SS prop noise question

[River - Runner]

My new 40 hp motor sometimes clatters when operating because of it's stainless steel prop. I was told that this was it and to me it always sounds like it's coming from the top of the motor rather than from below as I'm right on the motor - it's tiller steering. Throttle variations or slowly backing off on the throttle causes it to happen. My marine dealer and using the search feature here on this subject say this noise is common and harmless caused by the heavy stainless steel prop disengagement with the lower unit drive.<br /><br />OK - So that's the way it is.<br /><br />This is my question, thinking about it and which I haven't asked before. Why does this motor clatter when my other motor does not?<br /><br />The motor that clatters is a 2004 Mercury 40 M - tiller steering, with a Mercury SS 15" pitch prop. on a small light 14 ft. aluminum boat. The boat only weight is 160 lbs. (A picture of this boat is in my member profile)<br /><br />The motor that does not clatter is a 1988 Mercury 45 hp (Classic 50) - tiller steering, with a Mercury SS 14" pitch prop. on a medium sized light 16 ft. aluminum boat. The boat only weight is 525 lbs.<br /><br />Does this noise happen on the 40 hp and not on the 45 hp because of a difference in the motors or the motor to boat differences? Or what are your thoughts. Thanks for any replies.

 

[walleyehed]

Re: SS prop noise question

I've never heard any different noise due to a stainless prop. I would rather think you have a vibration, or harmonic result of some sort.

 

[Moody Blue]

Re: SS prop noise question

NO expert but have you tried swapping props ? I think the two are interchangeable. Try and determine if it is prop related or drivetrain related. Do you have the proper thrust washer installed for the prop ? Are you able to visually see any interference between the prop hub and gear case, trim anode etc ? Any play between the prop and prop shaft ? Just a few things to check.

 

[Dhadley]

Re: SS prop noise question

Merc sent out a service bulletin about prop "whinning" and said it was a hard noise to describe. Ask you dealer to show you the bulletin. It was for certian 25 to 50 hp models.

 

[jimmbo]

Re: SS prop noise question

With the motor out of the water and the spark plugs disconnected, spin the prop clockwise, when viewed from behind, and while doing so shift into forward. After it is in gear spin the prop again. If it locks up ignore this post. If it turns now making a racheting sound this might be the source of your noise. Merc uses a spring pressured clutch dog that stays engaged only when the forward gear is pushing torque thru it. When your decellerate the engine slows down, but the prop is now a turbine driven by the water it is passing thru. the clutch dog on the propshaft is now spinning faster the the forward gear. The design of the clutch dog and forward gear allow the dog to rachet. Not easy to explain, but easy to visualize withe gears in hand.

 

[River - Runner]

Re: SS prop noise question

Thanks for the replies. It is like jimmbo says and I am certain his explanation is exactly what is happening. I think it happens only when I am alone in the boat without other weight and happens because this boat is light and coasts when backing off on the throttle. I need more running time to know for sure. With this 40 hp motor on the small boat it is different and the power is awesome.<br /><br />This is my first question here and this forum is great. You can never stop learning about this stuff. Thanks.

 

[cobra 3.0]

Re: SS prop noise question

40 hp on a 160 lb boat! That thing must ZING! Although this may not be related, are you propped right? What's your top rpm now? Is it in the recommended range?<br /><br />Also, while I'm no specialist, I question the use of that heavy stainless prop on your light boat. I'd try an aluminum prop because I have a hard time believing that this noise is a good thing over the long haul.

 

[walleyehed]

Re: SS prop noise question

The SS prop has nothing to do with the weight of the boat, or it's use, it's just more efficient.<br />Why go back to an inefficient alum prop????<br />The SS prop is not the source of the whine.

 

[cobra 3.0]

Re: SS prop noise question

Because if Jimmbo's statement is correct on the ratcheting of the clutchdog, the heavier stainless prop's momentum is keeping it spinning faster and longer during deceleration of the motor. The lighter aluminum one would slow down faster and probably not cause the ratcheting sound.

 

[River - Runner]

Re: SS prop noise question

cobra 3.0 - I learned a lot getting this boat proped right. If you view my first ever post, I give the specifications of the boat. My dealer rigged the boat and installed a 19" pitch aluminum prop. They could not drive it W.O.T. and gave it to me and said be careful with it. I knew they would have trouble driving it because it was stripped down and it would really scoot with my 25 hp motor. I then loaded the boat like normal - front and rear electric trolling motors and battery and anchor up front. I then bought a digital tachometer for it and the rpm's were 4,200. The motor rpm range should be 5,000 to 5,500. My dealer was real good to work with and let me try any prop I wanted to. I had 5 props on, the 19", 17", 16", 15" and 14". I do not have a speedometer but the boat first seemed a little slower with the 14". I settled with the 15" running 5,200 rpm.<br /><br />The reason this light boat could not run a high pitch prop is because it is narrow and it does not get up on top of the water and skim along. It cuts through the water and tracks real solid with it's three runners on the bottom of the hull. The boat is 4 ft. wide and has a round chine - a smooth ride and a lot of fun - a muscle boat.<br /><br />They say running a aluminum prop it will not clatter, and if it does it's harmless. I'd like to hear any opinions.

 

[walleyehed]

Re: SS prop noise question

Once the throttle is set or slowly dropped to a slower speed, SS reacts no differently than aluminum. They are both strictly under the force of water pressure. The extra cup on a stainless prop will create just as much drag slowing down as a std aluminum prop.<br />I would make sure the hub isn't dragging on the inside of the gearcase. I've run SS and alum on both my Mercs and on my Rude...SS has never caused me any issues on the Mercs other than extra speed. Due to the testing of props that I do, I seldom have less than 5 or 6 props in the boat at any time, and the best thing I can say about using aluminum is it lightens the load and doesn't leave as big a ring on the carpet.<br />Back to the issue...There is NO reason to go with a more inefficient alum prop. Try another prop of the same pitch and see if it makes a diff.

 

[Moody Blue]

Re: SS prop noise question

WOW 40HP on a 14ft alum boat with a beam of 4ft. What keeps you IN the water ? Is this a tiller setup or remote steering ? Sounds scary

 

[River - Runner]

Re: SS prop noise question

The boat is a 1968 FD Alumacraft, the motor is tiller steering and the performance works out very well because the boat is narrow. That's what keeps the boat down. Up to 3/4 throttle is what I enjoy, beyond that you have to give it your undivided attention. I would never recommend someone to do this set up. Think safety first. The power and speed is at my limits and you have to know your limitations when running it. I have had this boat now 37 years and know how to operate it very well in all conditions. This boat is and will be only driven by me with the 40 hp motor on, and the motor comes off when needed.<br /><br />I will keep the 15" pitch stainless steel prop. The 16" pitch prop I had was aluminum and both ran the same rpm, meaning the 15" is more efficient.<br /><br />About the clatter - When I got the motor in Oct. of 03 and noticed the noise, the dealer said he had gotten a bulletin that stainless steel props cause it to happen, and nothing is to be done about it. His mechanic spun a prop on a motor for me by hand when in gear, and the ratcheting sound it made he said was the noise, and it is harmless to the motor.<br /><br />This noise happens sometimes to me with the 40 hp motor but never with the 45 hp motor, and getting back to my question of why?<br /><br />There is your concern about the prop rubbing on the housing. No it is not, and everything is factory perfect. I think it is happening on one motor and not on the other, is because of the motor to boat differences. I don't know if switching to different stainless steel props would make a difference, because they say it's the weight of the prop is what does it. It's all to be considered though.

 

[moderator1]

Re: SS prop noise question

I was just looking out my window before reading this and I have a comparison for you...<br />Consider the vanes of a wind-mill..Let's say those vanes are made of metal-what-ever type...and have a certain "Pitch". in a 20mph wind, that "pitch" allows "X" speed of the vanes.<br />Ok, now lets make those vanes out of a light wood, such as balsa, same shape and pitch...in a 20mph wind, the speed of the vanes will be "X"....the same as the metal blades because the "pitch" is the same. The vanes are being moved by the current flow across them, the same is true of your prop when reducing throttle...momentum is not turning the prop, the current flow through the blades is turning the prop when slowing down.

 

[jimmbo]

Re: SS prop noise question

Perhaps the 45 has a different type clutch dog, a type the has positive engagement in both forward and reverse and therefore doesn't rachet. The 15"SS vs 16"Alum pulling same rpm is probably due to more cup on the SS. It's not the weight of the stainless that could affect the sound but more likely the differences in mass.