Outboard motor transom design

Attached is a photo of my Skoota 20 fairing. You can see I also had an adjustable nacelle which could be raised/lowered depending on sea state.

A transom or fairing is essential. Gary is right, it isn't cavitation that's the problem. It's ventilation, as the outboard leg is not a good low drag shape. Apart from anything else you need to reduce spray/solid water from cascading over the engine at speed.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

I agree with the four or five others that recommend a wedge on edge with a flat bottom.
The ones I've build are about 80% as wide as the leg and seal tight against it with a rubber block. I use a finer wedge angle than the ones shown. The nacelle thing seems to work well on larger boats, usually looking for displacement speeds.

 

I have a twin canoe with a MinnKota 55 mounted between the hulls. There is alot of noise from the water flow around the 1 inch diameter vertical shaft. I made a smooth wing shaped fin that attached to the shaft that was below and above the waterline. The bullnose leading edge was about 1.5 inches diameter and it was tapered to a .125 inch trailing edge. It seemed to make matters worse!

Next I will try sharp leading and trailing edges with a 1.5 inch thick portion around the vertical shaft. I know putting an oar into the water on edge on makes little noise.

The twin canoes with the electric motor is a fun and versital little craft. I was able to pinch each hull with the connecting crossbars (amas?) and the boat moves very easily.

 

New Cat Build

I am aware that this thread is old but there is not much information about engine fairings for centrally mounted outboards on a cat. This is a power cat designed by Kohler. It will cruise in the low to mid teens with a 20 hp outboard. The fairing shown in the photos is not detailed in the plan and is my own design. I think the basic principle is fine but the geometry might need adjustment. The bottom of the fairing is at the designed waterline. I am assuming that the boat will squat a little under power. The outboard is set back some and that is central to my question. As you can see I left the bottom of the fairing extended back towards the outboard. My thinking at the time was that the fairing needed to be closer to the outboard. After reading about setback with jack plates I am not too sure. It seems that the farther back the outboard is the higher the outboard can be. I know there is a limit and my intention is not racing or adding tiny amounts to my top speed. I just want the outboard to run efficiently and maintain its bite on the water. The cavitation plate now is about 3/4" below the bottom of the fairing and I can drop it down another 2" with the bracket if I need to. Probably I will have to experiment with sea trials but I would like any comments from anyone who might have some experience. The boat might go in the water this summer.

 

The bottom of the wedge is at the design waterline ? The transom(s)are not immersed then ? Must be a lot of rocker to it. If that is the case, your prop is quite shallow, and ventilation is a possibility with the pitching motion of the boat. You could have made that fairing narrower, but it seems you have kept it to the same width as the jacking plate.

 

waterline

It may be hard to tell from the photos but the waterline is right at the chine at the transoms.

 

OK, you obviously have good clearance height with the tunnel then. Testing on the water will tell you if the engine height is OK.

 

You definitely need some sort of fairing in front of the engine. This is what happens without it. The photo is of a prototype we are testing right now, and we were in a hurry to get waterborne, so we didn't install any. Propeller will be in difficulties, when speed increases to point, where the bow waves combine at the motor.