Max RPM's duration at Sea Trial

Most owners will allow WOT for 30 sec to one minute. More that that is, in most circumstances, unnecessary. The longest I have ever seen at WOT was around 5 min. Some boats/engines sound/feel/handle ok at WOT and some just do not like it.

5 minutes has always been our limit on WOT and that's been the case for me for every boat I've owned. If it was a trawler with a continuous rated engine, then it might be different, but I've only owned planing or semi-planing boats.

When we bought our current boat the CAT mechanic hooked up all the gauges and had the boat run WOT for 15 min. That was the CAT standard seatrial test

I run the boat a few times a year at WOT for 5 min to ensure WOT RPMs and to make sure there is no temp creep and cooling system is functioning properly. Can't do that in one min

We had to replace a fuel inj pump last year. CAT mech had me run boat 5 to 10 min WOT several times to set inj pump correctly and match the two engines

Yes, I know those scared of running at WOT period. Perhaps too, if the engine is in very bad shape, their fear is justified. However, with a well maintained engine, brief runs at WOT are not a problem. 30 minutes, however, is just unnecessary and could be abusive to some engines.

As to babying engines, we run ours a lot at cruise, 70-80% load depending on the boat. We're not ones who run boats capable of 26 knots, with cruise of 20-22 knots at 10 or 12 knots all day. And definitely don't run boats with 41 knot WOT and 36 knot cruise at 10-12 knots all day.

5-10 minutes, 1 minute isn't enough to see how the coolers are doing, speed, rims etc. I know one popular surveyor who does it for 15-30 minutes. Way too much imo. However, the dealers do it on brand new, not even broken in engines for about 15 minutes, per manufacturers instructions.

An E rated engine as many planning pleasure craft are rated at least for CAT engines from a few years ago 8 percent at WOT up to 30 percent of max load. So nobody runs anywhere near that. But if you cruise longer distances at 70 percent or more of load that is way beyond the E rating

Since we tend to do longer distances to get the lifetime out of our engines and do an average of say 40 percent of load we cruise at 9 knots for maybe half the distance and plane at 20 knots which is maybe 70 of max load. That gets close to the E rating and saves fuel and is easier on the boat and machinery. Especially when it's nice out. Slow down and enjoy it.

I am happy with 3 to 5 minutes, will not go over 5 minutes.
On a boat that had MAN's that spent a great majority of their operating time between 68% to 80% loading, on an annual performance sea trial we would hook up the computers and run the boat at higher RPM settings than the 2150 max. cruise.
My practice was to increase the RPM by 50 at a time, and watch for temps and pressures to stabilize at each new throttle setting. So we spent a good amount of time at 2150, 2200 and 2250, before getting them above 2300 for a few minutes to ensure the engines would spool up like they are supposed to.
We were told to keep a close eye on lube oil temperature, as that would show an issue before cooling water temperature in most cases. So, once we had the engines on the pins we watched for temps to rise and stabilize ... if all within spec and not alarming we were happy with the result.

For a different perspective, worked on a project that required 8 hours at WOT for the final Ship Acceptation Trial (SAT). Everything was set-up, things went well, and they got bored at about 2 hours and in another hour everyone agreed that was sufficient and they all shook hands and called it good.

I would agree with 3 and no more than 5 mins on a trial. Not how they would typically run the boat, so why so much WOT? To get from the dock to open water for us in 15 mins in the Bay and they just recently took out the "no wake", though still won't run at speed through there. No need get "the finger" from everyone. So our engines get an easy warmup and cool down. When it is calm at the end of a trip like to put it on the pins for 3-5mins and run through the gauges to see how everything is doing. I know diesels are suppose to need WOT for X mins every X hours of operation. Don't keep a log, but figure we are close enough to that standard and the annuals show the engines are good, no leaks or bad oil test. I figure 5-15 mins of WOT is what one should need to outrun a pop up storm to get home...but in all my time I can count on 1 hand the times I really got caught needing to make that run. Having said that, 30 seconds for a diesel at WOT would not satisfy me that the systems were acceptable for purchase.

Why? Because if any issue is going to present itself in the near future, it will present itself at wot now usually

I test drove a BMW 740iL many years ago. After I drove it around, and my wife drove it around, I took it out and floored on a country road on my way back to the seller. A bubble of air in the coolant whipped through the S-shaped top hose into the plastic radiator end nipple, and snapped it off. The owner dropped the price $1500 and I bought it, then replaced the radiator for a few hundred bucks and about twenty minutes work (which was mostly draining and refilling coolant). I almost never floor my cars or boats, but definitely on a test ride.

Before I sea trialed my boat CAT had suggested I run the 3406C's at WOT for 10 minutes. That will show if the boat is over- or under-propped and also show if there any impediments to cooling water flow through the engines.

The selling broker had a fit about that (he was worried about us using up his fuel) but I told him if we couldn't do that WOT test I was not going to buy the boat.

The CAT tech who was on board to do the engine surveys backed me up on this. The selling broker relented, we did the 10 minute run and everything went well.

Now, I don't run at WOT for more than a few minutes at a time. It just FEELS SO **** GOOD!

Dumb question, what does WOT stand for and why we don't "push" a yacht a its maximum Power for a long time ?

A Cummins QSC8.3 comes in different horsepowers, and duty ratings, for the same basic engine. High Output is their lowest duty rating, for recreational boats.

A similar power level KTA19 has these duty ratings:

I often read blanket statements on boat forums that marine diesel engines can run at full power, or at some percent of full power, but in reality that varies with which engine, and which rating that particular engine came with. Recreational boat buyers don't seem to know the difference, and a lot of dealers either don't know or aren't telling customers. Recreational boat builder are putting the smallest, lowest-rated diesel engines in their boats, and letting customers assume they can run them hard to get the performance out of them. When I was shopping for a smallish diesel boat, I found a lot of tired engines and a lot of misinformation.

Well, the majority of diesels that go into planning motoryachts and sportfish from the various diesel engine manufacturers are the high output version of the engine. Almost all of the engine manufacturers state cruise rpm's to be 80% of load. Rarely do you see another HP/output rating in a planning hull motoryacht or SF. Now displacement hulls it's a mixed bag and many will have continuous duty engines.

Would 300rpm below max be more than 80% for most diesels? This was just from Cummins, others might have different specs.

Boats like mine with diesels that I looked at, had to run wide-open to reach planing speed where I get my best efficiency. Some ads mentioned cruise speeds just below what would require WOT, so they had to be cruising at or near WOT. Some could no longer get on a plane, but the engines had been checked out ok by a mechanic. On some forums, some owners insist diesels can run WOT, based on I'm not sure what information. A fair amount of used boats not that old, had rebuilt or replaced diesel engines. Newer boat next to mine in storage last fall had turbo-diesel engines replaced at $100k. Each. Then blew one on the test run, he didn't have to pay for that one.