H.P. of alternator

[25931]

I would be interested to know what is the effective consumption in horses. We have 1gm10s with 35 amp alternators and a 28hp Beta with 100amp .Am I right in thinking that the alternators take most power when the batteriws are flat & much less when batteries are fully charged ?

 

[mattnj]

you can work it out depending on the amps it is drawing, it probably never gets to its maximum rating, say a 50amp alternator, work on say 40amps max x 13.5v = 540watts (ish)

1hp = 745watts, so 540watts = 0.72hp (ish) and they are about 50% efficient (ish) so it actually probably needs twice that, 1.4hp (ish)

You can use the same simple maths to guesstimate your HP useage.

 

[nealeb]

...and the problem with a 1GM10 is that the alternator will be trying to produce that kind of output into the flattish battery that has just started it but while it is running at low revs; it takes so much power that the engine revs won't build up. That's why the saildrive version has a delay switch on it that effectively disables the alternator after the engine has started and then re-enables it after a short period, once the engine is a bit warmer and can cope with the load.

I had a 1GM10 in my previous boat and saw the effect - the engine wouldn't go much over fast idle after starting until it had had a while to top up the battery and reduce the load from the alternator. I only discovered the likely reason after comparing notes with a colleague who had the saildrive version in his boat.

You don't see the effect with a bigger engine but where the alternator load is a significant proportion of the engine output, and especially with an alternator that gives high output at low revs, it's a problem!

 

[vyv_cox]

My 55 Amp alternator goes to the maximum after a night of battery-operated refrigerator. Voltage before start is usually 12.0 - 12.2. This lasts about a minute, after which there is a steady decline to around 6 amps after 50 - 60 minutes. Sterling alternator controller, 330 Ah batteries.

 

[25931]

Thanks all & especially Nealab for answering a question which I was going to ask.