I have built a Footy from discarded 2-liter Pepsi bottles. One bottle is not quite long enough, so I married 2 together. A picture of the boat is on the Footy website, under “Pictures”. It is shown with the 12 inch mast (storm rig), which actually worked very well in a strong wind. However, it has been racing with a single sail having a 22 inch luff. A cat rig was used for simplicity; I don’t know if a sloop would give better performance.
We currently have 3 Footies sailing on our pond, of grossly different designs. We have been racing for the past 2 weeks, and the Pepsi Torpedo has won every race so far. This is probably because it is relatively light. It certainly is not due to sailing ability, since the other 2 skippers routinely win when we race Victorias.
Although I am probably perceived by most users of this forum as one of the high priests of the ‘technical’ approach, I thoroughly admire this type of innovative ‘trash-can’ thinking, especially when it works.
One of the formative influences of my life was an old man whose maxim was ‘an engineer is a man who can do for five shillings what any fool can do for a pound.’ How true. Very well done, and keep it up.
This design was actually based on a review of the Bob-About, which has a similar beam. The objective was to make it faster. There were several engineering objectives:
Reduce wetted surface with a cylindrical cross-section, which should also reduce turbulence.
Reduce submarining by moving the maximum beam further forward. This should allow it to carry a little more sail.
Round off the transom to reduce drag.
Eliminate rocker to provide more damping of the 1-second pitch oscillations.
Eliminate the fragility of a balsa hull, since our boats are frequently attacked by large fish.
By a strange coincidence, the form of 2 Pepsi bottles, back-to-back, provided a reasonable solution to all these problems.
There are no similar designs around here to compare results on an experimental basis, and it would be very educational to know if someone has conducted towing tests on various hull shapes at this very low Reynols number. The V-12 looks interesting, as it has almost evolved to a squared-off scow of maximum beam.
Another interesting aspect of these boats, is that the Reynolds number is very similar to a golf ball, so maybe a rough hull surface would have less drag.
yes i agree. i hear that the reason the rough golf ball surface works, is because it is spining; therfore, muckin with the laminar flow around the shape. was this talked about before? feels like day-shja-voo.
For starters, please note that the objective was to improve on the published Bob-About design, however at the present time there is no Bob-About in our area to determine whether the objective has been met. But it is faster than the other 2 boats at our pond.
To answer the individual questions:
Reynolds Number
The Reynolds number is a function of velocity, density, viscosity, and size (R=rhoLV/mu). The comparison with the golf ball works because the golf ball is much smaller, has a much higher velocity, and is passing through a far less dense fluid (air vs water). Consequently, if you work out the numbers, you will find that the Reynolds numbers fall in the same range. I don’t know if creating turbulent flow will actually improve the drag of a cylinder, as opposed to a spherical golf ball. I have seen mention of this effect on other modelling websites, but the issue has always been that the Reynolds numbers are too far apart. Here, there appears to be a match, because our length and speed are considerably less than any other models, so it might actually work. The effect of the dimples on reducing golf ball drag has been graphed in some data that appeared for a time on the Spalding golf website. The measured effect did not include spin, which added lift.
Fish attacks
We have been racing Victorias at this pond for several years, and they have frequently been attacked by large carp. We don’t know if the carp are trying to chase the boats away, or are trying to mate with them. Attempts to interview the carp have been unsuccessful.
while I don’t disagree with your “Reynolds” explanation, it seems rather odd to try to use two such different shapes, sizes and velocities to “match” performance characteristics. While both may have the same number - it seems to equate to a similar comparison - a ton of feathers to a ton of lead !
We have learned from the aircraft industry, that wing shape and thickness may be similar in all aspects, except when one is traveling sub and the other is traveling at super sonic speeds. Similarly, a NASCAR (for US fans) vesus a Formula 1 - both racing on a similar/same track wold probably have much different charaterisics - yet both could be considered as “race cars”.
I would expect that the Reynolds numbers, in order to mean anything, would require a comparison of identical objects moving through the same medium at the same speed, where only the foil shape would be different. Then, analysis and comparions would seem to make much more sense.
I am not a scientist, but logic would seem to support a view of Reynolds numbers all tested in the same (and identical) fashion - perhaps not. It’s just a personal view though, not trying to be argumentive.
wow, the carp phenom is incedible.
i wish to take my footy up to my buddies, but there are some serious muskelunge in there. one even bit his brother…no joke!
I’d be quite angry if a fish broke my R/C boat. Fortunately the local lake I sail at mostly has small trout, a foot or smaller (so the Footy will outsize them!), and the big carp tend to hang around the bottom only. Plus, the dock I sail off of always has at least 2 fishermen, and their bait is probably alot more tempting than my boat.
My experiments with a Bottle footy also lead to a favourable conclusion.
see the mariner thread for pics and details.
I have done drag testing with around 20 different footy hulls over an intensive 2 day period.I am keeping the results fairly close to my chest…but keeping an eye on what leaves a few key Footy drawing boards should show some clues as to what was learned.
Re Reynolds numbers and Golf balls.
Walt has it pretty right.The important part to realise here is that Water is 1000 times denser than air…this is why the speeds and flow can be compared.
Brett, I am very interested in drag testing. I was thinking about a small geared motor driving a spool of fishing line and dragging the boat (without sails) across the pond, and measuring the motor current at different speeds, then backing out the drag vs speed curve. We have been using some motors of this type ($15 each) in hop-starting our model ice-boats. I was also thinking of comparative testing with a cross-bar towing 2 hulls. Obviously, at this point it is all talk and no action. What method have you been using, and is it sufficiently accurate?
By the way, I have noticed an interesting characteristic of the Pepsi Torpedo hull. When overpowered going downwind, it doesn’t behave like a Victoria, which immediately makes a U-turn or a somersault after the prow goes under. Instead it plows along on its nose, because of the torpedo-shaped prow. I call it “Nose-Dancing”.
Walt,
I used a cross bar device towed from a dinghy.
Didn’t mearsure the actual drag…just compared all different combinations of hulls and studied wave forms.
Take a look at the Akela pics from Angus for some interesting waveforms.
“Nosedancing” yes have observed the same,“PipSqueck” does this as did the bottleboat,Akela looks the same to me from her bow wave.
See pictures of Akela - you’re 10% of the wayb to how (I think) the UHPB boat works. :zbeer:
Incidentaklly, the balance method was pioneered by Ljungstrom in Sweden in about 1935 and weas regarded ass a good cheap secondrate to a proper tank dymamometer at the time. Ljungstrom was a very prominent steaam turbine engineer whose hobby was yacht design. His opinions are geneally to be respected.
The Pepsi Torpedo is continuing to win the local races, but my competitors are getting more serious, and some inprovemet may be needed.
I was thinking of increasing the keel by 2 oz to take better advantage of the hull length, because some of it was wasted in overhang. This in turn would allow significantly more sail area. I was looking at changing the cat rig to a lateen sail, to get more sail area without increasing mast height, which would start to cancel out the advantage of the greater stability. Then I read some of Brett’s stuff, and it appears that he has solved that problem very nicely with his unconventional rig.
I have a question: it looks everybody that uses that rig has the mainsheet going to the forward end of the boom. Is there an advantage to that placement, as opposed to attaching it the same distance aft of the pivot?
I also noticed that when Brett had played with a soda-bottle Footy several years ago, he used the bottom of the bottle as the stern. I had used two top ends for bow and stern (this wastes waterline length, as noted above, but I had guessed that a square dragging stern was bad). I may build another hull using the bottom as the stern, to see how it compares. Maybe at the very low speeds encountered here, things are different.
Brett’s design calls for a very flexible main boom, to assist in the depowering action of the rig. Forward of the pivot is rigid, so it really is the only place one CAN attach a sheet.
Doug & Walt,
Brett’s concept suggests potential for aft boom flex to depower, but it is not required. The twist of the forward boom wire will also depower, but without increasing leach twist. So I think it is feasible to have an aft boom sheeting.
This could be done even with a fair amount of aft boom flex using the concept proposed by Brian in the “equilibrium rig” thread: http://www.rcsailing.net/forum1/showthread.php?p=40691#post40691
The main reason I chose a forward sheeting point was the convenience of not having to suspend the sheeting fairlead above the hatch opening
Today we had our weekly races. The competition is getting stiffer, as the Pepsi Torpedo won only 5 of 8 races. The losses were in stronger winds. It has become apparent that several deficiencies need to be corrected. When side-by-side with a more conventional hull, the other boat was going just a little faster in a strong wind. I think there are 2 reasons. The most obvious is that the Pepsi Torpedo doesn’t use the full allowable water line length of 12 inches, because of the natural shape of the double-ended Pepsi bottle hull. Secondly, it needs some extra weight in the stern to limit nose-diving, which is slow. Another problem was the weak sail servo (a micro-servo), which wasn’t strong enough to pull in the sail for a normal jibe in the strong wind.
The era of domination of the Pepsi Torpedo in our local races has come to a sad end. One of our group has purchased a V-12, which is obviously faster. The reasons are self-evident. It weighs about the same, but can carry more sail. It is built almost like a scow, with very full bows to prevent submarining. Today we had 2 Footy races before our regular Victoria races, which were both won convincingly by the V-12.
I am now looking at 1-gallon (or 4-liter) bottles, but all of them seem to have circular ridges, which make them unsuitable for hulls.