: Your scarf joint looks to be about a 2 to 1 ratio. Make them 8 to 1
: and they won't open up on you. Of course they have to be a solid
: fit and properly glued ......
: Bill

Interestingly, most of the "olde time" builders I know of have adjusted their notions of what scarf ratio to use; years ago it was 12:1 or even 16:1, now 8:1 seems to be the norm. The reason of course is the use of epoxy.

I took a look at scarf ratio from a theoretical point of view; in theory it should be sqrt(P/C) where P is the parallel-grain strength and C is the cross-grain strength. Since parallel-grain strength is about 10x cross-grain strength for most woods that would indicate an optimum ratio of about 3.2:1. Since figuring that out I have worked with ratios down to 4:1 and found the joint to equal the wood strength.

Of course, there are some caveats. A joint with a steep angle tends to slide apart when clamped so the clamping arrangement is more complicated, and there is always the concern that the glue could weaken with time or moisture. So based on that the 8:1 ratio makes perfect sense, and certainly wastes far less wood than 12:1. I usually use 8:1 below the water line and for wood that will be bent after scarfing, and 6:1 for unstressed joints above the water line.