Dual impellors; I need help!!!

[stan_deezy]

Hi guys!<br />It was my misfortune to buy a cheap Volvo Penta outboard and think I could restore it to its former glory......now I have come across a problem I have never seen and am stumped so I am throwing myself upon the mercy of the community in the hope that someone will have some answers.<br /><br />The engine was sold to me because "it wasn't pumping water". I stripped it down and discovered that the pipework wasn't connected up at all; so assumed this was the reason for non-pumping...wrong!! Here's a list of questions; if anyone can answer one or all I'll be very grateful (and very impressed!)<br /><br />1. This engine has two impellors in the water pump assembly (an arrangement I have never seen before). They are seperated by a metal plate and two gaskets. Why two impellors?<br /><br />2. Why does exhaust gas flow from the telltale (the hole under the clyinder block). I assume its exhaust gas; if I block the hole with my finger the underwater exhaust blows out air at a rapid rate of knots.<br /><br />3. With two impellors in a pump should they both be running in the same direction?<br /><br />4. Is there a rule of thumb as to which way the impellors should be running (no workshop manual available for this engine).<br /><br />5. Can I simply take the old impellors along to my local chandelry and get similar items or am I risking the wrath of the great lord Neptune?<br /><br />6. Can anyone tell me why there are two impellors and what logic is involved in this arrangement?<br /><br />7. Because the engine is a first time starter and runs sweet as a nut I am seriously considering and alternative cooling system if no-one can answer the above questions; it would go like this; electric pump mounted inside the hood with a pick-up pipe spliced into the old pump feed; has anyone else ever tried this and does anyone have any views on the pro's and cons of this system.<br /><br />Told you I had loads of questions!!<br /><br />I have got torn and tattered fingers from this engine but I feel she's too sweet an old lady to be put to rest so any help with this project would be much appreciated,<br /><br />best regards<br /><br />Bro

 

[Scaaty]

Re: Dual impellors; I need help!!!

As I don't know much about this engine (zip really), but something about pumps, its seems to me this is similar to a oil pump on a Harley. The two gears (impellers) turn opposite each other, water flowing into it from one side, the gear teeth carrying it around and pushing it out the other side when the teeth meet again and squeeze out the water. These need to have close tolerance, and that would require good gaskets and flat surfaces. And the exhaust sounds like its not a problem. Anybody else know much about this motor set up to help this person?

 

[stan_deezy]

Re: Dual impellors; I need help!!!

Robby,<br /><br />you may have helped me already!! Contra-rotating impellors would make perfect sense and would explain why the gimbal I bought the engine from couldn't get it to pump water; he had them both in the same way; and I have copied him, which makes me a bigger gimbal! Of course they would be contra-rotating and as you say the tolerances are very fine; the gaskets are new (I had to cut my own but it may be I used too fine a thickness of gasket material?) There is a polished metal plate dividing the two halves of the pump. My next question is the bobby-dazzler though: if the shaft is rotating clockwise as viewed from above my guess is that the lower impellor would be rotating clockwise and the upper impellor installed to rotate anti clockwise?<br />I must sound like a right muppet but this has even foxed my local outboard repair shop who have never seen a pump like this (although they have confirmed the bleeding obvious which is that the two impellors are in reasonably good condition!)<br />Thanks for taking the time to reply to my post and possibly pointing me in the right direction; I'll let you know how I get on..........

 

[stan_deezy]

Re: Dual impellors; I need help!!!

Ah, wait a minute......I have just re-read your reply Robby and have spotted the flaw in this; I wish there was a way to add photos or diagrams to this site with the posts......<br /><br />Imagine an normal water pump; sitting in its casing with an impellor: now imagine that for some strange reason the designer has fitted, instead of an outlet pipe connection, a thin metal plate and then above that plate has fitted another impellor which then has a housing atop it which connects to the pipe that feeds water to the powerhead.<br /><br />See my problem now? <br /><br />One rather large water pump casing with two impellors, one on top of the other, separated by a metal plate with a single hole in it....<br /><br />I still think you may be close to the answer with the idea of contra-rotation; if only I could figure out how to reinstall the system.<br /><br />Hmmmm, I wonder if I could do away with one one impellor and the lower half of the pump? <br /><br />I know I could fabricate a new pipe that was long enough to make up the missing distance, but why did Mr Volvo Penta designer put two impellors, one atop the other in the first place? Was there a good reason that I am missing?

 

[stan_deezy]

Re: Dual impellors; I need help!!!

Apologies to all I've just found the answer to myvarious problems by narrowing down my websearches.<br /><br />Now this makes you wonder why more engine designers didn't think of this; and having just rushed into the workshop and checked I find that not only does my engine pump water but it also sucks it up too from the bilges!!<br /><br />The following is acknowledged as being the Copyright of Bosunsmate.co.uk, who is eternally in my gratitude and for whom I may bear babies some day, as he has solved hours of head scratching!!<br /><br />"200cc twin cylinder 2 stroke. Water cooled, this motor was not much loved. Two unusual features could have made this a great motor, if they'd worked properly; a bilge pump fitted as standard, and twist-grip gear control. However, the water pumps were very weak - the bilge pump was a mirror image of the cooling pump, mounted back to back on the drive shaft, and suffered from corrosion and flimsy impellers. The bilge pump was utilised by pushing a hose with a strainer on the other end onto a pipe sticking out of the port side of the leg, just under the powerhead. The gear control was a good idea, but if you wanted to warm the motor up in neutral, you had to reach back and operate a lever sticking out of the cam box, just below the lower pan on the port side. Most people dispensed with the twist grip gear change by removing the slider from the cam box, and using the lever on the starboard side. The motor had chrome liners, and tended to suffer from bad storage. Intake controlled with reed valves."

 

[Scaaty]

Re: Dual impellors; I need help!!!

I'm glad you figured that one out! Seems kinda weird, but engineers always have been weird! Good luck with it now!

 

[fire831]

Re: Dual impellors; I need help!!!

My 1968 McCulloch 7 1/2 hp has two impellers also. 1- water pump, 1 - bailer.

 

[Dick S]

Re: Dual impellors; I need help!!!

Hi Bro scott atwater had that feature for a <br />lot of years then MCculloch bought them out<br />an used that on some of ther scot Mac motors<br />there where a lot of scott mac motors assembeld<br />in the UK. Are you sure yours is a volvo penta?

 

[stan_deezy]

Re: Dual impellors; I need help!!!

D ick, (swear filter blocked your name from my original reply, sorry mate)<br /><br />thanks for that; perhaps you are right and I might chase that lead too; might be handy for spare parts.<br />The engine is definitely marked Volvo Penta on the hood and the engine number ties in with a website on Archimede's Penta engines (also known as Crescents, Monarks and every combination of the above mentioned names!)<br />The great news is that she is now running like a sewing machine (not much good: I wanted to use her as an outboard engine LOL!!)<br />Seriuosly though, I removed the damaged water pump impellor and replaced it with the near new bilge pump impellor and made sure (unlike the previous "techy") that the drive pin in the shaft was actually in contact with the impellor. It is clear from the scoring on the central impellor bush that the guy had moved the driveshaft on reassembly and had dislodged the pin causing it to sit underneath the impellor (hence a very tight drive shaft to turn!!) <br />The bottom line is that an engine that was originally thought to be scrap has been brought back to life for a few pennies and seems to be a wee honey!<br />If only everything in life was as simple.......