Yamaha 9.9 prop

[katana750]

I recently gave up on a new Tohatsu 9.8 because of vibration issues and bought a 9.9 xtra long shaft thigh thrust yamaha.

very happy with the new outboard. I just had one small question I cant seem to get answered. It came with a 7 x12 inch propellar...roughly. I just finshed my break in on the motor and am noticing the rpms will not get any higher than 4500 rpm at full throttle. Manual says it should be at the high end of between 5000 and 6000.

Talked to the dealer and they said I wouldnt cause any damage to the motor running it at 4500.

Now every thing I have learned about props and pitchs from discus ions the rpms are indicating the motor is over propped and that is hard on the motor. I obviously dont run the boat at full throttle and I get roughly 5 knots at 3800 to 4000 rpm. The motor is quite smooth and pretty quiet at that range.

Just wondering if anybody else with this motor has checked there rpm on their motors. If running a motor that is over propped is hard on it. Is it hard on it only when at full throttle? or at all rpm ranges?

I can step down to a 5 pitch prop but it is 150.00. It is not very common and not many people buy it. I dont think it would get me in the high end of the rpm range any way.

My other thing is the manual covers all motors that are high thrust xtra long shafts to short shaft non thrust motors. It would seem to me difficult to compare rpm ranges between a small inflatable dingy with a short shaft at full throttle and slow moving displacement hull.

Thanks
Dave

 

[walt]

Now every thing I have learned about props and pitchs from discus ions the rpms
are indicating the motor is over propped and that is hard on the motor.

sounds like maybe something other than the prop pitch as 7 would seem to still be on the lower end or maybe mid end if that is a four blade.

Im also wondering if what us in the quote is just from discussion in forums like this or is this from a manufacture? The Honda 8 hp four stroke I originally owned had an 8.5 pitch prop which some would interpret to be "over propped" for a sailboat application but it ran just fine after 22 years. Parts were worn out (like the reverse lock) but the motor part was still strong. Somewhat leads me to think that what is said in the quote is true if you excessivly bog the motor down but I doubt this for the typical props we use in this application ranging up to maybe 9 pitch.

 

[katana750]

yes this was the site that got me first thinking about prop pitch. First with my two stroke yamaha....then the infamous vibrating nissan....still working on an outcome on that and now I have a motor I like I just want to make sure every thing is the best it can be. I am pretty sure its a three blade prop but I am going to check when I go back to the boat. I think everything is probably fine I just havent got the answer I quite wanted from the yamaha dealer yet. It seems pretty clear from everything I have read that if your motor cant reach the ideal rpm at full throttle... its over propped and that is hard on the motor....but I dont think its quite that simple. If I have a 150 hp flat bottom boat running around at full throttle and I cant get to the specified rpm that might be an issue but quite a bit different than me puttering around at 3800 at 5 knots.

I was just expecting something back from yamaha saying you have a displacement hull and it would be difficult for you to get to that rpm rating. Instead I didnt get a reason why this would happen just not many people switch to a lower pitch and the motor doesnt come from the factory with one but we can bring one in for you but then you have to buy it. I feel silly asking them same question. Just thought I would throw the question out here to see if people have similar results as me. If some body comes back and says they easily reach 5500 rpm with theirs it would tell me something is a miss. It might even be my tach but the numbers seem pretty steady.

 

[katana750]

Im also wondering if what us in the quote is just from discussion in forums like this or is this from a manufacture?

AfterI reread your comment you are right. It is only on discussions on forums that have said if you cant reach the specified rpms you are over propped and its hard on the motor. The manual mentions preformance fuel efficiency and engine life but it doesnt specifically state if your prop doesnt allow the engine to reach specified rpms it is hard on components. It says it should reach the upper half of 5000 to 6000 rpm. Maybe thats why I dont get the answer Im looking for. Maybe the fact my motor doesnt reach 5500 rpm doesnt really cause an issue. He did say he asked around the shop and that running it at 4500 wouldnt hurt it. It wasnt quite the definite answer I was looking for though

 

[FastOlson]

I put a lot of hours on our Yamaha 10 High Thrust OB (25" leg) on our prior 26 foot sailboat. I came with an 11.75" three blade prop standard.
It would push my 4K # displacement boat at 6 kts all day long, and burn a half gal./hour.
That model also had a 3:1 gear ratio, and was intended for displacement hulls.

Great engine!


What is your boats displacement and DWL?

LB

 

[walt]

what elevation are you at, did you happen to measure your peak speed?

Im just going from memory (which is getting old lol) but these numbers are ballpark with a 9.8 hp Nissan 4 stroke on a 1990 26S

At a litte over 5000 ft elevation, a four blade 5 pitch got the boat up to about 7.4 mph. A three blade 8.5 pitch got the boat up to about 7.1 mph (about a 4 % difference). The 8.5 pitch prop definately operated at a lower rpm than the 5 pitch.

I took that same three blade 8.5 pitch to about 500 ft elevation and hit about 7.6 mph.

I dont have any way to measure rpm but I saw more change from elevation than I did from changing props.

knots = mph/1.15 (I rememeber all these numbers in mph)

 

[thinwater]

Don't even worry about it. They have been the standard engine on many boats (including all PDQ cats) for many years, and they always die of something other than mechanical wear. Generally something corrosion related after about 15 years.

Go have fun!

 

[MaxinCalgary]

I have a H260 with a T 9.9 XL. At roughly half throttle if at 5.5 to 6KTS. Any more and I am plowing and probably putting strain on the transom mount. Cannot imagine the MAC weighs more. I do not believe running the engine at less than full revs will damage it. Remember that the primary use for these engines is to trawl at 1-2 kts on fishing boats that weigh a lot more than our boats

 

[Bruce31450]

There are so many variability in props. If it works and your fuel usage is good, save your $$$ the higher the RPM the more fuel you will use & you might get 1 knot more speed, is it worth it? Now if you were to put the engine in gear and it stalled or bogged down to misses in the engine yep too much prop. Unless you want a spare prop & to burn more fuel leave it alone. Mercury marine explains better than I can, here is the link ...

Prop Selector
https://www.mercurymarine.com › ...

 

[katana750]

Thanks for that... everybody seem to be on the same page. Its a macgregor 26 s so maybe fully loaded its 3000 pounds. Peak speed pulling the dingy is about 6.0 knots 3800 is about 5 knots. Elvevation here is just over 3000 feet. I will take fuel economy over speed any day. You will never be traveling fast anyway. Im pretty happy considering all the hassle I went through with the Nissan.

I just thought it was odd that this outboard is designed for this type of boat and it cant reach the specified rpm the manual is requesting.....but comparing the comments it seems its pretty comparable to everybody else. I must admit the bogging the motor thing confuses me a bit. Like Bruce 31450 says its not stalling the motor out its just not reaching the the higher rpms. Its just one of those things that you think hey this is pretty good....can it be a bit better? My goal is to run it at its lowest rpm and still make a bit over five knots. I think what I have is as good as it gets. Ill just leave it alone.

Thanks Everybody

 

[Sock Puppet]

This is something I've been thinking about more lately. When I bought the 9.9 Merc 4 stroke for my boat, I automatically bought a 4-blade high thrust prop for it. Never tried the factory prop. I sometimes wonder if I'm not getting the most out of it. Just picked up a GPS for the boat so I can track speed. What are you guys using for a tach on these motors? Depending on time this year I may throw the 3 blade prop on to compare. I sail at ~5800 ft or so.

 

[walt]

After using a high thrust prop myself I somewhat thought I had been duped by the internet for why I needed it. You can get led to believe that a conventional prop "slips" more and that you need a high thrust prop to fix this. This is the wrong interpretation of slip.. Slip has more to do with the drag of the hull. On these under 10 hp outboards, you just don’t have a cavitation problem with any style of prop. I got a little more speed out of the high thrust prop - pushing the boat way up into the displacement hull wall just wasting gas - and I think the main reason was that the lower pitch allowed the outboard rpm to get up to where the outboard was delivering more hp.

On the Mac Classics you just don’t need the full 10 hp and using a more conventional prop means lower rpm, less noise, less vibration, better efficiency. Way nicer if you’re motoring any distance at all.

On the other hand, if you were trying to push a Mac classic with a 4 hp, you might then need to go to a lower pitch prop simply because you need to get every bit of that 4 hp possible.

Not always the case but most of the high thrust props allow the exhaust to go through the hub in reverse rather than flowing over the prop blades and this does make a huge difference in how well the prop works in reverse. If you cant rotate the outboard to vector the thrust, I think one of those props that does really well in reverse would be valuable as you can keep the boat speed up for docking (keeps the rudder working) but then stop on a dime in reverse.

I can rotate my outboard and with the centerboard down a bit have great maneuverability going slow and find the reverse of my conventional prop more than adequate. One time I got the boat going full speed in forward and then put the outboard in reverse and timed how long it took to comfortably come to a stop. Only took 10 seconds with the conventional prop. Might have been half that with the high thrust prop I have however.

Just my opinion only – could be wrong but Im thinking more and more that “high thrust” for these under 10 hp outboards is partly a marketing term aimed at fishing boats and it means that you can troll at a very low speed. Do you really care about "hole shot" with these small motors? I don’t fish but I think my conventional prop moves the classic too fast for trolling even at idle but the high thrust prop would result in the boat moving slower at idle so better for fishing.

 

[katana750]

That is an interesting thought. I have propellers from the two stoke yamaha I wonder if they would fit on my new one...Pretty cheep experimant and it might even be better and it costs nothing to try.

 

[walt]

Its more complicated than just pitch since blade area affects things also ( F = ma.. two way to increase thrust. Accelerate some amount of water faster - increase "a" or move more water at the same acceleration - increase "m"). And then throw in the gear ratios.. Ive heard a lot of people say they really like the outboard you have on a sailboat.

Since you have a way to measure rpm (I dont), takeing some measurments like boat speed vs rpm vs wide open throttle would be interesting.

Maybe also time how long it take to stop from full throtle forward.

 

[BudGates]

Engines are designed to run at a particular speed for "optimum performance." Just because one engine is running faster than another for a particular boat speed doesn't mean it's burning more fuel. It could be less. Thus a higher pitch prop won't necessarily give you better fuel economy. A higher pitch can give a higher top speed while on a plane but it won't make a displacement boat go any faster. Speculation and opinion are useful and can be important but I'd be more interested in seeing actual data.

 

[walt]

And.. also interesting but probably hard to measure "apples to apples" is what does the EFI you have on the Suzuki do for efficiency. Probably good things..

 

[BudGates]

And.. also interesting but probably hard to measure "apples to apples" is what does the EFI you have on the Suzuki do for efficiency. Probably good things..

I've read that the EFI is more fuel efficient but haven't seen any data to determine how much. To conduct a proper experiment would require two identical engines, one carb the other EFI, and several props of varying pitch, diameter, number of blades, and blade shape. Considering my truck uses 30-40 gallons to get to the ocean one more gallon for the boat doesn't mean much to me.

I often park in rather tight slips so the most important factor for me is low speed control and maneuverability. I may eventually get a higher pitch prop but, for now, I think I'll stick with the high thrust prop that came with my outboard (4 blade, 5" pitch, 10" diameter). We'll see how it performs next week.

 

[katana750]

The only real data I have of the top of my head is
full throttle is 4500 rpm and 6.5 knots
3800 rpm is just about 5 knots...of coarse if I could get 5 knots at 3000 even better.
boat as a guess is 3500 lbs

Top end rpm really doesnt matter to me in the big picture. Except does it seem odd to anybody else that a brand new motor and prop specifically designed for big heavy displacement hulls and slow speeds does not reach the factory specified rpm of 5500 rpm that gives it the rated 9.9 hp. Thats the only thing that kind of rattles around in my brain. I talked to yamaha and the dealer and I thought I would get simple explanation. I never did. Yamaha suggested I trim my motor up a bit. I can try that but for the motor to be perfectly perpendicular to my hull in theory it should go down but it doesn't have any more play. Just for fun I was going to try the motor on my 8 foot infaltable and see what happens. If it doesnt hit 5800 with that something is wrong.

 

[walt]

the rpms sound low** but your peak speeds seem like everything is working fine. (** I guess.. since I have never measured rpm lol..)

your measurement (peak speed)
3000 ft el / 6.5 knt

My measurement (2010 Nissan 9.8, 1990 26S)
5160 ft el / 5 pitch four blade / 6.43 knt
5160 ft el / 8 pitch 3 blade / 6.17 knt
450 ft el / 8 pitch 3 blade / 6.6 knt

edit.. woops, my 3 blade must be 8 pitch
http://www.nissanmarine.com/accessories/prop.asp

Also from that link

The selection of the correct propeller will allow the engine to obtain it's full RPM as well as the designated horsepower. Engine horsepower is very closely related to the RPM that the outboard can obtain. In other words, if the outboard can not reach its specified RPM, it cannot reach its specified horsepower.

With this boat and the 10 hp.. full hp just means jaming the boat even farther up againts the displacment hull speed with little benifit

 

[BudGates]

My measurement (2010 Nissan 9.8, 1990 26S)
5160 ft el / 5 pitch four blade / 6.43 knt
5160 ft el / 8 pitch 3 blade / 6.17 knt
450 ft el / 8 pitch 3 blade / 6.6 knt

This would lead me to believe that a lower pitch can at times yield a higher top speed. That would make sense since you do have more thrust, and probably more horse power, pushing against the limiting hull speed. I'd be interested in seeing how the 5 pitch blade performed at 450 ft.

With this boat and the 10 hp.. full hp just means jaming the boat even farther up againts the displacment hull speed with little benifit

Very true! But it is nice to know that that extra push is there incase you have to fight against a strong headwind.