Raw Water System - Does this make sense

[Navi2016]

For some reason this doesn't make sense to me ?

I am in the process of getting it all running for the first time ( first time for me ), and when I was tracing the hoses this system shown in sketch is what I ended up with .... seems funny?

There is a"T" on the pressure side of the water pump">raw water pump hose that has a 1/2in hose - hard pipe - 1/2in hose connection to the top of the stuffing box. My stuffing box is approx 6ft from the cutless bearing and the prop shaft is 2 1/4in. With that distance do I really need a water connection ?

I guess the part that really got me wondering was that it was plumbed on the pressure side of the raw water pump

Comments ?

Thanks Navi

[Alan Mighty]

1. The T on the pressure side of the raw water pump leading to your 'stuffing box'

Depending on what your 'stuffing box' is, feeding raw water into it is unexceptional.

For example, packless/dripless shaft seals often come with such a set-up.

Two rationales are given:

a. feeding water under pressure into the shaft seal reduces abrasion (and is especially recommended for prop shafts that turn quickly and for boats that motor at a fast water speed that may not be conducive to adequate water entering the shaft tube from astern); and

b. such a T fitting allows the shaft seal (and the cutless bearing) to be flushed with freshwater (if a freshwater flush T is also established). See more at #3.


Hosing connecting the T to the shaft seal/ has to be rated for underwater use, meaning reinforced hose. I was unclear about your mention of hard pipe.

See the URL in #2 below.

2. The location of your vented loop

I am surprised about the location of your vented loop! Seems wrong to me.

Depending on the relative vertical location - with respect to sea level - of your engine and so on, I suspect you need a vented loop between the T and your shaft seal/stuffing box to avoid any chance of siphoning water into your engine from the shaft seal/stuffing box.

See for example the diagram shown by PYI on its webpage for the PYI T-kit to accompany a PYI PSS:

https://store.pyiinc.com/collections...products/t-kit

3. The lack of a facility for a freshwater flush

You may want to consider adding a T to allow flushing of your engine's raw water system with freshwater.

Flushing with freshwater (or with antifreeze, should you be in a location with air temp likely to fall below the freezing point of water) is one way to minimise the buildup of lime deposits in the engine's heat exchanger (and in some engines it reduces the speed of corrosion in the exhaust mixing elbow).

If you have such a freshwater flush T and you also have a T feeding water into the shaft seal/stuffing box, then flushing with freshwater before engine shutdown at dock has the additional benefit of putting freshwater into the shaft tube, meaning that the freshwater is around theshaft seal and shaft bearing/cutless bearing. If the water around the shaft is seawater, deposits of lime in the shaft seal and shaft bearing are common. Such deposits of lime will be brushed off the shaft when next it turns. And that brushing off is, in the case of the shaft bearing, done by the rubber of the cutless bearing. And the result is of course abrasion of the cutless bearing. If you can conveniently feed freshwater into the engine (and shaft seal/stuffing box + cutless bearing) just before shutdown, the formation of such lime deposits on the prop shaft are limited. And the life of your shaft seal and shaft bearing will be accordingly extended.