Scarf glue joints too thick?

I glued up the mast yesterday, the first of many scarf joints. the glue joint on the top looked good, but when I unclamped everything today I discovered that in the process of adjusting pieces and moving clamps around there must not have been even pressure all the way across the joint, and the glue joint on the other side is a heavy sixteenth of an inch in places.

Will this affect the strength of the mast? Should I consider taking the joint apart and trying again? I don't have enough experience with epoxy to know how thick of a glue line it can tolerate before you start to lose strength. (for reference, this was using MAS epoxy thickened with cellofil to mustard consistency, or a bit thicker)


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RE: Scarf glue joints too thick?

Epoxy can be quite thick and still give full strength, particularly thickened epoxy. If you are concerned about the look you can re-do it but it's a bit of a job getting it apart if it's fully cured.

RE: Scarf glue joints too thick?

Since the gap is so large, are you sure there is enough epoxy in the joint? Air doesn't bond well. Does the joint being mis-aligned cause the mast to be bent at the scarf?

If you want to sleep better, I'd take it apart and clean the joint of excess epoxy and rough the surface and re-glue it. Many joints on these boats just don't matter much, but mast joints really should be done carefully. The whole strength is the scarf, there's no other parts to share the load.

I'd use a gallon sized ziploc bag of boiling water (microwave the water in the bag), wrapped around the scarf, wrapped in a towel. Wait a few minutes, and then start teasing the joint apart with a thin putty knife. Use a hair dryer for more heat if needed, but not a heat gun on bare softwood.

RE: Scarf glue joints too thick?

If you're going to take it apart, just get your thinnest saw blade and cut it. A good Japanese razor saw will leave a 1/16" kerf. Even a big fat table saw kerf is well under 1/4". That much length will never be missed from the mast

You can either make a single cut and clean up both faces with a sander, or 2 cuts and completely remove the old gunk with the saw.

Also, nemo's right about there being no other parts to share the load, but it's pretty easy to add one. For under $15.00 you can get a foot each of unidirectional carbon fiber sleeve and biaxial fiberglass sleeve. You could leave the joint alone, slip the CF over the joint to reinforce it and slip the glass over the CF to protect it. Once the epoxy cures, a little careful sanding and fairing will leave an unobtrusive thickening at the joint. If you use heat shrink tubing to consolidate the layup the thickening will be thinner and there will be less sanding and fairing needed. This all assumes a painted mast, of course.

Good luck however you go.



RE: Scarf glue joints too thick?

There's definitely enough epoxy in the joint. If anything I overloaded it because I was concerned about starving the joint.  Theres no deflection in the mast due to the scarf, there's not even any twist that I can detect.

As long as the extra epoxy in the joint doesn't affect the strength, I'm inclined to take my chances leaving it the way it is.  The worst that can happen is I'll be making a new mast down the road. And if I do, I'll go the way of carbon fiber and epoxy sheathing.

A related question - is it preferable to have the grain lines of the mast oriented parallel to the centerline of the boat or perpendicular to it? The mast is laminated from several planks, so the grain isn't acually runs in several directions.

My thought was that it would better to have the grain oriented parallel to the centerline since the mast would be under the greatest stress when running before the wind.

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