sea water raw water colling won't pump after sailing fast

[njwilliams449]

We have a Bavaria 46 2006 with a Volvo D2 55. When it was slipped in May the shipwright installed a strainer on the through-hull we take sea water through (we don't use the saildrive inlet) which is solid for one half and slotted for the other - a common type I think. The water pump">raw water pump works fine unless we have been sailing fast. If sailing at around 8-9 knots, about a 50% chance it won't take up seawater, if sailing at 10-13 kts 100% likelihood it won't. Once primed with a litre or two of water into the black primary filter bowl, it works fine. We asked shipwright to turn strainer around so that solid side is now trailing edge rather than (as before) leading edge - in case it was producing a venturi effect) but this hasn't solved the problem. Once primed it works just fine, good flow-through of water, have checked primary raw water filter etc. Any ideas please?

[Suijin]

Remove it.

[njwilliams449]

We put one on because without one the intake was blocking frequently - one use in 4, roughly - from leaves etc in Sydney Harbour, or other material in the water. But thanks for the suggestion.

[Lepke]

Air leak, multiple bends in the plumbing, bad impeller, bad raw water pump shaft seal.
At least hundreds of boats use a strainer like you installed. I'd look for a plumbing install problem.

[DeepFrz]

https://marinehowto.com/external-strainers-omg/

You may want to look at these pictures.

[donradcliffe]

If it works after priming and there are no obvious air leaks, its probably the raw water pump. Take the plate cover off, check for missing impeller blades and grooves worn in the plate. You can replace the plate, or just turn it over for a quick fix.

[DumnMad]

Strainer helping the formation of turbulence and at speed air is getting into the intake?

[Ambler]

Try closing the intake valve, place the start key on a hook next to it if you need the reminder. See what happens then.

[Captsteve53]

When checking your raw water pump also check the internal cam plate has not been worn down as well as the front plate (mentioned above), also if not a new impeller then replace it !

Cheers Steve

[Captsteve53]

Forgot to say, what height is your internal filter, the top should be just above water line, even if wrong a normal rubber impeller pump will suck up water from elevated heights, Again check the impeller is not also slipping on it's own internal bronze bushing(seen that quite often)

Cheers Steve

[njwilliams449]

Thanks for the ideas, guys. I have replaced the raw water filter and seal and will do the impeller next, then work on through the list.

[taxwizz]

I have a similar problem on my Nonsuch 30 (although I have never managed to get it to 13 knots) with my A/C cooling water intake.
I installed a valve ahead of the water pump, below the water line on a "T" junction. I open it to allow water to flow out and fill the line, then close it. Not perfect solution, but it works.

[Terra Nova]

Provide photos of the sea strainer/plumbing installation.

[rwidman]

Let's think about this a minute:

It worked fine until the strainer was installed. After the strainer was installed, it didn't work right.

What could be the solution?

[sailorboy1]

It's turbulence going past the seawater inlet that is causing a void in the line. I bet that when the strainer gets primed it breaks the vacuum in the line.