Tennis ball speed log...

[Kim Whitmyre]

So I finally bought a 3-pack of tennis balls in order to use one ball and the container for a back-up speed log. Here's the particulars: it's calibrated for a 50' line (with extra length for angle and cleat distance above the water) and has a chart I printed out to put inside the plastic container that tells speed depending on how long it takes to be fully taut aft.

I need to make a secure attachment of the line to the ball and am open to suggestions. Monkey's Fist with the ball inside? I came upon a suggestion to thread a line thru the ball with a piece of stiff wire, but that would give me a ball full of water: no thanks!

[ssor]

A constrictor knot pulled very tight will work.

[Hwyl]

I'd go with the monkey's fist.

Do you have Ashley's Book of Knots?

[PaulC]

Do you have any of the synthetic mesh bags that oranges are sold in? I think one of those could be tightly sewn closed around a tennis ball.

[Kim Whitmyre]

I don't have ABK, just small volumes by Brion Toss and Barbara Merry. But I do have some directions for the Monkey Fist. The mesh bags floated thru my mind, too.

Some of that small diameter rainbow colored braided line in a monkey fist would seem to be the most durable way to trap the ball. How would you make a constrictor knot that would trap the ball? Seems like it would "pop" out the side, so to speak.:)

[Hwyl]

The constrictor knot is the beginning of a "Turk's Head" you could go the whole hog with that.

Does it have to be a tennis ball. You have a Wharram, couldn't you tow a naked Barbie doll?


http://icarai.net/IMG/jpg/2girls2cats.jpg

Technically you're not towing because the "log" remains stationary as you pay out the string.

[ssor]

The tennis ball is furry and flexible. The constrictor pulled very tight will compress the ball and give it a "waist". the fuzz will keep the knot from slipping. You couldn't achieve the same thing with a billard ball, it would pop out.

[Kim Whitmyre]

Does it have to be a tennis ball. You have a Wharram, couldn't you tow a naked Barbie doll?


That'd be a conversation starter, for sure :p

Well, I'll get to knotting and see what happens. 3 balls to a pack, so I have a slim margin of error. :)

[lagspiller]

Seriously, spring for a small, handheld GPS. I bought one a year ago and don't understand how I managed without. They give all kinds of data in addition to speed over ground. The handhelds are easy to take along on the boat, MC, car, hiking...
After a very short time I had all the relevant waypoints around my home sailing grounds and can run distances, time to goal, time or distance to next waypoint. I can correct sail trim with the GPS. Even if it a really simple one, I can plot a trip from the chart and read distances, speed averages, max speed, present speed.... and always have trip data at a fingertip.

I also used it on the Hadrian's wall trip in the same way. On the first part of that trip I never would have found the Hostel in Edinburgh if I hadn't plotted the long/lat before I left home.

Even my technically-skeptical wife got interested in the GPS while we were walking the hills of Border England.

[htom]

He's using the ball and line for a backup.

There is a way to trap a ball or box in a mesh of six square knots with a single uncut line, but I don't know a good way to describe how to tie it. There may not be a good way to either describe or tie it; I always end up drawing it out on a larger box, forming the mesh and knots, putting in the ball, and then tightening it.

Start by drawing a square knot on the bottom of the box, leading each of the four ends over a separate edge. Now draw each of the ends to the center of its square, and then make a (left or right doesn't matter, but each of the four goes the same way) right angle and over the edge again. Now lead each end to the center of its new square, where there's a right angle of the line, make a square knot, and lead the end to the top. You now have four ends at the top; draw them into the center and form a square knot. All of the ends are now consumed; choose where and how you want to break the mesh to form an attachment point or splice.

[Kim Whitmyre]

Lagspiller: yeah, I do have a handheld gps. Very handy, without a doubt. That book Ian McColgin mentioned in the recent compass thread has some great tips on using a gps, btw.

htom: I saved your method to a text file, I'll give it a try!

[Thorne]

I may not be totally clear on the device, but I assume you have the ball at the end of the 50' line, and time the interval between plop-down into the water and tautness?

Wouldn't all sorts of knotwork on the outside of the tennis ball change both the diameter and speed at which the ball moves through the water, or maybe cause it to "tow" at an angle?

Physics ain't my strong suit, but wouldn't you want the ball to be as smooth and round as possible, and attached to the log line with as simple/minimal a knot as possible? Otherwise you could use anything at the end of the line -- old shoe, wooden box, whatever...

[sharpie]

A small slit in the ball, just enough to push the line inside with a figure 8 stopper knot will do the trick. Water inside the ball is not a problem since the ball squeezes down on the line making a virtually water proof seal.

[Kim Whitmyre]

A small slit in the ball, just enough to push the line inside with a figure 8 stopper knot will do the trick. Water inside the ball is not a problem since the ball squeezes down on the line making a virtually water proof seal.

Great! I'll do this first. This is the simplest solution, I was just concerned that the ball would get too heavy and not float if it got full of water.

[sharpie]

Kim, since you are using this for back-up, it would not be used enough to get water logged. It will be used most often at first -- practice makes perfect. Even so, this sort of use will not be enough to cause problems.

[Thorne]

I'd try sealing the slit with PL Premium or whatever flexi sealant you've got kicking around -- that should keep the ball interior dry and still not interfere too much with how it moves through the water.

I always liked the old curved clear graduated plastic tube with the floating ball device -- not too accurate but fun to use.

[sharpie]

Thorne, I don't want to steal Kim's thread, but I am interested in your evaluation of your dory, especially as a sailboat. Any info you can offer would be much appreciated.

[Bruce Hooke]

Is this thing supposed to work basically like a traditional "rope log" where the end in the water stays fixed while you measure how long it takes for the rope or string to pay out (or measure how much rope or string pays out in a fixed period of time) or does it assume that the ball will make some distance through the water as the string is being paid out? If the former, I have my doubts about how good an "anchor" a tennis ball will be. Seems to me that it will be pretty hard to keep the ball from dragging along a bit, especially just after you drop it in the water. This is why they used something a bit larger in the old days. If the latter, it seems like the math could get really messy and not very accurate because of all the variables that would go into determining how fast the ball would move through the water. So, I suspect it is the former (that the ball is to remain still), but it seems to me that something with better water "catching" ability than a ball would be more effective. When I made one of these sorts of devices I used a traingle of plywood with a rope to each corner and a bit of weight added on one edge to get it to float vertically. When it came time to pull the thing in the tension on the rope would life the plywood up so that it skated along the surface of the water. Or at least that is what I remember. This was all 20 years ago.

I suppose a tennis ball might work if you basically held onto one end the string and dumped the rest of the string and the ball in the water all in one go, rather than trying to pay out the string. Of course tangles could be a bit of an issue if you drop the string in all at once...

[Thorne]

Glad I'm not the only one to be puzzled as to how it all works...

;0 )

Sharpie - see the new thread I've started on rigging pulling boats for sail in this section.

[Kim Whitmyre]

Here is the message (from where ? I don't remember) that explains the use of this log:

"If your boat doesn't sport a nifty electronic knotmeter, or the dang thing packed it in, you can use your GPS speed reading if you average the results over a few minutes for a reasonably accurate SOG (speed over ground).

If you don't have a GPS, or the dang thing packed it in, or you need velocity through water (to account for current effects on SOG), here's a little trick I picked up from "The Complete Sailor".

1. Tie a 51 foot 1/4” nylon braid line to a tennis ball and cleat the loose end off at the stern.

2.Toss the ball overboard (ahead of the stern cleat).

3. Time how long it takes the 50 ft cord (1 foot for cleat tie off and angle compensation) to become taut.

30 sec = 1 knot/1.2 mph/1.8 kmph

15 sec = 2 knots/2.3 mph/3.7 kmph

10 sec = 3 knots/3.5 mph/5.5 kmph

7.5 sec = 4 knots/4.6 mph/7.4 kmph

6 sec = 5 knots/5.8 mph/9.2 kmph

5 sec = 6 knots/6.9 mph/11.0 kmph

4.25 sec = 7 knots/8.1 mph/12.9 kmph

3.75 sec = 8 knots/9.2 mph/ 14.7 kmph

Now here's where I can start taking some credit for this wonderfully simple yet highly accurate (especially at slower speeds where most other devices suffer) idea.

We printed the speed/time chart (above) and put it inside the clear tennis ball container (facing out) so it was always handy, and have tied a lead to the cord bitter end for a manual 50' sounder.

We always take two speed measurements, and use it if they're identical. (The first one being slower could be due to a kink in the line paying out). If the first reading is slower than the second we make a third, and if the latter two are identical we use it.

The lead, line and tennis ball all stow away in the container.

So there you have it, the most reliable (no batteries or vibration/corrosion sensitive electronics) anti-theft/vandal resistant (when stowed) depthsounder/knotmeter combo available, at the lowest possible cost. "

So after cleating the line off, you toss the ball and line off the beam, perpendicular to the beam. Start your stopwatch, wait until the ball pulls the line taut, and then stop the watch. Should work like a treat! :p

[Bruce Hooke]

Thanks for the explanation.

That is roughly what I expected. I will be interested in hearing how it works out in practice and how speed measurements made this way compare with speed measurements made using electronic devices.

You would obviously need to be careful not to toss the ball too far out from the boat because that would have the effect of shortening the amount of rope that would need to pay out before the line pulled taught.

It does seem to me that you should toss the ball overboard in line with the cleat.

[Kim Whitmyre]

I threaded some New England dinghy braid (1.8mm?) thru the tennis ball, and rather than a stopper knot, pulled it around to itself and tied a buntline hitch. I feel much better now!:D

Granted, I can practically always just use my eyeballs for my daysailing, but for crossing the San Pedro Channel to Catalina, it'll be nice to know I can estimate speed should the gps eat all the batteries, or otherwise not work.

[Hwyl]

Seems like a complicated way to reproduce the old chip log.

I'm sure you knoew that's where the word "knots" came from, the number of knots run out in a specific time period. If you put a knot every 16.6 feet and run it out for ten seconds, you'd get a direct reading in knots.

And yet again in the spirit of my countryman; James Wharram, it would work much better with a naked girl.

[Thorne]

Here's the Knotstick, although they cost more than a can of three tennis balls..

http://www.knotstick.com/k4.jpg

and here's one post from the web that calls your setup a Cajun knotmeter -

"There's also something called a Cajan knotmeter which is done with a tennis
ball and stopwatch I have heard of. You drop the tennis ball fastened with a
known length of cord over the side even with the boat, then time how long it
takes the line to tension
up."