Drag measurements for Footy hulls.

Does anyone have any measured values for a Footy hull or hulls, designed on Hullform 9s and for which a predicted value graph has been obtained using the Hullform Pro drag calculations? My interest in this is that I have just started using these programs to design Footy hulls. In recent Forum discussions with Brett McCormack, the point came up that actual and predicted values for drag show some variations. This has been discussed before, by Angus among others, while WaltH has provided a few measured values.
Does anyone have any comprehensive data of this type, preferably from an already superceded design? I do not wish to encroach upon anyone’s latest design efforts (anymore than I would wish my own work to be copied).
I personally view the Footy boats as a chance to try my own efforts in the field of yacht design, a point I have often mentioned in previous posts in other Thread.
I would be interested in any worked out example, and also in an effective testing method which would not put me into a $million debt.

The easy advice here i9s a Ljugstrom yoke as suggested by me and used by Brett (unless he has an improved version.)

The original work is out of print and in Swedish. If anyone is sufficiently interesested I can attempt to get hold of it from the library of Sjofartsmuseet and translation is no problem.

I its basic version it is a very slick method of measuring the relative drag of two hulls. Take a very lightweight bar. appach to eaach end a towline, which is in turn attached to a Footy hull. Attach a furher towline to the middle of the ‘yoke’ bar. Tow the whole rig from this at a constant speed. For a simple measure of relative drag, adjust the position of the main towline until the bar is at right angles to the line of tow. The ratio of the lengths of the two hlves of the yoke gives the relative drag.

A little imagination will suggest all sorts of ways in which you can ring changes on this. Knowledge of a triangle of forces compulsory, but even biologists know about these (:devil3:)

Hope this helps as a starter.


Brett method…
At the bottom of my street runs a small stream,I find an area running in clear water at about hull speed or what ever speed I want to test at.
Hull is suspended in the current via a bit of string with some elastic in the middle…drag equals length of the elastic.
Since the hull is stationary you can sit back and look at the wave forms around the hull as long as you like and easily take photos if desired.
Crude but so far effective.

Brett - this is an excellent idea, far simpler than my fishing rod method.

Rod - some problems you will encounter with various towing methods, and some solutions that have been found:

You will find that a bare hull with weights has very little directional stability. I had initially added a skeg to make the hulls tow in a straight line. But then there is the issue of making sure the skegs are in line, and the difficulty of handling a hull that wants to lay on its side while the weights fall out. There is a better way. Add an 18" long dowel along the deck, protruding 6" forward of the prow, to attach the towing line. This will keep the hull going straight while towing, without the need for a skeg.

You will probably find that measuring the drag of a single hull gives a feeling for the shape of the drag curve, but is not accurate enough to discern differences in 2 reasonably efficient hulls.

If you use the tow-bar method, towing 2 hulls for comparison, you will find that the tow bar wants to droop into the water, which invalidates your measurements. This is especially problematic at slow towing speeds, where there is not much force on the tow line. To avoid this, make use of the dowel on the hull to support the tow bar. Drill a hole about 1" back from the end of the dowel. Use a very light tow bar (thin carbon tubing), with sliding grommets to adjust the lever arm. I use thin wire (#24 copper wire) to tie the dowel to the tow bar.

The sensitivity of the tow-bar method can be adjusted by moving the tie point of the towing string forward of the center, using a rigid piece to form a tee. I found that a 5" offset (on a 30" bar) gave sufficient sensitivity to see the difference between a soda bottle and a Razor. But the offset had to be moved down to 1" to see the difference between other conventional hulls. The angle that the tow bar settles into will give an approximate quantitative measure of the difference in drag (based on geometry), without a necessity to fine-tune the lever arms. But any bending of the tow bar will reduce the sensitivity.

But after all is said and done, the sails (and their balance and tuning) make a much bigger difference than the hull.

Have fun with your experiments

Many thanks, Gentlemen!
I too have a nearby stream with sufficiently open banks and car access, and I will most definitely try the suggested methods. I was trying to visualize using our pond (15 miles away) and was becoming aware of the difficulties of attaining constant towing speeds while running along the bank holding a fishing rod, while the traffic on the nearby highway was collapsing into fits of laughter. I am also going to try to get some numerical data to compare with the calculated graphs. What is the significance of the methods ‘Gerritsma 1981’ and ‘Gerrisma 1996’ which are used in the Hullform Pro program? There are no books in the city library dealing with this topic at all.

Just a minor comment–I Googled Gerritsma and found a lot of material on drag forces and the great difficulty in calculating them.
I have reached a tentative conclusion that empirical testing is the way to go, which is at least possible in Footys, but not so realistic in ocean-going oil tankers. I will stick with Footys.
Now, to start I need to determine the length increases in a very light metal spring— Brett’s rubber band sounds a bit too uncertain, and unrepeatable, given the amount of deterioration that I find in rubber bands. They become brittle in a very short time. I visualize a stick, marked with a scale, with the spring attached to the handle-end. The towing line would be attached to the other end of the spring, and extend on, to the bow of the towed boat in the running stream. The stretching of the spring can be calibrated using small weights in the range from zero to about 20 grams.
I will set it up and report here in this thread. I have a number of hulls to compare, starting with a Razor.

Or you could use something like this?:

Any scale, to be of any use for the purpose of determining Footy drag curves, would need to be accurate over a range of 0-50 grams, +/- about 1 gram. I do not yet know of any hand-held fishing scale for weighing minnows.
I have wound a very light spring which extends about 5.4 cm for a free-hanging weight of 50 gms.


Your decision to use actual empirical measurements is a good one. The drag of these little hulls has some terms which don’t appear in any normal hull drag analysis. Brett has mentioned this. In another thread, Flavio did some calculations on the drag of a Razor, which underestimated the measured drag by about a factor of 2.

I have a feeling that some of the additional drag may come from surface tension, which is generally ignored at higher speeds. There may also be other contributors.

Perhaps a trip to your local fishing shop would help. They will probably stock some stuff called Pole Elastic. It will more than likely be Latex, may even be hollow, but will certainly come in a selection of sexy colours and strengths.

This link to a shop in the U.K. http://www.subfish.co.uk/C/Fishing_Accessories_Pole_Elastic-(39).aspx will give you some idea of what to look or ask for.

I would expect the Latex to be a lot more stable than your common or garden rubber bands.



Are we about to become an adult site then? :devil3::devil3::zbeer::zbeer:

I have hopes. This does seem to be an area where it is possible to make a significant contribution, without the expenditure of millions of Euros, Pounds, CDollars or USDollars.

A straing gauge would be able to measure weights from 1g 0 100 or so. There is a simple adapter that will plug directly into a digital VOM so the display reads in grams. Or just read the raw value and refer to a conversion chart.

To Tomohawk:
Please describe the apparatus you are talking about, where it is obtainable, and its cost. I am only too aware that the measurement of small forces such as we would expect to find in the drag of Footys would usually require very expensive instrumentation–unfortunately this is a hobby for a retired biologist, who is willing to spend time, but not a great amount of money. As a clock hobbyist, I have an assortment of springs and the ability to wind spring wire into coil springs. It is in this area that I am looking for measurements as accurate as possible.
I have made one spring which extends about one inch under a force equal to about 25 gms, which, according to the drag forces predicted for Footys in the Hullform Pro program, would cover speeds up to about 1.4 Knots.
I attach a photo of the apparatus supporting said 25 gms, measured on a not-to-be-entirely-trusted kitchen scale which measures to 10 Kgms (photo attached)
I think I will re-wind the spring.

Going off thread - again!
Rod, if you lke clocks, heve you ever seen Harrison’s chronometrs in the National Maritime Museum in Greenwich? I love fine craftsman ship and clever engineering and I can sit and watch them for hours on end.

Euro Footy GP next year? It’s less expenive than you think. See http://www.flyglobespan.com/

where I can get the price of Hamilton ONT - Londoon Gatwick dwn to $750 Canadian withut really trying very hard. Charie Mann has travelled by Globespan and says that they do provide wngs, engines, pilots and food!


Unfortunately, I’ve never seen Harrison’s Chronometers, or his original tall case clocks with the Grasshopper escapement that he used in H1, 2 and 3. He abandoned it in H4, the final large watch-style chronometer.
I have however built and am running a modification of his grasshopper escapement in a tallcase clock where the movement was constructed from the remains of a modern “grandfather” clock. I put ball bearing races on all of the arbours, all carefully washed free of any lubrication, (the forces involved are so small that there is essentially no heat to be carried away), and made an endless chain to drive them, with a tipping mercury switch to turn on and off a small electric clock motor to rewind the weight. The grasshopper escapement hangs from a pair of suspension springs, while the pendulum in turn hangs from it, again by a suspension spring. The elaborate grasshopper arms are replaced again by springs, to that in the escapement action as the pendulum swings, there is no friction from any pivots. Because the whole movement has “no” pivot friction, it requires no oil, and therefore should never need to be stopped for cleaning and re-oiling. With no oil to deteriorate, the time keeping will not change due to the oil. The contact of the escapement pallets with the escape wheel teeth are correctly ‘touch-and-go’ with no sliding friction. The only troubles remaining are in the compensation of the temperature-compensating pendulum. It is an Ellicott pendulum with the compensation adjustable without stopping the swing of the pendulum.
Sorry to ‘run on’ like this but you sound like a man who would appreciate the problems of accuracy in small force mechanisms.


The advantage of using a strain gauge is the relative compactness and repeatability of the results, as well as the selection of a strain gauge to take precise measurements within a certain range. Measurements can also be quickly taken using electronic recorders and conversions can be done immediately through electronics or simple programs.

For you, I think it would be better to use your spring method, but see if you could get the spring inside a tube to keep things linear. I have used strain gauges (load cells) in the lab, but they are no longer as small as you would want to use in the field and a linear strain gauge would probably be the best thing for the average hobbyist (http://www.omega.co.uk/ppt/pptsc.asp?ref=Prewired_GP_Strain_KFG&flag=1) I would think. You get 10 in a pack, so you could share the cost.

Thank you for your reply about strain gauges, but I think that I will have to make my measurements while standing mid thigh deep in our local river Thames ( as we say, the other Thames). I checked my little local stream as suggested by Brett, but found it was too shallow at the moment and the fastest flow rate was less than 1/2 Knot.
Standing on slippery rocks thigh deep in the river, I think I would be better off not having electrical recording apparatus hanging around my neck. As it is, I will need my boat hull, the spring measuring apparatus, a float with 2 meters of cord and a stopwatch to measure flow rates, and a notebook to record the results. It would be too slow to return to the bank side for each change of apparatus.
However, the “Prince of Serendip” has struck again! While trying out all the pieces of fine spring wire in my scrap pile to make helical springs which would extend far enough to give meaningful changes in length by 25 gm forces, I found that the 20 lb stainless steel fishing leader used for downriggers and covered in nylon, which most of us use for the standing rigging of Soling 1M rc yachts, makes an ideal helical spring which stretches just about exactly the required amount. I attach a photo here. It is the same ‘tool’ as the previous photo, only the spring and the paper scale have been changed.
I do not follow your reference to enclosing the spring inside a tube for linearity of response. Surely that would just add to any interfering friction?

Rod - I do and I like. If health and finance permit, you really should try to see Harrison’s original machines. You obviously know very much more about them than I do, but I think that even with your knowledge you would be quite enthralled by the amount of detailed thought that went into them.


If the flow rate of the water doesn’t suite your experiment, you could build a sluice with a funnelled end to create a faster flow. The frame would also allow you to attach your load measuring device to hold it fixed. Put some wheels on it ald floats on both sides.