Slipper test sail

Daiqiri, thanks much. I'll add this to my file of useful stuff.

The New Haven sharpies were originally designed to work shallow oyster beds, thus the extremely shallow rudders. Some were equipped to lower the rudder deeper in the water, by means of pins on the rudder shaft, but that's an elaboration I hope not to need. Reuel Parker is considered the guru of modern sharpie design by many, including me, and he describes certain drawbacks to these plank rudders-- among them the tendency to lift out of the water when pitching, and the potential for being squirrelly, because of the balance area.

The rudders I've just put on Slipper are not as extreme as the one pictured in your link. Their aspect ratio is somewhat higher, and the balance area somewhat smaller. They are huge. They also extend about two inches lower than the hulls, which increases the draft slightly. If they do the job (and I'm really not expecting them to-- I'm just curious to see if taking the rudders in a direction opposite to the one that proved disastrous will improve matters) then I'll try trimming them along the bottoms.

I don't expect pitching to be a problem, since the original rudders didn't come out of the water even when we crossed big wakes. Also, I adopted an idea from Parker, and added a wedge on top of the rudder, which takes advantage of the stern wave to add area. I think the natural directional stability of cats will smooth out any tendency for the boat to be squirrelly, but I can always take off some balance area, if not.



As I said, they're huge. And rough, but I'm just experimenting.. If by some miracle they work adequately, I'll smooth them up into real foils.

 

Ray, thank you for being willing to experiment, and especially for sharing the results with us.
Daiquiri, thank you for sharing the spreadsheet for counter weighting kick-up rudders.
I haven't cut out the rudder for my Wa'apa yet (I'm currently building frames for hatches). I have drawn up full size patterns and fully intend to build a kick-up rudder, but it will be nice to know how effective the alternatives would be.

 

Darth, it helps me to think about this stuff if I rattle on about it.

If Gary says to use a kick-up rudder, that's what I'd do. He's a brilliant designer, and knows a lot more about these little boats than I do.

 

Well, this thread has been quiescent for a long time, but I've finally made some progress with the little cartop cat, so I wrote a blog piece on the current state of affairs, which can be read here.

In short, much of the advice I got in this thread was good-- I finally made a set of decent kick-up foils, and the transformation has been gratifying.



The foils look huge in this picture, but really, the shaped area has a chord of 12.5 inches, and a depth of 19" The profile is NACA 0012. The boat tacks much more certainly, and even though the only foils are the rudders, she goes to windward fairly well now.

A brief video from yesterday's test sail.