How far can you safely push the 3L mercruiser? - Maxum Boat Owners Club

Warpa1 · 08-08-2011, 08:24 PM

I had asked this on another forum, but they are more into the much bigger boats and very much stuck up their aholes

From what i understand the cruising revs are 3k and wot is 4400-4800, now its my understanding wot throttle is a bad thing for even 5 mins? So can you travel at 3.5-4k without over stressing the engine? This is on a 1750 from 1997, well looked after and very well maintained. Only reason i asked is because i normally run at 2.5-3k and opened her up to 4k for a few mins coming home the other afternoon, and i must say it was a whole new experience

The difference is speed seemed amazing and it also seemed to take the choppy sea a lot better as i just skipped the top of it instead of getting all the slamming.

Im just trying to understand that if 3k rpm is the cruising speed, why have neqarly 2k rpm left that could be used

seapuppy · 08-08-2011, 08:39 PM

Warpa
this is a very misunderstood item of boating...yes..running your boat wot for long periods of time will shorten the life span of the engine...however..for a few min..it won't hurt it ...some mechanic's feel that occasionally it's a good thing to blow it out and stretch the legs a bit every now and then.....but to run at a high cruise speed isn't a bad thing...my boat redlines or wot is about 5200 rpm...(5.7mpi engines)...I consistantly run it at 3800-4000 rpm as that's the best cruise speed I get for fuel consumption....ok..that sounds like an oxymoron but if I run at say 3200-3400 rpm...my fuel usage goes way up....
if I run her at 3800-4k rpm....my fuel consumption drops dramatically.....I can get twice the distance with half the fuel...running slower only pushes alot of water for my boat.....so..the objective is to find the best cruise speed and the most comfortable for the water conditions at that second.....(I'd say moment but up here that can change from second to second)....

your boat may be more comfortable at say 3600-3800rpm...but running it at wot for a blast or two won't hurt the engine at all...

the other thing to think about...if your lugging the motor and pushing alot of water ...that is the same in a car as going up hill...in 4th gear at say 1500 rpm....your lugging the motor and you will actually do more harm than if you dropped it a couple of gears ...get the revs up...and let the torque work for you.....

most marine engines like the higher rpm's....they are most efficient in that midrange to upper rpm than at lower rpms....
diesels are an exception to this rule...
SP

Warpa1 · 08-09-2011, 07:55 PM

Quote:

Originally Posted by seapuppy

Warpa
this is a very misunderstood item of boating...yes..running your boat wot for long periods of time will shorten the life span of the engine...however..for a few min..it won't hurt it ...some mechanic's feel that occasionally it's a good thing to blow it out and stretch the legs a bit every now and then.....but to run at a high cruise speed isn't a bad thing...my boat redlines or wot is about 5200 rpm...(5.7mpi engines)...I consistantly run it at 3800-4000 rpm as that's the best cruise speed I get for fuel consumption....ok..that sounds like an oxymoron but if I run at say 3200-3400 rpm...my fuel usage goes way up....
if I run her at 3800-4k rpm....my fuel consumption drops dramatically.....I can get twice the distance with half the fuel...running slower only pushes alot of water for my boat.....so..the objective is to find the best cruise speed and the most comfortable for the water conditions at that second.....(I'd say moment but up here that can change from second to second)....

your boat may be more comfortable at say 3600-3800rpm...but running it at wot for a blast or two won't hurt the engine at all...

the other thing to think about...if your lugging the motor and pushing alot of water ...that is the same in a car as going up hill...in 4th gear at say 1500 rpm....your lugging the motor and you will actually do more harm than if you dropped it a couple of gears ...get the revs up...and let the torque work for you.....

most marine engines like the higher rpm's....they are most efficient in that midrange to upper rpm than at lower rpms....
diesels are an exception to this rule...
SP

Thanks for that, it would seem that when i potter along at just under 2k rpm i am wasting fuel and possibly messing the ride up. I just wanted to be sure that the engine will be ok, fully serviced as i may have said, and also like you say i know cars do like to be red lined from time to time. I remember taking my mums car years ago for a long trip, could barely get 70 mph out of it, coming home it would cruise at 100. I know its not quite the same but my engine probably could use a good clear out as it takes 20 mins at 6 knots to leave my mooring and obviously the same getting back, and at times we dont even leave the limited area so itas on tickover all the time.

tackleshep · 08-10-2011, 06:48 PM

I usually keep my WOT for very short periods as I can't imagine red linning is good for it. With that said your leaving plenty on the table. I top out around 4500 RPM but run her regularly for our trips at 3800 to 4200.

Warpa1 · 08-10-2011, 07:00 PM

Quote:

Originally Posted by tackleshep

I usually keep my WOT for very short periods as I can't imagine red linning is good for it. With that said your leaving plenty on the table. I top out around 4500 RPM but run her regularly for our trips at 3800 to 4200.

So if i were to take a 30 min trip and alternate between 3.5 and 4k it should be fine?

ss3964spd · 08-10-2011, 07:23 PM

W1, if you able to, see if you can find out what the actual "redline" is for your engine. Redline represents the engine manufacturers' recommendation for the maximum RPM the engine was designed to run before parts start flying off and putting and end to your day. In your case it is reasonable to assume 4800 is the redline, but it might not be.

For your engine the listed 4400-4800 range represents what RPM's the engine should attain, assuming the factory installed prop is used, and allowing for, weight, wind, sea state, etc. The listed range is also a means to gauge if the engine is running correctly. If the drive is proped right, the engine shouldn't exceed 4800.

Most common failure of over-reving is valve "float" (when the intake or exhaust valves are no longer being completely controlled by their valve springs - not always the end of the world) or valve spring breakage (which allows a valve to drop into the cylinder - which will be the end of the world). Valve springs fatigue with age and cycles.

If your engine does not have a bazillion hours on it I'd buzz that thing at 4400 without concern for as long as I wanted to, and higher for shorter periods.

But it isn't my boat. You need to run it how you see fit.

Dan

vffb507 · 08-10-2011, 07:29 PM

I know I've been usually running 3000 to 3400 on our 3L for most cruising and such... Pushing her up to wot for only short jaunts - 10 min or so.. Now that being said I am also new to this... however previous owner told me this was his norm also since boat was new... But when fishin they went down around 1500 - 2000...
So far we've put about 40hrs?? on the motor since purchase and its running nice and smooth and even the oil is staying very clean so I fuguire all is running well.

seapuppy · 08-10-2011, 07:42 PM

as stated...WOT is different than redline....so...you can run it at WOT for short periods and it's ok....but not all the time..it will shorten the life of the engine just as much as lugging the engine....get the boat on plane....adjust the drive to free it up some...usually bumping the drive up a few times until you feel it free up and increase in rpm a few hundred revs....then adjust your speed for best conditions....ie cruise and sea state...at that point ...you should be getting the best speed...mileage your going to get for said conditions....

SP

Warpa1 · 08-11-2011, 07:33 PM

Thats some interesting posts! Weather permitting we will be out again this weekend and i plan on going to a ski area there the water is flat compared to what im normally in. I would have thought redline was 4400-4800, so that info was new to me, i do have an SS prop that was on the boat when i bought it, so no idea what difference it has compared to the standard.

My plan is to use the gps speed on my iphone to see what it can do, im also still trying to get to grips with the trimming as it all seems to make no difference? If i trim it up the revs rise a small bit, but nothing seems to change, or does that small rise in revs mean i have just speeded up a few mph and that is a good setting?