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Old 06-06-2013, 07:45   #16
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

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Old 06-06-2013, 07:52   #17
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

I was looking at some French designs yesterday (bord a bord, bretagne) and I found many of their designs were 25% between light and loaded. Not much adjustment for cat versus mono. Alloy boats.

As cheechako noted bridgedeck strength may have plenty to do in case of a cat. I think for a mono sail boat loading her up may result in huge alterations to forces on the rigging that may ultimately end up in failures.

I'd rather my boat were way 'too' light than a bit too heavy.

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Old 09-06-2013, 07:50   #18
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

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I'd rather my boat were way 'too' light than a bit too heavy.
Fine for a cat but for a mono sailboat it will tend to make the boat tender and ever more as fuel, water and food diminish daily. Cruiser tend to be built stronger and some choose to be slightly over loaded at the start of a voyage to reach a destination on the LWL.
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Old 09-06-2013, 08:32   #19
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

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Fine for a cat but for a mono sailboat it will tend to make the boat tender and ever more as fuel, water and food diminish daily. Cruiser tend to be built stronger and some choose to be slightly over loaded at the start of a voyage to reach a destination on the LWL.
I dig your sense of humour!

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Old 09-06-2013, 09:00   #20
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

how much weight it takes to lower your waterline by 1 inch

1. Determine waterline length (LWL).
2. Calculate waterline beam (BWL). Multiply .90 boat beam.
3. Multiply (LWL) (BWL) .75 = waterplane area (WPA).
4. (WPA) 5.3 = weight to immerse the waterline 1 inch.



Your boat has a 25-foot waterline (LWL) and a 10-foot beam:

.90 10 feet = 9-foot waterline beam (BWL)
25 feet 9 feet .75 = 168.8 waterplane area (WPA)
168.8 (WPA) 5.3 = 895 pounds to immerse the waterline 1 inch
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Old 22-01-2017, 11:42   #21
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

Resurrecting this thread in the hope of some advice. An Outremer 55L's CE plate shows "Maximum load" as 3550kg. This seems like a huge amount given that the displacement is shown as 8990kg.

Regarding the maximum load figure, does it generally include installed items such as generator, water maker, fridge, heads, etc.? If so, that indicates that the actual load capacity is a lot less.
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Old 22-01-2017, 11:57   #22
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

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Originally Posted by fxykty View Post
Resurrecting this thread in the hope of some advice. An Outremer 55L's CE plate shows "Maximum load" as 3550kg. This seems like a huge amount given that the displacement is shown as 8990kg.

Regarding the maximum load figure, does it generally include installed items such as generator, water maker, fridge, heads, etc.? If so, that indicates that the actual load capacity is a lot less.
I don't have the answer for you, but I highly suspect that the load data normally used is for buoyancy and they have not predicted what weight and resultant forces could do to the structure itself in trying circumstances. What do others think... or know?
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Old 22-01-2017, 12:40   #23
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

Unfortunately there is a lot of fiddling with the numbers to make aboat appear as though it has more capacity than it does. What you really need to know is the max design displacement, then get a certified scale to weigh your boat as it sits.

Just as an example, have you ever seen an option sheet for a new boat where the standard stove is a single burner no oven model, and its $100 to install a stainless three burner w/ oven instead? Well lightship displacement was probably done with the single burner, and you need to subtract the 200lbs of the real one when it is selected. The same is true for anchors (well typically an anchor is included lightship, but not an all chain rhode). If a carbon mast is an option, then I bet you lightship is with the carbon stick...

It all adds up, and it isn't all dishonest builders. Lightship is supposed to be 'empty' some designers take this more strictly than others. It is perfectly reasonable for a designer to spec lightship weight as the minimal reasonable weight achievable given the original options list. The fact that no boat is ever built that light isn't their fault.

It also costs the builder a lot of money to do a full weight study and follow that up with weight tracking. In high dollar racing it isn't unusual to weigh every piece of hardware as it is added to the boat, then track it in a spreadsheet. But cruising boats don't need this level of detail. Rough numbers, or professional estimates of weight are pretty reasonable, but the small variations add up (they never seem to overestimate the weight).

A travel lift with a certified scale however can tell you within a few minutes how much the boat as it sits actually weighs.

Max design displacement however is a very hard number with no fiddling around. You do not want to be over this number no matter what.
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Old 22-01-2017, 12:44   #24
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Re: How to calculate the load capacity supported a catamaran and a sailboat?

Considering that...
  • Many boats are already above their design weight (others mentioned this),
  • Many cats are too low in the water as-designed (not enough bridge deck clearance),
  • What is acceptable to one person may not be to another...
... I don't think the question has a calculable answer. The realistic approach is to ask how a certain boat is working out for folks.
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