Viking on the beach…

I guess you are correct. They definitely look too large for being only bypass pipes!

Don't mean to stray from topic. If needed, we can carry to a new thread.

"When properly designed," Very important text here.
"AND properly Installed" should be the next line of text.
I have witnessed some real FUBAR designs & installs that I would consider not helpful to the engines.

I have also witnessed some real FUBAR designs & installs that I would consider not safe.
Maybe a good thing, one of these mfg's is out of business.

It was reported that she was towed to the Viking Yacht Service Center in Riviera Beach, Florida.
About a three hour trip in towing.

Report says it has a few scuffs and a bent rudder as damage goes, and probably a hurt pride.

Are you saying that the underwater-exhaust back-pressure is less than it would be if discharged above water into the air?

That's exactly what I'm saying.

As a practical example, in my own boat, whose u/w exhaust were very carefully designed, the backpressure progressively increases from 0,4 PSI at idle to 1,1 at 1500rpm, when she starts going on the plane.
But at 1600, it goes down again at 0,55, and remains steadily around that value all the way to WOT.
Which defies logic, because obviously the gas volume at 2300rpm is much higher than at 1600rpm, and the exhaust pipes remain the same, so the backpressure should increase.
The only reason why it doesn't is that the scavenging effect also increases with speed, making the engines breath better.
As a result, engines actually run cooler anywhere between 1800 and 2000 rpm (normal cruising speed) than at 1500.

You should consider also that exhausts above the w/line are NOT straight from the engines, otherwise the noise would be awful.
Typically, they pass through a dustbin-style water trap feeded by the raw water coming out from the cooling circuit, and no matter how well built, those things do create a backpressure that increases with rpm.

I'm talking of wet exhausts of course - dry stacks are a totally different kettle of fish.

Actually, that's not what you said. You have now added the condition that you are comparing to a wet exhaust system, but with no numbers to compare to the numbers from your boat.

Yeah, as if the boat which is the subject of this thread (or any other sportfish boat for that matter) had dry exhausts...
My sentence which you call a condition I added was more like stating the obvious, but if you were in doubt I guess it didn't hurt to specify.

Ref. my numbers, I only posted them because the behaviour throughout the range is self-explanatory of how scavenging works.
If you have any others, either related to non-underwater wet exhausts (or also dry exhausts, if you wish), why don't you post them for comparison?

I have no numbers to post; that's why I'm questioning you about your statement that underwater exhausts have less back-pressure than above-water exhausts. The selling point of underwater exhausts on motor yachts back when it was new was in fact that they mitigate exhaust noise and fumes. I have never heard anyone from the boating industry try to sell underwater exhausts as a reducer of exhaust back-pressure.

I have heard of the scavenging effect on underwater exhausts of boats on plane and how that might reduce back-pressure. But that reduction has always referred to the back-pressure of the underwater exhaust system. No where have I heard that that scavenging effect reduces the back-pressure to less than an above-water exhaust back-pressure -- until now from you.

As far as your engines running cooler at 1800-2000 rpm than 1500 rpm, I would have guessed that at 1500 rpm your boat is in the hole (stern squatting, bow up} and you are in an over-propped condition. That'll make wet exhaust engines run hotter, too.

I see two reasons for that.
First, quite a few in the industry aren't even aware of it - which explains why some u/w exhausts are awfully built and detrimental to the engines performance. In fact, I did say "when properly designed", and I agree with CR that there are some FUBAR u/w exhausts around!
Second, it's not something that help selling boats, since very few boaters would appreciate that - if any.

That said, I can assure you that scavenging u/w exhausts are not something I made up after dreaming of it.
Just check out the outlet shape in boats from builders who know their onions, particularly in fast boats like Magnum, Pershing, Itama, Baia, etc.: it's immediately obvious that when the boat is on the plane those outlets run completely dry.
BTW, the gas flowing under the sides of the hull bottom also contributes to aerating the hull, reducing drag.

Of course, ultra fast boats with petrol engines and #8 outdrives are yet in another league, and use straight unrestricted transom exhausts - but that's an apples vs. oranges comparison, as well as dry stacks.

My understanding of the challenge with the exhaust for a sportfish is not as much for the acceleration, but more on the deceleration. I recall a lengthy discussion about side exhausts on these builds...

So, Catepillar, MAN, MTU, Cummins, et al don't know about the possible harmful effects of underwater exhausts, and yet, they still warrant their engines with those exhaust systems. Now, you are really reaching.

It's obvious that you can't prove that underwater exhausts produce less back-pressure than above-water exhausts. I'll keep looking for the answer, and if someone else knows, please let me know.

Leeky, it seems to me that you keep twisting what I write just for the sake of it, taking also the time to highlight bits on sentences in red, etc.
I'm not aware of (or can't remember, which is the same in practice) any reasons why you might have an axe to grind with myself.
But if you do, well, rest assured that I'm not interested at all to keep discussing with someone clearly unaware that denial ain't just a river in Egypt, if you see what I mean.
So, I will answer this last meaningless post of yours, just to give you the benefit of the doubt.
But don't hold your breath for any further replies from my part, if you will come up with any other twisted interpretations of what I wrote, because I'm not here to prove anything, neither to yourself nor to anyone else.
Of course, you are free to go on with your rants if that makes you feel better, anyway.
Just don't expect me to waste more time trying to understand their meaning and your weird train of thoughts behind them, because life is too short.

Having said that, back to your post.
My sentence which you highlighted was clearly referred to boatbuilders, not to engine manufacturers.
And I'm saying "clearly" because the former are the ones who design and install exhausts in their boats, not the latter, as I hope you know. And if you don't, well, you just learned something.
But some of them use u/w exhausts just because it's an easy way to reduce noise without internal silencers, without caring enough about the outlet dimension, position and design, which aren't so relevant for noise reduction but are crucial for backpressure optimization - and THIS is the reason why with u/w exhausts you can find the good, the bad and the ugly.

OTOH, it's true that engine manufacturers are well aware of the possible harmful effects of bad exhaust systems - ALL of them, not just u/w ones as you are suggesting.
In fact, they clearly specify the max admissible backpressure and only validate their warranty after checking that the installation fits their requirements.
Which BTW are related not only to exhaust backpressure, but also other things like sufficient e/r ventilation, fuel supply, raw water intakes, g/box mated (unless supplied together with the engines), and so on.
So, yes, Caterpillar et all have installation requirements for validating their warranty, and all boatbuilders must meet them.
But if you think that all the boatbuilders are equally good at skinning this particular cat, just because they manage to get the engine warranty validated one way or another, either you are very naive or you haven't seen enough different engine installations, methinks.

STATEMENT BY VIKING YACHTS REGARDING THE VIKING 92 BEACHED IN DELRAY, FLORIDA
On November 3, 2023, a 2017 Viking 92 Open Bridge Convertible (92 C) departed Palm Beach Inlet en route to Fort Lauderdale, Florida. Within an hour of departure, the 92 C encountered a fuel interruption to the generators and then main engines, at which point the 92 C lost power. It was set adrift as the captain and crew tried to resolve the problem. The crew tried to deploy the anchor by releasing the brake, but unfortunately the devil claw was still attached. It subsequently was lodged into the deployment chute rendering the anchor unusable. The 92 C came to rest on the shore in Delray Beach, Florida.
The vessel is now at the Viking Yacht Service Center safely blocked in our Riviera Beach yard since Sunday night. After our technicians inspected the 92 C, they determined that the incident was not the result of manufacturing or design error and that the damage was minimal. Once we receive the needed parts and materials, we expect to have the boat back fishing hopefully by the New Year.
Most importantly, the captain and the crew are safely ashore with no incident.

Question now goes to the specific details of the, " fuel interruption?"

I always have chain stoppers in the bow, but only keep ‘em in place/locked while crossing the to or from the Bahamas.
(Learned hard lessons from having old and simple boats in my 20’s, everything
could and did go wrong, frequently)

A sporty should have a lunch hook or better in the lazz, along with a bucket of chain and rode. You're in the Bahamas and dropping anchor, you'll likely want to back down close to the beach and toss that hook to maintain your proximity. In a pinch like this that same setup could have been deployed from the cockpit after tethering the rode to the bow.

Also brings to mind deployment of a sea anchor to attempt to maintain a parallel course. Both of these items are pretty much automatic to have aboard in my book.