Over exhuberant Jabsco

G

Graham_Wright

Well-Known Member
Joined
30 Dec 2002
Messages
8,185
Location
Gloucestershire
www.mastaclimba.com
Over exuberant Jabsco

Finally connected up my Jabsco pump last night (on the hard, fed from a tank).

I was amazed at the quantity of water ejected from the transom outlet.

It obviously has pretensions as a fire fighting pump.

It is 1:1 driven from the engine which is 1 1/2 litres.

Do I really need that enormous throughput?

Most boats I have seen under power seem to enjoy a gentle spurt now and again. This was like a horizontal geyser. Washed the van though (which happened to be in the firing line).

The exhaust system is as per recommendations;- water trap (low down), muffler and swan neck. The exhaust is commendably quiet - save for the gushing water. God help anybody's dinghy in the firing line.

I could reduce the pulley ratio or restrict the input or output of the pump but I fear that will not be good for it.

Just how much flow do I need through the heat exchanger (Polar)?

Can't correct the title;- exuberant
 
Last edited:
As you have a heat exchanger and (presumably) thermostat you are unlikely to overcool the engine, watch the gauge anyway: my 6 cyl ford will stay at temp even if the Jabsco flow is down to a quarter or less, so oversupply seems normal. You could restrict the inlet, just for convenenience, but it won't make any difference to wear. Only problem would be if you have enough pressure to blow a hose off, and then you will be introducing water to the bilge at a great rate.
 
I take it this is the right pump for the engine? Jabsco do make engine-driven bilge, washdown, and, yes, fire-fighting pumps.

My engine's half the size of yours, but the pump flow rate is extremely gentle.

Pete
 
What size is yor hose and intake? Typically a 3/4" hose will cope with a flow of 50l min, and a 1" hose 90l min. That is a lot of water to come out of a 50mm exhaust hose! You need to check the capacity of the pump against the requirements of the engine.
 
Graham_Wright said:
I am told this could cause cavitation which presumably would cause wear.

Restricting the outlet could be exciting!
Click to expand...

Inlet throttling can cause cavitation, although I'm not sure how serious this would be in a rubber impeller pump. The brass body will have very poor resistance to it so you are probably best not to risk it.

Restricting the outlet puts a high load on the lip seals in the pump, resulting in rapid shaft wear. Done to excess it will probably damage the impeller also.

What is the size of the pump? Bukh 20 engines use a 1/2 inch one that I suspect is a little on the small size. My Yanmar 3GM uses a 3/4 inch one, which supplies a more than adequate flow. The 2.2 litre BMC engine in my motor sailer has a 1 inch one but its drive pulley is only around half the diameter of the motor pulley.
 
You need enough water to cool the engine. but not too much. Although the thermostat should regulate the cooling effect, y9ou are pumping a lot of water into your exhaust. This may cause undesireable back pressure and affect the engine's performance. Worse, it may fill the exhaust so fast that you flood the engine if it does not start immediately. Many people warn against cranking the engine for more than a short period without it starting. With a larger pump this "short period" should be even shorter.
 
david_bagshaw said:
jabsco pumps are usually controlled by changing the cam, along with the pulley size, an not by constriction.
Click to expand...

What's a Jabsco cam? Sorry to be a thickie but I can't relate the word to the pump I have. It has an impeller which rotates in a cavity whose centre is offset from the impeller shaft.

Xylem's Jabsco Shop recommends 15 gallons per minute per 100 bhp.

Doing the sums, my 1.5 litre/36 bhp engine should need around 6 gallons per minute. A test last night at a fast tickover speed (no rev counter yet) revealed around 3 gallons per minute. This seems about right but subjectively the dollops of water ejected seem a bit over the top.

Next problem will be the heat in the engine space;- I was dripping!
 
The jabsco type pump has a flexible impeller running inside a round casing.the cam plate is an internal ridge which depresses the vanes on the impeller causing a vacuum that sucks the fluid into the pump. The thicker the cam the bigger the suction or lift.
The cam is usually held on with a screw from the outside of the casing and is simple to change.
 
scottie said:
The jabsco type pump has a flexible impeller running inside a round casing.the cam plate is an internal ridge which depresses the vanes on the impeller causing a vacuum that sucks the fluid into the pump. The thicker the cam the bigger the suction or lift.
The cam is usually held on with a screw from the outside of the casing and is simple to change.
Click to expand...

Thanks. I didn't investigate the construction when I stripped it to establish the direction of pumping. That seems an easy way of "tuning" the performance.
 
Graham_Wright said:
I am told this could cause cavitation which presumably would cause wear.

!
Click to expand...

Ah, yes, my jabscos are both below the waterline, so maybe not a problem for me. For my engine temp to rise the flow has to be just a dribble down the hull rather than a squirt 2 feet out. Dry exhaust, you see, so I know instantly if the flow is reduced.
 
Jabsco pumps are normaly run at camshaft camshaft speed (1/2 engine revs), this could be why you have a high flow?
Dont choke the inlet, the pump wont like it..
 
I visited Seawork yesterday and spent some time talking to an engineer on the Beta Marine stand. I raised the topic of water lock back-up continuously cranking a non-firing engine.

His opinion was that there was sufficient exhaust from the engine, even though it was just uncombusted air and diesel to blow through the exhaust train and expel whatever was injected into exhaust elbow.

I liked their injection system which was annular enclosing the gases (maybe everyone does that but I hadn't seen it).
 
You must log in or register to reply here.

Share this page

Top