changing hull length

If I wanted to build a larger hull than what the plan shows could I add equal percentages of length and beam? Suppose I started in the middle of the hull length and added half the amount of the increase aft and half forward. Then increase all beam demensions the same way and adding additional bulkheads as needed. This theory would be used on a 18 ft. catamaran. Is this crazy thinking or what???

Thanks, Don

 

Be vewwy, vewwy careful.... THe effects of changing plans can be insidious and unseen. If possible, talk to the designer of the boat first and try to get his approval, if not blessing, before proceeding.
I'm not saying it will automatically kill the boat, as many have done this with great success, but many have also done this and finished up with complete dogs of boats.
Steve

 

It depends on what percentage you are planning to scale to. The problem is that the factors influencing performance do not all scale equally- the law of mechanical similitude.

For instance, if you double every dimension the boat will be twice as long, have 4 times as much sail area, 8 times as much displacement, and 16 times as much stability. The ratios will be out of whack.

If, however, you're talking about a 5% change, it might not be such a big deal.

I agree- be careful.

 

Thanks guys for the insight. I was thinking about a 50% increase from 18 ft. to 27ft. I will continue to check around before I try it.


Don

 

For that kind of change, I'd forget the 18' plans and look for a proper 27'er. A 27 ft boat is a different animal from an 18 ft boat.

Not only are the proportions and stability quite different, but the way you use the boat is different. An 18' boat is a daysailer that can be capsizied, etc. A 27 ft boat will be more of a coastal cruiser or a high powered sportboat.

I'd say take note of the features you like from the 18 ft boat but design (or have designed or look for) something that meets the requirements for the boat you really want.

 

tspeer

You're right. A coastal cruiser is more what I want. A 9ft. extension may be to much to try. Thanks.


Don

 

hi Don, I took the hull shape of 18 ft nacra catamaran hulls and have stretched it out to 40 ft and doubled its circumfrence at it's widest point to 8ft. I've had it on the water but not under sail, it draw's only 6inches. I know nothing of the technacalities of hull design nor spelling, I just copy aspec's of already proven hull shapes ie polynessian, and I beleave if it looks fast and is light it will go fast. I have no idea what it's hull sped will be. I think with its length and draught and sailing in the chop and the roll of the sea state combined with a bit of wetted surface airation its pointless doing any sort of calcs. The wetted surface will be in a space you cannot do calcs . It's bound to go to fast for me and I'll be sailing around at about 50% effeicancy which is fine by me. If its about being on the water in a boat then maybe selling it and getting a bigger boat go with an existing plan, but if its about design, your design, with no real resale value then follow your instinc's . Good luck and I know my spelling and ideas are shocking.