What another water pump question?

[Tonyseagull]

I went through the search engine looking for a answer to my situation but have no luck... the question is in two parts ...I have an SJM with the metal water pump rotor...the water pump works the best when I put the rotor/impeller on the drive shaft upside down...the flow is considerably better with the impeller on that way...there is no wear marks on the impeller...


my friends 40plus FP pumps right down to a tick over...mine needs a couple of hundred more revs to keep the water flowing than his ...the cylinder on mine the water passages have been well cleaned and are free from crud and rust...

At half speed pushing my dingy the water flow keeps the engine just warm but at a slow trol I ocassionally have to bump up the tiller to increase the revs as the water stops flowing then the trol speed is too high...I have checked the water depth of the unit on the boat and it is on spec...any thoughts...or are the metal rotors/impellers not as efficient as the plastic??

[charlesp]

For an SJM to have been fitted with a metal impeller it must be a very early one. The metal ones were either aluminium alloy or brass/bronze, and a quick look in the workshop tells me that they were a little smaller, smaller in OD and smaller in section,and fitted to an altogether less substantial shaft. I have an alloy one that just doesn't work in an FV I'm putting back together, the corrosion (not wear) is such that turbulence or something is preventing the flow.

On the other hand I have an untouched FV - which I assume has its original pump - and that pumps right down to a slow tickover.

So I don't know!

[Tonyseagull]

For an SJM to have been fitted with a metal impeller it must be a very early one. The metal ones were either aluminium alloy or brass/bronze, and a quick look in the workshop tells me that they were a little smaller, smaller in OD and smaller in section,and fitted to an altogether less substantial shaft. I have an alloy one that just doesn't work in an FV I'm putting back together, the corrosion (not wear) is such that turbulence or something is preventing the flow.

On the other hand I have an untouched FV - which I assume has its original pump - and that pumps right down to a slow tickover.

So I don't know!

Yes I have noticed that the square drive shaft of this SJM is quite a lot smaller in section also the chrome main shaft is about half the wall thickness of my friends FP although the over all tube thickness of mine is the same, it also weighs quite a lot less...the top of this chrome plated shaft where the crankcase bolts on has a threaded hole were my friends seagull has the same lump cast on but no threaded hole.

I have a spare plastic impeller 4 with vanes and a brass impeller with six vanes cast on, but even if thees impellers would fit the lower housing the square drive shaft would be too sloppy to work...although the vanes on mine have no signs of wear...they have quite a lot of clearence with the sides of the water pump housing and I am assuming this is why my pump is not as efficiant as what I would like.

When I dismantled my SJM there was a white plastic disc on the lower face of the metal impeller pressed onto the drive shaft...does anyones manuals show a plastic disc ???...I can only assume that someone else had this same pumping problem as well and was trying to solve it as well...there is some circular scuffing that the plastic disc made so I have turned the baklite spacer/washer over that forms the joint between the gearcase and pump housing.

I have been thinking about obtaining a small block of nylon and making a replacement with better tolerances...any thoughts on a better way?

[Charles uk]

The water pump impellor is highly unlikly to be the cause of your problem & adding a "washer" underneath it is only going to restrict the flow into the pump chamber, the probable location of this crisis is after the water pump housing.

As an experiment remove the powerhead & try seeing how much water will pass through the cylinder via the water delivery pipe, with only 2 or 3 foot head of water pressure, using a funnel & a bit of plastic pipe.

[40TPI]

As an experiment remove the powerhead & try seeing how much water will pass through the cylinder via the water delivery pipe, with only 2 or 3 foot head of water pressure, using a funnel & a bit of plastic pipe.

Good idea. A stop watch and a calibrated jam jar under the end of the pipe might add a meaningful benchmark to the experiment?

If at the end of this a suitable increase in flow is not obtained is this the point to consider using the smaller Seagull trolling prop ( or even trimming down a std prop) ........


Peter

[Tonyseagull]

The water pump impellor is highly unlikly to be the cause of your problem & adding a "washer" underneath it is only going to restrict the flow into the pump chamber, the probable location of this crisis is after the water pump housing.

As an experiment remove the powerhead & try seeing how much water will pass through the cylinder via the water delivery pipe, with only 2 or 3 foot head of water pressure, using a funnel & a bit of plastic pipe.

That sounds very interesting...I will give that a try later in the week...I have a long shaft on the bench right now...I am begining to wonder who owns who with these delightful engines, they do seem to take over ones life!

[Tonyseagull]

As an experiment remove the powerhead & try seeing how much water will pass through the cylinder via the water delivery pipe, with only 2 or 3 foot head of water pressure, using a funnel & a bit of plastic pipe.

Good idea. A stop watch and a calibrated jam jar under the end of the pipe might add a meaningful benchmark to the experiment?

If at the end of this a suitable increase in flow is not obtained is this the point to consider using the smaller Seagull trolling prop ( or even trimming down a std prop) ........


Peter

I did not know they made a trolling prop...it probably would not make much of a difference to my cruising speed of the inflateable boat I have...I get about 4/5 knots at half throttle...anymore throttle just makes more carb noise and the boat does not go any faster...I really like the idea of having a second prop just for fishing.