September 23, 2016

Weighty matters

I thought I'd write a little essay on boat weights. There is a lot of misinformation I hear from my fellow sailors, so this is an attempt to educate. I'll try not to get too technical.

How much does your boat weigh?

Do you know how much your boat weighs? Probably not. "Wait!" I hear you cry - "I asked the Travelift guy how much it weighed when we last hauled out". Unfortunately the scales on a Travelift are usually just a pressure gauge which is measuring the pressure in the hydraulic system that is lifting the boat. And there is a scale reading in pounds but it's just a scaled pressure value. And they are never calibrated on a regular basis.

They are notoriously inaccurate for a variety of reasons:

- the manufacturer probably sets them to read high to reduce the chance of overloading the lift
- there is friction in the pulleys/wire ropes etc. which can vary from lift to lift
- straps and shackles are changed and they weigh less or more
- sometimes a lift will use 2 straps, sometimes 4 or more on the same lift. Sometimes big heavy spreader bars will be used so that 4 straps can be used. The gauge doesn't know that.

Every time we haul out, I ask the operator the weight. It's never close to the same amount twice. I'm talking thousands of pounds difference here. At one haulout in Australia, we got about 2500 lbs heavier in the four days we were hauled out. I don't think the new antifouling paint was that heavy...

What's the best way to find out how much she weighs at a given time? Lift it with a crane that has a recently calibrated load cell or put it on a trailer and take it to a truck weigh station, knowing the weight of the empty trailer.


Displacement of a boat is simply how much she weighs. When a boat is floating in water, she will displace (push aside), an amount of water that weighs the same as the boat (see Archimedes for further details). The weight or displacement of a boat will vary depending on how much fuel, water, food, and bottles of wine she has aboard at any given time.

"Brochure Displacement" This number you will see printed in boat sales literature. It used to almost always be simply "Displacement   XXX lbs". If you're going to quote one number for displacement of the boat, traditionally it was "1/2 load displacement". The naval architect would assume that fuel and water tanks were 1/2 full, some crew was aboard, some food, some gear, etc. It's an awfully fuzzy figure, because of the assumptions that about how much the food, gear, crew all weighed. They never assume that the boat is going long distance cruising with thousands of pounds of extra gear or food aboard. It's more of a "weekend displacement".

In reality sailboats are always heavier than brochure weights. Manufacturers want to keep weights down because a lighter boat is known to sail better than a heavier boat. Perhaps they don't lie as such, but they can be very optimistic in their numbers and justify it by saying they never weigh the finished boat. O.K. they lie. I would guesstimate that often sailboat brochure displacements were 20% lower than actual weights with minimal gear aboard.

Catamaran builders are even worse. Generally they would quote "Displacement" - but wouldn't tell you they were quoting Lightship Displacement. Lightship is N.A. talk for "Boat is empty of fuel, water, food, crew, and gear". So you get a nice low figure but it's not realistic for how much the boat weighs in service.

Recently the European Union decided that enough was enough, and cracked down. So now in CE marked boats you will see lightship displacement clearly indicated. I think they are supposed to actually weigh a few representative samples of vessels and not just guess at the weights.

A new Fountaine Pajot Lucia 40 catamaran has an "displacement unloaded" of 9800 kgs (21,560 lbs). She is actually 38' overall so that is a very heavy catamaran.

Compare that to a much earlier 1998 F-P Athena 38. Her displacement found in various reviews is between 11,000 and 12,230 lbs. Yes, catamarans have gotten much heavier in the past decades - but I bet that the Athena is probably a bit heavier than the brochure ever said.

The Lagoon 38 was a porker at 15,962 lbs. Empty.

The new Catana 42 Carbon (actually 41'). 8900 kg (19,580 lbs).

Ceilydh's Weight?
Haulout in South Africa

"How much does she weigh Frank?" I asked as we were finally hauled out by a single point crane. Frank said he didn't know because he doesn't have a load cell on his crane. Sigh! But he did say it's a 10T crane. He knows if somebody is pushing the limits of the crane at 10T and we were nowhere near the limit. He said probably 6 or 7T based on how hard the winch was working. Close enough for me.

Our boat is quoted by the designer as "3.5 T empty" i.e. 3500 kg (7700 lbs) and 6T loaded (13,200 lbs). Certainly she was very light compared to any other 40' catamaran.

But we added the bridgedeck cabin structure (~1000 lbs, even when built in a pretty high-tech way), a diesel engine instead of outboards (say 400 lbs weight gain), scuba compressor (100 lbs), scuba gear for 3 (200 lbs?), my tools (shudder), two sewing machines, a big RIB with a 15HP engine, canning jars, spares for so much, boxes of stainless steel fasteners, sometimes 100's of cans of food when crossing oceans, cat litter, shoes... She is down on her lines about 4000 lbs from her empty weight. I'd bet she is probably 14,000 lbs in typical cruising conditions. So when full of gear, she is lighter than similar size production cats that are totally empty!


We'll close with a quick discussion of a number found on boat registration certificates. You might find Tonnage, Gross Tonnage, and Net Tonnage depending on your country.

These have nothing, repeat, nothing to do with your boat's weight.

Tonnage is a measure of the volume of a ship. Historically cargo ships was taxed on how many tuns or casks of wine they could carry. It simply refers to the volume of the ship. Gross Tonnage is the overall volume of the ship, Net tonnage is the volume of the cargo hold and is the value big ships are taxed on by various authorities because it's the measure of the earning power of the ship.

So if the harbour master asks you for your yacht tonnage, give him or her the net tonnage, because it's the lower number, and there is probably some sort of harbour dues based on tonnage!

Wikipedia has a good little article on tonnage here: Tonnage

Further reading: A good article by Phil Berman, a long time multihull sailor and boat broker. It's remarkably honest about the real weights of production catamarans

- Evan

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