ChrisO wrote:
Both drogues and sea anchors are dynamic brakes, they need to move through the water to generate resistance. From USGS testing on series drogues, I would expect around 4-5 lb of resistance per cone.
So, it is not as though your kayak will be suspended from a stationary attachment point while loaded to capacity. Realistically, I would expect much less than a 900 lb max. load.
While in most conditions, yes, the load will be much less than the full weight of the loaded kayak, but one designs a drogue for the worst case scenario.
I'm not sure where you're getting the 4-5 lb per cone resistance. A 100 cone drogue is rated at a max load of 7500 lbs, which would be the maximum load (drag) imposed by the drogue under the worst case scenario. A resistance of 5 lbs per cone, assuming that is the maximum drag at highest velocity, would only provide 500 lbs drag, which does not seem to match the design data in the USCG report.
I've studied the USCG 1987 report several times over the years (and I've done a bit of solo open ocean sailing in larger boats) and my understanding is that the drogue load increases proportionate to the velocity and that the maximum load at maximum worst case velocity is calculated to be roughly equal to the displacement of the boat, until you start getting to very large boats (tens of thousands of pounds displacement) where the maximum load decreases to less than displacement of the boat.
From the USCG report:
Quote:
The design load is the maximum load that will be imposed on the drogue, towline and attachments in the event of a very severe breaking wave strike. A load of this magnitude would be encountered rarely if at all, possibly once or twice in the lifetime of the equipment. ... It will be noted that at a displacement of 7500 lbs. the design load is equal to the displacement and at a displacement of 60,000 lbs. the design load is 60% of the displacement.
Thus, my understanding of the report is that for the expedition kayak in question, the drogue and all attachment hardware should be able to withstand a load of 450kg, even if such a load would very rarely be encountered, if ever.