Power Vs. speed question

What kind of speed should I expect from my combination?<br />I have an "adventure" 9 foot speedboat. (shallow V) Like the Addictor, 245, but a foot longer.<br />It came with a Tohatsu 40, and would do about 31 mph.<br />I replaced that with a Johnson 75 stinger short shaft. Hydrofoil, no trim tabs. Motor as high as possible w/o ventilating. Tach is new, speed by gps.<br />The 12.25 X 13 it came with would turn 6200 rpm at 39 mph. <br />I put on a 12.25 X 15 and now the best so far has been 43 mph at 5400 rpm. <br />I've looked at the list of props for this motor, and it seems most people run more than 15" pitch, even on the 50hp motors. <br />I've also read of higher speeds from larger boats with the same power. <br />Seems odd that anyone could run higher pitch with this motor, or go much faster, since my boat is only about 300lbs w/o motor, gets on plane at 8~9mph, and trims nicely at speed. <br />I just wonder if Im in the ballpark speed-wise for what Im running. What do folks get with 75hp on "normal" length boats? Better than low 40's?<br /><br />Thanks, <br />-Dan

Re: Power Vs. speed question

I have a 1982 75hp johnny on a 16' fiberglass flat. It has a OMC 13x19 prop and has gps at 38 mph @ 5800 rpms. I believe the gear ratio is 2.33 to 1 (large gearcase). That would give me about 15% prop slip, which is about average. Hope this helps.

Re: Power Vs. speed question

I think you are having problems there<br />I have a 17' starcraft and run 38mph on the gps with a merc 75hp and a 18 pitch prop.<br />Try taking off the hydrofoil you don't need it on a boat that small with that much power.

Re: Power Vs. speed question

I agreee with bluewater19. I have a 15' homebuilt skiff and is fairly light for its size. I have an 89 Johnson 50 on it with an 11 1/2" X 19 SS prop & get get 40 mph out of it at 5700.

Re: Power Vs. speed question

There was a few years that the gearcase of the Stingers was smaller than the regular 60-75Hp models. Those used 40-55Hp props and that would account for the pitch differential. A 15p prop for the smaller 40-55Hp and certain 75 Stingers is quite steep and wouldn't work well on most boats.

Re: Power Vs. speed question

Originally posted by bluewater19:<br /> I think you are having problems there<br />I have a 17' starcraft and run 38mph on the gps with a merc 75hp and a 18 pitch prop.<br />Try taking off the hydrofoil you don't need it on a boat that small with that much power.

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The boat will not plane worth a damn without a foil, or trim tabs, or trimming the motor in, unless I put 30lbs of lead in the bow.<br />I believe it's because it's so short, it doesn't have a long heavy bow to pull the nose down. Trying to plane without the foil is like pushing a sheet of plywood through the water the hard way.<br />Thanks, though.<br />-Dan

Re: Power Vs. speed question

Speed is much more than power alone. Your biggest problem is lack of planing surface. On a 9-foot boat you don't have much lift working for you to keep the rig out of the water. As such you likely have a great deal of drag. You're likely and literally digging a hole in the water with too much power for the hull to handle. Seems counterintuitive, I know, but that's what happens. You have hit, and been turned back by, the "too much power" wall. I've seen it by folks who have put V4s on Glasspar G3s and 3-bangers on GW Invaders. Without proper set-up and addressing the hull issues you'll only get diminishing returns...<br /><br />As a reference: My 15-foot starcraft with a 75 Stinger does 41 on the GPS swinging a 17-inch pitch aluminum OMC prop. <br /><br />- Scott

Re: Power Vs. speed question

Originally posted by WillyBWright:<br /> There was a few years that the gearcase of the Stingers was smaller than the regular 60-75Hp models. Those used 40-55Hp props and that would account for the pitch differential. A 15p prop for the smaller 40-55Hp and certain 75 Stingers is quite steep and wouldn't work well on most boats.

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Im sure this is the case. A 13" prop will almost touch the cavitation plate. <br />I was thinking of going to a 11.75 X 17, or down in diameter and up in pitch. <br />I tried a composite prop for the 75, and it was way too big at 13 X 16 minimum pitch. I looked at the composite props for the 50 size motors, but the hub diameter is too small for my gearcase<br />What else can you tell me about this motor, can you recommend a prop strategy?<br /><br />-Dan

Re: Power Vs. speed question

Originally posted by Chinewalker:<br /> Speed is much more than power alone. Your biggest problem is lack of planing surface. On a 9-foot boat you don't have much lift working for you to keep the rig out of the water. As such you likely have a great deal of drag. My 15-foot starcraft with a 75 Stinger does 41 on the GPS swinging a 17-inch pitch prop. <br /><br />You have hit, and been turned back by, the "too much power" wall. I've seen it by folks who have put V4s on Glasspar G3s and 3-bangers on GW Invaders. Without proper set-up and addressing the hull issues you'll only get diminishing returns...<br /><br />- Scott

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I understand what you're saying, the short waterline is the reason it's so hard to plane without the foil, but it seemed to me that at speed Im riding on the last few feet of hull, just like any other boat. Why does length matter when 2/3 of the boat is in the air?<br />Thanks,-Dan

Re: Power Vs. speed question

Originally posted by WillyBWright:<br /> There was a few years that the gearcase of the Stingers was smaller than the regular 60-75Hp models. Those used 40-55Hp props and that would account for the pitch differential. A 15p prop for the smaller 40-55Hp and certain 75 Stingers is quite steep and wouldn't work well on most boats.

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Do you know the gear ratio for the small gearcase?<br />Thanks,<br />Dan

Re: Power Vs. speed question

The small gearcase was only found on the short-shaft models (15-inch mid). Gear ratio was 15:28.<br /><br />Boat hulls have a fair amount of lift built into them, whether it be hydrodynamic or aerodynamic. That lift is what allows the boat to ride on it's last patch of hull area. Without the hull dynamics ot achieve this the motor uses more of its power on lift and less on propelling it forward...<br /><br />- Scott

Re: Power Vs. speed question

I believe that would be a ratio of 1.87 to 1 (small gearcase), compared to the 2.33 to 1 on the large gearcase.

Re: Power Vs. speed question

Originally posted by Chinewalker:<br />[QB] The small gearcase was only found on the short-shaft models (15-inch mid). Gear ratio was 15:28.<br /><br />Thanks. I do have a 15" shaft.<br />This explains why the recommended props for this motor all seem too large. Propshaft spins faster on this motor than the typical 75.<br /><br />-Dan

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