Converting from raw water cooling to FWC on 7.4?

W

Windjammer

Well-Known Member
Nov 10, 2007
726
Acworth, GA
Boat Info
340 Sundancer 2006
Engines
8.1 Mercruiser V-Drives
Have got my eye on a used 340 with the 7.4's that are raw water cooled. The dealer says that these can be converted to FWC closed systems. Anyone ever had this done or know what is involved? Seems to me this might be closing the barn door after the horse has already escaped - i.e. with 6 or 7 seasons already on the engines would the corrosion/pitting process be well on its way?
 
Save your $$. That block will rot WAY after you sell it. Just check manifolds & risers and you're allset. I dont know why people spend the money on converting. Most conversions are 1/2 systems and you still have to replace manifolds & risers and also have to worry about a heat exchanger also. Engine blocks will rot out from salt water but not for MANY, MANY Years. I had a 1984 5.7 260 hp and it's been in salt for over 15 yes and no issues. I now have a 98 7.4 raw water and I am just replacing manifolds & risers for the 1st time this year,
 
The fact that the block won't rot away isn't a meaningful fact since the problem FWC solves isn't "engine rot" but rather that of minute bits of silt and corrosion that settle in the skirts on engine blocks over time. As this sludge builds up it gets packed tighter and tighter and as the level rises, coolant will not flow in the skirt area and the cooling system can no longer function and the engine overheats.

Windjammer doesn't tell us what year he is considering, but if the engine has been sea water cooled for 50 to 100 hours of running time, the general consus in the industry is that it is too late to make the change and you should just run out the sea water cooled engine. The time to change to FWC is on a new boat with very low time on the engine or at the time you repower.

On Mercruiser engines, riser and manifold maintenance and periodic replacement is an ongoing expense whether you are sea water or fresh water cooled.
 
It's a 2001 with about 300 hours...have also seen a listing for another that has "freshwater flush" - is this a factory option or an aftermarket system?
 
Might be either. Most likely aftermarket. Ask for details. There are also different kinds of freshwater flushers.

Esteban
 
I understand that about the buildup of crap that can block passage ways but again it will take a LONG time for that to happen as well. Its just my opinion. ALso like I said with fresh water cooling you pay around $1000 for it and you would need to replace the heat exchanger before the block went bad. Thats another say 1000 for a heat exchanger. That $2,000 so far. IF the block went bad and you needed to replace it that is 2,000 towards a block that even with fresh water cooling by now would probably need a rebuild anyways because of the number of hours on it. Thats the way I see it. I was thinking of the same things but thats why I didn't convert and personally IF I was buying even a new compl engine I would probably keep raw water cooling. (Less parts to worry about and especially overheating from a coolant leak) Don't get me wrong there are some pluses about fresh water cooling as well but for me they dont tip the scale in that direction.
 
fwebster said:
Windjammer doesn't tell us what year he is considering, but if the engine has been sea water cooled for 50 to 100 hours of running time, the general consus in the industry is that it is too late to make the change and you should just run out the sea water cooled engine. The time to change to FWC is on a new boat with very low time on the engine or at the time you repower.
Click to expand...

You guys just made my day. I bought a Raw Water cooled boat that was 9 years old with 170 hours. I enquired to two places regarding a FWC conversion (big bucks, since it was two engines), and BOTH said "don't bother, not worth it on your engines". After 2 years and 90 more hours on the clocks, I do wonder if that was the right choice. Also. . . I am worried about potential repower costs down the road -> but Ashton has reassured me.
 
Mine is a 98 with 260 hours and runs like a top. I also have a 1984 SeaRay w/a 5.7 and it still runs like a top(salt water cooled)
 
If I had the choice and it made technical sense to convert to FWC, I'd do it.

My experience with sea water cooled engines in Florida was about 8 years and 600 hours before we began to have trouble with the corrosion sludge build up. The way we maintain our boats I am certain we could have significantly extended that since the compression on the engines was still at new 7.4L specs.

Also, there are more components that just the block that function in salt water...the circulating pump, the thermostat, thermostat housing and valve all have shorter lives in sea water.

My final point has to do with resale value and location.......a sea water cooled boat in a coastal area will sell slower and for less money when other FWC boats are available......so you need to consider the value of your investment in both the boat and in a cooling conversion.
 
Don't you just hate it when Frank is always right?

Just kidding, but it's true. FWC boats around here sell faster. From what i can tell, that's true everywhere.
 
I too evaluated this conversion but after alot of research, I decided to forgo this modification for basically the reasons Frank outlined already. However, I decided to install a homemade flushing system which allows me to flush the motor with fresh water. Every time I return to the dock I hook up the dock water and run for 5-10 minutes. This leaves fresh water in the boat all week while it waits for me on the weekend!

There will be alot of debate on this topic. Many people feel this is a waste of time but I hope to extend the life of my rwc engine by some factor. I can't help but think that a clean water flush is better than leaving the salt water in the motor all week. To me it takes a few minutes to do this.
If I replaced my motor I would definitely convert to fwc but for me it is too late.

Mark
 
Boatrboy, What fresh water system did you install? I was looking for one as well but I hate the idea of cutting that water intake hose as it is very hard and wrapped tightly.
 
SAshton said:
Boatrboy, What fresh water system did you install? I was looking for one as well but I hate the idea of cutting that water intake hose as it is very hard and wrapped tightly.
Click to expand...

I made a set up out of bronze shut off valves and yes, I cut my raw water hose. I like the idea that I can now shut off raw water coming into the boat which has already come in handy already. My system shuts off the raw water with one valve (manual), and then I open another one which is connected to fresh water and start the boat. I can post some pics if you are interested. I did not like the Perko unit which does this automatically as I did not trust the plastic housing and the fact I really don't know if the flapper valve is closed all the way (spoken like a true engineer). Cutting the hose is difficult but I removed it completely over the the winter and cut it with a sawzall.

Mark
 
boatrboy said:
... I can post some pics if you are interested. I did not like the Perko unit which does this automatically as I did not trust the plastic housing and the fact I really don't know if the flapper valve is closed all the way (spoken like a true engineer). ...

Mark
Click to expand...

Hi Mark,

Yes, please post your pics if it's not too much trouble. I've been going back and forth on this issue since we bought our (raw water cooled) boat. I had the same thoughts about the plastic Perko valve ... I don't want it to be the only thing stopping my boat from sinking.

Thanks,
Charlie
 
boatrboy said:
I too evaluated this conversion but after alot of research, I decided to forgo this modification for basically the reasons Frank outlined already. However, I decided to install a homemade flushing system which allows me to flush the motor with fresh water. Every time I return to the dock I hook up the dock water and run for 5-10 minutes. This leaves fresh water in the boat all week while it waits for me on the weekend!

There will be alot of debate on this topic. Many people feel this is a waste of time but I hope to extend the life of my rwc engine by some factor. I can't help but think that a clean water flush is better than leaving the salt water in the motor all week. To me it takes a few minutes to do this.
If I replaced my motor I would definitely convert to fwc but for me it is too late.

Mark
Click to expand...

Mark,
I'm on board with you on this one. I had the same decision to make with my 320DA and looks like the answer is keeping the engines RWC and use the flushing system.
As for the valves you made, I think the only inconvenience is that you constantly have to jump in the bilge to switch those valves. I’m thinking about installing the Perko valves and run the hose from each engine to the transom storage locker where I hook up the water. This type of setup should help me avoid the hassle of open/close bilge, hook/unhook hoses and close/open valves. I do agree that those plastic valves don’t look too solid, but if it’s the only “automated” system available I don’t have too many options, but try it. I’m sure it should last for 2-3 years and for $70-$100 I think it’s worth it.
Currently, my engines have flush kit installed from factory. The only issue is that it works just like your setup, where I have to do things manually. Since I just got my 320DA I have to learn some details on how things work. If it turns out that I can just hookup the hose and run the fresh water without closing the seacock I don’t think I’ll have a need for Perko valves. I’ll just have to run the houses to the locker. This is one of the priority items on my list for spring.

Thanks,
Alex.
 
I just added a San Juan FWC kit to the boat I just bought with 94 hours on it. 90% use in the fresh water it was a good candidate. The tech installing it said all the manifolds and risers looked good and the kit I bought is the full kit. For the reasons Frank stated besides the block, the Circ pump, t-stat and hoses, + resale it's well worth the $2100.00 and piece of mind
 
Each case is different. If you plan to keep the boat long term it might make financial sense, but judging from the way most board members here upgrade/switch boats I don't see any gain. Peace of mind is just a term. I don't think many raw water cooled boat owners are losing much sleep over this.
 
that may be, but when you're not stuck in Canada with an overheating issue, piece of mind is a term I'd like to use knowing that my system is pretty solid and that Saltwater/corrosion issue shouldn't be at play.
 
Saltwater/corrosion issue still very much in play as far as the elbows and risers? Not much peace of mind there. To each his own I guess.
 
boatrboy said:
............
There will be alot of debate on this topic. Many people feel this is a waste of time but I hope to extend the life of my rwc engine by some factor...........
Mark
Click to expand...

I told you so :)

I will post a new thread with pics so aviod hyjacking here.
 
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