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Author Topic:   Outboard Gear Ratio

mkelly posted 01-08-2014 05:16 PM ET (US)
Am I correct in understanding outboard gear ratio at 2.5:1 will perform better at top end than a 2.0:1 ratio? Or do I have it backwards....also, I assume propeller size/pitch plays into the equation. Basically what I'm trying to do is get a motor with the largest displacement & gearing more for top end. Suzuki seems to have 2.5:1 ratio & a bit larger displacement than Yamaha (2.0:1), 150 HP.
Peter posted 01-08-2014 06:13 PM ET (US)
Not necessarily.
While I would not buy a 4-stroke 150 HP because I think there are better alternatives, if forced to choose between the Suzuki and Yamaha 4-stroke 150s (assumes access to service and price are not factors), I would choose the Suzuki simply because the Suzuki powerhead has more displacement. The lower unit gear ratio would not be a factor in my decision.
mkelly posted 01-08-2014 06:32 PM ET (US)
I did a search & found a ton of previous postings related to this issue, so I apologize for waking a sleeping giant. I think I'll consult with my prop guy as that seems to play into it. Thx
mkelly posted 01-08-2014 06:42 PM ET (US)
Peter, you would go with the ETEC or Mercury, I gather?
OMCrobert posted 01-08-2014 07:38 PM ET (US)
Simple answer is that 2.0:1 has a faster theoretical top end vs 2.5:1.
You will almost never see a Suzuki over 70mph simply because you run out of prop pitch on the high end 34pitch+.
It all depends on application. It does not matter if you boat can not go over 60mph+.
Binkster posted 01-09-2014 09:59 AM ET (US)
Antique Mercury racing outboards ran a racing gearcase with a 1:1 ratio. If the engine turned 6000 rpm the prop would also turn 6000rpm. In order to do this the props they ran were small diameter with a big pitch. If your outboard has a 2:1 gearcase ratio, and your engine will turn 6000 rpm max, safely, you prop will be turning 3000rpm. Now, if you have the same powerhead with a 2.5 gear ratio, you prop will be turning 2400 rpm at 6000 rpm The speed of the boat may not be greater with a lower gear ratio number, obviously with a larger gear ratio number you need a prop with a larger diameter, and maybe more pitch. I don't think displacement of the block comes into play, if the hp is the same. The setup of the outboard on the transom (height of engine and prop and angle of kickout/kickin) is more important that the gear ratio of the gearcase.
rich.
Tom W Clark posted 01-09-2014 12:58 PM ET (US)
There is no theoretical advantage to a higher ratio, though there may be a practical advantage, running out of available high-pitch propellers to meet the expected top speed, chief among them.
Gear ratio is something to be considered so I wouldn't take it out consideration but, as a practical matter, it is not a big part of the decision making process because there is a small finite number of motor models to be considered and they all off their own one single gear ratio.
Buy the 150 HP motor you want. It will have the gear ratio that it has.
If top speed if your goal, buy the most powerful 150 HP outboard in the class, whatever that may be.
OMCrobert posted 01-09-2014 09:58 PM ET (US)
The only advantage would be allowing the engine to get into the power range sooner while also allowing the engine to swing a prop with a larger surface area.
I have been on a 100ft fishing travel powered by less than 300hp (inboard Detroit). The 6.0:1 gear ratio made it work fine.

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Author	Topic: Outboard Gear Ratio
mkelly	posted 01-08-2014 05:16 PM ET (US) Am I correct in understanding outboard gear ratio at 2.5:1 will perform better at top end than a 2.0:1 ratio? Or do I have it backwards....also, I assume propeller size/pitch plays into the equation. Basically what I'm trying to do is get a motor with the largest displacement & gearing more for top end. Suzuki seems to have 2.5:1 ratio & a bit larger displacement than Yamaha (2.0:1), 150 HP.
Peter	posted 01-08-2014 06:13 PM ET (US) Not necessarily. While I would not buy a 4-stroke 150 HP because I think there are better alternatives, if forced to choose between the Suzuki and Yamaha 4-stroke 150s (assumes access to service and price are not factors), I would choose the Suzuki simply because the Suzuki powerhead has more displacement. The lower unit gear ratio would not be a factor in my decision.
mkelly	posted 01-08-2014 06:32 PM ET (US) I did a search & found a ton of previous postings related to this issue, so I apologize for waking a sleeping giant. I think I'll consult with my prop guy as that seems to play into it. Thx
mkelly	posted 01-08-2014 06:42 PM ET (US) Peter, you would go with the ETEC or Mercury, I gather?
OMCrobert	posted 01-08-2014 07:38 PM ET (US) Simple answer is that 2.0:1 has a faster theoretical top end vs 2.5:1. You will almost never see a Suzuki over 70mph simply because you run out of prop pitch on the high end 34pitch+. It all depends on application. It does not matter if you boat can not go over 60mph+.
Binkster	posted 01-09-2014 09:59 AM ET (US) Antique Mercury racing outboards ran a racing gearcase with a 1:1 ratio. If the engine turned 6000 rpm the prop would also turn 6000rpm. In order to do this the props they ran were small diameter with a big pitch. If your outboard has a 2:1 gearcase ratio, and your engine will turn 6000 rpm max, safely, you prop will be turning 3000rpm. Now, if you have the same powerhead with a 2.5 gear ratio, you prop will be turning 2400 rpm at 6000 rpm The speed of the boat may not be greater with a lower gear ratio number, obviously with a larger gear ratio number you need a prop with a larger diameter, and maybe more pitch. I don't think displacement of the block comes into play, if the hp is the same. The setup of the outboard on the transom (height of engine and prop and angle of kickout/kickin) is more important that the gear ratio of the gearcase. rich.
Tom W Clark	posted 01-09-2014 12:58 PM ET (US) There is no theoretical advantage to a higher ratio, though there may be a practical advantage, running out of available high-pitch propellers to meet the expected top speed, chief among them. Gear ratio is something to be considered so I wouldn't take it out consideration but, as a practical matter, it is not a big part of the decision making process because there is a small finite number of motor models to be considered and they all off their own one single gear ratio. Buy the 150 HP motor you want. It will have the gear ratio that it has. If top speed if your goal, buy the most powerful 150 HP outboard in the class, whatever that may be.
OMCrobert	posted 01-09-2014 09:58 PM ET (US) The only advantage would be allowing the engine to get into the power range sooner while also allowing the engine to swing a prop with a larger surface area. I have been on a 100ft fishing travel powered by less than 300hp (inboard Detroit). The 6.0:1 gear ratio made it work fine.