Twin I/O to Twin Outboard Conversion Project

Your risk of contaminating the vinyl ester is too high in my opinion.

Staggering the holes slightly would distribute loads better, no?
Although, radiusing and glassing would be easier if aligned.

Great stuff but I'm not convinced it's necessary.
( Tell us again why you're not using epoxy? )

I would use thickened epoxy but 5200 would do.
Are you using mechanical fasteners as well?

 

I ordered some Cabosil for the fillets. I tried the Coosa dust and it does give a rough texture . . .

As for the holes/cut-outs . . . I am talking about the cut-outs in the extension pod.

1)Transom openings . . .


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2) Underside of the swim deck . . . (both sides)
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I'll be making 'matching' cut-outs in the boat.

There will be about 50 bolts holding this to the boat, which will be drilled after the extension pod is set in place . . .
Here is a re-post of a prior illustration . . .

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A day of fillets . . .
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I got everything fillet-ed that could be done at this point. I'll be traveling for the next 10 days, so not much will happen until later this month.

 

Sharing some detailed drawings of the extension pod. These helped me with some internal design details and the total gusset, rib, and bolt count.

Yellow shaded are the boat's structure and the gray is the extension pod & related support structure.


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In total there will be 78 bolts holding the extension pod in place. The nice thing (IMO) about the design is that forces/loads are distributed among the transom face and the swim deck bottom and sides.

In addition to the 78 bolts, there will be 74 gussets and 16 ribs in the structural details. Distribution of loads/forces is the name of the game.

EDIT: Updated drawing file

 

I did a test fit of the extension pod frame to the boat. With this, I marked the cut-outs in the transom and the underside of thew swim deck.

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I'll be cutting the holes this week.

 

I really prefer fillets and tapes the same say for all primary bonding and no sanding. But if you wanted to test fit it before tabbing; it may make better sense.

I guess if you need the holes for test fitting; that makes sense for same to hole now versus after glass.

 

Where the pod goes up about 3”; you have a small stress risor; so you might want a large fillet there after you close it and you really ought to pre release an inside tabbing there for same reason.

I did a finger sketch for you. The tabbing would go up 3” or so and then be in the plane of the knees. You tack a 1x6 with ship tape all over it with hotglue to the knees and then probably two tapes say a 6” first and a 4” to the vertical section and the board. (4 sets it looks like) After they cure hard, remove the board and sand well and bond your bottom board to them. Green is pre release tabbing (poor drawing), blue is fillet under the filler board. Board is preglassed on inside I assume.

Just in case you get some movement there from engines; you don’t want a bottom only glass tabbing to take all the stress and square corners, etc.

You can get a better sense of the stress risor from this picture. All the engine weight bears down on the knees and this joint.

ps - you really need a fillet in there to do it properly; do the 4 fillets just before the tabbing

To be brutally honest; the bottom board also needs to be tabbed to the knees same way and best all at one time. Let me know if I am way off here, but at some point something has to be prereleased it looks like..unless you were going to do it all on the top, etc

 

Here is a view of the extension pod from underneath . . .


The force exerted on the knee brace will have to transfer to the main and secondary stringers and onto the base of the transom mating plate. So, I see your point ( @fallguy ). I think gussets on both sides of the stringers in that area should help transition the force from the knee to the stringers across a wider area.

I'll be getting the inside surfaces glassed over the next week or so, and then will be adding gussets. So, 6 more gussets.

 

Quick Update . . .

I have begun fiberglassing the internal areas of the extension pod . . .

A few imperfections, but it will be OK

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I am liking how the fillet areas are looking

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It always seems that my fiberglassing projects are during the winter months (Northeast USA), so temperature is a constant battle. My work area is usually around 60˚F, which is a bit on the cold side for curing. To get things moving, I 'tent' the structure and add some heat. Inside the tent is about 80˚F.

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I am trying to get as much of the internal glassing done before I flip the structure right side up and attach the remaining pieces. Once I do that access to the inside areas of the structure will get difficult.

As for resin usage on the 1708 cloth, I am finding that 1 ounce of resin is enough to lay down a base coat of resin (on the Coosa) and fully saturate 40 sq. inches of cloth. I cannot recall what my resin usage was on the swim platform that I did about 6 years ago, but it was probably about the same, maybe a bit less due to the vacuum infusion. Anyway . . . it looks like I'll be using about 12 gallons of resin in total.

 

You are running a bit rich on the resin, but 1708 is a bear. To reduce the usage; apply about 60% on the base, lay the glass and fin roll, then topcoat the other 40%. It can be done at a rate of 1 oz per 50 square inches, but in the cold it is turning to honey for you and always raises usage.

 

A couple of design changes for those following along . . .

#1) Ribs/Stiffeners . . .
I had planned on placing some stiffeners on the 3 stringer/girders of the extension pod to address compression loads and any tendency of them 'buckling' under such load. However . . . fabricating and glassing ribs in along the 'web openings' of the stringers looks like it will be too difficult.

So . . . instead I will place a set of horizontal ribs about mid-point along the girders to act as stiffeners. These ribs essentially shorten the the 'un-supported' span by about half, which increases their 'buckling strength' by 4X ( based on my 'strength of materials' class some 50 years ago !!! )

Horizontal ribs will be much easier (for me) to fabricate and give similar strength results.

2) Stress Riser Issue/Knee Braces . . .
I may add some external knee braces to the 'engine bracket' portion of the extension pod to address the 'stress riser' issue mentioned by @fallguy . My thoughts of the initial design, being as it is, were to reduce the static buoyancy of the extension pod. A couple of external knee braces at the engine locations should provide better transfer of loads from the bracket portion to the main structure (secondary stringers) of the extension pod. The fabrication aspect of external braces should be fairly straight forward. See below . . .

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Anyway . . . Those are my latest changes to the design, just to document them.

 

Will those external supports cause any induced cavitation on the way to plane?

 

Absolutely not

 

Someone brought up the aspect of change in the balance point (CoG) due to the higher center of mass of the outboards vs. the sterndrives. (The nerve of them )

Anyway, I had figured the CG would be a bit higher, but not terribly so, having not actually calculated it. So, I calculated it. (5.2" upward)

Here is an updated version of the CG diagram I had posted earlier in this thread. The CG moves 9" aft and 5" upward as a result of the conversion. It probably will be fine considering the extended planing surface and the 130" beam of the boat.
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Well, I will be very interested to hear how the higher CoG feels to you.

I drove a Double Eagle with twin Volvo-Penta inboard / outboards commercially for over a year, every day.
It was then converted to twin outboards.
The handling was very, very similar until the boat rolled considerably in a sea-way.
It would roll and then just hang there in the most disturbing way, like it didn't want to come back!
It always did but it was very disconcerting.
The waiting was not enjoyable ( especially with 14 soles on board! ).