Out Board Propeller Dynamics

Out Board Propeller Dynamics
In The fall of 2004 I purchased a boat and outboard. The company that built the boat also had the responsibility of mounting the out board. Unfortunately the company built the transom of the boat to high and this had a major effect on the boats performance. With the prop so close to the water?s surface the out board would ventilate ever time I tried to get the boat up on step. It would also ventilate on tight turns at high speeds. Unless I wanted to ship the boat back down to Seattle for modifications I would have to find a propeller that would operate effectively near the surface. The follow is some of the things I learned while solving this problem.
As the propeller rotates (fig 3-1) it forces water down and back as this is happening water must move into the void created by the spiraling blades. This creates a pressure differential across the blade- Low pressure on the back side and high pressure on the front side. This causes water to be sucked into the propeller and accelerated out the back (fig 3-3) much like a house-hold fan (fig 3-2). This action creates the thrust that drives a boat.

It is common knowledge that water boils at 100deg C (212deg F) at sea-level atmospheric pressures. Water will also boil at much lower temperatures if the pressure is reduced. This is the key to understanding cavitation. As an object move though the water at increasing speeds the fluid on the side and back of the object experiences a decrease in pressure. On a propeller this area of low pressure is on the back side of the leading edge.(see fig 3-1)
Pluse-

If this decrease in pressure is great enough it can cause the sea-water to boil (Vaporize). Once these bubbles of vaporized water travel down current to the front of the propeller they enter the high pressure side of the blade and are condensed back into liquid very rapidly. These small implosions of water vapor can cause cavitation burns on theblade and lower unit of the outboard. (see fig 4-21)
4-21

From a functional stand point these bubbles can compromise the outboards performance. As the water vapor travels from the back to the face of the blades they can decrease the water load on the prop (water contact surface with the propeller blades). This will cause the outboard to over rev and stall the boats forward motion. Cavitation can be caused by mechanical flaws with a propeller, improper prop design for a given boat type, it can also be induced by the out board ventilating.
Ventilation

Ventilation occurs when air from the surface or the exhaust system is sucked into the propeller. This often occurs during tight turns when the propeller is near the surface. It can also happen when the outboard is mounted to high on the transom. This can but the ant ventilation plate above the bottom of the haul making the plate ineffective at protecting the prop form air that is traveling down the haul.
Regardless of how the air is introduced its effect is the same. The air bubbles momentarily unload the propeller causing it to over rev. This also brings about a massive cavitation that further unloads the prop decreasing the amount of thrust the engine can provide. Once the ventilation induced cavitation subsides the propeller can again bight the water and resume normal operation.

When a given boat/outboard combo has a venting problem there are a number of ways to solve the problem. The most obvious is to mount the outboard lower in the water. When a propeller operates at increasing depths the environment it is rotating in is more stable and the prop is less likely to vent or cavitate. The anti-ventilation plate also becomes more effective at blocking air from the surface. This option can be undesirable because the outboard will cause more drag. Lowering the outboard will also increase the risk of hitting things like Rocks. The other option for solving the problem is to use a different prop design.

Stainless steal
One option is to but a higher quality stainless prop on. The blade of a stainless steal prop has a thinner cross sectional area then props of other materials. There for it disturbs less water as it rotates. Because the propeller its self creates less turbulence itis better suited for operating in unstable environments closer to the surface.

Rake
Rake is the degree to which the propeller blades sweep back (fig 4-5 and 4-6). The more aggressive sweep of the blades helps the propeller the grab and hold the water as it turns. This will not prevent ventilation or cavitation but it will make the propellers performance less likely to be effected by such events. The amount of rake a prop was is an option that can be chosen when purchasing a propeller.

4-6 4-5

Cupping
Cupping is when the trailing edge of the blade is formed with a curl in it. This curl or cup starts at the tip of the blade and extends a given length down the blades edge. This has the following effect on the propeller. As the propeller spins faster and faster water fills up the cup in the blade this ?wedge? of water is held in place by centripetal acceleration. This static water wedge affixed to the prop has two effects. First it increases the pitch of the prop. This is like adding a sixth gear to your car simply by driving fast. It will allow the out board to operate more aggressively at high speeds. The second effect cupping has on the prop is that it coats the load side of the propeller with water. When the prop gets into a venting or cavitating situation this layer of water prevents air bubbles from adhering to the propeller blades. This helps the propeller to maintain a constant water load which prevents over revving which could cause the boat to stall.

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