Righting moment

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Bertil Rafting
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Righting moment

Post by Bertil Rafting » Fri Sep 09, 2016 9:36 pm

Do anyone knows the righting moment at for example at 30 degrees heeling with and without water ballast 520 kg on the Macgregor 26M-2009?

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sunshinecoasting
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Re: Righting moment

Post by sunshinecoasting » Sat Sep 10, 2016 8:50 pm

30 degrees heel? for me 20 degrees is the righting moment to let out the main sheet and get back to under 15 degrees. :D

Bertil Rafting
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Re: Righting moment

Post by Bertil Rafting » Sun Sep 11, 2016 3:16 am

What I meant was a measurement of Macgregor26M stability with and without water ballast.

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NiceAft
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Re: Righting moment

Post by NiceAft » Sun Sep 11, 2016 4:01 am

This sailor will not sail his Mac without ballast; motor with certain precautions, yes, sail, no.

I am not sure what you mean by “righting moment”? If you mean the moment where a sailor says “OK,I'm scared to be heeling this far, time to bring her back up!”, well that depends on the sailor. I have intentionally sailed my :macm: in excess of forty degree's heeling (the Tilt-O-Meter stopped at 40, but I kept the boat kept heeling farther). You're not making much headway, but you give a great thrill ride to your guests who think sailing is dull. :evil: :D Later though, you have to clean the mess below deck. :(

Ray

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grady
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Re: Righting moment

Post by grady » Sun Sep 11, 2016 8:57 am

http://www.macgregor26.com/safety/safety.htm

Look at this page it states 130 lbs at the top of the mast. As far as with out balast you will have to tip your boat your self, or do math. I DONT DO MATH ON SUNDAYS!

Your center of pivot at that point would be 1/2 to 1/3 of the way up the hull. Get some measurements of mast and hull, water balast would be 100% out of the water so you can subtract that and you should be able to calculate what you need.

Bottom line
Stability with water; Great
Stability without water; Crap.

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Phil M
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Re: Righting moment

Post by Phil M » Sun Sep 11, 2016 9:19 am

I was caught off-guard with too much Sail and a gale force wind this summer. We ended up with the gunnels in the water for awhile. Fortunately I always travel with full ballast so the boat bounced back eventually, when I gave up and let the sails go free. :o
I NEVER travel without full ballast, sailing or Motoring. But then again I never use WOT either. (wide open throttle)

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Herschel
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Re: Righting moment

Post by Herschel » Sun Sep 11, 2016 1:48 pm

Do anyone knows the righting moment at for example at 30 degrees heeling with and without water ballast 520 kg on the Macgregor 26M-2009?
I think our Swedish friend is talking about these type of calculations.

http://www.marine.marsh-design.com/cont ... ity-curves

I haven't tried to plow into this kind of math for too many years now, but I suspect someone on our forum is up to speed on this stuff. The question involves calculating the righting force that the ballast exerts when the boat is heeled over.

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sailboatmike
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Re: Righting moment

Post by sailboatmike » Sun Sep 11, 2016 2:12 pm

I would do the calculation but Im just too lazy at the moment, I do love a good arithmetic problem to sharpen the mind.

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Judy B
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Re: Righting moment

Post by Judy B » Sun Sep 11, 2016 4:29 pm

Bertil Rafting wrote:Do anyone knows the righting moment at for example at 30 degrees heeling with and without water ballast 520 kg on the Macgregor 26M-2009?
Here's the quickest way to get an accurate measure of RM30 for any boat.
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Inclining

Most rig scantling formulas are based on the vessel’s righting moment at 30 degrees (see text). If that information is not available from the vessel’s designer, the most convenient way to get that figure is from the chart shown in Figure 5-24. But that chart is only an approximation for many vessels; if you want a more precise basis for your calculations, you can get it by inclining the vessel yourself. Start by setting a bucketful of water on the cabin sole, amidships, under the companionway hatch. Suspend a plumb bob from the hatch, into the water (the water will damp the swing of the plumb bob). Level the vessel laterally, as nearly as you can, and clamp a 5-foot batten in place parallel to the string. This will give you a vertical reference point later on. Mark batten and string at 4 feet down from the string’s attachment point. Tip the boat 3 degrees, either by swinging the boom out and weighting its end with water cans, or by having a bunch of friends stand outboard on deck. You’ll know you have 3 degrees when the mark on the string is 2½ inches away from the mark on the batten. Now measure how far outboard they were standing—say, 5 feet from the centerline. This would mean it took 5 × 700 = 3,500 foot-pounds of force to keel the boat 3 degrees. Finally, multiply that number times 10, to get the foot-pounds required to heel the boat 30 degrees, and plug that number into the RM30 formula in the text.

SOURCE Toss, Brion. The Complete Rigger's Apprentice: Tools and Techniques for Modern and Traditional Rigging: Tools and Techniques for Modern and Traditional Rigging (Kindle Locations 2920-2933). McGraw-Hill Education. Kindle Edition.

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NiceAft
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Re: Righting moment

Post by NiceAft » Sun Sep 11, 2016 5:34 pm

Thanks for the precise answer. Now, outside of the question being asked by a member of this board, why is this important?

I am not trying to be a wise a??, really, but again, outside of the question being asked by a board member, why is this important to us? I really would like to know its relevancy. There must be some reason this sort of information is needed?

Ray

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Judy B
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Re: Righting moment

Post by Judy B » Sun Sep 11, 2016 7:30 pm

NiceAft wrote:Thanks for the precise answer. Now, outside of the question being asked by a member of this board, why is this important?

I am not trying to be a wise a??, really, but again, outside of the question being asked by a board member, why is this important to us? I really would like to know its relevancy. There must be some reason this sort of information is needed?

Ray
The weight of the keel and the buoyancy of the hull form resist heeling. The maximum righting moment is how we measure the boats ability to push back against heeling caused by the wind.
So, maximum righting moment = maximum Load on the rig. You can use the righting moment (and some trigonometry) to calculate how strong the mast, wires, chainplates, etc must be.
Last edited by Judy B on Sun Sep 11, 2016 7:52 pm, edited 2 times in total.

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NiceAft
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Re: Righting moment

Post by NiceAft » Sun Sep 11, 2016 7:37 pm

So this is important in designing a boat, or if one is ever on a quiz show. :)

Thanks for the clarification Judy.

Ray

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Catigale
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Re: Righting moment

Post by Catigale » Thu Sep 29, 2016 6:35 am

If you know enough that this kind of data is useful, you won't be posting here.

:D :D :D :D :D

Serioulsy - the :macx: and :macm: boats sail so poorly hard over that anytime you are at 30 degree heel you should be trimming, reefing, or reefering to get the boat back on its feet.

The fast little S boats and guys like grady and Newell need not apply.

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sailboatmike
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Re: Righting moment

Post by sailboatmike » Thu Sep 29, 2016 3:01 pm

Yep once those big slab side get in the water causing drag its all over, upright is for speed, if you have to put the sponsors logo on the keel then its all show and very little go, which is what I have never understood, sailing is so slow anyway, why would you want to deliberately go slower,

But thats just me, some like watching the fish through the windows

Bertil Rafting
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Re: Righting moment

Post by Bertil Rafting » Sat Nov 19, 2016 7:06 am

I asked that question because i am putting an extra keel 200 kg connected to the dagger board to get more RM. I am using a keel and not a bulb to also get appr 15 % extra sideresistance,
which a bulb does not give (better than the bulb on Black Pearl).
Of course also have to build a new strong enough daggerboard, and also make enforcements inside the DB-trunk.
-I need to know the RM before my modifications as above to be able to calculate the new RM to see if the mask and schrouds is strong enough.
-Thanks for the tips I got from you.

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