JohnMB

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About JohnMB

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    Evanston
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    Sailing

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  1. JohnMB

    Physics of sail power.

    You have shown that some of your assumptions are wrong, there have been several suggestions as to which.. Things you might want to consider: -Why is the drag coefficient of a rectangular plate normal to the air flow greater than 1. -When calculating the drag force on a rectangular plate normal to the flow, if the plate is moving relative to the reference frame what velocity would you use to calculate the drag force? (air velocity relative to the frame, or air velocity relative to the plate, or some other velocity) Its not at all clear to me why you want to model a sailboat in this way. Usually I like models which give me insight into the problem, and help me improve the system I'm considering, its not clear how this model will do that. If you are interested in the energy flow, you are changing the velocities of the air and the water, but depending on the frame you work in you could be transferring energy from the air to the water or from the water to the air, either way you will be reducing the relative velocities of the two fluids. Personally I think way you are trying to calculate this you will end up throwing the baby out with the bathwater, as I think your simplifying assumptions will eliminate any useful insights, but I could be wrong.
  2. JohnMB

    Physics of sail power.

    The point about changing your frame of reference is that it sometimes helps you figure out what you have missed. If you don't get the same answer in the other frame of reference then this might point you in the right direction for your error. As several people have noted its is common practice to calculate sail forces using apparent not true wind, there is probably a good reason for this, and exploring it might give you more insight into the problem.
  3. JohnMB

    Physics of sail power.

    The dynamic pressure is not a vector quantity,and has no direction. It's a pressure, or a amount of energy per unit volume.... if you don't find its invariant under rotation you are calculating it incorrectly.
  4. JohnMB

    Physics of sail power.

    If you are using basic physics (conservation of energy or momentum) you should be able to redo the calculations in any frame, and still keep it all balanced. If you have set up the problem correctly, then you should be able to work in either true wind (frame pinned to average water position) or apparent wind (frame pinned to boat.) and still get to the answer. This may help you find where the error sits. Have fun.
  5. OK, so for most clubs Beer-cans is not a serious deal, BUT for some it is. I don't know about Muskegon, maybe the weds night series is the main deal for their club racers. So lets assume: 1. There are people at the club for whom the weds night series is the most serious racing they do, 2. The club still wants as many boats as possible to show up. 3. Any pro who is stepping in to drive will not care either way about the season standings 4. Bringing in a pro to help your score for a weds night series is definitely taking things too seriously There is no reason not to design the scoring to help make this work, it looks like the Muskegon weds night series is a high point series, one option would be to score boats based on the driver, if there is a different driver then the boat gets scored as a different boat, it lets the pro take the boat out and have some fun or run a clinic without annoying the people who care about the series scores. You could add complication by making allowances for regular crew to take a turn on the helm, etc. Sure we can tell people to stop taking weds night racing so seriously, but there is a subgroup out there that do want to take it seriously, let's encourage them to show up too :).
  6. JohnMB

    Warnemünde, Germany typical conditions?

    No. In Europe major cities are typically connected by single lane roads to help keep the train industry healthy. Google maps will give you good directions. I would recommend stopping in plzen on route.
  7. JohnMB

    do big race boats depreciate?

    milliard but as noted before Ted Heath made this obsolete in the uk in the early 70's
  8. JohnMB

    Worst anchor design I've ever seen

    And lets be realistic, on a broads holiday finding extra stuff for the kids to do is part of the fun for the parents :). The number of 'essential' tasks I had to do as a kid on cruising holidays was shocking, as a parent I'm convinced that 80% of the essential element was to keep me occupied...
  9. I never said you attacked their character i said you addressed you comment to the person rather than the argument. Your comment was an ad homines compliment. Not an ad hominem attack or (pace panoramix) an ad homimen fallacy. All ad hominem means is that you addressed the person (and by extension their character) rather than the argument itself.
  10. Crash's comment was definitely addressed ad homines (3 were explicit called out) I'm not sure why Lioness took it as an insult (maybe it was too subtle for me to understand.)
  11. JohnMB

    do big race boats depreciate?

    The UK killed it off in the 70's Ted Heath got fed up with the confusion caused by US vs UK billions.
  12. JohnMB

    Securing light wind sheets

    I would definitely not advocate tying off the sheet. The trimmer should be actively trimming all the time. And has been noted there are circumstances where having the sheet lead to a hand allows more flexible control of the sheeting angle. However, I do believe that where the lead is appropriate it is better to have the sheet lead to a fixed point on the boat (single wrap on a winch or a block) rather than having it lead to a hand. I do believe that there is a benefit in having the load from the said transmitted directly to the boat hull. The physics is complicated. My comments from the start were an objection to the oversimplicifaction of the physics, which treated the whole problem as a statics.
  13. JohnMB

    Securing light wind sheets

    I think that there well may be. There are certainly times when I do exactly what you suggest as it gives me a better lead, but when I am not able to get a better lead I prefer to make the sheet to boat connection as rigid as possible. I also prefer to make my bodies connection to he boat as rigid as possible in low wind conditions. This could all be psychological . I as for 'stating theory as fact' I thought i was being pretty clear. The fact that the losses are non zero is definitely fact, but as noted the losses may be negligible.
  14. JohnMB

    Securing light wind sheets

    The net impulse is the same only if there are NO losses in passing through the human (pedantic/ thrust is generally a measure of force, I'm assuming you meant impulse in the last sentence? /pedantic. ) The issue in low wind sailing is that even very small losses can be significant. since the force is transmitted through muscle, sinew and bone, there is some loss in the system compared to a metal winch. It may be very small but it definitely exists. The human will act as a shock absorber as will any material that is not purely elastic. It's possible the losses associated with the body are negligible, but they are definitely not zero, the bursa at the joints act exactly like shock absorbers. I suspect that the smaller the boat the bigger the issue.
  15. JohnMB

    Securing light wind sheets

    The issue with hand holding the sheet is that it can be lossy compared to having the sheet directly on a winch or a ratchet block. With a variable load (which is what you typically have with a kite) you want any forces transmitted directly to the hull, rather than transmitting them through a floppy human. This is also physics. The human can act as a low pass filter for some of the higher rate of change forces, and absorb some of the energy which otherwise would add to the kinetic energy of the boat. This is also one of the reasons you want the crew 'locked in' to the boat in light winds, and the reverse of this is what allows ooching to work.