Engine Loads and Fuel Economy (Alternators and props, oh my!)

[George Bean]

I have a Universal M35B (600 hours) with a 120 Amp Amptech alternator and a 3 Blade Flex-o-fold (15X9 pitch) propeller.  My fuel economy has been a mediocre .6 -.65 gal per hour.  I was wondering if going to a smaller alternator or going to a different prop pitch will improve my fuel economy.  I noticed that a lot of boats use 100A alternators, but if I'm using solar (200 watts and counting), perhaps something smaller, like 80 or 90 amps might be more appropriate.  Our MkII boats have a ridiculously small fuel tank and I looking for ways to improve cruising range.

On a related note:  Recently I overfilled my fuel tank and had fuel leak out the sending unit's gasket when we had a string of hot days.  Can I use Permatex gasket sealer rated for fuel?  My tank is polypropylene and not metal. 

[Ron Hill]

George :  When I had my 75A Balmar re-built the shop was only able to find a 120A or a 150A (same price) starter for it.  They installed the 150A because they thought bigger was better!!  I almost passed out when they told me.
Lucky that my Balmar ARS-5 voltage regular has the capability of restricting the alternator output (thru the field wire) to something much less than 150!!  I settled on 90 amps max. 

You might be able to do the same with your voltage regular?

A thought?

[cmainprize]

Hi George, that big alternator is only along for the ride unless you batteries need the amps.  If your batteries are full (solar or other) the effect on your engine is nill.  The biggest change you can make is a clean bottom good prop.  We are fresh water so we haul every 6 months for winter and and keep a really clean bottom.  We run a feathering J prop ( adjustable) and get similar fuel consumption.

We run 2400 rpm at 6.5 knots all day (calm conditions) and burn less then .75 gallon per hour.  We calculate our range at 1 gallon and hour for peace of mind. 

I figure our range under power is 25 gallons at .75 gallon per hour or about 215 miles.  Even if you could manage 15% better range you are only getting another 30 miles. 

[Mick Laver]

George,
I'd say your .60 to .65 gph is pretty good. We also have a Flexofold 15x9 and cruise at 2400. We will get anywhere from .69 to .89 gpm on longish (>60nm) passages depending on sea state. We have a 90A Motorola alternator.

Some friends who often travel with us on their 34 Mk II, Hakuna Matata, have a Flexofold 15x10, the standard 53A Balmar, motor at 2150 (to match our SOG), and get on average about .8 gph.

Doesn't sound like the alternator - or prop pitch - make a heck of a lot of difference.

[Stu Jackson]

I had a long phone chat with George.  I agree that his fuel consumption is rather remarkable, and very good. 

Many folks are so used to leaving the dock with full batteries, that they seem to miss the important considerations necessary when "you're out there," whether it be for a night on the hook or on a trip, wherever that may be.

One MUST think this way: "My house bank is at 50% SOC.  What is the best way to recharge them?"  Solar is one, but that's a separate subject from George's alternator question. 

One way is to have a BIG hunkin' alternator, sized way BEYOND the acceptance rate of a "standard" house bank, which, for our boats, seems to be around 400 ah.  The acceptance rate for this size bank at 50% SOC has been pegged by many skippers and experts at around from 60 to 80 amps.  So, a BIG 125A alternator will be doing just fine if fully loaded at cruising speed even after twp nights on the hook using 100 ah per day, which is, coincidentally, 50% of a 400 ah bank, right?

A nice big alternator will simply work cooler and longer than a smaller one.

I advised George to keep what he's got.

[Ron Hill]

Guys : To add to what Stu has mentioned - if you look at the larger alternators above 75 amps, you'll note on most that half of the diodes are mounted on a heat sync! 

A thought