Mercury lower unit gearing

[Rangerspeedboat]

On the Mercury website, the Mercury 9.9 4 stroke says that it has a gear ratio of 2.42:1. What does this mean?

I cant figure out if its 2.42 turns of the engine equals 1 turn of the prop OR if it means 1 turn of the engine equals 2.42 turns of the prop.

Due to 4 strokes lower RPM than a 2 stroke I would think that it meant 1 turn of the engine equals 2.42 turns of the prop, but I have begun to question that theory.

Can anyone make heads or tails of this. If you want to see what I'm talking about heres the link
http://www.mercurymarine.com/engines...pecs.php?ID=74

Its the mercury 9.9 big foot.

Thanks, Ranger

[CDK]

For one revolution of the prop the engine makes 2.42 turns. If it were the other way around, the prop would do more that 10.000 rpm at full throttle....

[Rangerspeedboat]

Thanks, Unfortunately that was the answer that I didnt want.

[CDK]

Next time, add to your post that you don't want to hear the truth. You'll get much more response that way....

[mudman]

I think that Ranger is thinking that the faster he spins the prop, the faster he can go.

[mudman]

Let me elaborate, you need to pitch you prop properly for your gear ratio. You will only be able to go so fast with a given hp.

[Rangerspeedboat]

I know about the pitch and stuff, what I wanted to do however is have an engine at low RPM spin a prop at higher RPM. This way I dont wear the engine out too quickly.

I thought that 2 stroke outboard were Higher RPM than 4 strokes. Well I guess not since mercury has a different gear ratio for 4 strokes that 2 strokes.

Heres a new question, what kind of kind of lower unit should I use for a briggs 10 hp engine? I was thinking a mercury 9.9 4 stroke, but I'm now thinking about a 2 stroke lower unit.

See my "Tiny Power Boat" thread in boat design for more info.
Please provide any info you can.

Thanks, Ranger

[mudman]

I may be wrong, but I think that your briggs is rated 10 hp at 3600 rpm. Lower the rpm and you will lower the hp. I'd say that you want the 1:1. maybe even 1.25:1. Either way, I'd be spinning the prop at 2900 to 3600 rpm. I might be inclined to get it to 4000 rpm tops, but in that case, I'd try and get more rpm from the engine.

Maybe others know more about props and gear ratio, I am no expert. I just know what I would do.

[Rangerspeedboat]

I believe that a Mercury 9.9 2 stroke has a different gear ratio, something like 1.5:1 . Would'nt that be better?
Also, could I just make up for gear ratios by prop pitch?

Mudman, I can get more RPM from the engine by removing the govenor. I have already done that, you can see the throttle setup in my other thread. So I can theortically get 5500-6000 RPM from the engine but I will most likely want to keep it down around 4000-5000 RPM.

[mudman]

I think that getting more than 4000 rpm out of the briggs is pushing it. Even 4000 rpm is pushing it. The engine is designed for 3600 rpm stock. It is an older engine that is not going to like being handled rough. On Briggs Racing website, they sell those go cart engines that are rated at a 3 hp at 3000 but go as high as 6 hp when the get to 4500 rpm. From there the curve starts dropping. It looks like they limit out a 6500 rpm at around 5 hp. I'm not sure what they do to those engines, but I think they are not stock setups. Probably check out briggs racing and see what they do to those things. If you want more or less rpm is will effect your gear ratio.

I think that if you run the engine at 3600 rpm and have a 1.5:1 ratio that it will work fine. Your prop will be spinning at 2400 rpm. I think that a 9X10 or 9X11 would suffice. Should put you in the 20 to 25 mph range. That is just a guess, other members on the board have much more knowledge about props and gearing than I do.

[Rangerspeedboat]

Thanks For the help. Now I know what kind of lower uint I should be looking for.

[pistnbroke]

mudman right on with his recomendations ..the 5 hp Briggs and Stratton outboard that they make has 1:1.54 gearing ....In truth getting a cheap lower unit for this project is more important ..all the main stream makes are about 2:1