HP vs. electric output ?

[Ken Secor]

My Catalina-22 has a 9.9 Merc OB motor. It's twice what I need for propulsion, but what about the equally important alternator output?

Charging the battery is also important and the amp output of the motor (alternator) is a major factor in motor selection.

I'll have to look at the manual (if there is one) to see what my 9.9 puts out - amp wise.

Day sailing? No problem. But if we're doing an overnighter or two, battery drain is an issue. Likewise, how long we need to have the motor on - simply to recharge a spent battery (assuming there' enough amps to start the 9.9) is a worthy consideration?

I assume virtually all motors are for propulsion - charging the batteries a distant second. I see sailing specific motors and assume hey have much larger generators than "typical"?

Most of us understand AH and battery maintenance/drain, etc. but are there motors designed or low HP and large generation out there?

I searched the boards and see little regarding this point (HP vs. electrical generation).

Did I miss it?

 

[sailormanbigd]

I think that the output of an outboard charging system is in dirrect relationship to the rpm of the motor as well. Something like specified max output at wide open throttle. If your overpowered you may not get anywhere near the max output rpm range? just my thoughts. Trade your 9.9 for a 6hp and some bucks and buy a solar panel with the difference?

 

[kloudie1]

These go on sale for $90 regularly.. will drive a 20 ampere 12 V battery charger .. Noisy, however. sooo go with a 4-6 hp engine and run this when ya charge the battery.
http://www.harborfreight.com/engine...800-watts-rated-portable-generator-60338.html

 

[Calif. Ted]

If it's a four stroke it's probably 6 amp maximum output, doesn't it have a retractable starter rope if the battery is low?

 

[Ken Secor]

I think output is directly related to RPM's on generators, but not alternators.

As much as I love Harbor Freight, I don't want to complicate things. I guess what I'm asking is; what's a "typical" OB motor's alternator capable of putting out in AH's - and are sailboat specific OB's designed to put out more AH's than a plane jane OB motor?

If so, what sort of AH output does either type - put out?

If I pull the cover of my Merc. 9.9, would that data plate or alternator state the output on the tag?

I'm not opposed to secondary generation, I just want to know the limits of "typical" sail boat motors in the 4 to 9.9 HP range are. 25 AH, 50 100 ?

 

[Bill Roosa]

More like 5 amps max. With 4 HP you really don't have much to play with considering it is designed to move water and not primarily charging a battery. you can also bet that it is a dumb regulator designed to recharge its start battery.

 

[Ray Bowles]

Most OB motors with less that 10hp that have charging systems are so low in output that you would need to motor most of a day at full power to replace a full days use of electrical power.
That might cover the stereo, vhf, electronics and reading lamps but I'd doubt it.
Look at what it takes to charge up a camper or small travel trailer 12v system.
Ray

 

[Sailm8]

We spent weeks at a time out on our C22 in the Keys. Had to add a second battery and a wired battery charger. The manual start 9.9 Honda OB just couldn't keep up with even the anchor light. We could easily go 3 -4 days without a charge with conservative use.

 

[walt]

I have a 2010 Nissan 9.8 four stroke outboard http://www.nissanmarine.com/products/9_8_4_specs.html and it actually has a fairly nice charging system. Although it uses a Magneto similar to most smaller outboards, it behaves much more like alternator/regulator setup. I.e., this outboard will put out near the full specified amps even at low RPM and it will not over charge the battery even at high RPM and it does NOT do this by burning off excess charge in heat.

If you look at the electrical diagram for this outboard, it shows a REC/REG where you would normally see just a REC (rectifier). In a discussion earlier this year, I was trying to figure out how this worked and ended up looking at US patents on this that are actually fairly old now and some of the regulator designs did things like replace the diodes in the rectifier circuit with a device called a SCR (silicon controlled rectifier) that allowed a control circuit that was monitoring battery voltage to select how long the SCR or ”diode" was turned on. For example, if the battery was near full, it might only turn on the SCR for a fraction of the time compared to when the battery needed charging. These old patents also addressed the issue of burning out the rectifier if the battery was disconnected while the engine was running so I suspect that may also be an obsolete issue at least on some outboards.. How the outboard produces near full current at low RPM I don’t know – but suspect it has to do with clever use of saturating magnetics.

I have been suspecting my Nissan 9.8 four stroke charging system actually worked much better than what you hear on the internet for a small outboard so I kept some records of what it did last week when I was at Lake Havasu. For these numbers, I used a charge monitor to measure voltage, current, amp hours and my battery is composed of two six volt golf cart batteries.

When I first put the boat in the water, the battery was fully charged (by a solar panel/charge controller), I ran the outboard immediately to see if the regulator was limiting current when the battery was fully charged. Battery voltage was about 13.7 volts and the current from the outboard was 0.3 amps at about ½ throttle and 0.4 amps at full throttle. Yep, the regulation works fairly well.

Another data point. A few days later I was down about 18 amp hours. I just started the outboard and let it idle. The battery voltage was 12.9 volts and the current was 4.5 amps. I.e., even at idle, I was getting 4.5 amps from the outboard! Under the same conditions, it would put out near the rated current at mid to full throttle. Ie, there was not much RPM dependence on current, the charging sytem was putting out near full current over the entire range.

I also found that the regulator starts to kick in at about 13.3 to 13.4 volts and when it did, the charging current would jump between a higher and a lower value. I’m not sure exactly where the outboard regulator really cuts back on charging but suspect it’s about 13.5 to 13.6 volts.

What I did find was that even with my two six volt golf cart batteries, during the week I never got much below 25 amp hours down and what would happen when motoring is that I would start out putting 5 or 6 amps into the battery but fairly quickly the battery voltage would get up near the 13.3 volt range and outboard regulator would start to cut back – basically at the regulator limit voltage, the battery acceptance was limiting things. If I had let the battery get a lot more discharged, the outboard charging systems likely would have contributed a significant amount.

On the other hand, I had a 1990 Honda BF8 and a lot of the advice here about the charging system not doing much, being dependent on RPM, etc was true..

 

[tommays]

Solar panel and outboard will take about 50 hours to charge a battery

 

[Barnacle Bill]

Most outboard alternators are 6 amp regardless of the horsepower. Smaller outboard might have a lighting coil which will supply DC electricity while the engine is running to run lights, etc., but they are not designed to charge batteries. Don't make this more complicated than it is.

 

[FourPoints]

my 1990 Tohatsu 8hp 2 stroke has a 70w output, at full throttle. at idle it puts out nothing (less than 10 volts) at 1/4 throttle it reaches high enough voltage to charge, and at WOT it (used to) put out 20v.

I replaced the rectifier with a rectifier / regulator a couple years ago, which is a dumb regulator limited to 14.4v IIRC. This protects the batteries from over charging since they are normally fully charged by the solar, and do not need the outboard power.

EDIT: Details of all of this are on my blog.