Block Heaters

Those are quite popular on sailboats in my area.

Most of us also fully winterize in case of a power failure so running the engines is not an option, unless the motors are set up to do that (e.g. impellers removed). Many of us don't start but do crank a bit just to reset the position of the valves etc. Some recommend against that short crank.

Don't air pumps usually have some kind of motor? I wonder what if anything people's engine manuals say about this.

My understanding of the reason is that in any stone cold start the engines are essentially spinning with no lubrication.
But the same result can be achieved by turning the engines manually, in a much slower and more controlled fashion.
With engines like ours, all it takes is a 32mm socket wrench with a long(ish) ratchet.
By turning them 180 degrees at a time, you are sure to close - in ALL cylinders - any open valve.

That is the good logic against the starter tap that I have heard. When you really get down to it we are probably talking about 4 months of lay up and can really get into the "weeds" on some of these questions. I have seen heavy equipment parked for a year at a time - I don't reckon any one is worried about that engine turning every few weeks .... when needed, they fire it up and go make some $$. And if it won't start they will shoot enough ether into it until it does that you could have a kidney removed.

Never had to store on the hard, but making sure exhaust is not holding any water would be important. Most boat exhaust systems will hold some water. Being on the hard would be a plus for underwater exhaust, they act like an moist air pump when in the water.

I agree with the principle, but I'm not aware of any way to do that on a boat with u/w exhausts while she's sitting in the water.
BTW, the u/w exhaust also implies an idle bypass somewhere above the w/line, that leaves a permanent passage for external air.

What I do with my boat (berthed in an area of the Med where freezing is not an issue at all) is just flush the engines with fresh water, whenever I know that she's not going to run for some time.
Does nothing to limit internal damp of course, but at least takes salt out of the equation.
If anyone is aware of a practical way to avoid internal damp, I'm all ears.

I use two plastic play sand pails push into the ends of the exhaust. I've lost one or two over the years but generally they stay put. I've seen some folks inflate a small/large ball in the tubes. Just don't over inflate. Generally most folk do nothing with the exhaust opening, based on what I have seen at our marina over the years

I guess you actually don't have a "full" underwater exhaust if you can do that, but rather round steel tubes sticking out from the stern of the boat.
Mine are amidship, moulded together with the hull bottom, and impossible to reach - let alone stick anything inside them...

Blocker heaters running at the dock should evaporate moisture internally. Our last boat had b/heaters which kept all toasty and dry when at the dock. Not sure if the idle bypass would eliminate all pressure on the u/w exhaust. Had to repair a cracked fiberglass pipe on one ours where the risers connects and looking at the water so close to the engine it was obvious that each wave or when boat rocks moisture would push towards the engine. Not good long term if sitting for long periods without running engines. The fresh water flush is a great idea, like the way you had yours plumbed.

Yet again, I see your point and agree with it in principle.
Fully underwater exhausts along the hull bottom are very popular though, at least among European boatbuilders.
And structural problems with them are reasonably rare.
On what boat brand did you experience the cracked pipe, if you don't mind me asking?

BTW, also block heaters are pretty rare around here.
For instance, I've seen plenty of boats with all sort of MAN engines, from different boatbuilders, but none of them (mine included) fitted the heater which is natively available from MAN as an option.
And while I can see why you would suggest to keep block heaters on while the boat is docked, since I can't remember to have ever seen less than 50°F in the e/room where I'm based, I'm not sure it's worth the hassle and the cost to fit them.
Also because I came across boats 10+ years older than mine, used more rarely, in colder climate, and also with no block heater, whose engines are still running strong...
Though I'm writing this with my fingers firmly crossed!

Just curious how you do that on your Post 50? I know my exhaust sits 1/2 to 3/4 submerged underwater at all times except when I am really fuel light which is not over the winter. If you seal the exhaust port with water in it, I would think it could lead to some suction with changing temps, etc which could push more moisture into the engine with submerged exhausts and sitting over the winter. Are you just fitting it enough to still allow air/water flow? On the hard, I absolutely see sealing the exhaust up tight.

The boat is a Choey Lee PH. I don't feel the vertical fiberglass pipe cracked on it own, there are hd t-bolt clamps on the silicone exhaust hose connector that were over tightened causing a few small cracks at the top opening. We discovered it when replacing the exhaust riser on that side. Had the same thing happen on another boat with surge tube connections.
This boat has block heaters, just never wired to the panel, it was kept in South FL until we bought it so no real need for block heaters. The er fresh air intake and exhaust on both sides have a hinged covers which makes heating the er easy and helps keeping it dry.

I don't store with a lot of fuel. I'm a contrarian on that. So Just the bottoms of the outflow are immersed. I figure closed or not the exhaust will always have water in them. I don't think I get any extreme temp changes because I sit in the water and my engine heaters help at bit in that area. Base on the inside angle of my exhaust system I always figured that at most 6-8 feet of the rear hoses had some water in their bottoms and the rest are dry. I'm more concern about the air flow and air surges from swirling moisture laden winds around the dock. Maybe I've been doing something that is actually harmful all these past years??

I think I've corrupted this thread a bit, Re reading it I think the discussion was primarily about underwater exhaust. Sorry for that.

Imho nope, nothing "wrong".
But I'm not sure I would bother doing anything at all, in your boots.
Aside from if and when the boat should stay on the hard, that is.

Ok, but in this case I wouldn't call that an inherent defect of u/w exhausts.
Just a poor installation (or re-installation) of a component that happens to belong to an u/w exhaust system...

Exactly, I think it had been re-assembled to replace the exhaust hose on the port side when the stb riser was being replaced several years earlier. The installer just cranked too hard on the t-clamp on the fiberglass pipe. Being u/w exhaust had nothing to do with the issue.