![]() ![]() Even with this diameter a 4-bladed prop is better than 2-bladed. At full throttle and speed of 11.7kts a 16" prop would have an efficiency under 60%.Īssuming the boat requires 10kW (13HP) to do 7.5kts you need to go up to a prop 3ft diameter and turn it at 400rpm to get above 80% efficiency. I would need to spend time to model the hull to determine the best prop but I doubt that you would have the room to swing it. On the other hand 11.7kts is a good speed for 70HP in that boat. At full throttle your prop is heavily loaded and efficiency has dropped off. Most boat props are a compromise to reduce draft so blades are heavily loaded. So if the prop is heavily loaded, it is better to have more blades. If the blades are not acting on enough area they will operate at a high angle of attack resulting in high prop slip. However you cannot just fit a a two bladed prop with narrow blades and get a better result. AR is blade area divided by maximum chord. Prop - The prop blades become more efficient as their aspect ratio increases. The benefit will not be much - unlikely to measure any increase in top speed. SO the thick rudder is ONLY BETTER if you reduce the area. If you reduce the length of the rudder, keep it at the same draft and make it thicker you can dramatically reduce the area and still achieve the same steering force. The best rudder has a thickness ratio of around 20%. Click to expand.Rudder - a flat plate is not a very good rudder. ![]()
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