Please Clarify Transom Rebuild Tutorial

[Chessie]

Being a design engineer (civil) by trade I am always leary about deviating from what the designer specifies as has been repeated on this forum over and over.

In the transom rebuild tutorial, it very clearly states to tab the cornerstone’s between the new transom, sides and bottom.

This is then followed by full sheets of glass beginning with the smaller and then increasing in size. I have always seen smaller to larger as the tutorial show.

The advice being given on pages 14&15 of the Grady White Overnighter build somewhat contradicts the tutorial. Even though it is being provided by well respected members, it contradicts the published “how to”.

viewtopic.php?t=66204&start=130

Can someone please clarify:

Tab or No tab corners.
Inner skin- larger to smaller or smaller to larger?

Thanks in advance.

Chris

[fallguy1000]

In all but the rarest cases or simple hole repair, glasswork is done largest to smallest. It is counterintuitive because our minds think shingle effect adds strength. In glasswork, thickness and no air entrainment are the key. And you cannot work smallest to biggest without air entrainment unless you are very good at laminating and making epoxy putties.

And when sanding, if your longest pieces are on the bottom; they never get sanded thru.

Tabbing is done one of two ways. Either with the same laminate as the transom or a separate piece. This also depends on skill level and time to layup. An experienced laminator may be able to tab with the transom laminates. An inexperienced laminator may need more time to do it well, Also depends on the size of the transom.

If you are asking these questions; my guess is you are an amateur. It would be best to tab to hull sides and bottom/stringers, then sand the tabbing with 40 grit. Then add the transom glass. In all cases largest first. Always precoat all raw wood before glassing. For a professional touch, prefill tabbed areas to be glassed with epoxy/cabosil putty with a 6" drywall knife to avoid air entrainment.

A pro can tab with the same laminates (if they are the right ones) and reduce the amount of glass and epoxy and time. But it is harder to do and much harder for amateur as the glass likes to pullout of the corners and must be rolled somewhat perfectly (inward) to avoid creating air.

[fallguy1000]

On the subject of tabbing before or after laminating the transom glass..there are a couple different directions that debate can go,,..

A composites engineer would have you tab at the end usually. What matters most is the size of the tabbing and that the area the tabbing lands on is properly prepared. This means good fillets, precoated wood and largest first. If the transom laminates were applied first; the area must be sanded well with 40 grit for key and just before tabbing; you would flat fill with putties.

It is rather difficult to sand fillets, and so for an amateur building, tabbing first ensures you are tabbing onto hopefully a just placed fillet and a good precoated new wood surface and good key on the hull.

Quality of the work is far more important than order of operations. Glass strength is not achieved by intertwining or overlapping tabbing and core laminates. Thickness and work quality are paramount. Work quality also means the laminate amounts and type.

If you laminate the transom first; you can probably keep working green on green without sanding and so the advantage to tabbing last is speed of work.....perhaps could be done other way, but tabbing is usually rougher..

[Chessie]

Yes, with respect to fiberglass, I am a 99% amateur but have done epoxy joints that have held for 15 years of abuse and get the point about sanding fillets versus working wet on wet. The techniques and materials here have held their own!!!

But the whole reason I asked was the posted tutorial here it specifies smallest to largest which is in line with what I have generally seen. It does not say if you are an amateur do it the other way. This to me, being a very black and white person, means the way it is written is the way it is to be done…..until I see an experienced person say something different; hence the request for clarification. It’s just my nature to make sure I do it right, no offense intended.

I do get the point about the air entrapment if the smaller pieces are allowed to dry and edges are not sanded to allow for proper contact of the next layer.

Does working wet on wet smallest to largest help eliminate this assuming proper care and technique.

As you mention quality of work is more important than order, but that is a given with any type of structural fiberglass.


Stay tuned, I’ll be asking more questions on my HyrdaSports thread

[Fuzz]

I have seen some top quality glass men say smallest to largest and they had good reasons for going that way. I have seen others go largest first also with good reasons. The guy I have the most respect for is a largest first guy. So I do not think it matters all that much. We are not trying to get that final one percent here. As Dan says the most important thing here is the quality of work. And even here we are not stressing things to the limits.

[fallguy1000]

Professional composites engineers specify longest first.

For a hole that is tapered; smallest first, or other places that make sense.

It is almost impossible to lay larger pieces of glass on smaller without entraining air. If you have done enough glasswork and willing to self critique; you know this to be true.

It is simple; a piece of glass 0.035" thick laying over another creates a margin 0.035" thick and that margin tapers to zero over some distance which in my experience is about 3/8" or so. This means some air is in the laminate. How significant a weakness? In a 2" wide bonding overlap; this is a weaker area of bonding of about 16%, call it 10% or maybe only 5%, but something greater than zero. Bonding largest to smallest; there is no tapered margin.

If you get enough specified detail from professionals and do enough glasswork; you start to understand the reasons.

The thing about epoxy is it is so strong that it doesn't make a huge difference, but you asked, so I answered; best practice is not the tutorial.

[fallguy1000]

Another best practice, or required practice is no hard edges. This means we do not end glass on top of itself. Why? It is so we avoid areas of significant strength next to areas of weakness.

So, when we specify laminate layups, they never end on top of each other.

So, transom laminates are tricky because the transom needs to be pretty strong all the way, right?

The transom is the one place where you could allow stress risors and laminate to the corners, but then end up with a stress risor in the corner. Laying longest first and reducing each piece by say one inch would also make sense as the corners would have all 3 layers or 4 of tabbing done.

Here is a crude finger sketch of what a pro would give you for a 3 layer layup of tabbing first and laminating second; he would note to putty sanded tabbing.

This is NOT your spec..an example

[fallguy1000]

You will note as a matter of semantics, there is no way to NOT entrain air in the drawing I made unless done in two steps. The air goes in on the first layer of purple transom laminate.

Like David says; these are all minor versus good work.

[Chessie]

Thanks everyone, appreciate all the info and clarifications.I’m a person who questions discrepancies only for clarification and to be 100% sure. Give me the 1% margin and I’ll find a way to be wrong every time.


That sketch just proves everything you need to know in life you learned in kindergarten and looks like alot of mine in work.

[fallguy1000]

The bow extensions I did had extensive professional cartoon drawings. I followed them best I could, but practicality also was a factor.

Good luck, I will help you best I can.

Dan