I have read many times that the RPM at WOT is very important, in terms of keeping it within the range outlined by the manual.
My question is:
Is there any harm in cruising along for hours at less than WOT, like 3/4 or 1/2 and showing 3500 RPM or so?
I am thinking as long as I can get up on a plane and keep it there, I would prefer not to be at WOT to keep the noise and fuel burn down.
Thanks
G
Outboard RPM
-
delevi
- Admiral
- Posts: 2184
- Joined: Fri May 06, 2005 1:03 am
- Location: San Francisco Catalina 380, former 26M owner
- Contact:
Geoff,
Not a problem. The WOT RPM per engine manual is to let you know what prop size to use to achieve the full output of the engine. If your RPM is too low, you need a smaller or lower pitch prop. If it's too high, you need a larger or higher pitch prop. This has no affect on crusing speeds below WOT. If, however, you don't have the optimum RPM at WOT, you aren't getting the most from your engine and not getting best fuel economy at any RPM range. You may also be overworking it. If you're in the range, or close, any RPM you run at is fine.
Cheers,
Leon
Not a problem. The WOT RPM per engine manual is to let you know what prop size to use to achieve the full output of the engine. If your RPM is too low, you need a smaller or lower pitch prop. If it's too high, you need a larger or higher pitch prop. This has no affect on crusing speeds below WOT. If, however, you don't have the optimum RPM at WOT, you aren't getting the most from your engine and not getting best fuel economy at any RPM range. You may also be overworking it. If you're in the range, or close, any RPM you run at is fine.
Cheers,
Leon
-
NiceAft
- Admiral
- Posts: 6806
- Joined: Tue Feb 01, 2005 7:28 pm
- Sailboat: MacGregor 26M
- Location: Upper Dublin,PA, USA: 2005M 50hp.Honda4strk.,1979 Phantom Sport Sailboat, 9'Achilles 6HP Merc 4strk
G,
I agree with what Leon said about the pitch of the blade, but that is only part of it (as far as I'm concerned).
I took your question to imply 3/4 of the way to red lining the engine on the basis of what the tachometer is showing.
If that is what you meant, then there isn't any problem with running your boat for hours. I'm sure you would not take a trip with your car and run it at red line (for any length of time). You would also check all of the fluids before taking such a trip.
Ray
I agree with what Leon said about the pitch of the blade, but that is only part of it (as far as I'm concerned).
I took your question to imply 3/4 of the way to red lining the engine on the basis of what the tachometer is showing.
If that is what you meant, then there isn't any problem with running your boat for hours. I'm sure you would not take a trip with your car and run it at red line (for any length of time). You would also check all of the fluids before taking such a trip.
Ray
-
Highlander
- Admiral
- Posts: 5998
- Joined: Wed Sep 21, 2005 8:25 pm
- Sailboat: MacGregor 26M
- Location: Maccutter26M 2008 75HP Merc. 4/S Victoria BC. Can. ' An Hileanto'ir III '
- Contact:
Beene
You tilt up that 75hp put a large plastic garbage bucket under it fill it up with water start the eng when the garbage bucket reaches its highest speed at the at WOT then you know you got the right prop & yeh hang on
If your planning on pulling alot of skier's you'll want a prop that will run max rpm's at WOT but you'll want a pitch that will give you lots of torq you'll give up top end speed for this
so as your not going to be pulling more than one person at a time you'll want a prop that will give you WOT/RPM's with the best possible speed but with enough torq to keep the boat up on a reasonable plane at about 2/3 throttle that way once up & planing you can throttle back and maintain almost topend speed & save big time on fuel My boat will cruise along at 1/3 throttle quite nicely but then the
has a semi V bottom & hard chines so she planes up quite easy & how you load the boat makes a big differnce to but their is lots of people on this site with your eng. on a mac m that they should be able to give you a heads up . A merc dealer with some used props Sandy Cove marine will maybe let you do try & error Then buy one new & a used spare 
So whats going on this wk-end do you want to come down & see my new pedestal guard before I post it Still pondering its design
You tilt up that 75hp put a large plastic garbage bucket under it fill it up with water start the eng when the garbage bucket reaches its highest speed at the at WOT then you know you got the right prop & yeh hang on
If your planning on pulling alot of skier's you'll want a prop that will run max rpm's at WOT but you'll want a pitch that will give you lots of torq you'll give up top end speed for this
so as your not going to be pulling more than one person at a time you'll want a prop that will give you WOT/RPM's with the best possible speed but with enough torq to keep the boat up on a reasonable plane at about 2/3 throttle that way once up & planing you can throttle back and maintain almost topend speed & save big time on fuel My boat will cruise along at 1/3 throttle quite nicely but then the
has a semi V bottom & hard chines so she planes up quite easy & how you load the boat makes a big differnce to but their is lots of people on this site with your eng. on a mac m that they should be able to give you a heads up . A merc dealer with some used props Sandy Cove marine will maybe let you do try & error Then buy one new & a used spare So whats going on this wk-end do you want to come down & see my new pedestal guard before I post it Still pondering its design
-
pokerrick1
- Admiral
- Posts: 2269
- Joined: Sun Aug 27, 2006 7:20 pm
- Sailboat: Venture 23
- Location: Las Vegas, NV (Henderson, near Lake Mead)
-
James V
- Admiral
- Posts: 1705
- Joined: Sat Jan 22, 2005 9:33 am
- Sailboat: MacGregor 26M
- Location: Key West, Fl USA, 26M 06, Merc 50hp BF "LYNX"
Not a problem as stated.
The problem becomes when you cannot get proper RPM's at WOT. Check your manual. On the Merc 50 bigfoot, like only able to get 3500 rpm's when you should be getting 5000 or over rpm's, engin shuts down on you.
You can run not at WOT. However the Merc manual recommends that you do so from time to time to help the engin. After this point it gets a little Tech for me.
I also do not like the extra noise at WOT but do like the speed.
The problem becomes when you cannot get proper RPM's at WOT. Check your manual. On the Merc 50 bigfoot, like only able to get 3500 rpm's when you should be getting 5000 or over rpm's, engin shuts down on you.
You can run not at WOT. However the Merc manual recommends that you do so from time to time to help the engin. After this point it gets a little Tech for me.
I also do not like the extra noise at WOT but do like the speed.
-
beene
- Site Admin
- Posts: 2546
- Joined: Tue Sep 19, 2006 5:31 pm
- Sailboat: MacGregor 26M
- Location: Ontario Canada, '07 26M, Merc 75 4s PEGASUS
Hi RayI took your question to imply 3/4 of the way to red lining the engine on the basis of what the tachometer is showing.
What I meant was 3/4 of WOT. My 75 spec range is 4500-5500. So I am thinking if I can get at least 4500 from a 14x13, then maybe that would be best as far as fuel econ at best psbl speed over my 14x11.
I won't know till I get a chance to do some testing.
Cheers
G
-
beene
- Site Admin
- Posts: 2546
- Joined: Tue Sep 19, 2006 5:31 pm
- Sailboat: MacGregor 26M
- Location: Ontario Canada, '07 26M, Merc 75 4s PEGASUS
Hi JamesYou can run not at WOT. However the Merc manual recommends that you do so from time to time to help the engin. After this point it gets a little Tech for me.
There will definitely not be a problem with me giving her WOT often enough, I am looking forward to it, just not for prolonged periods due noise and fuel econ.
Cheers
G