how scale can you get

[mark69Participant @mark69]

hi all lm planning to build a 1/5 scale one design keelboat at about 6ft long with a beam at centre of 14″ the full size has a 1 tonne keel
for this to sail correctly would l need to change the point of ballast
thanks for any help mark

[mark69Participant @mark69]

working from full size plans

[captainslogParticipant @captainslog]

Hi Mark,Surely iff the full size sails perfectly well with the ballast where it is why would you want to change it on the model? Its basically the same boat but scaled down. Personally I would leave it where it is and then think about moving it when you do the initial sailing trials. You will have a much better idea then whether it is in the right place or not

[mark69Participant @mark69]

Hi yes that’s what l thought but having never build a modal yacht before how would you scale down a 1 tonne keel ,do just devide by the scale or keep adding weight until the correct waterline is met but if your saying the full size sails ok so copy this why do some models have extended bulbs on there hulls ?
thanks …….. Mark

[Colin BishopModerator @colinbishop34627]

You have two separate issues here Mark.

Firstly, when you scale down a model the weight (or displacement) reduces by the cube root of the scale factor. So at 1/5th scale the model displacement wil be the original boat displacement divided by 5x5x5 or 125.

However, the area of the model reduces by only the square root so the sail area of the model will be the sail area of the original boat divided by 5×5 or 25.

This of course means that on on a model the sail area will be very disproprortionate to the displacement compared with the original vessel with l/25th of the sail area sitting on 1/125th of the the original weight. You can't add weight to the keel to offset this as it would drag the hull down below its correct waterline so the normal practice is to increase the 'mechanical advantage' of the keel by lowering the keel weight to help balance the increased relative pressure on the sails. And that is why most scale sailing vessels have external detachable keels even when the original boat carries all its ballast within the hull. They wouldn't work otherwise, unless reefed right down all the time.

Another way to explain it is to imagine a see saw balanced in the middle. If the weight at each end is the same then it will stay level. But it will also stay level if you reduce one weight but move the point of balance towards the original weight at the other end. Basically this is what you are doing with your model yacht with the sails and keel acting as the weights which are balanced around the centre of gravity within the hull.

There are of course other scale factors involved such as the wind strength on a model usually being proportionately much greater than on the real thing. All this is why you can see photos of sailing ships carrying clouds of canvas that no model could possibly match unless the keel is cemented into the bottom of the pond!

Hope this explains it a bit!

Colin

Edited By Colin Bishop, Website Editor on 21/11/2012 13:45:55

[Paul TParticipant @pault84577]

Colin's analogy of the see saw is one of the best explanations of this phenomenon that I have ever read.

Mark69: Your problem will be the size of the model which at 6ft will act more like a sailing dinghy than a model boat. I would suggest building the model with the correct scale sail area and then using removable ballast low down in the hull to trim model to suit and employing the keel simply to resist the centripetal force of the sails.

Paul

[mark69Participant @mark69]

Hi Colin and Paul thanks for your reply. great to get an answer l can understand in model terms thanks
Paul , this model this will be a true scale as it’s modelled on the full size of which l build so the sails mast, boom and rigging will have to scale but as we carry a 1tonne iron keel l will be using lead .
once lve done more calculations l will put some pics up
…. Thanks again …mark

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