Is the crossbar curvature restriction in F18 measured from the ends or hull to hull? Thanks

from the ends of the beams...from the rulebook

The F18 class rule set never fails to amaze me !

What kind of rule is this?

As it having 20 mm prebend in your mainbeam is in any way or form faster then less then 15 mm !

Just one more of those useless rules that only complicates the rule set. Really !

Wouter

The rule makers decided that curved beams were not desirable and thus designed the rules to control it. 15mm bend was the tolerance of "straightness" that they decided on.

I'm sure the international F18 rules committee are sorry that they didn't consult Guru Wouter when they drafted the rules and duely apologise.

Tiger Mike

Oh crap...you're right. The F18 rules are silly and certainly won't work! The people that created it must be stupid. The class will never grow or survive...

(ps...for those who don't know me...that was sarcasm)

Ohh yeah I forgot all those A-cats sailors are sailing with hugely curved beams and we all know that class ain't happening !

That BS works both ways guys !

Those rules makers were just a little more then overly concerned when they thought up that rediculous rule. That or scientifically challenged.

Hey, I find white hulls undesireable ! Shall I put in a proposal to ban that in the F18 class rules.

Apparently everything goes. <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

I mean really, the shape of the hulls and beams is largely unregulated (arguably a significant point in the way of performance) but I can't have more then 15 mm prebend in my mainbeam (totally irrelevant in the way of performance !).

Somebody has got their priorities mixed up here.

Wouter

typical...instead of asking why a rule that you don't understand might be in place, you assume you know everything and choose to redicule it and the class management.

It has to do with cost. Its much cheaper to extrude a straight section than a curved on. wouter, figure it out!

Could you get 15mm bend by cranking the dolphin striker? What boost would that give you or does it put undue pressure on the platform? Does it become a safety concern? Is the rule there to strictly ban TheMightyHobie18 type beams? What positive performance factors do curved beams give you?

I promise that this is not a sarcastic post, but am curious as to why a formula class would limit beam shape. There must be some good reason - cost maybe, but in the formula world you have choices that could reduce cost in other areas.....

I believe the reason this rule is in place is to control the height of the top of the mast above the water and thus heeling moment.

Is it a good rule, I'll pass on that one...

The Tornado class have the same rule. There the beams can be pre-curved by tensioning the dolphin striker strap.
I can see a scenario where someone pushing the edge bend their mainbeam and set it up with a shorther dolphin striker to stop it from hitting waves. Or get a curved extrusion, put on a dolphin striker and gets both dolphin striker/strop and rig higher.

I don't see any problem with having such a rule....and it is not exactly complicated is it?

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.




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.



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.



How would curved beams help you ?




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

Okay, I'll bite ...




Can anyone tell me why limiting the upward prebend of the mainbeam to only 15 mm is included in the F18 rule set.

And don't tell me it is cost as there is no reason why you can't achieve larger prebend values by cranking up the dolphinstriker tension.

also :



This quote assumes that there is some benefit to having curved beams otherwise no builder would even consider making extra cost and investing extra effort to get them. If such a reason is not present then having such a rule is useless as it would only ban something that nobody is considering anyway.

Please entlighten my ignorant behind ! <img src="http://www.catsailor.com/forums/images/graemlins/grin.gif" alt="" />

Wouter

Scooby,

How much difference would 15 mm make ?

Wouter

Rolf,

How much could you win here ?

No too mention that you have to change the beam landings to take the curved beam and thus can not used symmetrical beams anymore.

How much would be gained in speed by such an excersize ?

This is typical of todays a-scientific thinking that seems to be the fashion. I see it everywhere.

Some people BELIEVE that there should be an advantage to having a higher rig or more sailarea (think US I-20 sailors) and therefor they claim that it must be so deriding everybody else who questions it (guess who). Then when they finally done they find out that gut feelings are very basd advisors (I-20 example again, see the recent I-20 = overpowered thread).

Also why would you go for curved beams and all the hassle associated with it when you can just make the hulls higher at the mainbeam point ? The latter option is both simpler and cheaper. I guess we can expect a F18 rule on that soon. And later we find out again that the F18 is already fitted with alot of canvas and that placing the rig higher in effect does not result in a performance gain. But by that time the F18 rule set will have the size of the New York phone book. And that is my point.

There is something to be said for rule sets of limited size and leaving some things open. The F18 class used to be that way (WHEN IT GREW THE FASTEST) but now more and more rules (silly rules often) are added with no clear advantage. Where does this end ?

This 15 mm rule in itself may not be very interesting in itself as with the very large beams of F18's today you don't need more then 15 mm prebend, but the general concensus for adding rules has been set.

Wouter

Wouter,

there is probably very little to gain by having a curved mainbeam. I suppose they are out to stop "somebody" with the right funding from doing all sort of crazy stuff. You know how the psykology works in this, 'pro sailor X is doing this, so to keep up, I must do the same' and so on. It's silly, but I guess this is how the F-18 rulemakers think and what they want to limit.

Thanks Jake! I thought it was end to end. I was hopeing it was hull to hull. Didn't mean to start an argument about this. Just wondering how much I would have to alter the beam on a Hobie 18 to make it legal. Also looking at going with a self tacking jib config and a straighter section would help with that application. I think the saddles on the 18 have more than 15mm of rise from outside to inside. Thanks again for the info.

That is an interesting concept, Rolf.

As the F18 winners of the last few years have all been sailing a basic Tiger platform with maybe one or two upgrades in the sails. (Battened jib = Booth, F18 fathead mainsail, new cut spi).

So if this is psychological thing then their is no danger at all as as you say some guys first has to beat these sailing gods with such a freak design.

I again bring up the A-cat class. We have seen some freak designs here and they newer caused a major upset in the class. Afterall, such weirdness needs first to be proven against very good sailors on "normal" boats and that is no small thing to ask.

My point here is that the class itself is largely selfregulating on these issues without any need to such rules.

Wouter

hey wout

what business is it of yours again to pass judgement on a class that blows yours away in participation?


Rings of jealousy.