Why are ship large Skegs and Rudders flooded with water?

To my surprise, I am learning that in medium and large ships Skegs and Rudders are typically flooded with seawater (or even freshwater) instead of being 'empty with air'.

First, is that true in most cases?
If so, what is the main reason for doing so?
Is generally the structure watertight, carrying the same water, or are there openings in the structure and the water is free to enter and exit?

My next question will be how's the best way to deal with this in Stability calculations, but first I wanted to clarify this. Thank you for your advice.

 

I am equally surprised that some of our NA members have not replied to your question. In my case I can only speculate that flooded skegs and rudders might have a useful function..........If flooded components are a normal (?) design feature, what is the advantage?

 

I'm not aware of any ships whose rudder and skegs are flooded with sea water. In fact, I would consider it unwise since it is difficult to treat and inspect properly the inside for corrosion.

I've seen, in a few cases, to have them filled, and later emptied, with used oil to help preserve.

Maybe you can share where have you seen it? Maybe for vibration control purposes?

Regarding how to treat it in stability calculations: if open to the sea consider it as not contributing to flotation; if not open to the sea consider it as a filled tank or added weight (if not considered on the lightship weight).

 

Wild *** guess incoming.
Would a near neutral buoyancy rudder have less thrust on the bearings? It seems the near neutral buoyancy foils would have minimal effect on stability.

 

Where are you being taught that?

Given the preceding one, this statement seems disingenuous. As stated above, I also have seen very few free flooding or water filled appendages, mostly prop shaft skegs.

There is absolutely none. If free flooding, it is a maintenance, and ecological, nightmare. If "filled and sealed", there is a huge risk of temperature/density crush.

Pick your poison and go from there.

Are you calculating from the outside in or inside out?

 

Many of Philip Bolger's small boat designs had hollow, free-flooding rudders, centerboards, leeboards, and keel deadwood. If the appendage is to be hoisted, like a centerboard or leeboard, the weight required to give negative buoyancy to permit lowering would make raising difficult, whereas a lightweight structure can acquire negative buoyancy as it is lowered and lose the unneeded ballast as it is raised. In a fixed keel, a substantial underwater volume may be surrounded by structural timbers and side panels that are necessary for strength or lateral resistance but have no function of its own. If it were sealed and hollow, excess ballast would be needed to counteract its buoyancy, but if free-flooding the hull would lighten when hauled out, saving the cost of ballast and the difficulty of hauling.