EricFox
Member II
For the past two years my raw pump (Sherwood G-6) has had sporadic episodes where it fails to pump at engine start-up. I won't go through all of the debugging that I have done, but there are no blockages upstream or downstream, there are no water leaks, and the impellor is fine (swapped it out with no impact).
In fact, when it's failing to pump, I can detach the input hose to the pump from the strainer and put it in a bucket of water, but get nothing out from the pump even with the output hose disconnected - the pump can't pump hard enough to overcome what I expect is a small air lock in the input hose from the bucket (yes, I have swapped out the input hose too, and it's not kinked). If I wait several minutes, sometimes it can clear what I expect is the air lock, but only if I temporarily detach the output hose so that there is no back pressure. It typically fails after I have been sailing with some good heel - my hypothesis is that the water drops out of the through-hull when I'm heeled and the pump can't overcome the airlock when I do to start the engine. What I do in those situations is to detach the output hose from the pump to minimize back pressure and it usually starts pumping after a few seconds.
All of that so say that everything points to the pump itself, but when I have it apart I can't see any obvious problems (pics below) - maybe some light scoring if I squint just right, but that's it. Also, I'm almost positive that the shaft from the engine is turning based on the position of the key when I open the pump (and the fact that it does work most of time). There is no play rotationally or in-out on that shaft when I have the pump off the shaft.
I'm quite willing to replace it with a new Oberdorfer (N202-M16 for 1/2" NPT fittings from pumpsandpartsonline.com), but what is confusing me is that in all of the posts about bad Sherwood's seem to be about leaks, not about an inability to pull. Is the failure mode that I'm seeing consistent with a bad pump? Is there anything internal to the pump that might be bad that I'm overlooking? There doesn't appear to ne a whole heck of a lot to go wrong with pump if it's sealing, the impellor is good, and it's turning.
In fact, when it's failing to pump, I can detach the input hose to the pump from the strainer and put it in a bucket of water, but get nothing out from the pump even with the output hose disconnected - the pump can't pump hard enough to overcome what I expect is a small air lock in the input hose from the bucket (yes, I have swapped out the input hose too, and it's not kinked). If I wait several minutes, sometimes it can clear what I expect is the air lock, but only if I temporarily detach the output hose so that there is no back pressure. It typically fails after I have been sailing with some good heel - my hypothesis is that the water drops out of the through-hull when I'm heeled and the pump can't overcome the airlock when I do to start the engine. What I do in those situations is to detach the output hose from the pump to minimize back pressure and it usually starts pumping after a few seconds.
All of that so say that everything points to the pump itself, but when I have it apart I can't see any obvious problems (pics below) - maybe some light scoring if I squint just right, but that's it. Also, I'm almost positive that the shaft from the engine is turning based on the position of the key when I open the pump (and the fact that it does work most of time). There is no play rotationally or in-out on that shaft when I have the pump off the shaft.
I'm quite willing to replace it with a new Oberdorfer (N202-M16 for 1/2" NPT fittings from pumpsandpartsonline.com), but what is confusing me is that in all of the posts about bad Sherwood's seem to be about leaks, not about an inability to pull. Is the failure mode that I'm seeing consistent with a bad pump? Is there anything internal to the pump that might be bad that I'm overlooking? There doesn't appear to ne a whole heck of a lot to go wrong with pump if it's sealing, the impellor is good, and it's turning.