Estimating weight/displacement

scottnewcombe

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I am trying to get various people to 'spec' a new prop for my 27' Owens motorboat.
Most are asking for the weight or displacement but without any definite information how can I best estimate this??

The boat is 27' overall 8'6" beam but is a planing hull. Draws about 1' if you ignore the small keel. Mahogany on oak clinker built above the waterline with a plywood hull below the chine.

Boat is a bit like the old Chris craft US lake boats of the 50s/60s.

My guess is about 2.5 tonne?? Finger in the air stuff really.

lucky_eddie.thumb.jpg


(not sure how to make the picture bigger??)

Cheers

Scott

<hr width=100% size=1><P ID="edit"><FONT SIZE=-1>Edited by scottnewcombe on 12/11/2004 13:19 (server time).</FONT></P>
 

snowleopard

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method number one requires you to take accurate measurements of the underwater cross section at intervals of 1/10 of the waterline length. using simpsons rule you can calcualte the volume and hence the displacement.

method number two involves knowing the waterline length, waterline beam and draft (in metres) -

multiply these together to give the volume of a rectangular box that encloses the underwater part of the boat.

now you have to estimate what fraction of the box the boat itself actually occupies. you can do this as a simple guess or look at it from side, top and end-on to estimate the fractions in each dimension and multiply them together.

the figure you come up with will be between 6/10 and 9/10 this figure incidentally is called the 'prismatic coefficient'

multiply the volume of the box in cubic metres by your fraction and you have the displacement in tonnes when floating in fresh water. if you used the salt water line, add 10%

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Avocet

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If you're thinking of lifting out at any stage, hire a crane to do it. They have calibrated load cells on the crane lines so that they know when they're getting close to their safe limit. I've had Avocet weighed several times by this method (free with every crane lift!) and it seems repeatable despite using different companies.

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scottnewcombe

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Thanks Avocet

Tried that one but the chap who drives the lift has said that all he can tell is that if the boat is "quite heavy" the straps stretch more before the weight of the boat is taken up. Creaks a bit too!!

Hmmmm

Not a commecial yard, all run by volunteers. The lift is tested as required by lloyds but that's it.

Scott

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Avocet

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Ah well, worth a try! That said, it might still be a worthwhile exercise if the boat is easily accessible to a crane and you can share the crane with a few people (for something - dunno what!)

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Evadne

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You can get load cells that will hang from a crane hook, we have them at work for various purposes but I don't know if its possible to hire them from a crane hirer, it may be more trouble and expense than its worth. FWIW, my displacement sailing boat of 20' LWL (29' LOA), 7'6" beam and 4'2" draft displaces about 3.5 tons so I'd have thought your estimate is a bit on the high side. Perhaps a similar post on the Dark Side Forum would elicit a response if you've not already done so?


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