Out Board Propeller Dynamics :: physics boat propeller

Missing Figures 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).