90 degree elbows on raw water pump impeller?

crieders · January 23, 2012, 07:14:44 AM

Sorry to sicken everyone with this topic. Installed the new Oberdorfer in nice 25 degree weather. I used straight thread to barb nipples rather than the 90 degree to enhance water flow. This meant slightly longer hoses. A salesperson suggested this. I then talked with a senior at the US Merchant Marine Academy who told me it was better to shorten the hoses and use the 90 degree elbow nipples leading the hoses straight back. Soooo, who is right or does it matter at all?

Ken Juul · January 23, 2012, 07:49:44 AM

Not sure it matters. One possible advantage of the 90° fitting is if you do shed an impellor blade, the fitting might stop it, with the straight fitting the first stop is the Heat Exchanger.

Jim Hardesty · January 23, 2012, 08:48:25 AM

I'm with Ken. If it wasn't a problem, why try to fix it? Personaly, I always take salesmen's advice with a grain of salt. They may be right, but they are not engineers. But, don't change it. Let us know if you get more flow. The salesman may be on to something.
Jim

David Sanner · January 23, 2012, 02:02:55 PM

I would think a straight run, as long as you're only adding a couple inches to the hose, would have less flow restriction than a 90 especially since the ID of the 90 is less than the ID of the hose. However not sure that it will amount to much either way. One way to find out.

Stu Jackson · January 23, 2012, 02:21:38 PM

The engineering answer is that elbow friction losses are computed in what is called "equivalent feet of pipe." For small elbows in the 3/8" to 3/4" size, the actual losses to be calculated are from 3.2 feet to 1.4 feet, from the engineering tables. So, the elbows would be equal to simply adding another few feet of straight hose. Negligible, considering that the heat exchanger and the long(er) runs of hose are far more than the friction loss contribution from the elbows. Conclusion: it will make no difference.

Ron Hill · January 23, 2012, 02:48:35 PM

Cliff : I'd put a 90 degree elbow on the exhaust of the pump to the HX, for the reason mentioned by Ken !!

Stu Jackson · January 23, 2012, 03:59:12 PM

Quote from: crieders on January 23, 2012, 07:14:44 AM
Installed the new Oberdorfer in nice 25 degree weather.

Cliff, what you failed to clue us into is the solution to your OP in another thread: What happened to the "holes don't line up" dilemma? What did you do? How did you get it to work? We need closure!!!

crieders · January 23, 2012, 04:00:36 PM

Stu's observaton is what I think the fine young fellow from the Academy was trying to explain. Since I hooked it all up Sunday, I guess we'll see when the springtime arrives. By they I probably will have created some more interesting problems for myself !!

Stu Jackson · January 23, 2012, 04:02:05 PM

Yeah, but...

http://c34.org/bbs/index.php/topic,6731.0.html

Wha happened????

pablosgirl · January 23, 2012, 05:28:19 PM

Ken,

I have the factory Oberdorfer pump with the 90 deg nipples on the M25XP. I took out my HX last year to paint it and ream out the tubes. I found 3 impeller blades and part of a zink pencil in there. I guess the PO did not believe in checking the HX when he replaced failed impellers. So I doubt that the elbow did anything to impede the movement of the impeller blades travel to the HX. I believe that the HX diameter (3" as apposed to 2") would have a greater impact on the cooling capacity of the raw water (more surface area to transfer the heat). Thanks Stu for the engineering explanation!

Paul

Ron Hill · January 23, 2012, 05:41:20 PM

Guys : Believe me, the first place to look for a missing impeller blade is in the exhaust elbow of the raw water pump!!
It will probably be there.

That's what my limited 25 years of experience has taught me!!

mainesail · January 23, 2012, 07:44:12 PM

Quote from: Ron Hill on January 23, 2012, 05:41:20 PM
Guys : Believe me, the first place to look for a missing impeller blade is in the exhaust elbow.
It will probably be there.

That's what my limited 25 years of experience has taught me!!

Exhaust elbow? Or RW pump outlet elbow? I've yet to see a chunk of impeller make it through all four passes of he HX and end up in the exhaust elbow...? Usually they are stuck in the inlet chamber of the HX.

I've caught them in the elbows at the RW pump and very often in the HX. There is much "concern" about impeller chunks in the HX but there is little reason to be concerned. You'd need to chew up a LOT of impellers to plug the HX. Regular changing of zincs and impellers can nearly eliminate any chance of plugging the HX.

The far bigger issue is zinc crud blocking the HX from zincs not changed often enough.

The RW pump on my own engine has two 90's, one entering the pump and one leaving. She can still suck a 5 gallon bucket dry in under 60 seconds, at idles speeds. The elbows were designed into the system by Universal/Westerbeke. Nothing wrong with a straight barb but you now have more hose that can potentially chafe.

In my experience, with lots of Universal and Westerbeke HX's, is that a chunk of impeller is of little to no concern unless it gets stuck in the RW pump outlet elbow. If it makes it to the HX you're usually fine. Zinc crud is the far bigger "plugging" and flow reducing issue when it comes to HX's..

This impeller blade came through the elbow just fine and was fairly large.

This is the end where I find 95+% of HX plugging...

Here's one from two weeks ago.

Ken Juul · January 24, 2012, 04:31:56 AM

Having ruined more than my share of impellers, (thru hull not open, eel grass, plastic bags, the list goes on) I've found that the 90° elbow will stop and hold the big pieces. Small pieces go right past. I also think that a big piece left long enough in the elbow will eventually become unstuck and migrate to the HX. Moral of the story. When on the water, clean the elbow, replace the impellor and get back underway. Once back at the dock and things have cooled down, check the HX. Or you can just check /clean the HX annually which is a good idea anyway.

crieders · January 24, 2012, 07:26:56 AM

Stu, a friend of mine cut off a very small piece of the pump base on the upper left side, where that bolt is inserted in the engine. It then fit fine and the hole on the north side and the south end matched perfectly. Its real tight on there now. As to the debate over elbow vs straight, one "expert" told me, as others have said here, that the most important issue to to remove the HX and clean it out. I have never had an impeller break apart since I replace them every year and I replace the zinc every year but I probably should still remove the HX which I find an intimidating job.

Stu Jackson · January 24, 2012, 10:05:47 AM

Cliff, I've reported this a few other times: When we first got our boat, the engine would overheat. M25 with the OEM 2 inch heat exchanger.

http://c34.org/bbs/index.php/topic,1487.msg8117.html#msg8117

Since then I've recommended that folks remove their HXs if they think they have any issues because the port can clog up even if the tubes are fine. You'll never know that if you don't remove it.

The Critical Upgrades topic shows how to drain the coolant before you remove the HX.

We also upgraded to a new 3 inch HX a few years ago. Makes a big difference. Our HX is held on by two hose clamps and I inserted a rubber bicycle tube for vibration isolation between the tube of the HX and the mounting plate to avoid chafe.

Do a search on "mariachi" and there are three good links for help on doing the work.