fixing transom to top seam on old witchcraft

[colinJ]

Is denatured alcohol better than acetone? For such a large area I was think soap and water and a scrub brush blow dried with a leaf blower then acetone or denatured alcohol.

G10 is compressed and heat cured glass epoxy sheet that is available in a variety of thickness mostly fairly thin and relatively cheap. Its really strong and dense and certainly epoxy compatible and its really good for stuff like trailer eye backing washers at least in thicker sections.

[XstreamVking]

Soap and water for blush, alco or ace for clean up...

[colinJ]

A couple more questions:

How do I know that I have sanded enough? I am taking down everything that I am expecting something to stick too to light color fiberglass that is taking the dark oxidized relatively thin outer layer off out to 6-12 inches from any surface I am working on for the tabbing and layup.

Should the transom and knees go in together with peanut butter etc before being tabbed over or should the the transom be tabbed in first and then the knee install?

Do I need to add any tabbing or glass to the existing hull or rear skin before the transom goes in?

[XstreamVking]

Just remove any gellcote, fractured or loose glass and go out 6-12'' like you are. The transom is easier to lay up completely first then add the knees imo. As far as glass on the outer skin, about 1/4'' of glass is good to give a tough outer skin to mount the motor or jackplate on. Maybe add some if you are thinner than that. A layer of 1700 is about 1/8''-3/16'' thick.

[colinJ]

Hey Xstream, I met a guy recently that previously did solid fiberglass transoms on high performance boats, he apparently had owned a shop in the LA area at one time (maybe the 90's). It wasn't clear to me if he did the actual work or did design and ran the business but he did say he was a licensed engineer.

I was hoping to get some opinions on this process because if nothing else its a neat idea that would seem easy enough to replicate other than the CNC part.

The old wood transom cut out from inside or outside depending on the design, ground back to glass normal demo work and cleaning.

He would then create a cad drawing of the transom and use a CNC fabric cutter to cut out glass for the transom including tabs built in to the cutouts like the way a box is cutout and folded. He would fill gaps to the existing glass with peanut butter to a fillet and then use a combination of (1708/mat/roving/cloth) chosen based on the hull shape and desired thickness to build up the transom as a single layup onto the existing materials with epoxy or vinylester depending on the how the boat was originally constructed.

He claimed that the end result would be a little lighter or equal in weight but much stronger. There might have been more to it, I mean since the glass was cutout by CNC there might well have been lightening cuts built into the layup that would end up like dished 3d features.

Any opinions on this method?

the guy did say that this method was adapted from manufacturing process unrelated to boat repair and the method turned out not to be profitable

[XstreamVking]

If you made a grid structure on the transom it would be possible to make a solid glass transom, I would think that a 1/2'' thick trans (in the center anyway) with a grid would be plenty strong and have little to no flex. It would transmit a lot of vibration though...

[colinJ]

Purely as a mental exercise I figured out that a solid glass 1.5" thick transom in my boat (1200 square inches of area) would weigh about 87 pounds and use 35 layers of 1708, about 33 total yards. A laminated coosa transom should come in just under 50 pounds.

[colinJ]

I have all the materials in house for the transom/floor/stringers. Also I elected to go with vinylester for the laminating.

Here is my plan:
3 layers of 3/4" coosa, glued together with slightly thickened epoxy. I will sand everything afterwards to make sure there is no epoxy that can contact the vinylester. Then I will put a 1/4 radius all the way around with a router and put down 4 layers of 1708 on each side with vinylester.

Ok so with coosa which is water proof, do you need to put glass all the way around the edge like you do with wood? Does that edge add to the strength?

When the transom goes in I am going to put 2 layers of 1708 across the existing fiberglass skin then 2 layers of CSM to stick the transom to the skin with a nice big fillet of vinylester peanut butter along the seam.

OK so on the transom, no 1708 or CSM between coosa layers right epoxy doesn't need this for adhesion. The fiberglass is better to be laid on either side?

Next same thing for the stringer which will be 3 layers of coosa. I will put down a few layers of 1708 on the existing glass hull followed by a few layers of mat then the stringer can go in with a peanut butter fillet and get laminated on.

[XstreamVking]

You could have a problem burying epoxy under vinylester. Epoxy can go over anything but it does not like to be covered by anything but epoxy. I would use vinyl resin with 1708 and a 1.5oz mat between the layers of coosa. Mat against the coosa. Bi ply in between. Glass over all the coosa at least with mat. It is waterproof but needs to be sealed. Coosa to existing transom plan is good. Stringer plan sounds good too. Have you considered a box stringer on the keel? /``\ Shaped like this to support the floor better. Makes a good system for a single stringer boat. It is what my V-King has and has worked well for a lot of boats.

[colinJ]

Here are some progress picks.

So when you say that the coosa needs to be sealed. Do you mean that the layed up transom should be fully sealed before it goes in like with a layer of 1708 around the edge? I plan to put in a couple layers of mat along the full interior of where all parts will go along with the peanut butter on the edges. Actually 2 layers of mat first, wetted in then peanut butter around the edge then transom clamped in with a layer of mat wetted on the transom wrapping around. Then insert transom and clamp in with long reach clamps and also bolts and washers where the trailer eyes and drain plugs go to hopefully squeeze out the just mixed peanut butter so that I can really press it in with a spoon.

I realize that epoxy and vinylester aren't compatible I just figured that since there would be so little surface area to come in contact with the vinylester that it wouldn't make a difference maybe 1/32" and after all epoxy is a better at gluing.

If I go your route then would it be ok to use 3/4 mat, my local supplier is out of all but 3/4".

Also I like the idea of a box stringer but I think it would be a bit hard to lay up in this case, there is very little room under the floor which actually just goes between the center set of strakes, 32" wide. The unsupported span of the deck is only like 13". The original stringer is literally a 2x4.

What I am giving serious thought to is filling the area under the floor with semi structural floatation foam. Something like an 4 pound/cubic ft foam. The total volume is only about 4 cubic feet. I think I am already beyond overkill the original floor was 1/2" plywood with about 3/16" of glass only on the top.

[XstreamVking]

The coosa can be covered at anytime. As long as it is sealed. The 3/4 mat can be doubled up in however many layers as needed. Adding foam under the floor is going to lead to it getting waterlogged and making problems down the road. Hull blisters and added weight being two big ones. A little overkill is good.

[colinJ]

How high should the knees come up? For the keel stringer based on the material I have I can have those come up about 16" off the deck. For the other knees on the strakes should I just match that?

Also for the knee shape, should this just be a rectangle or more of a triangle with or without a flat top portion. I can conveniently have the knees come out about 6-8 inches from the transom. More than that is going to start to encroach on things. I was thinking to bringing the knees up as rectangles parallel to the transom and then come across the top of them with a flat piece of glassed coosa possibly the full width of the transom glassed into the hull sides horizontally.

[XstreamVking]

Your idea is sound and will work fine. The main ones in the center are the ones that do the most work. The others outboard of those could be smaller, or even left off. . Make them a shape that will work the best for your plans. Even small knees do a lot of work. You don't have to have giant knees to strengthen the trans. The ones in my stream are 20'' hi by 16'' long at the bottom and triangular shaped. Just 2 of them have been fine. Piece going side to side will strengthen the trans a lot, especially tied into the sides. You will have a bullet proof trans. All you want to do is stop any flexing........

[colinJ]

Hey Xstreem, The ocean engineer I occasionally work with is recommending your process for bonding the stringer parts with vinylester but epoxy for the transom sections. The thinking is that the glass will do almost nothing for strength in the middle of the core for the transom and that the bond line should be so thin that it won't impact the vinylester. He did say make sure it completely cures and sand off any spillage. Basically he is saying that I should add the additional glass to the outside instead.

For the stringer he recommended the vinyester and glass with one layer of 1708 on each half of every sandwich with mat between because of the joints (its 3 layers of coosa with overlapping joints). He is basically saying that in the transom all the strength comes from the skins and that the core material doesn't really have to do anything but in the stringers (again the original keel stringer was obviously made from a 2x4) that the core material actually does allot of work.

Despite his degree I don't think he has much real world experience when it comes to building things with composites (he does allot with metal).

Does this make sense? If I use the epoxy as described do you think I will run into any issues?

Thanks,

[XstreamVking]

It should be fine if you grind the edge back so that there is a very small amount of contact. All the glass on the transom does is spread the load. Engineers understand load and stress, so he knows what he's talking about.