Boom end repair - Scarf Recommendation

[tgillis103]

My son has acquired and will be restoring Comet #3561 (1960 by Lippincott). The clew end of the boom is rotten, about 1+"; cutting off 3" will reveal "healthy" wood.

The boom is of sitka spruce(?), 11'4" long X 3" tall (i.e. vertical) X 1 3/4" wide. The top face of the boom has a key-hole shaped groove for the foot of the sail to slide into (total depth of the groove is 13/16"). Then 8" from the tip of the boom, the top face is cut away, leaving a 1 5/8" tall by 1 3/4" wide by 8" tip. On this face the mini out-haul track is mounted. The forces on the extreme tip of the boom should be minimal; only a 1 1/8" cheek-block is mounted there to control the out-haul.

So the question is, at what scarf ratio should he splice-on the new tip? The boat came with a spare, but broken and round (2 5/8") max dia.) from which he will fabricate the new tip. The typical scarf ratios for planking (i.e. 12:1 or 10:1) seem extreme considering the "squareness" of the mating pieces (a vertical butt joint would have 2.84 sq-in. of mating surface) and the light loads.

Thanks for any suggestions and recommendations.

[Tom Lathrop]

My memory of the Comet is that the mainsheet attachment to the boom is well forward of any repair that you need and the clew is also forward of the rotted area. That said, there should be little stress other than longitudinal on the boom after a repair. I'd think any reasonable length of scarf would be ok. If the clew band on the boom is in the area of the scarf, a through bolt or two through the scarf to attach the track should be adequate for the job, even if the scarf is shorter than you would normally use.

[tgillis103]

Here is a photo of the boom end:

[John Meachen]

Traditionally mast scarfs us a 12:1 taper so that might be a good starting point.Not hard to do with a wood as soft as spruce.Creating the perfect match for the bolt rope groove might be the biggest challenge and would probably be most easily accomplished by making the new section in two halves and doing the machining at the earliest possible stage.It would take quite a bit of care to eliminate the possibility of a kink occurring but the project ought to be within the scope of a careful woodworker.

[Peerie Maa]

You should cut 18 inches away from the rot, as rot spores are usually active over that distance from the punky wood, then plan your scarf.

[Ian McColgin]

The leech pulls up at the clew, so there is considerable strain there. Just look at how in a puff the boom bends both flattening and powering up the sail's belly and letting the head power down as it twists off. So I'd scarf with some serious purpose.

To accomodate the major bend - more up and down than side to side - I'd have the scarff surfaces more horizontal than verticle. To save wood, I'd go with a cloths pin scarf ==>>===. 8:1 might be adequate but I think 12:1 is just as easy and so much better.

Once you have the scarf mating nicely, circumcise the male side so that the little flat at the end and the V of the female part leave a tiney pilot hole. Once the unit is dry, drill out and bank in a nice stopwater. This relieves some of the joint stress.

G'luck

[David G]

There's also another reason not to shorten up the scarf. The shorter the scarf, the more it will be like trying to glue end grain to end grain. Not a great formula for adhesion, no matter what the surface area, not even with epoxy. The longer the scarf, the more it'll be like gluing side grain to side grain. Much better adhesion.


"It is much easier to be critical than to be correct" -- Benjamin Disraeli

[holzbt]

http://i90.photobucket.com/albums/k253/holzboat/Clothes%20pin%20scarf/IMG_0449-1.jpg

clothespin scarf with 12/1 ratio

[Ian McColgin]

Finastkind, Holzbt.

[tgillis103]

The clothes pin scarf sounds like a good solution, will recommend starting about 7 1/2" in from the tip, i.e. just beyond the the notch and beyond (outboard) of the bolt rope slot . This will help minimize any chances of rot spores still remianing without significantly increasing the difficulty of the repiar b/c he won't have to fabricate and match the slot. Thanks for the suggestions.