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Old 28-10-2020, 16:16   #1
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Calculating load capacity

Hello,

My wife and I are in the process of closing on our first catamaran and I am looking for advice on calculating load capacity and distribution for our vessel.

We are purchasing a 2004 Maxim 380 with an approx. displacement of 6 metric tons. My logic tells me that if I can figure out the dry weight of the boat, then I can calculate the wet weight given the known fuel and water storage capacity of the boat. Subtracting this number from the displacement (i.e. 2205 * 6 = 13230) would give me the available payload capacity before I begin exceeding the displacement.

Is this an accurate way of figuring load capacity and has anyone come up with an intelligent way of spreading that load evenly in the hulls?

Thank you,
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Old 28-10-2020, 17:46   #2
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Re: Calculating load capacity

I'm not sure I understand your logic?


Yacht Displacement is the weight of the water that is moved when the yacht is out at sea” The weight of the water that is displaced is equal to the weight of the yacht and its load (fuel, water, provisions, etc). It is an indicator of how heavy a boat is. Displacement also includes ballast.

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Old 28-10-2020, 18:12   #3
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Re: Calculating load capacity

As a general rule...no. As-built weights are notoriously high compared to specs. Some of this is design optimism, some quality control during the build process, and a lot of this is additional accessories.


And it also depends on how low you want to go (bridge deck clearance, swim platforms, escape hatches). And then there is a whole seaworthiness and how she will rise to seas if heavy.


Complicated.
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Old 29-10-2020, 07:52   #4
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Re: Calculating load capacity

"We are purchasing a 2004 Maxim 380 with an approx. displacement of 6 metric tons."

I'm inclined to think that this is an interesting calculation, but probably not necessary.

There are bunches of different measures called "displacement," and they aren't the same thing as dry dead weight. In addition, starting with such a rough measure prevents your calculating something more precise.

Do you intend an unusual task for the boat, or are you planning on usual cruising for the two of you? The designer figured in expected load when determining the hull dimensions, so if you're not using the boat to smuggle gold you are good to go.

Distribution: You can break a 1,000 foot tanker by filling the fore and aft tanks but not the midship ones. That said, I doubt you are going to notice a change in handling of the boat unless you put all the cargo weight in one end. Sure, divide it by hulls and tend toward weight in the middle rather than each end, and again you should be good to go.

Perhaps think of it in terms of your moving about in the boat. Standing you up at the forestay or at the helm makes little difference.
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