Join Date: Jan 2012
Location: British Columbia
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Re: Bob Perry's take on Wolfenzee's dream boat
Tucking a rudder under the counter makes self steering and inside steering horendously more complex and failure prone. These factors in an offshore cruising boat, far away from marine supplies and boatyards, far outweigh any slight increase in speed. Having an outboard rudder has enabled me to build my self steering, far more effective than most horendously expensive models; mandatory on boat with inboard rudders, and my inside steering, for around $25. People who have never cruised off the beaten path on a shoe string budget are often totally incapable of comprehending such priorities, and thus unqualified to design for anyone on a tiny budget ,heading off the beaten path..
My wind vane, my autohelm and my inside steering all operate via the trimtab on the trailing edge of my rudder, a bullet proof sytem, far stronger and simpler than any attempt to power an inboard rudder with a servo.
What I have proposed is a set of nearly horizontal plates, on the sides of the outboard rudder, just below the waterline, tipped slightly up at the stern to follow the flow to the stern wave , to stop air from being sucked down the lee side of the ourtboard rudder ( like the ones on an outboard motor).Air bubbles would simply trail off the plates before being sucked down any further.
The turbulent flow of the transom is only on the surface. It doesn't go very deep, unlike the rudder. It has no effect on sucking air down the rudder. Again, the amount of water being diverted on the high pressure side of the rudder is a huge turning force, air on the lee side being far less relevant. Continuing the foil up a bit further can be done with any outboard rudder , easily.
When I put a rudder much further aft on my first boat, I simply removed the keel hung rudder, ending up with less wetted surface than I began with.
What is important is to make sure the ruuder rakes forward, to pull water up it, instead of air down it, not raking aft, which would invite air to suck down it, possibly on both sides. This is a common screwup by those who put trendyness ahead of common sense.
I met a Frenchman who had an outboard rudder on his 35 ft aluminium boat. He put two dagger boards alongside them on the transom, angled out 25 degrees.. When I asked him how effective the dagger boards were, he said that with the daggerboards down, it is imposible to get her to go anywhere but dead downwind. To change course he had to raise the daggerboards a bit. It made it impossible to broach.
Maybe something to consider on an offshore boat. Could be retrofitted.
Brent Swain, Boat designer, Builder, and author of "Origami Metal Boatbuilding"
Last edited by Brent Swain; 06-19-2013 at 05:59 PM.