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Posts: 633
Location: Madison, WI | Im looking to get a stainless prop for my merc 115 4-stroke and it has a 3 blade 13.75x15 on it right now. Now i know the #15 is pitch but is the 13.75 the only size that will fit or is there a range i should be looking at? I would like to maybe gain a few mph but good hole shot is a concern because my current prop is not great. i also had the same prop in 4-blade but it didnt seem to help. Any help? |
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Posts: 3511
Location: Elk River, Minnesota | Hiya,
What is your current Wide open throttle RPM? I have a buddy who has a fairly new 115hp and he has the same issue...very slow out of the hole. He is also running a 15 pitch prop.
A steel prop will help in both areas, but much of what you choose to do is dependent on the RPM's you have when lightly loaded at full speed. That is one of the best gauges to determine where you are at overall as a starting point. In most cases, if RPMs are high, hole shot is usually decent. If RPM's are low, hole shot is not as good.
So...if you can supply your speed and RPM's we can see what we can find out.
Steve |
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Posts: 633
Location: Madison, WI | my wide open rpms have been around 5100 and 38-39 mph. the boat is a tuffy 1760 |
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| 5100 is way to low. You should prop the boat for 6000 + rpms with just you in the boat then with a load it will go down from there. The specs I read said 5800 to 6400 max. Check and make sure your motor is at the right height before going to far. |
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Posts: 3511
Location: Elk River, Minnesota | Hiya,
after doing a bit of digging, the 15 pitch is right in the ballpark for what you should be at. With a maximum rpm of 6400, I would suggest going down in DIAMETER, though, which will get you closer to the ballpark for your motor RPMs. The holeshot will increase dramatically, and with the motor being able to turn the prop better, you might see an increase in speed as well.
This particular motor is not going to be a speed demon at all with a gear ratio of 2.33:1, It has the low end torque for pontoons and big eared props, but it also has the availability to use a smaller overall diamter to get things moving at speed as well.
NOTE: This is assuming there are props available in a smaller overall diameter, but the same hub size as well. I will do a bit of checking to see what I can find out in this area.
My buddy also has the larger diameter as well...stinks coming out of the hole, but going down in diameter will, in effect help hole shot as well since the motor can turn up to higher rpms quicker.
If you take a look at the mercury website, they have performance tests you can look at with something comparable to your boat. I looked at an alumacraft competitor 1750 which is probably a touch lighter than your boat, but they are reaching the 6100 rpm mark with a 12.8 x 17 pitch...a full inch smaller in diameter than what you have currently. I believe they were testing the "spitfire" propeller on those...Which is a 4 blade prop...that helps with hole shot and handling.
In most cases there is a limit to what the motor can do and RPM's drop dramatically if you increase diamter or pitch. For example, my recently traded yamaha 90 2 stroke could turn a 17 pitch prop to 5400 RPMs, but as soon as I put a 19 pitch on, it dropped to 4600 at trim... It just could not do it... So...by reducing your overall diameter (not hub size...just overall diameter) you can get an increase as well.
So...if you can get to a shop that has any sort of a demo prop program, you might be able to find a few props to try and head to the water and test them. One will stand out among the others for overall performance, holeshot and top end.. When propped well, the boat will just seem to float along at cruising speed... Nice and comfy, yet responsive when you need it.
Steve
Edited by VMS 9/18/2012 10:16 AM
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| Would 19u pitch be too much for that motor?
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Posts: 3511
Location: Elk River, Minnesota | Hiya,
I would say way too much.... That would send your RPMs further below where you are at currently.
Steve |
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Posts: 183
Location: Grand Forks ND | The 4 strokes must be way different. I have a 115 Opti with a 19 pitch prop and I am pretty near max on my RPM's, which is 5750. |
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| ditto the above--I have a 125 2-stroke so it's got a little more juice, but surprised it's that much. I run a 22pitch trophy 2 prop on a tin boat that's same weight as a 1760, and just hit max rpms wide open with just me in the boat. |
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Posts: 32959
Location: Rhinelander, Wisconsin | What brand and model is that prop? |
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| I would still be willing to bet your motor is at least one hole to low but probably more. Before doing anything else I would check to see that your cavitation plate is at or above the hull level. Then when on plane you should be able to see the top of the plate at or above water level. Then when thats right start playing with props. |
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Posts: 3511
Location: Elk River, Minnesota | Hiya,
In most cases when a boat is set up, the plate is set so that it matches the lowest portion of the hull. This is a great starting point if you run aluminum...any higher and most aluminum props cannot hold and will quickly ventilate.
I would highly suggest checking as mnpat said, but I would not bother moving the motor until you have a prop that is getting you much closer to your rpm range. A move of the motor up one hole is roughly good for about 100 rpms, so it is only a minute change and with an aluminum propeller, one bolt hole too high and the prop will blow out quite easily and performance is lost since very little trim can be applied before the prop breaks loose.
I too am interested in the exact model, as I wonder about the gear ratio and power curve. The ones I have looked at are 2.33:1 on the website, which surprises me that with a 15 pitch they are having a tougher time getting out of the hole than other models... I'd love to see the dyno power curve on this engine....that would tell a good story..
Steve
Edited by VMS 9/19/2012 8:50 AM
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Posts: 32959
Location: Rhinelander, Wisconsin | The 1760 is fine with the engine flat to the transom, most dealers rig it one hole up. Any more, and the prop will cavitate. |
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| The engine does appear to be mounted all the way down but unfortunately it is not something i can easily have moved as i wont be able to take it in anytime soon. I was not able to find a smaller diameter prop however i did find a good deal on a new 13x15 stainless so i ordered it because i already know how it performs and i need the prop by next week so i didn't have much time to shop
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Posts: 3511
Location: Elk River, Minnesota | Hiya,
You can do the move of the motor all on your own. All you need is your trailer, the tie downs, a few blocks and a tongue jack.
In a nutshell, all you do is lower you jack all the way down, then block the motor (motor level to the ground). Take out the top bolts and loosen the lower bolts holding the motor in place.
THen, start raising the tongue jack...this will lower the rear of the boat. It may take a little to get the motor to let go due to the silicone seal, but rock it a little and it will come loose.
Raise the motor to the desired hole, re-silicone and reinstall the top bolts. tighten and you are all set.
First time is always the toughest, but after that it will seem easy.
Steve |
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