View Full Version : Increasing/decreasing length of hull

Bill Baillie

12-08-2004, 11:17 PM

Wondering....

A little while back someone posted an explanation of what happens when you increase or decrease the length and beam of a sailing hull by a fixed proportion. I can't recall exactly what was said but I seem to remember that you can do one but not the other.

Can anyone fill me in on what might happen if you took an existing plan for a hull of about 26 feet and increased it to say 36 feet? Would it mess up the performance? How about a 36' hull decreased to 26'?

The reason that I ask the question is that I often see hulls in the 24 to 28 foot range that look very sleek and tidy. But a very similar hull in, say 32 feet or more seems to lose a bit of that look. I'm guessing that there is more to making a hull larger than just stretching it out and pulling the sides apart.

This getting into the area of personal aesthetics, but I would welcome any opinions.

Bill

Nicholas Carey

12-09-2004, 12:55 AM

The volume of a 3d solid, in general, varies with the cube of the dimension.

The volume of a sphere is 4/3 * pi * R^3, where R is the radius of the sphere.

The volume of a cube is N^3 where N is the length of the side; in general, a rectangular prism's volume is L * W * H, where L, W and H are Length, Width and Height respectively.

A cube where N=1 foot has a volume of 1 cubic foot. Double N and the volume increases to 8 (2*2*2) cubic feet. Triple N and the volume is 27 (3*3*3) cubic feet.

A boat's hull follows the same rules. Its volume varies far more rapidly than its weight: its displacement is based on its weight. The density of water is constant.

Significantly increase (say, more than 10%) any dimension in a boat's hull and the boat will no longer float anywhere near its lines.

Your rather oddly shaped cubic boat, when you double its size from 1 foot on each side to 2 feet on each side has increased its volume 8X, from 1 cubic foot to 8 cubic feet; at the same time the surface area enclosing it has gone from 6 square feet to 24 square feet, a factor of 4X.

Conceptually, while its volume has increased eightfold, its weight has only gone up fourfold. Rather than floating in, say, 6 inches of water (its a heavy boat), it now floats in only 3.

There, in a nutshell, is whats at play here. Whether or not the "performance" is improved or made worse depends on a lot of things.

A good general rule of thumb is that you can increase the length of a boat on the lofting floot by about 10% or so, just by increasing the station spacing.

[ 12-09-2004, 12:58 AM: Message edited by: Nicholas Carey ]

Not quite 3" waterline. You've 4X the area and 4X the weight, so same waterline. This still has you floating at 1/4 height of cube instead of 1/2 though. You are right with your math a little off.

Your biggest problem is your stability changes significantly when you narrow a boat.

Ian McColgin

12-09-2004, 09:09 AM

Just enlarging everything is almost certain disaster but one can often stretch an existing design a good deal.

Two methods are popular. We have one ferry and one steel hulled sail boat that were stretched simply by cutting the hull amidships and adding about 15% length right there. Given the extra length, the sailboat had a longer keel to hold more ballast and more topside area for dynamic stability so she was able to carry a larger rig on the same beam and, given more length, was faster.

In new construction, some have done much the same, just adding at the middle.

The other trick is to just spread the stations by some percent. This eases out the curves of the waterlines so there's no slab effect amidships.

In general this nearly 40% increase in length will be associated with more than 2-1/2 times the burden. This is so significant that all engineering and hydrodynamic aspects change and you really ought to be considering a new design.

Enlarging a hull by increasing mould station distances is a common practice fraught with dangers. I agree that, in moderation, this can be done successfully. Moderation means increases of, say, 5% - 10%. More than this and some serious problems will arise. How a boat behaves in a seaway is a function of the whole shape of the hull. If you alter the shape of a hull drastically without compensating for changes in displacement, centres of areas and volumes, weight distribution, wetted surface area, powering, etc., etc., you will have no idea how the boat will perform. And it may perform badly or even dangerously.

To find out more about this, I would suggest that you buy or borrow a copy of Skenes Elements of Yacht Design and read Chapter V, The Law of Mechanical Similitude. In fact, if you are considering messing about with a yacht design, I'd suggest that you read the whole book.

Pierre Gutelle's book The Design of Sailing Yachts is a good one to have on your shelf as well.

Bill Baillie

12-11-2004, 08:10 PM

Thanks for the info guys.

I have heard of people lengthening a boat by increasing the spacing of the stations by a certain amount and they seem to do it without a second thought. I wonder how many boats there are where this has been done. Is it something a fairly competent marine surveyor would catch?

Oh, and thanks for the numbers Nicholas, I had to reach way back into my college math classes, but it made sense.

Bill

Bill, Check out Seabird, 25'6" and Seagoer, 34' by Thomas F. Day around 1900. Seagoer is supposed to be a Seabird enlargment, but I don't think an "exact" one. There are posted topics on these very recently either here or in Designs/Plans. cbob

boatlover

12-12-2004, 04:02 PM

Originally posted by cbob:

Bill, Check out Seabird, 25'6" and Seagoer, 34' by Thomas F. Day around 1900. Seagoer is supposed to be a Seabird enlargment, but I don't think an "exact" one. There are posted topics on these very recently either here or in Designs/Plans. cbobYou might want to add NAIAD to the list. Day and Mower - 39' + a bit.

Regards,

Ed R

I have heard of people lengthening a boat by increasing the spacing of the stations by a certain amount and they seem to do it without a second thought. This just makes them thoughtless :D And even if the hull turns out ok you'd still have the sailplan to worry about, which will be tied, more or less, to hull volume and cannot be increased or decreased in a simple linear way.

brad9798

12-13-2004, 07:19 PM

For powerboats only, based on my own experience ... perhaps up to 10% MAX!!!

Check with the designer ALWAYS.

Bill Perkins

12-13-2004, 07:40 PM

Would 10% also be an appropriate limit for shortening a boat ? Michael ; I sat down to review chapter 5 of Skene's at your suggestion and it went quicker than I thought !I do need to sit down and read every page ;it's a great reference book that I've dipped in to ,but not compleatly read .

[ 12-13-2004, 08:03 PM: Message edited by: Bill Perkins ]

Would 10% also be an appropriate limit for shortening a boat ?" Bill Perkins You know the answer already, don't you? :D

Of course you do...

"It depends..."

Shortening a long, slender hull like a Rangely or a Whitehall by 10 % would be fine, but doing the same to an already pudgy hull like a Wittholtz catboat would not only have a detrimental effect on an already stiff boat, but would look awfully durned pecululiar to boot. As in most things, reasoned forethought and consideration are the best guides.

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