Twin Rudder Scantlings

[mishnish]

Hi!

I'm currently designing a shoal draught yacht at 55ft (16.8m) as a final year project at Uni.

In order to keep the draught down, I am utilising Twin Rudders, however I can't seem to find any information on how to size and position these appendages.

Larsson suggests scantlings for a single rudder setup but nothing that might help me size up a pair of rudders...

What things do i need to consider? In order to increase their effective length i want to angle them outwards, but what goes into the design of such a setup?

I would appreciate any pointers or references to helpful information!

Thanks
Chris d

[shu]

Chris,
I haven't any experience designing a twin rudder set up, so I'm not certain about sizing, other than they can be considerably smaller than a single rudder, since the leeward rudder is nearly always entirely in the water (and closer to vertical if your angle them outboard). And if the rudders are mounted under the hull you will get far less chance of cavitation than with a centerline rudder.

Once you do select the size, you can use the same scantling determination as for a single rudder. The scantlings will be less, since the leverage exerted by a shorter rudder is less.

That's about all I know, hope it helps. SailDesign should be able to help with sizing, and probably have something more intelligent to say about scantlings as well, since he's designed at least one open 60...
-shu

[sorenfdk]

A rule of thump says that the size of each rudder should be somewhere around 75% of the size of a single rudder.

[SailDesign]

We used to reckon that 55% of single rudder size was about right, assuming the twin rudders were correctly placed, and the boat was suitable. Scantlings were calculated for each rudder as though (!) they were single rudders - which they are.
Words of warning... Two "cruising" boats that I worked on with Rodger Martin were designed with twin rudders, and both owners went to singles after a couple of years. Tough to reverse.
Steve

[sorenfdk]

You could also do it this way:

Design a decent single rudder and calculate the developed sideforce at some typical heel angle(s) and speed(s). Then design two rudders that combined develop the same sideforce at the same heel angle(s) and speed(s).
This eliminates any rules of thump. BTW: The 75% I mentioned in my earlier post is a figure I got from a french designer, whose name I've forgotten. Steves 55% seems to be more reasonable.

The longitudinal position of the twin rudders should be the same as for the single rudder. They should be angled outward so the leeward rudder is as close to vertical as possible under most circumstances (which probably would be somewhere between 10 and 15 degrees from vertical). You can also align them with the flow, but this may be a bit difficult unless you know how much the boat trims when heeled (and remember that the direction of the flow changes as you move away from the hull along the span of the rudder!)

[mishnish]

Thanks guys...

thats certainally given me a nudge in the right direction. The idea about sizing two rudders to do the same job as one was what was brewing in the back of my skull, but as yet this is my first time with twins! (said the actress to the bishop(s))