Could you get 15mm bend by cranking the dolphin striker?
Yes, I have that much on my beam and it needs it otherwise I get a negative bend (downward) when the rig is fully loaded up.
Prebend in the beam is a way to reduce the bending stresses in the mainbeam while sailing and as such is very attractive. Picture it like this. When sitting on the beach the beam is slightly bend upwards because the laod on the mast step is only about 300 kg or so (mostly because of pretension on the stays). Now we go sailing and the mast step load shoots up to 1500 kg or more. If the beam was straight on the beach then on the water there will be a significant bend downwards and therefor there will be bending stresses in the beam.
Now picture the same mainbeam with prebend on the beach say 20 mm of it. When you go sailing the beam will first straighten out and eventually still go in a negative bend but less so. Therefor the bending stresses in the beam have been reduced.
On my beam I can easily go passed 15 mm upward prebend, but then again my mainbeam is not the most stiff beam of all catamarans.
What boost would that give you or does it put undue pressure on the platform?
It is not a performance issue but rather a contruction issue. You can use prebend to reduce the magnitude of bending stresses in the beam and have it more act like an exclusively axially loaded member.
Does it become a safety concern?
I can be. When the beam is straight then then you don't really now whether the dolphin striker strap is pretensioned or not. Additionally if you beam is to too well designed to handle larger and changing bending stresses then degredation of the beam is possible. However, if anything this will be a proces that takes a long time and you will be able to spot it by looking at crack growth.
Is the rule there to strictly ban TheMightyHobie18 type beams? What positive performance factors do curved beams give you?
How would curved beams help you ?
There must be some good reason - cost maybe, but in the formula world you have choices that could reduce cost in other areas.....
To be really honest, I think the F18's are currently already quite expensive. The price of the boats don't really seem to be a function of how they are build but rather of how much people are willing to pay for them. I hate to say it but if building light and under a much less restricted ruleset would be cheaper then we would't expect a certain formula class to be several thousand bucks cheaper then the F18's.
Besides, it should be instructional enough to note that no open rules class is using curved beams at this point. Apparently the benefit is far too little to justify any additional costs or effort.
Wouter
