How can I find out how stable my boat is?

NUTMEG

NUTMEG

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www.theblindsailor.co.uk
Without doing a practical test in F8!

My old Westerly had a ballast ratio of 48%. Which meant, I think, that she was very stable and would not capsize due to wind alone. With her two great lumps of cast iron hanging off the hull one felt reassured when pushing her a bit.

Nutmeg, being a mk1 Cornish Crabber 24 (not the new posh one you can stand up in, rather the old flush decked one that my wife hates), being a bit of a pet, does not get sailed as hard, but I have been wondering how stable mk1's are. I have had the lower rubbing strake under water in a gust but how far can they be pushed I wonder? Anyone got any ideas? Could they lay on their beam ends? Would they right if they were knocked down

View attachment 35931?

Is there any way of finding out?
 
Line to the mast head and pull down to induce heel. Once she gets to the point where you are finding it easy to induce more heel then stop as you're running out of righting moment.

Or get the GZ curves.
 
There is a set of calculations you can do to find the theoretical point of no return. I would have to search my attic for the college naval arc books. I would imagine there is a solid chance that the manufacturers might have done them though and might give them to you.
 
It's not just the ballast, but also the shape of the hull, that gives stability. Probably most informative would be the advice of other Crabber owners who have been out in rougher weather (I've only sailed one in moderate conditions).

You can capsize any boat, including a Centaur, but I would expect a Crabber to stand up to quite a lot and not be an easy boat to capsize.
 
Why not ask Cornish Crabbers? or the designer Roger Dongray to see if they have the stability calculations?
 
I gather that all records and paperwork on the old mk1s was lost/destroyed years ago. Will try a search for Mr Dongray though, that might be a starting point. I can see how tipping her from a rope to the masthead would give a good idea and boost/reduce confidence!

I have asked on the owners forum but deathly silence!

I have read of one that crossed the Atlantic twice, then the Indian Ocean and got as far as New Zealand before it sunk. Not sure if that's good or bad! Very big cockpit for oceans I would have thought. Several have circumnavigated the UK and crossed the channel so they can't be that dodgy, can they?
 
Stability is a complicated subject, in which ballast ratio is only one variable. The hull design of your boat probably relies on form stability more than ballast - which at a guess is probably only around 30%. On the other hand it has a low aspect rig with the area well spread out along the length, so it is unlikely to capsize as a result of wind alone. This is an indication of its ability to carry sail, and of course it is also affected by your centreboard which resists heeling when down (as well as restricting leeway). The other side of stability is the ability to right after being knocked down which is not only influenced by basic stability, but by what is known as the downflooding angle - that is when water starts getting into the boat. Your big open cockpit and low freeboard works against you in this respect. You can read more detail on stability on the RYA site which gives a good explanation of the issues.

In practical terms, the hull form and rig of your boat are based on well proven ideas that come from the world of working boats - long before tall rigs, narrow keels and pointy front hulls became popular. Not dissimilar from Slocum's Spray and a whole host of small fishing boats all round our coast and N America that are well proven at sea. You need to sail them flat - they don't like high angles of heel - hence the low rigs, but are buoyant and the long based sail plan allows easy (relatively) adjustment of sail area for conditions. However, they don't fare well by the measures that apply to modern hull forms.
 
In Oz the Yachting Australia authority give a lot of detail on stability requirements for monohull boats for ocean racing.
http://www.yachting.org.au/site/yac...3-16 Special Regulations Part 1 Keelboats.pdf
look for about page 63 and 64.
My little boat was tested by the pull down method for stability. No one on board. She floated quite high on the gunwhale.
It was tied up in the marina by bow and stern at rightangles to the wharf. The mast was pulled down by a halyard until it was near horizontal. A load cell was used to measure force required to hold the mast down. A spring ballance would do. Being a lift keel trailer saailer mine only just made the min requirment at 30kg at the hounds. The test is fine provided you do not get water into the cockpit or cabin when down.
This test of course does not reflect true stability situation when sailing ina light boat under 1 tonne where a crew of 4 can add a lot more mass holding the boat down in complete knockdown but also provide more righting moment when nearer upright. it seems to be a reasonable test giving some useful benchmark.
good luck olewill
 
Long ago and far away, someone gave me a spreadsheet that does some sums and works out interesting things for sail boats.

P.M. me with an email address and you can have a copy - just have to swap my numbers for a heavy Westerly and see what sums your boat brings out.
It contains macros, but I have never had any reason to suspect that they are anything other than what they say they are. Provided without warranty and for you to interpret any results for your own use.
 
Your Cornish Crabber has a hull form which is not dis-similar to a typical fishing boat.

Here is a very quick, easy and effective way of getting a pretty good idea re the stability characteristics of a fishing boat.


AA-SafetyatSea-Stability_zps816854b2.jpg



This is a system that is used by the FAO (and I obtained a copy of this document from them 12 years ago, when they were doing workshops on fishing boat design in the Caribbean)

Yes, it does make assumptions, and it is very generalised, but it is a useful 'first step'.

Basically, in general terms the longer the roll period the worser the stability - think of cross channel ferries, well laden, that seem to take forever to roll from one side, recover, and then go back the other way.
 
Honestly, the breadth of knowledge on this forum is staggering and the willingness to help fellow boa toners likewise. Thanks to all of you, some fascinating stuff there.

PM sent and suggestions will be followed up.

Cheers chaps.

Thank you.

Steve
 
A late addition may be of interest. There is an AVS estimation formula at http://dan.pfeiffer.net/10m/avs_calculation.htm. It is attributed to Peter Bruce in the "30th Anniversary edition of Adlard Coles Heavy Weather Sailing", with (i) a note of caution that it was not implemented fully (wrong hull draft) on a US Sailing page calculator (link now defunct) and (ii) an idea of the magnitude of possible errors.

HWS was first published in 1967, so the 30th anniversary would be 1997, but there was no UK edition in that year - perhaps the reference is to a US edition. In my UK 5th edition (1999), the same formula is given (in Chapter 2, 'The stability of yachts in large breaking waves', by Andrew Claughton) with the advice that it over- or under-estimates AVS by up to 10-15 degrees in some cases, and will not work for 'unusual vessels'. I do not know if later editions omitted it, or further discussed it.
 
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LittleSister said:
You can capsize any boat, including a Centaur.
Click to expand...

Can you, though? Not what I mean by capsize. I would want any boat with a bed on it that I'm sailing to be able to get knocked down with mast just about touching the water, and bob up by herself.

We got nearly that far once, with lee cockpit coaming well under, and close-hauled boom end in the water, and we never had a doubt that if we just held on she'd recover. I have seen a keelboat sink from a similar position, and I met a man who's cabin centre-boarder capsized (by which I mean she ended up on her side & stable) and who then required a rescue.

Will most of these small cruising centre boarders (shrimpers, memories, minstrel) self-right from a knockdown?
 
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jac said:
Line to the mast head and pull down to induce heel. Once she gets to the point where you are finding it easy to induce more heel then stop as you're running out of righting moment.

Or get the GZ curves.
Click to expand...

I once tried to test this on a small centreboarder, gave up as I was unable on my own to pull her down more than about 30 degrees from upright. Of course a year later I had the spreaders in the water whilst under sail- they even picked up seaweed. It was a lee-side-of-cliff williwaw - gust of wind blowing downwards not horizontal. She righted.
 
Will most of these small cruising centre boarders (shrimpers, memories, minstrel) self-right from a knockdown?[/QUOTE]

There's the rub. My westerly got laid so far over in a freak gust to the extent that green (well brownish, east coast) water poured over the coamings. Mast must have been pretty close to horizontal. What would happen to my old Crabber in similar circumstances. Although as has been pointed out, gaff rig has a lower centre of effort.

All fascinating stuff to ponder while nothing on telly!
 
jwilson said:
I once tried to test this on a small centreboarder, gave up as I was unable on my own to pull her down more than about 30 degrees from upright. Of course a year later I had the spreaders in the water whilst under sail- they even picked up seaweed. It was a lee-side-of-cliff williwaw - gust of wind blowing downwards not horizontal. She righted.
Click to expand...

Did it with a sonata (a long time ago ) to get the centreboard clear of the water. (Long story) I have aphot with 3 of us on board and seem to remember we used a spinnaker halyard backed up with main halyard and struggled to winch her down. Even had a large boned friend hanging off the genny halyard at maximum beam to help.

It did make us very confident re stability.
 
surely every decent ballasted boat would be designed to at least right itself from the horizontal? what sorts the men from the boys is how much further over the boat will go and still come upright (AVS) And as I understand ballast ratio is only a small part of that. Excessive beam is the guilty party to that equation.

I would also be more concerned about capsize from breaking wave action rather than a gust of wind. one can always luff or dump the sheets. again its not just ballast ratio, inertia comes in to it, as does tripping. for example, your centreboarder may well be better placed with the board up rather than down. that way the hull if pushed sideways can skit along the surface rather than trip over. Its like pulling the daggerboard up a bit to get through a squall in a dinghy. your leeway suddenly sucks but the capsize moment is greatly reduced.

suggested further reading

79 fastnet enquiry
'seaworthiness the forgotten factor' -marchej

but in the meantime, I wouldnt worry too much!! Im sure your crabber will take good care of you!
 
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