Aero foil rudder speed limitation

Hello,
My recently acquired planning TSMY is fitted as original with GRP aero foil section rudders.
All but my vessel were fitted with twin 210 hp diesels to exposed shafts supported by
'P' brackets.
However, my boat was custom built in 1976 and fitted with 300 hp diesels and using the same rudder pattern as the 210's.
The maximum respective speeds are approximately 21 and 28 knts.
Before I become too paranoid about the strange steering characteristic I am experiencing, would anyone be able to throw light on possible causes?
To this end are there limitations with any state of the boats trim or speed when using this design of rudder as opposed to a more flat section bronze rudder used on similar boats?
Thank you in anticipation.
Ian.

 

Hello Ian,

Wow, you increased the HP by 1.43 times and your speed increased by 1.33 times. What are the strange steering characteristics?
Was the motor support structure beefed up further? Are the shafts bending or bent from the extra horses? What about attachment points for the rudders...are they structurally sufficient? Is the prop the same? Does the rudder have sufficient inner plate area to keep it from deflecting under the extra rpm's? Do you get linear or tangenial deflection at the rudders? Is the shaft angle to the motors correct? Hmmm, I would probably ask those questions first to begin the process of elimination.

Good luck...
Jay

 

Hello Jay,
I'm sorry, my question was not clear. My concern is merely rudder design application and what goes on below the waterline at planing speeds. Regardless of whether the boats were manufactured with 210`s or 300`s, for 30 years owners have lived with the same short coming. At planing speed a slight hint of helm and the rudders will continue turning, and obviously very resistant to recover the ahead position. I do wonder if the naval architect made a conscious design decision to provide reasonable displacment speed steerage and only relatively straight line planing speed use. Unfortunately most of the boats passages are at 23 knts and when I think of the drag this design also presents I am open to suggestions either way. Or perhaps I should accept compromise more readily.
Thanks Jay I appreciated your rapid response.
Ian.

 

Wonky steering at high speed can be caused by cavitation down the rudder surface.

The simpletst cure is a fence at the top of the rudder that stops the air entrainment.

FAST FRED