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Old 24-07-2021, 15:35   #1
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Is it normal?

Is it normal for a keel boat to pull somewhat hard to starboard when going ahead under diesel.

I can't take my hands off the wheel for 3 seconds without the boat "leaving her lane to starboard. Not a gentle drift but a pretty honest veer. Within four to five boat lengths it would be a 90 degree turn.

In trying to intuit it, it seems the opposite should occur due to prop wash pulling the stern to starboard in ahead but not in my case.

Is it normal and if not so much what might cause it?

Mis-alligned Max Prop? The last boat yard seems to have had some difficulty getting it set. The same boatyard did a shaft strut fix and new shaft repair? Could that be out of whack given that the shaft HAS to go through the shaft aperture.

We haven't wrapped or hit anything and there are no untoward vibrations or noise while motoring.

If it's somewhat normal for a moderate fin keel boat to do this, I'm guessing it's the slight downward angle of the shaft and prop. I just need to figure out if it's something that needs to addressed at a planned haul out this fall.

all replies appreciated,

Kurt
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Old 24-07-2021, 16:15   #2
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Re: Is it normal?

No, it should not be happening. Should track pretty straight unless there's something else acting on the boat. A strong breeze from port and a roller furler on the bow could make the bow blow down. But not a sudden jerk to starboard.

Something's wrong, somewhere, and the prop seems unlikely. Having the strut off center, might cause the boat to pivot round her keel.

Hope you get it solved soon.

Ann
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Old 24-07-2021, 16:29   #3
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Re: Is it normal?

What is the make/model/year of boat?

Some boats have shafts mounted off center which can cause them to veer to one side.

Has it always done this or is it something new?
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Old 24-07-2021, 16:29   #4
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Re: Is it normal?

First thing is to determine which way the prop walk is. In the slip, with the boat securely tied up, put the boat in forward at idle. You should be able to observe water coming out to the side of the stern. Which side?
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Old 24-07-2021, 17:27   #5
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Re: Is it normal?

This behaviour is not ideal, but also not uncommon. In some boats (like mine), the engine is on the centreline, but the shaft tube and P-bracket are angled slightly to one side; my prop is about 4" off centre to port. Designing it this way means that the owner can remove the prop shaft and do all aspects of driveline servicing without having to remove the rudder.

The downside is that it adds a small yawing moment; with the wheel unlocked and unmanned, ours tends to veer to port. It's not prop walk. It's from the way the force vectors line up. Our engine is on centreline, well aft of the keel, and our prop is offset to port, so the whole drivetrain is cocked about 2° from the boat's centreline and applies a yawing moment that turns the bow to port. The prop wash being offset from the rudder centreline also causes a net torque on the rudder, so if I let go of the helm, the rudder will tend to move so as to tighten her turn.

I think the prop on the Islander Bahama 30 is offset to starboard, from what underbody photos I can find, so your case would likely just be the opposite of ours.

We compensate for it by just increasing the friction on the steering pedestal by a turn or so when we're motoring, so the wheel won't spin without a hand on it. The "dead ahead while motoring" wheel position is about a hand's breadth to the right of the "rudder true amidships" wheel position. It doesn't pose any real problem, and is a lot less annoying than dropping the rudder to work on the shaft bearings would be.
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Old 24-07-2021, 18:02   #6
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Re: Is it normal?

Or it could be builder playing fast and loose with how they install rudders like I found on these two
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Old 27-07-2021, 16:32   #7
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Re: Is it normal?

Quote:
Originally Posted by belizesailor View Post
What is the make/model/year of boat?

Some boats have shafts mounted off center which can cause them to veer to one side.

Has it always done this or is it something new?

'84 islander Bahama 30. Moderate fin keel. Shaft and prop in the open just ahead of a semi-balanced rudder (no skeg)


Kurt
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Old 27-07-2021, 16:36   #8
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Re: Is it normal?

Quote:
Originally Posted by marshmat View Post
This behaviour is not ideal, but also not uncommon.

I think the prop on the Islander Bahama 30 is offset to starboard, from what underbody photos I can find, so your case would likely just be the opposite of ours.

We compensate for it by just increasing the friction on the steering pedestal by a turn or so when we're motoring, so the wheel won't spin without a hand on it. The "dead ahead while motoring" wheel position is about a hand's breadth to the right of the "rudder true amidships" wheel position. It doesn't pose any real problem, and is a lot less annoying than dropping the rudder to work on the shaft bearings would be.

i've used that trick while heading up and out of the marina main channel. It's only moderately successful no matter how much I fiddle with the angles.


Thanks for the reply. It's one of those thing I can check when we haul out.


Kurt
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Old 27-07-2021, 16:42   #9
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Re: Is it normal?

Quote:
Originally Posted by MarkSF View Post
First thing is to determine which way the prop walk is. In the slip, with the boat securely tied up, put the boat in forward at idle. You should be able to observe water coming out to the side of the stern. Which side?

Our prop walk pulls the stern to port in reverse and pullls the stern to SB in forward so I guess the actual wash is the opposite of those two. Like I said above, in forward a small turn towards port could be easily explained by prop walk but our turn is more than slight and is to starboard.


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Old 27-07-2021, 17:19   #10
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Re: Is it normal?

Quote:
Originally Posted by KayZee View Post
'84 islander Bahama 30. Moderate fin keel. Shaft and prop in the open just ahead of a semi-balanced rudder (no skeg)


Kurt
It could be just be the way it is with your underwater configuration, ie the way the prop wash is interacting with the open semi balanced rudder.

The following test may prove / disprove this theory. On a calm day (no wind no waves) and with a clean prop and bottom, motor slowly and see how bad it is. Now slowly bring the rpm up until you just reach hull speed and observe again. Now go to full power (WOT) and if it gets much worse at WOT, it is likely to be caused by the increased prop wash. If so, it may not be 'fixable' but do check the alignment considerations made by others upthread.
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Old 27-07-2021, 19:18   #11
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Re: Is it normal?

I posted this many times before and this may or may not apply to your situation. When we first purchased out boat it pulled terribly when under engine power. The problem turned out to be bad engine to shaft alignment. Once the engine was aligned to the shaft properly, the issue resolved.
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Old 27-07-2021, 22:29   #12
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Re: Is it normal?

And if you put it in neutral while motoring, does the boat stop trying to turn? And when it is sailing, there is no increase helm on either side? Just trying to nail down this is prop wash or not.
Perhaps a bend in rudder post that you didn't notice before from damage to rudder from previous owner?
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Old 27-07-2021, 23:02   #13
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Re: Is it normal?

Quote:
Originally Posted by KayZee View Post
'84 islander Bahama 30. Moderate fin keel. Shaft and prop in the open just ahead of a semi-balanced rudder (no skeg)


Kurt


I’ve encountered this problem twice. The first was indeed the Maxprop, the yard had lost the original settings and when the skipper put it in ahead to drive out of the lifter slot for the seatrial the boat went ahead about a foot but laid the bow against the stbd side of the slot and the stern against the port side. We straightened up and tried again but same result so the shore crew pulled us out of the lifter dock and we got underway but the problem persisted, release the helm and she’d head to stbd with a vengeance. Reverse was not too dreadful so we reversed back into the slings and a different engineer re set the Maxprop and all went back to normal again. It was a 72’Derecktor.

The second is a 36 footer with a Perkins Prima engine and a big prop. Very quick under power but eternally needing a very strong hand on the tiller. Simply too much power through a too large propeller.
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Old 28-07-2021, 03:13   #14
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Re: Is it normal?

With trany in forward, which way does the prop turn when you are looking from behind it? If clockwise, then the stern will be pulled to starbord by "prop walk" and the bow will want to turn to port. If the prop is turning anti-clockwise, then the stern will be pulled to port and the bow will turn to starbord. (everything is opposite when you reverse).
Depending on size of the prop, number of blades, and pitch, boat underbody configuration, prop walk can be very strong.

I think some people call prop walk "prop wash", but its not the same thing. Prop wash is the water that the prop pushes.

If you know which way the boat "walks" in forward and reverse, you can use this to your advantage when docking. I had a long keel 40ft boat with 22" X 18" fixed 3 blade prop and 80hp diesel. By applying lots of power in short bursts, I could literally turn the boat 360˚ in her own length.
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