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Re: F18 crossbar regulations [Re: Wouter] #87028
10/23/06 09:05 AM
10/23/06 09:05 AM
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Posts: 12,310
South Carolina
Jake Offline
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Who are you 'quoting' here?

Quote


Quote

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.


Quote

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.

Quote

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.

Quote

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 ?


Quote

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


Jake Kohl
-- Have You Seen This? --
Re: F18 crossbar regulations [Re: WindyHillF20] #87029
10/23/06 09:10 AM
10/23/06 09:10 AM
Joined: Jun 2001
Posts: 12,310
South Carolina
Jake Offline
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Quote
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.


I don't understand why you are heading in this direction...I mean...if you are that concerned about fitting the F18 rule why not just buy an F18 boat?

Secondly, if the curved beam is all you have left that is not F18 compliant, as it stands within our regional racing in EMSA, I'm sure we'll collectively agree to score you evenly within the F18 fleet (one class) even with the stock Hobie 18 beams. You will need to replace that big Hobie 18 jib with an F18 jib though (as you mention you are intending to do).


Jake Kohl
Re: F18 crossbar regulations [Re: Wouter] #87030
10/23/06 09:53 AM
10/23/06 09:53 AM
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West coast of Norway
Rolf_Nilsen Offline
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Wouter,

we are about to cross over into a whole different subject, worthy of its own thread. "Fads and rages in catsailing". There have been plenty of such things that have come and gone. In the A-class canted hulls and canted daggerboards was all the rage for some time. "Everybody" wanted them. After a season or tow, the trend dies out becouse it usually is not as effective as claimed. I could mention other examples from the Tornado class, but rules like the "15mm" rule is stop teams from experimenting with them. Top teams do play pranks on each other and the competition by doing things like this.

Re: F18 crossbar regulations [Re: MauganN20] #87031
10/23/06 10:10 AM
10/23/06 10:10 AM
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North-West Europe
Wouter Offline
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Maugan,

You like 7 ?

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: F18 crossbar regulations [Re: Wouter] #87032
10/23/06 10:26 AM
10/23/06 10:26 AM
Joined: Feb 2004
Posts: 3,528
Looking for a Job, I got credi...
scooby_simon Offline
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Quote

Scooby,

How much difference would 15 mm make ?

Wouter


Very little.

If someone had "curved" beams that raise the mast by 1000mm than that would make a difference !

I'm guessing that they wrote the rule to stop people going out and making beams that were just there to raise the height of the mast.


F16 - GBR 553 - SOLD

I also talk sport here
Re: Okay, I'll bite .... [Re: Wouter] #87033
10/23/06 11:20 AM
10/23/06 11:20 AM
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Posts: 3,906
Clermont, FL, USA
David Ingram Offline
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Wouter, for the love of gawd, get laid!


David Ingram
F18 USA 242
http://www.solarwind.solar

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Re: F18 crossbar regulations [Re: Jake] #87034
10/23/06 12:19 PM
10/23/06 12:19 PM
Joined: Apr 2003
Posts: 440
Graham, NC
WindyHillF20 Offline OP
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Just curious? Not all that concerned about fitting the rules. I don't have the money to keep an F18 for racing and my 18 as a recreational playtoy, one boat must do it all. The talk of F18's racing the Tybee is what started my thinking. Could I show up with my 18mod and be allowed to race in the Tybee? I would assume no. Thanks for the comment on EMSA allowing the boat to run as a F18, I didn't think anyone would care. I'm soo not a threat to anyone but myself.

Re: F18 crossbar regulations [Re: WindyHillF20] #87035
10/23/06 12:45 PM
10/23/06 12:45 PM
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Sydney
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Wouter,

Do U think pre bend makes a boat stiffer and that therefore adds to performance?

it effects tow in-out of hulls, it effects canter on the hulls, it effects the twist in the hull, it effects mast hight..gee it effects quite a bit on a catamaran.

Do U think knocking anothers class that you are not involved in helps the sport?

Do U think the F16 rules are perfect?

I dont think you need to knock the F18 to benefit the F16, to me they are targeted at 2 different audiences..

The F18 rules obviously work..there are such a variety of different shapes/designs but basically they all get to the top mark at the same time. Maybe the 15mm prebend might slightly have something to do with this. There is a lot of professionals/experts in the F18 class that are not concerned that rule is in there becuase just maybe they think it is relevant.. or do U know better.

Re: F18 crossbar regulations [Re: WallyG] #87036
10/23/06 01:05 PM
10/23/06 01:05 PM
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St Petersburg FL
Robi Offline
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Guys please dont take Wouter's words as words from the F16 class. He is longer a F16 representative. So please for the love to the sport, do not take his words and judge the rest of us F16ers.

just my .02 worth

Re: F18 crossbar regulations [Re: WallyG] #87037
10/23/06 01:10 PM
10/23/06 01:10 PM
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Pre-stress doesn't increase stiffness. It will actually decrease the strength of something made out of steel or aluminum (i.e. materials with good tensile strength).

The only point of pre-bend on a cat is to get the geometry right - a little pre-bend during set-up to compensate for deflections when the rig is loaded up on the water.

Re: F18 crossbar regulations [Re: Rhino1302] #87038
10/23/06 01:25 PM
10/23/06 01:25 PM
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Pre stress does not increase stiffness but pre bend does. Pre bend from the dolphin striker stops the main beam bending and locks it into place.

When I say stiffness I mean lifting one hull and the other hull lifts as well..

Re: F18 crossbar regulations [Re: Rhino1302] #87039
10/23/06 01:37 PM
10/23/06 01:37 PM
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Essex, UK
Jalani Offline
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On behalf of the F16 class, I'd just like to echo what Robi has said. We in the F16 class are all very grateful to Wouter for all that he has done for the class. It can legitimately be argued that the class might not exist now without the work that he put in. However, Wouter stood down as an officer of the class earlier this year, his views are not necessarily the views of the F16 class and should not be seen as such.

The F16 class does not have any view on the rule set of any other class, the methods of any particular manufacturer, or the dominance of any one particular design or class over any other. It is not helpful to denigrate another class or design in any way as we need all the multihull sailors we can get and need to remain united as sailors with a common interest to further the cause of multihull sailing worldwide.

So, whilst respecting Wouter's right to a view, to dissent, to criticism, please do not think that he is representing the F16 class as a whole. Nor should anyone be under the impression that we are 'casting Wouter adrift' he was, and is, a valuable asset to our class and multihull sailing in general, with his enthusiasm and analytical abilities.


John Alani
___________
Stealth F16s GBR527 and GBR538
Re: F18 crossbar regulations [Re: Jalani] #87040
10/23/06 02:21 PM
10/23/06 02:21 PM
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North-West Europe
Wouter Offline
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Just to clearify. I'm my own (obnoxious) little person. What I write on these forums are my personal opinions only. In no way or form does it ever reflect what the F16 class thinks or does. As stated by others the F16 class and myself are totally different entities right now.

As you guys are all well aware I do like to throw the stick into the hen house once in a while. Well, maybe a little more often then once in a while, but that is just me and only me.

Now, people on this forum simply assume to much.

I can whole heartily disagree with an individual F18 rule and still be fully supportive of the F18 class and all its sailors. Disagreeing with a simple rule is never the same as total disregard for a class and its sailors. Still claiming such a thing is simply infantile behaviour. Want to shut me up ? Then stick it to me by showing why this rule is important. If you score a point then you can expect an appology from me within hours. Saying that I should just shut-up because F18 class has some magically brilliant people creating the rule set is just certified non-sense. I've been around long enough to understand how such rule sets get created and believe me, it is a whole lot less magical (or even scientifically justified) then what many may assume.

So why do I present my case in such an explosive fashion ? Because if I had done it in the most gentle and respectful manner then it would just have gotten ignored. That is the way internet fora work guys.

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: F18 crossbar regulations [Re: WallyG] #87041
10/23/06 02:53 PM
10/23/06 02:53 PM
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Wouter Offline
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Quote

Wouter,
Do U think pre bend makes a boat stiffer and that therefore adds to performance?


No, prebend is nothing more then a trick to fine-tune the stress situation in the mainbeam. Platform stiffness is not affected by it in any measureable way, neither positively nor negatively.

Pretty much the prebending of the mainbeam allows a designer to transmit all the mast step load to the dolphinstriker assemble and have the mainbeam act only as compression strut under the mast step loads. This frees the mainbeam up to take care of the platform stiffness. In effect, by doing this one can more economically load the mainbeam up and possibly make it lighter. It does not however improve stiffness in any way or form. It is a way to make the mainbeam break or damage threshold sit at a higher level or reduce weight on the setup. Neither of which is of much concern on a F18 platform with their huge (heavy) beams.


Quote

it effects tow in-out of hulls, it effects canter on the hulls, it effects the twist in the hull, it effects mast hight..gee it effects quite a bit on a catamaran.


All of these things are not affected. Meaning in the sense of being only affected in a totally negligible way.

Do a home experiment. Tension a 8 feet long line between two chairs at home. Now pull the middle of the line 15 mm upward (3/5th of an inche). Do you see the chairs moving ?

No, because the chairs will have moved by 0.0001 mtr. or 1/300th inche. And this is what I mean by being unscientic. Yes, it is true to some effect on two in is taken place, but todays populace is apparently challenged in recognizing the shear negligiability of such an effect. Note here how increasing your (partly inward facing) sidestay tension will have a much bigger effect on the toe-in of your hulls and even that will still be negligiable.

Isn't it wise to ONLY rule on things that actually do matter ? That is my point in this discussion.


Quote

Do U think knocking anothers class that you are not involved in helps the sport?


Why do you think that I'm knocking "anothers class" ?

Can I not disagree with some detail and NOT disagree with the overall setup or even disrespect a related group of people.

If I say that 1+1 = 2 am I then disrespecting and insulting all the people who dropped out of their high schools ?

What a weird way of looking at things.



Quote

Do U think the F16 rules are perfect?


Who introduced the F16 class ? I certainly didn't.

Why are you guys not confronting me on the actually point of discussion ?

Why is this 15 mm prebend rule in the F18 class rules ?

The rest appears to be only tactics to change the subject.


Quote

The F18 rules obviously work..there are such a variety of different shapes/designs but basically they all get to the top mark at the same time. Maybe the 15mm prebend might slightly have something to do with this.


Exactly the same argument can be had with respect to say A-cats where they do not have such a rule.

The fact that a class may or may not be succesful may well not be related to an individual rule like this. It is silly to argue that the succes of the F18 class somehow was related to having this 15 mm prebend rule. That is unless you also want to claim the opposite at the same time that NOT having this prebend rule was the cause of the A-cat class succes.

There is a logical conflict here. And that I call unscientific. It borders on anecdotal.


Quote

There is a lot of professionals/experts in the F18 class that are not concerned that rule is in there becuase just maybe they think it is relevant.. or do U know better.



This assumes that some unspecified person in another class is always to be regarded as more wise then a mere mortal like myself. But is it never shown why. I case some are forgetting; my background is such that it doesn't "underdo" for 99 % of the all the catsailors out there. I don't likje writing it down, but I actually do have a formal education in this field, I've done extensive math work on these boats and done simulations. The results of these have ended up in actually commercially build beach catamarans. Additionally I have set up a catamaran class from scratch, including analysing and creating a set of class rules. Maybe the question you pose should be asked in reverse, why do you think that I wouldn't know better ?

But all this is just another "changing subject" thing. Lets get back to the point of this discussion.

Why is it beneficial for the F18 class to have the 15 mm prebend rule ?

Wouter


Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: F18 crossbar regulations [Re: Wouter] #87042
10/23/06 03:35 PM
10/23/06 03:35 PM
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Pre-bend dosen't increase the load capacity of the beam/dolphin striker assembly. On the contrary, it decreases it somewhat.

To achieve the pre-bend you have to put the dolphin striker under tension. This puts the main beam under some compression and some bending. The rig load also puts the dolphin striker under tension, and the main beam under some compression and some bending (the applied bending moment is opposite to the pre-bend induced moment, though).

The net result is that the pre-bent structure has higher tensile stress in the dolphin striker and higher compressive stress in the main beam, although the tensile stress will be reduced or eliminated in the main beam. Assuming you were starting out with an efficient design, both the dolphin striker and the main beam will have to be made stouter to take the pre-bend.

Pre-stressing is a neat trick for concrete structures, because concrete has very poor (generally assumed to be zero) tensile strength. There's no point in pre-stressing a steel or aluminum structure except for deflection control.

Re: F18 crossbar regulations [Re: Wouter] #87043
10/23/06 04:56 PM
10/23/06 04:56 PM
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Clermont, FL, USA
David Ingram Offline
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Wouter just needs a little love guys.


David Ingram
F18 USA 242
http://www.solarwind.solar

"Do or do not. There is no try." - Yoda
"Excuses are the tools of the weak and incompetent" - Two sista's I overheard in the hall
"You don't have to be a brain surgeon to be a complete idiot, but it helps"
Re: F18 crossbar regulations [Re: David Ingram] #87044
10/23/06 06:40 PM
10/23/06 06:40 PM
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Yeah, he needs a kiss

from a locamotive.

Re: F18 crossbar regulations [Re: MauganN20] #87045
10/24/06 05:57 AM
10/24/06 05:57 AM
Joined: Feb 2005
Posts: 1,382
Essex, UK
Jalani Offline
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Quote
Yeah, he needs a kiss

from a locamotive.


Why make it personal? How is that constructive discussion? By all means agree/disagree/counter argue or be downright cussed, but make it personal? - I think it says more about you than Wouter....

Oh, and it's L-O-C-O-M-O-T-I-V-E


John Alani
___________
Stealth F16s GBR527 and GBR538
Re: F18 crossbar regulations [Re: Jalani] #87046
10/24/06 06:32 AM
10/24/06 06:32 AM
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Wouter Offline
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Rhino,

Quote

(the applied bending moment is opposite to the pre-bend induced moment, though).


Exactly ! So this is a method to arrive at a situation where the bending stresses are much reduced (against increased axial stresses) while sailing.

It is a commonly agreed fact that beams can handle axial loads far better then bending loads. Because of the individual mechanisms that translate either load in (max) stresses it is also easily proven that the resulting axial stresses are LOWER then the resulting bending stresses. For this reason the beams can be scaled a little down and be made lighter.

Wouter

Last edited by Wouter; 10/24/06 06:38 AM.

Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
Re: F18 crossbar regulations [Re: Wouter] #87047
10/24/06 07:42 AM
10/24/06 07:42 AM
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Quote
Why make it personal? How is that constructive discussion? By all means agree/disagree/counter argue or be downright cussed, but make it personal? - I think it says more about you than Wouter....

Oh, and it's L-O-C-O-M-O-T-I-V-E


You people are so effing uptight. Jesus relax. LAUGH maybe. I was just playing off of what Ding was doing.

Thanks for the spelling lesson, I'll remember to consult you next time I write my next whitepaper on an internet forum. :rolleyes:

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