Originally Posted by estarzinger
I can't see the section you are referring to?
I am suspecting that if you only slid 11 nm/day you had a current
against the wind.
Caught me, the current was in the range of 1/2 knot
. I slid 17 nm a day.
I meant Fig 15. which I agree does not indicate the rate of drift to expect. On reading the section which describes the mechanics of the proposed optimum conditions of how a wave passes by the boat, the comparisons of the loading on the towline, the reports estimate of how much more the load would be on a towline using a large chute one can estimate the rate of drift is 2-3 times that of a chute, it all depends on the size of the chute.
To generate a slick which is the turbulence produced by the keel
of a boat you must be at an angle to the wind and drift at a rate fast enough to generate a slick. Being at an angle to the waves causes you to be pushed back faster as the forward side of the wave is essentially moving at the same rate as the wave speed (as outlined in the report). Being at an angle also increases the wind load on the boat increasing drift rate.
The report states that the tension on the towline of a series drogue is relatively constant in practice. They also state that the towline of a chute goes slack with the passing of a wave, the boat yaws and could go beam on to the seas. I never experienced that, I was always head
on to the wind and seas. Perhaps that is why I did not slide back as far.
The line never going slack probably was probably due to the passing wave not having as much of an area of the hull
(bow on to the seas) to push against to accelerate the boat backward. The chute was deep in the water
and being so was not as affected by the back and forth motion of the water
as a wave passed through it. The only force would be the slope of the wave itself which the boat would attempt to slide down on. Being a lower force the boat would be more easily pulled up that slope. A lower force does not stretch the towline as much and then accelerate the boat forward when the wave passed causing the boat to overrun the line. The line going slack would cause the chute to partially collapse which to reinflate has to dragged back some distance. Overrunning the towline would cause the boat to turn side ways to the wind and the cycle would start over.
This cycle is exactly what the report states will cause excessive loads, chaff, break towlines, tear out cleats
, damage rudders or possibly result in a rollover. That is why they recommend a series drogue off the stern provided that you have the room. The report mentions a chute would be better at keeping a boat in one position if space was an issue.