What is 'displacement'?

Post by Ed » Thu Jul 19, 2007 9:19 am

Sure this is the kind of thing I should know but.....

what exactly is the 'displacement' of a boat.....and how is it measured?

I have been asked to confirm what the displacement of the Jollyboat is, which is down in the Uffa Fox books as 850lbs.

I (in my simple way) had always presumed that the 'displacement' was the weight of the water that was displaced by the boat at rest. In other words it was just a fancy way of saying the 'weight'.

But now I look at the figures....I wonder if it actually means the max weight (including boat) that it it designed to carry, including boat. This seems to make more sense with the figures quoted.

Any thoughts?

cheers

eib

JimC · Post by **JimC** » Thu Jul 19, 2007 9:57 am

I suspect it probably depends on context...It certainly ought to include the crew weight. Whether it should be a max crew weight, a typical crew weight, or the crew weight in J75 at 14:30 hours on 15th August 1967 is probably less precisely defined...

Trevor C · Post by **Trevor C** » Thu Jul 19, 2007 11:18 am

Displacement - the weight of the water displaced by the boat

Displacement- The Weight, in tons, of the water displaced by the boat and its equipment.

Displacement - The weight of water displaced by a floating vessel, thus, a boat's weight

Displacement
The weight of the quantity of water displaced by the vessel. The displacement of the vessel on her light draft represents the weight of the vessel ready for use including stores etc.

Post by **Rupert** » Thu Jul 19, 2007 12:19 pm

You should be planing anyway...

Post by Ed » Thu Jul 19, 2007 1:10 pm

Yeah....kinda new the deffinition I guess.....but it does seem to be a bit vague as to how it should be measured and as Jim says it is deppendent on the context.

Further research suggests it is another of those terms that should really be expanded a bit in use. I have come accross 'Max Displacement' or 'Light Displacement' or 'Empty Displacement' all of which makes much more sense to me.

Although in a dinghy context they seem to really make precious little sense. All in all - agree with Rupert.

cheers

eib

JimC · Post by **JimC** » Thu Jul 19, 2007 2:23 pm

> Agree with Rupert
at which point we start making distictions between static displacement, displacement at stead speed of X knots and goodness knows what else:-)

Post by **Rupert** » Fri Jul 20, 2007 9:08 am

I was thinking about this on the way to pick up the kids yesterday. The only way it can be the same for all boats is how much weight is needed (boat, rig, people) to make the boat float at the designed waterline, when static. As this is probebly the exact opposite process that the designer went through in order to decide underwater volume, the chances of coming up with the same figure that Uffa had in mind is in theory 100%, and in practice pretty much Zero...

chris · Post by **chris** » Fri Jul 20, 2007 9:14 am

I was just looking up something in the 1946 merlin rules and happened to notice that the illustrations etc that appeared in the Yachting World that year have a list of dimensions for the then new merlin. There are the basics ( length, beam, sail are etc) but it then says
Displacement 600lbs.
At the time the hull was 190lbs, minus weighted centre board mast etc
So the displacement did seem to include a couple of stout bodies on top of the all in weight of the boat.