Propping for WOT and 80% cruise

DM sent.

I have looked into this quite a bit over the years. I had CAT 3208s for 16 years and now C18s. There is the common folklore about running modern diesels at 80% load for long distance cruising. This is not what CAT recommends in their literature. Most of our CAT boat diesels are E rated engines. The following is the typical wording used throughout the CAT operations manuals and literature:

E Rated Pleasure Duty
• For infrequent use in variable load applications where full power is limited to one hour out of every eight hours of operation.
• Load Factor: Up to 30%
• Typical Annual Operation Hours: 250 to 1000 hours
• Typical Hull Forms: Planing
• Typical Applications: Pleasure craft, harbor patrol boats, harbor master boats, some fishing or patrol boats, sportfishers, motoryachts, and cruisers.

That means limited WOT running at rated load and rated speed up to 8% of the time (up to 30% load factor). But this load factor is your average load factor to achieve the CAT service schedule which is 10,000 hours for a major on C18s.

From the CAT literature on industrial engines that are actually a bit derated from our marine engines:


The load factor is limited to 35% for the average over all use not just WOT full load use. And next again from the CAT documentation is an example duty cycle for a D rated intermittent duty engine. This engine has a 50% duty cycle rating with a typical duty cycle shown:

This seems to me to be clear guidance from CAT on where the average engine load needs to be for long distance cruising. Also my CAT C18 manuals clearly provide guidance on my cruise RPMs and say I shuold cruise at 1900 to 1950 which on my 68 ft Viking Princess is a 65-70 percent load.

I believe that technically the Cat 3406E-800 evolved to the C-15. That would be its equivalent.

What I posted above is not CAT C18 specific. It is the load and duty rating guidance across many of the large CAT industrial diesels including the C15, C18 and C32.

What you posted has nothing to do with load at cruise or continuous load and CAT does indeed recommend 80% load as an all day, every day, cruise speed for the C18 and most all other E rated CAT marine diesels. What you posted states that you can run an E series Marine C18 at 2300rpms/100% load for 8% of the time during it's life, or for 1 hour every 8 hours.

Cummins is easier to find than CAT (will try to find CAT's later, nowhere have I seen CAT recommend rpm's for cruise on their modern diesels)
:
High Output (HO)
This power rating is intended for infrequent use in variable load applications with a load factor of 10-30 percent. Full power is limited to one out of every eight hours of operation. Reduced power operation must be at or below 80 percent load.

Engines with this rating are intended for powering recreational use vessels only. Use of High Output ratings in commercial applications will at Cummins' discretion void the warranty. Commercial use is defined as any work or employment-related used of the product, or any use of the product which generates income, for any part of the warranty period, even if the product is only occasionally used for such purposes.
Marine Ratings and Definitions | Cummins Inc.

What I posted has everything to do with load at cruise or continuous load. What I posted is also directly copied out of CAT literature and not from random mechanics or dock folklore. The images I posted directly show that when CT says 30 percent load factor, that means the average load factor should not exceed 30%. Other than the 8% of 100% WOT operation, you cannot just go to 80% and run forever. That gives an average load factor of way higher than 30%.

The following show how to calculate load cycle and ratings. Again directly out of CAT literature. Your Cummins example seems to say exactly what I am saying as well. Yes HO can do 1 out of 8 hours but the average load factor should be 10-30%. In my post above, look closely at the duty cycle figures, these show the WOT but also what is needed on average when not at WOT.

Let me try to make this a little simpler. CATs D rated engines say the following. Look closely at the figure which shows what needs to go with 10% WOT for a D rated engine. If your 80% all day to be correct, would require a B rated heavy duty engine.
From CATs literature for D rated engine:
Load factor limited to 50%.
Time at full load not to exceed 10% of the duty cycle or 30 minutes max, followed by one hour at Intermittent.
(see Figure 3 for typical IND-D Rating Duty Cycle)