carrying capacity v stiffness

[kidnapped]

I notice when wandering round marinas that most yachts are v. 'light' in the water, ie. they are floating well above there 'waterline'. I assume they are all like mine, very lightly loaded versus design load. Could some of this 'spare' capacity be moved to the keel as extra ballast thus significantly increasing the righting moment? I am also unaware of any yacht brochure that indicates the maximum design load capacity ... or have I just not been looking properly?

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[Twister_Ken]

In a modern yacht, engineered down to minimum scantlings, putting extra weight in the keel might overstress frames and floors, and might also expose rigging to strains it was not designed for (stiffer boat heels less, so sail is left up longer before reefing)

(I did say 'might',)

<hr width=100% size=1><A target="_blank" HREF=http://www.writeforweb.com/twister1>Let's Twist Again</A>

 

[qsiv]

A good current example is the new Mari Cha - she can be fully powered by either canting the 10 tonne keel, or by taking on 10 tonnes, of water. The crew know that utilising both options together (at full capacity) will, as the skipper puts 'pull the rigs out of her'.

I would guess that most boats aren't unduly 'light' on their marks, but due to the modern desire to build caravan like interiors the topsides have become huge expanses. In order to minimise this, antifoul has been taken higher, and the boot top higher still. In some cases one boot top isn't enough (X Yachts use about 3 blue stripes). At the same time designers use cove lines / cove stripes (Najad, HR, Contest) in order to 'bring the deck edge down', in visual terms. The visual trick does work, and the topsides look less cliff like.

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[Mirelle]

I think that is almost certainly the right explanation!

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[Guest]

In most cases the Naval Arch. designs the boat to a particular scheme / plan and palying about with it is not normally a good idea.
The main area of concern would actually be the stiffness would increase mast / sail load on the rigging and fastenings. Not from lack of reefing as another has put- but in all fairness depends on the skipper ! - but actually on the boat itself not heeling to the sails as designed.
The second area of concern would be the mounting and strength of such addition. Where would you put it ? Mould it to the keel ? This would most certainly strain the keel fastenings. Place in the bilge ? Safer but high up now and the ratio of depth to weight reduces the moment dramatically and starts to negate the whole exercise.

Simply put - my advise is to forget it !! Use the spare carrying capacity which nearly all boats have to carry that beer !!!! (not forgetting the wine for her ladyship !)


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[maxxi]

Have seen this done on two boats, an Endurance 37 & a Sunbird 33. Both boats had an additional moulding added to the keel containing lead ingots - one & a half tons and one ton respectively. In both cases this was declared a great sucess.

Both boats were 'proper boats' having encapsulated keels (wait for the flak) and both were owner built & rigged by professional boatbuilders, and had considerable redundancy in the rig and terminals.

Interestingly, the Sunbird's LOA & waterline later grew by 30" by virtue of a lengthened cockpit and whilst the accommodation increase was a considerable sucess, the boats performance on the water improved astonishingly.

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[AndrewB]

Puzzled.

How do you know they are light, or floating high? I'd be interested to know if there is a simple way of telling.

Incidentally, most owners carry the antifouling a little above the waterline.

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[Peppermint]

Thoughts in no particular order.

Weight takes power to move it. So if you add weight you need to add drive.

Stiffness isn't everything and some of it comes from buoyancy and hull shape.

Get the crew and gear onboard and see if it settles her down nearer the mark.

Designed waterline is not necessarily where the paint goes.

Working on the priciple that many designers get it right, I've sail precious few boats that didn't go better for taking a bit of casual weight out.





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[AuntyRinum]

Ah but where is the waterline? Is it the top of the antifouling defined once a year with masking tape, or is it a mark defined by the builder?

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[Jacket]

Sounds the right answer. Also, I don't know whether modern boats are epoxied below the waterline from new nowadays, but if so, thats normally extended several inches above the waterline. The antifouling is possibly extended up to the end of the epoxy, to hide the faint 'join line' that is otherwise apparent.



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[oldharry]

Good point Brett - the designer will have drawn a 'Designed Waterline' - but builders will have taken the a/f and bootopping quite some way above it, which makes the boat look as if it is riding high.

As we all know to our cost, particular types of fouling thrive at the waterline anyway.
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[MainlySteam]

Re: Puzzled.

<<<How do you know they are light, or floating high? I'd be interested to know if there is a simple way of telling.>>>

Generally, but not always, for a modern shallow canoe draft yacht one can get a good approximation of where its design waterline is by:

At the bow, which is frequently quite plumb, the design waterline is usually at the knuckle (which is often quite sharp) where the stem turns into the keelson.

At the stern, the design waterline is usually just such that it submerges the after end of the spade rudder.

For older shapes I don't think it is really possible to tell. However, for a 40 foot yacht say, with a large waterplane area it takes a lot of weight to increase its draft beyond another inch or two (assuming the design WL includes full tanks, depart port type condition). From memory, when our own boat of around 35 or so foot WL length was designed, 1 tonne increased its draft by about one and a bit inches, and that is about what we observe. So, unless the designer got it wrong, which would not be likely with a production boat, or it really is grossly loaded a larger yacht will be floating quite closely to its design WL except for trim issues. Trim is easier to affect by load, of course, especially at the bow, so because of load distribution the bow may be down with the stern up.

John

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[AndrewB]

Oh, that\'s all right then ...

... I can't see any yachts in our marina with their rudders sticking out the water.

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