Twin I/O to Twin Outboard Conversion Project

Structure Fab/Assembly Update . . .

After much consideration on how best to match the transom contact area (4 Coosa pieces) I decided to line the transom with cardboard 'facing' to mimic the fiberglass thickness. The cardboard facing gave about a 3/32" (0.9", 2.5mm) offset to everything and I then stuck the Coosa pieces to the cardboard facing. This seemed to work OK and about as good as I was going to get free-hand.

Here is the Coosa temporarily adhered to the transom and the edges/corners glassed.

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A couple of swift tugs and the whole thing broke free . . .

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It looks OK. I was having to add heat while the resin cured (57˚F outside temperature). When working in colder weather, I generally go with 2% catalyst.

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I set the center support piece in place to see how accurate the transom angle was . . .

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Looks pretty good . . . no bit gaps.

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Next steps will be continued assembly of the structure (mostly with fasteners at this point) and then another check of fit to the boat's transom before glassing.

 

Making good progress

Almost all my glass work is done sub 60 degrees. For smaller projects that don't get a full shelter we picked up a big ice fishing hut and a tube style diesel heater. Helps us get the project up to temp and held while it cures. Sometimes we'll even do the project on a table and drop it down over (has an open floor) and crank the heat with open windows. Its helped a lot for getting better cures in the cold.

 

Thx @comfisherman . . you are in Alaska (right?) In terms of fiber-glassing, where there is a will there is a way.

I did move my work area to a sunny spot . . . so the resin can 'solar cure'


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Ah yes the dark core definitely benefits from direct sun.

We loose intense enough sun to benefit in mid september... its just darl now. Gotta make our own liquid sunshine with big heaters.

Progress is looking good.

 

You have inboard dedicated hull . Adding extension to relatively weak transom . Yeah you will do many reinforcements .
This coosa aligned to the transom is zero worth mechanically .
As I said before what you really need is freeboards elongation or lets say hull elongation .Freeboard can take big loads .
Nice progress anyway.

 

@montero . . . when you say "freeboards" do you mean stringers?

 

One common aspect of nearly all I/O (sterndrive) to outboard conversions is that the boat's transoms were designed for sterndrives, known to be weaker in some regards than outboard equivalents. I'll have to do some searching as to the actual force/load comparisons. It may be helpful.

 

I know there is much talk about the differences in transom requirements of sterndrive vs. outboards, but have I wondered how much . . .

Here is my take on the transom forces of a sterndrive vs. an outboard w/ 30" shaft . . . Figuring 350 HP @ 41 mph (YMMV)



In general, it looks to be 66% more torque on the transom for the outboard configuration. (shorter shaft motors would be less)

 

No .I mean freeboards. Full beam extension of the hull .

 

Good analysis tpenfield.
Don't overlook the added weight and lever-arm of the o/b itself contributing to the transom torque load.
Also the acceleration load, i.e. getting up on plane may be higher than the static load while on plane.
However, your duo-prop is going to transfer more load through better "traction" than a single prop o/b.
I don't know how much debris you get in your waters there, but log strikes is a huge
issue here and may be a consideration in your neck-of-the-woods.

IMHO one can never have a "too strong" transom.

Great progress!

 

My apologies, but I am not familiar with that term . . . other than 'freeboard' which is the height of the deck above the water line. As far as a full beam extension, that ship has sailed.

@BlueBell . . . all good stuff. since we can assume 350 HP is what the engine can put out, I can probably calculate the typical force coming on plane. I am envisioning a graph of some sorts. Not a perfect science, but probably a good approximation.

Also, in discussing things with one of my engineering buddies, dynamic loads came to mind. I am not sure what boats typically try to design to (3G, 5G, etc.) . . . kind of like airplane structural ratings. I calculated the static forces (i.e. 1G) earlier on in this thread (3,450 lbs at top of transom IIRC). So I could see what 3G (10,350 lbs) and even 5G (17,250 lbs ) look like.

Some more progress today . . . not sure how much more I'll get done before darkness sets in.


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I'm letting the sun do its thing to cure the resin. (only about 50˚F outside)

 

How you guys call the board of the boat ? Freeboard , gunwale ?
@tpenfield how much is this coosa gramature and thickness ? I'm too lazy to dig .

 

Sidewall . . .

The Coosa is the BlueWater 26. . . Using it as a core material for the fiberglass. I have 12mm and 25mm thickness, doubling up as needed.

 

Engineering question that might be basic (I chase fish and then hit old rusty boats with a hammer for a living so only have a rudimentary understanding of engineering) question?

On the portion that will be the new transom. Will it be glassed across on the inside before the stiffeners are placed, or will they be put in place and glasses as a unit? Does it matter significantly for the strength?

 

I plan on glassing a couple of layers, in a continuous fashion, then fold in the glassing of the stiffeners and then a couple final layers of glassing, making the glassing of the stiffeners captive.

I'm not a big fan of glassing, sanding, then glassing an additional layer(s). I'd rather get the critical areas glassed up in one shot to the extent that I can. The transom will be a 2" core and 1/2" (total) of glass