Atomic 4 Fuel Consumption

[Sony2000]

Nobody maintains 3500 rpm under load. The engine was designed to do it for a short period of time. It was designed to do 2600 rpm cruising on a smooth sea with low winds. If there is a good chop and strong winds are not in your favor, then 3000 rpm is available to maintain control. Low rpm at WOT, is running the engine at maximum load on pistons, connecting rods, crankshaft, all the time. A 20-30hp diesel would be more appropriate.

[ndutton]

Theoretical and/or textbook figures aside, hull speed at 2000 RPM and 35 years of reliable service suggests I should stay the course and not fiddle with the prop or anything else. Seems I don't need all 30 horses to get the job done.

I don't run at WOT. I run her up to a speed and RPM she likes and let her cruise along. Any more throttle movement returns little benefit. She's happy, I'm happy.

I dunno Sony . . .
A new club around here and now a suggestion that a 20-30hp diesel would be more appropriate?.

I don't think I'd want to be defending that position on this forum.

[Ball Racing]

Nobody maintains 3500 rpm under load. The engine was designed to do it for a short period of time. It was designed to do 2600 rpm cruising on a smooth sea with low winds. If there is a good chop and strong winds are not in your favor, then 3000 rpm is available to maintain control. Low rpm at WOT, is running the engine at maximum load on pistons, connecting rods, crankshaft, all the time. A 20-30hp diesel would be more appropriate.

Nobody is going to have 3,500 rpms at hand to turn up too, even if for a brief moment unless they about are running around a 10x3 prop....
Math shows us say the Indigo prop:10x7.4 inches
Max rpm for most guys with this prop is 2,400 RPM so thats it nothing left at WOT
Now we want to free the engine from this destructive force of this too aggressive prop, so One inch of pitch gives or takes about 200 rpms,
so we need to free this engine to be able to reach 3,500, thats 1,000rpms away.
So basic formula rules we need 5 inches less pitch. 10x2.4Prop
Good luck with that.

As for the argument of load from not reaching that 3,500 rpm, well a little reading around here has shown this motor is basically a anvil, and I have yet to read a post that says I killed this motor from not letting it turn to the factory number.
Diesel engines will die a quick death if overwheeled, as well as car engines in boats that cannot free them selves.

But this engine with no compression and a flat torque curve seems to laugh at the wisdom brought on by experience from other combustion engines.

[positron]

I'm with Mr. Dutton on this one. 35 years of running with the same prop (12x7 2 blade) makes me very hesitant to change. That and the fact that everything south of the engine is nicely held together with gobs of rust and old paint. I have never had my engine to WOT and probably never will. It moves the boat to hull speed and is happy there for hours, at maybe 2000-2200 RPMs. She does what I ask, I keep my expectations low- we are both happy.

[romantic comedy]

I doubt that anyone here can make 3500 RPM in forward with a direct drive.
I have the Indigo prop and do 6 knots at 1800 to 2000 with a maximum of 2300-2400.

We are all over propped as far as the engine is concerned. That is just a problem with a direct drive and possibly no swing room.

A 20 to 30 HP diesel? I dont get it. That is less power then the atomic four. The diesel would have a reduction gear and be able to access that HP.

This subject is always fun and has been talked about many times.

fair winds

[Dave Neptune]

Apples and oranges

Guys, we actually have two diferant engines as far as the A-4 is concerned! We have the A-4RU (the RU is my term for the reduction model) and the A-4D the direct drive unit. One has the capability of getting to 3500 RPM's and making the 30+ HP available. If you can get this engine set up by the book it would max out at WOT 0~2" of vac at 3500RPM's. This engine setup would be excellent for almost any app that there is enogh power to achieve hull speed at 60~80% power. This would give you a good cruise "load" on the rotating parts and still have the power to get you through some rough weather.
Now for the A-4D, this engine was for those smaller auxillaries that require less than the 30+ available HP and the optimum set up is a far differant criteria. In these lighter boats 15~20 hp is all that is needed (and available) even with that little extra for punching through some rough weather. In these engines we limit the engine to a peak of around 2500RPM's or just over 20HP @ WOT. This max power of 20+hp still requires full throttle (0~2" of vac) and does so at a lower RPM. The engine and it's low compression long stroke and forged crank were designed for just such operation. The limiting factor for the A-4D's is the availability of props. As we (except Daniel and a couple of others) are using the A-4D in displacement hulls we are also limited to around 6+kts for hull speed and at these speeds the props still need to get the leading edges into clean undisturbed water for a good bight and that is the problem. The hull does not move forard fast enough (limited speed) and the prop winds up absorbing HP and just spinning in disturbed water that the preceeding blade just passed through. In these applications the A-4 engine is very forgiving and designed to be lugged down to the 15~20 HP available at the reduced RPM's.
As far as small diesels are concerned they're a great motor and a lot more expensive however if you are not propped to achieve "red line RPM" the engine will wear itself out or destroy itself in very short order! The gas engine is far more flexible as far as operating ranges just not as fuel efficient.
I have seen many deisel and gas engines ruined because of a poor choice of prop and operating range however other that a lack of performance or maintenance I have not seen an A-4 actually hurt BECAUSETHAT IS WHAT THEY WERE DESIGNED FOR.
I actually have one of the worst A-4D applications possible in my E-35MkII. My cruising weight is around 14,000 and I always tow my rowing dinghy to boot. I need the RPM range of the A-4RU to access the 30+ but with the direct drive I just can't get there due to the prop spinning in it's own wash. Fortunately Indigo has a prop that will allow the A-4D's to get to that 2500 RPM range. Mine works fine until it gets rough then I would like to have the extra few ponies, however the prop still gets all avaliable to the water as it does not loose it's bight at 5kts and 2500 rpm when its nasty.
My engine is certainly not set up to get the max that I need as it is plain ole' not available. My A-4D was lugged at 1700 max RPM's @ WOT until I bought her 28 years ago. I pulled the martec the PO's were using (they raced her a lot) and started playing with props. I tried at least 10 configurations of dia and pitch and got to almost 2000 RPM's. Then I came across the Indigo 6 or 8 years ago which got me 600 more RPM's and a smoother throttle, smoother running, happier driver, far more available power and lower fuel consumption. I can't think of many other engines that would of taken the PO's abuse from way overpropping for the first 14 years and my running it still at to low of RPM's for about 16 years until I came across the Indigo. Oh and by the way my "BEASTIE" (A4-d) is now on it's 43rd season and the head was off once to get her "UNSIEZED" and replace an exhaust valve when I bought it. Other than that the engine is ORIGINAL and still running nicely!!!!

Dave Neptune

[edwardc]

Dave,

Spot-on with regards to A4s with reduction drives. I have a 2:1 v-drive, a 12,800 lb 32.2 ft boat, and a 3-blade 15rh8 prop. At WOT underway, it maxes out at 3100 rpm, but is about 6.8 kts, which is 0.2 kts under hull speed. I'm probably under-propped.

With a rule-of-thumb of 2hp per each 1000 lbs of displacement, I need access to that hp near the peak of the curve from time to time. But I can cruise happily all day at 5 kts and 2000 rpm.

[sastanley]

Go Beastie Go!

Ed, does she sound good at 3,100 RPM?

After this weekend's prop cleaning and slime removal from the bottom, I can get 2,300 RPM at WOT, and 6.1-6.2 knots GPS speed (I am still re-calibrating the speedo from a recent transducer replacement) - this weekend I set it to 1,950-2,000 RPM and she sang along happily for hours! We had a light tail wind and the mainsail up for most of that run, so it helped unload the motor a little more. In flat water, 2,000 RPM is about 5.4 knots (as long as everything is clean!)

My old boat has never gone that fast, but I think I may be finally getting her tuned in a little better too. I am going to try to get some vac numbers while it is still clean, as I've never had a clean bottom & prop since the vac gauge install last summer.

I will note however, that I get a little bog/stumble when I throttle up, I suspect from a little trash in the idle jet. I can usually throttle thru it, but I always worry about it stalling out during a low-power maneuver..right now I add a little choke when I pop it out of gear, & that seems to help.

Always work to do!

[edwardc]

Ed, does she sound good at 3,100 RPM?

Yes!

...I will note however, that I get a little bog/stumble when I throttle up, ...

I used to have that problem too, mostly when not fully warmed up to 180. It went away when I used the air/fuel ratio gauge to adjust the jets ( idle & main) and the distributor to run smoothly just slightly on the rich side of 14.7:1

[sastanley]

Thanks, Ed. I bet watching Daniel go down the creek in his deadrise would be a treat.

Tonight, I tried Dave Neptune's test and I was able to avoid the stumble with a nice gradual throttle...she was warm and had been thru a range of RPM to get the vac numbers mentioned below, and we were almost back to the dock. Need to fiddle with the stumble on a colder engine.

I may get into air/fuel ratio gauges eventually, but right now I am just enjoying the boat...as the wife and I did this evening prior to the storms rolling thru. (well, if you call going to the pump-out station 'enjoyable')

I have some vac numbers on my notepad I'll post tomorrow..probably in my own thread so I stop hijacking this one.

[romantic comedy]

anyone feel like some math?

Take the indigo prop. What is the theoretical speed that it will provide at 2000 rpm, passing thru a solid? Its pitch is 7.4?

so 7.4 inches X 2000 rev/min X 1FT/12 inches X 1 mile/5280ft X 60 min/1 hr X knots/.869 miles = 16 knots

So we have 16 knots thru a solid. Now we need for slip to be counted in.

Empirically, I run at about 6 to 6.5 knots at 2000 RPM...

So actual/theory is 6/16 = .375 or 6.5/16 = .41


I had a 12 X 6 two blade that I got with the boat, a Tartan 34. It made 6 knots or so at 1500 RPM, as I remember.

Sticking the 6 inch pitch in my silly formula, i get a theoretical speed of 13 knots. using only the 6knots value, because this is silly, I get a slip of: .46

It does look like this part agrees to an extent.

Now the complications. There is prop loading, diameter, blade area, number of barnacle on prop, cavitation, deadwood, and a lot of other stuff to consider.

I just think that experience is the best way to find out what works. I think that everyone knew that already. I was just feeling a bit silly, and wanted to use the calculator, and my memory.

if i do it correct, there will be a photo of my prop below. It is the indigo prop, with some hitchhikers aboard

[toddster]

Guys... trying to follow this. WOT?


(hmm... maybe I should get some gauges that work / can be read.)

[Dave Neptune]

Wot

Todd, it's "wide open throttle".

Dave Neptune

[sastanley]

Dave...good analysis..I guess what we strive for in a displacement boat is "as much thrust as we can get without it cavitating/slipping"??

I can tell you any hitchhikers on my prop slow the RPM's down and piss me off.

P.S.> Dave, you are getting better with the technology & pictures and stuff...way to go!! - My prop with that many barnacles would be several hundred RPM off the pace.

[romantic comedy]

stan, do you have me and dave mixed up?

The boat barely moved with that many hitch hikers. That was after i hired a diver to clean the prop.

Since then I pull the shaft each season. Yes with the boat in the water. i have the diver do the hull after he pulls the shaft. I clean the prop while he scrapes, then he puts the shaft back in. Goes quick, and I know the prop is clean.