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Posts: 267
| I have a 1988 Ranger 680c with same year 90 HP Merc oil injected two stroke 3 cylinder outboard. Bought this rig last fall and am still working the kinks out. Teh Merc seems to run great having been just cleaned out and tuned up by a dealer. Last week, I got a trim adjusted top speed of 34.5 mph at about 4900 - 5000 rpm. The boat was fully loaded with two guys about 190 lbs each and accompanying musky gear. I have a 101 lb terrova on the front and a heavy previously installed home made rear deck extension in the back which probably add about 100 lbs total weight, too. It has a 12 gallon tank built-in the rear which was mostly full at all times. The prop is a stainless steel Quicksilver 2 4816990 A40 20P. A "whale tail" was installed by the previous owner. It doesn't jump right out of the hole but I don't think these older engines every really did. Is 34.5 mph at 4900 - 5000 rpm about where I should be or is there more left? |
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Posts: 3514
Location: Elk River, Minnesota | Hiya,
I would say you were adequately loaded but I'm betting you are over pitched by about 2 inches or so. That would explain to me why the whale tail was installed because my gut says the previous owner could not keep the boat on plane at lower speeds, thus needed the extra lift in the rear to keep the stern up.
If you are up to it, I would say try this:
Put just you and your gear in the boat, and top the tank off. Remove the whale tail and go for a spin. See what your numbers are at that point (RPM's, top speed, etc.). If you are below 5500 on your RPM's, I would say you are over pitched. My gut says you will see 5100 or so on your own and another MPH or two.
the key to a good set up is the ability to get up on plane quickly, keep it there under moderate RPM, and still maintain decent top end. It is a balancing act, and one that usually involves some compromise. If you want handling and hole shot, you sacrifice speed. If you want speed, you sacrifice handling and in most cases hole shot.
There are ways to get hole shot up. It is usually done with props that are vented. Mercury has an entire line of props that allow you to adjust the amount of venting, but I am unsure right off the top of my head whether or not the prop sizes you would be looking at have that option. What venting will do is allow exhaust gasses to reach the prop fins, essentially ventilating the prop in the water. This increases RPM's quickly, which at one point, the exhaust will be fast enough it bypasses the vent holes, and up the boat pops on plane. The draw back, though...harder to keep the boat on plane at lower speeds where exhaust gasses start to reach the prop fins again.
See what happens with just you. I'll help if I can.
Steve |
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Posts: 267
| Thanks Steve. If we're only talking a mph or two improvement, Ill prolly leave ger as is. I might add, tho, when the second guy in the boat sits in the back, it took quite a bit more time to get on plane.
Even though it doesnt make a ton of sense financially, I plan on keeping this boat for a long time and have been considering re-powering to a Merc 115 Pro XS. What might I expect performance wise with that motor? |
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Posts: 1096
Location: Hayward, WI | I would try a 17 pitch Splitfire, get it propped so you are running 5250-5500. The Splitfire has worked great on similar setups. |
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Posts: 3514
Location: Elk River, Minnesota | Hiya,
You are only increasing HP by 25 hp, give or take a few HP, which means you might see a gain of about 3 to 4 mph, so you will not see much more out of the rig.
A prop change for you would not cost much as you can find excellent prices on used props through ebay. I have purchased numerous props from there and if the prop didn't work out, all I am out is usually the price of shipping.
If you decide to keep the set-up as-is, in my humble opinion your are lugging your motor quite a bit. With any motor, if you are on the lower end of your Max RPM range, you are putting undo stress on the motor components, which over time will shorten the life of the motor. A $150 prop can get your performance up tremendously, and get your motor running optimally.
If you decide to try a couple of props, I would highly suggest a 17" ballistic (they don't make an 18") or a michigan rapture. both are great props. The rapture should be vented which would help your hole shot.
Both props would allow you to move the motor up a notch or two as well.
Steve |
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Posts: 532
| I would at least try a new prop, like VMS is saying. I have a 1994 681C with a 70hp Johnson it is loaded similar to your rig. 3 batteries, terrova, I got a 9.9 hanging on the transom, and I have a 23 gallon tank that is almost always full. With my rig I get no less than 33mph and no more than 35mph that's my average range for top speed so its very close to your speed with 20 less ponys.
If you plan on keeping that motor, I feel you cant do no wrong with playing with the prop alittle
Edited by anzomcik 6/9/2013 8:00 PM
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Posts: 267
| Thanks, guys. ill go ahead and play around w some props and see how it goes. Did a quick search and did find one of the ones recommended above on ebay. I need a spare prop anyway so cant hurt.
Man, with a nearly 30% increase in hp of the 115 Pro XS (or more - a salesman once told me that motor runs closer to 127 hp but who knows), I figured Id be in the low to mid 40's mph-wise with way more pop out of the hole and gas mileage than that old Merc. But I like that old Merc and am gonna keep it for now anyway. Thanks again. |
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