9.9 / 15 hp gear ratio

[KamikazeKid]

What is the gear case ratio for late 1970s to 1990s? I want to correlate engine RPM to prop RPM.

 

[jmendoza]

Re: 9.9 / 15 hp gear ratio

Do not have the manual here, but it's listed in the specifications section, hopefully someone here has a manual. My literature is 200 miles away at the lake where my cabin and boat are kept.

 

[AlTn]

Re: 9.9 / 15 hp gear ratio

from omc manual for 1978 9.9 evinrude....propeller gear ratio..12:29

 

[Rick.]

Re: 9.9 / 15 hp gear ratio

Gee that was easy. I was just getting ready to pull my leg and gear case apart so I could count the gear teeth. Actually I'm pulling your leg ...not mine. Interested to know what your going to do with that information though. Rick.

 

[Patate]

Re: 9.9 / 15 hp gear ratio

Rick, 12 engine revolutions, 29 prop revolutions

 

[Rick.]

Re: 9.9 / 15 hp gear ratio

thanks for that. I had it figured to 3 to 7 1/4. Never was any good at math. I'm still wondering why the question was asked though. Rick.

 

[KamikazeKid]

Re: 9.9 / 15 hp gear ratio

Isn't it 29 engine revolutions to 12 prop revolutions? In exploded view, drive shaft gear is quite a bit smaller than the propshaft gears. (That would make it 2.42:1, which would confirm what I've seen one other place.)

Since you asked, I'm doing a typical "engineer" thing - confirming that what everyone says is true. That a 15 hp kicker offers no advantage over a 9.9 hp. (I'm sure this will stir some comments.)

I set up a simple spreadsheet to calculate forward speed from engine RPM, prop pitch, and assumed slip. Even if I assume a 7" pitch and 40% slip, I still get 5.7 kts at 4000 RPM.

From what I've read, the 15 hp OMC gets the extra hp power by increasing max RPM from about 5000, to 6000. (Makes sense to me, since the bore, stroke, and porting are the same.) Since I'll achieve hull speed before 4500 RPM, I'll never see the extra 5 hp, so there's no sense in paying more. (Besides, who's going to run an emergency motor for hours, at 5000+ RPM?)

I had a choice to buy a rebuilt 9.9 for $1,300, or a rebuilt 15, for $1500. At first I chose the 15, but then changed my mind.

Thanks for the info!

 

[Haffiman]

Re: 9.9 / 15 hp gear ratio

For a kicker/trolling prop, try this one:
http://www.discountboatpropellers.com/product_propeller_info.html?products_id=97

 

[Rick.]

Re: 9.9 / 15 hp gear ratio

I'm kind of with you on this. What is kts? I've been of the opinion that for a kicker, small is better. ( my opinion has not included any math) Going a little further, would a 6 HP. not do the job even more efficiently? There is a large difference between a kicker and an emergency back up. For a back up I can see where more HP. might be more advantageous in wind and current. Rick.

 

[KamikazeKid]

Re: 9.9 / 15 hp gear ratio

kts: abbreviation for knots.

Here in Hawaii, "kicker" is synonymous with "emergency backup." Few people use downriggers, or do any slow trolling. Most small engines are for backup, and as you note, one needs to plan for less than ideal conditions.

 

[Haffiman]

Re: 9.9 / 15 hp gear ratio

Most trolling/kicker/emergency engines you see around are totally wrong propped.
They will normally never be able to pull above 5-9 knots, but propped for 15 and above.
As result too many are carrying to heavy a load, too many hp with too high fuel consumption around.
Most outboard manufacturers have a 'sail' propeller intended for 'hull speed', but one hardly see those on a kicker engine.
I wonder why?

 

[jmendoza]

Re: 9.9 / 15 hp gear ratio

Use a "sailmaster" prop designed for these by OMC, works great at below planning speeds.

At the local marina, they have a small 12 foot flat bottom launch for hauling boats to and from the main dock out to their mooring bouys. For the launch, since it's a tug, they use a 15 and the sailmaster prop as this gives the most low speed grunt and control when towing. They told me that over the years, they have always used a 15 instead of a 9.9 becuase it has more power, which they need for the tug. I found this out when I asked if they had any 15 engines in the bone yard, and they said the had far more 9.9 scrap from their rental fishing boat fleet.

 

[ChrisAG]

Re: 9.9 / 15 hp gear ratio

From what I've read, the 15 hp OMC gets the extra hp power by increasing max RPM from about 5000, to 6000. (Makes sense to me, since the bore, stroke, and porting are the same.) Since I'll achieve hull speed before 4500 RPM, I'll never see the extra 5 hp, so there's no sense in paying more. (Besides, who's going to run an emergency motor for hours, at 5000+ RPM?)

You should have gone with the 15 and propped it to reach 6000 rpm at WOT. That doesn't mean you would actually RUN it full time at WOT, but at least it wouldn't be lugging, which it certainly would be at your proposed 4500 rpm.

You can run those engines all day at 6000 rpm (which I did as a kid). However some people believe if you are over 6000 rpm you should richen the fuelil ratio.

Secondly, the 15 is not ONLY showing higher power at higher rpm, but throughout the rev range. Think of it this way; in the old days, GM offered 5.0L V8s with either a 2-barrel or 4-barrel carb at extra cost. You think the 4-barrel only gave power at the top of the rev range? That extra power was available any time you stomped on it.