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Old 25-07-2021, 22:11   #1
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Bukh DV20 and DV10 cooling system explained

Premininary explanation:
1) Direct sea water cooled diesels need to run with the cooling water in the block at about 70˚C. Any cooler and the engine is less efficient and prone to carbon buildup; any hotter and salt crystals will build up in the cooling water passages eventually causing overheating.

2) The Bukh cooling system has a thermostat that controls the outlet of water from the block and exhaust manifold into the exhaust mixing elbow. DO NOT attempt to run the engine without a working thermostat in place. That will NOT cure any of your cooling system problems.

3) The purpose of the thermostat is 2-fold:

a) To allow the engine to heat up to running temperature quickly by not allowing new cold water from the pump to flow through the block/exhaust manifold water passages.

b) Once the engine is at operating temperature, to maintain that temperature under differing engine loads.

Description of the cooling system:

1) The sea water impeller pump outlet goes through a short flexible tube to the copper water injection device which is bolted to the block by a flange with 2 bolts. On the block side of the flange, this device has a tube with several holes which extends about 100mm into the block. (It is a good idea to unbolt this device about once every 3 years and give it a good cleanout by immersion in dilute hydrochloric acid). If the flange is not sealed correctly to the block with a gasket, this is a potential water leak right onto the flywheel!

2) On the outside of the flange, the water injection device has a single tube branching into 2 tubes (or two separate tubes -older engines have the 2 tube version). One tube is connected to the water pump via a short flexible hose, and the other, via a flexible hose (called the bypass hose), to the intake side of the thermostat housing mounted on top of the exhaust manifold. The exit side of the thermostat housing connects via a flexible hose (usually with an anti-siphon loop) to the exhaust mixing elbow. Ok, that's the basic cooling system setup.
3) Now,
Proper startup sequence:
When the engine is cold before startup, the thermostat is closed and does not allow water already in the block and in the exhaust manifold to flow out and into the wet exhaust elbow. Therefore, ALL the water from the pump is forced into the bypass hose, then into the top of the thermostat housing, and then exits into the wet exhaust elbow to keep the exhaust gas cool so the exhaust system doesn't melt.

The static water in the block/manifold heats up quickly and you will see this happening within a few minutes IF you have a proper water temperature gauge. (I very much recommend having a gauge as the simple alarm light/buzzer only tells you after the fact that your engine is overheating, not what the water temp actually is inside the engine).

The water temp gauge will show the temp going up to about 85˚C before the thermostat starts to open, then the temperature will fall quickly to normal operating temp of around 70˚C and stay there.

At this point, the thermostat is allowing some of the water in the block/exhaust manifold to flow into the exit side of the thermostat housing and mix with water coming through the baypass hose.

The thermostat constantly balances the flow of water through the block/exhaust manifold against the flow through the bypass hose.

Engine too hot: more water is allowed to flow through the block, less through bypass hose; engine too cold, more water through the bypass hose, less through the block.

Without a thermostat, there is no way to control and maintain the correct operating temperature of the engine! It ould run too hot or too cold depending on the amount of restriction in various part of the system.


If this sequence is not happening (which you can't tell without a gauge), then the thermostat is not operating correctly and/or you have a serious restriction of water flowing through the block.


IMHO, it is imperative to occasionally do an acid flush of the Buk engine cooling passages. I do it about once a year. This involves setting up a constant flow through the engine of dilute hydrochloric acid using a small centrifugal pump and a bucket for about 30-45 minutes.

Just sucking up acid into the engine via the salt water intake hose, turning off the engine and letting it sit is not really going to do a good job of cleaning out the crud and salt crystals.

If there's interest, I can post a complete description of my flush out system.
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Old 25-07-2021, 23:03   #2
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Re: Bukh DV20 and DV10 cooling system explained

Is the raw water cooled Bukh thermostat really 70C? That seems awfully high as other similar raw water cooled engines (Yanmar etc) operate below 60C which significantly helps limit calcium carbonate build up in the cooling system. Any carbon deposits arising from the cooler operating temperature can be dealt with by operating at WOT for short periods. More inefficient at cooler temps - yes!

Is there any zinc anode in the Bukh block or head? If so, surely you remove them before flushing with HCl. I do concur with acid flushing though.

IMHO, raw water cooled diesels should be flushed with a quick fresh water flush when returning to the dock. I can't say I always did this but I do think it is an excellent idea if you can.
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Old 26-07-2021, 04:38   #3
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Re: Bukh DV20 and DV10 cooling system explained

My Bukh DV20 thermostat (its the standard one for that model) is stamped 70˚C. It is fully open at 70˚C. It starts to open at 60˚C. The engine temp goes down to 65˚C at times depending on engine load and sea water temp. I don't get a big salt buildup, but I do use the boat a lot. I agree that there is less salt deposit at 60˚C, but the engine is more efficient and has less carbon buildup at 70˚C, so like everything in life, its a compromise.



Yes, there is a pencil zinc in the block. Its about 10mm diameter and 70mm long. It gets replaced about every 3 months, and it gets taken out along with the thermostat when I do the acid flush.



I know some people flush their raw water cooling system with fresh water after every use, but in my case since I use the boat a lot (couple of times a week for day sails and 10+ times a year for long 2-4 week cruises), it would be a waste of time and energy.


Of course, the Bukh DV 10 and 20 were purpose built to be raw water cooled, slow RPM, and designed as lifeboat power plants, hence ability to hand start and simple cooling design. They were made with very thick cast iron crankcases and cast iron heads (no alloy in the engine at all) and, if properly maintaned, will last a very long time. Of course they are about 1.5 times heavier than similar sized modern high RPM small engines...
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Old 26-07-2021, 20:47   #4
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Re: Bukh DV20 and DV10 cooling system explained

Yea thats interesting that you have a 70oC thermostat. IRRC our Yanmar ysm8 one begins to open at 52oC. Can see why you'd need an acid flush every year as hot spots do occur

YSM8l burns clean as has pretty high compression but yes would be better still if it was converted to FWC & had 90oC thermostat. I toyed with the idea of FWC but it lasts so long anyway on RWC. I freshwater flush but P.O. never did & there was very little corrosion on it after 38 years so he kept up the anodes.

Can take a lotta punishment those DV20's alright but bad maintenance can still kill them same as any diesel.
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Old 27-07-2021, 02:20   #5
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Re: Bukh DV20 and DV10 cooling system explained

Hello Compass760,
I see youíre in Auckland. Iím down in Nelson with my Cavalier 32 Betelgeuse II. Mine is an early 1973 model, but had a major interior refit (added an enclosed head) in the 80ís before going offshore cruising for a year.

Re cooling of diesel engines: I looked into converting the Bukh to FW a while ago, but the added complexity, heat exchanger, 2nd pump, more hoses, etc. didnít appeal. My engine has lots of space around it, very easy to work on compared to a lot of nightmare boat engine spaces. Iíve had boats with heat exchangers and they need to be acid-flushed regularly anyway, so the maintenance is about the same.

The Bukh is a funny beastie. Did you know that both pistons come up and go down at the same time? To compensate for the crankshaft having two throws on the same side, there are geared counterweights (that adds some of the extra weight). I asked the makers about that and they said that it made the engines run smoother. So one piston is coming up on the compression stroke while the other is on the exhaust stroke, then one goes down on the power stroke while the other goes down on the intake stroke.

An interesting side effect is that the crankcase get pressurized on the down stroke and then has a vacuum on the upstroke. On top of the head, inside the valve cover, is a one way valve that is connected to the intake port of the head by a short hose. When the crankcase is pressurized, that valve allows the pressure to escape into the intake port. When thereís a vacuum in the crankcase, the valve closes. If the valve gets stuck closed, and the pressure cannot be vented through it, you get massive amounts of oil blowing by the crankcase seals (usually the front one behind the massive flywheel). Not many folks know about this little quirk.
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Old 27-07-2021, 03:16   #6
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Re: Bukh DV20 and DV10 cooling system explained

No didnt know that about the balancer, interesting.

An uncle had a DV20, seemed very smooth after our single lol.
Always started easy & went well his DV20 in a Pelorus 26 called Pumpkin. I think it ended up in Nelson after he died.

Forgot to say we get 3 years at least out of a small anode fresh water flushing when we return to marina.
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Old 27-07-2021, 12:38   #7
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Re: Bukh DV20 and DV10 cooling system explained

Thanks Nuku for a terrific explanation of these wee beasties. Iíd be very interested in your full explanation. My DV20 is about 35yo now and is proving very reliable, but does tend to blow steam, which I guess indicates hot spots in the block.
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Old 27-07-2021, 21:14   #8
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Re: Bukh DV20 and DV10 cooling system explained

"Thanks Nuku for a terrific explanation of these wee beasties. I’d be very interested in your full explanation. My DV20 is about 35yo now and is proving very reliable, but does tend to blow steam, which I guess indicates hot spots in the block."
You're very welcome. Here's the full descale method and after that some notes on my Bukh DV20:


First: If you haven't done a full descale before, it is a good idea to remove the water injection device from the front of the block and give it a good clean in an acid bath, then re-install with a new gasket. If the tube that sticks into the block is full of crud and partly blocked, the acid flush may not actually get all the crud out of it. For subsequent descaliing, after I remove the hoses from the water injection device, I have a thin flexible flat wire about 400mm long that I poke into both tubes. This wire will go all the way through the tube inside the block and ream out any salt deposits. I do this to avoid taking off the water injection device each time I descale.



Preparation: to descale you will need an acid-proof small centrifugal pump. I got mine, a magnetic drive with no seals to leak, from a craft beer/wine making supply place here in Nelson.

Then you need a hose from the input side of the pump to a 10L plastic bucket that will hold the water/acid mix. Try to keep this hose as short as possible.

The output side of the pump is connected to a single hose with a "Y" fitting at the end. The "Y" fitting has two short hoses which will connect to the 2 tubes of water injection device on the front of the block after you remove the hose going to the water pump and the bypass hose to the thermostat housing. You can use a single hose clamp on each hose.

You will need to remove the thermostat from the thermostat housing on the exhaust manifold and replace the cover of the housing. Then block off the bypass hose with a wooden tapered plug. Remove the outlet hose from the thermostat housing (the one that goes to your wet exhaust elbow) and connect a hose from that outlet fitting to the bucket.

Fill the bucket with fresh water. Now you have a loop: water will be sucked out of the bucket into the pump, then pushed into the block, through the exhaust manifold, into the thermostat housing and back into the bucket.

Secure both hoses in the bucket so they can't come out of the water!

Flush the salt water out of the engine with dock hose (see below step 8). I put a on-off valve on the end of the dock hose so I can control the water without having to turn it off and on at the hose spigot on the dock.
Turn on your pump (you may have to help the pump get a circulation going by sticking the end of a slowly running hose from your dock spigot into the pump intake hose briefly while that hose is under water in the bucket.


Now here's more of the process:

Descaling: Do it once a year if engine used regularly, more if not used much.



Procedure using the desalting kit described above and 30% hydrochloric acid:
1) Remove the bypass hose from flange on the water injection device on the block, block off the hose with a small tapered wooden plug.
2) Remove hose from salt water pump where it connects to the water injection device (make sure your salt water seacock is closed!)

3) Connect “Y” hose to both tubes (use 1 hose clamp on each hose)
4) Remove zinc anode from the block and screw in a blind plug (you don't want to cook the anode with acid!).
5) Remove the thermostat housing, cover and theromostat. Clean all in dilute acid. Put the housing and cover, minus the thermostat, back on the manifold with a gasket. (I make my own gaskets out of sheet gasket material from an auto parts store)
6) Remove hose on outlet side of the thermostat housing and connect a new hose which goes to the bucket (secure hose in the bucket with a cord!).
7) Connect output of centrifugal pump to free single end of “Y” hose. Connect extra hose to input side of the pump and secure that in the bucket.
8) Connect fresh water hose from dock to centrifugal pump inlet hose and flush salt water in the engine into the bucket. You can just hold the ends of the hoses together by hand to do this. Empty bucket and repeat. Empty bucket and fill Ĺ with fresh water. This is done so that all the salt water is out of the engine before you do the acid flush.
9) Fill bucket with freash water. Put inlet hose from centrifugal pump into bucket. Turn on the pump. Get circulation going using the fresh water hose (you must keep both hose ends held together under the water in the bucket for a bit, then remove the fresh water hose).

10) Once you are sure youu have circulation, add Ĺ to 1 litre of acid to the bucket. Circulate for 30-60 minutes.

11) Stop pump. Remove the inlet hose from the bucket and connect to the dock hose again. Flush acid/water solution into bucket with dock hose twice.

12) Install zinc. Disconnect “Y” hoses from water injection device. Re-install the thermostat in the housing. Reconnect all engine hoses. Open up the salt water sea cock and run engine and check for leaks.
10. Flush centrifugal pump and hoses with fresh water.
Good Luck!
If you have any questions, just ask and I'll try to reply soon.


Some details of my Bukh. Note that my Cavalier 32 has the engine mounted backwards with a "V" drive connected to the propshaft. That's why there is a thrust bearing.


Engine Notes:
Bukh DV20 serial number: 93640

Injectors rebuilt: new nozzles, 19-07-2019, 1,812 hrs

Minimum oil pressure with hot engine: 0.8kg/cm2

Running temp: 70˚C

Transmissions: on engine ZF-BW6 reduction ratios: 2.5 forward 2.75 reverse,

spare ZF-BW7 reduction ratios: 2.5 forward 2.75 reverse

R&D split coupling 202-153, flexible coupling 910-001

PSS seal: Saft seal model# 07-100-200R 2” shaft log X 1” graphite plug

Shaft thrust bearing: SA205-16G spherical self aligning, single row, eccentric lock 1”

Johnson pumps, (one on engine the other is a spare):
Johnson #353-9/16-35118-1, Shaft OD at impeller ⅜” Shaft OD at drive end Ĺ” with slot drive
Jabsco Impeller #808 B-1 V286, 6 blades 40mmOD X 19mm width X 9.5mm/3/8” ID
*Install pump with cam down

VDO Water Temperature Sender: #323803/001/006, 96C +/- 3C, 3W, 2 spade terminals: small is warning, large is gauge

Fuel Lift Pump: AC Delco 7971291, alternative part number is 461-199

Starter motors:

1) Paris-Rhone D9E51 K3 8, overhauled with new brushes 8/2020,

2) Bosch 829 D001315004 12V solenoid 0331 302060 − 560 12V 020 828, overhauled 1/2021

Alternators:
2 X Bosch 85A #9 120 060 009 K1-14V 14/85A, 92032637, Internal Regulator: EP 14V4 9 190 067 020, Sterling external regulator
Alternator belt: 7435mm X 10mm, Alternator pulley dia 75mm, flywheel pulley dia 160mm. ratio: 2:13, Engine rpm 2,000= alternator rpm 4,267
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Old 28-07-2021, 01:50   #9
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Re: Bukh DV20 and DV10 cooling system explained

Many thanks nuku. Comprehensive system and I’m sure my donk will thank you as well.
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Old 28-07-2021, 20:48   #10
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Re: Bukh DV20 and DV10 cooling system explained

Thanks Nuku great info. I also have a Bukh DV20 on my Hartley 32 moored at Waiheke. Very robust power plant. Yacht is a Ferro built circa 1976 by Sayer's boat yard in Whangarei. It has a similar V drive arrangement which I am trying to get more info on as its a reduction box which I think is a little unusual.

Is your prop shaft ratio same as your gearbox output?
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Old 02-08-2021, 23:26   #11
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Re: Bukh DV20 and DV10 cooling system explained

My "V" dirve unit is a 1:1, no reduction. It is a very simple unit consisting of an alloy housing with 2 parallel shafts about 150mm apart, each with a triple-wide chain sprocket. The sprockets are connected by a triple-wide heavy duty chain. The lower sprocket runs in an oil bath, and the engine salt water intake runs in a bronze tube through the oil bath to cool it.

Very compact and robust. Its 44 years old, original to the boat. I've rebuilt it with new bearings, oil seals, sprokets and chain, and it will outlast me if its maintained. These units were standard in all the original Cavalier 32's.
The "V' drive is solidly bolted to bearers and there is a double thrust spherical roller bearing to take all the thrust of the propshaft in forward and reverse. The thrust bearing is mounted in a transverse heavy-duty rib located just in front of the dripless seal.

There is a split flexible coupling on the propshaft so it is easy to take off when needed. Once the "V" drive output shaft is aligned with the propshaft, that's it no further dramas. The engine is mounted "backwards" and connected to the "V" drive input shaft with a short double universal joint shaft which can also slide in and out, so there's no alignment issues with the engine, which is mounted on anti-vibration mounts. The propshaft has a dripless seal.
The Cav 32 has a very generous engine space which can be accessed from top or front; heaps of room to get at all the engine bits.
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