My plan is to go with speer sections and see what happens.
The worst case is that they'll get weird or a bit unstable at high
speeds, in which case I'll have to replace them with naca sections. I
think the only way to see if they're going to work on the large scale
is to test them on the large scale. If they do work, it would allow
for a very simple shunt, and definitely simplify control via autopilot.
I think you hit the nail on the head: smaller rudders mean smaller
loads. I believe that one needs the longer rudders on the rudder-only
design because the rudders have to deal with all the leeway forces
while also handling the steering. Thus, Doug's boat is much happier
with the extra depth.
The rudder-only system is not impossible to design for, but it
apparently requires more foil than expected in order to keep laminar
flow. Perhaps there's an interaction involved with leeway prevention
that creates additional turbulence if you don't have enough foil depth.
If you took a lot of the leeway prevention force off the rudders,
they could surely be smaller without stalling. How much smaller? I
have no idea. That would take some playing around.
- Mike
On 6/18/2010 12:58 PM, Gardner Pomper wrote:
Hi,Thanks very much for the link. I have seen that paper and tried to work through it, but I did not understand why he did not include what the foil sections looked like. Now I know it was in the appendices, which is not part of the paper.Anyway, I really didn't understand any part of the paper, except the conclusion that the proa3 sections were the ones to use, so I guess I will try to reproduce one of those on my plywood proa. How do I know which one to use? 30012? 30112? something other than 12? Is it like sails, where at slower speeds, I should have more curvature?Related to this, I am not clear on how adding keels/leeboards reduces the forces on the rudders. I assume that you mean that you could just buld smaller rudders, but didn't Doug demonstrate that you need bigger ones. A central keel, at least, does not seem like it would prevent heading up into the wind. Also, Rob has said that you need the bigger rudders to get going from a shunt, so wouldn't we end up with big rudders + keel?- Gardner