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Old 08-10-2016, 19:27   #1
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Max righting moment question

Does any one happen to know the max righting moment for an eastward ho 24? I know it's a long shot. Having just got Brian toss "complete riggers apprentice" It looks like a little complexity is involved in finding it..I don't have an incompetent or friends available at the moment. Google hasn't turned it up either.also Looking at the sailboat data site what is a sail area displacement ratio of 13.1, bal/ disp 42.87%, displen 39.63 telling me? I know she's heavy and slow and hopefully safe. I'm in the process of evaluating her design for refitting and capeabilities.i figured I'd try and see if any one happend to know her max rm or rm at 30 deg. Thanks
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Old 08-10-2016, 19:38   #2
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Re: Max righting moment question

*inclometer ( not incompetent)
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Old 08-10-2016, 20:27   #3
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Re: Max righting moment question

The only real way to get an accurate result for your boat is to do an inclination test.

In your case I suspect just your weight moving across the boat will get you a rough figure, by using a halyard with a weight on it as a crude inclinometer. Multiply the heel angle by ratios to get an approximate RM30.

Even using RM30 for rigging calcs is a huge approximation and doesn't really account for dynamic loads.

Most cruising boat rigging failures are not due to the wire being overloaded or being too small to start with, they are due to stainless steel wire failing due to stress corrosion cracking or crevace corrosion and considerably weakening them.
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Old 08-10-2016, 22:08   #4
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Re: Max righting moment question

FWIW, you can build a serviceable inclinometer from a grade school protractor and a bit of string with a small weight on the end. And jerry jugs full of water on the gunnels can serve as heeling weights if no fat friends are handy.

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Old 08-10-2016, 22:21   #5
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Re: Max righting moment question

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...fat friends...
I think we prefer the phrase persons of respectable girth.
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Old 08-10-2016, 23:13   #6
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Re: Max righting moment question

Is there a rule of thumb as to the safety factor one applies after you have ascertained the maximum righting moment and calculated the rigging stresses?
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Old 09-10-2016, 00:14   #7
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Re: Max righting moment question

Are you interested in figuring out how well or quickly she'll come back from a knock down and/or how well she will resist it? It might be interesting to see this bit of a letter written to the original owner of my boat by Olin Stephens, the designer (pretty famous one too!) regarding metacentric height. (I think the original owner may have been concerned about the designs tenderness, which it is initially because of its narrow beam. But then you have to consider the weight and depth of the ballast as the heel angle increases.) His comment on the variability of righting moment is interesting. But I believe it was in in what Stephens wrote in Adlard Coles' book on heavy weather sailing that he has some comparative graphs of different hull designs' responses to knockdowns... or was it Marchaj's book... gotta search...
The specs on your boat sound like a boat designed for weathering heavier conditions. Your boat is heavy for its length, pushing the upper limits, but within them, of what Stephens felt was a good displacement to length ratio for an ocean-going boat. And you are not carrying much sail for that much weight. The specs on your boat are very similar to the Dana 24, which is a pretty well-regarded pocket bluewater cruiser.
DANA 24 sailboat specifications and details on sailboatdata.com
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Old 09-10-2016, 02:15   #8
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Re: Max righting moment question

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Is there a rule of thumb as to the safety factor one applies after you have ascertained the maximum righting moment and calculated the rigging stresses?
Its complicated, and depends on which design systems you use. The nordic boat standard (NBS) uses safety factors of between 3 and 2.3
For different shrouds and rigs, caps normally 3 and intermediates 2.3. But first you have to find out the loadings on each shroud for the various sail combo's.

The older skenes method is much cruder.

Chris Mitchell at AES has a great writeup about rig design here. Well worth a read.

http://www.aes.net.nz/?page_id=52

I made a pretty crude but very useful spreadsheet that I am using to analyse my rig. The results are facinating, but overall support the more simple rules of thumb, but show that the loads on the lower shrouds get pretty high with just a reefed main set. Overall the max static loads on my rig seem to be around 25-30% of break.

Fore and aft loads get much more interesting!

Sent from my SM-G930F using Cruisers Sailing Forum mobile app
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Old 09-10-2016, 03:18   #9
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Re: Max righting moment question

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Its complicated, and depends on which design systems you use. The nordic boat standard (NBS) uses safety factors of between 3 and 2.3
For different shrouds and rigs, caps normally 3 and intermediates 2.3. But first you have to find out the loadings on each shroud for the various sail combo's.
It's all about the righting moment, not the sails. Whatever the sails happen to be a sudden gust will heel the boat against the righting moment. For a blue water boat it means any sail alone can produce heeling force equal the righting moment. From there multiplied with the safety factor..

BR Teddy
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Old 09-10-2016, 03:46   #10
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Re: Max righting moment question

Thank you all, I've been looking at the Dana 24's specs I couldn't help but notice the similarites which is Comforting if. And that is a good read, I'm going to have to go over that a few times. Sailboat data has her ballast at 3000 my name plate reads 3700 lbs. so she's even a little heavier then listed. Possibly because she's got the. Volvo md7a as opposed to the palmer gas. I'm considering taking her offshore for some passsage making and seems to be a small boat built for it. Which brought my attention to the rig; chainplates terminate to the deck with just some washers and u bolts. They're about an inch from the toe rails so I'm gonna add some gussets or knees and either bring them to the hull sides or through the deck to the knees inside. So I bought the riggers apprentice to try to grasp a full understanding of rigging, a lot to learn still. I wondered why a boat that's seemingly designed so heavy full keel,tiller,3700lb ballast would have made the chainplates so skimpy. Which has led me where I am now to try to verify my wires,turnbuckles,tangs, etc are sized correctly and are my rig dimensions as stout as the hull design would suggest. Or do they follow suit with the poorly designed chainplates.........thank you all for your knowledge
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Old 09-10-2016, 04:48   #11
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Re: Max righting moment question

^^BR, I think thats about what I said, the RM stays the same, but the loads on the rig vary enormously depending on what sails you have set at the time. You figure the max loads and then apply the safety factor in the NBS. Skenes (If it is the same as Brion Toss's) I think applies a safey factor first (1.5xRM) then apportions the loads in a much more simplistic, but generally valid way, allowing another 2.5 safety factor to the shrouds afterwards.

Just genoa loads up the caps heavily, and just reefed main puts hardly any loads on the caps, most of it on the lowers and some on the intermediates. Full main and no headsail loads up the intermediates, as does just the staysail, or staysail/main combo's.

Then you also get the downwind load cases, where RM is pretty much irrelevant. Interesting things happen to the load on aft swept lowers with just a reefed main set dead downwind in 60 odd knots.
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Old 09-10-2016, 05:39   #12
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Re: Max righting moment question

Oh, according to a graph in Brion Toss's book a typical RM for a 22 foot wl boat would be about 5000-15000 foot pounds or so. 10000 foot pounds sounds like a decent start point, but best verified on any boat by an actual inclining test.

The NBS uses 27xRM1=RM30 to account for non linearity in the RM curve.
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Old 09-10-2016, 08:06   #13
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Re: Max righting moment question

Righting moment increases in foot lbs., As angle of heel increases right? So I don't see how the rm30 formulas work. Is it because rm increases proportionately and that built into the math formula?
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Old 09-10-2016, 08:47   #14
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Re: Max righting moment question

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Righting moment increases in foot lbs., As angle of heel increases right? So I don't see how the rm30 formulas work. Is it because rm increases proportionately and that built into the math formula?
It's not that simple. RM from the keel maxes at 90 degrees, but hull shape hasn't a huge amount to say about RM(max) and that occurs at 0-10 degrees. Rm(30) is generally considered the design point since it isn't generallyvery close to RM(max) and provides a solid place to start when you don't have a full S/n curve handy to work from.
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Old 09-10-2016, 09:02   #15
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Re: Max righting moment question

Eastward, do you have some photos of the chainplates and fittings? Is it possible they are through-bolted to chainplates on bulkheads below decks?

When I was starting my re-build of the mast suppport compression arch in my boat, I consulted an old friend I grew up with, a sailor and engineer. I was interested to know the force the mast would be (possibly) transmitting. "Well, it's a bit complicated but figure on the displacement." I found that shocking and hard to believe, but I am not one to argue with those who would know... The rigging would have to handle transmittng the weight of the boat... and then there is acceleration in a big gust... I never did get the formula to figure the safety factor that would take that into account.
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