transom and Stringers

jigngrub

jigngrub

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I'd go ahead and redo the motor mounts too just so everything would be new. The last thing you want is to do all this work and then find out in a year or 2 you should have replaced the motor mounts because now they're rotten and you have to pull the engine again.
 
C

CT River Runner

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What would you guys use to cut our thin the epoxy to get it to soak into the ply better? And when you do that do you have to add a little more of the hardener?
 
rickryder

rickryder

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A question frequently posed to our technical staff is "can I thin WEST SYSTEM epoxy so it will flow or penetrate better?" The answer to that question is "yes, but not without consequences." Many of the advantages of thinning epoxy are offset by disadvantages in other areas of epoxy performance. Thinning epoxy means lowering its viscosity. Low viscosity epoxy flows better, is easier to roll or brush, saturates fiberglass fabric quickly, and penetrates more deeply and more easily into porous surfaces like partially rotted wood. There are two methods of temporarily thinning epoxy. One is to heat the mixture and the other is to add solvent to the mix. The goal of both methods is to reduce the epoxy?s viscosity. This article explains what happens to WEST SYSTEM epoxy when it is thinned either by heating the components or adding solvent to the mixture.
Through knowledge gained from our comprehensive test programs and from 30 years of practical experience, we have learned that epoxy formulation is a balancing act. When one characteristic is altered?e.g. changing handling attributes by adding a volatile solvent?other characteristics like moisture resistance and strength are also changed. Our chemists formulate a well balanced, versatile epoxy that provides excellent structural strength and moisture resistance. If you elect to modify it, you become an epoxy formulator and need to understand the effects of your changes. Armed with the information in this article, you can decide if thinning epoxy is worth the tradeoff in performance.
Is thinning necessary?
There is a perception that epoxy needs to penetrate deeply into wood to be effective. Sometimes this is true, but most of the time it is not. Some common misconceptions are that deep penetration of epoxy 1) makes rotted wood as strong as new, 2) increases adhesion, and 3) makes wood more waterproof. The following is a brief discussion of these points.
1) Rotted wood impregnated with epoxy does not make the damaged wood as good as new. Deep penetration of epoxy into rotted wood will make the wood hard but it will not restore its original strength. This is not important if the rotted material is non-load bearing. A rotted door threshold does not need to be strong, just hard. However, when the wood fiber is damaged, wood loses its ability to carry loads and unless the fiber is replaced, it will not regain its full strength. A rotted deck beam or sailboat mast needs more than epoxy consolidation to return the wood to its original load carrying capacity.
2) Adhesion in all but the highest density wood is not enhanced by deep penetration of the glue into the wood. Research performed at the Forest Products Laboratory showed that adhesion to birch was increased slightly by using thinned epoxy. In lower density wood species like Sitka spruce or Douglas fir, the weak link is the cross grain strength of the wood. It does not matter if the epoxy penetrates 1/4" into the wood or 5/1000". The strength of the wood, the amount of surface area and the adhesive ability of the glue determine the strength of a glue joint. Most types of wood glue do not penetrate deeply, yet, if used properly, they can exceed the grain strength. Epoxy is no exception.
3) Water resistance of a piece of wood is not enhanced by deep penetration. Wrapping wood in plastic makes a pretty good waterproof seal without any penetration at all. Likewise, an epoxy coating on the surface is more water-resistant than a thinned epoxy coating that has penetrated deeply into the wood because, in most instances, the epoxy thinned with solvent is porous.
The USDA Forest Products Laboratory developed the Moisture Exclusion Effectiveness (MEE) test. It is a measure of how much moisture is absorbed by wood when it is continuously exposed to 100% humidity. Higher numbers mean the wood has absorbed more moisture while lower numbers indicate less moisture is absorbed. You can see that epoxy with solvent added is not nearly as moisture resistant as un-thinned epoxy (Figure 2). However, if you need an epoxy coated surface that is less of a vapor barrier, thinning WEST SYSTEM epoxy with solvent is a valid way to achieve this

The rest of the article can be found here; http://www.westsystem.com/ss/thinning-west-system-epoxy/
 
jigngrub

jigngrub

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If you're using the low viscosity marine grade resin there's no need to thin it, it will penetrate the wood well. The first coat will just about disappear after application and absorption.

As a suggestion I recommend you seal all of your wood with a couple of coats of epoxy after it is cut and fit and before installation and tabbing/glassing. This will give you experience and practice with your particular mix as far as mixing, pot life, and coverage/batch size. Start with small batches until you get a good feel for application and appropriate batch size. Epoxy will take a while to set and cure when brushed or rolled out thin, but it'll heat up and set pretty quickly in mass in the mixing pot. Once you get a feel for all this the glassing should go pretty easy.
 
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CT River Runner

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I appreciate all the info guys. I was planning on dry fitting my first ply piece, getting it right, then using that one as a pattern for the next piece. Then I am going to wet them both out a few coats. Would you guys put one piece laminated to the transom hull and then screw the second piece to that one after it cures? Or laminate the two pieces together out side the boat then slightly clamp them in place?
 
jigngrub

jigngrub

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Laminate both of your transom pieces together outside the boat, use your epoxy putty. Clamp or screw them together tightly, your adhesive should squeeze out of all 4 sides. After the adhesive has cured unclamp and install in the boat using you epoxy adhesive troweled to the wood and hull, clamp tightly. Again, your adhesive should squeeze out of all 4 sides. You can't overclamp or oversqueeze the transom wood to the hull, the tighter the better. Not clamping tight enough can create voids and air pockets that will reduce the bond.

Study some of the other transom and stringer installations in the different resto threads here before you start to give yourself a good idea of what you need to do.
 
jc55

jc55

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Nov 3, 2006
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We pick up best practices for ourselves along the way. Here's my transom. I strongly recommend Jig's advice, and building it "on the bench". To go further, encapsulating it as mentioned entirely before installation. When both pieces are together, you can use a dual action sander to round your edges slightly and make both layers match perfectly. This also reduces the edge checking that happens so easily with freshly cut plywood.

IMG_2333.jpg
 
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CT River Runner

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You guys have have been unbelievable to me! I read all those that have posted to me. Really good read. That's exactly how I will do it. I do have another question that's a little off subject? My old man has an aluminum boat with rivets? Quite a few are leaking. What do you guys suggest?
 
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CT River Runner

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Thanks wood, I'm about to order 170 biaxial cloth at 50" wide and 15 yards long. No Mat! That's the right cloth for epoxy correct?
 
C

CT River Runner

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When I sandwich the 2 pieces of ply together for the transom using tb-3 wood glue, will that bond to the resin coated ply, or will I have to sand it down before I apply the glue
 
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