design issues

[jorgoz]

I've been checking a lot of boatplans, gentleman's runabouts and step hydroplanes,lately and have a question concerning changing the beam of the older hacker period runabouts, with a smaller beam than more recent designs.

I found plans for a Hacker step hydro, called Slippery, and on the plans someones has written increased numbers for lenght and beam. He changed the original measures simply by making them 104 % of the original. The boat's originals are 14' 3 1/2" and 4' 9 1/2". Revised measurements are 14' 11" and 5" beam.

Would this change the characteristics of this design a lot ? And would it still be ok to go for 110 % giving me a boat of 15' 8 3/4" and 5' 3 1/4". Or would it be safer to stick with the 104% beam and increase the lenght to approx. 110% ? The increased beam would only be 5 3/4" over the orginal, i wouldn't think it would be a massive change, but then again i'm no designer.

Hope to get some feeback on this, don't hold back ;)

http://www.dngoodchild.com/5403.jpg

[Boatmik]

Cool looking boat.

Altering length and beam in proportion, particularly a smallish change as you are talking about will probably make little difference.

The only slight hesitation I have is that I would be tempted to increase the depth by the same proportion so the deadrise remains the same as the original as well. Otherwise making the boat wider without changing the depth will reduce the deadrise slightly. But still ... probably OK without it.

Michael
web page (http://www.ozemail.com.au/~storerm)

[Ross M]

Hi Jorgoz

Beautiful boat you have there.

One day I plan to build Ed Monk's (John Hacker's?) HORNET, the 15 foot step hydro I posted on one of your recent threads.

Here is what Ed Monk has to say about scaling boats in general: "Roughly speaking, a change in length of over 10 per cent should not be considered, and even less would be advisable with such boats as Hornet;"

At one time, I was considering scaling Hornet up so as to fit and carry a modern (OHV) engine.

I came to the conclusion that scaling up was probably unwise, for (at least) these reasons:

a) It must be performed in all three dimensions, otherwise the angles of the running surfaces are altered significantly. Changing beam without hull depth shallows the transverse angles; changing length but not depth shallows the longitudinal angles.

b) Scantlings are largely based on length, beam, hull depth. Changing any of these characteristics without beefing up the scantlings will inevitably yield a structure that is not only weaker, but is heavier and thus generates higher loads.

c) Changing scantlings quickly becomes a serious matter, with virtually every geometrical aspect of the boat affected.

d) Even if you manage to get the end result to perform properly, you will have probably destroyed the design provenance, resulting in potentially serious reduction in resale value.

I say look for a different boat or accept this one the way she is :D

Ross

[Matt Byrne]

Hey Jorgoz,

Where did you find these plans?

[jorgoz]

Problem with all of the designs of that era is the small beam. But in my country to build and sell a boat as a private person the design must be before the 1950's. If i use a more recent design i have to wait 5 years before i'm allowed to sell it. Or if the buyer wants to put his name down as the builder my problem is solved as well, but which buyer would do that. Bureaucracy :mad:

Thx Ross for the pointers, but i think the first 3 are surmountable, maybe i'm being a bit naïve. But the 4th is really important for me as well, so probably better to stick to the design.

Matt, i found the design here, http://www.dngoodchild.com/load_power.htm

George