by the port side of my skeg and rudder (know this as I had to pull it a coupleof years ago). So that likely explains why it pulls to port with increased rpms/ wash over the rudder.

To the OP - Mine still does it at lower rpms / boat speed as well, but it is not as pronounced. As mentioned, since my engine can rev much higher (thus having more available HP) this year, with the smaller / less pitched prop, it likley has more wash going by the rudder unevenly.
This would explain why it doesn't do it while sailing, as there is no extra (propeller induced) wash going by the rudder.

I guess I will live with it! Thanks for the great input!
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