Engine Cooling - Oversized Hose or No?

[cas206]

The MD7A on my "new to me" boat appears clogged with mud and gunk. I had no output cooling water when I moved the boat to it's new slip. I started with the Perko raw water strainer (the inside basket type) and verified I could push water through it. However, I decided to remove it and take it home to clean. The inner strainer was corroded at the bottom and was broke off from the base. There was about 2 inches of mud in the strainer.

The water flows from the strainer to the Reverse Gear (RG) unit. It comes out of the RG and goes to the engine mounted water pump via hard pipe. I removed the cover of the water pump and was only getting a trickle of water forced through the hose going to the RG. I took the hose off the RG and was able to scrape a bit of crud from inside the elbow that connects the hose to the RG housing. I'm getting a little more water out from the water pump. So I need to flush through the RG and then put a new impeller in the water pump and then try to force water through the engine to the exhaust.

Ok, on to my question. The hose that was fitted from the raw water strainer to the RG was over sized for the fittings. A past owner fitted a short piece of old hose to the fittings (both strainer and RG sides) and then clamped the over sized hose on to the old hose piece. My first thought was that maybe a larger hose would help protect against partial buildup and it might be a good idea to keep it. But then I started to recall that there is some relationship between flow speed, pressure, etc ... etc, depending on the hose diameter. But, the fittings feeding both ends of the hose should be the driving factor in what goes and and what comes out.

So I am seeking opinions on whether to keep the over sized hose and save a few pennies regardless of whether it actually is beneficial or not? Or do you think that operation of the cooling system is affected by the larger diameter and that that right sized hose should be acquired and fitted?

Thanks,
cas

[Ex-Calif]

Originally Posted by cas206

The MD7A on my "new to me" boat appears clogged with mud and gunk. I had no output cooling water when I moved the boat to it's new slip. I started with the Perko raw water strainer (the inside basket type) and verified I could push water through it. However, I decided to remove it and take it home to clean. The inner strainer was corroded at the bottom and was broke off from the base. There was about 2 inches of mud in the strainer.

The water flows from the strainer to the Reverse Gear (RG) unit. It comes out of the RG and goes to the engine mounted water pump via hard pipe. I removed the cover of the water pump and was only getting a trickle of water forced through the hose going to the RG. I took the hose off the RG and was able to scrape a bit of crud from inside the elbow that connects the hose to the RG housing. I'm getting a little more water out from the water pump. So I need to flush through the RG and then put a new impeller in the water pump and then try to force water through the engine to the exhaust.

Ok, on to my question. The hose that was fitted from the raw water strainer to the RG was over sized for the fittings. A past owner fitted a short piece of old hose to the fittings (both strainer and RG sides) and then clamped the over sized hose on to the old hose piece. My first thought was that maybe a larger hose would help protect against partial buildup and it might be a good idea to keep it. But then I started to recall that there is some relationship between flow speed, pressure, etc ... etc, depending on the hose diameter. But, the fittings feeding both ends of the hose should be the driving factor in what goes and and what comes out.

So I am seeking opinions on whether to keep the over sized hose and save a few pennies regardless of whether it actually is beneficial or not? Or do you think that operation of the cooling system is affected by the larger diameter and that that right sized hose should be acquired and fitted?

Thanks,
cas

Couple of thoughts come to mind.

- The mud probably comes from motoring off a grounding. Good to check/clean the strainer after any grounding/motoring off event.
- Unless the strainer or something upstream was seriously contaminated I dont think we have explained the low flow.
- I would dive on and check the inlet on the hull for build up. A screwdriver or pick and a bottle brush to clean up inside the through hull
- The hose you are describing is on the suction side. The larger volume of water in an oversize hose creates a need for a "stronger" lift/suction. A weak failing non-positive displacement pump (i.e. rubber vane pump) could be getting overwhelmed. Although with this limited volume increase root cause is inlikely.
- I honestly am not sure the function of the RG pump or how to check its condition but it sounds like you aren't getting flow or positive preessure from this unit. It could be worn out or it could be struggling with the lift it needs to do. Presuming the previous owner had this set up (with oversize hose) working at some point I would suspect the RG unit condition.
- I personally would throw a new impeller in the engine pump as it is cheap insurance and eliminates a factor. Check the cover of the water pump also as end plate wear is a common source of pressure leakage.
- I would also remove the hose from the water pump to engine cooler and check the flow there. That is the real key. What are you suplying to the cooler.

If flow is good there you may need to core out the cooler. I am presuming your initial diagnosis of no/low flow was observing the exhaust outlet. The final spot to check is the exhaust muff. Although blockage there would result in other, really bad, symptoms like water backing up into the head.

Its a good time to check it all from through hull to through hull and give yourself peace of mind.

Finally on hose size - for a given suction pump capacity, flow will be dertimined by the more restrictive orifice in the system - the original fitting sizes. The transition from narrow to oversize hose will result in reduced pressure and slower flow. Presuming the hose going into the strainer from through hull is correct and smaller, this will set volume and pressure for a given non-positive displacement pump. The expansion after the strtainer will reduce pressure and volume, the restriction going into the RG should set pressure and volume back unless you are cavitating inside the hose, unlikely at these pressures and flow rates.

My bottom line on equipment is that the engineers who design this stuff aren't usually dummies. Go with original sizes and specs is my basic call.

[sailorchic34]

While the larger hose offers less resistance to flow, I just don't trust hose without a hose barb to keep the hose on. I guess a little 4200 between the small hose stub and the larger hose would be OK. But even a blonde chic would use the right size hose ;-) Pressure drop savings using the larger hose would not be that much as the length of hose is small..

Really it's more important to get the heat exchangers clear of mud, etc. Also check the discharge ell for mineral buildup and at the hose fitting to the discharge ell. too

[Dennis.G]

Originally Posted by Ex-Calif

...The larger volume of water in an oversize hose creates a need for a "stronger" lift/suction...

Although I agree with your other points, the suction lift required at the pump will actually be less with larger diameter hose due to lower friction loss.

[Ex-Calif]

Originally Posted by Dennis.G

Although I agree with your other points, the suction lift required at the pump will actually be less with larger diameter hose due to lower friction loss.

I am referring to head pressure. All thrash pumps will have a table telling you how far they can lift a column of water based on hose size. Bigger hose size = bigger (heavier) column of water.

This is a factor primarily when choosing a wash pump. If you have a big lift to the bow on a high freeboard with a large hose you may be disappointed with the pressure available to wash the anchor chain. Reducing the hose size will ease the lift but reduce volume out the nozzle.

The very short lift here to the engine block is likely not a factor but I pointed it out for thoroughness.

In this case we presume the PO had this system working. What changed? Blockage, equipment wearout or equipment failure. That is the root of troubleshooting.

[Dennis.G]

Originally Posted by Ex-Calif

Bigger hose size = bigger (heavier) column of water...

I respectfully believe that you are not correct on this.

The static head is determined by the vertical height liquid has be be moved. The volume does not matter. The pressure of a column of water only depends on its height (when static). When pump moves the water you need to add the dynamic or friction loss head pressure to the static, which would total a higher head with a small hose.

[sailorchic34]

Oh dear, now I have to put my engineers hat on. Actually trash pumps base lift on NPSH or net positive suction head which is based on absolute pressure. So vertical lift is based on pump design and absolute pressure. A normal pressure gauge pressure at 0 psig is actually 14.7 psia absolute at sea level.

The self priming design of say the typical Goulds trash pump might limit how large a pipe could be, but its more a function of the impeller and inlet design then suction pipe size.

Dennis G. is correct in that the larger pipe diameter has a lower friction loss per flow rate. This applies even in suction lift applications.. Really most raw water inlets are below the water line so suction lift is not even a factor.

[cas206]

While awaiting for the jury to finish deliberating, here are some extra facts to add.

Originally Posted by Ex-Calif

In this case we presume the PO had this system working. What changed? Blockage, equipment wearout or equipment failure. That is the root of troubleshooting.

PO commented to me that very little water would be seen at exhaust indicating that he expected this as standard operating procedure. It is my opinion that a problem existed that he was not aware of.

Originally Posted by Ex-Calif

Couple of thoughts come to mind.

- The mud probably comes from motoring off a grounding. Good to check/clean the strainer after any grounding/motoring off event.

I found three BoatUS towing invoices due to grounding in the Alabama waters in the PO's name. So that is definitely a factor for the crud being in the system.

Originally Posted by Ex-Calif

- I would dive on and check the inlet on the hull for build up. A screwdriver or pick and a bottle brush to clean up inside the through hull

The boat was on the hard from being trucked to MD. The yard cleaned off the barnacles and muck and put a fresh coat of bottom paint. The thru hull strainer is fine. I also replaced the frozen valve with a safety seacock. When "open" water comes from the thru-hull side to the outlet. When "closed" the top side of the seacock is opened to the outlet. This way I can connect a hose to the top of the seacock and feed water, antifreeze, etc, from a bucket. This is how I started my trouble shooting. So the thru-hull, outside strainer are not really in the loop right now.

Originally Posted by Ex-Calif

- I honestly am not sure the function of the RG pump or how to check its condition but it sounds like you aren't getting flow or positive preessure from this unit. It could be worn out or it could be struggling with the lift it needs to do. Presuming the previous owner had this set up (with oversize hose) working at some point I would suspect the RG unit condition.
- I personally would throw a new impeller in the engine pump as it is cheap insurance and eliminates a factor. Check the cover of the water pump also as end plate wear is a common source of pressure leakage.

The Reverse Gear unit does not have a pump in it. Here is a manual link if really interested. www.bluemoment.com/manuals/Volvo_MD6A_MD7A_Workshop.pdf
The water pump mounted on the engine "pulls" water from the intake and through the RG and then "pushes" it through the engine and out the exhaust. I pulled the cover off the water pump with the intention of replacing the impeller. However, I wanted to make sure water could flow to the water pump housing before going further (especially if I end up pumping chemicals through the RG to clean out the crud).

Originally Posted by Ex-Calif

- I would also remove the hose from the water pump to engine cooler and check the flow there. That is the real key. What are you suplying to the cooler.

The connection from the RG to the water pump housing is metal pipe. The connection from the other side of the water pump housing is metal pipe to the engine. I was hoping to avoid messing with that pipe. But if I can't get things unclogged then I may have too. I am hopeful that multiple flush and soak cycles with vinegar/water will do the trick.

Originally Posted by Ex-Calif

If flow is good there you may need to core out the cooler. I am presuming your initial diagnosis of no/low flow was observing the exhaust outlet. The final spot to check is the exhaust muff. Although blockage there would result in other, really bad, symptoms like water backing up into the head.

That's the other thing, no muffler. I have that on my todo list. Right now the water/exhaust mixture goes into a metal pipe that makes a crazy rising loop before dropping down to the exhaust port.

Originally Posted by Ex-Calif

Its a good time to check it all from through hull to through hull and give yourself peace of mind.

Not really...lol...It was hot as heck today. I'm standing on my head to get to the engine with sweat poring into my eyes. But I get your intent and you are right, I want to make sure the cooling system is fully cleaned out and operating as intended before starting my cruising season.

Originally Posted by Ex-Calif

My bottom line on equipment is that the engineers who design this stuff aren't usually dummies. Go with original sizes and specs is my basic call.

Originally Posted by sailorchic34

I just don't trust hose without a hose barb to keep the hose on. I guess a little 4200 between the small hose stub and the larger hose would be OK. But even a blonde chic would use the right size hose ;-)

Even though you guys may be quibbling over the whys and hows, it still seems like two votes for getting the right hose. So I'm leaning towards that.

Thanks
cas

[sailorchic34]

That crazy rising loop, is your discharge ell. Your raw water is discharged into the elbow, which cools and muffles the exhaust. In cooling the exhaust, the hot exhaust causes the minerals in the sea water to drop out of suspension onto the inside of the elbow. Over time this will decrease water flow and is one cause of the raw water impeller failing. Checking it every 200-300 ish hours or so is a good thing...

You might also use phosphoric acid instead of vinegar. Really you'll not have much build up on the hoses outside the heat exchanger, exhaust manifold and discharge ell. Mainly as the water is pretty cool up to the heat exchanger.

[Ex-Calif]

Originally Posted by Dennis.G

I respectfully believe that you are not correct on this.

The static head is determined by the vertical height liquid has be be moved. The volume does not matter. The pressure of a column of water only depends on its height (when static). When pump moves the water you need to add the dynamic or friction loss head pressure to the static, which would total a higher head with a small hose.

I was wrong (once - LOl) and always willing to be corrected, as long as you agree to update this Wiki article - LOL

http://en.wikipedia.org/wiki/Hand_pump

Ther pertinent part is in Paragraph one.

Suction and lift hand pumps
Suction and lift are important considerations when pumping fluids. Suction is the vertical distance between the fluid to be pumped and the center of the pump, while lift is the vertical distance between the pump and the delivery point. The depth from which a hand pump will suck is limited by atmospheric pressure to an operating depth of less than 7 metres. The height to which a hand pump will lift is governed by the ability of the pump and the operator to lift the weight in the delivery pipe. Thus the same pump and operator will be able to achieve a greater lift with a smaller diameter pipe than they could with a larger diameter pipe.

I am guessing that the pump designer assumes the correct hose size will be used on the lift based on the fittings on the inlet of the pump.

The article goes on to talk about the types of pumps needed to achieve different suction heights.

Maybe I mixed suction and lift and I'll grant that but in it's ridiculous conclusion we attach an 8 inch pipe to a standard diesel water "thrash" pump inlet we aren't gonna suck it very far...

The attached is a sample of a sump pump spec. I presume the hose sizes are assumed by the graph creator to be suitable for the pump inlet fitting.

I could be easily convinced that if the column is too heavy the pump with either lift it, or it won't, due to suction available, and reduced performance at high lift may not occur at all. Too lazy to dig out the physics text. I do tend to mix Farmer John logic with book facts a lot...

Sorry for thread drift.

[sww914]

They prolly just couldn't get the right size of hose and intended to fix it later but it became out-of-sight out-of-mind. I've got a couple things like that on my boat and I suspect that most of us do. It won't affect cooling one way or the other if it's a bit too big, but too small will certainly hurt. Put the right size hose on to eliminate two extra potential failure points. Mechanic for 25 years.

[mrm]

Originally Posted by Ex-Calif

I was wrong (once - LOl) and always willing to be corrected, as long as you agree to update this Wiki article - LOL

Hand pump - Wikipedia, the free encyclopedia

Ther pertinent part is in Paragraph one.

Suction and lift hand pumps
Suction and lift are important considerations when pumping fluids. Suction is the vertical distance between the fluid to be pumped and the center of the pump, while lift is the vertical distance between the pump and the delivery point. The depth from which a hand pump will suck is limited by atmospheric pressure to an operating depth of less than 7 metres. The height to which a hand pump will lift is governed by the ability of the pump and the operator to lift the weight in the delivery pipe. Thus the same pump and operator will be able to achieve a greater lift with a smaller diameter pipe than they could with a larger diameter pipe.

I am guessing that the pump designer assumes the correct hose size will be used on the lift based on the fittings on the inlet of the pump.

The article goes on to talk about the types of pumps needed to achieve different suction heights.

Maybe I mixed suction and lift and I'll grant that but in it's ridiculous conclusion we attach an 8 inch pipe to a standard diesel water "thrash" pump inlet we aren't gonna suck it very far...

The attached is a sample of a sump pump spec. I presume the hose sizes are assumed by the graph creator to be suitable for the pump inlet fitting.

I could be easily convinced that if the column is too heavy the pump with either lift it, or it won't, due to suction available, and reduced performance at high lift may not occur at all. Too lazy to dig out the physics text. I do tend to mix Farmer John logic with book facts a lot...

Sorry for thread drift.

Umm... I think you indeed mixed things a little.
Assume a perfect pump (i.e. no leaks, no backflow) and think about it this way. On a suction side on each stroke you remove a certain volume of air. The same(*) volume of fluid will enter the suction pipe powered by... atmospheric pressure on the fluid surface. Of course, the larger the pipe diameter, the slower the vertical progress of the fluid, but with enough pump strokes it will get there... all the time pressed into the pipe by atmospheric pressure on fluid surface.
Now, once fluid gets to the output side you are indeed into weight lifting category and the more volume there the heavier it gets.

That was theory, assuming ideal pump. In practice, many pumps are not designed to run dry and may get damaged if it takes too long to get the fluid there or just have too high backflow when dry to even suck fluid to a needed height..

(*) another simplification here, not taking into account that air is compressible while water is not.

[shakey doug]

G'day All,

Is there such a thing as suction???? If there is please explain same.

[Strait Shooter]

Originally Posted by sailorchic34

Oh dear, now I have to put my engineers hat on. Actually trash pumps base lift on NPSH or net positive suction head which is based on absolute pressure. So vertical lift is based on pump design and absolute pressure. A normal pressure gauge pressure at 0 psig is actually 14.7 psia absolute at sea level.

The self priming design of say the typical Goulds trash pump might limit how large a pipe could be, but its more a function of the impeller and inlet design then suction pipe size.

Dennis G. is correct in that the larger pipe diameter has a lower friction loss per flow rate. This applies even in suction lift applications.. Really most raw water inlets are below the water line so suction lift is not even a factor.

Am I the only one that gets warm and fuzzy when women talk like this?

[mrm]

Originally Posted by shakey doug

G'day All,

Is there such a thing as suction???? If there is please explain same.

Pumps move media. When some amount is moved from pump intake, same or other medium tries to take its place (within general Physics laws).

If you want to call this phenomena suction, pull, or something else is probably open for discussion, but not being a native English speaker I bow out of this.