Here is an update photo of my new boom crutch.
Thanks for the ideas.
The kids are standing on the now treated bamboo. There was still a final coat of paint left, and ropes are not the final lines.
The two big spars hold the sail. A strap sewn every 18" onto the sail spreads the sail load along the entire spar. Within the sail spars, I installed aluminum tubes filled with dried bamboo and microbead. Then more bamboo is wedged around the aluminum pipes. They still flex, however final running rigging lashed every 36" is not shown yet.
All spars are epoxy sealed at the top and epoxy painted. I left a weep hole at the bottom and a boric acid greating for any visitors. I don't expect them to last forever. However, I can make another 50 sets before I even come close to the cost of carbon fiber.
The boom crutch is vertical in the background and has a triangular spreader bar giving it added strength. This piece of bamboo was originally bent near the spreader bar. The high horizontal force of the spreader made the pole straight. Had I started with a straight section of bamboo, the spreader would have introduced a bend.
Within the crutch there is another spar. This internal spar was made for me using fiberglass, epoxy, construction grade honeycomb, and Dyneema.
End fittings are different for each spar. The largest spar is using a new Harken spinnacker pole attachment insert. It simple locks onto the pin for the original boom.
The smaller pole incorporates a 3/4" stainless turnbuckle. The exposed eye bolt is free to turn, drain moisture, and locks onto a 1/2" U bolt through the larger spar.
Now here is where it was difficult. To install the U bolt I had to drill two parallel holes through epoxy plugs. Further, the U bolt is normally in compression not tension. To prevent the U bolt from pushing out the bottom, I made an adapter sleeve from stainless heavy wall pipe to transfer compression load to the top of the lower spar. Load is first spread to a large washer, then to a block of bamboo epoxied onto the top of the lower spar. To spread the loads more, the lower two meters of the large spar contains a section of that construction grade spar in the boom crutch.
Normally a boom crutch is fixed vertical. However, owing to the sizes, loads, and weights, designing the attachment of the boom crutch too some thought. Instead of permitting the boom to rise up and over the crutch, I decided to afix the two. This eliminates the need to untie and retie the boom. The rear radar tower and wind generator tower forced the boom cripple to be so tall, that it isn't practicle.
Therefore, the boom crutch is designed to pivot and handle both compression loads and tension loads.
The 3/4" boom crutch has a stamped graded shackle embedded with pin exposed. I used the other eye bolt from that turnbuckle and imbedded it into the mast step. The shackle pin then securely holds the crutch in place.
The top of the boom crutch has a 3/4" stainless swivel that also handles both compression and tension loads.
The kids in the photo are standing on the spar in the longterm storage position. The majority of the weight is still carried by the boom crutch using a block and tackle.
The only part I needed from the hardware store was 8" of stainless pipe.
Note: I still have on the stainless pipe clamps. These I tightened every day as the bamboo shrank on deck. I keep them in place for added strength.
Philip
Hot Buoys SV
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