Max RPM's duration at Sea Trial

Generalizations are so dangerous. Now, on all our boats, 300 rpm below WOT is around or less than 80% load. However, we consider 80% load to be our maximum running speed, except for brief WOT and we typically run 70-80% and then let off further for periods. I'd say a strict 80% or 300 rpm below or any such representations is fine in most planing boats if only doing it for a while. It's not an issue for most boat owners because they run a lot far less. We would never think of running 24 hours at 80% though without any variation.

Also, most boats have a quoted cruising speed from manufacturers and tests that reflect both being under the 80% but also a better fuel rate and would seem to be less taxing. For our boats that is 61%, 60%, 76%, 77%, and 70%. The ones at 76-77% are fast 40+ knots, planing boats. The ones at 60-61% are large 25 knot, 20 knot cruising, boats, and the one at 70% is in between as a planing boat but WOT only about 31 knots. These are so called "sweet spots" in the performance curves where the combination of speed and nmpg work out best. Working down from WOT these are typically the first speeds at which one sees a significant improvement in nmpg vs. WOT. Where the builders have quoted "cruising speeds" and "range at cruise" these are the speeds they've quoted because they make sense for many reasons.

So the speeds we'll actually run on a 10 hour or longer run, by boat:

Boats 1 and 2: 60-61% with occasional to 80% and occasional at 35% or so load.

Boats 3 and 4: 76-77% with occasional 80% and occasional 60%.

Boat 5: 70% with occasional 80% and occasional 55%.

If the engine is rated at 80% load, that is what I run it unless it provides incredibly stupid not to (for example 65% load gives you the same speed). All of the manufacturers tell me 80% load is perfectly fine to run them all day every day and it's best for them to run at constant speed, constant load as the temperatures all stay the same and the metals don't expand and contract. Look at EGT temperatures between 80% and 60% load for example. Look at how long generator engines that are run for very long periods of time last compared to marine engines, twice the amount of hours usually......

Every builder I have done speed tests for (with planning hulls) I have strict orders to NOT use the trim tabs so the baseline is the same between different engines, generally I see a 2 knot improvement or so with the tabs utilized and boat trimmed right, this is why you generally see lower cruise speeds in magazine tests.

Each engine is different. The Volvo D3-220 is a Diesel engine rated to cruise at 90% of WOT. Volvo recommends 4,000 rpm for WOT and 3,600 rpm cruising rpm.

90% RPM is not 90% load. It's probably around 70-80%.

How do they/you figure "load"? I like to use fuel flow, but Cummins specifies RPM rather than load.

If you don't have computerized load calculations, then the easiest way to estimate is the percentage of fuel flow to the percentage at WOT. It's not precise but it's far closer than RPM.

I've used fuel flow with turbine engines for years, just used to it I guess. I'm struggling to get Cummins charts to come up with this crappy hotel WiFi, would like to see if that percent looks about right for the RPMs they recommend.

There is not necessarily a direct correlation. One of the boats I run is a 2007 62' Sunseeker with 1100hp common rail Man's that holds 772 gallons of fuel, the computer calculates load and displays it on the display. Full of fuel 80% load will be 2030 rpms. 1/2 tank of fuel and 80% load will be 2180 rpms. But 150 rpm swing just by burning off 2500 lbs of fuel. It is supposed to turn 2300 rpms WOT, but slightly underpropped and will turn 2350 rpms full fuel/water which Man says is perfect, rather than being overpropped.

Hard to follow when it is not an apples to apples comparison. A QSC8.3 vs. a KTA19 discussion makes no sense as you wouldn't compare a Ford Fusion to a Ford F150, these were designed for completely different purposes.

Lots of available information if you spend the time looking, for Cummins:
Engine rated speeds from 2000 - 2800rpm, Cruise is Rated Speed minus 200 rpm
Engine rated speeds from 2801 - 3500rpm, Cruise is Rated Speed minus 300 rpm
Engine rated speeds from 3501 - 4500rpm, Cruise is Rated Speed minus 400 rpm

CAT makes it a bit more difficult by talking about Maximum Cruise Speed and Maximum Continuous Cruise Speed, both have limitations of operation.

There are pleasure craft diesel engines that have NO limitations on operation at WOT. When it comes down to it, you own the engine, after warranty it is really up to the owner to decide how highly loaded he wants to run his engine, with considerations for maintenance costs, engine life and resale value.

All this discussion on Engine Loads ignores that fact that it is tied into the propeller thrust. The load on the propeller drives the load on the engine which determines how much fuel is necessary to maintain that load. As CaptJ pointed out, the loads vary as fuel is consumed.

After a little yard time to have thruster tube glassed in and old bottom paint sanded down and 2 coat of Cukote it was time to open it up on the run home for 1 to 2 minutes. See attached pics. RPM's of 2359 Port and 2375 Stbd. Coolant temps were 192/190 respectively and 80gph at 100% load. 37.6knots.

She's a runner with room to spare!

Those are all perfect numbers. I like to see 2350 rpms on Man's even when they're rated at 2300.

Newly launched with a squeaky clean bottom ... those are good numbers. That "room to spare" will soon disappear with any growth or added load.

Hang on , when we have finished backslapping --------

Load never shows more than 100%. It can,t .
Therefore what was the rpm when the load just went from 99 to 100 .
How do we ( you ) know it's not actually overloaded in the screen shot ?

EGT at 2359 and 2375 would be nice to see as overloading would see a rapid rise into dangerous territory.

Not so fast with all you naysayers.
The load you see on your screen is a calculation and does not give you a full picture of the actual performance map. There is margin available in terms of fuel delivered to burn that you are not seeing and the really nice cushion of rpm - 59 port/75 stbd which is 67 rpm average.At this condition, exhaust temps are irrelevant because I can assure you they will be in spec.
Running real nice and plenty of cushion, bottom growth in the PNW is not the same as out East and you are not going to scrub that rpm in this well maintained boat. The boat is already in an owners possesion and has his gear onboard for a loaded condition.
I would like to see a two way average to eliminate any current effects.

I mean current. Wind at low to medium speeds not so much a factor.

When looking at speed data, I prefer a 2 way average, which includes the reverse course of the initial run.