Water retention / Displacement / Weighbridges

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glennytots

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A fellow Jeanneau Tonic 23 owner and myself are trying to reconcile the specified weight of his boat (1420kg), 2 axle trailer (450kg), 8 hp outboard (45kg) and a comprehensive list of all the other non-standard equipment, fluids and items on board (est. 195kg) with the overall weight as recently disclosed by a public weighbridge. Discounting the weighbridge being poorly calibrated, it would seem that the weight is about 250kg (approx. 12%) higher than expected (ie 2370 against an estimated 2110kg). This is perplexing to me as it has implications for the legality of my trailer / yacht / car combination .

In their brochure for the Tonic, Jeanneau specified a trailer with a 1670kg capacity allowing for outboard and a safety margin. This total is more or less exactly what my fellow owner estimates his load ought to be.

One possibility we have discussed is that perhaps the boat has gained weight over its 25 years due to water being held within the GRP. Can any forum members that have knowledge or experience of GRP water retention comment on this? Even if the boat was rarely dried ashore (not the case I believe), how much water could a 1420kg (inc 480kg ballast) grp hull absorb?

Another thought I have is perhaps the weighbridge has a high margin of error at what is perhaps at the lower end of its range. Any thoughts? Does anyone know how these weighbridges are calibrated/maintained and what margin of error they work to?
 
The boat could have just been overweight when built.

I've a friend who had his SB3 hull replaced because it was way outside tolerance. It was apparently just the way it was built.
 
The weighbridges I work with are + - 20k and are tested frequently by the relevant department who arrive with 20 t of test weights and a fork truck to position them.
Unless you are visiting a particularly remote and dodgy 'bridge I would count on it being pretty near spot on.
 
I cannot remember the precise figures but I do remember reading somewhere about water absorption in grp and the maximum percentage was pretty small, certainly well below 100kg on a 1600 kg boat. However, if the boat has a foam sandwich hull, the percentage could be way higher judging from a waterlogged foam sandwich dinghy I once owned.

The boats quoted displacement is usually an estimate from the designer rather than a weighbridge figure from the boatyard, but since weight = cost in boatbuilding, I cant imagine the French builder letting the hull weight go over by the sort of figure you are talking about.

A pal has a boat which has a stated displacement of 9 tonnes and which comes out at 12 tonnes on the club crane. The biggest part of the difference in his case is all the stuff he has on board. This is the most likely culprit
 
Other boat (mine) spends April to October in water. November to March or thereabouts in dry ventilated barn on the top of a hill . Weighbridge belongs to well used and run waste recyling company with local authority ties so likely to regularly calibrated.

I personally think that any boat builder building a trailerable boat should have it weighed with the basics on. It cost me a lot. With the plating on the trailer I should have been able to tow it with the Renault Espace I had, which did tow it Ok. I had to change it for an Isuzu Trooper. I think when I first had it weighed (2001) it was 2250 kgs. I will try and dig out ALL the weighbridge ticket.

Mine has has a boarding platform added which is probably another 20kgs or so which I forgot about.
 
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A public weigh bridge has to be calibrated regularly, at least once a year, and be within +- 20 kg at 500kg.

even our works axel weigher shows 110 kg with a 106kg me standing on it.
 
glennytots said:
A fellow Jeanneau Tonic 23 owner and myself are trying to reconcile the specified weight of his boat (1420kg), 2 axle trailer (450kg), 8 hp outboard (45kg) and a comprehensive list of all the other non-standard equipment, fluids and items on board (est. 195kg) with the overall weight as recently disclosed by a public weighbridge. Discounting the weighbridge being poorly calibrated, it would seem that the weight is about 250kg (approx. 12%) higher than expected (ie 2370 against an estimated 2110kg). This is perplexing to me as it has implications for the legality of my trailer / yacht / car combination .

In their brochure for the Tonic, Jeanneau specified a trailer with a 1670kg capacity allowing for outboard and a safety margin. This total is more or less exactly what my fellow owner estimates his load ought to be.

One possibility we have discussed is that perhaps the boat has gained weight over its 25 years due to water being held within the GRP. Can any forum members that have knowledge or experience of GRP water retention comment on this? Even if the boat was rarely dried ashore (not the case I believe), how much water could a 1420kg (inc 480kg ballast) grp hull absorb?

Another thought I have is perhaps the weighbridge has a high margin of error at what is perhaps at the lower end of its range. Any thoughts? Does anyone know how these weighbridges are calibrated/maintained and what margin of error they work to?
Click to expand...

It's not going to be water absorbed into the GRP itself, but a combination of overbuilding (resin and glass was cheaper then), and all sorts of odds and ends getting added over the years. The one place to look for "structural" water absorption is in the balsa sandwick deck.

There's probably only maybe 500-600 kg of actual glass/resin in the boat and this can only absorb 3% of water at most - certainly under 20Kg. The balsa in the deck sandwich could possibly soak up much more though.

Are you guessing the weight of the trailer - this could be part of the problem.
 
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