Season startup on a dry block

[MWJ68]

I’m trying to improve my understanding of the open coolant system. 
I know after dry blocking your motor (no antifreeze flush) you can put the boat in the water after layup is complete without a need to “prime” the block.
Here is where I’m lacking understanding: with an engine running normally, the water in the block heats to a level that creates the required pressure to open the thermostat. Cool lake water is then allowed to pass through the thermostat, lowering the water temp inside the block.
my question is: if the boat is placed in the lake after winter layup with a dry block, there is no water in the block to heat up and apply the pressure to open the thermostat. Then how does water get into the block after initial startup?

Thanks.  
 

[Woodski]

Thermostat operates via temperature not pressure.  T-stat has what is called a pill, it activates at a prescribed temperature.  You can verify by dropping one in a pan of water heating on your stove.  Use a laser pyrometer (available for ~$20 at hardware stores or Harbor Freight) to watch at what temperature it opens.  Open pan, no pressure.

Open cooling systems are equipped with a bleed or bypass that allows water to flow to the exhaust manifolds so they don’t overheat.

Sometimes the raw water pump will need to be primed or burped to get it to flow.

Edited October 2, 2023 by Woodski

[roeboat]

It's a common misconception that the thermostat lets cool water directly into the engine, when really it allows the hot water out of the engine. So when you start the engine with an empty block it fills up almost immediately. Also the thermostat doesn't have a perfect seal or it has a weep hole so the air will be displaced out. 

 

[MWJ68]

Thanks to you both for clearing this up. Now I understand. 
 

Additionally I now understand how dry blocking the engine and then running it off a bucket of antifreeze not only fills the fresh water system with antifreeze, it also fills the block. 
 

Thanks for all the help!