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Posts: 3
| My son in law just bought a 2000 21 foot sylvan pro select with a Johnson 150hp motor. Motor runs great, very responsive. The prop is a 3 blade aluminum 15X17p. This boat has an incredible hole shot but WOT with boat trimmed up is only 5000 rpm and according to GPS maxed out at 36 miles an hour. Am I wrong or should there be more top end. I have some prop knowledge and understand most of it. Any thought | |
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Posts: 202
Location: Angola, IN | I think 5k rpm's is plenty. The max may be 5500 or 6000 for this motor. Not sure. You could find out easily I'm sure on on iboats.com
So a 15 pitch should theoretically get you to 5350 rpm's and approx. 38 mph according to my calculations. How are you measuring RPM's? OEM dials are notoriously inaccurate. You might already be maxed out, or you could only be at 4k at WOT.
A good gauge directly linked to your ignition is worth checking out before you over rev your motor. | |
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Posts: 3
| Anybody have any insight in this. I double checked my tach with another one and the tach read the same rpm for both. | |
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Posts: 3511
Location: Elk River, Minnesota | Hiya,
I would bet the motor has a max of 5500 rpms, which I would say if you are lightly loaded (just you and a light load), you are over propped by about 2 inches. What this would do is give you a bit more "umph" throughout the RPM range if you punch it, but where you will notice it the most is in holeshot. Your is right on the money with about 15% slip in the prop. If you drop to a 15 pitch, you would lose a little on top end, but in rough conditions, the ability to get the boat moving can be paramount, especially if you are in big waves and needing to travel with following seas. The last thing you would want is to have a motor that is slow to accelerate with a breaker coming over the stern as you try to climb the next wave...that is a bad, bad situation...
Now...if you are loaded fully with a guest, his/her gear, a full livewell and gas tank getting 5000 rpms, you are probably good to go with the prop you currently have on it. With the gear ratio that motor has at 1.86:1, a lower pitched prop is normally needed since the spin of the prop is not all that much slower than the spin of the motor, thus a higher rpm is needed to turn the prop efficiently.
Am I correct in assuming you are running an aluminum prop?
Steve
Edited by VMS 9/13/2012 8:59 PM
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Posts: 3
| @ VMS, I am running an aluminum prop. I was told over the weekend at a boat ramp that the prop he is running is the same type that would go on a pontoon or deck boat. The blades look like large ears. Do you think a 4 blade stainless would help. He would really like to get more speed out of this. This boat has a hole shot that is just incredible. I thought my Ranger 690 with a 175 was fast out of the hole. He buried me in a race out the hole. Do you think 36 mph is slow for that boat? | |
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Posts: 3511
Location: Elk River, Minnesota | Hiya,
If you go 4 blade, you probably will not see any increase in speed due to the extra blade in the water. That extra blade would require you to lower the pitch further than just a couple of inches in order to turn it.
A good stainless will help some. The move to stainless will give you a performance edge in that the blades will not flex anywhere near as much as aluminum, and with the various designs that can be cast and forged into a steel prop, you can really dial in the rig.
I do have a thought, though, if the holeshot is really good, I would see about having the tach tested to ensure it is accurate. When holeshot is extremely quick, that is a great feature, but it also usually means the top end RPM is too high. If the tach is off and the boat runs full speed, it could greatly shorten the life of the motor. I do know there are many 2 strokes out there that have rev limiters on them, although I don't know if that particular motor has one on it.
To be safe, I would have him check the accuracy of the tach, and if it is off, get it adjusted and do a water test. A good baseline will be huge and if that tach is off, it could affect your decision on what to try..
Steve | |
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prop question