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Old 28-10-2014, 00:57   #1
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New boat pulls to starboard under power, prop?

Just bought a 46' fin keel, balanced rudder steel motor sailor with a 26" 16" pitch prop that under power pulls considerably to starboard, so much so on the delivery trip in strong seas burnt out the autohelm driver motor.

The previous owner who built her told me he sailed the boat so it didn't affect him too much but thought the prop was oversized, and hinted I may want to put on a smaller prop.

Under sail the boat is balanced beautifully but she is a motorsailer, or thats what I want to use her for.

I wondered if the balanced rudder was the issue with the prop wash driving the rudder to one side and maybe changing it to a skeg hung would solve the problem. As she is steel not an impossible job.

My last boat, also steel with a fin and skeg and a big prop didn't have these issues.

I saw on some previous threads that some posters had this issue and that they were told its normal, or prop not centred....

I need a course of action before just changing the prop.

Any tips on a UK, preferably south coast, expert who can work out a solution would be great.
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Old 28-10-2014, 04:55   #2
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Re: New boat pulls to starboard under power, prop?

do some searching about prop walk, it's normal
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Old 28-10-2014, 05:50   #3
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Re: New boat pulls to starboard under power, prop?

Perhaps engine and shaft alignment is worth checking. Our boat had a terrible pull when we first purchased her, which I assumed at the time was prop walk. Discovered post delivery that the so-called professional that installed the new engine for the PO's had forgotten to adjust the rear engine mounts and must have only finger tightened the front mounts. I replaced the rear mounts and realigned the engine and shaft and the constant pull under power stopped.
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Old 28-10-2014, 06:43   #4
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Re: New boat pulls to starboard under power, prop?

If she's pulling to starboard, it's probably prop walk. That is a very large propeller...and the larger the propeller and the greater the shaft angle, the greater the prop walk since it's a function of the greater relative pitch of a blade on (in your case) the downstroke arc of a given revolution. Essentially, they move more water than on the downstroke.

Have you run the engine at full throttle and checked the RPMs? It should run at @ 3000 rpm, depending on the specs for the engine. Technically, if you're not reaching full spec'ed RPMs then you're over propped. Reduce the size or reduce the pitch, both of which will reduce your apparent prop walk.

A skeg is not going to solve your prop walk problem, and in fact will be less effective at countering it, but it will reduce the stress on your auto pilot by taking the leading edge of the rudder out of the prop wash.
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Old 28-10-2014, 06:58   #5
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Re: New boat pulls to starboard under power, prop?

Any good prop shop should be able to calculate the correct size prop based on engine hp, max rpm, transmission ratio, displacement and LOA/LWL. Yours does sound too large.
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Old 28-10-2014, 07:17   #6
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Re: New boat pulls to starboard under power, prop?

What your calling prop walk is also called P factor.
P-factor - Wikipedia, the free encyclopedia
Before tearing out the old rudder and going to the expense of changing the design of the boat, I'd put an adjustable trim tab (also called a servo tab) on the current rudder, with that you can neutralize the "prop walk" it would have to be adjustable because this phenomenon is not there under sail, and I bet it's variable as well, the more motor power used, the more pronounced the torque.
Servo tab - Wikipedia, the free encyclopedia

If you want to correct it by rebuilding the boat, then I would do it by changing the thrust angle of the prop shaft, for example the aircraft we build has a 1.5 deg right thrust built into the motor mount to help counter the P factor. But I think the servo tab is easiest and cheapest.

On edit, maybe a simple as a power boat trim tab would work, you know the ones used to hold the bow down on a power boat. Fairing it in might be fun though.
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Old 28-10-2014, 08:01   #7
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Thumbs up Re: New boat pulls to starboard under power, prop?

Quote:
Originally Posted by a64pilot View Post
"... I bet it's variable as well, the more motor power used, the more pronounced the torque."
+1

First thing to check. Correlate amount of "walk" to power dialed up and you'll most likely find it is the prop size.

Fair winds!
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Old 28-10-2014, 08:16   #8
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Re: New boat pulls to starboard under power, prop?

My previous boat had a balanced rudder, very nicely balanced by the way and an offset prop and had no issues at normal cruising speeds. My guess is that the previous suggestions are spot on.
The one effect of the offset prop is that at low maneuvering speeds, IE, docking, turning in a narrow channel, etc, is prop walk causing exagerated yaw when accelerating either in revers or forward, but you can actually use this to your advantage when trying to get into a tough dock space.
I've been able to spin boats 360 deg in their own length just using prop walk.
Talk to a prop shop before doing any expensive hull work.
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Old 28-10-2014, 08:21   #9
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Re: New boat pulls to starboard under power, prop?

Is it just me, or is a 26" prop not only a big prop, but a pitch of 16" is also a lot of pitch for a prop of that size? Meaning either a BIG motor, or a large reduction ratio tranny?

Decreasing diameter means increasing pitch, but at 16" already, how much can you increase that?

What has the greater effect, increased diameter or pitch to "prop walk"?
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Old 28-10-2014, 11:56   #10
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Re: New boat pulls to starboard under power, prop?

I think I lost a post, electric and cable went down.

Beautiful vessel! Steel, soft chinned and fear. He did a hell of a job.

My humble take would be prop. dia.. Maybe due to lack of enough prop. bottom clearance causing more of an eddy effect. I wouldn't rework a well balanced rudder. A way out suggestion would be add a small trim tab just forward of the rudder rather than a full skeg. A slit angle might counter the walk but not effect sailing. Cheaper than changing shaft angle.

For it may or may not be worth?

Again a beautiful vessel.
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Old 28-10-2014, 12:33   #11
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Re: New boat pulls to starboard under power, prop?

My Mascot 28 does the same under engine power.Perfectly balanced under sail. It is a form of "prop walk".It is exacerbated by the substantial distance between our props & our rudders.A "normal"powerboat has the prop very close to leading edge of rudder.The water stream coming off a prop is always "Bent",but if your rudder is close to prop,the effect is negligible in forward gear.
In our case,that extra long distance between rudder & prop allows the "bent" water stream to push harder on one side of rudder. I "solved" it by putting an A/Pilot in that drives a Ray.M81130 electric ram attd to rudderstock (short tiller arm) & I use AP when powering.I doubt that a wheelpilot or tillerpilot would stand the high stress we are experiencing.I had to jam the tiller against my ribcage,or hook my arm thru the steeringwheel to steer for very long under power & yes it gets worse the harder you drive the engine! You could use the ram without A/pilot,& steer with a jog stick. You could install a "trim tab" on trailing edge of rudder,but,unless it is adjustable from cockpit,it will unbalance your rudder while sailing.I have always thought,but haven't tried,that a adding a skeg ahead of the rudder & carrying it fwd close to prop MAY decrease this problem,BUT this is only a guess.
By the way-Beautiful looking vessel.Is it a custom design?
Cheers / Len
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Old 28-10-2014, 12:42   #12
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Re: New boat pulls to starboard under power, prop?

I'm going to disagree with the replies about your prop being too large. Larger diameter props are more efficient. There is a trade off in sailboats between the larger diameter being more efficient under power and creating more drag under sail. Nonetheless, the larger prop will significantly increase efficiency under power. And it will increase your ability to power into heavy weather and big seas. For a very thorough reference on this I'd suggest Dave Gerr's Propeller Handbook.

I second A64's recommendation to install a trim tab on the back of the rudder. With this you can trim the rudder to be neutral under all conditions. This will handle your trim under power problem. It will also allow you to trim the rudder to neutral under sail. This will decrease the loads on your autopilot and reduce its power consumption significantly.
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Old 28-10-2014, 12:51   #13
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Re: New boat pulls to starboard under power, prop?



Servo adjustable tabs? I thought it was about sailing not space travel.
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Old 28-10-2014, 12:51   #14
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Re: New boat pulls to starboard under power, prop?

I'm still thinking a 26" diameter prop with a 16" pitch, is either way the hell overpropped, or a very slow turning prop. I would have expected a 6" pitch out of a prop that size?
If you reduced it to a 18" diameter, what would the pitch need to be to compensate for the decrease in diameter?
What contributes more to "prop walk" pitch or diameter, I'm thinking it might be pitch, but I'm also thinking in reverse too.
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Old 28-10-2014, 12:54   #15
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Re: New boat pulls to starboard under power, prop?

Quote:
Originally Posted by Cadence View Post


Servo adjustable tabs? I thought it was about sailing not space travel.
Not talking anything crazy, something like this would either prove or disprove the solution, without a lot of money or effort

Bennett Marine: The world leader in trim tab innovation, quality and service
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