Displacement weight versus actual weight

I've been looking at cradles and trailers to keep my sailboat on. The boat is an Alberg 30. The displacement is 9000lbs/4,082 kg. The boat is an early non liner boat. Everything is built into the hull so it should be lighter from my understanding. When I take into account everything in and on the boat, I come to a weight of around 6000lbs/2721kg. A member on another forum said his boat in cruising trim was 6600lbs/2993kg hanging in the slings. Why would I need a 12,000lbs/5443kg cradle or trailer to keep my boat on? That's double what the actual weight of the boat is and those heavy weight cradles and trailers are insanely expensive.

 

Displacement and weight are identical values. The first is the weight of the displaced water when the boat is floating. The other is the direct weight measurement of the boat. If the displacement is 9000 lbs, so is the weight. If it weights 6000 lbs, however you measured that, the displacement is 6000 lbs. How did you weigh the boat?

 

I haven't weighed mine. A member on another forum was told his weight (6600lbs) while the boat was hanging in the travel lift slings. I remembered the info but wish I had bookmarked the thread in the forum. I thought the manufacturer's stated displacement weight was the maximum weight a boat could have all in loaded to the max with gear, full tanks, persons, etc... My boat is very basic with empty tanks, no head, and no soft goods. No nothing except the furniture, drive line, and stuff built in the boat from the manufacturer. There's no way the boat weighs 9000lbs the way it sits.

 

There are two ways to find out the weight/displacement of the boat. Weigh the boat directly, is the easy one. You can calculate the immersed volume of the hull and multiply by the density of water and it will give you the weight. You can't reliably get a weight/displacement from rumors or hearsay.

 

Weighing in a Travelift is notoriously inaccurate unless it has been calibrated.
The displacement/weight the designer gives are with the boat floating on its "designed" water line.
That usually includes 1/2 the water and fuel capacity plus an allowance for some stores and personal gear.

SailboatData.com - ALBERG 30 Sailboat

 

It's hard to tell form sales literature what's what.

 

ok, none of this makes sense to me. look at this boats specifications. the rumba weighs 150lbs, it has a displacement of 225lbs, yet it is shown with 2 adults and 3 kids in it, and claims to be able to handle 3-4 adults. If I am reading this thread right, if the displacement is 225 and the boat weighs empty 150, then the boat can only hold 75lbs of crew and gear. Something doesn't make any sense. I've read in other threads that the 'capacity' of the rumba is 550lbs what is the explanation???

 

"Displacement" is a very confusing and even imaginative and amusing term.

For example, perhaps 'German Displacement' equals 1.25 x 'French Displacement'

And actual Displacement sometimes equals 1.5 x designer's Displacement.

I mean there are three forces of Chaos here:

1) different terminology or different languages or different usages and customs
2) imaginative commercial brochures of some shipyards
3) cheerful estimates impossible to believe

and to complicate things further
4) cranes for reasons of prudence often overcount.

 

Yes, there is the actual displacement and the legal displacement.

 

Here in the US, displacement is the volume of the boat, in lbs of saltwater; and displacement can vary with the loadout aboard which includes changes in trim. Designed, and typically advertised displacement is the vessel sitting on its designed lines with its designer selected weights (whatever those are, typically "burned out"...i.e. nothing but the fixed gear and fluid in the sumps) aboard. Hull weight for a portable boat is exactly that;... no rig, no rudder, no portable anything...what you need to pick up and put into the rooftop carrier. Capacity, in the US, is a federally defined condition.

Federal Register :: Request Access https://www.ecfr.gov/current/title-33/chapter-I/subchapter-S/part-183

 

I would think they are separating out the hull weight,which you may have to lift around,from the weight of the fully rigged boar when it is ready to sail.Things like spars,sails,rudder and assorted lines would make the difference.I would think the capacity refers to the occupants plus any "stuff" they feel a need to go afloat with.

 

There should be a marina/haul out facility in your area that has a calibrated Travel Lift.
It's simply a chart that lists hydraulic pressure vs weight for each sling.
The weight of the slings is then deducted to get boat weight.
I think the guy who said his Alberg was 6,600 is off his rocker.
That's a boat with a D/L ratio of almost 400, and has 3,300lbs ballast.
I would fully expect that boat to weigh at least 9,000lbs, if not more.

 

Don't bother with imaginary numbers too much. Design displacement is there to verify you are not too far from it to make your boat unsafe. The real displacement is and always will be the same as the weight of the boat regardless what's the liquid where it floats..

 

The weight (displacement) mentioned is that of the hull only. The other number mentioned is likely the hull, the rig, the centerboard and rudder, and some day sailing gear such as a cooler. My guess is it includes everything anticipated other than the crew. By describing its displacement this way, they can get a much higher Sail Area Displacement (S/D) number to hint at greater performance.

 

My guess is that the 6,600 lb weight is reasonably close to the truth. The 9,000 lb displacement figure is likely that of the boat loaded to capacity. Back in the Era when that boat was built, performance and light displacement weren't as much of an obsession as they became during the IOR days, on.

At that point sailboat manufacturers knew that racing was likely to become a major part of their product's life. And they tried hard to more or less imitate custom built racing boats of their day. But their boats had to be decent cruisers too. And they probably couldn't afford to use every weight saving building technique, the custom builders used, and keep their product affordable. So instead of mentioning the maximum displacement, they often went with the "light ship" one. That is the boat fully rigged with no crew or stores on board. They left it up to the public to imagine how many crews and stores were included in their displacement figure. The convention supposedly became that it included half maximum load. I think that the lighter the boat is supposed to be, the more likely the displacement figure is based on light ship loading rather than half or maximum loading. At least this is the assumption I make unless the brochure specifies the load condition in their displacement figure.

Now, for this particular boat.

There is a rather crude way of checking. It requires the boat to be afloat at its dock in dead calm conditions and no current. It requires at least two people, a tape measure, and a reasonably accurate bathroom scale. The procedure goes something like this:
1.) weigh yourself on the scale and log the weight (Do this at the boat with all of your gear on).
2.) Untie the boat and measure its distance from a fixed point on the dock.
3.) with the tape measure ready, (in your friend's hands), walk a given distance aft on the boat, then stop.
4.) have your friend measure how far the boat has moved from that fixed point, and write down this number. Do this at least three times, then average out the numbers you get.
5.) Divide the distance aft that you walked by the distance the boat moved, then multiply that product by your weight.

This will give you a rough estimate of the boat's mass (displacement).