Actual Weight Database

[jmh2002]

Originally Posted by owly

I agree that markings on the boat are the answer.....but "permanent" markings do not seem realistic. For them to be permanent, they would have to a physical shape change... a chine or something.

Vessels required to do so by their regulatory authorities have permanent Load Line markings as a matter of course. So it's entirely realistic, in fact it's entirely normal.

In the case of a steel commercial vessel they are welded to to hull. But in the case of a fiberglass production yacht they could easily be part of the hull moulding.

So it could easily be done for most production yachts, but it's not a requirement so they don't do it.

Here's an example below. For some vessels only a simple Load Line is required (the circle with the line passing through). For others the full Load Line may be necessary, which has multiple different load lines, for different water density, as well as different operational areas or times of the year. It is normally also accompanied by draft marks.

For production yachts a simplified version of this could easily be used, with graduated marks enabling immersion vs weight to be calculated, or even better just looked up in a table or graph in the owner's manual.

[Sailmonkey]

Originally Posted by owly

There is no excuse for travel lifts not having at least somewhat accurate scales. Load cell technology is well developed, and reliable. I have an ordinary (home built) shop engine lift "floor crane" that uses a hydraulic cylinder and electric pump, and have been able to calibrate things that at a specific boom angle and extension, I can work out the weight from hydraulic pressure to within about 10 pounds within it's 2500 lb lift capacity. (yes it's a big one). I would not expect travel lifts to be "certified" scales, but it's not unreasonable to expect that they should be within a few hundred pounds.




H.W.

There is a reason you don’t find much accuracy on travel lift scales......and you’ve already hit upon it with your engine hoist.

You need to have a specific boom length and angle for your engine hoist to measure accurately.

Same applies to a lift. Let’s use an example of a lift with four winches. Each winch is fitted with a load cell to measure the pulling force it generates. That’s the easy part, now let us lift a light wide boat. The sling angle may be nearly vertical, now lift a heavy narrow boat, the sling angle may be 45 degrees.

While it is possible to engineer a system that measures the angle of each sling tackle and apples the geometry to a correction, it’s still not going to be accurate for long (probably won’t last long either).

Best option would be to roll the lift onto some portable scales before and after the lift. But then what yard wants the added expense of operating and maintaining scales like that?

[owly]

Originally Posted by Sailmonkey

There is a reason you don’t find much accuracy on travel lift scales......and you’ve already hit upon it with your engine hoist.

You need to have a specific boom length and angle for your engine hoist to measure accurately.

Same applies to a lift. Let’s use an example of a lift with four winches. Each winch is fitted with a load cell to measure the pulling force it generates. That’s the easy part, now let us lift a light wide boat. The sling angle may be nearly vertical, now lift a heavy narrow boat, the sling angle may be 45 degrees.

While it is possible to engineer a system that measures the angle of each sling tackle and apples the geometry to a correction, it’s still not going to be accurate for long (probably won’t last long either).

Best option would be to roll the lift onto some portable scales before and after the lift. But then what yard wants the added expense of operating and maintaining scales like that?

The winches are not where load should be measured... That is actually a radically different situation from my engine lift where the load is entirely vertical. The only reason for the boom angle is so the load is at the proper distance from the cylinder... simple math..... Hydraulics is not an ideal way to weigh things due to friction.



Load should normally be within the structure where they measure only vertical load. It is not high tech complicated or terribly expensive stuff. I've worked around load cells before. You find them on all sorts of equipment, often mobile equipment.


The really neat and accurate way to measure boat weight would be to have a sort of pool you lower it into. Measure water level before and after, or fill the tank to an overflow port and measure how much runs through the port. But it would be expensive and impractical unless you had a LOT of boats to weigh.

[owly]

Originally Posted by jmh2002

Vessels required to do so by their regulatory authorities have permanent Load Line markings as a matter of course. So it's entirely realistic, in fact it's entirely normal.

In the case of a steel commercial vessel they are welded to to hull. But in the case of a fiberglass production yacht they could easily be part of the hull moulding.

So it could easily be done for most production yachts, but it's not a requirement so they don't do it.

Here's an example below. For some vessels only a simple Load Line is required (the circle with the line passing through). For others the full Load Line may be necessary, which has multiple different load lines, for different water density, as well as different operational areas or times of the year. It is normally also accompanied by draft marks.

For production yachts a simplified version of this could easily be used, with graduated marks enabling immersion vs weight to be calculated, or even better just looked up in a table or graph in the owner's manual.

For a catamaran, it's not about weight really, it's about how deeply they sit in the water. Weight is just a way to predict that, it isn't really what you care about in the end.


I would propose a simple scale stick system. Choose two standard measuring points, one fore and one aft. Measure the distance to water in "flat water", and refer to a chart. The fresh water / salt water, tropical water, arctic water aspect is irrelevant when depth of immersion is what you care about.

[Tupaia]

Originally Posted by owly

For a catamaran, it's not about weight really, it's about how deeply they sit in the water. Weight is just a way to predict that, it isn't really what you care about in the end.

If I have a 2 tonne cat with deep v hulls a la Wharram and a 2 tonne cat with round bilge hulls a la Woods. The first will sit maybe 1.5 times as deep as the latter. So you absolutely do care about the weight.

The whole essence of performance is based on the Power (sail area) to Weight (displacement) ratio.

Originally Posted by owly

I would propose a simple scale stick system. Choose two standard measuring points, one fore and one aft. Measure the distance to water in "flat water", and refer to a chart. The fresh water / salt water, tropical water, arctic water aspect is irrelevant when depth of immersion is what you care about.

For someone that seems so concerned by the accuracy of lifts your proposal is equally in accurate and requires someone (I assume the manufacturer) to produce the charts. Not going to happen. Also on a small cat, ei not a ship measuring fore and aft will not give always give false readings as many if not most catamaran foot prints are tear drop shaped and a depression at the bow does not result in the same uplift at the stern let alone the differences port and starboard.

To reiterate the above a really heavy but very fat 22 Tonne cat could possibly sit higher in the water than a 10 Tonne performance cat with skinny hulls. Do you include fixed keels, daggerboards? Furthermore if it is depth of immersion that you are looking for then the density of the water is very relevant.

I think maybe you have lost sight of your objective. Nice as it would be to have this data apart from a cursory spread sheet comparison I can't see what benefit it affords any one.

[Tin Tin]

Originally Posted by Tupaia

If I have a 2 tonne cat with deep v hulls a la Wharram and a 2 tonne cat with round bilge hulls a la Woods. The first will sit maybe 1.5 times as deep as the latter. So you absolutely do care about the weight.

The whole essence of performance is based on the Power (sail area) to Weight (displacement) ratio.



For someone that seems so concerned by the accuracy of lifts your proposal is equally in accurate and requires someone (I assume the manufacturer) to produce the charts. Not going to happen. Also on a small cat, ei not a ship measuring fore and aft will not give always give false readings as many if not most catamaran foot prints are tear drop shaped and a depression at the bow does not result in the same uplift at the stern let alone the differences port and starboard.

To reiterate the above a really heavy but very fat 22 Tonne cat could possibly sit higher in the water than a 10 Tonne performance cat with skinny hulls. Do you include fixed keels, daggerboards? Furthermore if it is depth of immersion that you are looking for then the density of the water is very relevant.

I think maybe you have lost sight of your objective. Nice as it would be to have this data apart from a cursory spread sheet comparison I can't see what benefit it affords any one.

I think the benefit the OP is looking for is to know the load carrying capacity of similar LWL boats. A Lagoon 40 will carry much higher designed load than a Seawind 1160 and this is important if you want to work out what gear you will be taking on a long term cruise. The speed penalty depends on the various design ratios of course, but more often crew competence plays a bigger part in that space.

[Chrisatlantic]

I had my 55 foot cat weighed twice in three weeks. On two different travel lifts at the same yard, same brand lifts and both relatively recent and high tech - indicated weight was different by 4000 pounds with no more than 100 pounds actual difference in the way the boat should weigh