Good running speed, opening her up

H

hack4alivin

Active Member
TECHNICAL Contributor
Apr 18, 2008
2,518
Joppa, Maryland
Boat Info
320 Dancer
Engines
Twin 350 V Drives
When clearing the 6 knot speed zone in our area I regularly throw both throttles to full to come up on plane. The boat accelerates rapidly to plane and I bring my throttles back till I am about 4000 RPMS or so. In that range I am doing about 25 knots. The boat is running great at that speed, and both the throttles have a long way to go to max out. The boat can very easily do much more, but I keep it there. The boat feels like a thoroughbred wanting to run, and I must admit I would like to let her go once.

The question is: Is running at 4000 RPM’s going to do premature damage to the engines? We are talking about dual 350 MPI Horizon, inboards here.
 
My mechanic says to never run the engines beyond 4000rpm for any period of time. Most setups max out at the 4500-5000rpm range (mine max at 4600rpm)

For me, cruise is at 3000rpm. Beyond 3800rpm, engine noise becomes notable, but the boat does run sweet. :)
 
I run the same setup as yours from around 3600 to 3800 RPM's and occasionally (3 to 4 times a season) open her up to insure everything is working right. I don't throw the throttles all the way down when getting the boat on plane. But I do give it some throttle - around 4000 to 4100 RPM's and then start pulling them back. The smartcraft on my Northstar shows the boat running best in the 3600 to 3800 RPM's from a fuel economy standpoint. I personally believe 4000 RPM's is too much. 75% of WOT is what I have always read and been told is best for the engines. Which is 3750 RPM's in our case.
 
I generally run my engine at 3400 RPM, which is 68% of WOT (5000).

I ran her home from Annapolis yesterday morning and made 26 MPH at 12.9 to 13.5 GPH at 3400. She was running great!

Michael
 
I normally push both throttles slowly until the boat come up on plane. I keep the rpm at about 3000-3300, cruising at 18-20 knts. However my engines are an older technology than yours (454, carburator).

I never pushed the boat at max speed.
 
hack4alivin said:
and I must admit I would like to let her go once.
Click to expand...

It's fine to open her up occasionally and run the engine at specified max RPM for a few minutes. If there were any problems, they'd probably show their ugly head in the very near future anyway, at a lower RPM. There's a reason they give you a "max RPM". That's the rpm you can safely run at. Obviously it will wear faster, but...

I can't believe you've never actually followed through with your urge to run it up!? You must have great self control. :smt001 I...just...can't...help...it. And with twin engines? I'd love to do that; someday.

Tom
 
Geez, I feel abusive. I run it at WOT often when we are just getting from point a to point b on the lake if we arent cruising around.
 
I run WOT occasionally, but not for very long. That has less to do with maintenance and more to do with 30 GPH fuel burn. I do push the throttle to the stop until the boat planes out and then ease the throttle to level out at about 3,000 - about a 30 MPH cruise.
 
I cruise at 3000-3500RPM. WOT RPM is 4800 for my engine, and I've heard 75% of that is the max recommended cruise speed, for Mercruiser also.
 
4k is pushing the longevity of your engines if that is a regular MO for you. I would keep it under 3800 rpm for extended periods if you plan on keeping the boat for some time.
 
I typically move my throttles up about 2/3 and then adjust to about a 3200 RPM cruise - that's about 26-28mph for me and is just slightly less than my 75% of WOT. Like Highslice said - the throttle position is more about how much gas do I really want to burn today..? If the kids are tubing I will throttle up with WOT and then back down to normal 3200RPM - just so we get up quicker.
 
My throttles have notches. Each notch pushes about 200-300 rpm. I generally move the throttles slowly to a point equivilent to 3200-3300 rpm, then back off one notch for 3000 rpm cruise once on plane. Works well.
 
I run mine between 3400 and 3600 rpm a true sweet spot for the 340 running about 27 to 30 mph ... sometimes I turn 3800 rpm ... still comfortable ... at 4000 rpm things get loud and you feel the strain on the engines but they are big blocks and turn 4600 at WOT.

305s and 350s should turn 5100 to 5200 at WOT and have no problem to run 3600 to 4000 rpm for extended periods whcih is right around the 75% of WOT mark.


If you ever want to get a feeling how hard these engines work, run the boat with an open bilge hatch! I did this last weekend with our mecahnic to check things out and almost went deaf from it.
 
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Alex D said:
I run mine between 3400 and 3600 rpm a true sweet spot for the 340 running about 27 to 30 mph ... sometimes I turn 3800 rpm ... still comfortable ... at 4000 rpm things get loud and you feel the strain on the engines but they are big blocks and turn 4600 at WOT.

305s and 350s should turn 5100 to 5200 at WOT and have no problem to run 3600 to 4000 rpm for extended periods whcih is right around the 75% of WOT mark.


If you ever want to get a feeling how hard these engines work, run the boat with an open bilge hatch! I did this last weekend with our mecahnic to check things out and almost went deaf from it.
Click to expand...

That’s the thing, at 4000 RPMs she is still purring, and seems ready for more….
 
Mercruiser lists the full throttle range at 4600-5000 rpms. Extended operation beyond 75% of your WOT rpms will decrease engine longevity. Gas engines are not diesel, and are not designed to be run for long periods at near max WOT. Running at 4000 rpms continuously may not be a good thing, unless you're propped to run at 5000 rpms or a little more fully loaded. I've always wondered about owners that say their boat cruises at "X" knots just where they're running their engines...

With that said, there are situations (like running in a heavy following sea with a heavily loaded boat) that my require more throttle to hit the "sweet spot" for short periods of time.
 
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My dealer firewalled the throttles on our boat when it was new to make certain the prop set up was right. Under minimal load conditions (fuel, water, humans, and gear) with the proper trim on a flat calm river, they turned 5200 rpm.

They have never seen that RPM since. Once in a while I'll spool them up to 4200 or so for a few miles (sure gets attention from the folks being passed in an open bow) they sound really good too...but not as good as the big block from my old 270
 
I just bought my 320 and I agree with you. 4000 is a sweet spot and seems to be the best for mileage. I was burning 32 gallons per hour and running right around 30 MPH. So that's close to 1 MPG. The boat runs nice and level at that RPM. When I back down to 3600 or 3700 it seems to settle in a little and leave more of a wake. My engines are 6.2 with warranty till 2011. Unless some one can convince me other wise or I run into a problem, I am going to continue to run at that RPM, seas permitting.

I did catch myself going as high as 4600 after getting on plane.. I was standing and looking at another boat. I looked down and I was taching 4600.. I felt like it would have easily gone over the 5000 mark.
 
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Bill

Theoretically, the lower the RPM you turn thru-out the engines life span, the longer your motors will last. But you also have fuel economy to consider, planing speed to consider, your desire to get somewhere fast, and then there's the "sweet spot" when your boat feels it's running best. So how do you sort all this out? :smt101:smt101:smt101

The "sweet spot" on my single 350 MAG powered 260DA is 4000 RPM just like it is your 320DA. But I don't run at 4000 RPM because my fuel flow is most effecient at 3800 RPM. So instead I choose that RPM as my cruising speed most of the time because the boat is still nicely on plane and is only running about 2 mph slower than it does at 4000 RPM. So I feel I am helping out the longetivity of my engine as well as saving a few dollars on gasoline. But that doesn't mean that I don't ever run faster than that. If I am in a hurry I'll run at 4000 RPM and I'll open her up and run her at maximum RPM for a few minutes occasionally as well. For me at least, the trick is not to run above 4000 RPM for any exended period of time.


Dave
 
I have always subscribed to the school of "engines need to be revved every so often to remain in good tune".
Every car I drive gets redlined once or twice a day and I run our 340 at WOT twice a year for about 5 minutes when we make back from our favorite cove.

I also believe that an ample warm up period ... about a mile for me ... is important before running up on plane and I also make sure to run the boat at 1200 rpm for about 15 minutes after coming off plane to cool things down.
 
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