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Author Topic:   Engine Resonance
jimh posted 07-12-2001 08:32 AM ET (US)   Profile for jimh   Send Email to jimh  
We are just back from 6 days of boating, which gave me the chance to finally get to know the new boat a little better.

I notice that the engines make a particularly abrupt change in their pitch around 4000 RPM. If you run at 3900 RPM the boat is on plane and the engines are rather quiet. Advancing the throttle to 4000-4100 RPM produces a modest increase in speed, no change in hull attitude or trim, but the engines begin to have a growl to them, making noticeably louder sound than they were just 100 RPM slower.

This growl persists until about 4300 RPM, when the engine note loses some of its edge and the sound goes back to a less raspy note, although not as quiet as at 3900.

It was suggested to me that this is perhaps being caused by a resonance in the air inlet silencer, a plastic manifold or baffle that covers the carburetor inlets.

Another suggestion was some type of resonance in the reed valves.

Any 2-cycle engine guru's with an opinion on this?

Bigshot posted 07-12-2001 09:35 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Engine is working harder, therefore louder. Some people will say to run a boat at lets say 4100 rpms for optimum cruise. I say find the sweet or happy spot. My happy spot on my Montauk with the 90 Yamaha is 36-3700. It is sooo quiet, boat has a great attitude, running 28mph on Gps and runs on fumes. Once I get to 38+ it gets louder so it is not as happy. Then as you increase it gets quieter due to a couple things. The hull is really planed out and engine is working less, and the wind is quieting things down plus you sometimes are slowing downdue to pulling the throttle back from 5000 to 4600. Sometimes what seems to be a second happy spot will slowly get louder as you settle in on that rpm. The faster you run it, the more gas you will burn and the shorter the life span on the engine. Does not mean it will run forever at idle. Ballpark though, run WOT constantly, 300+ hours. Run 3/4 1000+. Usually life expectancy on an outboard is around 1500 hours before they start getting tired. My 225 Ocean runner likes 37-3900. My 115 Merc liked 4200. My 140 suzuki liked 4100. Depends on the boat/engine setup but find that happy spot and use it.
SuburbanBoy posted 07-12-2001 10:32 AM ET (US)     Profile for SuburbanBoy  Send Email to SuburbanBoy     
For those interested in the nitty-gritty of two-stroke engines, I suggest any of Dr. Gordon Blair's texts. One of his books even has an outboard engine on its cover (but it is NOT just pertaining to outboards). The following link to the SAE website will help. Two-strokes are extremely sensitive to intake and exhaust tuning, with (as I remember) acoustic energy playing an active part in the tuning.

If the link fails, try searching on blair at

Blair’s bio (from the SAE website)
“About the author...A world-renowned expert on the two-stroke cycle engine, Dr. Gordon P. Blair is currently Professor of Mechanical Engineering at the Queen's University of Belfast. Professor Blair has served the University as Dean of the Faculty of Engineering and as Pro-Vice-Chancellor. In addition to his teaching and research work at the University. Professor Blair has been a consultant for many well-known companies such as Ford, General Motors, Mercury Marine, Volvo, and Yamaha. He has been named a Fellow of the Society of Automotive Engineers, the Institution of Mechanical Engineers, and the Royal Academy of Engineering. Professor Blair has been recognized by Her Majesty The Queen by the award of a CBE (Commander of the Order of the British Empire).”


Whalerdan posted 07-12-2001 01:18 PM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
Bigshot, where did you get those numbers on hours? Sounds pretty consertive to me, only 300 hrs at WOT.
Bigshot posted 07-12-2001 02:57 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
That consumer newsletter that does REAL reviews of boats over long term. Can't think of the name but is in black and white. They actually said like 250 hours if you run it pegged the whole time. They were mainly discussing bass boats and why the tournament guys get new equipment every year. Mainly because it is sponsored but also because they are beat from being run WOT. Believe it or not same thing applies to diesels, etc. Anything ran WOT all day does not live a long life. I have seen outboards with 2000+ hours on them but that is usually not the norm. Most motors will be destroyed by water ingestion or burn a piston from a lean carb before they "wear" out.
Whalerdan posted 07-12-2001 03:23 PM ET (US)     Profile for Whalerdan  Send Email to Whalerdan     
"Real Review?" I have a hard time believing someone sat in a boat for 250-300 hours with the throttle wide open.....OK I'm not that dumb but my point is I don't always trust "Consumer Report" type reviews. I'm sure running wide open causes more wear on an engine than, say 60 or 70 percent. But I still have a hard time believing it reduces it THAT much.
Bigshot posted 07-12-2001 03:49 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Ask around! A motor at WOT will last about 10% of the amount of time it should go. Ie: 5000 hours is expected life, it will last 500. The optimum life on a diesel is 90% throttle which can be a hundred or 2 rpms from WOT. Outboards are 70% throttle so 37-4200 rpms is max life. In that theory they should only last 150-200 hours but you can't obviously drive pegged all the time so 250-300 is reasonable. The consumer report mag is not some guy getting $6/hr driving pegged on lake x for 250 hours. They get their info from manufacturers and peoples testimonies. They also do the buyer satisfaction reports and Bayliner always comes in last even though they sell the most amount of boats(go figure). I hear the main problem with running WOT is the reeds get week, causing lean problems, etc. Mercury and yamaha, etc. have engines sitting in tanks pegged for weeks or months so they can see what breaks first and improve on it, I assume that is where their data comes from.
lhg posted 07-12-2001 07:21 PM ET (US)     Profile for lhg    
I have found that OB's last much longer if they run at lower RPM. This is one of the reasons I have overpowered Whalers. I can get a reasonable cruising speed, while the engines are loafing along. Hardly ever run them wide open, usually between 2000 & 4000 rpm max. On our recent trip with JimH, my engines were running around 2400rpm, while Jim's smaller 70's had to turn 4000-4500 to keep up. That's twice the piston wear, which has to translate to a shorter life.

Getting back to the resonance, I was told it's in the reeds. Can anybody else confirm or refute this?

Bigshot posted 07-12-2001 09:22 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Usually I believe it is the reeds. What size you running? I am very happy with my 90 Yammie. I had a 115 on another but it was a lot more thirsty. Top end was probably 45-47 where my 90 is 42-44. If I ever step up to a 4 stroke I think I will go 115 due to it is a 4 stroke so the lower the revs the better. You do not want to lug them either. This aint a highway with overdrive. Low rpm's can lead to lost powercurve and more stress, but highly unlikely in a 1000lb boat.
counterstrike posted 07-14-2001 10:22 AM ET (US)     Profile for counterstrike    
Could the level of sound increase be caused or not to say "caused" but the result of the engines reaching the begining point of the "power band" for that particular set up you have?
As the engine('s) reach the designed HP @ the tested rpm, the engine('s) are working harder to push the boat from dead to off plane, and as you increase your rpm and edge up to the lower end of the PB your engines are at a performance edge that the spark timing is at such a point that the combustion is not taking place at the preimium place in time, this can cause the engine to have a louder exhaust (timing is advanced and some of the tail end of the "flame front"in the combustion chamber is exiting through the exhaust). After the engines have passed this point in RPM the engine('s) RPM and timing change ever slightly and the engine('s) are now running at a more efficent power band pushing not as much weight (plaining) and the amount of air intake and spark advance is at a preimium.

could it be?

Bigshot posted 07-14-2001 09:07 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
where2 posted 07-14-2001 10:03 PM ET (US)     Profile for where2  Send Email to where2     
I would actually have guessed the original post had the answer "It was suggested to me that this is perhaps being caused by a resonance in the air inlet silencer, a plastic manifold or baffle that covers the carburetor inlets." The source of the resonance would be the reed valves.
Having listened to the growl of a '73 50Hp Evinrude running anywhere from idle to full throttle tied up at the dock (engine turning a 4 blade 9" pitch blade!), I can certainly say the reeds funneling through the carb venturis will make quite the racket. The silencers helped alot, but dad never liked to put them on since it was something to take off to tinker with the carbs. (there's more to this story, but I'll save it for another thread some day).
I vote for reeds and resonance.
counterstrike posted 07-14-2001 10:45 PM ET (US)     Profile for counterstrike    
I hope that jimh is talking about a *New('er)* set of OB's and not an older set. If the reeds on the new plants are that noisy then Im sure there would be a power loss and unusualy high fuel comsumption.

If there is not enough exhaust pressure running out the pipe to "pull" the intake fuel/air into the case you can get a "noisy" set of reeds (low compression = low /slow flame). Exhaust flow is very important on a [2-stroke internal combustion engine], too much back pressure and you get recycled combustion gas creating NOx and other corrosive by-products decreasing HP and increasing GPH.
If you have loud reeds on yer '73 ya might need to replace the Backers or the reeds themselves, and check to see if the Exaust ports are carbon'ed up.

Easy boy :-)

jimh posted 07-15-2001 09:32 AM ET (US)     Profile for jimh  Send Email to jimh     
The engines are 1987 vintage Yamahas, but to judge from their immaculate appearance, they do not have many hours on them.

We are just now developing a feel for fuel consumption rates, having run a little more than a tank of fuel (77 gallons) through the engines.

Interesting to me was the comment from Bigshot who mentions his 90-HP Yamaha likes to run at 3600-3700. My engines are similar 3-cylinder Yamaha 70-HP. Perhaps this is the "sweet spot" for this design.


counterstrike posted 07-15-2001 12:06 PM ET (US)     Profile for counterstrike    
Your most likey correct on that JH, You should "play" around with varying loads and find the "sweet" spot for each, Mark you RPM's, MPH and (GPH if available), Noting the angle of attack on the hull as well.
As for the Engine Resonance, I wouldnt worry to much about it. If you don't have carb covers on you might want to locate a set as they would keep unwanteds out of the carbs and engine,. If the Engine Resonance is that bad, you could locate some sound deadening material for the coweling's as this would quiet the engines down alot.


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