Being a cruiser kind of guy for the last couple years (cabincruiseronly.com, very funny), raw water boats that stay in the water definitely corrode fast, both the iron and aluminum parts. Most of my raw water cooling friends that flush have very little trouble. This also cleans the manifolds and risers (especially important with aluminum).
Closed cooling also works well, especially if you get a heat exchanger with the capacity to cool the manifolds as well. If you get a closed cooling kit, be sure to check to see if it's sized for manifold cooling. I think virtually every exhaust manifold, except Imcos, can be plumbed tis way.
Gary

Hey Nort,
You could just move to MO and boat with us on LOTO instead!:D

I flush mine everytime I'm in salt water. PITA but it's a necessary evil:( . A full closed cooling system is the only way to go if in salt. It would probably pay for itself in a few years.

I'd flush it even if it is a closed system.... if there are dissimilar metals connected by salt water... electrolysis will eat it alive... also check and change the zinc sacrificial anodes often.... Another problem is if someone has shore power close by.... if it is "leaking" electricity into the water.... you will suffer from it too. Even if you have a glavanic isolator in your shore power.... your neighbor could be causing corrosion on your outdrives.I'd invest in getting a hydro hoist.

Ok Guys I'm new here.... what does PITA and ttt mean?

Pain in the a$$, to the top

I have closed cooling and still flush it out everytime.

Thanks Checkmate

Closed Cooling
 

I have a 90 5.7 Mercruiser that I put a Monitor Products heat exchanger/closed Freshwater cooling system on i. It runs antifreeze in the block and manifolds, the Salt water is dumped into the risers.
It was a freshwater boat 3 years old when I got it, in 94.
It has lived 6 mos per year on a mooring in Casco Bay, Maine, as salty as it gets! I rusted thru one set of risers (these are salt water cooled, and were stock cast Iron). They lasted about 4 years in the salt, with one muriatic acid rod-out cleaning. I bought a set of take off SS cast stock Mercruiser risers on Offshore only(which I found the day I was searching, and have been a member since.. great board!)

The benefits of FWC are many, the manifolds and engine are 14 years old, and have no scale in them, the engine runs nice and cool, with nice temperature control, and there is no internal corrosion. Winterizing is very easy, as you don't need to drain and flush, and wonder if you got it all, just pull the ends off the heat exchanger, and pull the hose off the power steering cooler, and drain the risers with two petcocks. I usually flush the engine in the fall, but not always. I think the $600 that the system cost me has paid for itself many timews over in manifold and engine life. I used a system with my Alpha that used the Sterndrive pump, no seawater pump was added. I added an overheat buzzer/oil pressure buzzer when I did the conversion, it has saved me once from mussels clogging my intake. Best $50.00 I spent!

I think most people run raw water systems because the manufacturers want to keep the costs down. Most expensive yachts that run Crusader engines (GM) use FWC.. it is just cost that keeps them out of speedboats. In my opinion(and some marine engineers I know), you are crazy not to install a system if you are running on the ocean. That said, I ran a Volvo for nine years raw water cooled and it was okay.. though, it did go thru head gaskets, and a manifold.

My .02

Dave Leonard

Closed Cooling
 

I have a 90 5.7 Mercruiser that I put a Monitor Products heat exchanger/closed Freshwater cooling system on i. It runs antifreeze in the block and manifolds, the Salt water is dumped into the risers.
It was a freshwater boat 3 years old when I got it, in 94.
It has lived 6 mos per year on a mooring in Casco Bay, Maine, as salty as it gets! I rusted thru one set of risers (these are salt water cooled, and were stock cast Iron). They lasted about 4 years in the salt, with one muriatic acid rod-out cleaning. I bought a set of take off SS cast stock Mercruiser risers on Offshore only(which I found the day I was searching, and have been a member since.. great board!)

The benefits of FWC are many, the manifolds and engine are 14 years old, and have no scale in them, the engine runs nice and cool, with nice temperature control, and there is no internal corrosion. Winterizing is very easy, as you don't need to drain and flush, and wonder if you got it all, just pull the ends off the heat exchanger, and pull the hose off the power steering cooler, and drain the risers with two petcocks. I usually flush the engine in the fall, but not always. I think the $600 that the system cost me has paid for itself many timews over in manifold and engine life. I used a system with my Alpha that used the Sterndrive pump, no seawater pump was added. I added an overheat buzzer/oil pressure buzzer when I did the conversion, it has saved me once from mussels clogging my intake. Best $50.00 I spent!

I think most people run raw water systems because the manufacturers want to keep the costs down. Most expensive yachts that run Crusader engines (GM) use FWC.. it is just cost that keeps them out of speedboats. In my opinion(and some marine engineers I know), you are crazy not to install a system if you are running on the ocean. That said, I ran a Volvo for nine years raw water cooled and it was okay.. though, it did go thru head gaskets, and a manifold.

My .02

Dave Leonard

The Mercury closed cooling kits are $1500, plus 4-5 hours labor (each). Not seeing the value of that yet......

Still need to flush to save the exhaust parts! Thanks for everyone's input. I'm being edukated here! :D :D :D