Max righting moment question

[TrentePieds]

Quote: " I think I have a bunch of issue..."

Sounds like you do! But don't worry, there is nothing there money can't fix ;-0)!

A sprit sound sensible, though expensive UNLESS there was one originally and the mounts are still viable. Who knows what ignorant POs have up to along the way? It may be that with a sprit according to original specs, your fore and backstays will sort themselves out. Easy enuff to determined in advance simply by making a scale drawing showing the mast at plumb. 1/2 inch to the foot would prolly suffice. Then measure what you have and see if you will have enuff length (or too much length) with the rigging screws set at half extension.

Shouds are a kettle of fish of a different colour since raking the mast doesn't make much of a difference to them. Could even be that a PO retrofitted with a shorter mast. You never know till you measure.

It doesn't seem to me that there is any sense in spending money on ANYTHING until you've done yourself the scale drawing so you can know with certainty what the lengths of the various bits have to be. Then measure the existing bits. 50-foot tapes are cheep to buy :-) Then set up a table showing in one column what the DESIRED lengths are, and in another what the ACTUAL lengths are. Remember that the DESIRED lengths are pin-to-pin including the rigging screw.

Any bits that are too long, you MIGHT be able to shorten, depending on how much goods you have to work with. Any bits that are too short, you may have to replace. But just do these things systematically, one thing at a time :-)

The fact that your mast is raked, when it shouldn't be, may be because a PO picked up a roller furling jib for squat not having enuff nous to know that it was too long. The waterfront is full of people like that!

Make your scale drawing, then come back to us. You can pick up the profile from "Sailboat Specifications" on the Web

TrentePieds

[barnakiel]

Originally Posted by TeddyDiver

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

OK. Roger roger.

I think I understand what you are saying. At least I seem to understand the part where you mention the importance of the weights.

We are talking about different experiments though.

The way I have seen it done, a block was hoisted up the main halyard, a rope was pulled thru it and tied to a cell which was tied to a dock. Now the line was winched while the boat was allowed to float away from the dock (controlled by two lines run fore and aft). They winched an measured till the mast was about horizontal. They repeated the measurements again while easing the rope.

Not sure how many points they took. I guess about a dozen.

The data was then fed into a laptop and a curve was best fitted. It looked sinusoidal and the software was a plain spreadsheet software. It was pretty obvious then from just looking at the plot where the max was.

I cannot say how the data got processed afterwards but I know it was used to redesign the mast and standing rigging on this particular boat to bring it up to present IMOCA standard (present IMOCA have standardised rigging and keels).

So to say they did not just hang a water filled jerry can at the end of the boom, if this is what you mean by the test not being accurate.

Too bad my French is no match to those young guys skills in building and preparing the boats. I could have learned much more as this is exactly where the best boats are built and modified. I have never before seen so many well educated and talented people in one place. You may have noticed too that the peak of the IMOCA class are mostly engineers by trade.

BTW Given the look of the curve I think for many boats we can say the max is a range rather than a point. Notice most boats have very flat curves. Fewer have a semi-distinctive 'peak' that will probably too drift way to each side as the boat gets loaded with specific gear.

Cheers,
b.

[TeddyDiver]

Originally Posted by barnakiel

OK. Roger roger.

I think I understand what you are saying. At least I seem to understand the part where you mention the importance of the weights.

We are talking about different experiments though.

The way I have seen it done, a block was hoisted up the main halyard, a rope was pulled thru it and tied to a cell which was tied to a dock. Now the line was winched while the boat was allowed to float away from the dock (controlled by two lines run fore and aft). They winched an measured till the mast was about horizontal. They repeated the measurements again while easing the rope.

Not sure how many points they took. I guess about a dozen.

The data was then fed into a laptop and a curve was best fitted. It looked sinusoidal and the software was a plain spreadsheet software. It was pretty obvious then from just looking at the plot where the max was....

That sounds good and with much better accuracy than inclining experiment. The multitude of points is quite irrelevant, even three or four would be enough becouse GZ curves are quite predictable. Unless there's certain things like flooding in the cockpit or sizable deckhouse etc in question of course.

BR Teddy

[Eastward ho 24]

A lot of fish to fry here..the boat comes out in a month or so I was going to pull the mast and hopefully rerig with it down attack all my stays to the top of the mast and shrouds. Using stalock swagg fittings for the top leave the wires long for mechanical connections at the chainplates. Only thing I have to go by is the sailboat data site. Kinda start from scratch. I see some mast at the marina have a slight aft rake to them, and I assume with an adjustable back stay I could get it right. (Also I need to read more of Brian tosses book maybe find more understanding there).

[Snowpetrel]

Originally Posted by TeddyDiver

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

BR Teddy

Not what I have seen and heard, every small vessel stability book I have read has the results from an inclining test, calculation alone being considered to unrealiable and difficult.

For getting a rough GZ curve I would have thought just using the midships section would be way too aproximate to give any sort of valid result. I find it much quicker to knock up a rough hull shape in freeship and calculate it aproximately that way.

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[Snowpetrel]

Originally Posted by TeddyDiver

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

No, the inclining test is used to find the height of the centre of gravity (KG keel-COG), not metacentric height, which can easily be calculated from hull shape. Working backwards from the RM and displacment (hence the freeboard measurements) centre of G can easily be found. This can then be used in conjuction with the KN curves found easily from a program like freeship to calculate the GZ curve.

Most of the hydrostatics can be quickly found with a computer. Calculating COG can't easily be done, except roughly during the design stage. An inclining test is still standard to verify the as built stability on small commercial vessels.

[Snowpetrel]

Originally Posted by Eastward ho 24

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.

Not accurate enough. Use a long plumb bob, inside if there is a bit of wind or outside from the mast if it is glassy calm. Drop the tip into a bowl of water to damp its swings. Tape a ruler somewhere near the bottom of the plumb bob to read how far it has moved from centre and use trig to work out the angle of heel.


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[TeddyDiver]

Originally Posted by Snowpetrel

Not what I have seen and heard, every small vessel stability book I have read has the results from an inclining test, calculation alone being considered to unrealiable and difficult.

For getting a rough GZ curve I would have thought just using the midships section would be way too aproximate to give any sort of valid result. I find it much quicker to knock up a rough hull shape in freeship and calculate it aproximately that way.

Inclining test, if it's done as it is described in naval architecture, tells only metacentric hight in small heels of angle. What it does tell thou is to confirm if the computer calculated hydrostatics are to be trusted.. or not.
Using the midship section for an estimate for GZ's however is about +-10% accurate for conventional hull forms, giving usually a bit bigger result from the reality thus safe for calculating rig loads. This means the Cp of the hull doesn't chance too much with heeling.

For modern racers, with flat underbodies and wide transoms it's should be even closer to the reality but I must admit not having any experience with them.

True, importing the hull into Freeship give better results but there's much more to go wrong with the center of gravity with an older boat.

BR Teddy

[leftbrainstuff]

Originally Posted by RaymondR

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?

Factor of safety is valid for systems where weight is not an issue. Think elevators (10:1), lifting and chain tie downs (5:1).

For dynamically loaded systems fatigue is the primary failure driver. For example aluminum structures can be guaranteed to fail after 10^7 cycles. Crack propagation is another measure of end of life. This is how aircraft are lifed.

For rigging the design rationale is based on slender column assumptions. The failure mode is buckling. The role of the rigging is to stop the mast moving out of column and to transfer sailing loads through the boat.

The other complication in sizing rigging is to determine how much pretension is required to avoid any single piece of the standing rigging carrying no load. This will allow the mast to go out of column.

If you size your rigging to survive a capsize it will be grossly oversized. The impact loading is much greater than sailing loads.

As an engineer i take the following approach:

1) convert sail loads to simple vector forces acting on the mast
2) determine the reactive forces in the standing rigging
3) size the mast cross section to carry the compression forces
4) size the standing rigging
5) spec the attachment solution for the mast step and standing rigging.
6) then based on the use case I'll then add static and dynamic factors of safety and the useful life of the mast, rigging, attachment points, etc. If weight loading increases then you may need to recalculate loads.
6a) for a race boat this might be 1.2 factor of safety and usefull life is one race. ( in motorsport we sometimes have an fos of just 1.05)
6b) for my own cruising boat i use a fos of 3 and have a non destructive inspection regime of annually. I also have a destructive corrosion test running in parallel for new wire.
7) i might also build a fatigue model using miners law. If the vessel experiences loads and cycles which exceed the model then the life is reduced accordingly.

The alternative approach is just replace what's there. If the vessel has a good history then you already have a proven design.

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[Don C L]

I THINK that Eastward is interested in this.. finally found it...
this site is pretty good, there are more of course:
Understanding monohull sailboat stability curves | M.B. Marsh Design

note that these comparisons do not take weight aloft/rigging/sail area into account.

All other things being equal, for the sake of comparison the older designs, narrow and deeper keel, stay in the region of positive stability (or at 0) even when heeled (if you can call it that) at up to 180 degrees! And it is important to remember that wind doesn't capsize a boat, waves do. So while discussion of sail area and weight aloft is interesting in terms of performance, I THINK Eastward is more interested in the relative seaworthiness of his design.

[knockabout]

ask a rigger

[barnakiel]

If this is this boat:

EASTWARD HO 24 sailboat specifications and details on sailboatdata.com

then learning her max RM is just an intellectual exercise.

You can see she is just about right in dimensions. Ballast is about right, the structure is not too light and SA looks adequate.

Are you worried about her rigging? Does anything look too wee? Have there been any accidents? Etc?

Why the worry?

Given the size, your main trouble could be big waves in open water. And there is nothing to be done about that. It is a size function.

Our own boat is not much bigger or lighter than yours. She did whatever we asked her to do with only about regular amount of capsizing, getting wiped out and pooped.

So why worry?

b.

[knockabout]

I agree with barnakiel / stick to the rigging specs for your boat / if you are worried about age of your rig replace with identical but new / the only time we have up sized shrouds we lengthened the boom 1ft and the mast 6ft / added 1 headsail to run 3 headsails all to increase overall sail area but keep the boat balanced / we went to 6mm shroud / stability looks good on paper if you are knocked down in a swimming pool when the squall dies you self right / if you are knocked down at sea it might be a breaking wave that knocks you down not wind / requires different skills to manage and keep the rig on / but you should still self right / this boat is 21ft wll and built in 1893 / the deck goes past the vertical line surfing on the beam and she still self rights / staying on the boat is another skill and not damaging important equipment in the process helps as well / back to you original question look up the design specs for your boat if it all looks pretty standard the specs would still be the same / cases of beer are good to add more weight below if needed

[Snowpetrel]

Originally Posted by TeddyDiver

Inclining test, if it's done as it is described in naval architecture, tells only metacentric hight in small heels of angle. What it does tell thou is to confirm if the computer calculated hydrostatics are to be trusted.. or not.

An inclining test is not done just to verify the calculations, its an integral part of finding the ships actual as built VGC. Once this is obtained for the lightship its fairly easy to generate a fairly acurate GZ curve if you have the lines plans.

Without an acurate VCG it becomes just guesswork. And VCG is something that no amount of reasonable calculation can acurately determine on most complex crusing boats, though it may be possible to rely on just calculation on a simple racing boat with very careful weight control.

Using the midship section for an estimate for GZ's however is about +-10% accurate for conventional hull forms, giving usually a bit bigger result from the reality thus safe for calculating rig loads. This means the Cp of the hull doesn't chance too much with heeling.

Are you saying you multiply the GZ for the midships cross section by the CP to allow for the missing bow and stern sections? I'd be interested to see an example of the calcs and how it compares to the same hull using a more conventional approach.

True, importing the hull into Freeship give better results but there's much more to go wrong with the center of gravity with an older boat.

Regarding the COG, thats what the inclining test is for. It would be nearly impossible to retrospectively calculate it from individual weights and KG's for an existing boat. Much easier and more reliable to do an inclining test to find VCG.

Its pretty quick to knock up a simple hull in freeship if you have the plans or offsets. Even quicker to knock it up in Hulls and import it into freeship.

For the OP, I pretty much agree with B, it sounds like the rig has stood the test of time. I'd focus on making sure the chainplates are sound, the tangs and tang bolts are new, and that it is raked and tuned properly.

The boat looks like it might be a touch tender with that big deckhouse. Adding weight aloft is never going to be a good thing for most yachts, and can make them dangerously tender if taken to far.

You could end up down a pretty deep rabithole of facinating (but probably unnecessary) theory if you try to calculate the rig loads with any decent accuracy yourself, its pretty easy to get some quick checks, but hard to do accurate ones.

Saying that Id be very interested to see what you come up with by doing an inclining test. And running some numbers.

I might be able to plug some of the rig data into my spreadsheet to see how the loads compare.

[Eastward ho 24]

Yes that's the boat I'm talking about, And thank you to all for your inputs. We had a blowout tide all weekend 24 hrs dead low still, I've been listing in the mud. I think I need to try some of the methods suggested. Even if to get ac general idea.