Ok, so I've seen enough discussions here on sail trim, but not much about the purpose of prebend in the mast. Just if your sail is designed for it, you put some in. But why design a sail with mast prebend in the first place? What is gained?
Prebend is necessary in a boat with a fractional rig because it places the mast under tension. This strengthens the column, and prevents it from inverting backwards which is a terrible thing to happen.
If you think about a fractional boat with aft swept spreaders (and 99% now do), you can imagine that the shrouds and the backstay want to pull the rig back. That would be BAD, but more on that later. So why does it not bend back??? Mast prebend. The pre-bend in the rig bends the mast forward, and the tension placed on the mast makes it impossible to straighten out, or be pulled backward. see the crappy drawing. Think about it, if pre-bent like this and in column, there is NO WAY is can get out of this 'pre-bent' configuration.Hey @Jackdaw
Any chance you have time for one of your famous white board diagrams? I've never had a boat with mast bend (at least I don't think I have) and I didn't start this thread but I am curious as to what mast bend does.... and I have a fractional rig now but I don't think I have mast bend and I can't picture what you mean by "inverting backwards ...".
Thanks @Jackdaw
So I'm thinking of my Hunter 26. It has swept back shrouds but does not have a back stay. ....And if I'm understanding you correctly.... a pre-bent mast is designed to bend forward a little bit but the swept back shrouds pull it straight when you raise the mast. And the swept back shrouds also double as the "back stay" support.
My Hobie 16 has a rotating mast. I'll have to look at the mast more closely next time I set it up.Prebend is also common on rotating rigs. In addition to the support function, when a prebend mast is rotated, cloth is pushed into the center of the sail, making it more full. In addition, if the mast is a airfoil section (they always are), the mast itself adds to the draft. Thus, the combination of prebend and rotation becomes a powerful adjustment mechanism.
Not really a monohull thing.
Please re-read my two posts on why fractional boats have pre-bend. Anything you think you know about this from your 8.7 is wrong.Pre-bend is not about mast support. If your shrouds are properly tuned your mast is damned rigid. If you are having flex you have a poor design, improper gear or tuning error. Pre-bend is about sail trim ie flattening the main....
Did you cut and past that out of a tuning guide for beginners?? I'm sure I've seen that before. And you're miss-reading it.Actually....no
Adjust headstay to get amount of take wanted...another topic....set tension of backstay to desired level. Use cap shrouds to center the masthead in the boat and THEN tension the lowers. If fore and aft lowers,, tension forward first. This has two functions...first it takes sideways bend out of the mast...you should look up the sail track and it be dead straight..not straight will jump out at you. Secondly, this is when you add any prebend desired, to restate this is about flattening the main NOT tuning the rig. If you have aft lowers, then set them to desired tension. The aft lowers pull against the forward lowers and stabilize the mast further. THEN under sail 5-10 knots check rig tension.mast straightness etc and complete time...ideally repeat every couple of months...wire stretches and boat hulls shift about.