Re: Spade rudders vs blue water passage making
A friend had to replace the bearings on his Dash 34 with a spade rudder which had not all that many miles on her, and has never been offshore. It was expensive.
You can make a skeg as strong as you please, without a serious size or weight penalty, then use some greatly oversized gudgeons and pintles, with no real penalty. The weigh difference between going for 1 inch pins and1 1.2 inch pins is negligible, but the safety factor goes up tremendously. With a spade rudder you have more serious limitations.
As Bob points out ,a skeg hung rudder has a twisting load on ,over a much shorter distance ( the chord) than the cantilevered force on a spade rudder ( the length, a much greater leverage). The load on the hull bearing is huge ,and the catilevered load on the shaft at the top of the rudder, compared to the spread out loads on pintles, as it is on the shaft, and thus far more prone to fail on a spade rudder. If you have a pintle on the bottom of a partial skeg, it puts the bending load on the much stronger blade, rather than on the shaft .
Bob. What do you use for sleeves where the carbon fibre wears on the bearing? Bronze? Titanium , more affordable in Seatle and near Boeing Surplus? It seems that crevice corrosion under the sleeve would rule out stainless.
Then there is always the concern of fouling a line between the rudder and the hull, which is common sense which doesnt show up in computer calculations. A good low aspect skeg eliminates this problem, a liability on spade rudders.
On my first boat, a pipe dream designed by Kinny, in my early 20's ,I visualized a skeg hung rudder, 6 ft futher aft from the keel hung rudder specified. Then I thought "I have zero sailing experience, so I better stick to what the highly regarded designer specified. After sailing her accros the Pacific I did exactly that, a huige improvement.
Brent Swain, Boat designer, Builder, and author of "Origami Metal Boatbuilding"
Last edited by Brent Swain; 07-05-2013 at 10:55 PM.