Hi
The international load line convention is already in metric. Last time I was on an ABS classed ship, stability was in metric also.
The number given as a ships/boats displacement is the actual weight of water displaced. Which does not vary with temperature or water density. It is underwater volume, which varies.
The rule of thumb in the discussion, works only for boats of a certain size, shape and thence hull resistance. It is OK when most of you are comparing examples of the same or similar vessels.
Don't know why you want a rule of thumb anyway, when hull resistance, and hence power/motor required for a certain speed, is a pretty simple naval architecture/physics calculation. You can then allow factors for the intended use/weather conditions.
My boat, 2 ton 32 foot sailing cat, can get away with 1kw in flat water. But I use a 9hp outboard so I can get to windward in all but the worst weather.
Refer to Dave Gerr for prop sizing. Hull resistance, test tank data, for many types of vessel is public information.
The USA is not the world. But what can you expect from a country that is too primitive to even have universal public health care.
Regards
Captain Kerry Thomas
From: electricboats@yahoogroups.com [mailto:electricboats@yahoogroups.com] On Behalf Of tugs@earthlink.net
Sent: Monday, 24 March 2014 12:28 a.m.
To: electricboats@yahoogroups.com
Subject: RE: [Electric Boats] Re: electricboats
Oh darn, need to let American Bureau of Shipping and the USCG know that! Maybe the IMO needs to rewrite the Load Line Convention.
You must be thinking about the metric tonne which is just 1.6 percent less than the long ton and for purposes of stability and loading is equivalent in the eyes of class and flag. Because the difference is so small, the maritime world doesn't much care.
Point being, and to correctly answer the question, in nautical discussions, displacement is not normally given in short tons.
Displacement tonnage is a calculated volume of water entered into an equation in order to derive a tonnage number for stability purposes. It may or may not be the actual weight because that will vary with salinity and temperature so is very much a moving target in any event.
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