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Ask the Experts: Shaft Tubes

Author: Robert Ullberg
Q:What are "full-length" shaft tubes, and should I consider retrofitting my boat with them?

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With a gas model you don't need to worry about moving around a cord or many extension cords to reach your trimming destinations. Shaft Design The shaft is the tube connecting the motor or engine to the cutting head. Curved shaft models are generally lighter in weight and are easier to store. Trimmers with straight shafts or extended shafts provide better balance and allow the user to reach greater distances.

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Rubatex Pipe Insulation A:Full-length shaft tubes are basically an extension of the "shaft log." This is the tube that houses your shaft bearing at the point on your boat where the shaft enters the hull. What some builders have done is simply extend this tube all the way back to the propeller shaft strut.

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Rubatex 180 The biggest advantage that full-length tubes offer is drag reduction. At first this may not seem possible, in that the shaft tube is much larger in diameter than the propeller shaft itself. It stands to reason that the bigger the object, the higher the drag. The difference lies in the fact that the shaft tube does not rotate.

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Rubatex 180 Fs There are several factors that contribute to drag. Frontal area, surface condition (rough or smooth) and wetted area are the largest contributors. When a stationary object (such as a cylinder) is introduced to water flow, the water separates from the cylinder a little more than halfway around the cylinder. The surface condition determines where this separation occurs. The rougher the surface, the sooner the separation. This separation reduces the wetted surface and thereby reduces drag. (This is why golf balls are dimpled.)

Rubatex 1800 Even if a shaft is smooth and barnacle-free, when it rotates in the flow of water, the water tends to "stick" to the surface and does not separate. This, of course, adds to the wetted surface and drag. The non-rotating shaft tube, even with its larger diameter, can actually reduce drag up to 70 percent at high speeds.

Rubatex 320 If you decide that you would like to install shaft tubes, consult a marine engineer or naval architect. A few modifications will need to be made, including the addition of a water-injected shaft seal or stuffing box. Also, a cutless bearing should be installed in the strut to hold the aft end of the shaft tube in place.

Rubatex 373 Marine experts will also help you make sure the tube is sized correctly for your boat. A larger tube than necessary can produce too much air behind it and hinder the performance of your props. I like to actually "shape" the tube from a round to an airfoil shape. It's a little more expensive but produces far fewer bubbles.

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Robert Ullberg is President of Ullberg Yacht Design And Engineering in Winter Park, Florida. For more information, contact him at: ullberg@cull.net