Re: 4 blade prop vs larger diameter 3 blade prop with same pitch.
Rs2k, one of the reasons is that airplanes with props aren?t continually being pushed around like a boat is in the water. When you try to compare water density to air density there is no contest. In the air a plane that uses a prop is not continually being shoved around like a toy duck in the bathtub that your child is playing with. Most propeller driven airplanes have adjustable pitch props, we don?t have those on most smaller boats. When you are being pushed back by a large head sea the most important aspect of a propeller can be the ability to go slower and have enough blade surface area to resist as much of the push from the waves as possible, and a 4 bladed prop usually has more blade surface area than a comparable 3 bladed prop so it has more ability to handle this.The same is true when you are trying to stay on the back of a wave in a following sea so you don?t have the sea catch up to you and hit the transom and make the boat broach to the right or left. The more blade surface area the better chance you have of having enough thrust available from the prop to prevent these events from happening, especially in aluminum propellers, which normally do not have a lot of blade geometry.
Theoretically speaking (if you don?t use the 1 blade theorem) the most efficent prop has only two blades with a very large diameter turning very slow. But we don?t use two blade propellers on many boats because of the diameter that would be required to push the load it is attached to. That is why 3 bladed propellers became so popular, as boats increased in size and weight and hosepower, more blade surface area was needed to handle the extra thrust required for these vessels. The load on a propeller is directly proportional to the horsepower of the motor, the hull form of the vessel, the weight of the vessel, the speed it is traveling and the seas it is negotiating. Most everybody here talks about aluminum blade flex that is so bad, but when you have 4 blades you have more surface are to divide that thrust by, so there is less loading on any portion of the blade because of the fourth blade. There so many other reasons that can be discussed for why 4 bladed props have their uses that this could go on for a long time. None of this discussion takes into account blade geometry, which is a discussion all by itself and not suitable for the differences between 3 blade and 4 blade props.In short a good four blade propeller can go as fast as a 3 blade prop, can plane at a slower speed, can increase mid range speeds and can decrease prop slip because of the extra blade surface area it has. They are not a panacea for all boats in all waters, but they definitely have their place in larger boats that can and do encounter larger wind driven seas at times.
H
