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Posts: 152
| I have a '91 Tuffy Stinger with a '89 Mariner 40 hp w/14P prop. I believe the boat weighs around 900 lbs on its own, plus engine, fuel, musky gear, etc., my guess on the weight of the boat, engine & gear is around 1,200 lbs. Normally by myself I could get 24.5 mph out of it and with another 2 guys I could get 18.5/19.5 mph out of it.
This weekend several times I had 2 other guys in the boat (one of which is 350, so two guys essentially) and the boat could not even got on plane and plowed through the water at 12 mph. Switched props to a 12P and still could not get on plane at 14 mph. By myself, I could now get 26.5 mph out of it.
I am sure the boat was over the coast guard recommendations on extra weight, but I would still think that the motor would be enough to get it up and out of the water. The motor did not seem to be able to turn enough rpms or rev up all the way (which I understand is in part due to a 12P being harder to turn). Do you think think the motor is getting worn out (certain parts to be replaced?) or just a weight issue?
If money was no issue, I would have put a 50 hp 4S on it a while ago...but, like most, it is.
Thanks guys! |
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Posts: 532
| First 12P prop is easier to turn than a 14P.
You had to much weight for the motor to plane out, simple as that. Nothing is wrong with your motor (assuming it ran like normal after you went back to a light or normal load)
Everything has limits, while some people think a 40hp is a real strong motor, other do not. You now know that when loaded with that much weight you will have perfromance issues.
I have to ask did you try to adjust the weight so the boat was loaded correctly? |
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Posts: 152
| I always figured with the longer blades on the 12P, it would be harder to turn than the 14P. I think it was like 10 5/8" for the 12 vs 10 1/4" or something similar. Thanks for the heads up though. I just figured the motor should be able to push the boat better than that.
As far as having the boat "loaded correctly," if I understand what you are asking, you are referring to where I tried to shift weight to get it on plane or perform the best? If so, I tried getting the other guys towards the front/front middle of the boat to disperse the weight. It didn't have a chance with them sitting in the middle. |
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Posts: 3514
Location: Elk River, Minnesota | Hiya,
Sounds like your motor was over pitched to begin with using the 14 pitch as when you switched to the 12 you gained speed. My guess is your motor has a gear ratio of 1.85:1, so the 12 is probably good for just you, but would still be too much pitch for essentially 4 guys in the boat (with the 350 lb man).
Nothing is wrong with your motor...you just need a lower pitched prop for the boat when you load down. When you pitch up or pitch down in a prop, each inch of pitch is roughly worth 200 RPM. When you change diameter, each 1/4" is roughly worth 100 RPM. so...when you went down to a 12 pitch from your 14 (+400rpm) and went up in diameter 1/4" (-100rpm), you essentially gained 300 RPM overall. So...you gained some performance overall.
My suggestion: Get a 10 pitch propeller and see how that works. With the amount of weight you are trying to push, I would not be surprised if you are unable to get the boat to plane out... You are in a situation where I believe you are reaching the limits of the rig, and nothing but either a) a larger motor, or b) replace the entire rig are your options to gain performance with the kind of weight you are carrying.
Steve |
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Posts: 152
| Thanks Steve. I appreciate the info in the RPMs. I usually only have one other guy in the boat, but after playing around with Mercury's prop calculator, and listening to you guys, I think I should be getting a 10/10.5 P prop as you recommended.
I have been looking for 2000 or later newer 2 strokes or 4 stroke 50hp for the right price, some day maybe.
I will work on finding a 10P and see what my results are from there. Although a new prop might cost an add'l $130, it is still cheaper than having to buy a whole new engine. For now, I will most likely be grinding it out with the ol' 40hp mariner.
Thanks again everyone. Your opinions were reassuring.
http://www.mercurymarine.com/propellers/prop-selector/# |
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Posts: 32958
Location: Rhinelander, Wisconsin | Try an 11 pitch prop, and see if there's a major improvement. if not a 4 blade 12 stainless might be of some help. The Stinger is built on the hull now listed as an Esox LTD, and runs OK with a 40, but loves a 50 for heavier loads.
http://www.mercurymarine.com/propellers/elite/trophy-sport/ |
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Posts: 152
| Thanks Steve. So are you suggesting that a 10/10.5P maybe too extreme? I wish the 50 was an option, but I just graduated college and don't start at my new job until late September and most of those payments will most likely be getting appropriated to a wedding of mine and mortgage payments.
Also, commenting on the 4 blade, I didn't even consider it originally for some reason. I would think the increased "oompf" of 4 blades would help it plane out better at lower speeds. Do you think the 4-blade prop will sacrifice too much speed though? Obviously it will not be a speed demon if I have the boat loaded down like that though. I just want to try to narrow down my prop search before I start shelling out cash testing different props. I have been lucky in the past being able to find quite a few used ones at marinas though for about 1/2 the price.
I have big dreams of getting a newer Tuffy someday. |
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Posts: 32958
Location: Rhinelander, Wisconsin | You certainly can try a 10, but may find your RPM to be too high when running alone. |
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Location: Milwaukee | Ok, now I gotta ask, since the experts are listening.
I have an '87 Esox LTD, with a '79 Johnson 50, with a 13 1/4 X 17 prop. I also top out at 24 or 25 mph when alone in the boat. With a guest I lose a couple mph. I really feel it should be able to go faster. Sorry, no rpm gauge on this old motor.
Any suggestions regarding diameter or pitch?
Thanks for your advice!
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Posts: 3514
Location: Elk River, Minnesota | Hiya,
I'm guessing you might need to look at a 13-1/4 x 15. If memory serves me correctly, that motor has a gear case that is 2:1. That gear ratio will get that boat up and out of the water better than the aforementioned mariner, which I'm guessing has a 1.85:1 ratio but it will not be a speed demon by any means either. I had an alumacraft lunker which was about 100 pounds lighter overall than your tuffy (it weighs in at 750 pounds). I was able to run a 17 pitch prop on my johnson 50hp, but it had a 2.42:1 ratio, which allowed me to turn a higher pitch prop...only because the ratio has a higher first number, where the engine rpm turns a higher amount as compared to the prop.
When the pieces are designed and put together, they all have a limiting factor both for low end torque and top end performance. In the case of these older, heavier rigs, I doubt you will see any increase in speed, but you may be able to get it out of the hole better.
Without a tachometer, it will be hard to determine what pitch you really need, as getting the motor to the upper 1/3 of its rpm range is the ideal situation....even better if you can get it to reach the top rpm in the range. That motor should bring in 5500 rpms at full throttle, so under a light load (you and your gear), that would be what you would want to shoot for. Without a tach, that will be hard to do.
I still think with as heavy as that boat is, a 15 pitch might suit it better....but getting higher than your already posted speed will be a challenge. It may be possible just by getting the rpms in a higher range, but I wouldn't hold my breath.. Definitely worth trying, though...
Steve
Edited by VMS 6/29/2013 10:28 AM
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Location: Milwaukee | Thanks!!
I hope you realize how much your and Sworral's insights are to us on the forum. It is hard to trust some dealers and marine store employees, they are just trying to push product. Your information has become a trusted source. And you speak in a language that we don't need to be a outboard tech to understand! |
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Posts: 152
| I ditto what Dewman said above! Good to get some honest help, without a tailing sales pitch from a marine dealership. |
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