Nanni 3.75HE needs to be primed with raw water after haul out

[workmaster2n]

I've just purchased a new to me catamaran that has 2 Nanni 3.75HE in her. When we put her back in the water after slipping for survey, the owner mentioned we would need to prime the raw water. We did that by sticking a hose in the hole that feeds raw water from the strainer to the impeller, and she started sucking up water right away.

This isn't a huge issue, because I'm only going to slip once every year or two, but I can't for the life of me figure out why an engine would need to be primed like this . The impeller/water pump is very near or below the waterline. The strainer might be a 20-30 CM above the water line, but likely much less than that. There are saildrives (a first for me), so maybe something to do with that?
The anti-siphon setup is a metal tube that bends 180 degrees and has a perpendicular exit to the hull (like a tell tale on an outboard). The anti-siphon is between the raw water pump and the heat exchanger.

Any ideas why the engine can't "self prime" on the raw water side?

[fxykty]

We have Nanni 4.10HE engines in our cat and similar setup with raw water strainer bottom just below the waterline. When hauled out no water sits in the strainer (it drains out through the through hull) but there is water in the pipe between the strainer and the water pump. We do not prime as when the boat is in the water there is water in the strainer.

Your priming requirement might be because the strainer is above the waterline. You could just fill it and the inlet pipe, no need for an extra hose.

[skipperpete]

Nanni and Solè like to keep customers in the consumable spares loop and often they opted for raw water pumps that only took a hard to get metric impeller that’s mostly unavailable on the after market. It’s critical that the length of an impeller precisely fits the length of the pump body , if not, the water skips between the impeller blade and the pump end plate. When the pump is dried out, air takes the shortcut across the gaps and renders the pump non self priming but once the pump is filled with water it works ok…. Until the next haulout. The most common impeller to cause trouble is the 2” versus the 50 mm, the 50 mm impeller is .8mm shorter but same spline and about same diameter but the problem exists across most of the pumps we use from the smallest up to the largest ( on V16-92 Detroits ) you just need to measure the impeller with verniers and compare that length with the pump depth. Oring sealed pump end plates can also be capable of causing failure to self prime.

[deblen]

Sticking a hose into the raw water pump intake could possibly fore water into cylinders via exhaust valves-causing waterlock &/or internal damage.
Suggest you pour water into strainer instead.
Cheers/Len

[workmaster2n]

Originally Posted by skipperpete

Nanni and Solè like to keep customers in the consumable spares loop and often they opted for raw water pumps that only took a hard to get metric impeller that’s mostly unavailable on the after market.

The most common impeller to cause trouble is the 2” versus the 50 mm, the 50 mm impeller is .8mm shorter but same spline and about same diameter but the problem exists across most of the pumps we use from the smallest up to the largest ( on V16-92 Detroits ) you just need to measure the impeller with verniers and compare that length with the pump depth. Oring sealed pump end plates can also be capable of causing failure to self prime.

Interesting callout on the aftermarket impeller. I'll give it a look! Thanks for that.

[workmaster2n]

Originally Posted by deblen

Sticking a hose into the raw water pump intake could possibly fore water into cylinders via exhaust valves-causing waterlock &/or internal damage.
Suggest you pour water into strainer instead.
Cheers/Len

Excellent callout. Thanks for that