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Posts: 572
Location: Williamstown, WV | Okay so I'm looking for a new prop for my mercury 90hp 4 stroke. I'm looking for a four blade instead of a 3. My question is and yes I'm very new to this what in the world is the differences in the pitches? What I'm looking for is an aluminum prop, good take off, and add a little more speed maybe. Help.
Shawn | |
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Posts: 156
| Pitch is the theoretical distance a prop should travel with 1 revolution if there is no slip.
Lower number more revs and power. Higher number less revs, maybe more speed. | |
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Posts: 572
Location: Williamstown, WV | Also I have an 18 deep v Fisher boat. The prop that is on it is what the factor I guess had because I have not changed it. Hope this helps. | |
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Posts: 3508
Location: Elk River, Minnesota | Good evening,
First off, with any change in propeller characteristics, there will be trade-offs and a compromise will end up taking place. It usually takes place as a handling vs speed compromise.
To begin, as was stated, pitch is defined as the theoretical distance a prop would travel in one revolution. So...a 13 pitch prop would travel 13 inches forward in one full revolution of the propeller. To go faster, more pitch in a prop will usually do it, although the trade off is a loss of RPM's. The motor has to work harder to turn a prop of higher pitch, thus RPM's go down. It is much the same as changing gears on a bicycle. The bigger the gear, the harder it is to turn...
Also, when you increase the number of blades on the prop, the prop has now gained surface area that is interacting with the water. This allows the prop to work for you in terms of better handling characteristics (the prop will not lose grip as easily) thus turning is better, and you may not have to trim the motor down as much when making a moderate turn under speed. It also helps with getting the boat up and out of the water upon throttle-up. The trade-off, though...loss of speed...the reason is due to more friction between the water and the prop due to the extra blade. In most cases, switching to a 4 blade prop also requires reducing the overall pitch from what you currently have since more surface area will reduce RPM's as well..in most cases around 2 inches.
In all, finding the right prop is a matter of finding the right balance of speed and handling characteristics for your particular boating style. What all goes into finding that prop is primarily ensuring you are propping your motor so that it can reach the upper 1/3 of the RPM range (I believe for your mercury if memory serves me correctly would be max of 6000 RPM's) with the ideal being at max RPM under a light load. By propping your motor in this manner, you ensure some "wiggle room" so to speak when you start fishing with other people without having to change to a different prop.
This is just a touch of what can go into choosing a prop for your rig.... Aluminum options are pretty limited overall, but if that is what you choose to do, I would recommend taking a look at Solas brand props which are cast a little differently than your standard stock prop.
As a starting point, do you happen to know what RPM's you were reaching with your current prop? Also, if you can verify your motor's gear ratio and maximum RPM's that would be great too. Being you are running an aluminum prop, I would assume your motor is probably mounted one hole up off the transom, which is a standard starting point...that can be potentially adjusted a little depending on your rig and preferences.
If you have any questions, do not hesitate to ask. There are a few really good sticks on here who know their stuff on props and engines as well as these wonderful critters we fish for, and will be very willing to help out if and when they can.
Steve
Edited by VMS 1/26/2012 6:40 PM
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| OK, let me chime in with a similar setup. I have a 05 Tracker Targa 185WT. Dual console, full windshield, 4 batteries, Heavily loaded. I have a 90 horse merc 2 stroke with a 19p 3 blade merc prop. It'll spin 5200 rpm at 32 mph maxed out. With a 13p merc prop it'll do about 25mph at 5500 rpm not maxed out....didn't want to twist it any faster. I would be very interested as the what Pitch I'd need if going to the new 4 blade merc props that have recently come on the market | |
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Posts: 3508
Location: Elk River, Minnesota | Hiya,
Not knowing much about the prop you are looking at, and going off the numbers you listed, I would say a 13 pitch 4 blade could potentially work, given you might have to play with the height of your motor as it sits on the transom. With more blades, you increase the prop's interaction with water, thus lifting the motor will allow you more RPM's due to less lower unit in the water. That will give you some gain back in speed by going 4 blade. I would think you would be right around that 5500 mark and probably right around 32 - maybe 34 mph. Hard to say right off...
Steve | |
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Proppeller Pitches?