Re: no bottom end, hard to plane, good top speed
Hope your mathmatically apt, cause here ya'are, have fun.<br /><br />Maximum Boat Speed for a Propeller<br /><br />As a propeller turns, it's pitch determines how far it travels through the water. No propeller is 100% efficient so that in reality it "slips" by moving water aside rather than straight backward. For a planing hull shape, this propeller slip can be as low as 10%. For a displacement hull shape with a high speed propeller, this slip is usually around 45%.<br /><br />This means that the maximum vessel speed is a function of the shaft speed in revolutions per minute, the pitch of the propeller, and the amount of propeller slip.<br /><br />The Honda outboard motor described above will generate 5 HP at the engine shaft at 5,000 rpm which results in 2381 rpm at the propeller. Assuming no propeller slip, the maximum boat speed will be:<br /><br />Boat Speed = (RPM * 60 * Pitch) / 72913.2 = (2381* 60 * 7.5) / 72913.2 = 14.7 Knots<br /><br />If the propeller slip is 45% then the propeller's forward motion is 55% utilized so the maximum speed for this combination would be:<br /><br />14.7 * 0.55 = 8.1 knots<br /><br />However, since the maximum boat speed is 5.4 knots, the propeller will function with an inefficient 63% slip. As this shows, the pitch of the propeller must be decreased to between 5 and 5.5 inches to achieve a more normal propeller slip of 45%. In this case, the propeller diameter must also be increased in order to prevent the motor from speeding up beyond 5,000 rpm. As a general rule, if you purchase a typical outboard motor for use on a sailboat, you should instruct the dealer to provide the lowest pitch and largest diameter propeller possible consistent with the motor's designed operating speed and horsepower. The "standard" propellers provided with these motors will just not be efficient with a small sailboat. Either you will be running at 1/2 throttle and therefore never use all the available horsepower, or you will be running at full throttle and wasting the energy in propeller slip.<br /><br />Thoery for a small outboard on a sailboat, but the math is still the same.
