Propwalk Problems.

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allangray3

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Can any of you chaps help with a problem I am having with excessive propwalk on my Macwester wight (Bilge Keeler). It seems to me that the engine a Perkins 4108 has way to much power for the boat. My reason for thinking this is that even at tickover she seems to shoot up to 3 to 4 Knots with no problem. this makes close manuvering within the marina a bit of an underpants soiling ordeal. Put her astern and things get even worse as the propwalk makes the rudder practicaly useless. Does anyone Know what these boats where powered by when designed. EG engine and Prop size. hope you can help as at the moment berthing is a little fluid at the marina.
 
I don't know whether this helps or whether I'm teaching granny to suck eggs, but I had a similar problem when I first had my new engine fitted. It seemed she would shoot off sharply stern to port no matter how gently I used the engine.

The answer, in may case, was to manouvre mostly in neutral, using just short bursts with the tiller hard over. If she still insisted on going where I didn't want her, I'd turn the tiller in the direction she was wanting to go until I had control again, then turn her away where I wanted her to be (rather like turning into a skid.) If it still goes pear-shaped, because the wind is on the bow and making things even more difficult, you can always attempt to straighten out by going forwards.

The key to successful manouvering in tight spots, is to stay calm and try to look like it was what you meant to do all along!

I made friends with my new engine by taking the boat out of the marina and practising all sorts of tricky stunts with the engine while I was out in open water. Gave me the confidence I needed before trying it out in the marina with everybody watching.
 
Is there scope to reduce the tickover speed?
What revs is She doing as you just snick Her into gear?

You are going to have some prop walk whatever set up you have ref prop and eng power.
However I don,t think the "power" of the engine would make an awful lot of difference at "parking" speed! IMHO!
My current mobo suffers from prop walk. I have learnt to deal with the prob.
Actually my previous mobo was a right b**sted so I gained a lot of know how steering that little bugger backwards.
To help with my "new" boat I have adjusted the idle revs down to 450 rpm about 125 rpm less than when I got Her. This has made berthing a lot easier,just a thought.
 
There has been a lot about this recently, suggest you Search.

There were many more knowledgeable inputs than mine, but don't hesitate to PM me (don't be deterred by the MoBo Profile.)

sbc has your Perkins 4.108 in a 25-footer and admitted to having no control when arriving or departing. You may larf but I'm berthed right opposite!!
 
eascoastbernie, that,s exactly what I did with My previous boat, spot on.
It took me quite a while to "learn" the technique of nudging in and out of gear and adjusting the wheel whilst in nuetral and "blipping" the throttle.
Bit like the handling method needed to manuover an outboard driven dory etc.
 
Gosh. That's big. Not surprised there is prop walk at tickover! 4t or so of boat?

Tickover r.p.m. is pretty important then as it is likely to have a lot of horse power with it.
 
Would be surprised if they had any more than 25hp from new. I don't know if it is true, but I am guessing that prop walk is dependant on prop diameter. Can anybody in the know comment on this? Seem to remember some of the theories being debated in PBO some years ago.

It seems to me that prop walk is a black art that few comment on the causes (and therefore solutions) - you just hear people say 'well mine as a lot' etc.

My guess is that the size and power of the prop togther with the ability for the boat to turn (ie keel/boat profile; weight of boat; length of boat etc.) in which case if your rpm is at a reasonable minimum (you could probably go to 750 rpm - I assume the rev counter is working properly) then really you have to look at the prop. Don't think you want to consider the engine or the gearbox? After that it is all about learning to use it to its best advantage! Not always possible of course.
 
If the engine is ticking over normally, you should only get around a knot or so water speed. If you are getting 3 - 4 knots then the prop is way too big, and is also causing your propwalk problem.

The prop should not run at much more than 2000 rpm, and at that speed a 50hp engine in your boat would need a prop approximately 15x10 to acheive 8 knots. It is possible that an oversize prop has been put on to try to limit the top engine speed - not a good idea, as you have found!

To find maximum shaft speed check the gearbox reduction ratio - which should be marked on the makers plate, then work it out from the maximum engine revs. To acheive 2000rpm shaft speed the reduction on your engine should be at least 2:1.

Get the right size prop and I think you will find your walk problems also disappear! Mike Bellamy at Lancing Marine should be able to advise you how to sort your actual installation, so that it works properly.

Bu as others have said, 50hp is a lot of engine for a 30 footer. I would expect 20 - 30 hp to be ample for what you need.
 
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If the engine is ticking over normally, you should only get around a knot or so water speed. If you are getting 3 - 4 knots then the prop is way too big, and is also causing your propwalk problem.

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I think this is correct. Talk to an expert about your prop. The engine is still probably way more powerful than you need, but it is not the cause of the problem.
 
Give a burst ahead on full helm to get the stern swinging, then before she has started to move forwards, go gently astern. If you run out of turning momentum, give another forward burst.
As someone here suggested, try to plan a manoeuvre so as to use the walk effect, not fight it. Try going in stern-first, walking the stern. Or approaching head on in the direction so that going astern will walk the stern in, not out.
 
eascoastbernie, that,s exactly what I did with My previous boat, spot on.
It took me quite a while to "learn" the technique of nudging in and out of gear and adjusting the wheel whilst in nuetral and "blipping" the throttle.
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Yes, this method should always be considered.
Bit like depressing the clutch when manouvering a car.

However, if you do this in astern, then beware the problem of 'new' prop walk each time you engage astern.

Having got rid of it on your carefully considered astern approach, it can destroy the alignment if propwalk resumes, unless used deliberately to kick alongside.
 
You could 'start at the other end' to work this out as well.

Are you achieving full engine r.p.m. (or best part of)?

If you are then you do not have a prop that is too powerful for the engine.

However, the prop may well be too powerful for the boat!

It may be you can jiggle around a bit with diameter and pitch to improve this a bit (I would go down in diameter in this situation - you may get a bit of cavitation as a result when manoevering but sounds like you need it).

Would be surprised if you solve this unless you end up not transmitting your full 50hp by choosing an undersized prop.
 
A feathering prop (I use a Kiwi) seems to have very little prop walk.
Expensive solution maybe. Otherwise, learn to use the prop walk!!
 
What sort of prop do you have?

I replaced a 2 bladed fixed prop with a 3 bladed Gori-folding and it totally transformed the handling - prop walk is now almost non-existent and the more effective prop meant that the boat got steerage way much faster too.
 
Well Gents you have certainly given me a project for the winter. Will let you Know how I get on and the final solution.
Im going to put a strain gauge on her to take some measurments whilst I find the solution and will post results as this problem seems to intrest many. Once again thanks Lads.
 
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