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The Green Line Transom Joint

Posted: Sat Feb 20, 2021 1:21 pm
by jimh
In 13-foot and 15-foot and 16-foot classic era Boston Whaler boats, the top of the transom line across the stern is a joint line between the hull molded component and the deck or liner molded component; the two separate laminated parts join there. For that reason, I suspect that Boston Whaler prefers to keep the joint line free from radical changes in direction, so the transitions into and out of the transom notch were made with rounded edges.

At the top of transom, the line where the hull mold and liner mold meet is called the "green line." That name is used because at that joint there is no gel coat. You can see the green resin and laminates used to lay up the hull. In an archived thread Tom W Clark comments:

Tom W Clark wrote:All Unibond Whaler hulls have their two skins joined with what Whaler calls "The Weld" where the uncured 'glass and resin from the lay-up of the inner and outer hulls are sandwiched together.

This "weld" is basically a lap joint that cures into a monolithic joint between the two skins.

I cut up an early 13-foot Whaler for fun some time ago. The "weld" at the transom was a bit more involved as the plywood has to be accommodated as well, but the 'glass cloth still inter-folds in a thick build up of fiberglass.

Historically the joint at the transom was then detailed by hand (after the hull was removed from the molds) to remove just enough gel coat to expose the resin beneath. Gel coat would actually be added as touch up if needed to get a nice even line across the transom where no rub rail would cover it.

This joint was called the "green line" and it was a point of pride with Whaler to leave it for the world to see.

The "green line" only appears on the small whalers up to the 16' 7" hulls and is only seen on those hulls produced until the early 1990s. Since then Whaler has adopted the labor saving (read: cost cutting) detail of simply running a rub rail across the transom joint or covering the joint with an aluminum extrusion.

In another archived thread, additional information about the green line joint is given:
doobee wrote:When the deck and hull molds are separated there is flat horizontal flange, or moustache, all around the boat. The flange is cut off then ground down smooth. Whaler refers to the green line along the transom as the "transom weld". The reason they leave it exposed is to show off the unibond construction and the fact that there are no mechanical fasteners holding the hull and deck together, as there would be on a single skin boat. If the line does not come out neat, they will touch it up with gelcoat to make it look straight. Sometimes the weld will come out more white than green and they touch it up to make it more green.

An example of a beautiful green line joint is shown in Figure 1, below:

Fig. 1. A green line joint at the transom of my 1976 SPORT 15. Also note how all transition in the joint line are made with gentle curves; there are no 90-degree bends in the green line joint. Photo by the author.