Seawater pumps and their vagaries ..... speculation and a question!

[RichardS]

This is just my thoughts about seawater pumps having had another experience re-lauching a couple of weeks ago and following on from a comment made by
sailaboutVic after his recent re-launch.

It seems that seawater pumps failing to pump reliably is quite a common theme on PBO. Posters have usually changed the impeller but without recifying the
problem. The answer is very often that the seawater filter screw-on lid is not making an airtight seal and remedies such as silicone/vaseline, a new lid, a
tighter lid etc usually seem to remedy the problem.

When we acquired our boat, I replaced impellers and installed Speedseal Life pump covers but still one of the engines would not always prime reliably until
I properly greased the filter lids. They now both seems to be 100% reliable ..... unless the boat has been lifted out of the water, and then all bets are off.

I am guessing that the hit and miss re-priming of the pumps when the boat is lowered back into the water is strongly dependent upon the height of the filter above sea level. My cat sits very high in the water and the filters are well above sea level. I haven't measure it but I'm guessing that the top of the central tube in the filter is perhaps 9 - 12 inches above sea level and this is presumably affecting the priming procedure once the water has drianed out through the saildrive inlets which are well below the hull.

Last month's re-launch went like this:

Stb engine started. No sign of any exhaust water. Blipped throttle. No sign. Blipped again. Water starts.

Port engine started. Some water appears but then stops. Throttle blipped. Water starts.

We start to leave the hoist and I say to SWMBO to nip across and check both exhausts. She looks over the stern quarters and says both are OK.

We set off. After 30 seconds I ask SWMBO to check again. She asks why as she's only just checked. I ask her to humour me. She looks over the Stb side and says it's fine. She looks over Port side and says "It's stopped. Nothing there"

I guess that's it's airlocked so pop her into neutral and blip again. SWMBO confirms that the water flow has re-appeared.

We check again all the way back to our berth but everthing is fine now. I start the engines the next day and the water flow is instant, as it always is .... until we have our next lift out.

I wonder if others have a similar experience? If so, the moral clearly is "When leaving the hoist keep a very close eye on the exhaust/s and don't assume anything".

Richard

 

[Len Ingalls]

Try filling filters with water before you start up first time. May help to prime system.
Suction side needs to be airtight for sure.Greasing lids is good but indicates lid seals may be shot.
Are suction hoses double clamped & tight fitting before clamping?
A couple of feet lift is nothing for an impeller type pump if there are no suction leaks.
You have installed Speedseals so pump covers should be good,but check cover O-rings & apply a bit of silicone grease to tips of impellers when re-installing in spring. You do remove impellers for winter?
What about the "swash"? plate that compresses the impeller blades? Worn?
Are the pumps belt driven & slipping?

http://www.marinehowto.com/
Hope this helps.

 

[sailaboutvic]

Richard have describe the problem we had to a T .
When we was first launch and checked the exhaust , water pumped , not long after leaving the quay it was reduced to hardly nothing ,
Luckily we had room just to drop the anchor .

I filtered a speedseal disk last year so checking the impeller was quick , which shown it was fine.
Next I undid the sea water filter mine it the vetus type with the see thought lid with the cross on top , almost impossible to undo . I removed the lid and filled it with water until it run out , replace the lid , this time water now started to come out of the exhaust , it seen fine until it was in gear and under presser , then every few second it would start again , so I removed the filter lid again this time I greases the rubber ring .
This seen to have sorted the problem .
Anyone with a sail drive need to check and keep a close eye on the water from the exhaust after launching until they are happy it working fine .

Tip.
The lid on these filters do seen to go on so tie that it a pain when it come to remove them , mine is in a place where too close to the panel to get any grip , the guy who fitted it need shot in , my way around this is I cut out some ply the size of the lid and remove a cutout cross in the middle from the ply ,
I also screwed screw all around the cut out cross into the ply ,
so now I just place the ply wood on the lid and use a screwdriver between the screw to lever the top open .
Not the best solution but it works .

 

[vyv_cox]

I agree with the posts above - almost any small air leak will prevent the pump from sucking as it should. However, I can add another. Air will enter through the shaft seal if the lips or shaft are worn. Replacing the seal fixes the problem briefly but if a groove has been worn in the shaft it may be a temporary solution.

 

[sailaboutvic]

I agree with the posts above - almost any small air leak will prevent the pump from sucking as it should. However, I can add another. Air will enter through the shaft seal if the lips or shaft are worn. Replacing the seal fixes the problem briefly but if a groove has been worn in the shaft it may be a temporary solution.

I take it you mean the shaft seal on the sea water pump ?

 

[Len Ingalls]

Richard have describe the problem we had to a T .
When we was first launch and checked the exhaust , water pumped , not long after leaving the quay it was reduced to hardly nothing ,
Luckily we had room just to drop the anchor .

I filtered a speedseal disk last year so checking the impeller was quick , which shown it was fine.
Next I undid the sea water filter mine it the vetus type with the see thought lid with the cross on top , almost impossible to undo . I removed the lid and filled it with water until it run out , replace the lid , this time water now started to come out of the exhaust , it seen fine until it was in gear and under presser , then every few second it would start again , so I removed the filter lid again this time I greases the rubber ring .
This seen to have sorted the problem .
Anyone with a sail drive need to check and keep a close eye on the water from the exhaust after launching until they are happy it working fine .

Tip.
The lid on these filters do seen to go on so tie that it a pain when it come to remove them , mine is in a place where too close to the panel to get any grip , the guy who fitted it need shot in , my way around this is I cut out some ply the size of the lid and remove a cutout cross in the middle from the ply ,
I also screwed screw all around the cut out cross into the ply ,
so now I just place the ply wood on the lid and use a screwdriver between the screw to lever the top open .
Not the best solution but it works .

Yes. It appears to be same. Thank you.
Good idea for cover removal tool.

A plumbers strap wrench or oil filter wrench may work also. https://www.amazon.ca/BO13010-Constrictor-Aluminum-Strap-Wrench/dp/B0011E4QU4

 

[Len Ingalls]

I agree with the posts above - almost any small air leak will prevent the pump from sucking as it should. However, I can add another. Air will enter through the shaft seal if the lips or shaft are worn. Replacing the seal fixes the problem briefly but if a groove has been worn in the shaft it may be a temporary solution.

+1

 

[Len Ingalls]

I take it you mean the shaft seal on the sea water pump ?

I think he does https://coxengineering.sharepoint.com/Pages/Waterpump.aspx

 

[Deleted member 36384]

I used to work for a charter company in the late 80s, for 5 years, and the filters were mostly the hull mounted tube style on top of the seacock. The high level, clear view styles were just being introduced with new boats. Priming issues then started and it was always to do with the lid and seals (when I say issues, they were not significant, usually just a lid off, clean and grease and back on again). If it is a significant and regular issue with you, then perhaps changing to a hull mounted inline filter might be a solution. ASAP stock the Seaflow range: -

http://www.asap-supplies.com/raw-water-intake-strainer-hull-mounted-402024

My own boat has a similar hull mounted strainer but with a knurled cap that is secured hand tight, no need for strap wrenches or spanners, a sister yacht has one with a lid that is just held on with butterfly screws. Seals are simple flat sheets of rubber cut to fit the profile of the cap. They are very simple arrangements and work efficiently and reliably.

 

[sailaboutvic]

I think he does https://coxengineering.sharepoint.com/Pages/Waterpump.aspx

Yes I know , I was just pulling his leg

 

[RichardS]

Very interesting thoughts ..... thanks guys.

I should just add that I don't think I have any problem per se. As long as my boat stays in the water, the water starts pumping through the exhaust as soon as I start the engine at idle speed, even if the boat has been left alone for the 6 months over the winter. What's more, since I've had the boat from new, I can see that the output from both engines even now appears to have as much volume as it did when the engines were brand new.

To answer Len, I have tried priming the filters and that certainly does work .... but I don't think that it should be necessary unless certain engine configurations (relative heights of sea level / filter / impeller / exhaust elbow) mean that such a procedure is always advisable because of drain back / siphoning out of the entire seawater system.

I don't remove the impellers over winter (neither do I remove the impeller from my outboard, of course) but this never causes a problem when I re-start the engines after the 6-month dormancy. They were extremely well greased with silicon grease when I inserted them along with the Speedseal cover and PTFE bearing. However, I might well trying re-greasing them before re-launch next time I lift out. If the water flow is then immediate it would suggest that the delay in output is due to the impeller not being very good at pumping air at low revs unless recently greased. Perhaps they are all like this to some extent but my relative heights are exacerbating the issue?

Richard

 

[BobnLesley]

...Air will enter through the shaft seal if the lips or shaft are worn. Replacing the seal fixes the problem briefly but if a groove has been worn in the shaft it may be a temporary solution.

A slight thread drift, but we've extended the life of two impeller pumps now by replacing their specified single-lipped seals with the equivalent twin lipped versions; they give you two water/oil stops and these sit either side of that wear-groove.

 

[lw395]

Even if a pump seals perfectly, it can take a long time to prime.
The air it is sucking expands, so the pump becomes inefficient as soon as there is a pressure differential.
A rubber vane pump does not squeeze the return side vanes down to zero, so a certain amount of air is just going around with the vanes.
This doesn't matter with water, which doesn't compress much, but it's inefficient with air.
Couple that with a large volume strainer and priming takes a long time.
Add the slightest leak and it's all over.
I just chuck some water in the strainer before launch, then open seacocks after launch.

 

[Len Ingalls]

I don't remove the impellers over winter (neither do I remove the impeller from my outboard, of course) but this never causes a problem when I re-start the engines after the 6-month dormancy. They were extremely well greased with silicon grease when I inserted them along with the Speedseal cover and PTFE bearing. However, I might well trying re-greasing them before re-launch next time I lift out. If the water flow is then immediate it would suggest that the delay in output is due to the impeller not being very good at pumping air at low revs unless recently greased. Perhaps they are all like this to some extent but my relative heights are exacerbating the issue?

Suggest you remove impeller during storage.
1. This will prevent the blades from taking a "set" & being very inefficient at start up in Spring.
2. Gives you a good chance to closely inspect impeller for cracks/impending failure.
3. Even a brief dry run time may damage impeller to various degrees.
4. Leaking pump shaft seal should show water stain at weep hole on back of pump.

Speedseal is great at saving the impeller from wearing the front cover,but impeller can wear on blade tips & against rear of pump housing. Speedseal also makes it quick & easy to remove/replace/inspect impeller at any time.

I refer you to the Maine Sail & Cox links that I provided in prev. post re RW pump & impeller.
Impellers are a "wear" item, & should be treated like oil,belts,filters,etc.

 

[RichardS]

Even if a pump seals perfectly, it can take a long time to prime.
The air it is sucking expands, so the pump becomes inefficient as soon as there is a pressure differential.
A rubber vane pump does not squeeze the return side vanes down to zero, so a certain amount of air is just going around with the vanes.
This doesn't matter with water, which doesn't compress much, but it's inefficient with air.
Couple that with a large volume strainer and priming takes a long time.

Some interesting thoughts there LW which I don't recall being expressed before but which make sense to me.

Perhaps with my volume of air to shift I'm getting too nervous too soon. Even though I've got Speedseal Lifes I don't really like the thought of the impeller running dry (although I'm sure it's not actually dry in the true sense of the word) which is why I blip the throttle which I know will always do the trick.

It might well be that both pumps are actually pumping air as well as they possibly can and that it's going to take say 100 pump revolutions to clear the air. If I blip the throttle the water appears sooner ...... but it's still 100 revolutions.

I'll try re-greasing the impellers next time I'm out, but I'll also perhaps hold my nerve and just leave it at tickover. If open the engine hatches and watch the clear plastic tops of the filters I could actually see the seawater appearing out of the top of the central pipe so I would know that it's just a matter of time and I need to hold my nerve.

Richard

 

[RichardS]

Suggest you remove impeller during storage.
1. This will prevent the blades from taking a "set" & being very inefficient at start up in Spring.
2. Gives you a good chance to closely inspect impeller for cracks/impending failure.
3. Even a brief dry run time may damage impeller to various degrees.
4. Leaking pump shaft seal should show water stain at weep hole on back of pump.

Speedseal is great at saving the impeller from wearing the front cover,but impeller can wear on blade tips & against rear of pump housing. Speedseal also makes it quick & easy to remove/replace/inspect impeller at any time.

Thanks Len. I'm sure that removing the impellers is a good approach .... but a few years ago a friend of mine had his boat burn to the waterline in the marina in the South of France and those photos have left a lasting impression on me. There was no-one on board at the time so the marina had to start moving adjacent boats and the flaming boat very quickly.

My current marina office also keep a spare key just in case they have to move the boat for other reasons, such as happened in a storm 2 years ago in our previous marina 10 miles away when some of the pontoons started to break loose and they had to start moving boats very quickly.

Even if there are no impellers fitted the boat would be saved ..... but at what potential cost?

It's probably a margin call but I do sleep better at night with the impellers in place.

Richard