Proper raw water cooling flow rates.

[GILow]

Hi all,

I am in the middle of rebuilding our engine cooling system.

The engine is fresh water cooled with a raw water heat exchanger for the engine cooling and a smaller "in line" cooler for the transmission.

Engine is a 96HP Nissan ED33, but has been geared and propped to max out around 85 HP. Transmission is a velvet drive 71c.

I usually run the engine at around 1150 rpm, which is about 40 HP according to the engine power curve.

Marinisation was by a company called Diecon, who are no longer doing marine conversions but were pretty common in Australia for a long time.

So, today I pulled the water pump">raw water pump and noticed that the backing plate was on back to front. Now, that MIGHT have been my doing, I've replaced the impeller twice as part of routine maintenance since we bought the boat at the end of 2012. But judging by the build up, it's been back to front for longer than 4 years and I assume I would have replaced it without looking all that hard when changing impellers in the past, just returning the shiny side inepwards.

I only noticed because I was cleaning up the plate to try the find the model number, in doing so I noticed the cam profile on the plate, and of course, found the model number on the inside.

Anyway, I have looked up the model number now (Johnson F5B-8) and I have discovered that it's flow rate is about half of what is recommended on the xylem systems web site for an engine of our HP. (Xylem reckon I need around 80 litres a minute, this pump is more like 40)

Our engine has always needed everything to be exactly right in the cooling system, otherwise it does get hot. In fairness though, when everything is right I can motor along at cruising revs all day without a problem.

So, can anyone share their wisdom on raw water cooling flow rates and what they have found to be satisfactory. Also I would love to know how much extra tolerance people have gained in their cooling systems by increasing the flow rates. Although I intend to keep our cooling system in good condition, I am aware that it may be difficult to do so when we are cruising.

Matt

[Jim Cate]

Matt, can't help ya on the flow rate query, but it is a common practice to reverse the cover plate when the original "inside" becomes worn enough to allow leakage past the edges of the vanes. That may explain your inside out situation.

Jim

[GILow]

Interesting... there is a definite profile engraved on the inside of the cover panel, but it looks so regular I assumed it had been cast that way.

Perhaps not. Perhaps it is actually serious wear. The engine has 2000 hours on it, and I assume the pump is original. So it could be wear.

I've cleaned an polished it all up using some 2000 grit wet and dry, I shall see how the water flow looks tomorrow. The results will be a little skewed by the fact that I have managed to remove two right angle elbows from the flow path, and fitted a new nitrile impellor, all of which should certainly help. Diecon had gone to a lot of trouble to tuck all the cooling hoses up tight against the block which led to some insanely complex plumbing. I simplified the fresh water side of the cooling a year or so back, but now it is the turn of the raw water side to get the same treatment.

Still worried about the discrepancy between the xylem recommendation and the pump capability though.

Matt

[NCboatrx]

At 2,000 hours I suggest that you just replace the raw water pump. This is about the official service life of a brass rubber impeller pump. As you state that this pump is undersized, I suggest getting one that has a greater output.

Without knowing the cooling system, the surface area of the heat exchanger and the engine's heat rejection ability it is impossible to know exactly what you need.

[tkeithlu]

You've made some very thoughtful changes to increase heat transfer within the bounds of the pump. How about adding (1) a very complete cleaning of the inside and outside of the heat exchanger core, (2) a look at whether the thermostat is constricting flow, and (3) a cleanout of the coolant passages combined with new coolant?

[gjordan]

Double check your stats on the flow to make sure it really calls for twice as much water flow as you have now. If it does, you probably need to buy the correct pump and keep the old one as a backup. You say it cools fine at cruising RPM, so I assume that means it doesnt cool well at full throttle? I ran my Perkins 4-154 for 8 years like that without any trouble until I had to keep up speed going through the Panama Canal. When you need HP, you really need it ,and if a few hundred dollars for a bigger pump will give you your rated HP you will be a safer cruising boat. If I had fried my engine in the Canal, it would have cost me over $1000 for a tow. The rest of my cruising was fine with about half of the engines rated HP. I dont like oversize engines in cruising boats but sometimes they come in handy. Just my 2 cents worth.

[a64pilot]

I'd replace the pump with a larger one. If you start out with excess capacity then your fine with a little growth on the inlet, a little clogging in the strainer and a little wear on the impeller, but if you start out with no margin then your not, everything has to be perfect, all the time. When it will get you is when you need the excess HP that you don't normally use, and then it won't be available, cause at normal cruise you likely only need about half cooling capacity anyway. Cooling needs and HP made track each other rather close
That is why I will occasionally run mine flat out for 5 min every so often, I will accept a 5 deg rise in temp, but no more before I start cleaning things.

[S/V Alchemy]

Originally Posted by tkeithlu

You've made some very thoughtful changes to increase heat transfer within the bounds of the pump. How about adding (1) a very complete cleaning of the inside and outside of the heat exchanger core, (2) a look at whether the thermostat is constricting flow, and (3) a cleanout of the coolant passages combined with new coolant?

Good idea. I wouldn't assume upsizing the pump was sufficient. I would examine the cooling system in totality, including the size of the Perko/Groco filter, carefully measuring where the real (loaded) waterline is in relation to it, looking out for sharp bends in the hose and examining the exhaust set up for unnecessary backpressure.

Then I would flush out the heat exchanger.

Then I would consider for this size engine an exhaust gas temperature (EGT) sensor. It allows you to compare RPMs to engine "effort" and can help to reach the sweet spot of torque versus RPM versus prop pitch.

What I'm suggesting is a holistic approach. Your RPM at first glance seems low to me for cruising speed, perhaps suggesting either overpropping or too much engine for the boat. I would be interested were I you in plotting a fuel use map.

Only after the inspection of the "frictional" elements in your drive train, from pushing water needlessly to prop pitch settings, can you really judge if a bigger volume pump (which may not be suited to your block in terms of wear) is the solution or merely a partial masking of a more systemic problem you now have an opportunity to address.

[Redpcx]

You Mention 40 HP at 1150 rpm, according to the engine power curve.
Unless you have a prop that adjusts while running, you probably are using much less than 40. The engine power curve shows the MAXIMUM hp it can produce at a particular speed. GPH is a better curve to guess HP.
The prop likely uses less than 40 HP at 1150 engine rpm. 40 HP takes about 2 GPH in a diesel engine.

Cooling ability varies with engine speed. Cooling requirement varies with HP, which varies with either the square or cube of the prop speed.

I think you said indirectly that your engine will not pull its rated HP and speed.

This is an overload condition, and will cause overheating. A true classic, over pitched prop. Fix this first, if your engine will not pull rated rpm, it will overheat, and run dirty.

[johnny AoB]

The wear on the cover plate can contribute to shortening the life of the impeller. If the pump turns out to be the right size and is still in reasonable shape you might just get a new cover plate. I would suggest that you get a speed seal (Welcome to Speedseal) cover since they include a Teflon washer that makes the impeller last longer and can allow you to run a little longer dry. I have a buddy that bought two of them for his new cat with new Volvo engines. He said it has been one of his best purchases.
-johnny

[Jim Cate]

Matt, how is your r/w pump driven? Belt or direct? If belt, could changing pulley size raise the pump rpm relative to engine rpm enough to increase the flow?

But again, if the cover plate is worn the output will be compromised. New cover plates are available and not too expensive. That would be my first "fix", rather than re-engineering the whole cooling system which perhaps worked fine before the wear occurred.

Jim

Jim

[GILow]

Hi all.

Gee, some great stuff here, thank you all. It will make a very good check list to make sure I have not missed anything.

I am still curious to know what sort of flow rates people think are normal. I can't help thinking a web site that sells cooling components have a few reasons to recommend a bigger pump than may be strictly necessary, but some of those reasons are good.

If anyone has an engine of a similar output capacity I'd be particularly interested to hear what size raw water pump you have fitted.

Matt


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[donradcliffe]

From Xylem website:

What cooling water flow rate does an engine need?
On average, petrol engines and fast-running diesel engines with direct cooling require a raw water flow of about 36-39 litres (8 - 8.5 gallons) per minute for each 75kW (100 bhp) of engine power. Engines with indirect cooling require more flow: about 65-70 litres (14.5 - 15.5 gallons) per minute for each 75kW (100bhp) of engine power.

I ran a few numbers, and have to agree with them. If you are running 100 hp with 15 gpm on the raw water side, you will have a raw water temperature rise of 117 deg F. Add that to a 85 degree sea temperature and you are up to 202 deg F at the exit of the heat exchanger--too close to boiling for my tastes. Even with a counterflow heat exchanger there isn't much margin for fouling or losing an impeller. I'd be more comfortable with 75l/min at WOT/max RPM, which would give you a raw water exit temp of about 172 degrees in 85 degree sea water.

[BruceS]

My 4-108 Perkins has the "9" pump whereas you say yours has the "8".
I'm thinking they are the same except yours is bare shaft (Pulley drive) whereas mine is cog driven by camshaft.
4-108 is supposed to be about 50hp from memory but I only run it at 1900-2000 so maybe 25-30hp.
I think both pumps are 3/4" ports?

[GILow]

Originally Posted by BruceS

My 4-108 Perkins has the "9" pump whereas you say yours has the "8".
I'm thinking they are the same except yours is bare shaft (Pulley drive) whereas mine is cog driven by camshaft.
4-108 is supposed to be about 50hp from memory but I only run it at 1900-2000 so maybe 25-30hp.
I think both pumps are 3/4" ports?

Thank you Bruce, yes, that sounds about right. And unfortunately does support the theory that mine might be too small. At least, it does not disprove it anyway.
Mannum hey? How's the river flowing? I saw it last week and it still looked pretty brisk at Murray bridge.