Probably a silly question, but...

[WayneS]

After 3 week-ends of using my new (to me anyway) boat, my first with such luxuries as an inboard, separate heads etc....I have a little questions regarding prop walk. 9If that is what you call it)

Under motor power the boat tends to turn to stb'd quite severely (If I let go of the tiller of course), however, if I stop the engine while at speed, the boat continues on in a straight line. So, as far as I am concerned, it is the thrust from my 2 bladed folding prop that is causing the turn.

The big question is, can anything be done about this. The only thing that I can think of is to realign the drive. If this is the only way, is there a method to determine how much I should "bend" the drive.

Hope that someone out there has had this problem before and managed to solve it.


Cheers

Wayne

<P ID="edit"><FONT SIZE=-1>Edited by WayneS on Wed Mar 27 15:16:17 2002 (server time).</FONT></P>

 

[chrisc]

i am afraid that this is the natural thing that props do.
deaper in the water the prop the more efficient,so the
upper part of your prop is working less efficiently than the
bottom so the two forces are not balancing each other out.
boat turns to stbd = clockwise rotation of prop (facing forward)

 

[Miker]

I have a two blade feathering prop which replaced a two blade fixed prop. I never experienced any prop walk going forward, until late last season. The problem was a piece of plastic wrapped around one of the blades.

 

[AndrewB]

Yes, that sounds likely.

WayneS shouldn't be feeling significant propwalk going forward at speeds over 2kts, unless he has an offset drive - very unusual other than in old wooden yachts. My guess would be the prop isn't working quite right, either fouled or not opening correctly.

Realigning the drive just isn't practical.

 

[Guest]

Wayne, what you have is not prop walk, you only get that effect when starting to go astern. The problem is to do with the distance your prop is from your rudder. What is happeneing is the wash from the prop is striking the portside of the rudder making it want to turn to stb. This is confirmed by the fact as soon as you stop the prop whilst still underway the water runs evenly over the rudder making the boat go straight. Unfortunately theres not a lot you can do about it!

 

[oldharry]

Afraid I don't agree at all, Peter. Waynes problem is a definite case of bad prop walk, from the causes clearly explained by other posters - unless there is actually something wrong with the prop. Prop walk stops the instant you take power off anyway, and although the wash against the rudder does compound the problem to a small degree, Waynes problem as described is undoubtedly caused by the well known 'paddle wheel effect' of his propellor, and familiar to many of us.

Some boats suffer from it much more severely than others, and it has to do with prop size, configuration of the keel, rudder and propellor speed. And every single propped boat will do it to some degree. Unfortunately some boats are just plain 'vicious' under power and there is little can be done about it without expensive major alterations.

Knowing what will happen as a result of prop walk is an important part of handling boats under power, and has featured large in both the theoretical and practical parts of every test and exam I have had to take in boat handling.

One 46 footer I used to skipper the prop walk was so severe (large direct drive prop, running 1800 rpm on a 120hp engine on a shallow drafted hull), that it in reverse the boat would only go sideways intially. This had to be allowed for when backing out of fuel berths etc, as she would turn nearly 45 degrees on to the berth if given too much throttle before she gained appreciable fore and aft movement. Forward gear was not so bad, but even so a light throttle was needed until she was making a couple of knots. Extremely useful to be able to do this getting out of tight corners - provided she was berthed the right way round, and judicious use of the throttle and gear lever from stationary, would allow me to spin her right round in her own length when occasion demanded! A similar 36 footer, with a different and slightly deeper hull/rudder shape, but using the same engine/prop installation would hardly 'walk' at all!

Now, what to do about it is another matter - its usually virtually impossible to change the shape of the hull and rudder configuration, so the need is to look at prop size and speed - which involves changing gearbox ratios, and prop size which is expensive, and needs professional input from someone like Mike Bellamy at Lancing Marine. I have known changing to a 3 bladed prop improve matters - or make it worse! Rudder shape almost certainly comes into the equation also - but here you would need the designers advice, as rudder design is a quite complex issue.

 

[Guest]

in my experience,old harry has it right. i'm guessing wayne's boat has a left handed wheel, that is, the direction of propeller rotation is counter clockwise when going ahead looking forward, then prop walk will cause the boat's head to go to starboard. if wayne has a right handed wheel , then i would expect propwalk to move the bow to port when going ahead, as it does in my boat.

when backing the effect is the opposite-- the right handed wheel when backing tends to back the stern to port (bow to starboard) a helpful property that can be used to advantage when docking. i would expect wayne's boat, if it has a left handed wheel, to back to starboard (bow to port).

the only means of which i'm aware to eliminate the prop walk effect is to shroud the propeller with a "kort nozzle", a common practice for tugs on the mississippi river which also seems to add to the propeller's thrust. the nozzle is little more than a short tube around the circumference of the propeller which forces the water to flow in the fore and aft direction only. i've seen some outboard motors so fitted with nozzles as an after market accessory. i've not seen a kort nozzle around an inboard powered sail boat's propeller, doubtless due to the added drag the nozzle would cause while sailing.

krgds

pearlfish

 

[drawp]

Prop walk is a fact of life because of the way propellors work - the degree of prop walk just varies from boat to boat. As long as the helmsman is aware of its effects it can be used as an aid to manoeuvering under power - getting into and out of berths and turning in a tighter radius in one direction than in the other. But perhaps Peter_aird has a point if WayneS has a balanced rudder. Could it be that the prop wash 'unbalances' the rudder? WayneS, when you let go of the tiller, does it swing violently across to port thus turning your good ship to starboard? Or does the tiller remain midships?
As long as there is nothing mechanically wrong with your prop, my advice would be to live with the prop walk and use it to your advantage where possible.