Scaling a Boat Design - slighty larger or smaller)

[Louie Campbell]

I read over the thread "How to Shrink a Skiff Plan" and want to ask a question about something Terry Haines touches on in post #45. This is not exactly a hypothetical question; for my third boat I plan to adjust a semi-dory design just in this fashion.


If I'm working from a boat's offset tables and spacing between mold sections (that is all three dimensions) and if I create a new set of offset tables and mould spacings by multiplying every offset (width and height) and every mould spacing by the same factor (say, 1.10 for a boat 10% larger in every direction), will the new boat perform in a fashion similar to the original boat? Will the new boat perform in a 10% heavier sea in a fashion similar to the original boat in a sea without the 10% increase in intensity?

Thanks for any help.

[mcdenny]

No. Scaling x,y,z by 1.1 will make the boat volume and weight 33% larger. It will be quite a different craft. It might be a different in a good way but if you can't figure that out before you build the boat you shouldn't do it.

You can usually get away with making the length (but not width or depth) up to 10% greater.

The best advice is to get plans for a boat in your desired size from a reputable NA.

Another route is to learn NA software like Freeship. Model your new boat there and learn enough about what makes good boats good to evaluate the hydrostatic data Freeship calculates. This can be fun if you are a geeky retired guy like me but...

[Peerie Maa]

You can scale the length by increasing the station spacing by a small %, however as Denny says do not also scale beam by the same proportion. Small changes in length do not require any change to the beam, however if you want to redesign the boat with significantly larger length, beam and hence stability will scale by a different ratio, as will weight and scantlings. So then you need to look for a different boat.

[Ian McColgin]

There are so many designs in the universe that you really do not need to wreck one design on the route to your perfect boat.

[Louie Campbell]

Good Heavens! Look at the number of posts you guys have. You need to get busy building boats and quit responding to crack-pots like me.

Seriously, Thanks for the responses. I'll look into getting Freeship and do some experimenting but will likely just try to locate another boat design or go with the one that's close enough. I am an old f*** with plenty of time to explore and mess-around with designs and Freeship might be just the ticket for me.

Again, thanks for the replys. Who knows, you might have saved my life!

Louie