Design vs Actual displacement

[sailjumanji]

Originally Posted by clockwork orange

I have been weighing a few boats at the marina where I work and it seems to me that our travelift is showing about 1600lbs over which is not surprising really. When you think of it they are not really scales per se but rather 2 hydraulic pressure gauges which read the front and back straps. I don't know for sure but I suspect that everything after the cylinder rods is probably being counted in the weight you are seeing, the cables blocks and wet straps which I could easily see adding up to the 1600lbs discrepancy I am seeing. I would like to weigh something with a known weight like a one design race boat. A couple of other factors are that what the designer calculated and what the builder built are two different things and most production boats come in heavier than the designer intended. Boats are usually designed with half fluids and most of the items that it would be expected to have.

What amount of weight are you talking about? If you need a known weight, take a trailered boat to a certified scale, weigh it, and then weigh it on your travel lift.

Here is the other thing we find when chasing down weights. Luckily, sometimes we actually get to talk to designers and builders. Ask them where in the process they actually weighed the boats, with the reported weight. Many of the trimaran and small cat builders have weighed boats. But what you will find when questioning them is that it typically is before the engines, batteries, mast and boom, sails and rigging go on. They are basically weighing at the production/layup side before its rolled into rigging. Funny thing is that the initial part of the conversation - when you feel you are questioning their reported weights - is very defensive. Then when you tell them you just want a fair shake for their owners, because "x" thousand lbs for a 30 ft cat seems very light and will result in a fast PHRF rating. Then they start backtracking with everything that got omitted, because it hadn't been added yet when the boat had been weighed. No manufacturer wants to be known for producing heavy boats, or certainly not the ones that want to say their boats can actually be sailed.

[Dsanduril]

If you want an example the other direction, the last travelift operator called out that our 50’ cat only tipped his scales at 10,000 pounds! But to fit our 27’ beam we were on a 150 ton lift and even at ~11-13 tons actual weight our boat was less than 10% of the lift’s capacity. Given that they’re only really reading a couple of pressure gauges with unknown provenance don’t expect anything close.

[Jim Cate]

I've often thought that one needs to consider the angle that the straps make when lifting the boat. A narrow boat in a big travelift means that the take off angle of the straps is very different than when lifting a beamy catamaran where the straps go straight down from the lifting pulleys... and it seems that the geometry would mean higher relative pressures for the narrow boat and hence a higher indicated weight.

I've had very different readings when lifted in a 35 tonne machine compared to a 100 tonne one... and I don't really trust any of them.

Jim

[MondayNever]

Originally Posted by SVTatia

I agree 100%

^^^^This!

I have some numbers for the Contest 46, and since we are quaren-tained, lets crack our heads:

LWL = 39ft (rounded)
Waterline area = ((.67*39) * 12 (assumed beam at LWL)) = 313.56
313.56 * 5.333 = 1,672 - This is the pounds per inch of of immersion for the Contest 46.
In other words, for every 1,672 punds you loaded in, the designed waterline sinks 1 inch.

Therefore, if the designed displacement is 33,960 pounds and the current displacement is 46,000 pounds, the difference is 12,040 pounds over, therefore your current wateline should be

12,040/1,672 = 7.2 (or more - see below).

Is the designed waterline 7 inches below the water?

-----------------------------------------------------------------------
Ref. Ted Brewer:

WATERLINE AREA: The area of the LWL, usually expressed in square feet. It is not always easily obtained but can be calculated roughly for a sailboat by the formula : .67 x LWL x Beam. It is more accurate if you have the Beam WL rather than the Beam(Max), of course. Knowing the LWL area is essential in working out the following calculations.

POUNDS PER INCH IMMERSION (PPI): The weight required to sink the yacht one inch. It is calculated by multiplying the LWL area by 5.333 for sea water or 5.2 for fresh. The PPI usually increases as the hull sinks into the water as the LWL area is also increasing due to the shape of the hull above water.

Thanks for doing the math for me! Unfortunately, I don’t know exactly where the designed waterline lies. I’ve got ~6” of anti foul above her current lines, but that probably doesn’t mean much as it’s entirely possible that the painted waterline could have been raised. The only indication that I’ve got that she’s sitting low is that some of the output thruhulls (sink drain, scuppers, etc) are submerged by a few inches. But if I were the designer, I would have spec’d these for a lot more than 4-5” above the designed waterline anyway, so maybe this is just a fundamental disagreement between them and I about where thru hulls should be placed.

Any other ideas about how I can tell where the dwl is located?

[SVTatia]

Originally Posted by MondayNever

...

Any other ideas about how I can tell where the dwl is located?

Thats a tough one. Usually, builders place a marker at the bow and stern of where the WL should be, but these can be hard to find on an older boat as it may have been sanded thru or filled with paint.
The only suggestion I have is to go from the bottom up. If the designed draft is 6.42, measure that from the bottom of the keel up on a trully vertical plane and place a mark on the hull.

[SVHarmonie]

Originally Posted by MondayNever

Any other ideas about how I can tell where the dwl is located?

Somewhere, someone might have a drawing of the hull with the DWL marked on it...

[MondayNever]

Originally Posted by SVHarmonie

Somewhere, someone might have a drawing of the hull with the DWL marked on it...

Presuably Dick Zaal or Contest have the actual drawings with measurements included, but the ones I've found online do not. There is a line in this one that I could try to eyeball against my own boat, but there will be at least a couple inch margin of error, which as the esteemed SvTatia calculated on the first page, could be a few thousand pounds in either direction.


I should (and now will) contact the yard to ask for detailed drawings, but I was wondering if there was some other trick for determining dwl to which I was oblivious.

[MondayNever]

Originally Posted by SVTatia

Thats a tough one. Usually, builders place a marker at the bow and stern of where the WL should be, but these can be hard to find on an older boat as it may have been sanded thru or filled with paint.
The only suggestion I have is to go from the bottom up. If the designed draft is 6.42, measure that from the bottom of the keel up on a trully vertical plane and place a mark on the hull.

When I measured the draft on the hard I came in at ~6.25.' I'll grant you that this was not the most precise of measurements (as I was not doing it with attempting to determine displacement in mind). At that point I pretty much disregarded the sailboatdata quoted 6.42' draft as being something that was either wrong or had varied over the production run, given that the Travelift readings suggested I ought to be significantly deeper than 6.42'.


Thanks for the tip on the markings. I'll see if I can find any then next time I'm at the boat.

[Cheechako]

My 47 mono Passport was down 5" on the waterline when we had her loaded. Surprisingly, transferring that gear to our 42' Cat right across the dock the cat was only down 1.5" +

[SVTatia]

Originally Posted by Cheechako

My 47 mono Passport was down 5" on the waterline when we had her loaded. Surprisingly, transferring that gear to our 42' Cat right across the dock the cat was only down 1.5" +

Thats interesting, but there is something missing you did not mention - it seems it was already down - how much did the Passport go up right after you removed the "cargo"?

[SVHarmonie]

Originally Posted by Cheechako

My 47 mono Passport was down 5" on the waterline when we had her loaded. Surprisingly, transferring that gear to our 42' Cat right across the dock the cat was only down 1.5" +

If your Passport went up 5" and your cat with down 1.5 you are either missing a part of the story, or you have have changed laws of physics that have not been violated since Archimedes took his famous bath.

[chrisr]

Originally Posted by SVHarmonie

If your Passport went up 5" and your cat with down 1.5 you are either missing a part of the story, or you have have changed laws of physics that have not been violated since Archimedes took his famous bath.

reading between the lines, i think his point is that the passport was down 5" on DESIGNED WATERLINE and the cat was 1.5" down on DESIGNED WATERLINE

the inference being that different designers allow for more or less extra weight when setting the designed water line

cheers,

[billgewater]

The immersion figures discussed should be available from the designers naval architect. The pounds per inch (or metric units) is an essential feature and the variability of the "constant" should also be well known as would be the variability with heeling and or pitching. These are fundamental factors in boat hull design. Archimedes would have something to say about all this. Think about large transport ships and Plimsoll lines etc.

On some data sheets the pounds per inch immersion is given at the designed LWL, but as mentioned by others it can be highly variable. An excellent example is some catamaran hulls that are actually concave near the LWL whereas most hull forms are either vertical or convex. (depending on your perspective) and if you also consider the "pointiness" of a normal bow to the common flatish overhang at many sterns then it is understandable that the immersion constant can change dramatically with immersion.

The designer may not wish to release this data for commercial reasons but if you provided a set of simple measurements e.g. from the gunwale to the current water line or from the keel bottom up, then they should be able to tell you exactly what the current displacement is. (Unless the hull displacement form has been altered.)
If you can get your hands on the offset tables for the hull form of your boat, then any competent naval architect can provide the data you are chasing. (use is made of Simpson's rule etc.)
However, is it worth worrying about?

[Pottasch]

My Personal Experience- The design was 8000 lbs. The original builder told me Actual should be 13,000. When I hauled out it was 22,000lbs. In disbelief i called the company (not the boatyard) that tested that lift. They said that lift was tested within the month and they stood behind the numbers. 6 months later the boat yard bought a brand new lift. same boat under same conditions.. it weighed 16,000lbs. I'm going with the weight from the new lift, but i have no confidence in any of them.