A "tap" on the inlet of the raw water pump is a common "upgrade". Due to the water I boat in and the type of boating I do, I went one step further and converted my system to a thru hull inlet with an external screened scoop.
Wether the inlet is thru hull or thru the drive, the ability to have a feed into the raw water system has a number of benefits.
1) ability to run on land without "muffs"
2) ability to flush your system with fresh water on a regular basis if used in salt water
3) Quick winterization ( if "fresh" water cooled,i.e., if you have a heat exhanger)
Regardless of the inlet location or raw vs "fresh" water cooled, the ability to flush your engine everytime you use it will greatly lenghten its life. When I bought my current boat in 2005 ( a 1969 model) a raw water cooled engine used exclusively every year in salt water, it still had the original engine in it. While it no longer had its original elbows and manifolds, the block was still it good shape. The original owner had installed a pair of bronze valves and a "T" fitting so that he could shut off the flow from the drive and connect a garden hose for fresh water.
I should point out, that the safest way to "inject" fresh (tap) water into the cooling system is by use of a 5 gal pail and a short lenght of hose. To use this system, stick another garden hose ( hooked up to a source of fresh water) into the 5 gal pail and let the hose run free. Stick the short hose from the "T" fitting into the bucket. Turn water on. When 5 gal pail almost full, start engine and let the engine suck water from the pail as it wants it. Hooking a hose from a faucet directly to the inlet of the water pump has some risk (to the pump).
The setup goes like this... starting from the water line from the drive.
bronze hose barb.... that will thread directly into a....
1/4 turn bronze and stainless ball valve (these are not cheap!)
bronze short nipple from valve to...
bronze "T"
bronze hose barb threaded into "T" to hose to the inlet of the raw water pump.
bronze hose bib. ( to connect hose for flushing) on the third leg of the "T"
Bronze hose bibs ( i.e., that which a garden hose attaches to) can be hard to source.
I use brass ones and inspect them annually, and replace every other year, regardless. Iusually source these from my local hardware store and use the "boiler drain" types as these seem to be made of better materials. Once got 4 seasons out of one and it was still in good shape, i.e., no "red" color on the inner surfaces.
The "best location" for the tap varies from boat to boat, just make sure that it is in an easy to get to spot both for hook up and for visual inspection.
To flush engine... shut off the 1/4 turn valve, attach short hose to bib on "T", put other end in pail. Open the bib (garden hose) valve. Run water into pail..... etc.
To use boat.... disconnect hose, open 1/4 turn valve, see sea water come out of hose bib, and shut off hose bib valve.
Most parts available at local marine supply store. Make sure you preserve the hose size with the fittings, for example, my 5.7L specifies 1 1/4" dia hose (ID) so, all fittings are for 1 1/4". Use double 100% stainless hose clamps on all hose to barb fittings below the water line of the boat, i.e., all.
There are some products out there that are designed to make it easier to do this, i.e., fresh water flush your engine, however, I've avoided them as they all rely on some sort of automatic check valve, that I believe is an unacceptable risk... also some are made of plastic, and for me plastic below the water line is a no-no.
You need to be careful with the fresh water flush, as most garden hoses will not flow water fast enough to supply water to the system at much above idle. Keep a sharp eye on the water level in the 5 gal pail while doing this and either control engine speed or shut engine off BEFORE the pail runs dry.
Been using this system on all of my boats since 1983... it works.
