Extending a stearn to mount outboards

I'm repowering an old Gibson 36 with a pair of Merc 60hp outboards. Mounting the outboards requires a 2' setback from the existing stern and 2' above the waterline. Existing stern only extends 7" below water at the edges and about 12" below at the center.

My intent is to use 2 layers of 3/4 ply to build a box 2'x2'x8; bolt it to the existing stern; and expect the back side of that box to provide foundation to the new transom's engine mount.

My question at this point is an understanding of how much buoyant force is exerted by the box I am adding. 2'x8' 'hull' that extends 7" below waterline?

Presuming a bullet-proof bond between addition and existing stern (including 4 sets of 4 thru-hull bolts top and bottom) To guesstimate the amount of additional bracing required i'd like a idea of how many pounds of flotation that box gives. The engines combined weigh 625lbs.

thx much

 

Quite a lot to consider apart from the bouyancy of the "box". The structural integrity of both the bracket, and the hull where it will be joined, the height of the installation, the effect of shifting weight aft, the selection of a suitable shaft length, gear ratio and prop, etc. You need to get all right to avoid disappointment.

 

go metric and its easy .6x2.4x.15 =??? is the extra displ in cu.m or kgs in fresh water

But I agree with Mr E, you have greater worries than that

Richard Woods

 

Mr Efficiency gave the best advice. Strength and buoyancy of the box is of lesser importance than whether the existing transom can handle the new cantilever you plan to hang on it. You need to look at the balance of the whole boat and what your addition will do to it. My first guess is to look at an extension of the existing hull that maintains a zero sum buoyancy to match the moment of the new weight.

 

If the previous motor was a circa 80hp 4 stroke, adding twin 60's will add a good bit of weight. The box itself will add weight. The buoyancy added is about 62.5 pounds per cubic foot. If 2 x 8 x 7/12 @ 62.5 per, call it 580#. If 12 deep in the center, then 2 x 8 x (7 + 12)/(2 * 12) @ 62.5 per, call it 800#. Either way, you're looking at about a 20,000# boat if memory serves, so you are not going to notice any trim change. The weights and moments will balance out near enough for an 8 knot houseboat.

 

Highly appreciate the replies -- based on the stamp on the engine (a Chrysler 318) someone was able to return these stats:

ABOS Marine Bluebook:
Make Gibson Boats
Model Year 1971
Model 36
Model Type Houseboat Full Hull
Length - Feet 36'0"
Beam Length 12'0"
Pontoon Length x 36'0"
Weight 9600

I'm figuring the engine and outdrive weighed ~1200 lb so i'm pretty sure I'll be needing to add ballast after launch but I'm not too concerned about that mainly because the only performance characteristic i'm concerned with is fuel consumption.

I have no expectation of ever getting this thing back up on plane - i'd much rather spend the day tooling along a 5kt than to spend the $$ to go 'fast'.

I've lost a little sleep over the concern that I'm drastically changing the stress point of the transom but, again... based on the fact that I'm content to operate at much lower speeds I'm just going to go so far as adding bracing from the interior of the transom wall down to the floor joists that used to hold the engine and watch for signs of stress after I'm back in the water.

So it's very reassuring to hear the calculations come out as they do.

I think the next most important feature is the props. Keeping in mind the lack of need for speed -- that I'm patient enough to not need the burst of power out of a dead stop, how do i choose the prop? And how do I optimize the depth that i set it?

and again... thanks much for the time.

 

You probably don't need 2 x 60 hp if there is no need for speed, 2 x 25 would likely suffice if you can get a "high thrust" version in them. The only time you might appreciate extra horses is in a strong headwind. One 60 would be enough for puttering around if you don't need the security of twins.