5 ms·
"The general hope was that the roller ship would slash travel times by virtue of its low-drag design. Sadly, an attempt to cross the English Channel in 1897 re
by mojomark 3y ago
"The general hope was that the roller ship would slash travel times by virtue of its low-drag design. Sadly, an attempt to cross the English Channel in 1897 revealed that the roller design had one unforeseen drawback. In the water, the rollers tended to drag a great deal of water up with them as they rotated."
Super fun. I'm a marine engineer and design vessels. I've seen a lot of whacky designs over the years and never encountered this one!
Apparently, from the above quote, the culprit for increased drag was skin friction (in this case, that meant water clinging to the surface of the rollers and being lifted out of the water - kind of like the reverse effect of a Tesla turbine which exploits skin friction between moving air and a stack of rigid discs on a rotor). That would certainly cause propulsive inefficiencies.
However, from my view, the primary source of drag here are the "hollows" between the rotor discs. If you were to look at a plan (top) view of the vessel as it moves through the water you would find significant water pressure gradients between (at the aft and fore end of) each wheel. When you have multiple wheels in series, you have oscilating high and low pressure zones along the length of the waterline. That's a tremendous amount of energy lost to water turbulance.
When you look plan views of hydrodynamic hullforms at the waterline (whether monohull, or multihulls like catamaran/trimarin/SWATH) or even aircraft foil shapes, you don't see "wavy" form lines for this very reason.
I will say that these inventers were encroaching on the SWATH hull shape. The author states that this roller disc hull would not be stable due to the high CG. However, SWATH hulls are known for their stability in high sea states.
- LorenPechtel 3y agoSo many things are obvious once pointed out. I never would have thought of pressure gradients but once you did it's obviously a very uneven surface and you don't do that in anything contending with a lot of drag. How do you see this as a SWATH? This is a catamaran that combines buoyancy and propulsion. I do agree it has a high center of gravity, but that's a relative term--what counts is the ratio of height to width. And a catamaran or SWATH has a much wider base than a typical ship. Wikipedia says SWATH ships require complex control systems--why? Yes, they're very load sensitive because the small contact at the surface means a lot of vertical movement to compensate for weight changes, but couldn't that simply be handled with some pumps and tanks? Make the ship always weigh X--take on water or pump it out as needed to maintain that X.
- usrusr 3y agoI'm not a marine anything, but I'd assume that the control system Wikipedia mentions are ballast tanks that need to reflect any change in load, load distribution and even wind load I'd guess. Sinking a little deeper won't do much to compensate increased load because the small surface area means little change in displacement per change in vertical position. Active trimming pumps.
- LorenPechtel 3y agoYes, but how is that all that complex by modern standards? Dealing with known weight, simply pump the right number of gallons. If your ship can change weight rapidly you might need some fairly big pumps. (I'm thinking of a warship--VLS equipped ships can throw an awful lot of weight into the sky awfully quickly. But on the flip side you can dispense with a whole bunch of stuff meant to deal with the missile exhaust--your VLS tubes go all the way through the ship, when the missile lights it simply blows a panel open on the bottom of the cell and the exhaust slams into the ocean. And it's a safety advantage to be able to jettison a missile without having to launch it or rig a crane.)
- usrusr 3y agoMore complex than a hull shape that is perfectly fine with just sinking in in a little deeper when load is added or redistributed. The control system you don't have or need is inherently less complex than the control system you do need. It's not exactly a problem impossible to solve, SWATH have already been operated half a century ago, but it seems to be rarely worth the effort.
- Someone 3y agoI don’t understand why they would think this to have low drag. - Displaced volume wouldn’t be lower than that of a traditional ship (likely even quite a bit higher because the wheels would add mass and the hull would have to be stronger than one that rests on the water) - Surface area would be higher than that of a single hull. Also, looking at this as a variant on the paddle steamer (https://en.wikipedia.org/wiki/Paddle_steamer#Seagoing_paddle_steamers https://en.wikipedia.org/wiki/Paddle_steamer#Seagoing_paddle...), didn’t we already know in 1897 that screw propellers are a lot more efficient? SS Archimedes (https://en.wikipedia.org/wiki/SS_Archimedes https://en.wikipedia.org/wiki/SS_Archimedes) was built in 1839. So, what was the thinking? It being easier to have low rpm in steam driven engines, and that compensating for those disadvantages?
- eternauta3k 3y agoPart of the drag scales with the relative velocity between the surface of the wheel and the water. If they move with ~0 relative speed, drag is reduced.
- bluGill 3y agoPaddels can he just as efficent as screws but only with mechanical comylexity which makes them unreliable.
- s1artibartfast 3y agoWheels add mass and cross sectional drag to my car, but rolling more than makes us for it. I assume the thought was similar. Some of the energy is lost to friction, some is lost to the motion of water. Imagine a ship where the hull sufface has the same relative velocity as the water. It would behave the same as a frictionless ship.
- usrusr 3y agoIsn't it rather obvious that they thought friction was the main (or the sole!) component of drag? If displacement waves didn't exist, or did not cause meaningful drag, floaty wheels might even have taken over already in the age of sail.
- lttlrck 3y ago
- 082349872349872 3y agoFor the high seas I had thought a design criterion was self-righting; is that only true for smaller vessels?
- s1artibartfast 3y agoI don't even think it is true for small vessels. Catamarans and outriggers are absolutely not self righting.
- adonovan 3y ago> the culprit for increased drag was skin friction You raise the question of whether Teflon-coated rotors might have improved the efficiency... but isn't the skin friction also the source of propulsion? Would eliminating the tendency to "drag a great deal of water up" also eliminate the tendency to make headway?
- rokkitmensch 3y agoDevil's bargain. The friction losses you save in dragging water up are more than compensated for by the loss in propulsive efficiency.