Skid Fin Design Help

[McustoM]

I am working on an outboard hydroplane and am stuck on which skid fin design to use. I am looking for input on some different designs that have been tried and tested. The fin will flat (no curve allowed for outboard racing) and be mounted at around a 6 degree angle (As the unlimited boats do). I realize that there will have to be multiple designs to test as all boats react different so I am just looking for some help to get some baselines for the first fin. Here are some of the questions that I have:

1. The fin will be angled down but what is the optimum angle to do this?
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2. How deep in the water should the fin go from the bottom of the sponson runner? I know that the scale is a little different but I believe this can be calculated from a ratio of total hull length (my boat is 11.5 feet long).

Thanks in advance for any input! Offshoreelectrics has some great info that has helped me out so far in my build. -Branden

[carlcisneros]

by the sounds of it you are going to be driving the boat yourself. (being it will be 11.5 feet in length.)

If that is the case, then make multiple turn fins with angles starting around 7 degrees with
each additional turn fin having one (1) more degree angle than the one that was tried.

as far as depth in concerned, check with the limited builders (full sized boats) and they can realy steer
you in the right direction. This goes for the turn fin patterns as well.

The ones that I have seen on the 5 liter boats all have an angled leading edge on them with some of them
having vertical (straight) trailing edges and some that have the same angle on the trailing edge as they have on the
leading edge.

[McustoM]

Thanks for the input carl! 7 degrees seems to be a good baseline to start. I am not sure whether or not to go with the same angle on the trailing edge or just to go straight up. Anybody have any theories on this part of the design?

[carlcisneros]

It all depends on what you want the boat to do in the turns Branden.
The straight fin will give a little looser feel and the angled one (trailing edge on both types) will get you a more
of a tighter to the water feel. (so to speak. if this made sence to you)

[McustoM]

That makes sense I know understand why some designs that I am seeing on the rc forums would work well that have the initial angle going back and then near the bottom curve to the bottom flat part of the fin. This curve would make the last part of the fin want to stay tighter than the initial angle. Here is an example of a fin that I am talking about http://www.accu-techrchardware.com/RTF-100.html

[carlcisneros]

yepper
that style of turn fin really digs in and plants the sponson on the water preventing the boat from sliding out in the turns.
The forward/backward angle if you will will also control how the sponson with the turn fin rides on the water.
with the turn fin kicked back, the sponson will dig in the water, with the turn fin kicked forward, the
turn fin side of the boat will be riding on the fin itself. (the turn fin sponson will be out of the
water riding on air and not skimming on the water.)

hope this made sense to you.