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mikehoyt 10-22-2002 04:35 AM

Displaced weight vs Dry Weight
On another email list there is a lively discussion on whether a sailboat dry weight (measured by crane) should be the same as the advertised displacement of the boat.

Some say that the crane weight is not accurate because it measures tons and because of some extra gear used in lifting that was not accurately reflected in measurement. Others say that the advertised displaced weight is usually optomistic or just wrong.

Well the discussion then got theoretical. So the qwuestion is this....

Is the weight of a boat dry supposed to be the same as that of the water it displaces? Ie. If we took a boat and measured teh weight of the water it displaced and then measured the dry weight of the same boat should they be the same.

Please explain why or why not.

I suspect most of the reason is that the designers come up with the displacement and THEN the builders build the boat and it might not behave exactly as forseen.


windship2 10-22-2002 07:52 AM

Displaced weight vs Dry Weight
I always thought that why there is a displacement and a weight is that when a boat is designed the statistics are figured out to be whatever they may be for any given boat. Ie. this is how the boat will perform.This being designed displacement. Now, When you load the boat with water, fuel stores or just stuff you might have, this extra weight will have an effect on it''s performance and will vary from it''s designed statistics. This being weight.
I don''t know exactly how much truth there is to this but it makes sence to me.


WHOOSH 10-22-2002 08:16 AM

Displaced weight vs Dry Weight
Mike, there are several useful discussions on the topic of displacement in Offshore Sailing by Bill Seifert. The essential points he makes are as follows:
1. There''s no commonly agreed to definition used by all boat manufacturers when citing ''displacement''. (Remember: Bill worked for many years in boat building, with Tartan, TPI (J Boats, etc.) and Alden).
2. The most common practice is to "list displacment at a datum waterline rather than at the actual floatation plane." (p. 170)Essentially, the designer has no absolute knowledge of how the workers construct the boat, only of his/her design...but selecting a datum waterline allows computation of displacement - and yes, that is the weight of the water the boat will displace.
3. The actual weight of the boat after construction can vary from one hull to the next considerably. Generator? A/C? What sail compliment? Capacity of the DC electrical system along with its many components? Moreover, builders change materials and construction methods over time. And since builders don''t print up brochures for each hull, the displacement figure of a given boat can''t be completely accurate even if the boat sat on its datum waterline.
4. Builders also don''t agree on the tankage being referred to at this ''design displacement''. "Remember that 60 gallons of diesel and 200 gallons of freshwatere add a ton of displacement and that''s before food, cltohing, spare parts and, oh yes, crew." (p. 170). FWIW, for extended cruising we estimate each permenent crew member brings 1000# aboard the boat.

For these reasons, numerous writes on this subject (not just Bill but also Nigel Calder & Beth Leonard come to mind) recommend purchasing a Performance Package from U.S. Sailing. This will include info typically available on a sistership''s IMS Certificate and provide a relatively accurate report on the displacement of that sistership for a known condition of loading. With adjustments, this is a good way to assess the weight of your own boat.


DuaneIsing 10-22-2002 09:27 AM

Displaced weight vs Dry Weight
Most of you know this, but in case this helps at all -

The term "displacement" as used in the boating world may have all the aforementioned confusion assigned to it, but in terms of simple physics, the weight of the water that a floating object displaces is equal to the weight of the object. You are actually displacing a certain volume of air, too, which contributes very minimally minimally to the bouyancy (that''s the way helium balloons work - the rubber plus helium do have mass, but less mass than the air the inflated balloon displaces).

So, if you add mass (weight) to a floating boat, it has to sink lower to displace an equivalent amount of water weight. If you remove mass from the boat, it has to float higher and displace less water. That''s the only way equilibrium can be maintained.


Denr 10-22-2002 11:15 AM

Displaced weight vs Dry Weight
Me and my good friend Archemedes while sailing along one day came up with the very theory DuaneI-life-sling is referring to.

DuaneIsing 10-22-2002 01:22 PM

Displaced weight vs Dry Weight
Damn, Denr. You mean to tell me someone stole my idea before I even had it?!

Archemedes had a screw named after him. Did you help him with that design?

Fair winds,

DuaneIsing 10-22-2002 01:27 PM

Displaced weight vs Dry Weight

BTW, my last name has been mostly pronounced with the "s" as a "z." Did you ever realize how many words end in "-izing"?

Imagine all the names I could have for potential offspring: sympath, maxim, tantal, etc. If you care to publish a list of funny ones (keep it sort of clean), I bet you could top 100 easy.

I know you have a great sense of humor. Here''s your chance.

[apologies for hijacking this thread]

Denr 10-23-2002 05:01 AM

Displaced weight vs Dry Weight
Yes as a matter of fact Archemedes coined the phrase “Loose Screw” after me!

Yeah, your offshorespring could have anyone of the following top ten names based on contributors to these bulletin boards.

Antagonizing-after denr
Winterizing-after nauticalrich
Cauterizing-after marybeth
Stabilizing-after whoosh
Mesmerizing-after jeffH
Benehuntalinaizing – you know who you are
Sanitizing – after waternut
Prioritizing – duaneIsing (will this guy ever buy that boat?)
Terrorizing - after tsenator
Sailkoteizing-After sailormitch

[no apologies for hijacking this thread]

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