Has anybody out there ever attached some sort of speed measurement device to their merlin or other dinghy?

Can you do it on the cheap? - I'm only thinking of using it to learn about tuning and I'm sure it would be illegal to be fitted during racing.

Haven't even read all of this article by rohan veal but might be relevant to you.

Some years back there was A SPEED PETO available the you put down the CB Slot and it gave hull speed in a sort of thermometer  funnily enough I saw one in a chandlery in Vejle in Denmark recently it had dust on it! It was V Old technology so I doubt it would transcend any electronics rules!

What about using a handheld GPS kit. There is a guy from Dalgety Bay who sails a Moth and he has tried it with some success.

Hmm, I was considering the Pitot Tube, but you've got to measure the difference between the static pressure at the depth you're measuring from and the pressure resulting from the water rushing into the tube.  The Pitot tube would give a nice quick response but having a big glass tube in the boat might be a little hazardous.

I dismissed the GPS as not giving a responsive enough measurement but if I took my boat somewhere big like Draycote and did a series of timed runs with various rig settings I could possible get quite good data.  The GPS could also give me heading information.

Fascinating article about using GPS to examine your course after a race. Just the sort of thing for our techno-savvy fleet.

What about using the speedo alone to judge rigging settings on the water? Is the wind ever steady enough to show you that the rake adjustment you just made was faster or slower? Or is no speedo accurate enough? Do waves ruin the results?

Thats what I was thinking about GPS but If you got the right water (pref flat, non-tidal) and did a few runs and took some averages you might get some good stuff.

Perhaps you could alternate settings and do 10 runs.

Why not find a tuning partner you sail boat against boat and only one of you does something different at any one time thus you can find out what works or does not. You need to trussst the guy though! It removes the need for kit and maths etc.

I sometimes use a total head tapping which I incorporated into the leading edge of my rudder. A length of plastic tubing connects the tapping to the space above some dyed water in a container (at the bulkhead) forming one arm of a manometer. The other arm is another length of clear plastic tubing ending at the gooseneck or higher. By putting different coloured bands at intervals along the clear plastic tube you can easily note the head and so estimate the speed.
I only connect the tubing to the rudder when I am afloat to reduce initial depth of tapping considerations but a lot of damping is required to reduce the effect of waves &c..
My primary requirement is to see changes in speed (for tuning purposes)not absolute speed but I would comment that unsteady winds, crew position variations, resistive effect of waves &c. serve to cause continuous changes in velocity which makes life  difficult.
I will use this kit at Salcombe this year if there are any big winds just to see how fast we go. I see no problem with the rules whilst racing since it is not electronic, maybe GGGGGGGG could comment. For interest, I could calibrate it to use in conjunction with my compass to provide indication of changes to VMG without any on-board maths.

How do you calibrate it?

if you put the other end of the tubing somewhere it would be good for colonic irrugation as well

The high the voice the greater the speed - until!

Nice one John,

That's what I wanted to hear!!!!,  do you have any photos?

Thanks

Jon

Hi Jon.

Sadly, I do not have any photos but look me up in the book and ring (or whatever) with your address - I will send some sketches.
I thought of a rather nice demon tweak to improve sensitivity at the low speed end (swot up Bernoulli's equation)- by allowing a catinery  (now you cannot patent it !!). The VMG part will take too long to explain - maybe you can work it out.  If I had a regular crew who did not require supervising all the time (and if I did much sailing!) I might have used it to good effect to move up the fleet. A new main sail is likely to be better!!

Cheers..........JJJJJJJ