Antonio
With that size sail on a boat that is easily driven you should get away with rudders 150mm long and 500mm immersed. Space then 4 to 5m apart.
If the boat is heavy and sluggish then the rudders will need to be bigger. If it can get over 10kts now then suggested size should work.
They will ventilate if you do not fence them and hence lose effectiveness at higher speed. There will be an air pocket on the windward side when ventilating. The height of the fence probably best placed level with the bottom of the hull.
You should have some means of trimming their angle with respect to each other so they can be fine tuned. Ideally it should be easy enough that it can be done while on a course but at minimum you should be able to find a happy medium and leave it set at that.
I provided the best section I could come up with
here:
You can scale it to suit and use as a template or I have posted a link to coordinates previously.
Rick
On 19/08/2010, at 6:10 AM, Antonio Tutusaus wrote:
Hello everybody:
This summer sailing has brought me one step closer to building my planned harryproa mini cruiser/weekender.
My brother in law rented a Beneteau (10m. monohull) for a week. It was nice, but all we managed to achieve were peaks of 8 knots running in a very strong wind. Most of the time we had trouble keeping 6 knots in full sail.
Also, the angle to windward was never smaller
than 50 degrees. And in the rocky coast north of Barcelona he was psychotic about hitting a rock with our 1.8m draft. So we missed many of the best bathing sites.
So I will probably build a harryproa in the next two years. BUT, first I will convert my pacific proa TITAO into an almost harryproa:
1) Add rudders
2) Add aerorig
3) Add engine
The key element of a harryproa that I won't incorporate for the time being is the all weight to windward since my ama is not buoyant enough.
The first step is rudders. I plan to build two rudders attached to windward side of lee hull. They will be symmetrical fore aft and asymmetrical lee-wind (almost flat to lee). They will be linked by rigid bars and have a single steering wheel, rod, rope..(don't know yet). They will therefore be in opposite sinc.
What I have really no clue about is positioning (distance from center) and size of the rudders.
TITAO basic data:
Mainhull length: 6.7 m
Mainhull width: 0.44m
Mainhull height: 0.9m
Mainhull asymmetrical lee-wind. Slight rocker, flat deck
Overall width: 2.56m
Distance between akas: 2.42m
Ama is V shaped with slight rocker.
The aerorig is yet to be built. I have no ideas as to sail type or area. TITAO has always sailed with a batten less, boomless, triangular sail of 11sqm.
I appreciate any ideas, considerations, advice or criticism.
Toni Tutusaus
Barcelona
Rick Willoughby
03 9796 2415
0419 104 821