Displacement at D.W.L.

[Venchka]

You all are pretty smart. No doubt you all can explain this to someone with Old Timer's Disease like me. Speaking strictly in terms of sailing craft. Is it the on the water with a bit of a load displacement for optimum performance? Empty but rigged for sailing displacement? Maximum, not to exceed or you die displacement? A little bit of all of the above?

One old totally confused enquiring mind needs to know.

[ 04-03-2004, 05:42 PM: Message edited by: Venchka ]

[Meerkat]

This is a SWAG...

DWL is the waterline at the "as designed" displacement. I have read that this often may not include the weight of groceries, water, fuel, books, CD's, sun tan lotion, clothes or bodies. However, as most boat designers tend to design something they hope will be built and be useful, there must be some allowance for the organic content.

I think a far more interesting number is pounds per inch of displacement and what the maximum WL is. IIRC, the Pardey's Seraffyn had 900#/inch of displacement, so a lot of load carrying ability. I've also noticed that Dudley Dix quotes a "displacement light" and a "displacement heavy" and I would guess that the difference between the two is the available load capacity.

[JimD]

Ted Brewer defines displacement this way:

DISPLACEMENT: If you weigh the boat on a scale, that is her actual (not advertised) displacement and it is the weight of sea water that she will displace when she is afloat. Most designers figure displacement when half loaded (the boat, not the designer) with stores, liquids and crew.

Displacement can be expressed in pounds, long tons or cubic feet; one ton = 2240 pounds = 35 cubic feet of sea water, at 64 pounds per cubic foot. Fresh water weighs only 62.4 pounds per c.f. so a boat taken from sea water to fresh water will sink into the water and increase her draft slightly. For example, a boat weighing 7500 pounds will displace 117.19 cu. ft. of sea water or 120.19 cu. ft. of fresh water. The difference is 3 cu. ft. so, if her waterline area is 150 sq. ft., she will sink 3/150 of a foot (about 1/4") when she is moved from salt to fresh water. It is truly insignificant for most sailors, unless you are skippering a 90,000 ton tanker.

[Frank E. Price]

I believe it's up to the designer, though I'm sure there is a specific requirement/definition in commercial vessel plans.

I've been studying Warram's plywood catamaran designs recently, and he provides a LWL and the empty displacement and the payload weight; so presumably his/their LWL is at the empty displacement. As his boats have raking sternposts and stems, the LWL will change substantially as displacement increases, although there is no loaded LWL provided.

I've never seen more than one LWL (or DWL) in published specs from any small boat designer. Presumably a DWL is the LWL at the given displacement on the plan, but it sure would be nice if the designers would tell us if that displacement includes any load, or part of it, or all the rated load. Maybe that's just one of the things they expect us to buy with the plans.

Frank

[George Roberts]

The Eun Na Mara gives a weight (1500#) and a displacement (2300#). The 800# seems reasonable for 2 crew and supplies.

[Aramas]

That's one of my pet peeves. I'd like to see designers give empty weight and designed displacement (the displacement to which the scantlings are designed) at least. Most don't.

I'd also like to see stability curves for empty, half load and full load like in a trim and stability book for any commercial vessel.

On the down side, the more information a designer provides the more dumb questions he has to deal with tongue.gif

[Venchka]

Amen, Brother Aramus!

Dry weight with all required gear to make it go built of reasonable materials. In the case of trailer boats, at least then you know what trailer to buy and if your tow car has the gumption to tow it.

Safe working cargo should be essential, I would think.

I guess there is a liability thing happening.

But then everytime I look at boat plans and I look at my detail design drawings the boat plans are incomplete by comparison. Forces boatbuilders to think. Invent. Innovate. Improvise. Moan. Drink. Cuss. :D

Thanks, everyone. It really is either too broad or too specific a question to answer in general terms. I'll go back to the source with my questions about my boat.

[mmd]

The definition of Design Water Line (DWL) among the colleagues I have been fortunate enough to work with, and of most of the designers whom I have met, has always been the depth and trim at which the hull will float in salt water when loaded with the following:

* estimated (calculated) hull mass including structure, joinery, outfit, and spares
* estimated (calculated) mass of rig, sails, lines, and spares
* estimated mass of crew compliment (number assumed by designer as the "normal" number on board)
* estimated mass of crew effects (clothes, bedding, personal effects)
* estimated mass of crew consumables (food & non-tankage beverages)
* calculated mass of all tankage (fresh water, fuel oil, lubricating oil, hydraulic oil, sewage, etc,) at half full capacities

This is a highly subjective waterline, as it is dependant upon whether the designer was tediously accurate in his weight calcs or whether he uses factors or just plain guesstimates. Also, whether the builder is fussy in adhering to the construction schedule and whether the client decides to generously add to the equipment list.

You know; "it depends". :D

I agree with what has been said above; the designer should, in the accompanying design notes, indicate whether the LWL is in salt or fresh water, how many crew are factored into the displacement, what is the status of the tankage load, etc. I also agree with the comment made that more info should be given in published boat specs. I have in front of me a copy of the British yachting rag, "Yachting World" which, in its reviews of new yachts, lists:

* profile, sail plan, interior & deck plans
* laminate schedule (sorry folks, 'glass boat :eek: )
* designer, builder, and marketing agent(s) names and addresses
* polar graph of speed under sail
* bar graphs of speed per rpm, decibels in various locations on the boat at cruising and maximum rpms, stowage volume vs compartment volumes.
* dimensional and capacities data on the subject vessel and those of three similar vessels in its class.

This is the way it should be done! I just don't understand why we here in North America are given such little information about the boats we may want to buy. Do the marketers of North American boats feel that we are too ignorant to be given the data? Or that the Europeans are so much more sophisticated than we and can understand such technical stuff? Sheesh!!