Changing the impeller on a Sherwood

tonywright · June 18, 2015, 10:31:50 AM

I made a rookie mistake last weekend. Despite all the usual pre-departure precautions (including checking for water being pumped out the stern - oddly it seemed fine when I checked), for some reason I did not open the thru-hull for the raw water cooling. So after motoring out into the main channel, I heard the temp alarm go off. Quickly realized what I must have done, anchored and shut the engine down. Sure enough, thru-hull was firmly.

Assuming the impeller would be shot after the 10 minutes or so of engine running, I got the spare ready, and removed the pump housing (cursing those rear-facing bolts in the process). The good news: the impeller was fine. I could have just opened the thru-hull and continued happily on my way. But no, now the housing was off, and the impeller in my hand, I had to reverse the process. I think it took the better part of two hours to finally get the job done.

So I am thinking that I must have the technique all wrong, so looking for some hints and tips. (I am seriously thinking about an Oberdorfer of course, but apart from that solution).

The main problem: getting the O-ring to stay in the groove while I put the housing back in place. Every time I would try this, the O-ring would pop out and hang loose. In the end, the first mate (who I am now thinking of promoting to chief engineer) suggested each of us using a kebab stick to hold the O-ring in place while we carefully pushed the housing back over the impeller, and then remove them before pushing the housing home the last 1/4 inch. Good job the galley was so equipped!

But is there a better way?

Impeller on the shaft or in the housing first? I found putting it on the shaft was the only way to keep the key in the slot, especially since the shaft had stopped with the key slot at 5 o'clock.

A couple of things I think I learned:

Put a cloth into the sump under the engine to cover up the drain holes. The shaft key loves to jump out when you are a little careless putting the impeller on, and it would fit easily down a drain hole. Also have some needle nosed pliers on hand to remove said key from the crevice between the belt and pulley just below the pump.

Lining up the brass plate: the shiny surface showed that the impeller compresses at approx. 12 o'clock, so made sure to put it back that way. Have a spare gasket on hand so when you tear it on your 52nd attempt to get the impeller back, all the effort is not in vain.

Push the housing over the impeller starting at 12 o'clock, so that the vanes are compressed there. Angle it in slightly at the top, then press down and slide it into position. (it is at this point that the O-ring pops out of position and taunts you by hanging loosely over the impeller). Pull the housing back, put the O-ring back in place and start over...

Post a large notice on the helm station to double check that the thru-hull is open.

Again, looking for a better way to do all this, at least until I cave and get an Oberdorfer...

Jim Hardesty · June 18, 2015, 12:31:59 PM

Tony,
I know some that keep the ignition key on the seacock handle.
Hope you had a nice sail any way.
Jim

KWKloeber · June 18, 2015, 12:54:51 PM

Tony,

Nope, you're doing it all correctly -- so long as you keep enjoying the feeling of beating your forehead against the bulkhead due to a Sherwood pump. It's state of the art to be used in a marine environment. NOT. Keep going with that, until the shaft seizes and snaps off the fork on the end of your camshaft. Then you'll really enjoy the pain of beating your head once again. Is all that worth "saving" $232.00 over the life of your sailing season(s)? -- and if you aren't sailing for a month and have a bill for an engine job to show for it?

http://www.c34.org/wiki/index.php?title=File:Camshaft_after_Sherwood_pumps_seize_up.jpg
http://www.c34.org/wiki/index.php?title=Why_do_some_of_our_engines_have_different_Oberdorfer_sea_water_pumps_than_others,_and_what_about_that_cir-clip_%22holding_the_impeller_on%22%3F

ken

Stu Jackson · June 18, 2015, 01:00:08 PM

Ken has recently given us some good food for thought on the severe potential downsides to continuing use of Sherwood pumps.

It's here on the forum and we've both posted his input to the wiki.

What's your ENTIRE engine worth to you?

Ed Shankle · June 18, 2015, 01:18:54 PM

Tony,
1. try a light smear of silicone grease or just vaseline to hold the oring in place
2. I put the impeller in the housing first, then roughly orient the slot angle to the key on the shaft close enough so I can line it up and slip the housing on. then finally orient the housing holes to the pump housing holes
3. when putting on new gasket, I put the screws in, extending just the enough for the gasket and plate to hold in place, and follow the same above steps

A few years ago I managed to strip the threads of the top right housing hole. So I got a longer screw. reamed the hole and installed the screw front to back, with a nut and washer on the back. Due to the shape of the housing the screw head couldn't sit flush, so I ground it down on one side enough so that the remainder would sit flush.

regards,
Ed

tonywright · June 18, 2015, 02:48:52 PM

Jim

I do put the key there. For some reason the reminder didn't work this time. I did check that there was water coming out the exhaust though. Proves that that particular check isn't good enough. It also doesn't seem to be true that running "dry" destroys the impeller in 2 minutes or less. Maybe I had enough water in the system still to keep it wet.

Ed, I think I did it your way the last time I did this. But this time the key kept jumping out of the slot. I figured Vaseline might destroy the O-ring over time. But maybe I got that wrong? Silicone might have worked.

Ken and Stu make a powerful case. I may just have to consult with my new chief engineer. When I acquired Vagabond, the original pump was leaking with just 43 hours on the engine. The replacement has held up fine for the last 8 seasons, but maybe it's because we sail in fresh water? Sounds like I may have just been lucky so far...

Tony

Fred Koehlmann · June 18, 2015, 07:10:34 PM

I'm sure there must be a good reason why you all close your thru-hull when not motoring, but why? I've done it occasionally when I was putting in the winter anti-freeze when out of the water, but even then I open it to drain. Is it that you close all of them when not on the boat, in case of breakage and the subsequent leakage? I can't ever remember closing any of our thru-hulls when away, even on my dad boat. Maybe I'd do it for really long absences.

Ron Hill · June 18, 2015, 07:15:40 PM

Tony : YOU NEED A CHECK LIST to follow.

Whether it be a Sherwood or an Oberdoffer or a Johnson --- you'll screw up an impeller if you don't open the raw water thru hull when you start the engine. No magic or surprise in that!!!!

A thought

Stu Jackson · June 18, 2015, 07:51:48 PM

Quote from: Fred Koehlmann on June 18, 2015, 07:10:34 PM
I'm sure there must be a good reason why you all close your thru-hull when not motoring, but why? I've done it occasionally when I was putting in the winter anti-freeze when out of the water, but even then I open it to drain. Is it that you close all of them when not on the boat, in case of breakage and the subsequent leakage? I can't ever remember closing any of our thru-hulls when away, even on my dad boat. Maybe I'd do it for really long absences.

This gets into Philosophy 101: Does one close all their seacocks when leaving the boat?

It is a simple question that has been asked & answered on just about every boating forum known to man.

The answers are obvious, and probably not worth repeating. Because there are only two: Yes or No.

Your boat, your choice.

Personally, I like this question better: "Why am I even considering keeping a Sherwood pump on my valuable engine?"

Ron Hill · June 19, 2015, 07:13:50 PM

Tis easy to Damn something, but it takes some effort to help people improve an item and make it better!!

A thought

Ken Juul · July 02, 2015, 02:02:43 PM

Yes changing the impeller on the sherwood is easier. That said I can change my Sherwood in less than ten minutes. Close the thru hull, pull the cover off, the brass plate usually stays stuck to the paper gasket, no need to change it. If not, quick sand and use a spare. Gasket on, brass plate on, impeller on. Lube the impeller with some veg/olive oil (in the galley in easy reach) squish the cover on. Don't worry about vane alignment, they self align on the first rotation. Tighten screws. If in doubt, this take a little extra time, remove the intake line and fill with water, hold thumb over it to reattach. OPEN THRU HULL. Restart engine.

As far as the pump seizing...The weep hole will give you plenty of warning. It should be checked with the engine running at least once a month. Catch it early enough and it's and easy rebuild.

tonywright · July 02, 2015, 07:28:44 PM

Ken, you did not mention the O-ring. That's what I found to be the worst part of it. It just would not stay put in the groove. Any tips?

Tony

Ken Juul · July 03, 2015, 04:22:46 AM

The o-ring should stay seated. Be careful with it, the first time I did it, I cleaned it with a rag and it stretched making it a tad too big. Used my spare. Now I just remove and inspect it, toss if bad. Clean the metal cover and reinstall. If it doesn't want to stay in I hold it in place with my fingers as long as I can. It should stay in place that last 1/4" or so. Two or 3 dabs of RTV or similiar liquid gasket could also be used to hold it in place, just makes cleaning up the next time harder.