Gear Ratios on Lower Units

[MrWalleye73]

Hey all,

Should have a boat again next weekend, but as I wait for that day, I've been thinking. My new motor has a 2.0:1 gear ratio and my old one was a 1.86:1. Not a huge difference I know, but it still is. I'm guessing with the 2 ratio, I should in theory get more ooommpphh off the line, just like when auto manufacturers use say a 3.73 in their truck with the towing packages. Anyways, if so, what would that mean for props? Just want to include that in my thought process when trying out different props for it. Since it's same boat, but different motor, I'll have to do some different thinking compared to all I learned with the old motor.

Thanks all!!

[REW]

Many of the new motors are running taller gear ratios, because they have more torque than their predecessors.
As a result, you generally have to run different props that motors with a lower gear ratio.

With many of these new motors, you want to run larger diameter props to take advantage of the better engine torque of the motors. Then, using the larger diameter motors, you need to adjust the pitch to optimize the boats performance.

Take care
REW

[MrWalleye73]

Thanks REW. I will try some larger diameters.

[staylor]

...years back I did an "extreme" test. Two motors considered to have around 15 hp each. One was an early 1980s Evinrude 15, the other was a fully restored 1951 Merc KG4 Hurricane. The Evinrude was running around a 2:1 ratio, and the little Merc ran a non-shift gearcase running 1.3:1 ratio. In a 14 ft open aluminum cartop with me and 2 kids on board both motors ran within 2 mph at a top speed around 21 mph. But the Merc took a good 30 seconds to slowly get the boat on plane while the Evinrude with its lower gears and bigger prop jumped on plane in around 1/2 the time. The Evinrude would also plane with 1 kid on a tube- the Merc could not plane while pulling the kid and tube. The Merc did have one advantage- it's tiny little 7 inch diameter prop- needed because of the 1.3 gearing- allowed it to troll slower than the Evinrude with its much larger prop. Of course, the Merc could only do this for about 20 minutes before it would start to load up its plugs from the rather weak spark put out by its old fashinioned magneto at low rpm.
Doug

[MrWalleye73]

Okay, so the theory is that the taller gear ratio will provide a little more push out of the hole, well not theory, it should just do that.

I had a h*ll of a time with props and engine height with my last outboard, and never seemed to get it right. I did find the last year I owned it (before it blew) that the tach was reading about 400 rpm's lower than what the Evinrude software said (looking at actual hours at certain RPM's). So I never really propped it right from the get go, I had more room to play with on the top end. But at least I was still running in the recommended RPM range at WOT. Not the reason the powerhead blew.....oil pump.

Just hoping the new Yamaha allows me for a little more top end. I always could jump up on plane very easily. See, this no boat/I'm waiting for my boat thing is making me crazy. Guess I'll ask some prop questions after this weekend! It is being rigged with the digital tach, which is good.

Thanks for defining this better for me!

[staylor]

...if you want the best hole shot you should prop to the top of the rated rpm range with a normal load. If you run light, just don't open it up all the way, or switch to a different prop for light loads. My son runs a heavy glass 19 footer w/120 Evinrude. he likes to take his friends fishing- all 6 of them with gear. He's dropped down in pitch 6 inches from his normal light load prop, and the little 120 has no problem at all pushing the heavy load up on plane. If he's alone he can't apply more than 3/4 throttle before it over-revs.

Of course, the other way to pull a good hole shot is just going to a larger engine to overcome any gear ratio problems. Here's a video of a fellow who has done this- a world record hole shot with lousy 1:1 ratio top end gears. Note how his added power cures the problem....but it looks like he might have plug fouling set in if he tries to walleye troll....
http://www.youtube.com/watch?v=yxM-u...e_gdata_player
Doug

[MrWalleye73]

Ha....cool video. Talk about FAST!

I'm gonna prop this one to get max RPM's as much as possible. But in turn mess around to get as much top end as I can. Always will be some compromise here and there. Usually just two or three in the boat, with all of our trolling gear, downriggers and the like. Since I have a new outboard, I'm sticking with this boat for a while longer, and want to get it right. I'll post up some info when I do some test rides. The installing dealer seems to be real keen on making sure the motor will be at the correct height. And he emailed me today and said he has a plethora of 3 and 4 blade props we can try. He said he put aside the afternoon to test ride with me. Never had one be so willing to do so much, and he didn't even get the sale on the motor.

[staylor]

...since he has a pile of props and will let you test run them! There aren't that many dealers like that around anymore. Please post the results here since any comparison prop test info is useful to all of us here.
Doug

[MrWalleye73]

Will do Doug.....he's installing the injectors and HPDI pump on the motor today and tomorrow (LOOOOONG story), so we should be good to go on Saturday.....long ride to get the boat.....6 hours! And I have to take a boat to get to the boat....LOL. Best I ever got with the old FICHT was using a 17 pitch, and hole shot was tremendous. But when I went to a higher pitch, hole shot was still okay, but I never did gain any top end, only lost RPM's at WOT. I'm just thinking with a proper setup etc, I might be able to swing an 18 or 19 and get the boat closer to 50 mph. But we shall see, it all may still just be the same. Thanks for the info and interest!

[MrWalleye73]

Well, the boat runs! Or should I say, the motor is running, and very well. We tested out a 14 1/2 x 17 Aluminum Solas (that I bought). The old motor ran best on a 17". I figure it's going to be good for a spare. Things noticed. A little bit of blowout in turns, which can be eliminated by trimming down and or slowing down, but I don't wanna do that! Also, some blowout only when we'd hammer down the throttle from idle. Slow application of power was no issue. Hole shot was great. Motor ran right at 5500 R's, so right at the top end. Speed? I didn't have the GPS on board, next time I will. Only....he had nothing available in 17" in stainless, only 15 and 19. Oh well. So, remembering what I know, a good stainless should help rid the blowouts. And I'm sure there's some slippage going on. Motor is mounted on #2, just like my old one. Anyone able to say which Yammie stainless is good? Or another make? Just need a bit more grip.

I missed my boat.