Engine quits in reverse, so fix the alternator
 

This has to take the cake as the oddest issue I have had in some time:
I went to back out of my slip and the engine quit :confused: I tried a couple more times after running at full throttle forward against the lines with no issue. Third time was the charm, got out and went on our way. Our top speed was 3.8 knots at full throttle :(
As soon as we cleared the breakwater sails went up and we managed 6 knots or so. I concluded from this the bottom might be a little dirty but the prop had fouled for sure.
The next morning I cleaned numerous barnacles off the prop and noticed a large improvement in speed headed home. I also noticed that like the day before, the engine was surging. A quick loo showed the battery voltage and charge rate varying with the surge. My ARS-5 regulator has an LED display that shows the desired voltage and the actual voltage. These should be within 0.1 volts, but if you caught it at the right time the difference was nearing 0.8 volts or more :confused:
What the issue turned out to be was corrosion in the fuse holder for the sense line to the battery. This was causing the sensed voltage to read low, then the alternator would ramp up to "fix" it, and then the voltage would be too high - rinse, repeat. With the extra load on the alternator plus the barnacles on the prop, it seems like at idle in reverse it was enough to cause the engine to quit.

Joe, how big is your alternator? It must be quite robust to stall the engine. However I don't really think that the alt could absorb enough power at low RPM's to really load up enough to stall a good running A-4. Just my opinion and curiosity.

Dave Neptune :cool:

I didn't think so either. A 60 amp Balmar should be a bit over 1 HP at full field. It seemed to be a combination of that and a badly fouled prop. Cleaning the prop and cleaning the contacts for the sense wire seem to have the issue fixed, if it comes back I'll be back to the drawing board.

Hi Joe,
Just as an after thought you might want to pull a spark plug wire one at a time and see if you have a dead plug or something. A sheen on the water might also indicate unburned gas. Had something similar happen to me this year, although restart was easy with choke, I had reduced power and speed in forward as well. I replaced my plugs this spring and stalled a few times in reverse, felt her slower to come away from the dock, and slower to gain speed after switching to forward....it was a plug, dead number 4 plug and it was new.

Just a note: i changed that plug with a new one and no change, so I changed wires, cap and rotor....again no change. Then, as an after though, I put in one of my old NGK XR4 plugs that were used in the previous engine for years. Fired it up and working fine. Really ticks me off when new plugs come in a box non serviceable. These plugs were picked up at a local parts store.

My recollection is an alternator imparts an approximate 1HP load for every 25 amps of output.

I was just doing the raw conversion, the alternator puts out 840 watts and 746 watts is 1 horsepower, assuming no conversion losses. Maybe there is more lost than I thought??

Joe, even at a 2 Hp load if you had applied even the slightest throttle it should not of stalled IMHO. At approximately what RPM did you experience the stalling?

Perhaps it is time to consider a look at your idle settings. How do your plugs look after a bit of idling? I don't like the way the transfer from idle to hi-speed is set in the late model A-4 carb as in many cases I have found that band to have a lean spot at the start of the transfer.

Dave Neptune :cool:

"Accidental Test"
 

Recently, while taking a late afternoon harbor cruise, I accidentally performed a "test" which may add a little interesting info to this conversation. My alternator is a 105 amp, single wire Delco which needs to be revved to about 1500 rpm to actually start charging. After this initial "firing" it will charge at any engine speed. On this particular occasion, I started my A4, let it warm up for a few minutes, and then headed out. After a few minutes of 800 rpm cruising, I thought that the engine was running even smoother than usual. A glance at the voltage gauge showed about 12 volts, rather than it's normal 14.5 volts. After shifting to neutral and revving the engine to the required 1500 rpm, the voltmeter went to 14.5 and the boat felt "normal" again. This brief test did show me that the alternator load can indeed be felt.
Tom

I think we need to also take into account that the alternator is not 100% efficient. There will be losses due to belt friction, bearing friction, heat in the alternator, etc.
This site - https://nacfe.org/technology/high-ef...-alternator-2/ - reports efficiencies of 55%-60% for standard alternators.

So to produce that 840 Watts output you need about 1400 Wats input - a little less than 2 HP. As Tom points out, this is noticeable.

Best,

Peter

PS - thermodynamics gets you every time...

The three rules of thermodynamics:
1. You can't win
2. You can't break even
3. You have to play