Is it OK to Stretch or Shrink an Existing Boat Design?

A bit of a hobbyhorse here.

The picture right is of my Goat Island Skiff (GIS) design. A nice, quick sailing, spacious, but easy to build sailing skiff. People tend to think it looks pretty nice too.

A fellow who built one of the GISs wanted to shrink the boat because of limited building space and that someone gave the poor fellow the advice that …

“Generally it is OK to shrink a boat by a factor of 10 or 15%”

So my customer went ahead only to find out that this is really, REALLY bad advice.

Also the sailmaker changed the sailplan a bit – but didn’t bother to decrease the sail area for the much shorter and less stable boat.

The poor builder had a terrible time trying to make the boat work.

In the end I reviewed the situation and made some relatively simple suggestions about getting it all to work OK.

Just a bit too frequently lately I’ve read various postings on the net along the line of:

“Generally it is OK to increase or decrease the length of a boat by 10 or 15%”

It is bad advice for a motor boat and really, really bad advice for a sailing boat.

How does the person making the claim know how close the designer has gone to the limit on a particular boat? And will that line be crossed with the change.

Also are they allowing for the different relationships of the parts as in a sailing boat. Centres of effort, gravity, buoyancy, lateral resistance.

Do they know that by reducing the size of a boat by 15% all round the stability is reduced to 50% of the original?

So I’ve added three things to my webpage.

1/ A preamble to the problem
2/ An actual case study of someone who built one of my Goat Island Skiff (GIS) designs but changed a number of things because someone advised that it is “an OK” thing to do.
3/ Some advice about how to make the changes more safely in some boats.

The articles are here on my FAQ article

Some experts deserve a big kick up the bum.

And that is the end of my rant!

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