How can I calculate the tonnage

of a lifting keel schooner 80 x beam 14 x draft 4 to 9 metres,
the waterline is 54 metres x beam waterline 11.4 x hull depth 2.65
thank you

 

Don´t you think it would be necessary to know more about the boat?
To start somewhere, what tonnage you mean?

 

There have numerous methods for measuring and calculating tonnage over the last eight centuries or so. One which was widely used for yachts in the UK is Thames Measurement.

Thames Measurement for yachts: (L-B) x (B x 1/2 B) / 94
L is length and measured in feet. I don't on how L was/is measured. It may be from stem to sternpost.
B is beam and measured in feet.

 

the displacement

 

If you have other similar vessels to use for reference, and you can calculate the block coefficients for these, then you could perhaps use this calculated block coefficient for an initial estimate of displacement (?)

Block coefficient = volume of displacement / (waterline length x waterline beam x average draft)

Otherwise calculate the section areas and run them through Simpson's Rules.

 

You would need to know the hull geometry to know the displacement. Do you have a lines plan or a 3D model?

Do you just want a rough estimate based on the type of hull?

 

just a rough estimate, i have a lines plan, i have just finished the offsets table, no one seems to want the old designs so i am designing more modern hulls

 

what block co efficient do you suggest for this hull

 

peter radclyffe, if you have a table of offsets or a body lines drawing, and if you have AutoCAD, I can tell a rather quick way to get some very correct hydrostatic values​​.

 

thank you TANSL but i dont know how to use autocad

 

this is the hull
http://www.boatdesign.net/gallery/showphoto.php/photo/21912/ppuser/29904

 

No 142 at 80 metres

 

http://www.boatdesign.net/gallery/data/500/medium/DSC03640.JPG

 

For what purpose are you doing this? If I was in your shoes I'd try to get a pretty darned good answer out of some hydrostatics software or at least through Simpson's rule integration of a bunch of stations.

If you want to be super-rough with it at this stage, use a prismatic constant of something like 0.6 (I can't tell from this angle how realistic this is, but it's most likely going to be within 25% of this) and get the real midship area then use 0.6*Amidship*waterline_length+keel_volume. Really, though, it's not likely to be so hard to get something better than a rough guess here.

 

thanks Mike, what does this mean