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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About boatmik
On the "round Australia trip" I found myself employed by a tiny business in Adelaide - Duck Flat Wooden Boats in Adelaide.It was an eye opener - It became clear that one could build a boat for a fraction of the cost of current racing boats.My ideas hinged around high performance, easy building, fun to sail and reasonably cheapToday Storer Boats are built in all countries and we have active groups on Facebook for the following groupsGoat Island Skiff Open Goose Storer Boat plans Really Simple Sails