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!

2 thoughts on “Is it OK to Stretch or Shrink an Existing Boat Design?”

  1. I have a kit for the Eureka 155 canoe, carefully cut by JO Woodworks. As soon as weather permits, it will be assembled. I’m an old guy, happy to have such a project. I’m curious, if I cut 6” off the butt ends of each of the ten panels, the canoe would be one foot shorter. What side effects (paddling, stabilitet, etc.) would that cause?

    Reply
    • Big reduction in stability.

      The middle of the boat is flat with a lot of volume.

      The ends are narrow – and a boat built to that shape for its full length will fall over immediately.

      So cutting out the middle will compromise stability … big time! Same for reducing length by scale – it has a big effect and also means the panels won’t meet up correctly.

      Good question to ask!

      Reply

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