STABILITY QUESTION

saltwater_gypsy

saltwater_gypsy

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Now back in Scotland . Boat for sale
www.saltwatergypsy.com
The boat berthed across from me is HEAVILY overloaded .

Its an Achilles 9 and its draught should be 5ft but is currently 6 feet!! The owner swears that the heavy load increases the stability but I'm not so sure. The load lowers the centre of gravity in the water but also raises the centre of gravity of the boat itself. There are probably other safety issues.
Any experts out there to give guidance before he sets off for the Azores?
Photo:
http://i300.photobucket.com/albums/nn14/hectou/IMGP1669.jpg
 
My back of the envelope figures give a weight of well over 2500kgs (over 2 tons in old money) to sink it 30cm (1 ft). Not only will the extra wetted area reduce performance, but that amount of weight high up is not good! BTW the photo suggests it has not sunk that much. The only way of really knowing is to compare the design LWL with the actual (or the reciprocal the midships freeboard).
 
I think you are very right to question the stability. If he has added weight to this extent it is unlikely that he has added it below the dynamic centre of bouyancy. Basically this is the centre point about which the boat rotates in the water when it rolls. If the added mass is above this point it will reduce his angle of vanishing stability quite drastically. It is also important to calculate this additional mass relative to the centreline athwartship and logitudinally. if added to the centre if the boat in simple terms it has no significant effect on pitching moment but if away from the centreline, even if balanced fore and aft it could negate any reserve bouyancy in the bows and stop the boat rising to a big wave. This in turn could lead to a pitchpole. A third consideration quite apart from this is how secure the "ballast" is. In the simple case of lots of tins of beans and Fray Bentos pies, they may be balanced now but if the starboard beans decide to migrate to port in a blow it could well be the death of him. Frankly he is being very dangerously stupid and needs to be educated out of it before he kills himself and anyone else stupid enough to sail with him.
 
I doubt that the Achilles opposite you is a full 12" down on his designed waterline.
Here is a photo of a sistership for sale on Yachtworld.
http://www.yachtworld.com/core/listing/d...hoto=1&url=
They say that her displacement is 3,200 kg, and her draft is 1.70 m. (ie 5' 7").
But who should you believe re displacements? /forums/images/graemlins/confused.gif
Boatshed also have one for sale, with a draft of 5'6", and a displacement of 7,000 kg (!) at http://www.boatshed.com/achilles_9m-boat-22676.html
 
I would guess it looks about 3 inches down on her normal waterline, which is still a lot of extra weight. Mind you if most of it is food and water, she will gradually get lighter as it is expelled slowly through the toilet outlet. /forums/images/graemlins/laugh.gif
 
ADDENDUM
I got the basic draught from March YM p98. (1.52m, 5ft).
The owner told me that the boat now draws 6ft which I believe because it is WELL down on its lines. The boat is full of stuff!!!
The basic question which concerns all long distance cruisers in small boats is "does this degree of overloading give more, or less stability?"
 
It depends entirely on where the weight is stowed.

The increase in draft will raise the CoB. If the additional weight is stored low down (below the CoG) then it will improve stability. If it is significantly above the CoG then the stability will be reduced.

So provided the additional weight is stored low down it would not have a significant affect on stability.

In terms of stability adding a Radar, or furling genoa, will probably have as much impact
 
I know a couple who have been living on their She 31 for the past 10 years or so. And they literally have everything but the kitchen sink on board, and hence they have had to raise the waterline by at least 8", perhaps more.
The poor boat is (even more) wet when going to windward as a result of such a dramatic decrease in freeboard....
Yet they crossed the Atlantic 9 years ago (from England via the Canaries), and since then have been all around the Caribbean, incl a trip up to New England via Bermuda, and encountering some pretty horrible weather along the way.

Re the stability question :
The most basic stability formula is BM = I / V
where BM is the distance from the Centre of Buoyancy to the Metacentre,
I is the 2nd moment of area of the waterplane area, and
V is the volume of displacement for this waterplane area.

As the draught increases, the volume of displacement will increase accordingly, however the I of the waterplane area is probably going to increase at a much slower rate, unless the hull has extreme flare (especially amidships) at the load waterline.
Hence it is usually the case that the BM will decrease as the draught and displacement increase.

We have KM = KB + BM, and KM = KG + GM

where :
KM is the height of the Metacentre above the keel base,
KB is the height of the Centre of Buoyancy above the keel, and
KG is the height of the Centre of Gravity above the keel.

And I am getting all tangled up trying to explain the rest, so maybe it is easier to let the Wiki explain it here http://en.wikipedia.org/wiki/Metacentric_height - they can do it much better than I can!

So, to summarise, as you load up the boat, all else being equal, the GM will probably decrease.
The righting lever GZ will also decrease, as it is a function of GM. And hence it is quite probable that the angle of vanishing stability (AVS) will also decrease a bit. And the boat will become less stable.

This is assuming of course that all of the added weight is securely lashed down and cannot move. If it is not lashed down, then if the vessel suffers a knockdown (or worse), then all of this ballast will shift, and exacerbate the situation.

Edit - Addendum : Here is a stability story for you, re the transatlantic rowing boats.
In one of the races (I think 2 years ago) quite a few boats capsized in rather rough seas.
They are supposed to be self-righting in the event of a knock down (or even a capsize) so long as the 'whaleback' type bow and stern buoyancy compartments are closed (in the same fashion as the buoyant superstructures on the RNLI lifeboats).
However some of the crews found that they were not self righting if the crew were sheltering inside the cabin.
When the boat gets knocked down or capsized, the crew weight inside the cabin ends up on the cabin side or even the roof (as it is 'portable' ie not lashed down), thus improving the 'stability' in the capsized condition.
And if the boat will not self right in this condition, then the crew have no option other than to flood the cabin and swim out - which then makes the boat even more stable upside down.
 
people usually answer stability questions in terms of heeling moments, and certainly thats what matters if the only pressure is from the sails and the keel cant slide sideways. But in real sea conditions there are lots of other effects, some more important. For example, its often said that once you get above about 40 ft, a modern cat is much less capsize prone than modern monos - the reason being the energy needed to invert is way greater. And energy transfer from breaking waves is a major cause of capsize.

Following this logic, I've read modern designers commenting that the stability of a lightweight modern mono design can be greater than an older heavy one, because it gives to the waves. Thats what would worry me about the Achilles. The extra weight will as like as not have lowered the c of g, but it will also have increased the boats inertia and therefore reduced its "give" to the waves. Extra push on the sails, more sideways resistance from the boat and over she goes.

Plus of course - is the structure designed to take that weight bearing in mind that whatever it was capable of when new, it will be less now.
 
Without doing a lot of measurment and simulation it is very difficult to say whether the boats stability is better or worse than as designed, al we really know is she is almost certainly being operated outside her design requirement, which may be dangerous and may not. Mind you many would suggest because we don't know that is in itself dangerous. Relatively simple inclining tests will show what the current GZ is which can be a basic guide thow more work would be needed to calculate the current angle of vanishing stability and that would not be that cheap which will also have changed. On the otherhand an experienced sailor may well be able to get a feal for her sea worthiness by sailing her in moderately testing circumstances. The real bottom line is go outside the original design spec and you are in unkown territory.
 
My wife and I have lived on board an Achilles 9 metre for 9 years and come through the French canals twice. The draught is 5ft 7ins. The retrousse stern should be clear of the water or the speed of the boat is badly affected. I think he should do an audit of what he is carrying if there is a concern it is surprising what some boats carry and never need.
 
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