How many layers of epoxy to reinforce a bowsprit?

[Greenheart]

Thanks Mr Q, I'll be sure to.

Half the trouble is finding a photographer with any idea how to keep the horizon level. My mate with a waterproof phone, couldn't keep up with the Osprey in his Europe dinghy.

 

[thinwater]

It's a relief to know it is a dinghy. That's a lot of sprit!

First we need to guess the tack load. My crazy guess from my beach cat days is that it is on the order of 100-150 pounds. I know it is more than you can hold. The second question is the angle of pull. It's probably more up than sideways, but this depends on the heading.

My first thought is that without a bobstay, this thing is going to whip around a lot. That is far more than the head load on the windsurfer mast and more concentrated, and they bend a lot in a blow. I doubt it will break easily, but maybe. I would definitely want to use the center proton, where it is thicker. The U-blot should be the portion where the wishbone attaches. After the first test sail in kight winds, I'd be thinking about whether I needed a bobstay.

As for the u-bolt, there are two dangers. Localized crushing, and concentration of the bending moment. Crushing could be solved with a 1/3 round section cut from the mast. It need only go about 30% of the way around and 8 inches long should be plenty. Taper the ends and sides at ~ 12:1 and bond. Should look slick. As for distributing the load, that is harder to guess. I don't know whether these sections generally break in tension or compression (buckling). I think it is tension. In that case, a few layers tapering over a distance of 2 feet seems reasonable. It could be that the center of the mast is already beefed-up, since that is where the wishbone attaches, and nothing is needed. My guess is that crazy bending will clue you to add a bobstay before it is in danger of breaking.

And this opinion is worth what you paid for it . Good luck! Sounds like fun.

 

[Greenheart]

...this opinion is worth what you paid for it . Good luck! Sounds like fun.

Thank you, I feel surer than I did.

All the same, the prospect of beefing up the carbon locally, and getting it right if it's going to do any good, is a wee bit daunting. I'm alarmingly bad at even basic epoxying.

I begin to revert to my earlier idea, about whether (as a very simple means of load-spreading/reinforcement) I could buy about five feet of close-fitting drainpipe. If I can't make it close-fitting enough, I could carve a narrow section out, and wire the thing exceedingly tightly round the carbon tube, making sure that the reinforced section lies midway either side of the U-bolt...or perhaps sticking out more one way, than the other?

Although, I can imagine that breaking the circular form of the pipe might catastrophically reduce its resistance to bending forces. Talked myself out of that already.

I'm wondering, when a spar is made of, eg. 65% carbon fibre, what is the other 35%?

Just took a look at the RS Sailboats website...the RS 400 bowsprit is available in carbon or aluminium.

£50 buys something called an "RS400 bowsprit rub rail; mounted on the deck to guide and support the bowsprit." What isn't available to buy, is the presumably tubular, permanent support structure inside the hull, which keeps the bowsprit pointing straight.

I reckon you're right about the forces being more upward than sideways - lots of photos online showing how much skyward bend there is in bowsprits.

I'll just keep thinking. Lucky it's so damn cold, or I might be out there testing the whole ill-considered thing to destruction. :biggrin-new: