Yanmar Genny motor

[BobFord]

Information please- I am keen to make up an electric pump as a standby, in the event of failure of the water pump">Raw Water Pump on the Yanmar 3TNV-70H ,engine, that drives a generator. Existing pump is a Jabsco MC 97 impeller and the engine runs at about 3500 r.p.m. I need to know the required flow, output , of the pump. By the way , anyone that has issues with 'chewed ', impellers ? I think that I have a cause , not mentioned in the manual. Look at the cam in the pump body. The leading edge may be sharp , due to wear. Cam can be removed , eased with fine Emery and even reversed. Time will tell if i'm on the right track !!

[skipperpete]

How far above the waterline is the raw water pump on your genset?
The problem with genset raw water pumps is that the engine hits 1500 or 1800 rpm straight out of the gate and if there’s any drainback at shutdown, the pump always starts dry. Its a really familiar problem on motor cruisers around the 70-100 foot length, to gain accom space the generators are often high up in the engine room with as much as 1.5- 2 metres between pump inlet and waterline. The problem is also compounded by the below water discharge of the seawater and a dry exhaust outlet so no visual flow warning. “Globe” polyurethane impellers provide a partial fix, they can run dry for long enough to pick up the prime.

[BobFord]

Cheers Pete , Interesting observations. I note that r/water pump is 350mm above w.l. and wet exhaust is at w.l. Sea chest has a clear cover and is 900mm above w.l. When impeller has failed ,it was clear that no head of water was seen. The sensor located at exhaust mixer, will cut engine within 15/20 secs.

I assume that there is normally a 'trap', of water in the feed hose and system from the sea chest to exhaust mixer. System is now 2700 hrs old and has never suffered from 'destroying impellers. Now (3) in short time.

Testing my theory re the "sharp" edge on the pump cam ,at the moment.

Do you have any idea of the normal pump output rate ? I do see an electric pump as a viable option.

[skipperpete]

With 2,700 hrs of normal impeller usage, I’m in agreement about the sharp edges theory.
Are the impeller failures a slow loss of ability to pump and then a major collapse when suction is lost and the pump runs dry or just a few blades worn or broken? The raw water pump delivery is dictated by the speed at which the pump is driven but I don’t often see a 3TNV spinning a 2 pole generator, they usually are a lot slower than 3,500 (1500-1800) with a slightly larger pump in consideration of the lower pump speed.
You could get a bucket and do volume measurement at the exhaust outlet with a good impeller in the pump then find an electric pump with the same delivery. I’m sure that others on the forum will be able to provide more info on the modification to electric based on their personal experience in this area.

[BobFord]

The pump has not been a progressive loss. Catastrophic is a good description . This is the amazing part of the event , Multiple blade loss.Running use say 250hrs, next 150 , next 50 last 5 !! The edge of the cam did have a very small 'nick' on one corner. I guess that it was so thin that the blade caused the apparent loss of material. The resultant 'sharp' corner , then caught each blade.

[jtsailjt]

An advantage of having an electric pump onboard is that, if sized appropriately, it could be used for either your genset or engine. I have a spare Whale Gulper onboard that I used last year to “vacuum” out 2 of my water tanks that had developed a thin flexible skin that was floating around inside my tanks after I scrubbed them with a long handled brush. I don’t know if that pumps capacity would be adequate but if my raw water impeller repeatedly failed like yours are and I couldn’t get to a location where I could buy a new one, I’d give it a try. I’m partial to using diaphragm pumps whenever possible because, though they usually aren’t real high capacity for their size, they are very reliable and forgiving of both running dry and particles in the water that might clog or destroy other types.

But, I’d like to have a nickel for all the raw water impellers I’ve either inspected or changed out that had nothing wrong with them. Your genset went 2700 hours before it developed a taste for eating impellers. I’m inclined to think your filing off a sharp edge, or if necessary replacing the pump, is very likely to solve your problem and you’ll return to the days of dutifully inspecting your impeller once a year and find that it looks just like it did last year when you inspected it.