Twin I/O to Twin Outboard Conversion Project

Also thinking about lateral bracing since most of the structure is oriented front-to-back.

 

20 lbs savings to make it a significantly more complex layup.... oof.

 

I'm still thinking about web holes, but not sure the fabrication trade-offs would be favorable.

Here (below) are the latest set of pictures. I should have some Divinycell arrive today, which I'll use for the core of gussets, stiffeners, etc.

I'll need to think about the best way to fiberglass this (complex) shape, mostly concerned about the internal surfaces. I know that I'll have to do it in sections, but wondering about the voices of experience in this forum. I know @fallguy glasses various boat structures . . . probably a few others.

I plan on adding gussets and stiffeners, which I will pre-form (Divinycell) ahead of time, then install/glass in right after I put a layer of 1708 on the base surfaces. The external surfaces I am less concerned about, since it is more straight forward.

Anyway, comments and ideas on the approach to fiber-glassing all of these internal shapes are sought and welcome !

I consider myself only mid-range in fiberglass experience, having done 3-4 boat/fiberglass projects over the years to various boats. . .

• Anchor locker design/fab (2010),
• Structural repairs (2014),
• Extended swim platform (2019).

Progress pictures . . . I'll need to install some weep holes and do some gap filling before fiberglassing.
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Most all my internals were glassed with db1700.

If anything was load path related, it can be glassed with 2x db1200 or 1708, but all my pieces were pretty small.

My boat gained some weight using 1708 for tabbing, maybe 200 pounds more per hull. But I always feel pretty confident in rougher seas. Use two or three pieces depending upon load path, I’d say. You may want to pre-release the tabbing for the splash pan, if you have one. Minimum 2 tapes, but more for bigger loads.

I did post cure both hulls in a 40 foot oven. The reason I mention is taking parts off the vac table with two layers of 1700; they were a little gooey, for lack off a better term. You could use an electric blanket over plastic vac bag if you wanted the pieces to be stronger.

I don’t know what that shape is with the holes. If you attempt to vac bag that; you’ll be really sad as those cutouts should be done after. They will suck all the resins from nearby, so if you are bagging that; put the pieces back in, or you could try to cut the glass out, but you’ll have bridging troubles. Anyhow, maybe that is something else.

Please forgive me, I did not follow along a lot lately. I took my boat 2000 miles or so from Minnesota to Corpus Christi, and have been busy as heck with a bad left foot too boot.

 

I think I will cut some web holes in the girder-like stringers, as well as add some ribs to strengthen them.
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Something like this . .

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I'm also going to add some lateral support . . . details TBD.

 

Don’t make those cutouts until after you laminate if you are wet bagging.

 

I'm not planning on vacuum bagging, but thinking I should have a layer of fiberglass laminated before adding the ribs. Maybe I could use peel ply to get a textured surface for additional glassing. (?)

How many layers of db1700 (or 1708) cloth/resin are you applying to the boat hulls you build?. My understanding is that 1708 is going to yield about 0.05" thickness or 3 layers for 1/8" thickness.

 

It depends on where it is going.

For the tops of my transom, I overlapped 8 layers of db1700. I wrapped 4 layers on the outside of the hull and 4 on the inside, so 8 over the top, then I squared it off just for fun.

Walking decks all got 2xdb1200 both sides.

You math is pretty close, 1708 is a bit under 0.045/ or darn close to 1/8” depending on methods.

 

Thanks . . .

I was planning on 1-2 layers of 1708 on the inside and 3-4 layers on the outside surfaces, with 6 layers on both sides of the transom, wrapping around the top.
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Maybe an extra layer or 2 in the high stress areas (bolt locations, etc) . . .

 

Reverse the transom top so the longer pieces land first and over a good radius like 1/2” to start as it will reduce some as you go. Then the top of the transom is 12 pieces of glass and about 1/2” thick. It cannot be done in a single go.

1708 is a bear to wetout for 12 layers; it will NOT wet down through and the substrate needs to be saturated and basically drippy.

I think it is just bad drawings, but you need radii and fillets for all.

For a 6 layer layup each side; you might reconsider db1700. You are going for 150 oz and can achieve almost that at 8 layers of the easier to work fabric.

If you plan to take table wetted pieces to the part, you’ll need two people and want to carry them over on vac bag used for the wetout. Even that will be hard to do.

I also recommend patterning each piece using 6mil plastic and not the fabric. Longest first, tape the plastic down and then sharpie mark the desired dimension and even consider how you’ll cut the corners with the plastic. I did my transom using the fabric as the patterning and regretted it as the db1700 edges would fray easily pulling masking tape. Main areas were all good, but it was really slow versus using plastic.

 

I drew a finger sketch with 3 layers of laminate. For a 6 layer layup; overlap about 1,5” per layer, 4” minimum. This is 13” for layer one, then 11.5”, etc. After laminating one side, grind any imperfections and putty fill to make the thing flat before laying the other side. You cannot really alternate as that will drive air into the top edge. You will probably be a little unhappy not using vacuum. Anywhere 3 planes meet is always a bit tricky to get to lay down well, so make sure all those edges have good radiuses.

On mine, I did end up making some patches now and then on corners, but I really am glad I overlapped and have 8 on top when I’m hitting some bigger waves at 12kts. And I don’t see any stress signatures on my transom after 300 ocean miles.

 

I also only use a 6”x3/4” bubble buster roller. And when rolling over the top; you can only roll in one direction which is toward the overlapping end. This can pull the bottom inside. If it does, then push the bottom inside only on the bottom overlap to close any air gap made by rolling the top. I’d recommend the same or similar distance inside, like 14” minimum or up to 4” least overlap plus 2” per layer which is 18”. The standard is 2”.. but the last layer needs some glass to lay down. If you have trouble with the end laying down well, wrap it in plastic and pull 5’ of masking tape around it in a couple spots. I seem to recall doing that on the final pass if the 4” overlap piece wants to lift off for you.

 

For my corners, I used an every other approach, so one corner I’d flop one way and the next the other. If I recall, I used a bit of epoxy putty now and then if it got wonky. So have some buckets ready to mix small batches of it. Lay it out thin on a board and a batch of slow might last you a couple layers..etc.

 

Yes, I got to do some fillet and rounding of square edges.

Progress update . . .

I cut the web holes. I did it before any glassing, as I want to wrap the cloth through the web holes.


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I need to get some core stock for the stiffeners . . . and maybe some sort of routing tool to round the corners of the Coosa. Then got to fillet the corners.

 

I have figured out some lateral bracing members that should do the trick . . . 4 pieces that connect to the girder/stringers, the top plate, and the butt end of the planing surface. (cross section illustration)

They sort of go like this . . . Right where the planing surface ends (butt end).
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