Hi
The hull speed formula doesn't work for narrow boats and non standard prismatic boats, simply because those parameters were left out of the formula, for simplicity. When most boats were a similar L/b ratio and similar co-efficient it worked fine.
A hull speed formula that works for boats such as multihulls can, and is, derived.
Kerry Thomas.
From: cpcanoesailor@yahoo.ca [electricboats]
Sent: Sunday, 15 October 2017 4:38 AM
To: electricboats@yahoogroups.com
Subject: [Electric Boats] Re: displacement & semi-displacement hull performance
For any hull type, theoretical hull speed is only theoretical, not a law. The shape of the hull, especially the length to beam ratio has everything to do with how much theoretical hull speed applies. Or said another way, hull shape and length to beam ratio determine how steep the power/speed curve is at theoretical hull speed.
Tugboats really are displacement vessels, with a very steep rise in power needed when they approach hull speed. So steep, that they will push up huge bow waves instead of climbing onto a plane.
Multihull sailboats, like trimarans or catamarans, on the other hand, have very narrow hulls (at the waterline) for their length, and have only a nominal rise in power needed when they approach hull speed. Often so nominal that there is no noticeable difference in how the hull interacts with the water as hull speed is approached and then exceeded. There is even an argument that above hull speed, these hulls are not really planing, but so what? They go fast with not much energy required.
My understand is that semi-planing hulls trade some hull lift for a softer, more comfortable ride and better handling in rough conditions, but are still based on a lower length to beam ratio.
I noticed the Wye Island challenge entrance categories all mention limits for length to beam ratio.
(Can you tell I like skinny boats?)
Tugboats really are displacement vessels, with a very steep rise in power needed when they approach hull speed. So steep, that they will push up huge bow waves instead of climbing onto a plane.
Multihull sailboats, like trimarans or catamarans, on the other hand, have very narrow hulls (at the waterline) for their length, and have only a nominal rise in power needed when they approach hull speed. Often so nominal that there is no noticeable difference in how the hull interacts with the water as hull speed is approached and then exceeded. There is even an argument that above hull speed, these hulls are not really planing, but so what? They go fast with not much energy required.
My understand is that semi-planing hulls trade some hull lift for a softer, more comfortable ride and better handling in rough conditions, but are still based on a lower length to beam ratio.
I noticed the Wye Island challenge entrance categories all mention limits for length to beam ratio.
(Can you tell I like skinny boats?)
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Posted by: Kerry Thomas <kjt@outlook.co.nz>
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