Blogumulus by Roy Tanck and Amanda Fazani

Tuesday, February 23, 2010

There’s lots of maths in a boat.

Specs for the Haiku by Iain Oughtred:
LOA: 9.15 m - 30' 0" Beam: 2.36 m - 7' 8" Draft: 0.40 m - 1' 3" Sail Area: 31.32 sq m - 337.00 sqf Weight: 1590.90 kg - 3500 lbs Displacement: 772.73 kg - 1700 lbs

People build boats by eye and experience all the time. One day, I might. But right now I follow the plans. That means I take a series of measurements and turn them into 3-D shapes. This is when I wish I was metric. But I do love how my brain feels after struggling with turning numbers into a spatial reality. And there are some interesting things to learn. For example, the numbers above mean the boat will only go 7.34 knots (or more than 8 miles an hour!)

The formula:
v = 1.34 x √LWL
"LWL" is the length of the waterline in feet, and"v" is the speed of the vessel in knots (1kn=1.15mph)

I don’t understand the formula well enough to explain it, but I think it says a boat creates a hole in the water that it cannot climb out of. But it can push it forward at a speed that is defined by the length of the boat, or the distance between between the bow wave and the stern wave.

Other interestingly named numbers that affect speed are prismatic coefficient (explains how sharp-ended a boat is, knife versus brick going through the water) and wetted surface (how much of the hull’s surface comes in contact with the water creating drag.)

When I try to wrap my head around this stuff, I can’t think of anything else. And that is a good thing, at times.