Slightly different outboard - transom question

[M-S-D]

Hi knowledgeable forum. This may have been asked before but couldn't spot it quickly.

Believe I understand the concept of the captivation plate to be level with the bottom of the transom.

Having a scout around the hard standing, all the boats with outboards have a hull which resembles a flattened "V", i.e .the lowest point of the hull is lined up with where the outboard would be. So a nice easy measurement.

Now, for our sins, we are converting an old Dawncraft that in the past had an inboard engine, and the hull here is not the "V" style, but is "cut out" style. i.e. about 1.5 feet either side of centre is the lowest part of the hull, and lined up with where the outboard would be is about 4 inches higher up.  __/------\__
So my question is, should we be taking the measurement from the lowest point of the hull which is to the side, or from the centre which is higher up?

Additional info, this is just for pottering about on inland water ways. Also intention is to add an "outboard pod" so the outboard will actually sit say 16 or so inches behind the rear of the boat.

We probably have enough "play" to go for a long shaft or an extra long shaft, and advice here may clinch it.

Just as an aside, it seems to me from measuring the actual outboards, that the captivation plates for say a 20" long shaft actually seem to be more like 22 to 23 inches. Maybe this is just manufacturers building in some extra to allow for angle etc.

Thanks in advance.
 

[Tony Brooks]

My thoughts:-

On no boat is it a good idea to have the transom masking the prop so you need the prop lower or all but lower than the lowest point on the transom. That puts the anti-cavitation plate at or a little above the lowest point of the transom. I don't think the measurement has anything to do with the plate except its an easy way to describe what is ideal from a prop efficiency point of view. Now with your hull the water will have a lot of length to get into the tunnel well before the prop so as long as the prop has no transom immediately in front of it any height should be fine. I would npt put it too deep though to avoid damage from hitting things - OBs do not always spring up on impact!!

 

 

[Alan de Enfield]

Just make sure that the prop is not 'masked' by the transom and that it has a free flow of water.

 

Edited March 10, 2018 by Alan de Enfield

[Phil Ambrose]

Look at it from a slightly different angle  when in reverse the flow of water (and thrust) should be free to pass under the hull, if the thrust/flow of water strikes the transom it means you have no stopping power because the thrust is absorbed by the transom. 

Phil 

[M-S-D]

Thanks Phil and all