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[A 1978 SPORT 15 with 50-HP will be] even better! I use a 48--same 737cc--and it's plenty fast. They are easy to work on also. I did have a problem finding the right propeller. I had two OMC propellers: 12x15 and 11x17. The 15-inch pitch propeller revved too high and the 17-inch propeller lugged. No one offered a 16-inch pitch propeller. I ended up having a propeller shop add some cup to the blades of the 12x15 propeller and that put engine speed right where it belonged. Now I can have a person seated in the bow and the boat jumps right up on plane. I tend to fish alone but when I do take someone along that makes a huge difference. As great as the 15 footers are, they are still small boats. Weight placement is critical.

Regarding the prop choice [for a SPORT 15 with 50-HP]: The dealer here recommended a 14 pitch (I think it's a 10-1/4 diameter) for my 2000 Mercury 40 hp on my 1978 Sport 15. Perhaps the 50 hp motors use the larger diameter. My gps readings with 2 adults (total of 400+ lbs) aboard with 1 6-gallon full fuel tank were 34.5 mph with a bit of throttle left. I was definitely surprised the performance was that good. The motor is mounted properly with the cavitation plate a bit above the hull line.

Unfortunately initial prop choice is a guess at best. Once the boat is fitted out and loaded the motor will soon let you know of any shortcomings. What is most important is a quality tachometer. Then you will know if the motor is operating within it's recommended rpm range. With the exception of the SOLAS brand, most propellers are offered in two inch increments of pitch. Usually that is fine, but in my case, it was not. On my other boat, an 18 foot aluminum skiff with a 30hp Evinrude, the same thing occurred. I needed an in-between pitch and SOLAS offered one. It ran beautifully. But for my 48hp nothing was offered anywhere. Someone suggested getting the over-revving prop cupped and it worked.

I had a 1976 SPORT 15 with a Mercury 50-HP. Top speed was about 34-MPH.

Propellers for smaller engines are generally available in one-inch pitch increments. As I recall, I tried three propellers, in pitches of 13, 14, and 15, before settling on one.

Fortunately, at 50-HP you can use an aluminum propeller, and smaller aluminum propellers are not as expensive as the larger stainless steel propellers that sell for about $600

Initial propeller selection can be made with good accuracy by estimating the speed potential of the boat based on its power and weight. Crouch's method of predicting the speed of moderate planing hull boats has shown to be very reliable in predicting the speed of Boston Whaler boats. Use a hull factor of 180 for a conservative estimate of speed. See the topic in the ANNOUNCEMENT section of this forum at

http://continuouswave.com/forum/viewtopic.php?f=7&t=41

Once an estimate of top speed has been made with a reliable basis like Crouch's method, the propeller pitch can be estimated by using a propeller calculator. Use a factor of 10 for the SLIP. Use the upper one-third of the manufacturer's recommended full-throttle speed range as the engine speed. Propeller pitch estimated in this manner should be very good. Again, see the topic in the ANNOUNCEMENT section of this forum at

http://continuouswave.com/forum/viewtopic.php?f=7&t=41

Choosing an initial propeller based on a speed estimate with Crouch's method and a propeller calculator will give you a very good estimate for the pitch of the first propeller for your sea trial testing. I would not refer to this method of initial propeller choice as a guess. It is a very good estimate based on principle of hull design and physics. It will be much more accurate than a random guess. At best, your initial choice using this method will be the perfect propeller.

Propeller diameter is generally not a variable because propeller diameter is limited to the outboard engine's propeller aperture. For a 50-HP engine there will be a limit to the diameter. Propeller makers tend to make propellers of appropriate diameter and pitch for the horsepower range the propeller is intended. As a result, since you have little control or choice of propeller diameter, it won't be a factor in your estimates of propeller choice.

No evaluation of a propeller can be made without an engine speed measurement by an accurate tachometer.

While a top speed of 34-MPH may not sound like it would be very fast, because the SPORT 15 is such a small boat and has very low freeboard, the sensation of speed is increased. Cruising along at 34-MPH in a SPORT 15 provides a very satisfying feeling of speed.