Gordon wrote:
..

Non contact centrifugal impeller. If you are tank tesing they wont
pump water unless you take the prop off.


I don't understand this. Can you explain a little more?
Thanks
Gordon
Yeah, sure, no problem.

A normal raw water pump uses a vaned rubber impeller in an assymetric
chamber. The base of the chamber and the cover plate need to be close
fitting and the pump works by compressing the vanes moving back fromn
the outlet to the inlet nearly flat so that far more water (in the gap
between the vanes) is moved from inlet to outlet as the rotor turns than
can get back on the side of the chamber with the bump. The pump has
rubber rubbing on metal at a couple of thousand RPM. The only thing that
stops it destroying itself instantly is the plentiful supply of cold
clean water its pumping which cools and lubricates it. Remove the
water or add a little sand and it will chew itself to bits faster than
you can yell 'STOP'. This is a positive displacement pump, i.e. its
output is proportional to its speed (as long as it isnt overloaded) and
it pumps from a very low speed.

A seagull does things rather differently. The water pump chamber is
cylindrical and cincentric on the drive shaft. In the chamber is a hard
four vaned rotor running on the shaft with a top plate around the
shaft of about 2/3 the rotor diameter. Water enters around the shaft in
the middle of the rotor at the bottom and is spun to the outside of the
pump chamber where it exits upwards via the gap between the rotor top
plate and the chamber wall. The closest the rotor ever comes to any
stationary part is about 1/16" so as far as the pump is concerned it can
run dry all day with no damage though the powerhead will get pretty
unhappy after a minute or two with no cooling! It also isnt much
bothered by a bit of sand or sediment as long as the liquid its pumping
is fairly runny, its happy.

Where the trouble starts is it cant lift the water enough to get it to
the powerhead at low revs and also it cant pump effectively if it gets a
big air bubble in the chamber so the top of the chamber *must* be
totally below the water line even when moveing. If you are testing the
engine in a small tank, it stirs in too many bubbles so the pump doesnt
work worth a damm and the engine may overheat. Take the prop off, no
stirring, no bubbles and you can test it all day with no problems. You
want to test the engine under load, moor alongside with your stern
pointed towards open water please, rig a strong bow spring to keep you
there and test the engine on the boat. One can keeep that up for as
long as people near by are willing to put up with the noise.


--
Ian Malcolm. London, ENGLAND. (NEWSGROUP REPLY PREFERRED)
ianm[at]the[dash]malcolms[dot]freeserve[dot]co[dot]uk
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'Stingo' Albacore #1554 - 15' Early 60's, Uffa Fox designed,
All varnished hot moulded wooden racing dinghy.