Curved Rudder a problem ?

[djh1962]

Hi,

Our rudder on our Atlanta 8 Metre bilge keeler is curved to port, when view from rear. When we start to motor faster the tiller becomes heavier until at about 5 knots we really struggle to keep it in a straight line, she just wants to head to starboard! The prop which is a three blade fixed type seems large is it the prop too large or is this purely down to the rudder? Any help on this is gratefully received.

Many Thanks
DJH.

Merry Xmas to All... /forums/images/graemlins/smirk.gif

 

[sailorman]

Evening & welcome

sounds to me that she is "overproped" my sons Cinder 22 had the same problem
i cut 25m/m off of the Dia of the prop & its now very much better & no reduction in speed, transom doesnt try to bury itself either

check out prop calc sites

 

[donm]

Do you mean that the blade has a curve in it or simply that it is not parallel to the tiller?

 

[FAITIRA]

Re: Curved Rudder a problem ?

If the rudder is warped, bent vertically, yes it,s a prob that needs solving, you will obviously, have a severe bias.

 

[maxi]

I don't think that the curve will bring any significant problems.

With regard to the helm becoming heavy, that can be easily fixed by attaching a 'small' fillet on one side of the trailing edge of the rudder, which side will depend on the 'hand' of your prop - but trial and error will prove which is correct.

The fillet should be say 14" long and about 3/16" thick at the trailing edge, tapering to nothing over about 1/2. I.e a wedge of cheese shape.

This has worked brilliantly on our Mirage 37 which showed the same symptoms as your boat. Easy job next time you dry out.

 

[djh1962]

Re: Curved Rudder a problem ?

Thanks so far for replies, for further info the curve is uniform on the length of rudder, with no sign of a knock, thats why i was wondering if it was meant to be like this. It sails fine even at 5 knots.

br DJH

 

[jimbaerselman]

Re: Curved Rudder a problem ?

Your rudder curve to the port will cause the helm to pull itself to port (assuming you have a tiller!) or, if not resisted, to turn the boat to starboard. The force exerted on the helm will depend on water speed over the rudder, so, either prop revs ahead, or boat speed, or both.

You can check the effect by dropping into neutral while going as your maximum speed. If the turning force still exists (albeit reduced) your rudder shape is the problem.

A temporary fix is to fit a 'tab', a strip of bendable metal extending beyond the trailing edge of the rudder (probably at least 5cm, but that's a guess). Make it bend in the opposite direction to the rudder curve. Only trial and error will tell you how much to bend it - the objective is to make the total structure look 'straight' to the water flowing past.

But the real question is - what caused the bend? Is it design? or is there a structural fault? Design is unlikely. Speak to the manufacturer or another owner. Investigate.

As to OK while sailing, very often the turning moment due to heel angle would hide the turning effect, which is, of course, much stronger when the prop is additionally pushing water past the rudder.

Overpropped? Unlikely. But you could soon tell - full throttle would not be able to give you maximum revs when you're at speed, and there'd be clouds of black smoke . . .

 

[Bodach na mara]

Re: Curved Rudder a problem ?

Yes, the curve in the ruder is acting as a trim tab and setting up a starboard helm. It is working the same way as the trim tabs on aircraft, which are used to reduce the force that the pilot needs to exert on the control to maintain a flight attitude. You need to get it sorted. As you have noticed, the force needed to counteract the effect is speed dependent.

In the 60's, I sailed on Torridon, a 9-metre built by Anker and Jensen at Asker in 1912. She had a slight curve in her rudder. This showed up by her being neutral on the helm on port tack, however she was a right handful with weather helm on starboard.

 

[djh1962]

Re: Curved Rudder a problem ?

Hi All,

Merry Xmas .....!Appreciate the replies and its now of to the blacksmith to get it straightened out will see how that performs then change to two blade prop that was with the boat if required. Good to have your knowledge and help out there, thanks to all!!

Tight sheets!...Dale /forums/images/graemlins/grin.gif

 

[JohnBowles]

Re: Curved Rudder a problem ?

My previous boat was an Atlanta 8.5. I had the plate rudder re galvanised and it came back with a significant curve. Perceived wisdom was that the curve would cause problems so I had it straightened in a press.