Ok, the obvious answer is - well, I don’t know. You tell me
Let’s assume that tyres could be something like revolving sides of the ship, or anything revolving that’s in contact with water.
you’re kind of describing a paddlewheel boat.
If you imagine this on the road, you can see the difference(friction of the bottom).
Something like belt driven engine, but the belt would be used as transmission between the surface and the ship.
I don’t see this working. I don’t think you’d be able to get enough friction going between the boat and the tire to overcome the resistance of the water. So the tires would be spinning but the boat wouldn’t be moving forward. It’s pretty much what happens when you try to drive a car on ice or mud.
What if sides were only revolving, normally propelled? Magnetic bearings
Snowmobiles do this to go across open water on lakes but they get a running start on the ice. They also need to keep the speed up to get across. Again they aren’t designed for it from the start. Getting the boat up to speed to get the top of the belt out of the water would be an obstacle. Paddle wheel is about the best description for this idea.
Ships are already a heck of a lot more efficient than land vehicles. It’s probably not worth it to do much of anything that makes the ship more complicated and expensive, just to cut fuel costs down a little bit further.
It sounds like you want the wetted surface of the ship to have zero speed relative to the ambient water, presumably in an attempt to eliminate the skin friction drag due to the viscous nature of the water. However, I’m pretty sure skin drag is minor compared to form drag, which is caused by the need to exert force to push water out of the way at the front of the boat (and water being reluctant to fill the void left behind the boat, which creates a vacuum that tends to decelerate the boat). Form drag won’t be affected by your proposed mechanism.
Your system would also somehow need to eliminate water from the shipward side of the tires or belts, or the skin drag will simply manifest there instead.
Is that true? I would think the difficulty of moving through water (relative to air) and the inefficiency of propellers would work against ships. How do they compare to freight trains in terms of ton*miles/gallon (or whatever metric is usually used for such things)?
This I was already aware of, but I just could not remember (associate) before starting this thread. I guess this settles the case, but I’m looking foward to all the ideas risen from this pondering.
Frictional resistance varies from around 1/2 the total in fast merchant ships to 85% or more for very big and slow ones. The traditional term for non-frictional resistance is ‘residuary resistance’ which includes ‘form’ or eddy resistance and wave making resistance. Those are somewhat different things but have been traditionally been grouped together because subscale models in towing tanks can correctly model both but not distinguish between them, and OTOH if residuary resistance is being modeled correctly frictional resistance isn’t (they don’t scale the same way, so the model’s frictional resistance is subtracted, its residuary resistance scaled up to ship size, and ship’s theoretical frictional resistance added to get total estimated resistance for the full size ship).
For actual boats as opposed to ships, wave making resistance can dominate to a greater degree.
Although back to ships, even though high speed ships typically have a smaller % of their resistance coming from skin friction, it’s typically still a larger amount per tonne-mile of cargo than for big slow ships where frictional resistance dominates %-wise. So innovations to cut frictional resistance (like air lubrication systems, etc) are often seen applied to ship’s where frictional resistance is a relatively low % of the total.
A bunch of spinning devices covering the whole wetted surface of the ship doesn’t sound too practical at first thought though.
Way back when I was reading a magazine on air lubrication systems but there was a cost associated to operating the compressors. A company then put wings on the test vehicle to basically lift the boat partly out of the water at it’s intended speed and they achieved the same reduction in “resistance” but with significantly less cost.
However, this was a small boat, not even a ship.
That sounds butt-clenchingly terrifying.
Probably not as terrifying as Jamie's Aqua Bike | MythBusters - YouTube
Air lubrication systems have been applied to ships in recent years. An example is Mitsubishi’s MALS system. The article is about a 145m long 21 kt ferry, IOW a fairly fast ship in terms of speed/square root of length, so where a fairly high % of resistance is from wave making rather than skin friction. But the system still results in a claimed 5% net reduction in fuel consumption, which in the real world is significant.
http://worldmaritimenews.com/archives/113949/japan-mitsubishi-heavy-installs-innovative-mals-on-ferry-naminoue/
:smack: Ignorance fought, thanks. I’m used to dealing with smaller/faster vehicles where form drag dominates; I should have done some checking before assuming that the same applied to container ships.
It’s not as nuts as it sounds. People regularly have water-crossing contests, and there’s a world-record for longest distance covered on water by a snowmobile (212 km). Lotsa videos here. Note from this particular video that you don’t even need much of a running start (about ten feet of beach sand), provided you’ve got plenty of power and traction.
So you are basically asking if a ship could go faster with a treadmill strapped to its hull.
Can we infer anything from the behavior of an aircraft in a similar configuration?