7.4 MPI Cruising RPMs

[Bounty]

My boat Bounty (340 DA 2000) has twin 7.4 MPI with vee drive and closed cooling system. The data plate on the engines indicate WOT (wide open throttle) of 4000-4400 rpms. What does this really mean in terms of sustainable RPMs for cruising for durations of roughly 1-1.5 hours? Can I safely cruise at 4000 RPMs when sea conditions seem to warrant it?

 

[aquadancer]

I had 7.4MPI Horizons in my previous boat, 98 370DA. Our fast cruise was 3400 rpm at about 20kts. Real sweet spot for the engines seemed to be around 3200 rpm - the sound not too strained.

I would not cruise for any sustained time at more that 75-80% of WOT.

 

[osd9]

Roger:

The first thing you need to do is to confirm that you can reach the recommended WOT RPM range. It's better to be closer to the top. If you can't reach the WOT range, then you need to determine the cause...boat, engine, props, etc...

Once you have determined your WOT, than, as Jeremy stated, 75-80% of WOT should be where you would want to cruise.

 

[skolbe]

You will start burning some oil off at that kind of RPM's. I would suspect at least a quart of oil at 1.5 hours. The engines are built loose on purpose. The mechanic manuals cover the oil burn rates.

 

[Bounty]

Roger:

The first thing you need to do is to confirm that you can reach the recommended WOT RPM range. It's better to be closer to the top. If you can't reach the WOT range, then you need to determine the cause...boat, engine, props, etc...

Once you have determined your WOT, than, as Jeremy stated, 75-80% of WOT should be where you would want to cruise.

Would that be 75-80% of 4400 or 4000? (the "range" is 4000-4400.)

Sometimes she planes best at around 3900, other times much lower, depending on sea and wave conditions. I carry a pretty heavy dinghy on the stern which of course makes it a bit tougher to get on plane, although no problem. She will do at least 4400 although I *never* run her there.

 

[goblue]

75-80% of engine manufacturer's WOT is the correct answer. Faster than that and you are adding lots of premature wear to the engine.

 

[comsnark]

My W.O.T. is 4500 rpm.

75% of that is 3400 rpm. My boat (5.7LX with Alpha I drives, Raw salt water cooled) drives very well at that setting. Minimum planning RPM is about 2800 RPM, but you can tell the boat is barely on plane. 3000-3200RPM the boat is running fine.

At 3900RPM, my engines are starting to really scream (as in: get loud!). I would not run them there for a full 1.5 hours. Is that really the minimum RPM at which your boat gets on plane? Does that change if you take the dingy off? Maybe you have too much weight at the stern.

 

[osd9]

Roger:

The first thing you need to do is to confirm that you can reach the recommended WOT RPM range. It's better to be closer to the top. If you can't reach the WOT range, then you need to determine the cause...boat, engine, props, etc...

Once you have determined your WOT, than, as Jeremy stated, 75-80% of WOT should be where you would want to cruise.

Would that be 75-80% of 4400 or 4000? (the "range" is 4000-4400.)

Sometimes she planes best at around 3900, other times much lower, depending on sea and wave conditions. I carry a pretty heavy dinghy on the stern which of course makes it a bit tougher to get on plane, although no problem. She will do at least 4400 although I *never* run her there.

Roger:

The 75-80% number is with respect to the WOT that your specific boat achieves, so long as your boat achieves a WOT within the manufacuters range. As said earlier, it's best if you can reach the highest number that the manufacturer recommends for a WOT, that way you would be loading the engine less.

In your case, if you reach 4400 RPM WOT than you should have no problem cruising at 3500RPMs. If you "need" 3900 RPM to "maintain" plane, you may want to consider adjusting your load, or working your tabs a bit. That's closer to 90% of WOT than Iwould be comfortable with and seems a little high.

 

[Bounty]

In your case, if you reach 4400 RPM WOT than you should have no problem cruising at 3500RPMs. If you "need" 3900 RPM to "maintain" plane, you may want to consider adjusting your load, or working your tabs a bit. That's closer to 90% of WOT than Iwould be comfortable with and seems a little high.

3900 is quite high for me too. Usually she planes fine between 3200 and 3600. Sometimes we get big following seas which seem to require more zip to keep us nicely on plane. This is usually, at most, 3700-3800. This is rare.

Anyway, thank you to all who responded to this post. It was very useful and I very much appreciate it.

 

[goblue]

As was pointed out earlier, 75% percent can change during the season depending on the condition of the bottom for example. If WOT goes from 4400 down to 4000 due to weight, bottom, etc., the the new cruise becomes 75% of 4000 or 3000 RPMs. Above that new number and you are overloading. This is why most manufacturers recommend that you test your WOT periodically, say monthly so you can keep operating within a safe loading range.

 

[Hampton]

2005 w/ 8.1 Horizons

W.O.T. yesterday afternoon - Port - 4850, even with throttle pulled back 2 knob widths. Stb - 3640.

Top Speed - 33.0 Knots GPS, 37 MPH SOW.

This confirms my suspicions that the Port Prop has been trimmed down during tuning at "Cubano Bros Prop Jobs," (Made that part up).

Fresh Water - Full
Waste Water - Empty
Fuel - 5/8
1 Adult

 

[Alex D]

My experience is that this boat the old 340Da, runs best at about 3400 to 3600 rpm, which with my 8.1s engines gives me 26 to 30+ MPH.

I would say that a sustainable rpm for your boat is 80% of 4400 rpm (which is the max. rpm) therefore 3520 rpm, so I would not be too concerned to run cruise her at up to 3600 rpm ... above that you get into a zone where increased engine strain and wear is present. I certainly don't think 4000 rpm is good for extended periods of time.

The interesting part of this x% of max. rpm is that often the lower HP engine has the same internals than the higher HP (and therefore higher rpm) engine, but max. rpm diffes.

For example the 8.1sHO has a max. rpm of 4800 rpm. 80% of that is 3840 rpm and that should be sustainable. The 8.1s has a max. rpm of 4600 rpm. 80% of that is 3680 rpm. Now we know that the 8.1s and the 8.1sHO have the same rotating internals, only the cylinder head castings, camshaft and the fuel and timing curves are different.
Since wear and max. rpm is determined by teh strain the rotating assemply can bear, both engines should have a max. sustainable rpm of 3840 rpm.

You can see the rule is somewhat flexible.

 

[goblue]

My experience is that this boat the old 340Da, runs best at about 3400 to 3600 rpm, which with my 8.1s engines gives me 26 to 30+ MPH.

The interesting part of this x% of max. rpm is that often the lower HP engine has the same internals than the higher HP

For example the 8.1sHO has a max. rpm of 4800 rpm. 80% of that is 3840 rpm and that should be sustainable. The 8.1s has a max. rpm of 4600 rpm. 80% of that is 3680 rpm. Now we know that the 8.1s and the 8.1sHO have the same rotating internals, only the cylinder head castings, camshaft and the fuel and timing curves are different.
Since wear and max. rpm is determined by teh strain the rotating assemply can bear, both engines should have a max. sustainable rpm of 3840 rpm.

You can see the rule is somewhat flexible.

While it's true that HO 8.1s have a higher WOT, they may not last as long because the HP is made at high RPMs. To take advantage of the increased output you have the run the h**l out of them. There is no such thing as a free lunch. High HP, small displacement diesels for example don't last as long as normally aspirated, large displacement, low HP engines. It's the same with gas engines as well. The more HP you crank out of a given engine size, the shorter its life is likely to be.