Engine Cooling - Oversized Hose or No?

[Jim Cate]

G'Day Cas,

No experience with Volvo (thank goodness) raw water engines, but the raw water cooled BMW D35 on our previous boat developed enough scale in its cooling passages to severely obstruct flow. This only took a few hundred hours of operation. The remedy suggested by the local BMW mechanic was to warm up the engine, close seacocks, remove hoses from inlet and exhaust and route to a bucket full of HCl (swimming pool acid from local hardware store), diluted around 2:1. Then run engine for a few seconds to start circulating the acid mix through the whole of the cooling circuit. Let sit for an hour or so, repeat to refresh the acid in the engine. Continue doing this until the addition of fresh acid does not result in audible "gurgling" within the engine as the new acid attacks the scale buildup. Oh yes, remove any anodes from the cooling system... the acid will destroy them!

This worked for us for years, and from your description of symptoms, might well work for you. Despite all the "learned" discourse above, I firmly believe that the use of a slightly larger hose on the inlet side of your pump will have no measurable effect upon system operation. Constriction of passages within the engine most certainly will.

Cheers and good luck,

Jim

[Cheechako]

Lets... see... MD2B I believe it was....

[Ex-Calif]

Originally Posted by sailorchic34

Going way off topic now...
.

So Ex-cali is right and so is Dennis G. But your talking about two different things...

Probably clear as mud now.

Yeah but.... Mom...

Who's more right? - LOL

Can't we all just get along....

BTW - if OP opens the impeller cover, opens the seacock and water comes out I would suspect the engine is below sea leavel. So this conversation is a bit of a tempest in a teapot.

And yes, I agree there are two definitions and phenomena.

[Ex-Calif]

Originally Posted by barnakiel

Have MD7 here. No oversize items anywhere, just plain & correctly sized fittings and you are fine.

Worth converting to fresh water unless you already have it.

BTW this engine does not push a lot of cooling water thru the system - not as much as some newer Yanmars and other such likes. Just test the thermo and the sensor and you are fine though.

Cheers,
b.

Great point. Sometimes it is hard to determine how much water should be coming out withi the exhaust.

Also to OP. invest in a cheap IR sensor - I picked one up at Home Depot type place. You can shoot it at the engine block and heads an get a good idea of actual temps and temp differential around the engine. Probably wont get a definition of what the temps should be but you can monitor for temp changes.

[cas206]

Status Update (for those keeping track).

Hose appears to be 5/8". The installed hose was 3/4". Local marine sources did not have the hose required. So I've ordered it.

Meanwhile, took the water pump cover off. Looks like one blade was missing from impeller. I'm working blind as I need to reach around front of engine and work on aft facing water pump by feel. So I don't know if the missing blade dropped into the bilge somewhere. Replaced impeller. Can't test until I get new hose. (I had left the old hose pieces at home so couldn't get a temporary connection to test.)

Looking at the manual shows two bolts holding thermostat in place in a easy to reach front location. I think I will remove it next and see what I can see of the engine cooling chamber.

I'm a bit shy in going heavier with the acids. The cooling water hose goes to the reduction/reverse gear first. The output of this unit goes via hard pipe to water pump and then to engine. I'd be leery of pumping acids into this reduction/reverse gear unit. So I'd have to find a way to break into the hard pipe after this unit and before the engine intake.

In order to reverse flush, I suppose I'd have to remove the impeller again

cas

[cas206]

Originally Posted by Ex-Calif

Great point. Sometimes it is hard to determine how much water should be coming out withi the exhaust.

Also to OP. invest in a cheap IR sensor - I picked one up at Home Depot type place. You can shoot it at the engine block and heads an get a good idea of actual temps and temp differential around the engine. Probably wont get a definition of what the temps should be but you can monitor for temp changes.

Appeals to the geek in me. I might try that. There appears to be a fairly new looking temperature gauge installed with sending unit in the engine head.

[sailorchic34]

Right on the heavy acid. It's only for the engine block and not thru the pump, if you do an acid flush. Muriatic acid works, but the engine block internals will rust as soon as you flush with water. Using phosphoric acid (hardware stores have it in the paint, naval jelly isle.) puts a phosphoric oxide coating on the metal and prevents rusting when flushing. Used to passivate stainless steel pipe. Good stuff..

Back to Pumps 101 for a minute

Centrifugal pumps need a positive NPSH (net positive suction head) number at the pump inlet for the pump to.. well pump. Captivation happens if the NPSH is lower then the minimum required by the pump. The fresh water pumps (and some raw water pumps) are true centrifugal, where pressure is developed as water is imparted velocity by the impeller vanes

The rubber vane RAW water pumps, rotate but they are more like a sliding rotary vane type positive displacement pump. And positive displacement pumps ignore NPSH, They laugh at it. Its not an issue. Ex-CALI'S comments were based on a positive displacement pumps in an open suction. Depending on the efficiency of the PD pump, there would be a point when too large a pipe would effect pump lift in a vertical lift application like in a well.

As most raw water pumps in boats are flooded suction, suction pipe size is irrelevant as long as its no smaller then required by the flow rate of the pump. In a high vertical lift application like a well, with a PD pump, Pipe size actually is important or can be depending on application.

Nuff on that.

[Ex-Calif]

Originally Posted by sailorchic34

Right on the heavy acid. It's only for the engine block and not thru the pump, if you do an acid flush. Muriatic acid works, but the engine block internals will rust as soon as you flush with water. Using phosphoric acid (hardware stores have it in the paint, naval jelly isle.) puts a phosphoric oxide coating on the metal and prevents rusting when flushing. Used to passivate stainless steel pipe. Good stuff..

Back to Pumps 101 for a minute

Centrifugal pumps need a positive NPSH (net positive suction head) number at the pump inlet for the pump to.. well pump. Captivation happens if the NPSH is lower then the minimum required by the pump. The fresh water pumps (and some raw water pumps) are true centrifugal, where pressure is developed as water is imparted velocity by the impeller vanes

The rubber vane RAW water pumps, rotate but they are more like a sliding rotary vane type positive displacement pump. And positive displacement pumps ignore NPSH, They laugh at it. Its not an issue. Ex-CALI'S comments were based on a positive displacement pumps in an open suction. Depending on the efficiency of the PD pump, there would be a point when too large a pipe would effect pump lift in a vertical lift application like in a well.

As most raw water pumps in boats are flooded suction, suction pipe size is irrelevant as long as its no smaller then required by the flow rate of the pump. In a high vertical lift application like a well, with a PD pump, Pipe size actually is important or can be depending on application.

Nuff on that.

I dont think we will close on this because the definitions are all over the place, we are mixing thrash pumps, vane pumps and PD pump theory all together and it is clear as mud. Do you really mean "captivation" and not cavitation?

Also I really like wikis definition whch I consider cerrect and descriptive. Suction is how high you need to raise the fluid to the pump - lets assume suction is a real word and means negative guage pressure. Lift is total height you need to lift the fluid. i.e. to the outlet of the pipe, nozzle etc.

The distance to raise the fluid before the pump is usually limiting - most pumps simply cant lower the pressure on the inlet side that much - especcialy low cost consumer pumps. Once the fluid is delivered to the pimp inlet there are lots of pumps thatmwill creat a very high lift (pressure side)

For boats it is somewhat moot. The lift ain't that high and in fact many water pumps are below sea level...

[cas206]

My take away from this is that in the ideal situation, pipe size doesn't matter. In the non-ideal case, resistance has to be factored in and that is where the effect of hose size is felt. In my application, it is probably negligible.

I prefer to see the system in energy terms. The pump is the only thing that does work that imparts energy to the fluid (disregarding heat transfer from hot engine to water) to either increase potential energy (rise in height) or kinetic energy. The rest is just semantics.

In a vertical column situation, a bigger pipe means more water, means more potential energy is required. This implies that less energy is used as kinetic, so the water moves slower. Pressure (static? dynamic? total? or what'em'call'it) defined as force/area will be the same regardless of pipe size

P = Force / Area = mass * g /Area = density * (Area*height) * g / Area
= density*g*Height

Practically, the Bernoulli model breaks down when pushed to the limits. Cavitation is one example of things pushed too far.

--cas

[cas206]

This is the cooling system route. I have a water hose feeding the input of the system. I disconnected the water pipe from the outlet side of the reverse gear, reduction unit, and verified that only a very small amount of water dribbles out. The reverse gear, reduction unit, is described as a water jacketed system. The actual mechanism is lubricated with engine oil. So my next step is to try and get some sort of chemical through the water jacket to clean it out.

cas

[Sandero]

More water changes per unit time offers better cooling. It can be limited by internal pipe size or pump ports... in which case larger hoses mean little.

One mechanic tried to convince me to change my 2" diameter exhaust hose to 3" diameter despite it being an OEM size and having worked for 20 years just fine. Still have the same diameter hose. Engine works fine!