Design vs Actual displacement

[MondayNever]

When we hauled our recently purchased 1995 Contest 46 I expected her to be heavier than the design displacement of 33,960 pounds. She's got davits, a genset, washing machine, and myriad other systems that have accumulated over the last 25 years. We also had roughly a half load of diesel and water. But even so, when the Travelift operator called out "46 thousand!" I was pretty surprised. That's 26% over! I don't have reason to believe she's water logged as the hull sounded with flying colors during the survey and everything was dry when we replaced 9 thru hulls, so I think this is just a combination of designers underestimating and the sheer combined weight of hundreds of little gadgets and spares that accumulate after the boat leaves the builder's yard.


It would be really interesting to compile a list of other boat models' actual vs design displacements and some basic boat-specific details. It's not information that I was able to find anywhere else. I'll start:



___________



Model: 1995 Contest 46
LOA: 48'
Beam: 13.75'
Design displacement: 33,960 pounds
Actual displacement: 46,000 pounds


Particulars: Half load of diesel and water. Extensively cruising equipped but only minor to moderate accumulation of loose junk.

[Ben245]

That seems in line. Hull displacement is typically hull only with structural furniture and nothing else. In our case:
Passport 545
39, 000 lbs Stated Displacement
4,000 lbs Interior equipment like W/D, AC, Reefer, Electrical and electronics, plumbing & heads, steering, engine, generator, running gear
+ 2,000 lbs personal effects, tools, spares, bedding
+ 4,000 Lbs from 550 gallons water/fuel
+ 2,000 lbs rigging, spars, sails,
= 50,000 lbs all In (49 and change when weighed)
Right with you at 26%!!!

[MondayNever]

Originally Posted by Ben245

That seems in line. Hull displacement is typically hull only with structural furniture and nothing else. In our case:
Passport 545
39, 000 lbs Stated Displacement
4,000 lbs Interior equipment like W/D, AC, Reefer, Electrical and electronics, plumbing & heads, steering, engine, generator, running gear
+ 2,000 lbs personal effects, tools, spares, bedding
+ 4,000 Lbs from 550 gallons water/fuel
+ 2,000 lbs rigging, spars, sails,
= 50,000 lbs all In (49 and change when weighed)
Right with you at 26%!!!

Designers of cruising boats really don't take into account fundamental equipment like the engine and heads? A genset and AC is an optional, but plenty of systems aren't.

[SVHarmonie]

There are two things going on here.

One is the market driven tendency for manufacturers to quote the lightest possible weight for their boats. Most builders who even want to pretend to be reputable will quote "light ship" weight, ready to sail but empty of all fluids and provisions. If there are any builders who quoted the weight of the hull alone without engine or interior furnishings as the weight of their boats, they are being downright duplicitous, to the point I wouldn't do business with them. There might be others, but Amel has always quoted their boat weights as full load weight, ready to sail.

The second issue, is that travellift hydraulic gauges are NOTORIOUSLY inaccurate. Accounting for fluid weights, our boat has varied in weight by almost 10,000lbs (~25%) from one lift to another. The only way to get an accurate weight is with a calibrated scale, usually on a crane. Since this is a expensive PITA, it is usually reserved for racing boats who need to prove they are within weight rules.

[MondayNever]

Originally Posted by SVHarmonie

There are two things going on here.

One is the market driven tendency for manufacturers to quote the lightest possible weight for their boats. Most builders who even want to pretend to be reputable will quote "light ship" weight, ready to sail but empty of all fluids and provisions. If there are any builders who quoted the weight of the hull alone without engine or interior furnishings as the weight of their boats, they are being downright duplicitous, to the point I wouldn't do business with them. There might be others, but Amel has always quoted their boat weights as full load weight, ready to sail.

The second issue, is that travellift hydraulic gauges are NOTORIOUSLY inaccurate. Accounting for fluid weights, our boat has varied in weight by almost 10,000lbs (~25%) from one lift to another. The only way to get an accurate weight is with a calibrated scale, usually on a crane. Since this is a expensive PITA, it is usually reserved for racing boats who need to prove they are within weight rules.

I take you at your word about what you've experienced, but a 25% error range across travelifts is hopefully out of the ordinary. I'll update this thread the next time I haul out at a different yard.



And while you're here, how 'bout you post up some numbers?

[StuM]

Extract from the post here:
https://www.cruisersforum.com/forums...ml#post2683146


quoting Bob Perry:
"I don’t trust Travel Lift weights. They are notoriously wrong."

[chrisr]

does the boat float on (or near) the design water line ?

if really 26% heavier, she will float a lot lower. if not floating lower then the chances are the travel lift weight is over

cheers,

[SVHarmonie]

Originally Posted by MondayNever

I take you at your word about what you've experienced, but a 25% error range across travelifts is hopefully out of the ordinary. I'll update this thread the next time I haul out at a different yard.



And while you're here, how 'bout you post up some numbers?

Here you go: our numbers:

Our 53 foot Amel Super Maramu was quoted in the sales brochure as: "35,845lbs (loaded)" Other numbers bounce around the internet that are close to this number and are of unknown provenance, but this was the official number from the designer. Our hauling weights (corrected to full tanks) have averaged 39,000lbs. I can easily believe we have 2.5 tons of "stuff" aboard. Heck, I probably have a quarter ton of tools alone.

I can't think of any one thing of significant mass that has been added to, or removed from, the boat compared to the day she was launched at the factory.

If our boat had been built by Beneteau, and quoted "light ship" that would have been, -(2205 lbs of water)-(1097 lbs of diesel)-(500 lbs of anchor and chain)-(???) something less than 32,000 lbs.

[sailjumanji]

Originally Posted by SVHarmonie

There are two things going on here.

One is the market driven tendency for manufacturers to quote the lightest possible weight for their boats. Most builders who even want to pretend to be reputable will quote "light ship" weight, ready to sail but empty of all fluids and provisions. If there are any builders who quoted the weight of the hull alone without engine or interior furnishings as the weight of their boats, they are being downright duplicitous, to the point I wouldn't do business with them. There might be others, but Amel has always quoted their boat weights as full load weight, ready to sail.

The second issue, is that travellift hydraulic gauges are NOTORIOUSLY inaccurate. Accounting for fluid weights, our boat has varied in weight by almost 10,000lbs (~25%) from one lift to another. The only way to get an accurate weight is with a calibrated scale, usually on a crane. Since this is a expensive PITA, it is usually reserved for racing boats who need to prove they are within weight rules.

Extremely well written, and I 1000% agree on both counts. We have same issues with the multihulls. I am on a rating committee for offshore multihulls in the Gulf Coast, mostly Texas. Manufacturers are all of the place regarding weights they report, and some have wide swings for the same models.

As far as travel lift gauges are concerned, the weight scale is only good enough to show them when they have completely lifted the boat. 20-30% inaccuracy - yeah, I believe it.

If it is a trailerable boat, we have them take the boat/trailer combo to certified scales, launch the boat, and come back and weigh the trailer. Those weights come in extremely tight and repeatable. regardless of which certified scales are used. For a big cat, I have yet to come up with an accurate program. Mostly we look at construction, interior, equipment and comparison to similar boats, and work up a weight guess. And in the world of PHRF rating, its weight, LWL, sail area and foils primarily. The rest is just tweaking.

For my Seawind, the manufacturer provided a waterline vs displacement diagram. Given where the empty boat floated on the lines, the manufacturer displacement looked pretty close. I wish more manufacturers would do this.

[Orion Jim]

Originally Posted by SVHarmonie

There are two things going on here.

One is the market driven tendency for manufacturers to quote the lightest possible weight for their boats. Most builders who even want to pretend to be reputable will quote "light ship" weight, ready to sail but empty of all fluids and provisions. If there are any builders who quoted the weight of the hull alone without engine or interior furnishings as the weight of their boats, they are being downright duplicitous, to the point I wouldn't do business with them. There might be others, but Amel has always quoted their boat weights as full load weight, ready to sail.

The second issue, is that travellift hydraulic gauges are NOTORIOUSLY inaccurate. Accounting for fluid weights, our boat has varied in weight by almost 10,000lbs (~25%) from one lift to another. The only way to get an accurate weight is with a calibrated scale, usually on a crane. Since this is a expensive PITA, it is usually reserved for racing boats who need to prove they are within weight rules.

Agree. If you don’t have a calibrated scale you are just guessing. I spent my career in the aerospace industry and calibrated freight scales were recertified twice annually. If there is no certification, or it is out of date, you cannot rely on the result.

[Macblaze]

Originally Posted by MondayNever

When we hauled our recently purchased 1995 Contest 46...

Ooh... Are you going back into the video business?

[Ben245]

Originally Posted by sailjumanji

Extremely well written, and I 1000% agree on both counts. We have same issues with the multihulls. I am on a rating committee for offshore multihulls in the Gulf Coast, mostly Texas. Manufacturers are all of the place regarding weights they report, and some have wide swings for the same models.

As far as travel lift gauges are concerned, the weight scale is only good enough to show them when they have completely lifted the boat. 20-30% inaccuracy - yeah, I believe it.

If it is a trailerable boat, we have them take the boat/trailer combo to certified scales, launch the boat, and come back and weigh the trailer. Those weights come in extremely tight and repeatable. regardless of which certified scales are used. For a big cat, I have yet to come up with an accurate program. Mostly we look at construction, interior, equipment and comparison to similar boats, and work up a weight guess. And in the world of PHRF rating, its weight, LWL, sail area and foils primarily. The rest is just tweaking.

For my Seawind, the manufacturer provided a waterline vs displacement diagram. Given where the empty boat floated on the lines, the manufacturer displacement looked pretty close. I wish more manufacturers would do this.

Originally Posted by SVHarmonie

There are two things going on here.

One is the market driven tendency for manufacturers to quote the lightest possible weight for their boats. Most builders who even want to pretend to be reputable will quote "light ship" weight, ready to sail but empty of all fluids and provisions. If there are any builders who quoted the weight of the hull alone without engine or interior furnishings as the weight of their boats, they are being downright duplicitous, to the point I wouldn't do business with them. There might be others, but Amel has always quoted their boat weights as full load weight, ready to sail.

The second issue, is that travellift hydraulic gauges are NOTORIOUSLY inaccurate. Accounting for fluid weights, our boat has varied in weight by almost 10,000lbs (~25%) from one lift to another. The only way to get an accurate weight is with a calibrated scale, usually on a crane. Since this is a expensive PITA, it is usually reserved for racing boats who need to prove they are within weight rules.

A builder publishing hull displacement and weight are two different things. Displacement is the weight of the volume of water the boat displaces at a certain draft usually measured in inches from a baseline. Most custom and semi-custom builders publish light displacement and will tell an owner the impact on displacement as both design and equipment decisions are made. The range of "ready to sail" displacements can have a pretty wide range. Agree that travel lift scales can be inaccurate but the bottom line is most all boats will weigh more on land than their stated hull displacement, and 20-30% more is not at all uncommon.

[sailjumanji]

Originally Posted by Ben245

A builder publishing hull displacement and weight are two different things. Displacement is the weight of the volume of water the boat displaces at a certain draft usually measured in inches from a baseline. Most custom and semi-custom builders publish light displacement and will tell an owner the impact on displacement as both design and equipment decisions are made. The range of "ready to sail" displacements can have a pretty wide range. Agree that travel lift scales can be inaccurate but the bottom line is most all boats will weigh more on land than their stated hull displacement, and 20-30% more is not at all uncommon.

If a boat weighs 30,000 lbs, then when floating it will displace 30,000 lbs of water at that identical load/equipment/tankage condition. A boat that is in light displacement condition will weigh in air the same as its displacement in water, in lbs, tons, kg, or whatever.

[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.

[SVTatia]

Originally Posted by SVHarmonie

There are two things going on here.

One is the market driven tendency for manufacturers to quote the lightest possible weight for their boats. Most builders who even want to pretend to be reputable will quote "light ship" weight, ready to sail but empty of all fluids and provisions. If there are any builders who quoted the weight of the hull alone without engine or interior furnishings as the weight of their boats, they are being downright duplicitous, to the point I wouldn't do business with them. There might be others, but Amel has always quoted their boat weights as full load weight, ready to sail.

The second issue, is that travellift hydraulic gauges are NOTORIOUSLY inaccurate. Accounting for fluid weights, our boat has varied in weight by almost 10,000lbs (~25%) from one lift to another. The only way to get an accurate weight is with a calibrated scale, usually on a crane. Since this is a expensive PITA, it is usually reserved for racing boats who need to prove they are within weight rules.

I agree 100%

Originally Posted by chrisr

does the boat float on (or near) the design water line ?

if really 26% heavier, she will float a lot lower. if not floating lower then the chances are the travel lift weight is over

cheers,

^^^^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.