|Topic Review (Newest First)|
|11-28-2011 06:37 PM|
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
|11-28-2011 05:05 PM|
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?
|11-28-2011 03:50 PM|
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.
|11-28-2011 03:41 PM|
Originally Posted by dabnis View Post
|11-28-2011 01:30 PM|
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?
|11-28-2011 12:34 PM|
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?
|11-27-2011 10:57 PM|
|11-27-2011 08:38 PM|
|lillia28||I had thought you were kidding. I did not mean to offend. To drive a bigger prop, you need more low end torque, not high end horsepower. Or, as has been mentioned, you could change the gearing of the transmission. I just picked up Don Street's "Ocean Sailing Yacht" and he talks about the British Sea Gull outboard, which has a really low gear ration and swings a big prop and generates a lot of thrust. In short you want to increase displacement (bore and stroke) to increase torque. Props are most effective, roughly speaking, below 2,000 RPM. Increasing horsepower by the means you discussed, will allow the engine to breath better, rev higher and produce horsepower at the top end, exactly where you don't want it for a propeller. Typically when you increase horsepower, you have less at lower rpms and more at higher RPMs, leading to the "peaky" nature of racing engines. The best torque producers are big single cylinder thumpers. That's why many boats had an optional 10 HP diesels like the Yanmar YSM 12 (1 cyl), vs. the 30 HP Atomic (4 Cyl). Less HP, more torque and a lower gear ratio. This is why tractors have four cylinder engines and Formula 1 cars have v12s and the like. For planing hulls, you can make use of high horsepower, as drag is reduces when on plane and out of the water, but for a displacement hull once you are at hull speed, drag rises dramatically and the bow wave gets bigger, but speed doesn't increase. Both the Macgregor 26M and the Hunter Edge have a planing hull so they can use the 90 hp outboards. My 28 displacement hull will get to hull speed (6.4 kts) with 10 HP. No matter how much additional horse power it won't go faster than 6.4 kts. The YSM in my boat swings a 13 inch propeller, vs. the 9 inch you quoted for the Atomic. I hope this makes sense.|
|11-27-2011 07:44 PM|
Originally Posted by SloopJonB View Post
|11-27-2011 05:31 PM|
Actually, answering WHY is easy. What made me think of it was reading a thread about props. Apparently an optimized A4 prop is 9X7 - that is barely more than the blade in my wife's hand mixer. I was simply thinking about some mild mod's that would increase the torque to a level that would allow it to swing a decent sized prop while still keeping the RPM down.
Thinking about all the 33+ footers around 10K Lbs out there with A4's, swinging a 9X7 prop seems pretty feeble.
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