Yacht ballast

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21 January 2011 at 16:07 #29530
roy nottrodt
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@roynottrodt88198
I have a double size model of a star class yacht built from the Model Boats plan. The original model was 22″ but mine is 44″. How do I calculate how much lead to add to the keel, do I simply double the original amount or is there a formular to follow?
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21 January 2011 at 16:07 #3691
roy nottrodt
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@roynottrodt88198
22 January 2011 at 10:34 #29551
Ian Gardner
Participant
@iangardner62867
I think it’ll be original weight x2x2x2! Goes up quite steeply when you increase the size of models.

Ian
22 January 2011 at 11:05 #29553
Colin Bishop
Moderator
@colinbishop34627
If you square the dimensions then the volume is cubed!

It’s this effect in reverse which makes it hard to scale down sailing ships as the dispalcement goes down by the cube root but the sail area only reduces by the square root so the smaller model is proportionately over canvassed.

Colin
22 January 2011 at 11:25 #29554
Bob Abell 2
Participant
@bobabell2
Hello Colin

This argument has always confused me!

You say…….”If you square the dims”….etc

But Roy has only increased his model by doubling the original length?

I`m confused already!

More clues needed, please

Bob
22 January 2011 at 11:45 #29555
Bob Abell 2
Participant
@bobabell2
If Roy doubles the sail area….he just needs to double the ballast

Are you saying this is incorrect?

Bob
22 January 2011 at 12:09 #29556
Colin Bishop
Moderator
@colinbishop34627
If you double the dimensions then you will triple the internal volume and thus the displacement. So you would need to treble the ballast. That’s how I’ve always understood it!

Also, think of it the Archimedes way – as you get older you usually get fatter so you need less bathwater to reach an acceptable waterline although I thiink the iceberg principle comes in here too somewhere, 90% submerged and all that…Just make sure you can still reach the plug.

Colin
22 January 2011 at 12:13 #29557
Ian Gardner
Participant
@iangardner62867
When increasing the size of a sailing model the sail area increases by the scale factor squared and the displacement by the scale factor cubed. It works in your favour when increasing the size but not so good decreasing. Vic Smeed expains it very well in one of his books.

It’s quite fun to play around with figures and see what weights models would come out at when size changes.

Roy’s boat will have a sail area 2×2 the original and a displacement of 2x2x2 the original weight.

Ian
23 January 2011 at 14:39 #29568
Telstar
Participant
@telstar
Hi Ian You have explained most elequently Froud’s rule, discussed and denigrated by some ‘experts’ elswhere on the forum.

Bravo, your dead right though

Cheers Tom
24 January 2011 at 10:06 #29580
Ian Gardner
Participant
@iangardner62867
Hi Tom,

After many years of thinking about (not necessarily getting round to building!) model boats this seems second nature to me, but I didn’t know it was Froud’s rule. I spend more time than is good for me working out what effect changing the size of models would have- many people I know think I should get out more!

Regards,

Ian
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