Amended single rudder for small sailing cat

[aussiebushman]

Before I get dumped on (not that anyone on this forum would do that!) I'll explain the situation briefly:

1 This boat as described a year or more ago, is nothing more than a bumble around job for inshore waters - it is just a way of getting me back on the water after my 6m tri was stripped and torched. I recovered the amas and used them to build the little cat.
2 It will NEVER race though with a 16' skiff mast it should sail fairly well.
3 When I put it into my farm dam to check the water line etc it became obvious that with the outboard and me on board, it would need more buoyancy so I have modified the stern sections of the hulls - work still in progress
4. The original twin rudder system was off an A Class cat and was very difficult to fit the cross beams and tiller because the outboard was in the way and I wanted a boarding ladder (the latter can now be moved to the much wider stern of one hull). The attached picture shows the original setup that was quite impractical. That is why I want to redesign the arrangement for a single central "kick up" rudder mounted on the aft crossbeam. Please see picture of the unfinished assembly.

My question is: Is the "balanced" rudder likely to be big enough to steer the boat under sail? Also, would there be any advantage placing the rudder stock/tube on the forward edge? Another silly idea might be to place the two original rudders side by side, say 1' apart?

Please scr#w me gently - this boat is nothing fancy - just an escape for me and the dog

 

[Blueknarr]

My question is: Is the "balanced" rudder likely to be big enough to steer the boat under sail?
The nessicary rudder size is determined be the submerged portion of the hull, not the means of pulpution. It would be impossible to determine nessicary rudder size without a full study of the hull dimensions.
Test it out.
Also, would there be any advantage placing the rudder stock/tube on the forward edge?
That would remove the "balance" of the rudder making it harder to steer.
Another silly idea might be to place the two original rudders side by side, say 1' apart?
I can't think of advantage to rudder so close together.

Please scr#w me gently - this boat is nothing fancy - just an escape for me and the dog[/QUOTE]

 

[Richard Woods]

A central mounted rudder will be less efficient than hull hung. And obviously more prone to damage. Its not clear from your photo what you have, but you don't want more than about 20% area in front of the stock. Otherwise it will be an overbalanced rudder, not a semibalanced one (which is what you are after)

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[aussiebushman]

Thanks - exactly what I needed to know. The rudder(s) were the original A Class and if you look at the first picture you will see where they were mounted on the hull transoms. While I agree this is more efficient it makes the linkages and a tiller extremely difficult to arrange, due to having an outboard motor mounted centrally. Hence the idea of one central rudder instead.

Where I placed the new rudder stock in the second picture was where it was originally located relative to the overall width. In view of your comment. I'll move the stock further forward, leaving around 80% as the trailing portion. Does that make sense?

 

[bruceb]

Have you tried it with just one rudder on one float? It isn't designed to heel, and one decent sized rudder might be all you need. Sure would be simple.
B

 

[aussiebushman]

Not a bad idea Bruce. I agree that one rudder should be adequate. I'll check how the linkage would work if it is mounted on one hull, but a sleeve and tiller assembly for one centre rudder is by far the simplest and the steering tiller can be set up to clear the outboard tiller. It also keeps it out of the way of the main-sheet fixings

I have significantly widened the stern sections of each hull to increase the buoyancy and because these additions are faired onto the hulls, they have the effect of channeling water through a smaller gap between the hulls than the original hull widths. This should greatly increase the efficiency of the rudder. As Richard advised, I have also widened the rudder and changed the position of the stock to provide about 80% on the trailing edge for a "semi-balanced" result

 

[bruceb]

Almost any rudder/rudders will work if sized and located properly. Closely spaced hulls produce significant crossing waves that meet under the boat and can end up producing turbulence around any center mounted dagger boards/rudders/engines. They have to be mounted deeper than you might expect to function. If you can run the boat as is at your expected speeds, you can observe the location of the waves and plan accordingly.
Keep it simple
B

 

[aussiebushman]

Thanks Bruce - I appreciate your advice. Actually, the rudder blade has been remade not only to make it much wider than the original A Class, but it also sits slightly deeper in the water. The gap between the stern sections of the hulls is now about 1.5 metres