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?
Why design a boat with prebend?
- Thread starter Daveinet
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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 ...".
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 ...".
If you think of a mast held at the base (straight buried fence post) and start to pull it forward with just the headsail force it bends forward. Next raise the main, which the pulls to aft on the mast, but not as direct of a force as the headsail.
What you don't want to happen, is flexing and moving of the mast at the Base.
So you put a little"pre-stress" in the mast to prevent "buffeting" fore and aft.
Most who change from jib to 110% Genoa don't realize they just put more forward stress on the mast, fractional rig or not.
Jim...
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.
Now image the mast without pre-bend. Straight. Without the forward bend the rig load can make the mast bend backwards (inverts), and that will slacken the lower diagonals that go the the spreaders. Thats the BAD part. These shrouds support the middle of the mast, and keep it from snapping under large loads. If the go slack, the middle of the mast loses all support.
Some fractional boats have baby-stays to help prevent this.
Some fractional boats have baby-stays to help prevent this.
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.
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.
More like this.
The mast is a straight spar, but after it is up, tightening the UPPER shrouds adds the prebend.
Then tightening the LOWERS both add support to the middle of the mast, and stops any more bend.
Thats why you ALWAYS do it in that order.
Now you are safe to sail. Even without a backstay, the only way your mast could 'invert' is if the mast could stand up straight again, which impossible with the lowers snug and in place.
Jack has done a great job of explaining prebend. This is a link to a course on Mast tuning and why?
Although the teacher used a "diving board" analogy, read the prebend section.
http://usitalongbeach.org/Sail_Trim___Mast_Bend.html (it is pre-stressing)
It was interesting that a Sailmaker must know the prebend to cut a sail for best performance.
I personally know that because I have in-mast main furling ( different opinions on prebend vs in-mast performance)
Also note the difference between mast Rake vs Prebend.
Jim...
Although the teacher used a "diving board" analogy, read the prebend section.
http://usitalongbeach.org/Sail_Trim___Mast_Bend.html (it is pre-stressing)
It was interesting that a Sailmaker must know the prebend to cut a sail for best performance.
I personally know that because I have in-mast main furling ( different opinions on prebend vs in-mast performance)
Also note the difference between mast Rake vs Prebend.
Jim...
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.
Not really a monohull thing.
My Hobie 16 has a rotating mast. I'll have to look at the mast more closely next time I set it up.
^^ Generally only the masts with swept back spreaders can be prebent. My Prindle 16 was straight, but my Stiletto 27 had about 9 inches of bend (measured in the center). The shame is that the ability to de-power by under rotating the mast would be very useful on smaller cats.
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....
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.
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.
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.
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.
The effect of static pre-bend is CUT INTO THE SAIL. It comes this way from the sailmaker. When your mast is static with its pre-bend, the sail has its FULL SHAPE. Not flat. Bending the mast with the backstay flattens it. Pre-bend is not part of this at all.
Until a fractional mast is bent, it is as loose and as bendy as a noodle. Its this pre-bend that gives it its rigidity. Then the sailmaker cuts extra cloth into the luff to match this bend to give the full static shape.
Mainesail, I've appreciated and followed your advice a bunch of times, but I think Jackdaw has this one right -- and maybe NOT just on fractionally rigged boats, either.
I think the key is this: A mast is "nailed down" at the deck (deck-stepped masts for sure, and more-or-less true for keel-stepped masts, too). And the inner and outer shrouds and spreaders pretty much "nail it down" at the hounds. In theory, that should stabilize the whole mast. And it does, when the boat is stationary on its lines on a calm day. But under real-world sailing conditions, including gusts and waves, the mast will actually oscillate between these points, especially between the aforementioned two points. In moderate winds that's no big deal, but in big or extreme winds, it can be. And ironically, the same adjustment that "nails down" those two points more securely -- increasing the tension on all the shrouds and stays -- can actually make a straight mast LESS stable, because it's now under more extreme compression, which means that any bending oscillation will be exaggerated by the tight wires. I think that when masts fail in storms without a shroud or stay breaking first, they usually deform into a kind of "S" shape along the way to disaster.
Pre-stressing the mast with controlled pre-bend -- putting and keeping it in a "C" shape -- permits higher shroud and stay tensions without risking an "S" deformity, at least fore-and-aft. That's where the analogy of a diving board, or a plank spanning a creek, comes in.
I'm happy to learn more about this by being corrected! ;-)
I think the key is this: A mast is "nailed down" at the deck (deck-stepped masts for sure, and more-or-less true for keel-stepped masts, too). And the inner and outer shrouds and spreaders pretty much "nail it down" at the hounds. In theory, that should stabilize the whole mast. And it does, when the boat is stationary on its lines on a calm day. But under real-world sailing conditions, including gusts and waves, the mast will actually oscillate between these points, especially between the aforementioned two points. In moderate winds that's no big deal, but in big or extreme winds, it can be. And ironically, the same adjustment that "nails down" those two points more securely -- increasing the tension on all the shrouds and stays -- can actually make a straight mast LESS stable, because it's now under more extreme compression, which means that any bending oscillation will be exaggerated by the tight wires. I think that when masts fail in storms without a shroud or stay breaking first, they usually deform into a kind of "S" shape along the way to disaster.
Pre-stressing the mast with controlled pre-bend -- putting and keeping it in a "C" shape -- permits higher shroud and stay tensions without risking an "S" deformity, at least fore-and-aft. That's where the analogy of a diving board, or a plank spanning a creek, comes in.
I'm happy to learn more about this by being corrected! ;-)