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Old 26-11-2012, 22:25   #1
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Atomic4 Gearbox Ratio?

I looked at a boat today(35 foot ,long keel) with an Atomic 4. The owner seemed to think that it had a 1;1 gear box, and I think that would be very unussual for a heavy displacment sailboat. Is there a place for a data plate that would show the ratio? Another question?????. For a replacement engine does a 7/8" shaft limit the horspower that can be used to replace the anemic4? How much horspower can a 7/8 shaft take before it becomes marginal?_____Grant.
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Old 27-11-2012, 04:34   #2
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Re: Atomic4 gearbox ratio?????

I don't recall seeing 1 that had other than 1:1. They tend to swing a small prop at high revs.
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Old 27-11-2012, 05:23   #3
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Re: Atomic4 gearbox ratio?????

According to Wikipedia the reduction gear is 2.04 to 1
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Old 27-11-2012, 05:49   #4
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Re: Atomic4 gearbox ratio?????

The required Shaft Diameter will depend, in part, on the (torsional yield strength) of the shaft material.

See ➥ http://www.abycinc.org/committees/P-06.pdf

And ➥ Marine Machining & Manufacturing - Sizes & Diameters
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Old 27-11-2012, 06:51   #5
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Re: Atomic4 Gearbox Ratio?

I recall my Atomic-4 being 2 point something. It was a V-drive. Pretty easy to calculate the ratio by turning the engine over by hand.

A 7/8" shaft is small for even an Atomic-4. Depends entirely on the unsupported length in most sailboat applications. Torsion limits come into play at shorter lengths. Of any material, shafts lengths of 30 to 40 times diameter become problematic for vibration ... for preferring to become a pretzel rather than spin the prop. 20 to 1 will not vibrate in a proper installation.
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Old 27-11-2012, 09:01   #6
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Re: Atomic4 Gearbox Ratio?

Atomic 4 has built in reversing gear which he might have referred to as 1:1 gear box. Some models have 2:1 paragon reducing gear. All are lubricated by the same engine oil. 2:1 reducing gear is easy to spot as its bell shaped round, it sits behind the square reversing gear housing(the one with a shifting lever)
Pics of the v drive and reducing gear are at the bottom of this page...
Moyer Marine Atomic 4 Engine Rebuilding and Parts

I would go with 1" shaft if you're putting a larger engine, although i think A4 with 2:1 would push a 35 foot hull pretty good with the correct prop selection.
If you decide to stay with A4, look if this thing has isolation mounts. A4s are commonly installed w/o rubber isolation mounts. Its a pain in the ass, on some boats like my C&C, everything vibrated when idling or motoring, like rigging, doors, hatches, stove, pots...
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Old 27-11-2012, 09:33   #7
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Re: Atomic4 Gearbox Ratio?

A "direct-drive" A4 is indeed 1:1, as far as I recall. I have a 7/8" shaft with a two-bladed Gori folder at 11 x 8. It replaced the "stock" Michigan Wheel fixed 12 x 6 two-blader. This is on a 33 foot fin keeler. I get about 6.1 knots in flat water, but doing 5.5-5.8 is quieter and far more fuel efficient.

A heavy 35 foot full keeler with direct drive is indeed a little underpowered for forward gear, but probably has great stopping power in reverse, which is desirable.

The "Paragon drive" is 2 something.

The "V-drive" is something else again.

In short, the Atomic 4's gear ratio is dependent on what is bolted on the business end. As far as I can remember, as all my manuals are winterized at this moment.

Take it for a sea trial and do stopwatch and SOG tests near a nav aid.
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Old 27-11-2012, 09:40   #8
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Re: Atomic4 Gearbox Ratio?

phorvati, thanks for the link to Moyer Marine. From the photos and the diagram, I think it had the reduction gear behind the reverser. I just could not find any data plate or marking that indicated what it was. I didnt think that the owner was swinging a prop at 2200 RPM. The shaft is bronze, and has very little distance from the coupling to the stuffing box. If I happen to buy this boat(a big if) and replace the anemic4, it will be with a fairly small diesel. The arpature sp? will limit prop size anyway, so a big engine would be a waste. ___Thanks for the help from all of you._____Grant.
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Old 10-12-2012, 11:12   #9
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Re: Atomic4 Gearbox Ratio?

The coupling on a 2 to 1 Atomic 4 installation has 4 bolts. 1/1 is 3, I believe. So that is an easy check. Or you can put it in gear, mark the shaft, and turn the engine over exactly one revolution with the hand crank. If the shaft mark is right back where it was, you have the 1-1 transmission. If it is halfway around from where it started, it is the 2.05 or whatever to 1. No crank? You can use a 1/2" drive ratchet and I think about a 13/16" socket. Grind two slots for the crosspin on the flywheel shaft. For easier turning, remove the spark plugs. Good excuse to check them and/or replace them.

I don't think the 7/8 shaft should be a major problem with most common A-4 replacement diesels, but certainly a 1" shaft would be preferable. Make sure it will fit before you go for it, though.

I don't care for gasoline engines. Yet, I have an Atomic in my liveaboard Cal 2-27. I would love to repower, but then my new engine will cost about 4 times what I paid for the whole boat, Atomic 4 included. So we have an uneasy coexistence. The Atomic is quieter than just about any diesel, with no perceived vibration. Gas is always easy to find, when inland or coastal cruising, than diesel. If nothing else, I can wade ashore with a suitable container and walk to a gas station. Oh, and my motor is raw water cooled. A disaster waiting to happen, after almost 40 years of having seawater pumped through the block, but so far so good. A diesel is cheaper to run, but I only use the A-bomb to get in and out of the marina, and I probably burn about a tablespoon of gas doing it, so no biggie. As long as it keeps running after a fashion, I won't replace it.

My suggestion, then, unless you are paying enough for the boat that $8,000 or so for repowering is small potatoes, or unless you do a lot of motoring, is if the Atomic runs okay, just keep it in good repair until it seems to be not up to the task anymore. And since you will already be carrying a tankful of gasoline, you have a good excuse to buy an 8hp kicker for emergency use, and an outboard mount for it. Then if the A-bomb goes boom or clunk or whatever, you can at least finish motoring back into your slip with the outboard. A gently used outboard can always be sold, if you later on go ahead with a diesel replacement for the Atomic.

Give the atomic a good evaluation before you make up your mind to get rid of it. In other words, it it ain't broke, don't fix it. If it ain't broke, don't break it, either.
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Old 11-12-2012, 08:52   #10
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Re: Atomic4 Gearbox Ratio?

^ ^ ^

Good advice. The Atomic 4 is still the better choice for a) owner repair and servicing; b) quiet and simple operation; and c) economy over the short-haul.

It is not a good choice for a) people who forget to install or run a blower; b) people who motor enough to use more than a tank of gas in a season (the range of an Atomic 4 and a typical under 35 footer is less than 100 NM per tank); and c) people who are not mechanically inclined and/or sensitive enough to do the minor lawnmower/chainsaw level of maintenance needed to keep the A4 running indefinitely.

It is stronger and better situated than an outboard, however, and is a great choice...still...for the typical Good Old Boater. Diesel repowering is non-trivial and can easily exceed the cost of the boat (these days). In addition, diesels aren't ideal for "switch on, go head to wind, switch off" short runs. That's why the A4 persists, 27 years after the last one left the factory. They are still the best inboard for the buck in the typical "occasional sailor" category most North Americans fill.
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Old 11-12-2012, 09:47   #11
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Re: Atomic4 Gearbox Ratio?

Quote:
Originally Posted by S/V Alchemy View Post
^ ^ ^

Good advice. The Atomic 4 is still the better choice for a) owner repair and servicing; b) quiet and simple operation; and c) economy over the short-haul.
I agree with a) For owner who likes to tinker, A4 is nicer. You can tinker with the diesel as well, but time/effort and consequences related to tinkering are significantly increased.
There is something to be said about b) the quiet operation. After some 8 years, I finally took the A4 out and put westerbeke 40.
I was amazed to say that the diesel is much much quieter. In neutral or in gear. The only thing i can relate the observation to is isolation mounts.

Atomic 4 had standard size isolation mounts I bought them and installed them new in 2008, in hopes it would improve vibrations and noise. Bushings Inc. - DF-2200 Series Dual-Flex Marine and Industrial Engine.

And it did, but the diesel installation improved the noise even more.
On the diesel installation I bought another brand new set (oversized)
http://www.jamestowndistributors.com...51124&type=pla
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Old 11-12-2012, 15:16   #12
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Re: Atomic4 Gearbox Ratio?

That could indeed be the case. I'm sure if you went to the lengths I'm going to (soft isol mounts, an Aquadrive, loads o' sound deadening paint) I am going to on my new diesel install with an Atomic 4, you'd get the same result.

Frankly, I don't notice the A4 being loud unless I try to get that last knot of speed. Given what that does to the gas consumption, I generally consider 5.5 knots "sufficiently fast" in the 33 footer. If there's wind, I sail, and if not, I'm not usually driven to redline the A4.

Nor the diesel, really!
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