#96783 - 01/25/07 11:07 PM
Re: Please give your opinion on ... SMOD/ OD / For
[Re: Wouter]
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Carpal Tunnel
Registered: 11/12/02
Posts: 5558
Loc: Key Largo, FL & Put-in-Bay, OH...
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Quote:
Yes I quit this public project for an F12, which is not the same as my personal F12 project.
Wouter
Good. I think trying to design a boat with public input has to be almost impossible, whether for children or for adults, because 20 different people are going to have 20 different ideas and opinions. It's sort of like trying to write a song by committee.
Edited by Mary (01/25/07 11:10 PM)
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#96784 - 01/25/07 11:39 PM
Re: Please give your opinion on ... SMOD/ OD / For
[Re: Wouter]
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addict
Registered: 08/20/02
Posts: 545
Loc: Brighton, UK
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Quote:
Two complete hulls out of 3 sheets ? Show us how you do it, because it will really suprise me if you can do that and still have sufficient freeboard. I don't think that that can be done and as such I think you have a bullshit argument there.
My design is from 3 sheets of ply it has a total volume of 240l per hull (480l per boat) a surface area of 4.15 sqm it is 250mm wide 450mm tall.
edit - thats over 1000lb total displacement for the non metric among you (if anyone is still reading this thread)
Gareth
Edited by grob (01/25/07 11:47 PM)
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#96786 - 01/26/07 06:35 AM
Re: Please give your opinion on ... SMOD/ OD / For
[Re: Mary]
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addict
Registered: 08/20/02
Posts: 545
Loc: Brighton, UK
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Grob, maybe I missed it somewhere, but how long is your design?
12ft
Gareth
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#96790 - 01/26/07 01:43 PM
Re: Please give your opinion on ... SMOD/ OD / For
[Re: grob]
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Carpal Tunnel
Registered: 06/16/01
Posts: 9582
Loc: North-West Europe
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My reasoning goes like this : Quote:
The Centre of effort (CofE) of the sail is say 2.5m high,
Mine is ALOT higher.
Your centre of effort of the sail is consistant with a 3.90 mtr luffed sail on the F12. That would give the boat a 5 sq. mtr. sailarea if the minimal required aspect ratio is 3
This last measurement is important as below aspect ratio's of 3 you loose alot of sail efficiency, almost to the extend that more sail area is almost offset by the loss of efficiecy.
For comparison a laser 1 rig has aspect ratio of 4 and the wave has 3.6
With a 5 sq. mtr. rig the boat will never sail at 15 knots speeds. It will be much slower, thus in turn reducing sailforces. In one way this is a good thing as the pitching is reduced to such an extend that it will be of no consideration. The performance is what is being hurt here. Texel rating of 172. This is about the same speed as the laser 1 with a 4.7 sq. mtr. rig that is the kiddies boat for 50 kg and less.
In summary I think 2.5 mtr leverage of the sail power is far to small.
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The centre of buoyancy (CofB) of the boat is say 1.8m from the stern
If the boat is level on its intended waterline, yes. There abouts anyways.
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So in order to balance out the forces, the crew could move his weight (60kg) back to the rear beam
Yes, I've done those calcs as well. Still you assume a static balance while I focussed more on dynamics involved with wind gusts and the courses I'm interested in are the downwind course where accellerating the boat during a gust will increase the force far beyond the 200 N of the static balance.
Just assume a 10 m/s gust hits your boomed out 5.0 sq. mtr. sail directly from the rear when running square downwind and making a low speed gybe. The sail force generated by that LITTLE sail alone will be 250 N already. If the crew isn't at the back of the boat when the gust hits then the boat is about to tip over, especially if the deck dig in and pin the boat down.
In summary I think static calculations aren't dependable enough to concluded that worse case scenarios are well covered.
And of course any boat can be made controllable by just slapping a smaller rig on it. I admit that I had the design goal that the boat HAD to beat all laser dinghies and the Hobie 14 and Wave in performance, as such my designs NEED to have more sailarea then 5.0 sq. mtr. and also have higher sail aspect ratio's then 3. Indeed if this performance design goal is dropped then there is no reason why even a 10 foot design can be made that won't tip over.
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Of course the extra pitching moment of a longer hull is a good safety device, but with such a lightweight low drag, low CofE boat which will tank along at 15 knots without any danger of pitchpolling is it really required?
And that is where the argument goes awry. This boat WILL NOT tank along at 15 knots as your sail area will be far to small to produce sufficient sailforce WHILE SAILING at 15 knots. It can produce that sail force during a gust when the boat is almost motionless but it can't when the boat itself moves at 15 knots.
In arguments like these I advice somebody like yourself to develop you mathematical models further, they still allow conclusions that are in direct conflict with eachother and as such lead to errornous conclusions. Your models are not yet well defined.
I have a model that takes into accounts all influences like changes in angle of attacts etc and the one clear conclusion was that with each reduction in sailarea the boat because disproportionally slower. Even at 9.00 sq. mtr. (my 3.90 mtr design) I could achieve 10-11 knots at maximum. Which is fast but a 5.00 sq. mtr. sail area at a significant lower aspect ratio will be well below 10 knots (around 6 knots or so).
This is actually why apparent wind sailing is a different form from just yachting about. With each increase in boatspeed your sails produce MORE drive despite having a smaller angle of attack allowing you to go just a little faster again. Just as with pointing ability, each and any speed increase that you can achieve is advantagious as you disproportionally improve pointing and speed that way.
The adagio indeed is "speed is everything !"
Now having said all this. We can still build this 12 foot version, as I wrote I developped both simultaniously, and just see how much sail area it can really carry before becoming scary. The numbers and models are just here to base our decision upon but we may go against them if we decide to do that.
My only real question to the 12 foot supporters is what they expect to win by going to 12 foot instead of 12'10" ?
It isn't weight savings, it isn't cost savings, at least not in significant quantities. It sure as hell isn't performance improvements or even optimal crew weight capacity. Apart from the name being F12 instead of F12'10" what are the arguments for a 12 foot hull ?
Wouter
Edited by Wouter (01/26/07 01:47 PM)
_________________________
Wouter Hijink
Formula 16 NED 243 (one-off; homebuild)
The Netherlands
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#96791 - 01/26/07 02:06 PM
Re: Please give your opinion on ... SMOD/ OD / For
[Re: Wouter]
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Carpal Tunnel
Registered: 11/12/02
Posts: 5558
Loc: Key Largo, FL & Put-in-Bay, OH...
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Quote:
It isn't weight savings, it isn't cost savings, at least not in significant quantities. It sure as hell isn't performance improvements or even optimal crew weight capacity. Apart from the name being F12 instead of F12'10" what are the arguments for a 12 foot hull ?
Gosh, Wouter, now you are asking to start going round and round again. 
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#96792 - 01/26/07 02:25 PM
Re: Please give your opinion on ... SMOD/ OD / For
[Re: Wouter]
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addict
Registered: 08/20/02
Posts: 545
Loc: Brighton, UK
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Quote:
Your centre of effort of the sail is consistant with a 3.90 mtr luffed sail on the F12. That would give the boat a 5 sq. mtr. sailarea if the minimal required aspect ratio is 3
Again Wouter you are talking about your design not mine, as I have said all along I would use a 7m2 windsurfing rig on a 460 mast. The gust response of these sails is very good I would not expect to see the situation you describe.
This is a similar rig to that used on the international moth that won the world championship a few years back so perfromance is not as bad as you are trying so hard to make out.
Gareth
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