Testwiki:Reference desk/Archives/Miscellaneous/2023 October 16

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October 16

Fastest multi-deck boat in 1940?

Legend of Korra has a situation where they are trying to get away from a Crazy Waterbender, and the boat (which appears to have two decks) speeds up very quickly. Given that LoK technology sort of runs in the 1920s-1940s punk situation (ending with something that is definitely at the A-Bomb wonderweapon level), I thought I'd ask about the fastest a boat of that time period could go... Naraht (talk) 19:04, 16 October 2023 (UTC)

The USCGC Fir (WLM-212) could run at 12 knots, but I don't know how well the ship could accelerate. It was not designed for high acceleration. I suppose with 1940 technology, when aiming at high acceleration, a much lighter multi-deck boat could have been built, resulting in much lower water displacement and thus better acceleration, while still using 1,350 shp engines or even more powerful ones.  --Lambiam 19:48, 16 October 2023 (UTC)

Template:USS of 1938 could manage 42.8 knots (49.2 MPH), although that was a full-sized ship. The British MTB 102, a 1937 motor torpedo boat which had a crew of twelve, could make 44 knots (50.5 MPH). Civilian boats were rather slower, except for racing motorboats which generally only had a cockpit - Miss England III of 1932 managed just under 120 MPH and her competitor, Miss America X, did just under 125 MPH. Multi-deck motor yachts were more sedate; most of the inter-war yachts I looked at didn't exceed 15 knots (17 MPH). Alansplodge (talk) 21:18, 16 October 2023 (UTC)

Using the drag equation, I find that the magnitude of the initial acceleration under full power of a boat initially at rest is given by

$a=\frac{P}{mv},$

in which $P$ stands for the power, $m$ for the mass of the boat and $v$ for its top speed. To apply this, you need to convert the given values from horsepower, tonnes and knots to SI units.  --Lambiam 07:08, 17 October 2023 (UTC)

That formula may be a useful rule of thumb, but it's not hard physics: first, it assumes that maximum effective power is available at top speed; second, it assumes that thrust is independent of speed.

BTW, the drag formula isn't very useful for boats. For a surface vessel, the drag coefficient depends not only on Reynolds number, but also on Froude number and therefore speed in some complicated way. Practically speaking, big ships have a drag proportional to speed, but for fast boats, when getting close to the hull speed and when dealing with planing hulls, things get complicated. PiusImpavidus (talk) 09:55, 17 October 2023 (UTC)