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Mast Pre-bend


steve_garlick

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Hi all,

I'm wondering if more mast pre-bend would allow me to power up my sail better.

I'm just learning the basics of tuning the 5.8, and reading White's book, it seems that if I increase the tension in my diamonds, and increase the rake on the spreaders, it will allow the mast to bend more, and give me a fuller sail.

I'm currently running 35mm rake on the spreaders and shrouds are 'middle' tight.

what's the perceived wisdom on this. I should add that my sail is old and baggy (shapeless really) and total crew weight is embassing (195 kg) with the weighty part being at the back. I'm figureing I'd like a fuller sail, that my crew can flatten off with the downhaul, and just leave mast rotation alone at the moment.

REgards,

Steve

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Steve yeh you can pull brebend into a 5.8 mast,by cranking down the diamonds, but when the wind gets over 15 knots (even with 195kg on the side ) you'll be over powered,if you need shape in your main try sanding the battens so there thinner and bend easier.however if your main is clapped out you my be up for a new sail!

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Steve, Increasing prebend will flatten your main. Try and get it to stand up a bit. Decrease spreader rake, whilst maintaining diamond tension. The reason for maintaining diamond tension is to keep the mast stiff..... So the head doesnt lay off early. You will also want to rake your mast a little further forward than the lighter teams.

I ran a bit on the heavy side also when I had a 5.8 but can't remember the settings.... Too many years ago. Give Brisbane Cat Center a call and ask them what the basic factory settings were and stand it up from there. I'll ask some of the past 5.8 gurus at our club and see what settings they come up with.

PS. Where are you based.

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if you set your boat up on the bank and with the main sheet and the down haul crank the shit out of them then so as to pull the sail out of the bottom of the mast. you will see that the mast has now got heaps of bend in it and the sail is flat. this goes with what steve is saying (tornado). we are a heavy crew as well 165-170kg and we did find that raking the mast forward gave us a better boat at the nat's this year. look at your batten weights.

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Tornado is correct for a wing mast bending across the major axis of the mast.

A Nacra 5.8 with a loose foot and positive mast rotation tend to sail upwind with the mast heavily rotated.

we bend the mast accross the minor axis to depower the boat as the wind builds. The diamond wires control how much bend the mast will have across the minor axis. loose wires allow the mast to bend more which will flaten he sail out. tight diamond wires will make the mast stiffer and and create a fuller sail.

this is why we back the diamonds off in heavy weather.

Does that make sense to you Steve?

[This message has been edited by HooD (edited 28 March 2006).]

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Let me see if I've got this. Prebend increases the mast's "bendiness" on the major axis, so flattens the sail.

With the spreaders in a less aft raked position, diamonds add stiffness on the minor axis. Going to windward, mast is rotated about 90* so tight diamonds will give a stiff mast, hence a fuller sail. Loosening the diamonds allows the mast to bend more, so flattens the sail and depowers the rig.

Skippered my first race last weekend with Megan as crew, came last but had a great time. Gotta love this boat ! Now I just need to learn how to go faster!

Thanks,

Steve

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