Converting from direct raw water cooling to closed loop

[GetItDone]

Hi all,

I've been having a mare with my little Volvo Penta mb2a/50s which is basically a Honda BF 75 7.5hp on a sail drive. We had an engine failure, and upon stripping the power head the corrosion from the salt has eaten through chunks of water jacket in the alu block & head. I've got 4 spare PH's and one of them is the exact same.

Given I'm going to rebuild the whole thing, I'm not keen to repeat this in a few years because the same issue has occurred. I obviously don't know if previous owners flushed properly etc. but whilst I've got everything out and I've got an opportunity to do something about it I am sorely tempted.

My thoughts are to use a raw water to closed loop heat exchanger from another setup, though I haven't yet found one suitable that isn't new and rather expensive. I've also read about people using heat exchanges from old boilers, but I don't know how I feel about this with regards to salt and reliability.

I think it would be fairly simple to plumb it all in with a pressurised header tank, heat exchanger, 12v electric water pump for the closed loop, and then running the raw water through the existing raw water pump & exhaust.

Removing the compacted salt from the water jackets only to find holes in them is not something I'd like to repeat

Many thanks in advance for any advice.

 

[Tranona]

What you are describing is similar to the Martec systems martec.se/en/fresh-water-cooling/martec/ That were popular before HE cooled engines became the norm. they are still available so might be worth exploring their universal kits. An alternative that might work is keel cooling.

 

[pete]

I did this on a Yanmar 1GM10 several years ago. I used a cylindrical heat exchanger of an old Worcester combi boiler (about the size of a large vaccum flask) that had a copper coil with steel outer casing.
I fitted a small expansion tank above the heat exchanger but didn't make it into a presurised system. The raw water went through the copper coil and the engine water with antifreeze inhibitor through the steel outer tube.
I made a pump housing from a 4" x 4"x1" thick block of ally and fitted a ford fiesta water pump front that ran off the alternator belt. I also had to make a small pulley for this. This system worked for around 8 years for me and AFAIK continued working well for the boats new owner.
I had to change the raw water alarm sensor with a fresh water on as the engine ran slightly warmer.
When PBO was a proper DIY magazine they did a write up on making this with plans for a Petters mini twin engine, August 1999 P B O number 392 by Mike Parker
As my job was installing central heating I collected these heat exchangers from the old boilers that I removed.
The most difficult thing was making the pump body but I didn't want to rely on an electric which would have been much easier.

 

[Graham376]

What you are describing is similar to the Martec systems martec.se/en/fresh-water-cooling/martec/ That were popular before HE cooled engines became the norm. they are still available so might be worth exploring their universal kits. An alternative that might work is keel cooling.

I used a Martec kit on a MD11C many years ago, in our Berwick. Not cheap but worked well.

 

[PCUK]

Any remote mounting heat exchanger will do the trick. I did the same on a BMC 2.2 in the middle ages!

 

[rogerthebodger]

I marinized a ford petrol engine when I lived in the UK and I went to Lancing Marine. You could also try Bowman heat exchangers in Birmingham

A water-cooled exhaust manifold is quite easy to fabricate from stainless steel I had one on my current boat

 

[GetItDone]

So, I've been mulling this over the past 12 or so days, have spoken to a few people about their solutions, and think I've come up with something that might work for me.

Firstly, there is an additional challenge that I hadn't considered. The powerhead on my saildrive is designed to be raw water cooled as part of an outboard (It's a Honda BF75 7.5hp on a Volvo Penta MB2/50S). This means that the cylinder head has two coolant water outputs, one on a hose which is fine, and one right next to the exhaust port that joins the manifold. The exhaust port goes down into an exhaust box where the raw water flows around the exhaust chamber like a jacket and then is pumped into the exhaust and expelled down into the water lock exhaust and eventually out of the stern using engine exhaust pressure. This means that, based on the current design, if I pump closed loop coolant through there it's going to go out of the exhaust regardless of how I route hoses.

So, in my mind there are effectively two options:

1) Modify the cylinder head to move the coolant feed from going into the exhaust and instead go into a hose barb, and then use the hose input on the exhaust box to keep pumping the raw water into it.
2) Build a new exhaust that circulates the closed loop coolant water to cool the exhaust, but directly expel the raw water some other way.

I think I am leaning towards option two. Not having the raw water going out of the exhaust completely dose away with the risk of it backing up and flooding the engine cylinders, and means I can run electric pumps rather than the quite old and difficult to maintain mechanical one currently on the engine (which was already added after market, and not particularly well I think). This would mean a slightly strange constant stream of raw water coming out of this fitting, but not in any significant way I don't think.

This means that I'll be looking to do the following:

1) Install a new through hull fitting above the waterline for raw water exit.
2) Build a new simple exhaust box that sends the exhaust gas from the cylinder head directly down the exhaust hose to the existing water lock without mixing in any water, and which circulates the coolant water in a jacket around the exhaust box and then back out through a hose barb.
3) Install an electric coolant pump into the circuit along with an expansion bottle and a plate heat exchanger.
4) Circulate closed loop coolant through the circuit using the electric pump with the expansion bottle in place as a reservoir, using the existing thermostat in the engine to control coolant flow.
5) Use the existing raw water inlet through a strainer to another electric pump and through the other side of the plate heat exchanger.

I'll also be installing a temperature sensor and gauge on the closed loop circuit to ensure it's not overheating, particularly given it's now also cooling the exhaust.

My main question is, can anybody think of a reason why pumping just straight exhaust gas and not any water through the exhaust waterlock might be an issue? Worst comes to it I'll route the exhaust hose straight to the exhaust exit on the hull, but this way seems easier if it won't cause any damage.

I'm aware this may seem a bit crazy, but the raw water system on this engine has caused no end of troubles, and having stripped a few of them down the amount of salt damage to the cylinder heads is significant. Electric coolant pumps are, I believe, more reliable than the standard mechanical raw water pumps, and having a closed coolant circuit should stop any additional damage to the aluminium block/head.

 

[Daverw]

I would expect the main issue would be the exhaust temperature, the raw water significantly cools this before the water lock, think how hot your car outlet manifold pipe gets

 

[GetItDone]

I would expect the main issue would be the exhaust temperature, the raw water significantly cools this before the water lock, think how hot your car outlet manifold pipe gets

Sorry, perhaps my description didn't make it clear. The exhaust manifold coming off the cylinder head would still have a water jacket, its just that the water jacket would now be part of the closed loop and not dump the water used to cool it into the exhaust; rather send it back around to the heat exchanger. I think that, hopefully, this water cooling jacket would keep the manifold cool enough. Thoughts?