Drag measurements for Footy hulls.

To Tomohawk: I live in a city, and access to the river is limited to just people on foot, so any large sluice would need to be transported back and forth to the nearest carpark, and then by hand. It couldn’t be left in situ, as the vandals would destroy it within hours.
When I get set up within a day or so, I will attach photos of the intended site and you’ll see what I mean. The parking area is for a couple of soccer fields on the river’s floodplain land, about 3 kilometers from my house. Access to the water itself is limited by bushes and trees growing thickly along the banks. I am a flyfisherman, so am fairly familiar with most of the accessible places.
To Angus: If you get a chance to see the contruction of H4, you will observe one of the most complicated mechanisms devised by man. Harrison must have been desperate. For real elegance of design, look at the chronometers of Arnold, admittedly designed some 15-20 years later, but the difference in principles is enormous. I have long wondered just what happened to Harrison that made him abandon the H1-H3 design and switch to H4. Unfortunately, nothing seems to have been recorded, and Harrison’s own writings are just incomprehensible. He suffered from some mental problem that turned his written words into near gibberish. A fascinating historical and medical problem!

Rod: I was aware that T4 was incredbly complex, but I obvioisly underestimated just how complex! The problem is that when you see it in the museum, it’s just another clock in a case and there isn’t rrally much apart from details of the fnish to say it didn’t arrive on a Dutch East Indiaman. For sheer visual efffect, however, the others are unbeatable.

As complex machanisms, what abut the Analytical Engine of Babbage?

Cheers

I mentioned Harrison’s problems with written language, and looked up the reference. Here is part of the first sentence of a manuscript he wrote-- the first sentence was 25 page long.
Do you wonder that he had so much trouble with the Board of Longitude?

Apropos the Babbage engine, the London Science Museum now has a modern reconstruction that works. I’ve seen in action. The curator responsible for re-creating the machine, a man called Doron Swade, wrote a book about it, whose title I can’t recall. In it, among other things, he suggested that one of the reasons that the original never came to fruition was that the manufacturing capacity of the firms contracted to make the bits wasn’t up to the level of accuracy and consistency required.

Russell

On Harrison’s prose, you may or may not be aware that judgements of French curts are typicallly expressd - reasoing and all - as a single sentence. The longest I have been calleed upon to ttranslate was about 30 pages long. Horrendous.

I have been experimenting with the problem of retro-designing an existing boat into Hullform 9s, when the lines are not directly available in machine readable format, such as my “Glass Petrel I & II” which were carved out of pink foam on an “eye for a hull shape” principle. I started with an original “Razor” and did a crude estimate of its hull shape into Hullform. I then took photographs of the actual hull, as built. I then transferred the photograph into the Paint program, where I eliminated all extraneous parts of the photo, and sized the photo to a 12" hull length on the screen. I then took the Hullform line image, and used “Print Screen”, Control X, “Open Paint”, “Control V” to get the image into Paint. In Paint, I sized the image to 12", the used 'Copy". I re-opened the Paint photo, “Paste” and moved the line image to superimpose it on the photo. After further adjustment of the lines in Hullform, I repeated the procedure to compare the newly adjusted lines to the photo of the actual hull.
The resuts are shown in two attachments, showing the deck view and the side view.
Unfortunately, in all of this confusing and tedious procedure, I naturally forgot to “save” my modified Hullform lines within the Hullform program, and so have it all to do over again, tomorrow when I am not so tired!!!

Sorry for all that work, Rod…unfortunately my original Razor Hullform file is long gone. I think I posted it years ago on this forum, so it’s remotely possible that someone might have saved it.

Does anyone out there have the Hullform file for Razor? If so, please send it to Rod!

Bill

to mudhenk27: Thank you for thinking of all my travail, but I have restarted my endeavours. I dont know if you intended it, but my Razor, made from thin plywood, measures just 29.4 cm rather than 30.4—sort of the opposite of “diagonal in the box”! I had wondered why my Hullform lines did not quite fit the hull deck photograph!
The procedure I’m following is quite laborious, with many steps and dimensional resizings, as my computer screen image of 15 cm is a measured 14.5 cm. Frustrating!

Rod - If you are working from an existing boat then you can derive lofting points that can be entered into most boat design software programs from paper tracings of the hull panels. Once you have the tracings you will need to create a template for the rocker profile and the curve of the deck line down the centerline (if applicable, some deck lines are straight). You will also need to create a series of parallel lines at regular intervals perpendicular to the waterline that wrap from one panel to the next. This is fairly easy to do on a side panel but the bottom panel is tougher. A strip of paper, say 1/2 inch wide +/-, is used to create a “tick strip”. The strip of paper is held perpendicular along the curved lower edge of the side panel tracing and the parallel lines on the side panel are “ticked” or marked on the strip. These tick marks can then be transfered to the the bottom panel along its common edge with the side panel. The rocker template should have parallel lines drawn perpendicular to the waterline at the same intervals used for the side panel. A tick strip can be laid along the rocker template’s curve to get the line spacing. This line spacing can then be transfered to the keel line edge of the bottom panel. Lines drawn on the bottom panel can then connect the tick marks. The intersections of these lines drawn on the hull panel tracings with the tracing’s edges will give you accurate lofting points.

Neil,
I examined your suggestions about the process, but I think what I’m doing here is easier for me in that I’m not a draughtsman and don’t really have those facilities. What I’m doing is adjusting the Hullform lines to match photographs of the boat. It is of course trial-and-error, and is a bit laborious, going back and forth between the Hullform program and the photos of both the boat’s hull and the lines in the Paint program, making successive changes until I’m “satisfied” with the matching. I attach here three views showing the hull with the Hullform lines superimposed. I think that the method will work equally for rounded hull shapes where no original plans are available, like my “Glass Petrel I & II”. The stern view suffers from parallax problems–I will take the photo at much greater distance and fully zoomed in, so that stern beam and midships beam will be more correctly sized.
The ‘object of the exercize’ is to obtain drag curves in the Hullform Pro program and then compare with the drag obtained by ''towing" the model in our local Thames River, where I should be able to get adequate water depth, a range of flow speeds and smooth flow. I flyfish the river, and in most places it is mid-thigh deep. It should be possible to compare hulls both with and without keel and rudder, and by subtraction, obtain drag curves for keel and rudder separately.
I have been enjoying our little interchanges on the other thread—a bit of light-hearted teasing is sometimes needed in the Forum when people get overly serious about Footy differences. For some of us its purely a hobby, for others a potential future business, so the development of the Footy class can have major implications for the future. My own thought is that nobody anywhere is ever going to ask me to design an America’s Cup racer, but I can try my hand in Footys without breaking the bank.

Rod - The reason that I suggested a a lofting technique is because there is always a bit of distortion with photographs so your system may not render an accurate hull shape. Of course, many of the Razors that have been built are probably not true to the original plans since there is, in this class, such a broad range of building skills and materials in use.

My impression of the Razor from photographs and sailing reports on the various Footy sites is that it is a rather unbalanced design. It’s sharp entry and low reserve bouyancy up front contribute to it’s tendency to wheel up windward when over-pressed. Bill H.'s later designs in the Cobra series were better performers overall and those panel templates are still around.

Are you planning to do drag tests in heeled trim as well as running trim?

Neil,
Yes, I will make the process as complete as I can. It was pointed out earlier, that it would be so expensive to use a major hull-testing facility for this job that no-one would ever do it for a “toy” boat. I don’t think I can make any contribution to Footys by my ‘racing’—I’m still learning, somewhat late in life, to race in Soling 1Ms, but as there are only two of us in our local club who wish to race regularly, I have very little “starting” experience. In a fleet start, I usually get ‘bullied’ out.
Retro-designing to photographs of actual boats may not be as accurate as designing properly, but as you point out, only professionaly designed and mass manufactured by vacuum-moulding boats are likely to be faithful to their actual designs.
The numbers that I produce by this method may be useful to others.

Rod,

We are anxiously looking forward to your testing. There is very little real information available, and all of it is in the upright (not heeled) condition. The heeled measurement is probably more important, since that is the condition of the boat during most of the racing. But it is more difficult to perform in a controlled manner. My initial testing wasn’t concerned about heeling, since it was concentrated on a soda bottle, which doesn’t care if it is heeled or not.

Strangely, my soda bottle is still competitive in our local races, frequently winning. I attribute this not to the hull, but to the sails and the enormous keel and rudder, which allow it to be controlled in the barely perceptible wind at our local pond.

I will have to do some more exploring of the mechanisms of the Hullform program before I’m confident I’m doing everything right.
It will be difficult to heel the actual model duing the towing tests–I can concentrate all the interior weights such as the receiver servos and batteries under one gun’l, but that will produce only a small amount of heel. It may be possible to reduce the weight of the keel-bulb (but not the volume or shape) and place that weight up on the mast to heel the model without changing it’s total weight. I realize that in tank testing, the model is held at the required angle by a framework attached to the moving carraige, but that technique will not work in river-towing.
I would welcome any suggestions.

I thought about the “heeled” problem, and came up with a possible solution.
Rod

This is a very original solution. It can also be adjustable with a bowsie on the heeling string.

But if you just want to test the hull, I would suggest an incomplete model, with just the hull, and use weights inside the hull to simulate the weight of the bulb and radio gear. These weights could be held in place to simulate heeling and pitch attitude (adjust at home in the bathtub). The problem I had was that the weights wanted to move when heeled, despite velcro attachments. My early test models had minimal decking, and tended to swamp when heeled. More recently, I have used carved foam blocks, with a little tape, to hold the weights in place. The hulls are much easier to handle without the keel and rudder. But this method will need some trial and error to get the hull pointed in the right direction. Note that a bare hull will not go straight unless the tow attachment point is far ahead of the prow (I have used a long stick taped to the deck). The addition of keel and rudder may not necessarily improve this problem.

But if you have a hull already built, go out and get some data (any data), and you will see what is possible and what isn’t. If you only get a set of data on a commonly known hull, not heeled, it will already be more than is available.

Flavio has done some simulation of the Razor hull, which he shared with me some time ago. Unfortunately, my hard disk died last November, so that information has been lost, but he may still have it.

While my Razor had a firmly attached keel, my Glass Petrel I & II have detachable keels and detachable bulbs. I can always saw the keel off the Razor after I get numbers for the keel-and-rudder condition. The rudder is detachable anyway. One advantage of this method is that a model can be tested with all radio and batteries and even rig fully in place (assuming a windless day on the river).

On the weekend, I went out on the river and got some preliminary numbers for the drag of my Razor, “Big Red One”. I discovered that I will need deeper and less turbulent water than in my first attempt. I also found that I need to use extremely light rubber bands, and four of them attached end to end, to get enough extension to make reasonably consistent measurements. I also need to make my measurements away from the disturbance created by my own legs in the water, so I mounted the rubber bands and a ruler on the end of a T-shaped stick, 3 ft long, held out sideways at arms length.
The still-attached keel was too close to the bottom, where turbulence was created by the head-sized rocks. The drag on the boat varied continuously, making it difficult to decide upon the drag-force measurement.
The water velocity was measured by dropping a 2 inch fishing float on the end of a 7 ft cord, and timing when it reached the end of the cord with a stopwatch recording to 1 / 100 th second. 4.12 seconds equals 1 knot.
I attach here some preliminary results.

Rod - This is a good start. Keep up the good work.