understanding displacement

I have seen many designs where displacement and weight are different. These designers or sales people can't grasp the concept that they are, or should be, the same. For example in http://www.multihulldesigns.com/stock/23daytri.html , the displacement is about 40% of the weight. Displacement is the weight of the displaced water by the submerged part of the vessel. It equals the weight of the vessel.

 

Displacement = weight of boat, that makes sense to me. The confusion is when some people use it for the boat by itself, while I understand it to mean the boat, stores, fuel, and passengers.

Gary

 

I'm quite sure Kurt knows the relationship between displacement and weight. What he's actually giving you are two things most designers don't: the empty weight of the boat itself and the maximum design displacement. How you get from one to the other will depend on what you put aboard!

BTW, displacement does not equal weight under all circumstances. Dynamic lift results in displacement < weight, for example.

 

You mean, weight is 40% of displacement.

I would prefer it if all designs included the dry weight and design displacement, as then the consumer would have a better idea about how much of the design displacement is to be made up from crew, stores, tankage, gear.

DC

 

I think designers tend to use the term "weight" when it's something you'd determine out of the water by weighing it. "Displacement" is reserved for when the boat is floating.

It's too bad there isn't a system of standardized definitions for small craft mass. In aviation, one determines the "operating weight empty" by weighing with the fixed ballast, unusable fuel (the fuel that remains trapped after the tanks are drained), full oil, hydraulic fluid, etc. In other words, everything that's necessary for flight, but without the things that vary like payload, passengers, fuel, water and crew. Then there's maximum takeoff weight, maximum landing weight, and maximum zero fuel weight (loaded with cargo, but without any fuel in the wings to reduce the wing bending moment). The intended limits are precisely spelled out, even for light aircraft.

Ships have similar definitions for light ship displacement, gross tonnage, deadweight, etc.

With boats, it's not clear if the "dry weight" is to include essential items like sails, safety gear, and removeable but standard items (cushions, stowage compartment covers, fuel tank, outboard motor, anchor, etc.). Is it the weight as it comes from the manufacturer or something more akin to aviation's operating weight empty?

There seems to be a policy of slanting the displacement figures toward whatever will sell best, especially toward the low side to make the boat seem lightweight and with the expectation that people will overload it anyway. And most people don't know what their boat's actual weight is, either loaded or not.

 

Even if there is no standard within the industry, it sure would be helpful to the consumer, be it the builder, or eventual owner, to know what the dry weight includes.

This could be either an extensive inventory, for an extensively detailed design, or it could be a general (generous) allowance. Either way, if the designer makes this specification, comparisons between designs could be better informed.

There's nothing like hopping aboard a boat with a sack lunch and finding that she's exceeded her design displacement.

DC

 

No boat has a fixed displacement. None.
It is a moving target, and should be quoted on plans as "Displacement to DWL" or "Displacement at half-load" or whatever. A weight estimate showing the derivation of that number can be handy to show the punters as well.
On the lines plan, the "to DWL" type is great, beacuse that is the magic weigt that floats the boat to its lines. If you want to know what the boat weighs when empty, then a "lightship Weight" should be provided. It is a term recognised by most YDs and NAs all over the world.
Steve

 

Thank you, Steve, for suggesting the clear standards of Displacement to DWL and Lightship Weight. It would sure make comparisons simpler if designers would present their figures in a consistent format.

Regarding "lightship weight", is their any "standard" for what items are included in that weight? In a sailboat, does it include the sails and rig, but not the outboard? In a raceboat do you discount the galley, but include it in a cruiser? Perhaps this is where the designer really needs to include a summary weight estimate.

Seems like often designers presenting plans for unbuilt boats are overly optimistic regarding lightship weight. Then it apears you have a lot more available payload to settle out at DWL than you actually do. Or you find out the payload published for the boat in "fully loaded condition" has the hull(s) immersed well beyond the published DWL. Can think of a couple micromultihull designs with published plans that play a little fast and loose with displacement figures.

Chris

 

In fact, there is ISO 8666 standard.

 

Interesting - it defines everything in terms of mass, sidestepping the displacement issue altogether.

The light craft mass includes
- heaviest outboard that can be fitted
- batteries
- cushions & mattresses
- anchors, warps & chains
- fenders, docklines
- mast, boom, spinnaker poles
- working sails, but only if they are supplied as standard

Not included are removeable items like
- cutlery, crockery, kitchen utensils, linen
- charts & loose navigation equipment
- tools & spare parts
- additional sails
- personal safety and lifesaving equipment
- liferaft
- fuel & water
- people

But there's a "performance test mass" for power boats that adds in personal safety equipment, 25% - 50% fuel, working sails, anchors & warps, but still leaves out most of the removeable items. (I wonder if most people realize just how stripped the boat is for the performance specs.)

Then there's the ready-for-use condition, which has all the equipment on board and tanks filled; the fully loaded ready-for-use, which add the people, personal equipment and liferaft; and finally the maximum load.

Why does one never sees the ready-for-use conditions quoted - they're required for the owner's manual? The difference between the maximum load and the ready-for-use load is the real payload that's useful.

 

My take on displacement has allways been a little confused so please allow me to bounce this off of you all!
I will use silly numbers to keep it simple for a boat like my trawler.
1-design your hull and set your DWL a little light
2-get your rough numbers from the calcs or your math (disp & moment to trim)
pretend the hull as is displaces 70,000# with 7,000# to trim 1 inch
3- calculate actual weight of hull and superstructure per building method
eg..780 sqft of 3/16" Alu and so on say this gives you 11,000#
4-make a detailed weights spreadsheet adding ALL things the boat will carry
with all tanks at 3/4 full say this is 18,000#
so total would be 11,000+18,000=29,000#
5-take all the weights you have including the hull proper and divide by 7,000
29,000/7,000=4.14 go back to your hull and sink her 4.14" to the now LWL
your math should say something like 81,000#

So the question is.... Does she displace 81,000#
OR 81,000+29,000=110.000# for an all-up weight
My gut tells me it's 81,000# Displacement is only a term describing the pounds of sea-water displaced by the immersed volume of the hull as determined by the setting of the LWL.
All this of course revolves around a educated guess at the DWL to start with!
I hope I was able to describe this well enough so you folks could see what I'm getting at!
This I think is one of the most basic things one needs to understand to be able to design properly so she floats on her lines on launch day. Your client will love you AND you can put her in your portfolio
Tell me if I'm all wet! 8Knots

 

The DWL is determined by the displacement. The salt or fresh water displaced by the hull weighs the same as the boat.However, there should be two different DWL for fresh and salt water. 70,000+11,000+18,000=99,000 that is the weight of the water displaced and the loaded displacement of the vessel. I assume that everything means cargo and passengers too.
How do you come to the conclusion that the boat will sink 1" for each 10% of added weight? Also, you make no allowances for changes in trim. If the boat won't float on her lines, the underbody has to be redesigned or some heavy things need to be repositioned. The process is very simple but tedious. You figure the underwater volume of each section and the weight of everything in that section. The weight and displaced water aft or forward of the CF must be equal. If they are not, something has to be changed.

 

Respectfully I disagree....The LWL or load waterline is determined by the total volume of displacement. The DWL is only a tool. I may very well be wrong I am no designer at the professional level. I understand that salt and fresh will affect the displacement values, I was trying to keep it simple.
also the numbers posted are purely for demonstration. when designing a hull, whether using software or Simpsons rule you end up with a displaced volume based on an arbitrary WL that you as the designer set. then it is simple to figure how many pounds of weight it takes to "sink the hull equally" I went blank and can't remember the official term. you are correct in this makes no compensation for trim. use that number for at rest only. you are 100% right on for & aft trim this needs to be done on the drawing table rather then expostfacto. I truely believe the more acurate you are with your weight tables and there correct position in the hull proper the more confident you can be on launch day! As you said all this is easy but tedious!
I hope this sounds right Gonzo we'll get her figured out
8Knots

 

8 knots,

the term you are looking for is TPC or TPI (tonnes per cm or tonnes per inch). TPC is calculated purely from the waterplance area x 1cm x density.

tedious? get a decent hydro program and you soon forget all of that simpsons stuff 1 4 2 4 2 4 2 .....something something something.... /3 lol

Of course crap in = crap out.

 

8knots -
Afraid I don't get your take on displacement at all. You suggest your bare hull + superstructure weight will be 11,000# and the weight of everything else will be 18,000#. Your would be trying to design a hull that displaces 29,000# (or 453 ft^3 of saltwater, or 465 ft^3 fresh water). I don't understand what the 70,000# figure has to do with anything.

A simple way to start would be to assume a waterline length (LWL), max waterline beam (BWL), and maximum depth (D). Draw a hull profile and datum waterline that you like using only those parameters. Now choose where your section (body plan) of maximum area will be - probably somewhere between where the profile is deepest and the waterline beam is widest. Draw in a hull section using the waterline beam and hull depth picked off from the profile and waterline views.

Find the area of that immersed section (A). Pick a prismatic coefficeint (Cp)that's reasonable for the type of boat your trying to design. If your hull isn't too radical, just start with 0.55 if you don't have an optimal or target value. Multiply immersed max section area*waterline length*prismatic coefficient (A*LWL*Cp) to get an approximate design displacement for your hull.

Take the area of the datum waterplane you just drew * 1" depth and convert volume to pounds to get Pounds Per Inch Immersion (PPI) for your hull at level trim.