February 10, 2011

The stretch of doom

NAVAL ARCHITECTS ARE DRIVEN MAD by people who want “small changes” made to existing designs.

“Just another three feet in length,” some hopeful says, “and she’d be perfect for me.”

“Just six inches less draft and I’d be able to get across the sandbar.”

I believe yacht designers receive special counseling for this. They’re taught not to pull their hair out, or strangle the potential customer, even if the latter move would improve the human gene pool. They have to explain, as gently as they can, that changes like that mean starting all over from the very beginning.

People who want to build their own boats are especially vexing. Because venturesome sailors have such individual requirements and are usually close to broke, they are often tempted to buy stock plans that a designer has drawn up for a small boat and enlarge them on a photocopier. And when disaster looms, as it will sooner or later, they blame the designer. What they don’t know about is the law of mechanical similitude, a very interesting law that applies to boats of similar shape. Interesting things happen when you alter the size of a boat.

Let’s say you double the size of a vessel evenly all around. Here’s what happens:

— Length, beam and draft increase 2 times.

— Wetted surface area increases by 4 times.

— Interior volume increases by 8 times.

— Weight increases by 8 times.

— Stability increases by 16 times.

Now think about that. The new boat would be 41 per cent faster and could carry four times as much sail. But the point is that even small changes in proportion cause large changes in stability, buoyancy, maneuverability, accommodation, handling, and seaworthiness.

So if you want a boat that’s five feet longer, remember the law of mechanical similitude. Find a boat that was designed from scratch to be five feet longer in the first place. Don’t be tempted to economize with the stretch of doom.

Today’s Thought
Architects are pretty much high-class whores. We can turn down projects the way they can turn down some clients, but we’ve both got to say yes to someone if we want to stay in business.
— Philip Johnson, Esquire Dec 80

Boaters' Rules of Thumb, #158
It’s been established as a rule of thumb over the years that the maximum horizontal pull a person can exert on a line, given a good foothold, is about 150 pounds. If you’re pulling down, then your maximum pull equals your own weight, of course.

“My neighbor’s dog keeps barking all night. I can’t sleep. I’m at my wits’ end. What can I do?”
“Buy it from him. Then HE won’t be able to sleep.”

(Drop by every Monday, Wednesday, Friday, for a new Mainly about Boats column.)


Micky-T said...

The four letter word that haunts most tradesmen when spoken by those that do not know....just.

Can't you just...
Well if you just...
I just need...

Matt Marsh said...

That's sound advice, John.

Now and then, the question comes up: "It's all drawn on computer now, can't you just scale everything automatically?"

Well, yes, we can. If there's a hull you like, but you want it a few feet longer, it takes only a moment to change the hull surface in the CAD model. The crew will stay about the same size, though, so the interior layout has to be modified to make proper use of the extra space. That takes a bit longer.

If you change the length, of course, you've now changed the displacement and the loads, so the structure has to be modified. And by "modified", I mean every scantling in the boat has to be re-calculated as if it were a new design. This takes quite a lot longer.

Now that you've changed all the scantlings, the weight calculations have to be re-done. Tanks might need to be moved. The engine room configuration will probably change. The turning moment's different, so a new rudder must be designed. And so forth...

So yes, please set your desired size, budget and design goals in the Statement of Requirements, before your architect starts whirling around the design cycle. If you can't pin down what you want the boat to be and do, then building a new boat is probably not for you.