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Old 09-10-2016, 09:04   #16
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Re: Max righting moment question

Ok so your saying after about 30 degrees rm doesn't increase that much more. Meaning from 0 to 30 it increases sharply then kinda levels offfrom 30 to max. Or am I saying that backwards?
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Old 09-10-2016, 09:25   #17
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Re: Max righting moment question

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Originally Posted by Eastward ho 24 View Post
Ok so your saying after about 30 degrees rm doesn't increase that much more. Meaning from 0 to 30 it increases sharply then kinda levels offfrom 30 to max. Or am I saying that backwards?
You kinda have it right.... Once you start passing 30-45, you have all that weight of the rig further out on the moment arm... Lookee here...
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Old 09-10-2016, 09:33   #18
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Re: Max righting moment question

Ah ha I see I didn't take into account the counter weight of the rig, as heel increases.
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Old 09-10-2016, 09:59   #19
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Re: Max righting moment question

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Ah ha I see I didn't take into account the counter weight of the rig, as heel increases.
You want that bird to stand up after every dip sip !


(Impressed with your rig investigation... kudos...)
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Old 09-10-2016, 10:01   #20
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Re: Max righting moment question

Narrow hull with high ballast ratio has RM max close to 90deg. The wider the hull with lighter keel diminish the angle. Extreme case is multihull where the RM stars to decline when the weather hull rises of the water ~10 deg more or less. Most moder sailboats has their RM max around 60 deg of heel.
Metacentric height is inaccurate with large angles of heeling becouse it's not a static value in this case. You can, thou it's not a simple task, make estimate of the GZ curve. Just calculate the center of gravity and make a section drawing of B max. Draw waterlines at different heel angles so the area under the waterline stays the same, calculate centers of these areas, B's. Then measure the distance btw center of gravity (G) and vertical line drawn from the B. You get this way approximate GZ curve. The distance is the righting arm, multiply with G and you have RM . Not too complicated but needs some work

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Old 09-10-2016, 10:10   #21
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Re: Max righting moment question

Excellent post Teddy !
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Old 09-10-2016, 10:14   #22
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Re: Max righting moment question

It can be measured at the dock.

Why don't you measure it?

b.
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Old 09-10-2016, 10:20   #23
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Re: Max righting moment question

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Originally Posted by barnakiel View Post
It can be measured at the dock.

Why don't you measure it?

b.
Are you talking about inclining test? Results are all over with boats, even with ships under 60meters inclining experiments are unreliable.

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Old 09-10-2016, 10:23   #24
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Re: Max righting moment question

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Originally Posted by HappyMdRSailor View Post
You kinda have it right.... Once you start passing 30-45, you have all that weight of the rig further out on the moment arm... Lookee here...
PS-Toss book indispensable!
The curve you have shown applies to a certain family of designs (beamier, flatter, fin keel.) Do you have the pictures of the curves for the old CCA boats handy? (I know I am insufferable when it comes to the classics...) Their righting moments stay in the positive over more of the curve.
...and yes, good reminder to keep the weight aloft down. It gets multiplied by distance.
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Old 09-10-2016, 10:37   #25
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Re: Max righting moment question

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Originally Posted by TeddyDiver View Post
Are you talking about inclining test? Results are all over with boats, even with ships under 60meters inclining experiments are unreliable.

BR Teddy
But this is how it is practically done. At least in boats where small differences lead to major consequences.

I have witnessed a test on an IMOCA racer. The boat was tested once empty and again with all ballast shifted (water, keel and stacked sails).

Do you say experiments are a LESS reliable data source than computer calculations?

Why do you say heeling test is unreliable?

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Old 09-10-2016, 10:46   #26
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Re: Max righting moment question

Quote:
Originally Posted by barnakiel View Post
It can be measured at the dock.

Why don't you measure it?

b.
Easy enough right??? (no pun intended)


Quote:
Originally Posted by TeddyDiver View Post
Are you talking about inclining test? Results are all over with boats, even with ships under 60meters inclining experiments are unreliable.

BR Teddy
Agreed... somewhat... at least the curve can be drawn to as far as you can weighted heel... But at least you have an accurate RM to that point, and a good clue from the curve... I always thought it would be awesome to weight a boat to 15ish and put a force gauge on a halyard to pull her over to 45+...

Quote:
Originally Posted by Don C L View Post
The curve you have shown applies to a certain family of designs (beamier, flatter, fin keel.) Do you have the pictures of the curves for the old CCA boats handy? (I know I am insufferable when it comes to the classics...) Their righting moments stay in the positive over more of the curve.
...and yes, good reminder to keep the weight aloft down. It gets multiplied by distance.
Ah Don... you are INDEED insufferable !

I just quickly grabbed that from the net to illustrate aloft weight influence v heel... I'll look to see if what you're insufferable for is out there!
HS
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Old 09-10-2016, 10:50   #27
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Re: Max righting moment question

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Originally Posted by barnakiel View Post
But this is how it is practically done. At least in boats where small differences lead to major consequences.

I have witnessed a test on an IMOCA racer. The boat was tested once empty and again with all ballast shifted (water, keel and stacked sails).

Do you say experiments are a LESS reliable data source than computer calculations?

Why do you say heeling test is unreliable?

b.
I guess he means past the point of direct data measurement???
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Old 09-10-2016, 11:40   #28
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Re: Max righting moment question

Quote: "Does any one happen to know the max righting moment for an eastward ho 24?"

The righting moment is defined as the horizontal distance twixt a perpendicular through the boat's Centre of Gravity and a perpendicular through the boat's Centre of Buoyancy multiplied by the boat's displacement, and that distance changes from second to second. Therefore it is of no particular interest to a skipper. True enuff - it has a maximum value, but what that is, is of no consequence to a skipper.

I rather think you are thinking of righting FORCE. What that is depends on the size of the boat and a host of other things, since one of the elements in the calculation of righting force is the volume of the hull that is submerged, i.e. the momentary displacement. There again, why would you care? You can take it for a dead cert that it's ADEQUATE. Else the boat would go glug-glug! You can also take it for a dead cert that it is equal at any given fraction of a second to the force of the wind resolved to act through the rig's Centre of Effort as it exists from second to second. When it is not, the boat simply changes its angle of heal and bring the two to equallity. Skipper needs to do nothing about that.

So are you worried that at RM(max) the Righting FORCE, opposing the pressure of wind in the sails, will rip out your chainplates or in some other way bust you standing rigging?

I think you can take it for granted that the standing rigging including chainplates are adequate for anything you can throw at them. By the time you are heeled so far that RM(max) heaves into sight, you'll have wet your knickers, and not necessarily with briney, or you'll have started sheets :-)

The Eastward 24 has a SA/D about the same as TrentePieds, i.e. she is "permanently reefed", and you can take it for a dead cert that in standard configuration, and "as designed", she'll take better care of you than you can of 'er. If your rig, as designed and built, has been well maintained, you have nothing to worry about, and you should go sailing instead of treading the boat's designer on the toes :-)

Now if you have, say, crevice corrosion in a rigging screw - THEN it's worth your while to worry. But that has nothing to do with the theoretical aspects of rig design.


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Old 09-10-2016, 12:11   #29
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Re: Max righting moment question

Quote:
Originally Posted by barnakiel View Post
But this is how it is practically done. At least in boats where small differences lead to major consequences.

I have witnessed a test on an IMOCA racer. The boat was tested once empty and again with all ballast shifted (water, keel and stacked sails).

Do you say experiments are a LESS reliable data source than computer calculations?

Why do you say heeling test is unreliable?

b.
The problem with inclining experiment lies in the fact it's used to calculate Metacentric height, not RM. Boats hydrostatic features (ie submerisible hull shape) vary far more than ships in different heeling angles thus the metacenric height doesn't remain the same. True you could incline a boat to much greater heel but then the weights used start to influence on the displacement then the results would be false too.
Why some racing rules demand such test is questionable, maybe just some rule writers tried to feel them smart, which they obviously are not

BR Teddy
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Old 09-10-2016, 12:20   #30
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Re: Max righting moment question

Lots of good info here thank you.i will give it a shot measuring it. I need to make my way to the marine store to get an inclometer. My rig now is very poor I'm two blocked and my lowers are still very loose,plus the po just put on a cdi flexible roller with brand new 150% headsail( sails great in heavy weather with just the 150%, been out in 20 knot winds comfortably), however My mast is raked very far back. I think he may have ordered one for a eho24 with a bow sprit (I dont have a bowsprit) and to compensate the mast has been pulled back. It's about 9 inches aft if I measure the main hylard straight down to the mast boot.so I think I have a bunch of issues to sort out not sure what order I should do them in. I was thinking of adding a bowsprit allowing me to keep the roller 159% forward and adding another forestay,and a Solent stay with running back stays. Does this sound like I'm getting crazy with it ? I'd hate to get rid of the brand new 150%. But reefing the 150 down it handles poorly. My uppers are 3/8 turn buckles 3/16 wire. And lowers are 5/16 turnbuckles 5/32 wire, back stay is 3/8 turnbuckle w 7/32 wire. and the forestay I can't see to measure. I haven't been sailing long and this is my first boat. Any thing about these numbers jump out at any one? I was thinking I was just gonna use 1/4 inch wire with 1/2 turnbuckles on everything( uppers lowers fore and back) for ease of replacements and simplicity.
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