Posts: 531
| I recently picked up a new to me Ranger 681 with a 1995 johnson 70hp.
WOT 5600 rpm, I cant find the WOT range but my manual says min 5700rpm (dont know why there isnt a max, but there isnt)
GPS speed 33mph with "normal load" the boat will see.
2.42 gears
13.75 dia 17 pitch SS prop (unsure of brand, couldnt find it on the prop)
The rig does require use of trim to plane out, it isnt the quickest out of the hole, but i have owned boats with slower hole shots. So I am OK with out of hole preformance, would like to improve it but i can certaintly live with it. Top speed I am happy with, i dont need a rocket (i understand a 70 is slightly under powering this hull) but also i wouldnt want to lose any speed if the prop is changed.
So with all of this, I am unsure if i should drop an inch or two of pitch, to get my RPM only to lose or maintain MPH.
any thoughts? |
Posts: 3480
Location: Elk River, Minnesota | Hi again,
RPM range: 5000 - 6000
So..if you are seeing 5600, you are right in the meat of the rpm range. Question is, though, how heavily loaded is your rig at that RPM? Are you lightly loaded (you and just your gear with full tank)? If you are lightly loaded, I would say you are either A) over pitched or B)too large of diameter.
With the 2.42:1 ratio, you have a motor that has some real "umph" behind it as the motor can get into a higher RPM band for turning the prop. It allows for a higher pitch prop in many cases as well.
I'm not too familiar with the weight of the 681, but I am guessing there is a bit of weight to it, so getting the boat moving is important. My gut says to try a smaller diameter prop in the same pitch. You are only 400 RPMs away, in which if you were to find a 13-1/4 x 17 3 blade, you would gain that 300 RPM's right there, which would allow the motor to spin up a little faster and potentially get you out of the hole a little quicker. And...being a 17 pitch, I'd doubt you would lose any top end and would not be surprised if you actually gained a little. Then, if that works, you might be able to raise the motor one hole on the transom without too much loss of handling to gain another 100, putting you right at the top of the RPM band. Aluminum props don't allow for much vertical movement of the motor as they tend to break loose in the water quite easily due to a lack of cupping on the tips and trailing edge.
A prop I would recommend to try would be a solas amita 3. It will flex a little less than a standard OEM prop, is thinner, and is cupped more on the trailing edge, which will grab water better and stay hooked up a little more in turns. It will be much more efficient of a prop as compared to the OE Johnson aluminum. If you go stainless, I would say a ballistic propeller in the 13-1/2 x 17 or a rapture (if you can find one) in 13-1/4 x 17.
The other option would be is to drop your pitch by 2 inches and go to a 15 pitch in the same diameter. What this will do is help your hole shot dramatically (bigger fin, less pitch=higher RPM) but you will tend to lose a mph or two on top end.
The key:...determine what you want the boat to do for your particular style of fishing, and go from there. Don't be afraid to try different sizes and brands of props to find one that works best for what you want. If you can find a dealer with a demo-prop program, you would have many options of props to consider. Also, Ebay is a good option for used props and in most cases you can play around with the only loss to you $$ wise is shipping.
Steve
Edited by VMS 3/16/2013 2:57 PM
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