Weight and displacement

[wallythacker]

At one time I knew the answer to this.

Is weight for a boat the same as the displacement for a boat?

[Mediator]

Yup. As long as it's not a submarine.

[StuM]

Yes, but tonnage is a different thing entirely

[wallythacker]

So when I subtract the published ballast from the published displacement the figure is the weight of the boat with engine all systems and sails, but not personal items. Is that right?

Does anyone know if the tanks are supposed to be filled when the boat's displacement is calculated and published?

[smith8273]

I'm another one that is curious, what displacement is...

I assume that it is the completely fitted out but empty boat; in other words sails, rigging, head and engine counts — water and fuel in the tanks, knives and forks do not.

Is the anchor and rode part of the displacement? Stove?

[valhalla360]

Originally Posted by wallythacker

So when I subtract the published ballast from the published displacement the figure is the weight of the boat with engine all systems and sails, but not personal items. Is that right?

Does anyone know if the tanks are supposed to be filled when the boat's displacement is calculated and published?

Depends on the specific displacement you are looking for. Usually the ballast is part of the displacement.

I believe empty would be light ship displacement. There would also be maximum design displacement which would be full tanks and maximum allowable cargo. Might be a few other variations.

Displacement is the weight of water displaced by the hull...which for most practical purposes equals the weight of the boat.

[djmarchand]

Realize that any builder's published displacement numbers are highly suspect and almost always low. Some builders are pretty rigorous and some take it lightly shall we say.


The worst I have experienced is an Island Packet catamaran which weighed 18,000 lbs on the Travelift scales but IP calls it 12,000 lbs.


David



David

[Don C L]

It's my understanding that the published data on displacement is essentially what the designer calculated with all tanks empty and no rigging or sails. So it would be the weight of the hull and ballast and interior cabinetry. I am attaching a portion of the letter that Olin Stephens wrote to the original owner of my boat to point out that even the designers don't know for sure exactly how the boat will actually turn out once it is floating given that they are not sure what builders will do, or owners.

[Pelagic]

Originally Posted by wallythacker

Does anyone know if the tanks are supposed to be filled when the boat's displacement is calculated and published?

When a new boat or ship is sea trialed and also tested for stability,(Inclining Tests) it is tested at:

Lightship: All tanks empty-no cargo
25% Tanks and Cargo
50% Tanks and Cargo
75% Tanks and Cargo
100% Tanks and Cargo

These are often tied to contractual performance gaurantees, as well as Stability requirements.

They also form the Data for measuring "Displacement" at any loading, thru calculating the amount of immersion as measured by the draft marks (fore/aft/midship)

From that Data they calculate what is called TPCI TONNES PER CENTIMETER IMMERSION for that hull design.
Very important for doing draft surveys in bulk carriers...Also knowing your weight if being lifted out of water

If the vessel is built under a class standard, those tests and numbers are independently monitored and verified.

[StuM]

Originally Posted by wallythacker

So when I subtract the published ballast from the published displacement the figure is the weight of the boat with engine all systems and sails, but not personal items. Is that right?

Ballast is generally the weight of the keel (speaking very loosely). Subtracting it tells you what the rest of the boat weighs. Ballast/Displacement ratio is an indication of stability.

[wallythacker]

Originally Posted by djmarchand

Realize that any builder's published displacement numbers are highly suspect and almost always low. Some builders are pretty rigorous and some take it lightly shall we say.


The worst I have experienced is an Island Packet catamaran which weighed 18,000 lbs on the Travelift scales but IP calls it 12,000 lbs.

David

I'm hoping that doesn't happen with her other boats I'm dealing with. The first one is spec'd at 12,500lbs. My new trailer is rated for 15,600lbs. The next boat I'm moving is spec'd at a weight of 14,500lbs.

This could get interesting...

[Simi 60]

No.
A boat can have a displacement of 10,000kg but weigh 5000kg alloying it to carry 5000kg of stuff.

[StuM]

Originally Posted by Simi 60

No.
A boat can have a displacement of 10,000kg but weigh 5000kg alloying it to carry 5000kg of stuff.

Depending on which displacement you are talking about. The above is true if it is "light ship displacement". If it is "maximum displacement", then that is not so.



My Belize is specified as 8,600kg light ship displacement and 11,500kg maximum (or Full Load) displacement.
But maximum load including crew is specified as 12,030kg.

[SV Siren]

I am blatantly stealing this from Robert Perry's facebook page. He wrote a "little" tutorial talking about displacement..July 18, 2018.


Grap a cup of your favorite beverage..it takes a while to read.


DISPLACEMENT 2
I thought we should start this series of mini tutorials on numbers with a discussion of what is meant by “displacement”. This is a very important number in the world of yacht design. To make it easier for me I will contract this to “displ” from time to time. Do not confuse “displ” with “DSPL”, totally different and we can discuss that later.
I am frequently asked, “What is the difference between displacement and weight?” Simply put, when it comes to yachts there is no difference. In our whacky yachting vernacular “displacement” means “weight”. Technically displacement is the immersed volume of any hull. In the US we measure this in cubic feet. Take that volume. Multiply is by the density of salt water, 64 lbs. per cu. Ft. and you have the weight of the boat. If you sail on fresh water you can use 62.2 lbs. per cu. Ft. times the immersed volume. The exact density will depend on the salinity of the water. Your boat will float deeper, i.e. have less freeboard, in fresh water than salt water. More on this later.
Designers will determine the displ for a new design from the hull lines drawing. In the pre computer days this was done by measuring the immersed sectional area at each station and running the areas through a formula called Simpson’s Multipliers. There are other formulae for this but I have never known a designer who did not use Simpson’s. Of course today, with a computer program you get displ with the push of a button. Who knows how the computer gets the number? But we all seem to trust it. I will say that in doing the calculation by hand with a planimeter, a fancy tool used to measure square footage of shapes, you get a very viscera feel for the shape you are producing.

When I was 15 I could not afford a planimeter. They were about $100 used in a pawn shop if you could find one. So the first time I had a complete set of lines and wanted to find the displacement of my design I drew a 12” square grid on tracing paper. I laid the grid over the body plan of my lines drawing and I counted the squares, estimating for the fractions of squares in some places. I plugged my areas into Simpson’s and with my slide rule I went to work on the calculation. I was a very poor math student in high school. I think my problem was dealing with abstract math problems. But when I applied math to yachts I had an idea of the numbers I was looking for. For instance, I knew my 32’ double ender should weight between 12,000 lbs. and 15,000 lbs. based on similar boats I had studied. So I was shocked when my calculations were completed to find that my 32’er weight more than twice as much as any similar design. I’d make a mistake somewhere. I can’t recall how I figured my error out but in time I did. I had measured and input the entire sectional area of each entire station right up to the sheerline! I had sunk the boat! I should only have measured up to the DWL. I put the correction in and recalculated and came out right where I had estimated. I always start with a guess at the answer I am after. I am an extremely good guesser when it comes to boat numbers and this way an error is immediately obvious. When I had a staff I would give a calculation to a helper and tell him before he started what the answer would be. You can call it “showing off”.
Very often there is a curve on the hull lines drawing showing the immersed volume. This is called the “curve of areas” or “displacement curve”. You can clearly see it on my example set of lines here. If the designer is working by hand he can plot the areas he gets. If he can’t draw a fair curve through those plot points then there is an error at that station’s area.
Displacemet can also be measured in:
Long tons 2,240 lbs.
Short tons 2,000 lbs.
Metric tons or “tonnes” 1000 kg or 2,200 lbs.
I use long tons for my D/L calculation.
Do not get confused with “gross tonnage” or “net tonnage”. These are terms used to describe the volume of a commercial ship and used for determining classifications, cargo and harbor fees etc.. These numbers have nothing to do with the weight of the boat and everything to do with the volume inside the vessel.
If you have a boat designed in the IOR era you may find the term “DSPL” in your documents or specs. DSPL is an IOR term that represents an estimate of the boat’s displacement. It will be close but it will not be an accurate displ. Under the IOR it was advantageous to have the rule’s DSPL be heavier than the actual displ of the yacht. The heavier you measure the lower your rating. More DSPL for a given rating means you can have a longer “L” or more “SA”. DSPL was calculated from depth measurements taken at the mid section below the sheer times “L”. To increase the depth measurements some owners would take their IOR boats into fresh water where the boat would float deeper, i.e. heavier”.
There is also often confusion over the IOR “Ton” classes. You can go back to the fourteenth century when ships began to be measured for cargo taxes and see early tonnage rules. “Ton” referred to “tun” which was the size of a standard barrel. Ships were classified by how many barrels they could carry. By 1694 ships were being measured to determine “tonnage” with very simple formulae : (L x B x D) /94. Soon boats were being designed to take advantage of this rule so they carried more and paid less taxes. In 1845 the Royal Thames Yacht Club introduced the Thames Tonnage Rule to handicap racing yachts. In time this morphed into the classes, quarter ton, half ton, three quarter ton, one ton and two ton that eventually the IOR would adopt and base on IOR ratings. A typical one tonner probably weighs between 14,000 and 17,000 lbs..
( If this history of rating rules is of interest to you I suggest you get a copy of Peter Johnson’s fabulous book YACHT RATING.)
Published or brochure spec’ed displacements present a problem. I have had the honor of writing design reviews for SAILING magazine for over 35 years. I used to do four boats an month. Now I do three designs a month. This means I have taken a good, hard look at around 1,630 different designs. Published displacements are often very optimistic and sometimes just plain wrong. I want a displ that reflects the weight of the boat as it sits at the dock ready to go sailing. Designers often use a “light ship” displ to represent the weight of the boat just as it leaves the factory. The boat will never weigh this once it is launched and outfitted. I use a “light ship” displ but I also use a “loaded” displ to represent the boat’s weight when it is ready for a long voyage. But the displ I am most interested in is the weight of the boat as you would typically find it any given weekend, at the dock, ready for a nice afternoon’s sail. Designers work from a “weight study” to approximate these weights. The validity of the weight study will vary from office to office and design to design. Designers usually hate doling weight studies. The bottom line is this: If we go to the marina and walk the docks and I give you a buck for every boat floating light on its DWL and you give me a buck for every boat floating heavy on its DWL I will end up with a big pile of cash. Boats weigh what boats weigh. Just go with it. You can’t sail a brochure.
With an accurate displacement calculated from the hull lines the designer can now launch into a series of other calculations all depending on displ and then make changes where he thinks is necessary in order to get both the shape and the numbers he wants.
How can you tell what your boat really weighs?
You can either take freeboards fore and aft on your boat (don’t stand on the boat when you are doing this) then call your designer and have him compare your freeboards to the designed DWL freeboards. This will allow the designer to adjust the actual waterplane and calculate the actual weight of your boat. This is a snap with the computer and a PITA by hand. Or, you could rent load cells and weigh the boat hauled out in the boatyard. I don’t trust Travel Lift weights. They are notoriously wrong. Of course the Travel Lift operators all think that designers are dreamers. Of course we are. How the hell cold we design a boat without a dream?
I think that about covers it. Displacement is not an “opinion”. Displacement is not “subjective”. My approach to writing about it may be subjective but the facts presented are not opinion. They are honest numbers derived from age old formulae. I’m very happy to answer any questions you might have but if you want to challenge me you had better come loaded for bear. Contributions to the subject at hand are encouraged. Always.
Wasn’t that fun?

[StuM]

Originally Posted by SV Siren

There is also often confusion over the IOR “Ton” classes. ...
In 1845 the Royal Thames Yacht Club introduced the Thames Tonnage Rule to handicap racing yachts. In time this morphed into the classes, quarter ton, half ton, three quarter ton, one ton and two ton that eventually the IOR would adopt and base on IOR ratings. A typical one tonner probably weighs between 14,000 and 17,000 lbs..

What's a "typical one tonner"


The original One Ton Cup was based on the 1899 Godinet Rule, not Thames Tonnage Rule. They were basically 6m dinghies and would have been a lot lighter than 14-17,000lbs. The last to race as "one tonners" were 40-45ft keelboats. (The last Bruce Farr designed 1 tonner was a 40 footer and weighed in at about 12,000lbs)



As you say the rules morphed constantly. It wasn't until the mid '60s that the 1, 1/2 and 1/4 tonners came under RORC/IOR rules.



I've raced one of the last Bruce Farr design 1/2 tons (Design #93) for years
(We "won" the Coral Sea Classic from Cairns to Port Moresby in it two years ago)