D1-30 Seawaterpump - Impellers break quickly

[umu]

We are currently going down the Intracoastal Waterway in the US and put on a lot of engine hours over the last weeks.
After a complete failure of a fairly new impeller (only 45 engine hours) I replaced it and after another just 25 hours the new impeller already shows cracks on the ends of a few blades. Before I never had any visible signs of wear on the impellers even when replacing them after >200 hrs.

So far I did:
- checked the heat exchanger bundle while cleaning out the blades from the first failed impeller: clean
- checked the amount of seawater that gets pumped thru (found somewhere it should be 18 litres per minute at 1200 revs): ok
- checked seawater filter is clean and kept clean
- checked exhaust elbow which is only 1.5 years old and shows no signs of blockage
- checked cam in the pump for sharp edges due to wear: ok
The two impellers that broke are from separate batches (one from Johnson OEM, one from Volvo) so I guess a manufacturing fault can also be excluded.

My repertoire gets exhausted - any ideas what is going on or which direction to look into?

 

[Billjratt]

I suspect the incoming water is abrasive, possibly due to shallow depth? maybe a wrap of old tights round the intake filter mesh will highlight something.

 

[Cardinal]

If the IW is rather shallow, might your inlet be taking in small sharp-shelled crustaceans and/or sand which is cutting up your impellers?

 

[umu]

Thanks for your response!
While this might be a cause I do not think it is - we are going with a buddy boat that also has a Volvo with a saildrive, he has no issues whatsoever.
Nor did the shop where I bought a spare impeller and asked about ideas regarding the issue indicate this as a possible reason (which they would most likely know as tons of boats going thru).

I suspect the incoming water is abrasive, possibly due to shallow depth? maybe a wrap of old tights round the intake filter mesh will highlight something.