Twin I/O to Twin Outboard Conversion Project

I did some force calculations assuming a typical power curve and boat speeds. 4700 lbs looks to be a high number while coming up on plane and 3,000-3,500 while on plane (these would be maximum numbers).

(HP=F x V) 1 HP=33,000 Ft-lbs/min


.
If I apply these numbers to my force calculations . . .

.
This gets to a '3.5G' kind of number.

This exercise is helpful in finalizing the internal structure of the extension pod and the transom reinforcements. (What's the tensile strength of biaxial fiberglass/VE resin?)

 

You will have power of 6 ton x leg length , transom will be backward twisting at acceleration . I dont understand why your transom at pictures isn't going down like "normal" transom . It make whole build weakier.

 

The largest forces are likely to be those imposed by moving at great speed over large waves. A hard landing can be several g's.

 

Yes, I have been thinking about the internal structural details to handle impacts with the water. I'm thinking a hard landing could be 3-5 G's . . . Not sure if any applicable analysis has been done/shared on this forum, but worth a search.

 

Probably can apply some basic kinematic analysis to the issue of hull slamming . . .

Basically considering how far out of the water a boat goes after coming off a wave ( probably lands in trough) and how far the boat goes into the water upon impact . . .

So for example, if a boat hull is 5 feet out of the water and goes into the water by only 0.5 ft upon 're-entry' . . . it would be a 10G impact.

 

I am quite certain those studies have been performed, but your math checks out. I know aircraft structures within the cabin are designed for 20 g's. Radio mounts and the like.

 

Typically, when boats slam in short, steep waves and confused seas the transom experiences the lowest forces.
However, every slam is a cycle of loading and unloading.
Over time, this fatigues transoms sometimes leading to catastrophic failures.
This may be avoided by rigid, robust design and construction.
Did I mention there is no such thing as a too strong transom...?

 

My Formula 330 Sun Sport was fairly light for its size . . . We would catch air if going 30 mph (ish) in a 2-3 foot chop. Here is a picture of my boat about as far out of the water that I ever got. I think the props were still in the water, but not the hull.

The Cruiser Yacht 338 (current boat) is the same size but a lot heavier (2,000 lbs more) and has a deeper 'V' hull than the Formula. I have not caught any air in the boat, not even close.

As you say . . . the transom typically remains closer to the water. Mid-ship seems to take the most of the impact.
.

 

Closer to the water and also much, much closer to the centre of rotation.

Props leaving the water is more than a magnitude of difference in what your concerns are.

Just to be clear to others, that is not the boat in this thread.

 

Yes - here is the boat in this thread. . .

Cruisers Yacht 338 Sport Series (mid-cabin bow rider)
32'-11" 11,000 lbs. dry weight, 22˚ Deadrise

.
I'm not sure this boat could 'fly' even if it had wings.

By contrast my previous boat (that could fly)
Formula 330 Sun Sport, 33'-0", 8,900 lbs. dry weight, 20˚ Deadrise

 

And how many HP?

 

The Formula 330 was 2 X 330 Hp

The CY 338 is/was 2 X 350 HP

 

Progress Update . . .

I've got most of the big pieces cut & fitted. The transom pieces remain.
.

.

.
I'll be working on the internal structure over the next few days. First order of business it to cut some access holes. There will be holes in the transom face and the top plate for fabrication and assembly access, as well as any maintenance needs (bilge pump, etc.). I'm trying to keep the access holes fairly small, yet large enough for access. I came up with this . . .

.
I sketched out a few patterns . . . something big enough for a person (me) can get their torso through . . . and finalized on the rounded corner pattern shown above. The Yellow lines are the cut-out and the Purple lines are going to be a 4" high stringer grid, which will give the flange area some 'muscle'. There will be a matching grid inside the boat's transom to provide strength from both sides. I'm also working on the internal structure details (ribs, gussets, etc). I've got some Divinycell on order for the internal bits.

Everything gets glassed, which will be challenging in some regards, considering the shape complexity . . . we shall see.

 

Terrific progress TP.

I can't decide whether to mention it or not but this is your last chance to complicate things further and cut out some lightening holes in the internal panels.
Single, 8" or 10" holes would lighten the unit without sacrificing strength to any significant degree, improve circulation, and look impressive.
Your call.

 

I was thinking about the weight savings with a few 'web holes' . My 3D modeling had such. It might not save much weight (20-30 lbs) . . . I just have to think about what pattern makes the most sense. I'm also planning on adding some ribs to the big pieces to address buckling forces.