Knocking noise at full speed

[mtbeatle]

Hi there,

I have just bought a boat and tried it out for the first time today.

It is a Pioner Maxi with a 15HP 4 stroke Yamaha outboard. It is about 7 years old.

There were four of us in it to start with and it seemed to work perfectly from slow to full power.

I then went out in it myself so it was obviously a lot faster (about twice the speed) and when I got up to full speed something made a 'knock knock knock...............' noise.

Im not sure its the outboard itself but was certainly coming from the rear of the boat/engine/propeller area.

Any ideas? Is this something to worry about?

Many Thanks

Craig

 

[mtbeatle]

Re: Knocking noise at full speed

Hi,

It has been suggested that because my propeller has a few chips on it and one of them fairly big, that this could be the cause. Any thoughts?

Thanks

Craig

 

[robert graham]

Re: Knocking noise at full speed

Better check it out before running it any more. Is the motor actually running good and strong, but just knocking? Check the condition of your spark plugs for any signs of detonation(maybe lean fuel mixture caused by plugged carb), or very white/clean look to the plugs. You just don't want to chance frying a piston due to detonation/overheating caused by lean fuel mixture. I'm not sure if a chip in the prop would cause a "knocking" sound, but it's worth a look-see. Good Luck!

 

[BaileysBoat]

Re: Knocking noise at full speed

Are you sure you're not dragging a mooring line? A metal clip at speed makes quite a racket banging the hull.

 

[mtbeatle]

Re: Knocking noise at full speed

Hi,

Thanks, definatley not dragging a mooring line........

The outboard was running strong and apparently perfectly with the four of us in it at full speed. When I got in it and ran it at full spped by myself however the knocking started. It only stopped when I eased off on the throttle slightly.

This causes me to think it has to be something to do with the speed I was travelling at as I was probably going twice as fast on my own as opposed to having an extra 3 people in it.

It is a four stroke engine so it wouldn't be effected by a 'lean fuel mixture' would it?

 

[robert graham]

Re: Knocking noise at full speed

Lean mixtures are bad for any motors. I'd still take a look at those spark plugs for signs of lean or overheating, just to be sure. The knocking could be in the lower unit or drive shaft...maybe drop the lower unit and take a look. Maybe a warn drive shaft bushing?

 

[robert graham]

Re: Knocking noise at full speed

Have you checked your engine oil level?...maybe too much oil in the crankcase?

 

[Silvertip]

Re: Knocking noise at full speed

I don't know if that engine has a rev-limiter on it but if it does, the engine may have hit the rev-limiter which could give you the impression something is wrong. A rev-limiter activates when the engine rpm reaches some pre-set rpm (use 6000 RPM as an example). With a full load of people, the engine cannot reach 6000 rpm so things appear normal. With just you in the boat, the engine now has much load and can rev more freely -- too free in fact so it reaches the 6000 rpm mark and the rev-limiter activates to prevent the engine from spinning faster. It does this electrically by interrupting the ignition. That makes the engine sound funny. The only way to know if this is the case is to install a tachometer. For your engine I would recommend a Tiny Tach which is self contained, self powered and has only one wire to connect.

 

[Chasmorgan]

Re: Knocking noise at full speed

I had the same knocking on my yamaha 15hp 4stroke after installing a new 4 blade solas 9.25 x 8 prop. When I replaced it with the original 3 blade prop the knocking went away! Just to be clear the knocking with the 4 blade was only at full throttle. I could back off slightly and the knocking would go away. Hope this helps.

 

[robert graham]

Re: Knocking noise at full speed

I had the same knocking on my yamaha 15hp 4stroke after installing a new 4 blade solas 9.25 x 8 prop. When I replaced it with the original 3 blade prop the knocking went away! Just to be clear the knocking with the 4 blade was only at full throttle. I could back off slightly and the knocking would go away. Hope this helps.

That sounds like it could just be your problem!....You might not think just a prop could do that, but maybe?..live and learn!

 

[mtbeatle]

Re: Knocking noise at full speed

Thanks for everyones help.

A few others have suggested the engine is reaching the rev limit. I can confirm the engine does have a limiter.

It seems the most popular suggestion is to try a new prop.

I currently have a 9.25 x 8 fitted so I think I will try a 9.25 x 9 or 10

Thanks again !!

Craig

 

[Chasmorgan]

Re: Knocking noise at full speed

Upadate. I visited with my local prop shop where I bought my four blade solas prop about the knocking noise and he too suggested that my motor might be over reving at wot. He asked if the prop was otherwise meeting my objectives of better hole shot and better load carrying capacity ie: two adults. I confirmed that both objectives had been met. He suggested he put a little cup in the blades to lower the rpms. It took him about three minutes. I'll report the results when the weather warms up.

 

[99yam40]

Re: Knocking noise at full speed

I would think the easiest way to tell if it was over reving would be to hook up a tack to it and run it to see what WOT RPMs were.
That would help you decide if a prop change was needed and tell you how much of a change

 

[Silvertip]

Re: Knocking noise at full speed

You sholdn't need a tach to tell if the rev-limiter is being hit. The engine begins to miss and a popping noise is heard at roughly half the frequency of engine rpm. Since you know what prop pitch is, you know what the lower unit gear ratio is, and you know what the rev limter rpm is, run those numbers into a prop calcualtor using about 12% slip and that would tell you what the "theoretical" engine rpm is. Bet it comes out close to the rev-limter rpm.

 

[Chasmorgan]

Re: Knocking noise at full speed

Well with the cupping the knock is gone! I installed a sendec tach and at wot it read 5200 rpm. I lost a little top end but otherwise I'm happy.

 

[robert graham]

Re: Knocking noise at full speed

Glad you got it fixed with the prop, but I'm still curious exactly what was making the knocking sound?....maybe some kind of aeration or cavitation around the blades of the prop???

 

[99yam40]

Re: Knocking noise at full speed

5200 may be a little low on those 4strokes, what is the WOT spec for that motor?
If you lost top speed and RPM then your motor is struggling to get there and may cause a problem.
adding a little cup should have increased the speed, but only if you can keep the RPM. May be a little too much cup

See what a set up like yours ran and what prop they used
http://www.yamaha-motor.com/outboard/products/perfbull.aspx

 

[robert graham]

Re: Knocking noise at full speed

The book says 5000 to 6000RPM's WOT, so 5200 is within range....

 

[99yam40]

Re: Knocking noise at full speed

Yes, but they say it is best to prop it to the top end of the spec, and it also depends on how it was loaded when he tested the RPM. If it was all by his self then it will be way off when he loads it up with more people and gear.

I would prop to high end with a light load and then check what it does with a max load or at least the load you normally run to make sure it can still get withing spec at WOT. do not want to cause motor problems if you can help it