Outboard motor question.... Your opinions please..

[Fish Chris]

Okay, here's the deal; I had owned my old 12 ft aluminum, with a Honda 9.9 4 stroker, that I bought brand new, for 10 years. Lord knows, I put a TON of hours on that little motor. It was always dependable, but it was always the slowest darn 12 ft'er on the water. In fact, it often had a hard time getting plane with just my fat butt, and my tackle. Throw in another guy, and forget it.

Now call me a slow learner, but it took me all 10 years to finally start to feel like maybe my boat would have benefitted from a lesser pitched prop. I never did have a tachometer (because I didn't find out about those little easily attachable tach / hour meters until a year ago)

So, I ended up giving that boat and motor to a friend (on the stipulation that I could still use it at Bodega Bay next Summer..... Don't want to put my new baby in the salt).

So anyway, here's my question..... After all of the thousands of hours on that old Honda, with darn near every one of them ran at the very close to the same top RPM..... even if that RPM was way lower than optimum..... would it be a good idea to turn around and drop the pitch down 1, 2, or 3 increments (depending on what the tachometer says its getting right now) and maybe bring those RPMs up 500, 1000, or even more than it has ever ran, in its whole long life already ? I know this sounds like a scary proposition, but keep in mind, if it were only able to get say, 4300 RPMs before, at full throttle, technically speaking, this is putting a lot more load on the motor, than a lesser pitched prop, which allows it to run up to 5500 RPMs (peak performance for most out boards)

I guess in a nutshell I'm asking, is it better for this motor to run at a higher load "that its used to", or to jump up to an RPM level that its not used to, but which produces a much easier load on the motor ?

Just trying to decide if I should recommend to my buddy, to get a tachometer, and possibly changing the pitch on the prop, after all these years ? He often goes out with his brother, and I'm sure it would really help to get them up on plane......

Your opinions please....

Fish

[Bass Smacker]

   I don't know about out boards. But I do know that if you buy that cheery car from grandma.

Yopu know the onr that she only drove on Sundays and never over 35mph and tri and hot rod the motor. You will pop the upper compresion rings on the ridge in the upper cylinder.

But on a boat motor I have no clue

         

[Catt]

Fish Chris

Given the cost of buying the tachometer and a new prop I serious doubt you see any major benefit on a 9.9/4-stroke or 2-stroke for that matter. Now if you friend is not too concerned with cost I would also suggest looking at put new reed plates under the carburetor. Another concern with any outboard is getting it set at the proper height on the transom.

[RatONaStick]

I was in the exact same situation with my little boat and a 15hp Johnson 2 stroke, which I'm sure is much older than your Honda. I went from a 9.5"x10 prop to a 10"x7 and it made a world of difference!!! The boat only does 14mph or so and was way over propped. Like you, it had a hard time getting on plane with just me and my gear, with two people it would plane sometimes or not at all. Now with the new prop it literally jumps out of the water with just me and will plane with two people in all situations.

To answer your question, it is better for the engine to be in the proper rpm range with a lesser load. That engine was designed to run in a specific rpm range, it will not hurt the engine by allowing it to run in that range, even if it hasn't seen those rpms yet.

Don't get caught up thinking the engine is used to running at a certain rpm, it isn't true. As long as you don't over rev the engine it will not cause any harm.

[Fish Chris]

Ya' know RatONastick, you don't have to convince me how much difference a pitch or two can make ! I learned that right off the batt, with my new Yamaha 25hp ! Crazy as it may sound, 1 pitch drop, causes an 800 RPM increase on my new boat, bringing it right to the 5500 peak performance RPMS's, at 80% throttle.

My only concern was, that my old Honda might be so used to being over-propped, and never seeing more than 4300 or 4500 RPM's, that even raising it to the "proper RPM's" might be a shock to it.....

But my auto mechanic said the same thing you did. In fact, he said that because of being overloaded, and "under revved for so long, it might be pretty carboned up too. He said that allowing it to spin to what it was designed for, might even clean out some of that carbon buildup, and cause it to run much better than it has in years ! I hope it doesn't run "TOOO" much better.... Or else I might be kicking myself in the arse for flat giving it away ! :-) ....not really. He's a cool kid.... Plus, I can still use it for certain stuff.... or heck, even as just a back up, should my new boat ever need to go to the shop for anything.

Thank you guys,

Peace,

Fish

[RatONaStick]

Your mechanic brings up a good point. When engines are lugged below their power range there is a tendency for them to build up an abnormal amount of carbon deposits.

There is also a tendency to go through spark plugs real fast.

I couldn't believe how much better my 15hp Johnson ran through the entire rpm range after the prop change, plus it's not eating plugs anymore.

If/when you do change that prop, go ahead and change the plugs too.

Stick a big one for me!  

[HPBB]

Had a problem like that with my 15/9.9 Merc on a 15 foot boat. It just wold not plane and always seemed to be down in RPM. I don't have a tach, but the GPS MPH was only flirting with 10mph. Firts thing I did was get a set of DOLEFINES, those hydrofoil fins that go on the lower unit. They look like little wings. well that made a huge differenace, boat planed fast and ran better, and at only $30 a lot cheeper than a prop. I also got one of those mini jake plates top raise the motor up. with these changes the boat runs around 15 mph now and the rpm's are up.

Jim

[Fish Chris]

Hello HPBB. I agree with you that those hydro-foils make a HUGE difference....... Only problem is, I put one of those on the old Honda within the first few weeks of its life, and while it certainly helped it get on plane, it was still always the slowest 12ft aluminum on the water.

With my new Yamaha G3, I put a 'fin' on it in the first week also, and while it might have helped it just a little bit with planing, the fact is, my new boat has a 25hp, which is the max for my 14 1/2ft aluminum, and so I can pretty much power it right up on plane, even without adjusting my power trim. But changing the prop pitch made a MUCH larger difference with my RPM's.

BTW, you can get pretty decent alminum props for these smaller motors, for only $50 or $60..... And heck, I think its a good idea to have an extra prop (your old one) around anyway.

Peace,

Fish

[ThomasL]

Fishchris,where are you finding those props,i have a fourstroke merc 15 hp and

i believe it has a 9 1/2x10 prop.I have tried to find stainless for it but cannot.

[Fish Chris]

Hey Thomas, iBoats.com has tons of props, both aluminum, and stainless. I have a Michigan Wheel Vortex aluminum on my Yamaha, and I love it.

Peace,

Fish

[mudcatwilly]

You're better off keeping the rpms as close to optimum as possible.  If you over prop the motor, you could eventually tear up your lower unit.  That was advice I got from my boat mechanic.