I would like to try a few things on a small scale, just to eliminate the truley bad. That should reduce the number of things to try on the large scale.
I would like to try Speer sections on a modification of Todd's rudder mechanism, but altered to allow canting and lifting. I see the following advantages:
1) the lower support reduces shaft loads
2) eliminate the rudder shafts altogther, since you just build the rudder blades
3) can put in rudder stops, since the rudders never have to rotate 180 degrees. this can avoid accidentally turning them perpendicular while going fast
I accidentally found a post in another forum that indicated that the problem Rob had with canting rudders was that the sideways water flow jammed the rudders against the support before there was enough backwards pressure to cant the rudders back.
I was thinking that the direction the rudders should be canted is directly related to the boom position. When you want to shunt, you rotate the boom to the other side. It would seem like there should be some way to have that automatically put a force on the rudders, so that when the boat comes (briefly) to a stop, the rudders would automatically cant in the right direction. That would be really neat if we could figure you how to do that.
Have you noticed how easy this is when all we have to do is talk about it, but not build it? <grin>
- Gardner
On Fri, Jun 18, 2010 at 1:51 PM, Mike Crawford
<jmichael@gwi.net> wrote:
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