why are the bottoms of some subs red?

Sometimes it's no other reason than tradition ... the bottoms of ships are always red, so ...

But for subs that sometimes spend a lot of time on the surface, such as the Kilos, the red color is caused the the anti-fouling and corrosion (read, barnacles, etc.) treatments put in the paint to kill said marine life.

It makes more sense if you sort of think of the problem in the other direction.  The red paint, as is noted above, is anti-fouling paint.  In terms of maintenance, durability, and keeping the sub as fast and quiet as possible, it would make sense for the entire hull to be painted that color.  But a red sub would be mighty conspicuous on the surface - or even below it.  (The red on the few subs I've seen is a remarkably bright, almost orange-ish color.  In clear water on a calm, sunny day a submarine painted that color would be visible to a human being in an airplane even if the sub were quite a few feet under water.)   So the upper part gets painted black (or some other camouflage color). 

I imagine (though some submarine expert may correct me here) that the whole hull initially gets painted with red primer, and the black gets applied on top of it.  I don't know whether the finish coat of red is the same stuff that's used to prime the whole hull; I rather doubt it. 

 

In my experience anyway the whole boat was usually "primered" in anti foul paint which is generally red-brown in color but not always. The above waterline is painted in a anti foul black. ( Some Boomers had haze grey sides and sails while on Westpac patrols in late 60s and early 70s) 

Nobody usually sees the lower half of a boat so the don't paint it black. Saves weight and $$$. I have seen overall black boats. Depends on the boat, the yard, and the conditions.

Judgeing from the general disrepair and poor condition the Russian Navy keeps their boats in I would guess they don't overpaint their lower hulls.

Thanks guys! That makes a lot of sense.

Joshua

The hulls are now painted completely black now because it is cheaper. The red antifouling paint isn't used much anymore. The hulls are hydroblasted to remove sea-growth and painted over with several coats of the boot topping black. Saves time and money.

<BLOCKQUOTE><table class="quoteOuterTable"><tr><td class="txt4"><img src="/FSM/CS/Themes/default/images/icon-quote.gif">&nbsp;<strong>subfixer wrote:</strong></td></tr><tr><td class="quoteTable"><table width="100%"><tr><td width="100%" valign="top" class="txt4">The hulls are now painted completely black now because it is cheaper. The red antifouling paint isn't used much anymore. The hulls are hydroblasted to remove sea-growth and painted over with several coats of the boot topping black. Saves time and money.</td></tr></table></td></tr></table></BLOCKQUOTE>

I was about to say that... <img src="/FSM/CS/emoticons/icon_smile_wink.gif" alt="Wink [;)]" />

Actually the primer is a different paint (with different properties and chemical recipe) than the anti fouling paint.  The primer of choice for a long time was red lead.  During WWII the Mare Island Shipyard developed the Mare Island Epoxy Primer system, which was and still is available in several different colors: primer red; black; buff; etc.  Several coats of primer were appied, each one a different color, to ensure complete coverage.  The antifouling paint usually has copper or zinc in it, which the  marine life doesn't like.  During WWII some US Fleet Subs were painted in an overall black scheme; some fleet boats had the gray paint job.  Both paint jobs used black anti fouling paint - red was easier for an aircraft to see.