Propping for WOT and 80% cruise

rtrafford · Nov 13, 2025

Capt J said: ↑

I have done engine start ups and required seatrial on well over 100 sets of new CATs with many different CAT technicians as well as higher ups from CAT, mostly for well respected large scale Yacht Builders. I've also been the Captain that CAT used to set up the joystick parameters or fine tune them for a few new yacht models for yacht manufacturer(s) for the CAT 360 system. I am very well versed in what CAT engineers want and don't want in regards to their engines, from the CAT engineers and top technicians.

The entire discussion was Caterpillars rated cruise speed when at cruise and this is someone else's thread searching for answers to his propeller situation AT CRUISE RPM's IE 80% load or 1900 rpms or betteron a set of 3406E 800HP engines, not your thread, which is indeed 80% load (the answer the OP was asking about). 80% load is where CAT allows them to be run all day, such as if you are delivering the boat in the ocean from one place to another, unlike say WOT which can only be run for an hour at a time, or 30 minutes depending on engine. It can be run anywhere else under that as well. But if you blow a motor and they see in the ECM printout that you've been cruising the engines at 85% rated load when you do cruise them, or the engines are never run above 40% load, your warranty is not covering it. There is absolutelty NOWHERE in the CAT display that will show you as an owner what your average lifespan load is, nor is CAT concerned about it as far as warranty goes. No yacht ever runs at cruise rpms all of the time, so the overall load works out usually to much less than CAT asks anyways. You're reading from the CAT manual, but not totally understanding what you're reading.
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....and by all means, run your Cats the way that you see fit and proper to do so. You don't need anyone else telling you how to manage your systems. As a very basic rule of thumb that does not apply to DDEC's, as you advance in running time beyond 3-4k hours on any machine, you should be monitoring oil sample reports, exhaust temps, oil consumption, sounds, etc. The assembly of these points of data will help the engine speak to you in terms of when it wants a makeover. There is no magic litmus in a handbook by which to live. Two engines from the same factory run that have the same type of use and care will respond in their own way regarding longevity. For instance tomorrow morning your starboard engine could lose #1. Meanwhile Port might be just fine pushing you along for another 1500 hours.

DOCKMASTER · Nov 16, 2025 at 11:37 AM

Interesting read of this thread for sure. I run my C-18's (1136 hp) at varying RPM's based on sea conditions, time I have, my mood on any given day, etc). I find I typically cruise at 1800-1850 most often. This is 65%-70% load for me. 80% load is around 2025-2050 rpm if memory serves. I just did a 740 mile trip and mainly was around 1850 rpm most of the time. If I have lots of time I may slow way down to 8-10 knots then run them up to 80% load toward the end of my run to clear them out. I'm not worried about how many hours to overhaul. Unless I get an unusual failure (knock wood), these CATs will almost certainly outlive me. It is more important to focus on good maintenance and care then to worry about 60% vs 70% loads IMHO.

Regal400 · Nov 18, 2025 at 2:22 AM

Great info shared guys much appreciated.
I have c18s on my sportfish - just hit 4k hours
Cruise her based on load typically it’s 76-80% anywhere from 1900-1965 rpm.
Engines fall into a grove at 1950-1965 and overall avg fuel burn seems to be the same from 1850-1965 so sea condition permitting 1950-1965 it is!
Love my cats

DOCKMASTER · Nov 18, 2025 at 1:42 PM

Engines fall into a grove at 1950-1965 and overall avg fuel burn seems to be the same from 1850-1965 so sea condition permitting 1950-1965 it is!

When you say fuel burn seems the same, are you talking about gals/mile? My gals/mile increases as I speed up but not much relatively once I get in the 60% + load range. I burn roughly .15 gal/mile more with every 10% load increase. So 80% load is about .3 gals/mile more than at 60% load. If going long distances this obviously can add up to quite a bit more fuel.

Also curious what boat are your C-18's in and what speeds do you see?

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Propping for WOT and 80% cruise

rtrafford Senior Member

DOCKMASTER Senior Member

Regal400 New Member

DOCKMASTER Senior Member