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Go Back   SailNet Community > On Board > Gear & Maintenance > Engines > Gas > Atomic 4
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  #31  
Old 11-28-2011
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Quote:
Originally Posted by dabnis View Post
Perhaps an alternative to modifying the engine could be experimenting with props. I would think one would want to have as much prop surface available as diameter would allow, perhaps a larger diameter with less pitch? If diameter is limited perhaps 4 blades with less pitch? It appears the A4 makes about 30HP?, the problem appears to be getting the power efficiently transfered (thrust) into the water, kind of like having little skinny tires on a car with 300 HP in it? Dabnis
That seems to be what they've done with that Indigo three blade. Also, the 30HP figure for A4's is maximum rated output, not real HP at useable RPM. I suspect that at the RPM that most A4's run at in the real world, they are not producing more than 10 or 15 HP. Also, as as been noted previously, in a boat particularly, torque, not HP is what counts.
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  #32  
Old 11-28-2011
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Originally Posted by lillia28 View Post
Well like the farmer said the first time he saw ballerinas toe dancing: If they wanted taller girls, they should have hired taller girls.
At some point it is easier and cheaper to sling in a diesel than to custom build a a-4 with a custom transmission. I know with Airplanes, which face similar constraints, they end up installing huge 500+ cubic inch motors to get 250 HP, low enough to run a propeller without a gearbox. Besides if the 9.5 inch prop is getting you to hull speed, what is the advantage of the bigger prop? Lou
I had no intent of going anywhere near the extremes you mention. Changing or adding reduction boxes etc. are way beyond the scope of what I had in mind. Obviously spending more on an A4 than changing to a diesel would be foolish.

I was merely curious about what effect the mods I mentioned would have when incorporated into an engine that was ALREADY being rebuilt. For the most part they would only add some work to the rebuild, not a lot of cost (other than the Moyer upgrades mentioned)

As to hull speed, if your engine will attain hull speed against a stiff wind, tide and/or current, then agreed, no benefit. However, around here, there are BIG currents, BIG tidal flows and sometimes, even big wind. I doubt a stock A4 will push a 10K Lb. boat against them anywhere near hull speed.
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Sloop, on Moyer's site they refer to about 2,400 RPM as being a "Good" cruising engine speed:
Moyer Marine Atomic 4 Engine Rebuilding and Parts

I guess that if one could pull the biggest prop (diameter or maximum pitch without cavitation) at that speed that is about all you can ask of the engine in stock form? As mentioned earlier the engine may not be the most robust in stock form? The modifications you mentioned would increase pressures, hard to tell exactly how much?
One way to tell if the engine is up to the mods is to try them and see if it stays glued together. Anything else may be an educated guess?

Dabnis
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Old 11-28-2011
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The A4 has no issue on the 8000# Cal 29 with a 12 X 8 prop normal speed is 1400 RPM and 6 knots it will reach over 2000 RPM and stern squat the boat about 18" deep at which point your going 1 knot faster and burning 3 times the fuel

I have to think this is OK as its 41 YEARS old with only a valve job BUT i will be changing to a 12 X 7 FLEX-O-FOLD this winter as the feel there 12 X 8 will have more bite that the current 12 X 8

On a 1970 C&C 35 again there are a bunch still A4 powered without issue and my friends 1970 C&C 35 does just fine with the 12 HP BETA twin cylinder diesel

If you ever had to deal with one of those GIANT hand crank or common belt driven starter/generator old style diesels on a cold day you would think much better of the A4
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Last edited by tommays; 11-28-2011 at 07:01 PM.
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