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Old 28-10-2014, 13:06   #16
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Re: New boat pulls to starboard under power, prop?

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Originally Posted by a64pilot View Post
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.
Most Pilothouse Motorsailers have quite a bit more HP than the same length sailboat. My Mascot 28 has 28HP 3cyl diesel & is 28ft long.It came with a 17d x 14p 3 blade@ 2:1 red. & rolls 3200 @ 6.9kts if I cram it.Handles like a powerboat around the dock,etc. Average 30ft sailboat 12-15HP with a 2 blade prop. Different situation. Jus sayin.
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Old 28-10-2014, 13:28   #17
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Re: New boat pulls to starboard under power, prop?

See my 38' sailboat with a 44hp swings I was told an 18" prop pitched to 8" for 8 kts at full throttle. I think 2.14 to 1 reduction, but not sure as I pulled that number from my head.
Pitch is number of inches forward movement with one revolution, I think, right?
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Old 28-10-2014, 14:24   #18
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Re: New boat pulls to starboard under power, prop?

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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
I was just being a wise ass. I got the link on trim tabs?

The more sophistication on a vessel the more chance of failure or at least expense. The OP has a beautiful vessel I hope he gives it to me. I guess that's a snowballs chance in hell.
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Old 28-10-2014, 14:28   #19
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Re: New boat pulls to starboard under power, prop?

Nothing complex about a trim tab. They have been a standard way of building wind vane steering for about 50 years. Note - and this is valuable- if you install a controllable trim tab the autopilot can be linked to the trim tab instead of to the rudder shaft. This will reduce loads on the autopilot by 90% plus. The autopilot only needs to control the small trim tab, the trim tab drives the rudder. This is a very efficient system.
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Old 28-10-2014, 14:38   #20
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Re: New boat pulls to starboard under power, prop?

I would try backing off the power till the boat goes straight. You will save lots of money on fuel.

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Old 28-10-2014, 16:35   #21
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Re: New boat pulls to starboard under power, prop?

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I would try backing off the power till the boat goes straight. You will save lots of money on fuel.

Sent from my SCH-I415 using Cruisers Sailing Forum mobile app
The OP doesn't sound like someone running balls to the wall? People motor sailing just seem to be out for the ride, not fast sailing not fast motoring. I'll wager a bet he only runs at 60% even motoring?
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Old 28-10-2014, 16:40   #22
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Re: New boat pulls to starboard under power, prop?

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The OP doesn't sound like someone running balls to the wall? People motor sailing just seem to be out for the ride, not fast sailing not fast motoring. I'll wager a bet he only runs at 60% even motoring?

And I'm thinking with that prop, it's not a little motor hooked to it either
Be interesting to know the motor / tranny combination
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Old 28-10-2014, 16:43   #23
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Re: New boat pulls to starboard under power, prop?

Are you sure that's a 26" prop? It just doesn't look that big in the pic for a 46 ft boat...? How big is that round zinc above it? Must be a 6-7" zinc if that's a 26" prop....
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Old 28-10-2014, 16:54   #24
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Re: New boat pulls to starboard under power, prop?

Looks like a big prop to me, it's probably on a 1.25" shaft? I don't know what it's called, but the zinc may be closer to the camera distorting perspective, looks like a lot of pitch too, so his numbers seems right?
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Old 28-10-2014, 17:16   #25
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Re: New boat pulls to starboard under power, prop?

How I'd go about figuring out this issue if this were my boat-

For a start, I'd decide if the prop was correct for my type of motoring. The way to do that is by checking your spec'd rpms under way. Motor up to par but not getting your max rpm's? Then you are most likely over propped and figure that out.

Now the prop walk thing- as A64pilot mentions, the downward moving blade takes a bigger bite than the upward moving one does as they all do and some boats let you know more than others. Meanwhile, when you are stationary and give it a hefty bit of throttle, does the stern move sideways a bit before picking up any significant forward movement? If it does, there is the best example of prop walk issues.

I may be misunderstanding your situation, but it sounds like your main issue is not that the ass scoots sideways when you open the throttle from a stop but that there is just a general out-of-trim issue while underway under power. That is where I'd put my energies in regards to your problem. First is to check the alignment of everything, if it is in alignment or not moving the engine to starboard should take some of the pull out if not be able to eliminate most all of it.

If the boat sails fine and only pulls while under power, the more that can be adjusted out regardless of the true culprit it will not only steer better but cruise more efficiently as well. Fitting an adjustable trim tab will make it handle better but is still going to add a drag penalty that does not necessarily need to be there.
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Old 28-10-2014, 17:21   #26
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Re: New boat pulls to starboard under power, prop?

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Originally Posted by a64pilot View Post
And I'm thinking with that prop, it's not a little motor hooked to it either
Be interesting to know the motor / tranny combination
Even at that it's a SWAG. Given his apparent displacement I'd be for a big shallower wheel. Lets face it he won't be jumping out on a plane. If the dia. is the cause of his crabbing he could reduce the dia. and break even, drive wise, and at least reduce the frustration at the helm. I still want him to give it to me. Guess I'm back to the snowballs chance in hell.
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Old 28-10-2014, 17:35   #27
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Re: New boat pulls to starboard under power, prop?

With a left-handed propeller, I need a three-degree rudder to port to maintain a straight forward course. Perfectly normal. Fortunately, my rudder system can maintain a constant angle so there isn't any need to fight the prop walk.
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Old 28-10-2014, 19:59   #28
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Re: New boat pulls to starboard under power, prop?

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Originally Posted by a64pilot View Post
Looks like a big prop to me, it's probably on a 1.25" shaft? I don't know what it's called, but the zinc may be closer to the camera distorting perspective, looks like a lot of pitch too, so his numbers seems right?
Boy I hope it's bigger shaft than that if it's really 26". My 18" prop had 1.5 diameter shaft.
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Old 28-10-2014, 20:11   #29
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Re: New boat pulls to starboard under power, prop?

Hello a64pilot
Apparently I made a BOO BOO trying to reply to you.I'm new at this forum.
The reply that I did send is below.


Hi a64pilot That is correct. Theoretically you move ahead 8" in your
case for each rev of prop.There is always some slippage,especially when
starting out using lots of throttle.
The vessel has to "get going" fast enough to move the 8",& that takes a
bit of time on a heavy displacement sailboat,compared to a light
powerboat. Same principle as spinning ur tires-easy if u have some hp &
are starting from 0 mph-harder if u punch it at 40-50mph,with same hp.
Displacement boat props are usually limited to 1000 rpm or less @ the
prop,by reduction gear.This is because a disp boat has a limiting
natural hull speed based on w./line length.It takes very little hp to
eventually attain hull speed-less hp just takes longer to get up there.
It takes a horrid increase in power (& fuel consumption) to go beyond
hull speed.
With 44hp,I am surprised your prop doesn't have more pitch & it should
be a 3 blade.
Everything is a tradeoff though. More Prop = more drag when
sailing,unless u have an expensive "folder". I spent my working life
around our "small commercial fishing fleet (35-65 ft).We all chuckled
when sailboats came in (sometimes from across the ocean) & could hardly
stop @ the dock,let alone maneuver & dock. I don't chuckle anymore,after
some experience with way underpowered sailboats- LOL I feel sorry for
them trying to maneuver in those things!
I am glad I found my little motorsailer. It's decent under
sail--5-6kts,though slow off the line as it disp 10K # in 28ft. But it
has 28HP & a 17d x 14p 3 blade prop & handles like a power boat.Won't
win any races,but it's all tradeoff.
Tks for reply-I'm new on here.
Cheers /Len
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Old 28-10-2014, 22:15   #30
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Re: New boat pulls to starboard under power, prop?

My .02$,
Set the autopilot to a rudder trim tab to drive the balanced rudder. This has been accomplished successfully on many sailing boats and works fine while under power as well. Driven by either wind vanes and/or smaller ram drives. The smaller rams use less energy.
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