is a bit of a saying hanging round the net at the moment that it is OK
to extend or shrink an existing plan by 10 or 15% and it will be OK.
Or that it is OK to shrink or expand all dimensions by 10 or 15 percent.
THIS IS COMPLETELY WRONG!!!!!!!!
Here's why ...
The above statement is so completely wrong that anyone stating it and holding it to be true should wash their mouth out with cabosil.
What it should say is that an old time boatbuilder, apprenticed in the trade as a young man and now with 30 years of building boats can make the decision that such a shrinkage might be OK for a PARTICULAR hull.
Can I show you all what it does if you decrease the dimensions to 85% of the original.
Carrying capacity will go down 0.85 cubed = 0.61 - you lose 40% of the carrying capacity. - so your 14ft boat which carried one person now only carries 0.6 of a person. Or that carries two people will only carry 1.2 people
Stability will go down 0.85 to the 4th power = 0.52 - so you lose half your stability. So the O.6 of a person is very likely to get very wet.
And if it is a sailing boat the sail area is now 0.85 squared = 0.72.
So stability has reduced by 50% but sail area has only been reduced by 28%
Still with me!!!!
My sense is that the quote about increasing or decreasing length is a misquote.
THE ACTUAL ORIGINAL QUOTE IS SOMETHING LIKE:
THE LENGTH OF A BOAT MAY BE INCREASED BY 10 or 15% without changing the depth and beam without damaging the performance and handling of the boat and has some capacity to improve both.
And Bolger was talking about work that he was doing so he could adapt sail areas, mast positions and gear distribution. It's not for anyone to just go ahead and do it for themselves even in the case of motorboats.
If any of you have read my article in the most recent Amateur Boatbuilder magazine - you will know exactly why I am over-emphasising this point!
A client took it to heart with one of my Goat Island Skiff designs. It was unsailable and the client was asking me why (without telling me he had shrunk it initially). You can read it here
So if anyone quotes that one at the top to you ... ask them how they know it is true!