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”.
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.
An interesting topic. The Prijon Capri 1 and Dayliner S for comparison. The Dayliner S is longer, heavier, 1 cm more narrow, yet the total capacity is 10 kg less than the shorter/lighter Capri 1. I guess it depends also on intended use and handling characteristics.
Capri 1- length/width 367/62 cm, weight 20 kg, cockpit 92 cm, capacity 90 kg
Dayliner S- length/width 387/61 cm, weight 22 kg, cockpit 88 cm, capacity 80 kg
I just have to pipe up and say that in engineering, the factor of safety is part of the design, and not left up to the end user of the product. So the strength of the item should be that much higher than the max load it is designed for. Factor of safety of 1 means it is designed to fail right at the expected max load/conditions. Factor of safety of 1.5, 2, etc increases that margin. I would think (hope) that the capacity listed is more for handling characteristics, and that you would be fine all the way up to that listed max. I would also assume they use a pretty high FOS.
I don’t know anything about boat building, but in my opinion it would be criminally negligent for any company to list the max allowable load for a boat at the actual failure point (or swamping or nosediving or any other point). Failure (or sinking or inability to maneuver) of a kayak could easily lead to death. And they KNOW how some of these kayaks are used, and how little many of the users know/care about safety. Plus, how fast are you going when you hit that rock? How big were those waves? Stability and ride characteristics are another thing. Will it nosedive if you overload it? Will someone drown if they load to one pound over the published load capacity? I would hope that the numbers are guides, and intended to help people choose the boat that will best suit their needs, and not a hard and fast “impending disaster” number. So while that 0.7 factor is probably a good idea, to give you the best paddling experience, I seriously doubt if loading right to the limit is an issue other than leading to a more ponderously-handling kayak. And lack of fun.
Using 3D modeling, as someone mentioned above, does not guarantee the design is good, safe, or in any way appropriate. Computers are not magic. It just means it wasn’t done on paper, and the engineer could roll it around on the screen. It’s easier to verify the design is good, but you still have to do the engineering. I did this for a living. Easier to do it right, correct. But it’s a tool. FEA is great. Unless you use the wrong load cases (and we’re dealing with Mother Nature here, so what are they?), in which case it’s downright dangerous. So what someone said above about what loads are assumed, and why do they still build test boats - BINGO! You can’t skip the prototype phase. That’s when you find out if your assumptions were correct.