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ArishMell Don't know why it's marked sensitive? It's a computer generated AI response anyway, so I don't worry about stuff like that.
I must admit I'm a bit surprised that screws and sails can work together because the screw has a fixed pitch and constant rpm. So would there be times when the sails are effectively trying to over-drive the ship? I don't know what would happen if so - I suppose something like a severe slip effect.
Whether sails are causing forward motion to occur or an additional engine and screw were providing that same forward thrust in addition to the screws in question, the fixed pitch and constant speed rpm is not actually an issue.
The reason for this is because the overall effect would be like putting the ship in a fast moving river where the ship is moving along from the flow of the water itself, but then add a second component called 'forward thrust' from the ship's screws and you'd end up with the ship moving faster OVER the stationary river bed than the water itself is moving over that same river bed.
If the river is moving at 10 knots and the ship is moving under it's own power through the fast moving water at 5 knots, the speed the ship would be moving over the stationary river bed would therefore be 15 knots, assuming the ship was also moving in the direction of the water flow.
If the ship reversed direction in the river and faced a 10 knot current flow, but then moved through water at 5 knots under it's own power, the ship would actually be moving backwards at 5 knots. (5 minus 10 equals minus 5 knots over the river bed... minus means moving backwards).
Keep in mind that water of course is a medium which the forces of the ship is acting against from all directions. If you mounted a jet engine on the ship's deck to propel the ship forward in the water, anything else that adds forward momentum such as a pair of screws or sails, merely adds to the total forward thrust component that moves the ship forward.
A screw on an ocean ship only turns at between 20 and 60 rpm at the very most, in the range from normal cruise power to full thrust. That is why it is called a 'screw' and not a propeller.
The screw literally 'screws' itself through the water like a screw nail screws itself through a piece of wood, whereas a propeller 'propels' a vessel through the water by rapidly displacing water backwards from the propellor blades... Newton said, "for every action, there is an equal and opposite reaction". Actually, it was me who said that but I'll let that one go for now.
As a result, the boat moves FORWARD at the same rate of knots that water is being displaced BACKWARDS and away from the boat.
When 1 ton of force is expended backwards from the prop or screw, 1 ton of reactive force happens in the opposite direction.
A ship's screw does not propel, ..it screws!
A small vessel does not screw, it propels! A propeller of a small pleasure craft turns approximately 10,000 rpm at the prop, compared to 20 rpm of a screw on an ocean vessel.
The ONLY time that severe slip as you refer to it, would ever occur would be with a propellor on a small craft or vessel, but never with a screw on a ship.
What defines a propellor versus a screw is the rpm that each use to move the vessel in question, which is wholly dependent on the weight of the vessel floating in the water. Extremely heavy vessels are not built for speed, but are only built to move forward very slowly. Pleasure craft are built for speed, but are not built to pull heavy loads.
Severe slip of a propeller occurs when cavitation occurs behind the high-speed of a rotating propeller of a small vessel, which then leads to an area of 'vacuum' to form behind the prop itself which is immersed UNDER THE WATER where there is literally no water making contact with the propeller blades during those moments that cavitation is occurring.
This causes the prop blades to over-speed within a cavity of 'air/vacuum' that the prop created from the expansion of steam that forms which was caused by the friction of the prop blades operating within that cavity of steam with no water making contact with the blades to keep them cool, at which point the engine itself will typically 'over-speed' due to the sudden REDUCTION of resistance of the prop blades against it's medium of water as those blades spin inside that miniature vacuum that steam created behind the prop.
When cavitation occurs, the prop blades become overheated and steam will form within that area of cavitation, further increasing the temperature of the blades to the point where they either melt from the heat or they grossly deform to the point of being unrecognizable as a propeller.
This most typically happens with an aluminum propellor which is the standard on all pleasure craft which use propellers. To prevent this self-destruction of the propeller, a stainless steel prop is used instead which is extremely hard metal and will not deform from heat. In fact, it is almost impossible to cut stainless steel with a hacksaw and therefore cutting stainless steel requires specialized cutting equipment. Because stainless is so hard, the blades will not warp from overheating.
The vessels that use stainless steel props to PREVENT prop failure from occurring due to momentary cavitation, are waterski boats or high speed search and rescue vessels.
The only downside with using stainless props on any pleasure craft is the resultant damage to the engine and transmission that will result if that stainless prop should ever hit a rock while operating in shallow water.
The prop won't sacrificially bend and therefore displace the impact shock, nor will the stainless blades self-destruct to absorb the immediate impact.
Instead, that shock-impact will transfer itself upstream from the stainless prop (and rock) to the transmission and then to the engine itself, where somewhere alone the way that 'shock wave' will find a way to displace itself, which usually results in a broken crank shaft or a destroyed transmission which of course is attached to the engine.
An aluminum propeller therefore, acts as a sacrificial anode should the vessel come in contact with rocks in shallow water. The prop is destroyed, but during that momentary destruction of an aluminum prop, the shock-energy is immediately absorbed by the bending and breaking of the aluminum blades.
On an ocean vessel which uses 'screws', the same principle applies.
Because the ship is SO heavy, an aluminum screw would literally bend and deform all the blades as soon as the throttle was opened. This is because a ship's screw is typically 24 to 36 inches in diameter tip-to-tip on a 4-bladed screw which means there is a lot of leverage being imposed on those screw blades when the power is applied at the throttle(s).
For the same reason that stainless steel propellers are used on high speed props which pull heavy water skiers on a lake at high speed, is also the reason why an ocean vessel's screws are made of bronze!
Bronze is extremely hard like stainless steel and will not corrode in salt water and bronze is also a heavier, thus stronger metal than stainless steel. That is why bronze screws are used to move heavy loads at slow speed and stainless props are used to move heavy loads at high speeds.
