Beam and depth increase

[wudenbote]

Fellow wooden boat lovers: Thanks for all the input I have received...it is much appreciated. I will continue to bug all of you until my project is completed. The boat I am going to build is a modified Pacific dory with an OAL of 20' 8". My question is this...I would like to increase the beam and depth by 20%. Am I correct in assuming that stability would not be an issue if I increase both the beam and depth and thickness of the frames by 20%? Using a percent figure should keep all relationships the same. The boat will be conventional marine ply over hardwood frames. At present I am building the strongback. Thanks for all the help.....and I ALWAYS finish my beer :-)

[rwatson]

Sounds like a really bad idea - basically, you are suggesting increasing the depth by the same amount as the size of the frames, which means the inside diameter of the frames stays the same, and the frames extend another 20% out. That is both unnecessary and weight adding.

You cant just apply an abritrary percentage to a design. You have stability issues by raising the centre of gravity,, and you are not increasing the length of the boat by 20% ..... and it goe on and on.

Get another boat design with the interior room and beam that you need - but modifying a design in this manner is a really bad idea.

[terhohalme]

rwatson,
I don't think you got it right? Wasn't it a 20 % growth in molding and siding of frame? Sounds reasonable.

Your judgement of stability issues is based on what?

Increasing of beam by 20% will make her metacentric height (stability) about 20% higher. For:

BM = I/D

and I will grow 1.2^3 times and D will grow about 1.2^2 times. There is still room for a 20% rise of gravity...

Here BM is hight from bouyancy to metacentric, I is second moment of water plane and D is displacement.

wudenboat, you knew the new design is then about 1.4 times heavier too? You should use 16% thicker plywood (yes, hard to find).

[rwatson]

Oh yeah - sounds good easy.

I roughed up a quick sketch of the result.

Doesnt look like too many problems to me.

[terhohalme]

OK, now I know how the pacific dory looks like.