New mast rake and prebend

[allene]

My Lapworth-36 was dismasted on May 28 and I just had the new mast delivered. The mast is 45 feet long and P is 35 feet. There is an adjustment plate at the mast end with a range of 1.1 inches. The builder suggested putting the plate so that with it all the way back, the heel of the mast would be directly under where it goes through the cabin top, up 69 inches.

The question is what should I set the rake and prebend to be. One possibility would be to center the adjustment plate and put rake and prebend in the center of what is possible with the heel in that position so I would be able to tune either equal amounts. The mainsail will be made to the new mast. The mast is 7 inches cross section front to back.

If I understand the physics of the mast-prebend-heel position correctly, that might be a spot like 1.5 degrees of rake and 2 inches of prebend.

I read somewhere that prebend should be between 25% and 50% of the mast section. I read somewhere else that it should be 100% of the mast section, which would be 7 inches. On post 1174 Brion said and inch or two on a Catalina with a 42' mast, which is similar to mine.

There are two questions here. One is where I should drill the holes for the adjustment plate. Second is what should we shoot for on the rake and prebend.

[Stumble]

Sadly I don't have the foggiest idea... I have only played within the constraints the builder left for the owners I have never tried to define them. My guess is that if you start out by matching the rake and prebend of your old sails you will at least be in the right neighborhood. But I suspect it just depends to much on the boat to make sweeping generalizations.

Just from personal experience, the J-35 used to sail with reasonable amounts of rake and 2" of prebend, then someone figured out that 16" with almost no rake was faster...

I get the builders suggestion though. That location would be zero rake, which is the absolutly furthest back the mast will ever be, and much further than I would expect. I might suggest putting the back edge between that position and directly under where the mast would sit if it was centered in the partners... but that's the best I can do.

[allene]

Two points. First, the old mast was wood and I cannot measure it as it exploded in the collision. Also, the new mast is aluminum.

Second. Just to be clear, the mast builder suggested that the mast such that with zero rake and zero prebend the adjuster all the way aft. In other words, straight under where it goes through the partners. Then applying rake would give prebend from that position and if that was too much prebend, the adjuster could be moved forward.

[Brion Toss]

Hello,
The good news is that you are in a position to set an optimal position for your mast. The bad news is that determining that position, in terms of both rake and bend, can be tricky.
Let's start with rake. Although it is in practice largely an aesthetic consideration, rake can affect helm; if you had lee helm before, for instance, it might have been at least in part because the mast was raked too far forward. But it might also have been because of faulty hull balance, or a rudder anomaly. Eliminating other balance-affecting suspects is important.
Bend will also affect balance, because it affects mains'l draft. At least as important, it reduces the tendency for a spar to shudder, or to invert, which is at least as important, even if you never adjust mast shape under way. And if you do, bend changes how a mast reacts to tuning inputs like backstay tension, spreader angle, etc.
For any boat, I like to consult with the owner and the sailmaker before tuning, but for your boat I'd be inclined to put in 2 to 3 inches of bend. To get this fair, from the keel up, I'd set the butt about an inch aft of where the rake would put it, as opposed to setting the mast for rake, and then trying to bend it forward at the deck, after the partners had stiffened it and locked it into place.
For more on this, see the new Apprentice, my tuning dvd, and/or Selden's tuning guide. You might also consider calling for a consultation.
Fair leads,
Brion Toss

[allene]

Thanks Brion,

Thanks for your input, it is very helpful.

In talking to the mast builder, I find that my rake will be 1.5 degrees as he supplied the forestay. Two things I did not mention was that the mast has forward and aft lowers so it can't invert. Also, the sail will be made to fit the new mast. Everyone says take my best guess and then go sailing.

Allen