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#54150 - 08/01/05 03:36 AM trap line....  
Joined: Jul 2005
Posts: 32
max Offline
newbie
max  Offline
newbie

Joined: Jul 2005
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maybe is a stupid question but why don't use the trapeze line to righting the boat after capsize....
it can be do a piece of line with a dogbone and an hook...
using the trapeze u can do more leverage with the mast or not?
thanx
max

-- Have You Seen This? --
#54151 - 08/01/05 03:44 AM Re: trap line.... [Re: max]  
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Tornado_ALIVE Offline
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Tornado_ALIVE  Offline
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Melbourne, Australia
Got to be able to reach a trap line from the top hull first. Even harder on a Tornado

Just as easy to have your righting rope tied to the dolphin striker and throw it over. Don't believe there would be any difference in righting momentom either.


#54152 - 08/01/05 05:43 AM Re: trap line.... [Re: Tornado_ALIVE]  
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arbo06 Offline
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arbo06  Offline
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South Florida & the Keys
My guess would be that there would not be any advantage to using the trap line. The point at which the righting line touches the hull and leads to the sailor is the limiting factor. If the line could go directly from the mast hound to the sailor with out touching the hull, there may be an advantage.(?)


Eric Arbogast
ARC 2101
Miami Yacht Club
#54153 - 08/01/05 07:41 AM Re: trap line.... [Re: arbo06]  
Joined: Jun 2001
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Jake Offline
Carpal Tunnel
Jake  Offline
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Posts: 12,279
South Carolina
No - no mechanical advantage from where the line originates. Just more or less loading on the line or boat parts. You can safely eliminate the line from the equation because, in a sense, nothing is moving and the sailor, the righting line, and the boat can be treated as one solid unit. You are simply looking at a balance equation with the mast on one end and the sailors on the other pivoting around the hull in the water. Placing the sailor's body weight further away from the boat and the closer to the water will result in more righting moment.


Jake Kohl
#54154 - 08/01/05 08:45 AM Re: trap line.... [Re: Jake]  
Joined: Aug 2004
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Simon Offline
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Simon  Offline
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Cheshire, UK
So Jake, how far out on your F18 dagger board would you stand to get the most leverage? Do you think it is strong enough to take the weight of helm & crew if you stand at the end of the board?

Simon


Simon
Shadow 067
#54155 - 08/01/05 09:47 AM Re: trap line.... [Re: Simon]  
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Jake Offline
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Jake  Offline
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South Carolina
The boards are pretty heafty. I usually go to the tip of the board - I've righted my F18 boat three times by myself by standing on the tip (twice with the crew still on the dry hull). It does not give me any indication of being overloaded when I do so (I weigh about 170lbs). However, putting both of us on the end of the board would probably be a bit much but we've never needed to do that.


Jake Kohl
#54156 - 08/01/05 09:53 AM Re: trap line.... [Re: Jake]  
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Rolf_Nilsen Offline
Rolf_Nilsen  Offline

Carpal Tunnel

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Posts: 4,451
West coast of Norway
Shouldn't the boards be plenty strong for you both to stand on them? After all, when you are double trapping 2.5 meters from them your righting moment (which the board counteracts) is far larger than your combined weight?
I havent'd done any calculations, just an opinion.

#54157 - 08/01/05 10:37 AM Re: trap line.... [Re: Rolf_Nilsen]  
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bvining Offline
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bvining  Offline
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Atlanta
Rolf,
You bring up a good point, are the forces the same or greater?

I've never used daggerboards to right an HT, I assumed they would break, and I wouldnt think of using my daggerboards to right my A cat, but I just went down stairs and leaned on my A cat boards, and they didnt move. I just assumed that a 1.85kg hollow carbon board wouldnt hold my weight, but would it?

Any engineers out there care to comment? If it doesnt break during sailing, does it mean you can stand on it to right the boat? Or are the loads different?

Are the forces more fore and aft as opposed to sideways?

Bill

#54158 - 08/01/05 12:52 PM Re: trap line.... [Re: Jake]  
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Posts: 141
steveh Offline
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steveh  Offline
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Panama City Beach, FL
Jake, how does being closer to the water help? If you have a playground see-saw and hang under the seat instead of sitting on top of it, you don't gain any mechanical advantage.

Rolf, the dagger doesn't counteract the heeling moment, it counteracts the leeward force generated by the sail. In fact, the windward lift force acting on the dagger increases the heeling moment. However, the dagger has a smaller moment arm than the sail, so its contribution is proportionally smaller.

As for the magnitude of the force, assume a 15 knot cat with a 3 ft long dagger (exposed below the hull) with a chord width of 16in with a lift coefficient of about 0.15 at a 2 degree angle of attack.

L = 1/2* CL * density * speed^2 * area
L = 1/2 * 0.15 * 1.99 * (1.69 *15)^2 * 4
L = 384 lbs

That force will act roughly 1/3 the distance from the hull to the tip giving a 384 ft-lb moment. For an equivalent moment, you could have a 128 lb person stand on the very tip. Factoring in structural safety factors, a higher design speed, etc, etc, etc, it looks to me like the dagger would be plenty strong enough to stand on.

#54159 - 08/01/05 01:04 PM Re: trap line.... [Re: steveh]  
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Jake Offline
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Jake  Offline
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South Carolina
Quote
Jake, how does being closer to the water help? If you have a playground see-saw and hang under the seat instead of sitting on top of it, you don't gain any mechanical advantage.


Just like proper trapeze form, getting yourself more parallel to the water surface moves your center of gravity out as far as it can go to achieve maximum leverage.


Jake Kohl
#54160 - 08/01/05 01:07 PM Re: trap line.... [Re: steveh]  
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Jake Offline
Carpal Tunnel
Jake  Offline
Carpal Tunnel

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Posts: 12,279
South Carolina
Quote
That force will act roughly 1/3 the distance from the hull to the tip giving a 384 ft-lb moment. For an equivalent moment, you could have a 128 lb person stand on the very tip. Factoring in structural safety factors, a higher design speed, etc, etc, etc, it looks to me like the dagger would be plenty strong enough to stand on.


Steve, why 1/3 the distance? Most boards are straight section and not tapered.

After seeing the hell my boards were going through at Performance Race Week last year (boat leaping clear into the air in 4' waves and coming back into the water sideways) I didn't feel to worried about putting my full weight on the tip! Most boards are designed with a pretty big factor of safety.


Jake Kohl
#54161 - 08/01/05 01:49 PM Re: trap line.... [Re: Jake]  
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steveh Offline
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steveh  Offline
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Panama City Beach, FL
Simply a fudge on the lift distribution. It won't act mid-span because of the tip loss. I guess 3/8 to 7/16 would be more accurate, but 1/3 of 3 ft makes for a nice, easy number and it makes for a worst-case for figuring what kind of weight you can put on the tip.

Gotcha on the position. Hadn't envisioned it that way.

#54162 - 08/01/05 02:08 PM Re: trap line.... [Re: steveh]  
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MauganN20 Offline
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Uh oh, looks like another engineer.


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