View Full Version : Closed water cooling system verses a raw water cooling system

Dan Stephens
09-02-2002, 07:04 PM
"I am sure that you guys are getting to know me well by now. I finally got my 350 engine running. It runs on a reverse rotation and i must say that it runs rather well. Now i have the Dilemma of desiding which way to set up my cooling system. I have been reading online that a closed fresh water cooling system is better because if you are running in salt water the engine will last longer. But i have also been aproached with the question, what do i do if the boat starts over heating. I have been told that an open raw water cooling system will give me better results and i have been told that lubricants can be run through the engine that will help slow down the erosion process when running my boat in salt water. The only down fall i see is making sure that i have the proper anti siphoning device set up in line with the raw water system so my engine does not get flooded out and my boat doesn't sink. So i guess what i am asking is, What kind of system would you guys run in your boat if you wanted it to last? Would a raw water system be ok if i decided to port my boat in salt water or should i run a fresh water closed system? Look forward to hereing from you guys and want you to know that i really appreciated all your help believe me. I hope that you guys also continue to follow my progress because this truly is a refurbish job from start to finish and i am sure that lots of other people can learn from my experiance through your site.

Dan Stephens
[email protected] ("")"

09-03-2002, 08:54 PM
"I'd vote for fresh water cooling, Dan. As usual, it comes down to dollars vs. durability. If you plan to keep your engine more than two or three years, it would be worth it in a salt water environment.

If I had invested thousands of dollars in a good marine engine, I sure wouldn't like the thought of hot salt water circulating around the engine's bare metal parts. The idea that FWC systems are more prone to overheating is probably due to their increased complexity and maintenance requirements, causing the heat exchanger and other pieces to be neglected.


Dan Stephens
09-03-2002, 09:03 PM
My wife and i just discussed your answer and we've dicided that fresh water cooling would be the best way to go. I totally agree with your suggestion and appreciate your quick response. Look forward to talking to you again soon.

Michael Hamilton
08-07-2003, 03:16 PM
" Hi guys...First time contact... I was blessed with a gift. A 23 ft. Mako inboard circa 1980. It seems to have a running hot problem. I've checked the risors and manifolds and made sure they were open. It has a raw pump-belt drive. I rigged a fitting to the raw water intake so I could attach a hose. When I do this the engine runs hot-220 degrees plus. If I put the hose in the water tank up front of the engine, it never runs hot. My question is are the two systems separate. If I fill the expansion tank should that fluid stay and does the raw water intake cool the manifolds? The fluid seems to disappear from the tank. Is there a common denominator? Also what is the purpose of the valve at the raw water intake? "

08-07-2003, 06:29 PM
" Mike, it is supposed to be two seperate systems. The fluid in the expansion tank should be a sealed system like your car that cools the engine, and transfers its heat via a heatexchanger, which is cooled by the raw water pump. The raw water also cools the manis and is then ejected out the exhaust. These two systems as I said before are seperate if all is well. The valve at the raw water intake is a safety measure. It can be closed if the boat is moored and it can alsoo be used as an emergency bilge pump as follows: Shut valve, disconnect raw water hose and drop this into bilge. The pump will pump any water in bilge thru system and overboard. This is in an emergency only. Obviously when done reconnect hose and open valve to restore raw water to engine: As for fluid escaping from the expansion or fresh water side:It can go many places none of which are good. Pressure test the fresh water side, try to locate leaks and hopefully it willl end your overheating. Kim PS The most common areas are manis and risers or blown gaskets, which will result in water in oil or crossflow between fresh and raw water sides. "

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Thomas Lura
02-19-2008, 12:56 AM
"Hi: I was hoping someone on here could help me with a cooling problem I seem to be having. I recently acquired a 1966 Stevens V-drive flatbottom ski racer. It is equipped with a very hi-performance chevy 348 w/409 heads the original owner spent an awful lot of money refurbishing this boat only to give up on it when it over heated. I agreed to purchase the boat after looking it over and seeing that something not sure what didn't look right with the routing of the cooling hoses. It has a cam driven water pump which I am told is only really working when the boat is at low rpms the main cooling is forced into the system under power. I did not witness the overheating but from what I could gather from all the questions I had asked was that one side of the engine ran very hot while the other seemed alright. What I am looking for is a diagram for a system that I have tried to describe if anyone out here might be familiar with it?? Any and all info would be greatly appreciated. Thank you for your time. Tom I can be reached at [email protected] ("") I do have some pictures and can take any that are needed to helping identifying my problem."