Kayak Max Capacities Company Comparison

I know when looking at Max Weight Capacities of kayaks, each company calculates it slightly differently. Do you calculate the ‘actual’ max weight capacity differently for different companies, and if so what do you do?

For instance, I’ve seen many people say “multiply the listed max capacity by .7(ish)” to get the actual/performance max cap. I’ve also seen many others say “multiply the listed by .7 and then subtract the weight of the kayak itself”. Are there some companies that you should or should not subtract the weight of the kayak itself? Are there some companies that are known to overestimate/underestimate their weights? Do you use different multipliers for different companies?

One example is the Old Town’s Castine 140, which lists a Max Capacity much higher than WIlderness System’s Tsunami 140 despite being narrower (and other than that they appear to have a very similar shape). Their ‘listed useable weight’ is a big more comparable though.

It’d be cool to make a chart of the “Performance Factors” to apply to each company’s listed max capacities.

UPDATE:
For those interested, i called Wilderness Systems between all my work meetings today. The man on the phone said that their listed “max weight capacities” are the max capacities you can load into the boat and have it perform as expected, and they have already subtracted the weight of the boat out.
So in otherwords, their listed Max Weight Capacity is equivalent to some other companies’ (like Old Town and Ocean Kayak) listed “Useable Capacity”.

We have an OT rec-kayak and I feel the rated number is good and maybe even a little conservative.

The advice to only load to about 70% of the listed capacity is a good way to make sure you have a safety margin. The weight capacity is for ideal water conditions. I think weight capacity is just marketing BS. For example the safe load in a boat decreases dramatically as wave size increases. If you get some water sloshing around in the boat, even if that water doesn’t put it over weight, it will make the boat easy to swamp. You need to take weight capacity with a grain of salt. If you are concerned with how much weight a boat can hold, you should get a bigger boat.

Yeah who wants to use a rope at it’s rated capacity. The kayak people have no mathematical formula it’s just opinions I’m sure.

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A lot of manufacturers use 3d models and FEA so it’s not just a guess

So the limits they use are?

They still make test boats because?