Long Shaft on Short Shaft designed Rib

[Bigplumbs]

Amongst my small fleet I have a nice little Valliant D340 Rib that currently has a 15 hp Mariner 4 stroke on the back that is pull start and fixed trim. I use this boat for various small trips and would like to switch out the engine for a Power Tilt and Trim and Electric Start. I have found a nice one (20hp 4 stroke) for a reasonable price but it is long shaft, This little rib is designed for short shaft.

So with a 4 inch difference I was wondering if I could plant say 2 inches on the top of my Transom and then only have 2 inches too long at the bottom of the hull. Would this be silly or would it go ok.

Interested in views particularly of anyone that might have done something similar

[Andy]

Well...... what I have been taught is that the ventilation plate, just above the propellor, has to be, near as damit, as close to the bottom of the bottom of ya transom as possible. From what you say this wont be the case ?

[ChrisParish]

From a physics point of view the torsional loads on transom would be greater than designed, as would the drag of the boat.
It would all “work” (for a given interpretation of work) if you took it easy but you may well get a lot of splash back and your fuel economy and performance would suffer.

If you had the long shaft engine already then I would give it a crack, but I think that personally I would keep looking for a short shaft engine.