Cruisers Forum

Join CruisersForum Today

  This discussion is proudly sponsored by:
Please support our sponsors and let them know you heard about their products on Cruisers Forums. Advertise Here
Thread Tools Search this Thread Rate Thread Display Modes
Old 11-10-2009, 14:28   #1
Registered User

Join Date: Aug 2008
Posts: 7
Alternator Rating

The altetrnators on my 44ft MY are rated to produce output at 1000 RPM. I have heard people say that you must run the motor over 1000RPM to generate any current. It seems to me that the rating would be for generator speed, which is overdriven on my motors. One characteristic of an alternator over a generator is that they put out more at lower RPM, or that is what I have heard in the past.

Does anyone know what this rating means.

Captain Dave

davefm3 is offline   Reply With Quote
Old 11-10-2009, 15:57   #2
Registered User

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 18,337
Mine kicks in at a specific rpm, BUT once it kicks in, I can rev down to any rpm and it keeps on delivering quite some amps. Sure, if rev'd up I get next to full output.


barnakiel is offline   Reply With Quote
Old 13-10-2009, 13:54   #3
Registered User
S&S's Avatar

Join Date: Jul 2009
Boat: 48' 1963 S&S yawl
Posts: 851
Images: 6
Alternators deliver more current at lower RPM than generators (dynamos).
That said, they both drop off at low RPM.
S&S is offline   Reply With Quote
Old 13-10-2009, 14:48   #4
Registered User

Join Date: Sep 2009
Location: Baltimore, USA
Boat: Irwin Citation 39 'Chesagansett'
Posts: 159
Also, there are 'self-excited' alternators where charging doesn't kick in initially until a threshold RPM level is reached. It will then charge at lower RPMs, once it's in this state.

I'm presently running one of these (a Delco single wire alternator, it's supposed to be the spare). When I goose the throttle after starting the engine, the load that suddenly comes on the engine from the alternator is obvious.
steve_hendry is offline   Reply With Quote
Old 13-10-2009, 15:36   #5
Senior Cruiser
GordMay's Avatar

Cruisers Forum Supporter

Join Date: Mar 2003
Location: Thunder Bay, Ontario - 48-29N x 89-20W
Boat: (Cruiser Living On Dirt)
Posts: 37,596
Images: 241
See the earlier (very short) discussion at
Gord May
"If you didn't have the time or money to do it right in the first place, when will you get the time/$ to fix it?"

GordMay is offline   Reply With Quote
Old 13-10-2009, 15:38   #6
Registered User

Join Date: Dec 2006
Location: Seattle
Boat: Cal 40
Posts: 2,603
Images: 7
You need to find the output curve for your alternator. They all increase with rpm to a certain extent. Some are almost flat after a certain rpm, look at the output for the red line Balmar alternator in the second link as an example.

(Edit: Gord beat me to it with his own link)

Car and Deep Cycle Battery Frequently Asked Questions (FAQ) Section 5

cal40john is offline   Reply With Quote


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
Star rating unbusted67 Forum Tech Support & Site Help 21 17-08-2018 11:09
Thru Hull Valve Rating Question Pete the Cat Plumbing Systems and Fixtures 5 03-11-2008 12:24
alternator nelsonsmoody Electrical: Batteries, Generators & Solar 14 30-08-2008 08:29
rating gunkholes (Chesapeake) schoonerdog Other 1 14-08-2008 07:07
AH rating for WalMart MarineMaxx 29? amytom Electrical: Batteries, Generators & Solar 9 10-07-2008 04:42

Advertise Here

All times are GMT -7. The time now is 09:16.

Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2020, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2020, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.