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Posts: 633
Location: Madison, WI | I have a 2007 Merc 115 4-stroke on my Tuffy 1760. The current prop I have on it is a solas 13.75 x 15 stainless. My top speed is 38-40 and the hole shot is a bit slow with a heavy load. However my biggest concern is the rpm at WOT, it only runs 5000-5100 depending on trim? My motor should be at 6000 or better so I figure I'm not propped correctly. I would like to stay with stainless so I cannot afford to buy a bunch of props to try out. Anybody have this motor? If so what size prop should I try for best performance? I don't need anymore speed but would like to improve hole shot and definitely get my rpm's up?
Thanks - Dan |
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Posts: 3508
Location: Elk River, Minnesota | Hi Dan,
Without seeing anything else, I would say you could stay with the same pitch prop, but drop the diameter down to 13-1/4" which should help hole shot a touch, but increase your speed a bit. What physically is going on is you are reducing the prop's surface area, which means less friction. With a stainless prop, you won't lose as much bite on the water either. What the smaller diameter should do for you is increase your overall RPM's by roughly 200 if you went with the same brand and model of prop.
If you are unable to find props with this diameter in your hub size (I'm guessing a 4-1/2" hub so you should be able to find them as it is a pretty standard size for mid-size outboards...) then a drop in pitch to a 13 would be in order...
BUT...before doing any of this, how is your boat normally loaded when you are running the 5100 or so? How are you trimmed...as far up as you can go before either porpoising or losing grip, or are you below that point?. If you are not trimming up much, that will be a huge issue as you want to trim up as far as you can go before porpoising or the prop starts losing grip or doesn't gain any more speed (you'll know because you should be able to both feel and hear how the engine responds. In most cases, when you trim up, you should see a substantial increase in RPM.
If you are heavily loaded with the numbers you have given, I would first start by setting up the boat if you were fishing alone with just your gear....it is at that point you want to maximize the rpms, so when you do get that buddy that brings everything and the kitchen sink along, you don't have to change props and the boat should still perform within the given parameters by Mercury. The heavy load will be reason for the slower hole-shot, thus when lightly loaded, the boat should jump up on plane in little time....a vented prop also aides in this area as well.
Given your numbers are under light load, the last thing to consider is moving the motor up off the transom more. I am not very familiar with Tuffy's models, and from previous discussions like this, I believe they make their transoms a tad taller which can negate how much you can move your motor up. Some of their models already have a set-back built into the hull which also affects what you can do as well. Steve Worrall knows tuffy's well and I would bet he can give some insight on the transom and what is possible.
IF you are able to move the motor up...even one bolt hole, you should see another gain of about 100 or so rpm, which is getting you closer to that max value. If your rpm numbers are under that heavy load, I would bet you will be darn close under light load and trimmed.
Good brands and models of props to try: Michigan ballistic or rapture, mecury has a couple as well in props that are high rake as well. When finding props, ebay is a great source for used props in great condition. And...the nice thing is about the only thing you are out if you try a prop and it doesn't work is the shipping. In most, if not all cases, you can the same money back out of the prop as you put into it.
Steve
Edited by VMS 2/2/2014 4:34 PM
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Location: Not far enough north! | Dan,
I have the same boat different motor (75 opti). Last summer I raised my motor 2 holes and gained 2mph and a couple hundred RPMs. My opti was mounted all the way down. Moving it up was the right move for me and the boat performs better than it did. I couldn't go any higher than that though, I was a little worried about water pressure the cavitation plate rides about an inch above the water with a light load. It helped the hole shot by about a second... |
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Posts: 633
Location: Madison, WI | The motor is currently mounted all the way down and yes I trim all the way up until I start to lose speed. The boat is heavy so I really never have any porpoising
Edited by danmuskyman 2/2/2014 7:08 PM
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Posts: 1096
Location: Hayward, WI | I would check your tach, that motor should have a 2.07 ratio and your rpm/mph numbers don't add up. You should be running 5800-6400 rpm if motor is a 2007. To get these motors up to to RPM best options have been Mercury Vengance prop or an aluminum 4 blade. They don't like a lot of rake or cup.
Edited by kjgmh 2/3/2014 9:28 AM
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Posts: 633
Location: Madison, WI | Would dropping to a 13" diameter but 17 pitch 4 blade be worth checking? Wouldn't the added blade and drop in diameter help in hole shot but and the added pitch increase speed. Or am I way off on this? |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
In your case, if you went to a 17 aluminum in a 3 blade, you would see about the same RPM's as what you have now. Adding the 4th blade would reduce those rpms about 200 or so, but then reducing diamter to 13 would increase by 300 (1/4" diameter is roughly equivalent to 100 rpm). So overall, you would see a rough decrease in 100 or so rpms, give or take some as these are general guidelines and not hard and fast numbers. Hole shot would probably decrease as well since you are trying to get the motor to turn a higher pitch.
I think if you reduce your diamter to 13-1/4 x 15, you would gain roughly 200 rpm there, then by moving the motor up 2 holes, you will gain another 200 rpm. This would get you to roughly 5500 given the same brand and design of prop.
When checking your numbers from original, I too am wondering if your tach might be misreading, or the speed is not a gps speed but a pitot tube speed. Theoretical speed calculation on your motor with the given numbers would put you at roughly 37mph without any sort of slip. Given a 12% slip would give you a rig only going roughly 33...
If we can check the tach and make sure it is on, then get numbers from there, we can get a better idea of what to do...
Steve |
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Posts: 633
Location: Madison, WI | Talked to the local marina and they agree we should check the tach. They also feel I should stay at 15 pitch but drop to 13 diameter. I have a 3 blade and a 4 blade aluminum prop at home that are 13x15 so if we ever get soft water I will report the readings from there. I think ill try the 4 first. Thank you vms for all the help! |
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Posts: 633
Location: Madison, WI | We'll I took the maiden voyage last night with the 4 blade aluminum 13x15 prop. Hole shot was good however I don't have any musky gear loaded since season isn't open. The rpms however still max at 5100 trimmed as far up as possible? Every time I touch the trim switch it would jump to 5500 or so but then back to 5000-5100? Is my tach really off or is my motor lagging? Top speed remained the same at 38.5ish. |
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Posts: 3508
Location: Elk River, Minnesota | I'd get the tach checked...going to a lower pitch should have an increase in tach reading. And...in your case the numbers are still not matching up.
Edited by VMS 4/11/2014 6:18 AM
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