Portofino
Well-Known Member
Ask the guy to do this - ? Simples
Should I have my boat lifted by an undersized travelift crane?
PilotWolf
Well-Known Member
OK bad typing late and after rum!
Actual mass of the boat or the GRT/NRT might have been better. The last boat I had lifted had a GRT of over 65 but actual weight on the lift was around 46 ton.
What I was suggesting is that the travel lift is rated in mass but the OP is using the GRT rather than the physical weight of the boat. (I think - been a long day!)
W.
Bandit
Well-Known Member
NRT and GRT are not the same as weight ie mass.
You need the displacement ie the dry weight of the boat plus fuel, water, additional equipment and stores.
Most manufacturers weights are estimates very few boats are weighed with load cells.
You need the displacement ie the dry weight of the boat plus fuel, water, additional equipment and stores.
Most manufacturers weights are estimates very few boats are weighed with load cells.
JumbleDuck
Well-Known Member
Gross registered tonnage is a measure of volume, not of mass, and referred originally to the number of tuns (ruddy great barrels) which a vessel could carry.
Hurricane
Well-Known Member
This might help
JW is slightly larger and probably with the same amount of clobber on board.
With full fuel (4000 litres), a couple of times, I have asked our travel hoist operator how much she weighs.
On our hoist, they can only read the weight when the winches are actually lifting (presumably some kind of brake is engaged when it is static)
So the operator needs to climb onto the hoist to read it.
I say this because I'm not sure of it accuracy.
Anyway, under these conditions, the operator recons that she weighs in at 43T
I've always condidered that she is 50T - rounding up etc for safety.
So, in your case, it dhould be OK but I don't think I would take the risk.
Our yard also rigs double strops (4 in total) - I would have thought that would be prudent for yours as well.
Hope that helps
JW is slightly larger and probably with the same amount of clobber on board.
With full fuel (4000 litres), a couple of times, I have asked our travel hoist operator how much she weighs.
On our hoist, they can only read the weight when the winches are actually lifting (presumably some kind of brake is engaged when it is static)
So the operator needs to climb onto the hoist to read it.
I say this because I'm not sure of it accuracy.
Anyway, under these conditions, the operator recons that she weighs in at 43T
I've always condidered that she is 50T - rounding up etc for safety.
So, in your case, it dhould be OK but I don't think I would take the risk.
Our yard also rigs double strops (4 in total) - I would have thought that would be prudent for yours as well.
Hope that helps
jfm
Well-Known Member
ok I get that aspect of your thinking ( though it isn't what you said!). Alas Deleted User is referring to weight not volume when he says his boat is 45 tonnes or whatever number he said
Scuse the pedantry but mass and weight are different things. The relevant thing to a purist engineer here is weight not mass. Ok you'll get away with mass of course; I am being pedantic
D
Deleted User YDKXO
Guest
Thanks Mike. My boat is obviously a bit shorter than yours but I think it's a bit beamier as well so I guess the weight won't be too far off what your P67 weighs. Whatever it actually weighs, it's close to 40t or just over by a few tonnes and therefore it's going to exceed the travelift capacity. Yup, good point about doubling up on the strops. I'll check with the yard I'm planning to take it to
JumbleDuck
Well-Known Member
Ahem. If he says 25 tonnes he is referring to mass, not weight or volume!
jfm
Well-Known Member
Bandit
Well-Known Member
Marinetravelift reckon its accurate +/- 5% it gives value when hositing is stopped, they reckon its most accurate as it clears the water.
hartcjhart
Well-Known Member
their crane is able to do it(after the safety margin) but is it wprth the risk, if they will confirm in writing that any problems will be covered by their insurance then go for it
Mariner69
Well-Known Member
If you take the water length of the vessel, times the beam and the sea water draught floating and multiply them together and then by 0.95 and 1.025 you will have a rough (within 10 per cent) estimate of the weight (displacement). This will give you the weight that has to be lifted.
The lifting equipment should be marked with the Safe Working Load (SWL) which in the UK has a safety factor of 5 by law. Seamen making ad hoc lifts have traditionally used safety factors of 6 to be on the very safe side. An overload of 100 per cent (IIRC) is allowed fore a one off emergency lift on cranes but........... has to be recorded and can cause problems with HSE/MCA et al especially if it goes wrong. The owner of the lifting equipment takes the 'risks' and pays the consequences if it goes wrong so you should be interested in their insurance in the first instance.
I trust this helps.
The lifting equipment should be marked with the Safe Working Load (SWL) which in the UK has a safety factor of 5 by law. Seamen making ad hoc lifts have traditionally used safety factors of 6 to be on the very safe side. An overload of 100 per cent (IIRC) is allowed fore a one off emergency lift on cranes but........... has to be recorded and can cause problems with HSE/MCA et al especially if it goes wrong. The owner of the lifting equipment takes the 'risks' and pays the consequences if it goes wrong so you should be interested in their insurance in the first instance.
I trust this helps.
MapisM
Well-Known Member
Mmmm... I wholeheartedly agree on your definition of "rough" for such measure of a boat displacement - much more so than the "within 10%" bit.
That aside, at risk of sounding a bit academic, I can't see the mathematical sense of stating a rule where two multiplying factors should be considered.
I mean, why not just say "then by 0.97375" instead of "then by 0.95 and 1.025"?!?
BartW
Well-Known Member
I agree that there could be done a certain estimation for a certain hull shape this way, but there must be something wrong with your coëfficiënt
consider the volume of water:, boat WL length x beam x max draught thats much more than the real water displacement,
the coëfficiënt should be something like 0.6..0.7 imho
MapisM
Well-Known Member
Yup, you should consider the average, rather than the max draught (that depending on whether the hull has a deep keel or not might be VERY misleading).
Trouble is, you can only guess what the average draught is!
Trouble is, you can only guess what the average draught is!
Nick_H
Well-Known Member
What units are you using?
Using SI units and your formula, I estimate my 57' boat to weigh about 70 kgs
I also can't get any sensible result in tonnes, kgs or lbs by starting with mm, feet or inches.
jfm
Well-Known Member
LOL i suppose nick that if you use metres you will dimensionally get an answer in tonnes because a cubic metre of pure water weighs a tonne. But the formula is evidently nuts because the coefficient should be 0.6 to 0.7 as Bartw said, and the draft has to be taken without including protrusions. Much easier to use ferretti brochure which says 45 tonnes
Mikes boaT is about 17 x4x1 X 0.65 Bartw coefficient = 45 tonnes
Mikes boaT is about 17 x4x1 X 0.65 Bartw coefficient = 45 tonnes
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D
Deleted User YDKXO
Guest
Mariner69, thanks but that's not true. Broadly speaking there are 3 types of safety factor applied to lifting equipment, safety factors for overturning stability, safety factors for structural component failure and safety factors for lifting gear (ie ropes, chains, strops etc). It is true that safety factors for lifting gear are generally 5 but the two other safety factors can be much less. The minimum allowable safety factor for stability may be as low as 1.25 and for structural failure 1.5 although manufacturers may use more conservative figures than this. However to make a broad statement that a 100% overload is allowed as a one off for an emergency lift would be very dangerous for many types of lifting equipment.
In the case of the 40t boat travelift in my marina, I guess that overturning stability is not a primary consideration but structural failure certainly would be so you could take the view that 60t is the maximum lift before the thing starts buckling which is probably why they've got away with lifting boats heavier than mine in the past. However, I very much doubt that their insurance policy would cover an accident in the event that the SWL is exceeded any more than mine would and in any case, I really wouldn't want to rely on their insurance policy anyway.
Mariner69
Well-Known Member
Ok guys.
The measurements are in metres and calculate the box of water the vessel is sitting in. The 0.95 is the block coefficient and 'yes' 0.65 would be the factor for a fast hull shape but to err on the safe side the 0.95 gives a larger volume. The 1.025 is the specific gravity of sea water in tonnes per cubic metre. So vessel 10 metres long, 4 metres beam and 2 metres draught would give a volume of box of 80 metres. Using a Cb of 0,65 would give a volume of 52 cubic metres and the displacement 53.3 tonnes. The Cb at 0.95 would be for a full hull like a bulk carrier but would err on the safe side.
Reference safety factors, I agree they vary according to the use intended so for a mooring system then a factor of 2 might well be used at the design stages and equipment chosen accordingly. Lifting is specific in having a declared safety factor.
In any event, I agree, if there is any doubt go elsewhere.
First rule of insurance is maximum premium and minimum pay out so the cynic might well expect the insurance company to decline the claim if they can.
Good luck to the OP with whatever route he choses.
The measurements are in metres and calculate the box of water the vessel is sitting in. The 0.95 is the block coefficient and 'yes' 0.65 would be the factor for a fast hull shape but to err on the safe side the 0.95 gives a larger volume. The 1.025 is the specific gravity of sea water in tonnes per cubic metre. So vessel 10 metres long, 4 metres beam and 2 metres draught would give a volume of box of 80 metres. Using a Cb of 0,65 would give a volume of 52 cubic metres and the displacement 53.3 tonnes. The Cb at 0.95 would be for a full hull like a bulk carrier but would err on the safe side.
Reference safety factors, I agree they vary according to the use intended so for a mooring system then a factor of 2 might well be used at the design stages and equipment chosen accordingly. Lifting is specific in having a declared safety factor.
In any event, I agree, if there is any doubt go elsewhere.
First rule of insurance is maximum premium and minimum pay out so the cynic might well expect the insurance company to decline the claim if they can.
Good luck to the OP with whatever route he choses.
JumbleDuck
Well-Known Member
Ahem too!
I'm an engineer who works with physicists. Mixing up weight and mass is almost as good for winding them up as referring to centrifugal force ...
