How do you gauge your speed

Discussion in 'Laser Talk' started by Skygod1, Jun 8, 2007.

  1. viktor188924

    viktor188924 New Member

    Likes Received:
    0
    Trophy Points:
    0
    My dad uses that for biking, running etc. Great idea! It lacks a VMG sensor, but whatever. It costs about $70 US as opposed to $200-$600 for the velociteks. If you get a data recorder add-on you can surely mark, if you're going around a practice course, where each mark is and you can see how your speed went up/down on the upwinds, downwinds, reaches on your computer.
     
  2. AndyDove21

    AndyDove21 New Member

    Likes Received:
    0
    Trophy Points:
    0
    How about a water pitot tube?

    Simplest method I can think of, that would at least give some indiction for little money would be a short length of flexible tubing mounted on one side of the tiller plate, or better still, on the transom.

    The bottom needs to bent though 90deg and faced into the oncoming water. Put a bouyant bead in the pipe, and crimp the top so it can't escape (but air can). The faster you go, the more water is forced into the tube, and the higher the ball sits. Use pipe strong enough to stay erect, but weak enough to shrug off the mainsheet as it passes.

    No idea whether it would work in practice, but seems to in my head...:confused:
     
  3. Ross B

    Ross B Guest


    I also have vang return, JC Clew Strap Safety Line

    and somewhere in there are carbon booms, 1 or 2 piece wing masts, better sails, middle boom sheeting, middle traveler, open cockpit, but none of that is serious because it would dramatically change the class, and possibly be not good for the immediate future of the class
     
  4. Chainsaw

    Chainsaw Brmmm Brmmm

    Likes Received:
    0
    Trophy Points:
    0
    Re: How about a water pitot tube?


    This sounds like the best idea yet! Though I would leave off crimping the end of the tube.

    Imagine the excitement at the club after the racing when you re-tell the story of how you popped your ball out of your tube as you surfed down a huge wave!

    "Hey, guys! I popped one off this afternoon!"
     
  5. Chainsaw

    Chainsaw Brmmm Brmmm

    Likes Received:
    0
    Trophy Points:
    0
    Yes yes absolutely, Ross, we must think of the future of the class.

    Now, on a serious note: What we also need is a large disc that fits onto the gooseneck pin to stop the cunningham line jamming between the boom the gooseneck.
     
  6. Ross B

    Ross B Guest

    just put a lot of tape around the gooseneck so it is flush with the boom

    or we could just do like lots of other boats, and Finns, and have the boom bolt to the mast, on the Finns, the boom does not rotate side to side, I am a firm beleive that our does not need to either
     
  7. Skygod1

    Skygod1 Member

    Likes Received:
    0
    Trophy Points:
    6
    So how does bolting the boom to the mast indicate speed? Funny thing I s I totally get the apparent wind concept. I think the tube would just spray water in the air.
    The line with a small disc and spring is reminiscent of the old airspeed indicators on the old airplanes. I was wondering though if sailing up current would give you a false indication?
     
  8. Ross B

    Ross B Guest

    it doesn't indicate speed, I just answered Saw's question.
     
  9. Looper

    Looper New Member

    Likes Received:
    0
    Trophy Points:
    0
    If we agree that you have to bear away as you become headed by increasing speed I would be fascinated to know if anyone has ever tried to calculate a theoretical optimum angle of sail for making winward progress. I think there are too many variables involved to ever try and determine a result empirically. But say perfect flat water, 78Kg Helmsman, fully sheeted main, boat sailed perfectly flat, centreboard down, sail settngs adjusted for pointing. Is it possbible that there is some optimum theoritical angle of sail? And does that angle of sail change with changing windspeed? I guess this more of an academic question than practical.
    I imagine within a certain windspeed range there may be 2 different angles depending on whether you choose to bear away and plane or not? I understand it is pretty difficult/impossible to plane closely hauled in a laser? Is there a theoritcally determinable minimum true wind speed at which it makes more sense to bear away and plane at a more oblique angle in order to maximise windward progress? Or is maximum windward progress always achieved non-planing closely hauled?
     
  10. Skygod1

    Skygod1 Member

    Likes Received:
    0
    Trophy Points:
    6
    Since this horse isnt quite dead yet, I want to beat it some more. So I thought I would take a different tack. So my question is: If under ideal conditions and sailing as close to the wind as possible; If there is a 20 knot wind what is the best attainable speed I can expect? What percent of the windspeed can I expect to use as boat speed? Same question going down wind ?
     
  11. gouvernail

    gouvernail Active Member

    Likes Received:
    17
    Trophy Points:
    38

    Just for jollies...

    I will go after the upwind speed question.

    There is a buoy well up the lake from our harbor.
    In medium wind I can get there in under a half hour.
    When the wind builds enough it can take me 45 minutes to get there.

    In medium wind, my sailing buddies all get to the buoy ahead of me.
    In heavy wind I usually get there first.

    The only guage of Laser speed is a fleet.
     
  12. Chainsaw

    Chainsaw Brmmm Brmmm

    Likes Received:
    0
    Trophy Points:
    0
    a function of sailor body weight.
     
  13. Skygod1

    Skygod1 Member

    Likes Received:
    0
    Trophy Points:
    6
    Weight is always a factor in any moving objects speed and efficiency. So lets work with the optimum weight the Laser was designed for, 185 pounds. I understand that there isnt anyone out there with a speed indicator strapped to their Laser so humor me.
    If you are tooling along in a steady 20 knot wind, going upwind; what is the best speed you expect to obtain?
     
  14. Chainsaw

    Chainsaw Brmmm Brmmm

    Likes Received:
    0
    Trophy Points:
    0
    As we discovered earlier, there needs to be a distinction between speed and VMG.

    In either case, you'd need access to polar diagrams for a percentage of the lasers currently sailing the world, and the weight of their sailors.

    You might then be able to work out a vague average of what you could expect.

    But having said that, the heaviest sailors are not always the fastest downwind or the lightest the fastest upwind. Ben Ainslie is just one example of that.

    How could you factor in the skill/ability factor?
     
  15. Skygod1

    Skygod1 Member

    Likes Received:
    0
    Trophy Points:
    6
    No skill or ability should be considered. Just the numbers. I am sure there are boats that are more efficient than the Laser and that there are reasons why. Not just the ability of the helms person. I am just looking for an approximate (at best ) percentage of loss of performance due to design. I realize that you can only typically sail faster than what the wind drives you, is when surfing down a wave. So when sailing in typical conditions approximately how much slower than what the wind is blowing is the Laser moving accross the water? Is it usually 3 to 5 knots less or is it 20 percent of the value of the wind. Is there greater loss in lighter winds or when it is really blowing?
     
  16. LPW

    LPW New Member

    Likes Received:
    0
    Trophy Points:
    0
    i might be wrong, but if u were sailing the boat flat and fast upwind and not planning, u would be goin at hull speed of about 5 knots??
    or maybe im totaly wrong
    L
     
  17. Looper

    Looper New Member

    Likes Received:
    0
    Trophy Points:
    0
    I was thinking about that question a bit more (optimum pointing angle). It should be a fairly simple trigonometric puzzle. Given a starting point to the problem e.g. a given boat (say a laser, but any boat really) can point at 45deg to true wind when it is doing 5 knots true hull speed in a 15 knot true wind. So the edge of the no sail zone is 45deg under these conditions. For every knot that the boat picks up in true hull speed it must bear away by some trigonometrically calculable amount to stay outside the slightly headed no sail zone. Its true directional velocity can be fractionally divided in a windward (0 deg to true wind) and leeward (90deg to true wind) component using pythagoras theorem. At some point along the continuum of bearing away with increasing true speed the windward fractional component of the true hull speed should be maximised (unless it actually does not even vary along this continuum - I am not even sure that it does). It is years since I was at school and I don't have any text books but this seems to me to be relatively simple trigonometric problem. If I manage to figure the answer I will post it.

    Also, Skygod1, you may know this already but in some hull configurations boat speed doesn't always have to be less than wind speed. Here is a foiling trimaran clocked at 44.8 knots in a 25 Knot north-easterly
    http://www.hydroptere.com/
     
  18. John Gilmour

    John Gilmour Member

    Likes Received:
    0
    Trophy Points:
    6
    I think this discussion has gotten a lot more compicated than it needs to be. It is an intereting theoretical point that when the boat goes faster, it cannot point as high relative to actual wind. From the standpoint of the sailor trying to go fast, all that matters is apparent wind. Don't worry about true wind. My understanding is that you should always sail to according to apparent wind.

    John
     
  19. abenn

    abenn New Member

    Likes Received:
    0
    Trophy Points:
    0
    OK so I did the trigonometry for you LooserLu. Note that this is only a pure trig problem I'm solving here - not making any claims that it really applies to laser sailing or anything else - just done out of interest.
    So the basic problem I solved is that, given a boat will have optimum drive at a particular angle to the wind at 0 speed, and then as it picks up speed will need to bear away to keep the angle to the apparent wind constant at this same angle, how does the true pointing angle vary with speed and more importantly how does the VMG vary?
    I worked out the equations as :

    (1 + f*cos(TA))/f*sin(TA) = tan( 90 - TA + OA)

    .. where f is fractional speed (speed = f * wind speed),
    TA is true angle to wind,
    OA is optimum drive angle.

    I stuck in an OA of 30deg just to see how it comes out and at an f of 0.3 this gives a TA of about 48deg, so as LooserLu says that seems about right.

    But the VMG chart then says the best VMG is at an angle of about 68deg !! and I don't think that sounds right. Any suggestions as to how to vary the model to make it more realistic ? I hope my attachments worked.....
     

    Attached Files:

  20. abenn

    abenn New Member

    Likes Received:
    0
    Trophy Points:
    0
    I just realised whats wrong with that model. I just did the calculations for each value of boat speed, 0, 0.1, 0.2 etc, ie assuming that the boat could go at basically any speed. Obviously as you go faster, bear away a bit, go faster, bear away etc, there will actually be a limit to how fast you can go but how do I work that out ?
    When I did the calculations first, at OA = 45deg, it comes out that at f=1, TA = 90deg ie on a beam reach, which sounds plausible. With OA=30deg, you can see from the chart that the speed can increase to >1, ie faster than the wind speed, before you reach TA=90deg. Back to the old "planing up wind" discussion ??
     

Share This Page