Displacement weight or Scale weight?

[Canopy Locked]

This may be a very stupid question.., but is there a difference between "displacement" weight one often sees quoted on brokers adverts and the actual weight one discovers when the boat is lifted by a hoist with a scale?

Thanks
Rob

 

[aquaplane]

There shouldn't be but there often is.

The displacement quoted is often the builders weight ex boatyard. A cruising boat will weigh more with all it's kit on.

No stupid questions, I hope my answer isn't either.

 

[sarabande]

no. Displacement is the amount of water that the hull moves out the way when it is immersed. It is equal to the mass (very very nearly = weight) of the whole boat.

So if you had a boat on a trailer. and drove the trailer and boat onto a weighbridge and they weighed 6 tonnes, and you knew the trailer weighed 1 tonne; the weight of the boat is 5 tonnes, and therefore the displacement will be also 5 tonnes.

Brokers often quote a displacement figure from the builder which is without fuel, water, sails, anchors, etc. That is the source of difference.

 

[rob2]

Not really, although many boats ended up a bit on the heavy side when the build was completed. The design displacement weight includes such things as fuel and water in the tanks and basic equipment, but by the time we've added an extra anchor and chain, tender in the cockpit locker, spares, instruments, whatever most will be a bit heavier.

The other thing to watch out for is the variety of tonnage systems employed which aim to measure the cargo carrying potential and thus harbour dues, etc. Register tonnage is one of these and will be displayed in any boat which is or has been Part 1 registered. Just as an example, a 24ft boat with a displacement of around two and a half tons may have a regster tonnage of over five tons. Older books will often quote Thames Tonnage for a design and this is a similar system, but with different measurement criteria, based originally on how many tuns of wine could be carried.

To be honest, if you're looking at towing on a road trailer or maximum SWL for a boat lifter, then there is no substitute for weighing her in the condition she will be in for the journey/lift. The most impressive private towing rig I've seen was a short truck with sleeper cab. Everything that could be removed from the boat was in the back of the truck and the mast slung on top!

Rob.

P.S. the designer's displacement figure should not be an empty hull, as he is designing the boat for use with the necessary kit. It may be that some builders missed the point and just weighed the first boat!

 

[ditchcrawler]

The other thing that you also need to remember is that as manufacturers often list their displacement as a bare boat not only is the displacement more when the boat is loaded but the draught can be more also.
My own boat a Moody31 is listed by the manufacturer as a draught of 1.52m.The actual draught is nearer 1.7m and others with the same boat confirm this.(Moody Owners Association)

 

[Tranona]

There is now an ISO for calculating design displacement which is part of the RCD, so quoted figures for post 1998 boats are potentially accurate, but they are "light" and cruising equipment, fuel, water etc can easily add a ton to a mid 30's ft cruising boat.

Older boats were often very different from their quoted displacement because of the lack of standardisation in the method of calculation and the lack of control over the materials used in building the boat. Marketing "hype" would also play a part - do customers value light weight or heavy duty? That might have an influence on the figure used in published material.

 

[vyv_cox]

The other thing that you also need to remember is that as manufacturers often list their displacement as a bare boat not only is the displacement more when the boat is loaded but the draught can be more also.
My own boat a Moody31 is listed by the manufacturer as a draught of 1.52m.The actual draught is nearer 1.7m and others with the same boat confirm this.(Moody Owners Association)

And, as has been said by others, there is a good deal of difference between the maker's displacement and what it weighs. The displacement of my boat quoted in the maker's brochure is 5.8 tons but in the slings it weighs almost exactly 7 tons.

 

[Canopy Locked]

Many thanks for that Info gentlemen...