ballast ratio is an expression of...what?

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Shearwater

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It seems from boat reports and adverts that a high ballast ratio is a Good Thing..... i.e. ' a stiff 53% boat'...etc. Wud appreciate an explnation of what it is a ratio of. In May's PBO a report on the Hunter 23 said the owner added shingle in bags to settled her deeper as he regularly sailed solo - before I haul bags of stones down to the marina, does it follow that extra ballast is necessarily beneficial to the stiffness of the boat (as in reducing heeling when solo)?
 
Ballast ratio is the percentage of the total displacement of the vessel that is ballast. On it's own it doesn't tell the whole story. Partly the stiffness depends on the ballast ratio, but also depends on how low the ballast is in relation to the point about which the vessel rolls. Lever arms and all that! A high ballast ratio suggests the vessel will stand up to her canvas in a blow, but it can also lead to a choppy motion and put unnecessary strain on the rig.
 
It's a pretty crude measure of a boats stiffness. Stability can depend on where the ballast is carried and can also be influenced by hull shape. It's just one measure amongst many of a boats character.
There's an article that covers the various formulae on the Salty John website, under 'cruising resources' which may be of interest.
 
As tugboat says it gives you an idea of how the weights is distributed between keel and boat. But it does not give you an idea of the hull shape which is very important as well. Nor does it tell you the position of the weight in the keel.

Just take the example of say making a boat out of cellophane. It would need a very large amount of ballast to sink it to the designed displacement. Therefore the ratio would look really stiff. But then it is only cellophane. It is therefore also a measure of how little you are buying.

So you can see why marketing types think it is so important?
 
I would be very carefull about adding extra ballast to make your boat stiffer. You will be putting extra strain on your standing rigging and mast, IMHO the rig is set up for the boat with the ballast and boat load in the calculation. Just My opinion
 
A keel boat needs to have two factors addressed. The first is most noticeable and that is stiffness. ie resistance to heeling with wind on the side. It is significant from dead flat to medium mounts of heel.

Stiffness is achieved by hull shape reacting with the total weight of the boat. So really flat bottomed hard chine will force the boat to float on the sides of the hull and the total weight then presses the centre of thee hull down causing a righting force.
A catamaran is an extreme example of the shape giving the stiffness.

A heavy weight at the bottom of the keel works like a pendulum. Initially it does not have very much righting force until the heel gets to around 45 degrees when the mass on the bottom of the keel tends to right the boat. This is the second factor ie the ability to self right the boat from knockdown.

Now the two righting powers do interact. ie the weight on the bottom of the keel adds to the total mass of the boat to provide the stiffness even at very small heel angle.

Ballast under the floor will add somewhat to the self righting ability but is far less valuable than bottom of the keel ballast. But if we are only concerned with stiffness then ballast under the floor is nearly as good as ballast on the keel bottom.

The selfrighting ability of a boat is also dependant on hull shape and even the superstructure shape. ie consider the shape of the boat in the water when it is on its side. A large wide cabin becomes part of the hull and if the centre of bouyancy then is like near the windows then the keel bottom ballast as well as the in hull ballast all work to varying degrrees to make the boat stand up again.

So stiffness is what we want for comfort and self righting is what we need for safety but hopefully never need.

The ballast ratio then only tells part of the story. There should be curves published describing the self righting force at various heel angles. (through to inverted).
A catamaran has huge stiffness 0 ballast ratio and no self righting ability.
A round hulled boat may have wonderful self righting ability with a huge ballast ratio but heel at the slightest puff leaving you sailing on your ear all day. It all depends......

My little trailer sailer has a 100kg lift keel and about 200kg under the floor. It has adequate self righting at least from 90 degrees. I have tested it many times with the aid of a recalcitrant spinnacker.
But the boat has a relatively round bottom so that it is quite tender. It requires crew to sit on the gunwhale for any sort of performance in any wind. So movable crew is the best ballast. olewill
 
Totally agree with all you say olewill as usual. I would only add that a boat with high ballast ratio is more likely to have a high angle of vanishing stability (i.e. the angle at which it will self right) than one relying on its shape or like a dinghy, relying on crew hanging out of the right side. Your example of the cat is excellent. No ballast, very stiff indeed, but angle of vanishing stability is quite low.
 
Agree with others comments but also beware of what is the ballast.

On most AWB's the ballast ratio is the weight of the Keel v Total Weight of Boat but when I looked at an X boat and concluded that the hull must be very thin I found out that the internal steel stiffening U frame is added to the ballast. Now on most AWB's this internal stiffening is made up of GRP ribs and stringers and is not part of the ballast weight.
 
Thank you for the explanations - I now understand a lot more. I won't add shingle, I shall take more friends than usual and get them to lean out over the sides (well, the winward one anyway!) to stiffen the boat. Thanks partic to the note from olewell and something on Salty John's website has gone onto my must have list.... the tiller tamer seems excellent value and just what I want!. Tks again.
 
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