How to SCARF join a stringer?
 

How to SCARF join a stringer? The end a 3/4" stringer is rotten (from the bulkhead). Most everthing else is good so I want to repair the end.

If I understand scarfing 8 to 1. If the string is 10 inches high am I suppose the make the angle 64 inches long? Seem too extreme....

I'm guessing that would be a ratio? For every 1 inch of thickness, you need to taper back 8 inches? I would guess that means a 3/4 inch wide stringer would need to be tapered back 6 inches.

+1

So 80 inches... that is out of control.

Width, so 10:1. 10 x .750 = 7.5"

It is not the 3/4" thickness of the stringer that determines the scarf joint, it is the width of the board which is 10" in this case. The best solution would be to just cut the rotted stringer out completely between the two bulkheads at each end of the stringer. Glass in a new stringer between the bulkheads with 1708 cloth and vinylester or use 1700 and epoxy. You will need to rough up the existing surfaces with 36 grit and clean everything to insure a good bond. The bottom edge of the new stringer should be beveled to match the bottom hull angle, and the top edge of the stringer should be rounded to allow the cloth to drape smoothly over it.

I should add that the 10" dimension governs the scarf joint as that is where the load is on the stringer, from the bottom of the hull trying to bend. There is really no side load on the stringers 3/4" dimension.

There are plenty of threads on tabbing the stringers in place.

If i understand your question you are referring to a scarf joint for the wood? Not sure what the ratio for the wood is but for the fiberglass something in the 8 or 10 to 1 is fine for VE. In other words in the fiberglass lamination on the stringer is 1/4" thick you will taper the area about four or five inches. If possible it would be better to replace the whole stringer though, hour for hour you'll have as much time dealing with with the joint as you would just cutting the whole thing out and install a new stringer. May not seem like it but trust me thats how it works out plus you have peace of mind.

if i have to scarf wood i make the cut diagonally at about forty five degrees from top to bottom and over laminate the joint.