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Thread: Mercs and megaphones--thinking the unthinkable?

  1. #51
    modifiedoutboard OUTBOARDER's Avatar
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    Quote Originally Posted by zul8tr View Post
    The basic physics relation (Drag proportional Velocity squared and Power proportional to Velocity cubed) I stated still applies, it's the K that is variable with different conditions of boat operation. You stated that "The boat is dragging less in water" which is contradictory to what I stated in my posts above and implied when using the formula that "all else remaining constant". "K" is NOT constant. The calculated value based on a single velocity and Hp was used for the increased Hp to estimate velocity to keep consistent with your approach. There are many variable conditions going on to apply a single K value to all operation conditions.

    Relations noted here (K usually known as Cd - coefficient of drag):

    http://en.wikipedia.org/wiki/Drag_(, physics)


    Examples for drag and power (air drag used here but same applies to water drag and Cd still not constant but used that way for easy relative to compare calculations)

    http://phors.locost7.info/phors06.htm

    Let's get this post back on track regarding megaphones!

    once again for planing hulls that run high speed = k * sqrt of hp
    more than one boat builder has used it to predict speed of a boat for a change in hp

    let me go one step further with my real life john boat experince
    15 hp = 25 mph
    25 hp = 36 mph
    50hp = 44mph over powered Carolina skiff J16 with 1998 Johnson 50 hp went 44 mph

    as in real life John Boat example
    k can be treated as constant over a small interval

    the cubed HP is for Discplacement hulls like Ships.
    Last edited by OUTBOARDER; 05-13-2014 at 09:41 PM. Reason: cause
    ANTHONY McCULLOCH
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    They Want it cheap..............

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    Quote Originally Posted by GHMiller View Post
    Jeff, your are correct. That is what I'm trying to figure out. I think that is where the "black magic" comes into tuning with megaphones. Once you hit on the "right" porting, flow, carburetion of your set up, now how does your megaphones affect all of this? By changing length vs. cone degree you need to get the power band tuned into where you desire the rpm range to be. With our snowmobiles this was easier because all we had to do was change the clutch settings to obtain the desired power band to rpm ratio. But with outboards you are somewhat limited to what changes you can do. I believe that is the obstacle with tuning an outboard whether it is an expansion chamber or megaphones.

    So if I understand this right, by having a short wider cone you raise or have a narrower power band with higher rpm's? Thus increasing horse power but not torque? By having a narrow longer cone then you would have a lower or flatter power band with lower rpm's? This would increase your torque but not horse power? Where would your compromise be? Maybe a longer but wider cone?
    Frank Volker has a new thread about a flathead on the Quincy dyno. He says the pipe temperature fluctate between high on the first pulse to low on the suction wave. He thinks temperature readings are to problematic to be much use.

    Yesterday Mercedec Formula 1 tried a megaphone on the turbo outlet ( looks short with wide divergence ) to increase the noise. It didn't do much.

    It does however beg the question, Ars all racers just 4 year old boys, sitting in a cardboard box and going vroom, vroom,vroom.

    John

  3. #53
    modifiedoutboard OUTBOARDER's Avatar
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    Default Pipes are Colder on water or Vacuum is assisting scaveging or a combination of both

    Quote Originally Posted by Dixon263 View Post
    There is something that I've never been able to figure out and never heard a good explanation either. Here's the deal, I own a Stuska water brake dynamometer (130 hp max, 14,000 rpm) and have dyno'ed many four cylinder 44 cu in Mercs in quiet form. They dyno real close to 40-41 hp in stock, quiet form with a great horsepower curve. I chose to dyno directly off the driveshaft to reduce unneeded wear on the "D" Quicksilver or 45SS unit. A friend with a 12' 6" hydro saw 79 mph many times but never broke 80. We put his motor on the dyno and pulled 40.6 hp in the 6500 rpm range consistently. Then we installed a set of Parker megaphones and saw 57 hp with a wide power curve that was right in the range the motor would be run at for top end. A trip to the water saw some odd results, 79 mph and not a tiny bit faster. The pipes still made good hp to well over 7000 so we were definitely in the higher hp range. With megaphones, at top speed, the motor was developing close to 50% more hp, but was not able to go ANY faster.

    Before I present a possible theory, here is another example. I modified a '44 with single ring Turner pistons, mild porting used the larger KC carb instead of the KA's. The dyno showed 46 hp, in quiet form again. A trip to the water showed a max top speed of 85 mph, quite a difference from my usual 78-80 bests. So what is going on here? These tests are not flukes but consistently this is the way it will go every time. So here is a possible theory. After decades of racers using megaphones on Mercs, is it actually possible that somehow the high speed air that flows across the pipes is cancelling the 50% gain shown on the dyno? I realize that this a big pill to swallow, but something is really wrong with this picture. 50% more power being applied to the prop at top end and not any more speed at all? I have a friend that manufactures the 'Hurricane' 27 hp industrial leaf blower that will move a large volume on air at 160 mph, I guess it would be interesting to set it up in front of the dyno with a megaphone Merc running at 57 hp and turn the air on. Any other ideas on this one?
    spray some water vapor also???
    Pipes are Colder on water [(apart from water injection) watercraft tune jacketed exhaust by manipulating water temp surrounding pipe]
    one way to correlate engine performance under both conditions aside from conjecture would be a egt gauge.
    ANTHONY McCULLOCH
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  4. #54
    Sam Cullis Mark75H's Avatar
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    Quote Originally Posted by Dixon263 View Post
    There is something that I've never been able to figure out and never heard a good explanation either. Here's the deal, I own a Stuska water brake dynamometer (130 hp max, 14,000 rpm) and have dyno'ed many four cylinder 44 cu in Mercs in quiet form. They dyno real close to 40-41 hp in stock, quiet form with a great horsepower curve. I chose to dyno directly off the driveshaft to reduce unneeded wear on the "D" Quicksilver or 45SS unit. A friend with a 12' 6" hydro saw 79 mph many times but never broke 80. We put his motor on the dyno and pulled 40.6 hp in the 6500 rpm range consistently. Then we installed a set of Parker megaphones and saw 57 hp with a wide power curve that was right in the range the motor would be run at for top end. A trip to the water saw some odd results, 79 mph and not a tiny bit faster. The pipes still made good hp to well over 7000 so we were definitely in the higher hp range. With megaphones, at top speed, the motor was developing close to 50% more hp, but was not able to go ANY faster.

    Before I present a possible theory, here is another example. I modified a '44 with single ring Turner pistons, mild porting used the larger KC carb instead of the KA's. The dyno showed 46 hp, in quiet form again. A trip to the water showed a max top speed of 85 mph, quite a difference from my usual 78-80 bests. So what is going on here? These tests are not flukes but consistently this is the way it will go every time. So here is a possible theory. After decades of racers using megaphones on Mercs, is it actually possible that somehow the high speed air that flows across the pipes is cancelling the 50% gain shown on the dyno? I realize that this a big pill to swallow, but something is really wrong with this picture. 50% more power being applied to the prop at top end and not any more speed at all? I have a friend that manufactures the 'Hurricane' 27 hp industrial leaf blower that will move a large volume on air at 160 mph, I guess it would be interesting to set it up in front of the dyno with a megaphone Merc running at 57 hp and turn the air on. Any other ideas on this one?

    I agree, the most likely culprit is the prop ... or possibly something else in the set up. It doesn't have to be a speed induced drag problem; there are lots of things that can cause a boat to "hit a wall" speed-wise.

    Set up for circle competition most 44's are propped for 8400 rpm ... might be running above the power peak to gain some acceleration.

    If you propped for exactly the power peak, you are probably propped too slow. If you are not running a spray shield/baffle in front of the carbs, 80 mph intake might be affecting the carbs.
    Since 1925, about 150 different racing outboards have been made.


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    Sam Cullis Mark75H's Avatar
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    Quote Originally Posted by champ20B View Post
    One thing I used to think about what really happens inside a megaphone is......
    I think you should read the Brinkman explanation until you understand it.
    Since 1925, about 150 different racing outboards have been made.


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    Quote Originally Posted by Mark75H View Post
    I think you should read the Brinkman explanation until you understand it.
    Key words: (I used to think), as to what happened. I have other ideas too. But no one really knows the exact answer.

    Afterall, it is still as good of a theory as any. They are all really no more than theories when one gets down to it, particularly when it comes to megaphones. As similarly stated on another reply, "if the two (Ps) [props and megaphones] were understood, one could win every world championship".

    I did, however, come up with a nifty way to find the right exhaust tune potentially.

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