Cobra 750 prop walk ?

[Nagaina]

Hi, I have a Cobra 750 and would like to know if anyone has come across the problem of the tiller pulling really hard so as the boat is wanting to turn to port when going ahead under power. The more power I apply on the throttle the worse it gets. If anyone has come across this problem or not could you please let me know, as it could be something that has been fitted to my boat and not to yours, or just might be a common fault with the 750 and I may never sort it but I dont think so. When shes sailling theres no problem. I can put up with the prop walk in reverse but it seems to have it in forward and has already broke one autohelm because of the very strong pulling on the tiller. Can any one help please.

Boat= Cobra 750
Engine = Bukh 10
Prop= 14x11x3

 

[twisterkai]

Initial impressions are that your prop appears to be far to big (assuming a gearbox ratio of 2:1), so I'd be inclined to contact one of the propellor companies who'll be able to advise you of a correct size. Our boat, whilst being slightly longer than yours probably has a similar lwl (21'6" or 6.55m) but is almost certainly heavier and has a slightly more powerful engine (13.5hp) and is currently fitted with a 12"x9" prop. Previously, she had an 11hp engine which was struggling to throw a 14"x10" prop (both had 2:1 reduction gearboxes) and we experienced similar problems.

Secondly, a lot of Cobras were home completed so it may be worth checking that the stern tube is in straight - ie check that the aft end of the tube isn't off to port relative to its hull exit point.

Good luck.

 

[Nagaina]

Thanks for that, the prop could well be the answer, dont think its the stern tube its a factory finished model but you never no.

 

[twisterkai]

No worries.

Hope it's a good suggestion. Let us know how you get on.

Cheers.

 

[simon_sluggett]

I have a 750 and it is not a problem for me ,tho the tiller does pull a little and gets stronger the more the tiller is turned, but its pulling it back to the centre more. Though I wouldnt strictly call it propwalk as this is caused by a sideways force caused by rotation of prop.its where the wash from the prop is acting on the rudder.Its because the prop is right close behind it..You may be overpropped but you would find your engine would struggle to power up to optimum revs and also smoke a lot. I will measure mine when it comes out in a couple of weeks and let you know what size I have.
You could try the cobra owners site , they have a technical forum which may answer your question. Though you may have to join to access it, for a few squid I think.

 

[Nagaina]

Thanks a lot, I'll put another posting in asking the question regarding prop size.
Cheers.

 

[jackho]

May not be of any consequence but I had a similar problem (all be it wheel steering) caused by (unknown at the time) weed growth along one side of the boat.
The more power you applied the more she "dug in" to pull you round.
A good scrub and a little fresh anti-fouling solved the problem.

 

[Nagaina]

I've had the boat for 3 years now, when the boat is fist put in the water at the begining of the year with fresh anti-fouling it is just the same. I'm glad to hear you got yours sorted, I think after 3 years its time I got mine.

 

[Duncantaz]

Hi,

I have the same problem with my Trapper 500 with a 14HP engine and hand steering uder power for any length of time is very tiring. Apparently the T500 doesn't have room for a large prop so i'm not sure that that is your problem. Prop walk going astern is also quite significant however the rudder on the 500 can be turned backwards which helps to counter it. I'd be interested to hear what solution you come up with.

Good luck.

 

[Nagaina]

Hi,
I will of course keep you informed and hope you get yours solved too. Do you know the dimensions of your prop (diamiter / pitch 2 or 3 bladed) and how much clearance there is between the tip of a blade and the hull also the angle of the prop shaft ? it would of interest to me to know. As I keep finding more information about props all this seems to be a great factor towards the problem, not to definately say it is in your case or mine, but to try and find a common denominater could answer the question.

Thanks.