how accurate are travel lift strap weight gauges

Is that a fair representation of its condition in the straps at 19000 lbs?

Normally the load on a crane is measured with the rope tension. This is a pretty acurate measurement. But this instrument has to be calibrated regularly. Ask the lift operator for the date of the last calibration. If its long ago, the indicaton may be worthless. 6000 lbs difference on a 13000 lbs boat is impossible, if your boat performes according to spec.

As HTM09 says, is your boat performing to spec?
It could well be the brochure figures are wrong. How often have you seen a sales brochure or operational manual give the wrong numbers. Often they are generic, especially on Italian boats. Check direct with the builder.
OTOH, could it be water intrusion? Is it cored?

Water soaked core...?

Solid fibreglass does not hold water, so I am asking.

Hi,

You will find that these two weights are the same.

The boat will displace an amount or volume of water equal to its total weight, this of course varies a little bit between FW and SW.

Cored GRP

The core of a multi layered GRP can only take a significant amount of water or humidity if it has an open cell structure. This only counts for balsa core. As balsa is a natural wood, it has an open structure and can take a lot of water and therefore a lot of additional weight.

Our early GRP made glider aircraft had balsa core for strength and weight savings. They took on so much additional weight due to soaked up humidity, that their useful load decreased more and more, until most of them could not take the weight of the pilot anymore. We had one Phoenix glider in our Club, that could only be flown by kids with a max weight of 100 lbs (including parachute).

Balsa core is still used by some Italian yards for their fast boats and for some racing boats were Nomex is to expensive. But even in the worstest case, that could explain only a weight increase of maybe 1000 or 1500 lbs.

But most yards nowadays use closed cell PVC foam as core material. This core material is unable to take on much water, because of its stucture. Even if a hull suffers from strong osmosis, this will not explain a weight difference of 6000 lbs.

Additional Items

Base weight by builder PLUS options of generator, fuel, swim platform, anchors and rodes, larger engines, flybridge, cockpit hardtop, etc. Could be huge difference but still seems excessive.

Good info there HTM09.
I did know of a boat, built in Taiwan, that would not perform to spec, in fact very far off spec.
I think it was an Angel Pilothouse built in the early 1990's.
The unfortunate owner discovered the problem was water intrusion plus issues with the props.
The problem never sorted out completely and it was sold.

Don't take my head off folks,, but i have to believe a travel lift gauge is an estimate at best. Is there a truck scale available? What's the significance of the supposed weight differential, if performance is on target?

My catalogue weight is 57,000lbs at 36 knots top end. If I can make that speed, I would suspect the travel lift, before the boat??

Hi,

I would have to politely disagree with you there.

There is no reason that a properly rated and calibrated tension load cell used on a lifting device cannot be as accurate as a properly rated and calibrated beam type one used in a platform scale.

Rope tension measurement systems

We do calibrate the weight measurement systems (cranes) of our self loading container vessels and on the heavy load carriers quite often, as the measured weight is relevant for the ships safety. For that purpose, we have some calibrated heavy weights in stock (big solid pieces of iron), as our lifting devices range from 50 to 750 metric tons. The maximum allowed difference has to be below 3 %. This checks are watched by the German authorities (SBG, TÜV). To our surprise, those systems are very accurate and last very long. Very seldom, we see differences of more than 1 to 1,5 %. No commercial crane in Germany, land, road or ship based, works without such a measurement system. Anytime safety is concerned, tolerances must be kept to a minimum.

Who said anything about net or gross tonnage? Don't add deadweight to the mix as that is another measurement altogether.

K1W1

I would have to agree with you. I was thinking more in line of the typical boat yard lift which probably isnt calibrated with commercial accuracy you describe?

I have had my boat put on the hard for periods ranging from a week to two months, added absolutely nothing to it while there, and had it "weigh" 2-5% more when lifted going back in than it "weighed" when taken out of the water soaking wet. Has gone the other way too. 80,000 pound boat, plus or minus!