Hi,

I have a Yamaha 8hp 2stroke with the factory aluminum prop. I wonder has anyone here ever put a non factory prop on their engine and did they see any performance changes ?

Maybe there is a better design out there

What kind of performance change are you looking for?


On small outboards like that, the most common reason for a prop change (over here, at least) is to improve the trolling characteristics. This tends to split 2 ways, some guys with small 2 strokes go to a lower pitch so the engine can be run with higher revs and still maintain a slow speed. This helps reduce the plug fouling and stalling that happens if you run constantly right on the idle speed stop.

The other change is to go to a so-called "hi thrust" prop. I'm not exactly sure what this means, they look to me like a larger diameter prop with an increased pitch. I see it done mainly on 4 stroke engines, I think the logic here is that the increased pitch gives more "bite" at low speed for moving heavier boats.

I know one guy who runs stainless on a small outboard (25 HP). He claims it's for increased durability and lower wear when running in rivers where the silt eventually will sand down the aluminum blades. I think that's a waste of money unless you operate in those conditions for hundreds of hours a year. Personally, I doubt I can keep a prop long enough to have this problem, my props usually die of Rock Rash long before thin blades become an issue.

If you're looking to increase top end speed, I think you'd be pretty limited in what you can do with prop changes on an 8 HP. The torque and revs are so limited on a small engine like that, that I'm not sure it would be worth the cost. There's a lot of scope to make improvements on outboards in the 120+ HP range by changing props, but with an 8 HP I think there are hard limits to what you can do.

I'm not even sure if you have many options. I see the stock prop is an 8 inch X 7 pitch, but I'm not seeing any other pitch options available in 8 inch to fit Yamaha in the online catalogs of the 2 places I've bought from before. What are your options for increasing diameter or pitch?

Grouse

Perfect timing with this one! I have a Honda 8hp 4 stroke which is used to push an orkney longliner 16. I used it to troll on lomond a couple of days ago and found it just a wee tad to fast. Engine runs great in every way but I did get the impression it may have been idling just a shade on the fast side.
I took it to a local engine repair shop and it was tested in a tank. It appears it may just be a tiny bit on the fast side but it seems that could have something to do with a sticky thermostat. The engineer at the work shop made a very small adjustment to the carb and there was an immediate reduction in noise accompanied by a reduction in prop revs. However while it was fine to be left on that low setting it wasn't at all happy when the throttle was turned up and it stalled. After a bit of trial and error the carb had to be returned to almost its original setting. The end result was that only a very minor reduction in speed (if any, yet to be tested )was achievable and a change of prop seemed to be the prefered option for a more noticeable speed reduction.
He wasn't a honda dealer and wasn't sure if there was an alternative prop with less pitch available for the 8hp engine. They are also quite pricey items by the looks of things but if the boat isn't moving at the correct speed next time i'm out I will have to bite the bullet and shell out for one. If available that is !

Anyone else find the Honda 8hp too fast to troll ? I can recollect using a honda 9.9 to troll once or twice in the past and thought it a bit fast.

RR, I'm with you, when it comes to trolling speed, there's no such thing as too slow.

I believe there's a similar constraint with my 3 Johnson outboards that you have with the Honda. My mechanic explained when I asked about setting the idle lower that you can only set the idle so low and then you get beyond the range that the ignition can retard the spark. So you end up stalling when you try to set slower than the "lowest" setting.

With Honda, I see PropMD.com says that there isn't a lower pitch available as speced for the 8 HP (9X10), however the 10-15 HP range shows a 9.25 X 8 pitch available. You'd have to find out from your mechanic or email a supplier to see if your shaft size on the 8 HP is the same as the 10-15 range. If it is, I'd think you'd have clearance to add a .25 of diameter to go to the lower pitch.

Otherwise, if you REALLY want go-slow, add a trolling plate. I must come clean and say I hate the looks of these things, but they do work and work really well. I ran one for two trips to Canada on a buddy's boat and he had a 50 HP Yamaha 4 stroke tiller. Unbelievable, but that 50 HP with the plate deployed would troll down so slow that you wouldn't move if pointed into the slightest breeze. To get 1.2 MPH, I had to go up about 400 RPM off of idle. If you raised the plate the boat would accelerate to 10 MPH, so that's how much the plate is taking off the speed.

Personally, I went the other route with my main fishing boat and added a 4 HP aux outboard as a kicker. I got it off of Craigslist for $300, an 86 model Johnson 4HP that looked like it had been used about 3 times total.

I just got back from Canada where I ran that little guy for 10 hours a day and even on a relatively small boat 15 foot alu v-hull), it trolls down to near nothing. It's also a nice reliability factor to have a second "get home" option if the big outboard finally meets the rock with his name on it.

Grouse

Thanks Grouse, I'll explore the prop change a bit further but when you consider the cost and the loss of top end speed that will cause, it might be worth trying to get my hands on a 3 or 4 hp second motor. I have been planning to get out on the bigger lochs and it will, as you say, provide a 'get home option'. It's a long row from Luss to Balloch!!

Cheers
Jim

Have you considered a trolling plate?

Grouse

Yes, I did have a look at them but they too are pretty expensive when you consider the cost of a small second hand auxilliary engine.

Thanks again.

Jim

Thanks for the replies.

I will probably get a buy a new Yamaha prop..I was asking for a bit of magic to get a 20% performance increase from that type of engine.

Funny ye talk about trolling speeds...I noticed this year I caught all my ferox/trout at faster than normal trolling speeds...maybe 1.5 to 2.5 times idleing speed...all my takes where when I went from a normal speed to a little burst of acceleration.

Thanks for the replies.

I will probably get a buy a new Yamaha prop..I was asking for a bit of magic to get a 20% performance increase from that type of engine.

Funny ye talk about trolling speeds...I noticed this year I caught all my ferox/trout at faster than normal trolling speeds...maybe 1.5 to 2.5 times idleing speed...all my takes where when I went from a normal speed to a little burst of acceleration.