Tips, etc. Home





Keeping water out of the engine

Water-traps and syphon-breakers are all very well, but the way to keep water out of your engine when on passage in open water is to shut it out with valves. Most serious cruisers we know have had water enter their engines from the exhaust line, into the exhaust manifold and thence to one or more cylinders. This happens when the boat pitches excessively and the water already in the exhaust line and water-trap is given a helping hand by gravity and momentum to invade the engine.

Placing a ball valve in the exhaust line immediately below the injection bend solves this problem - if it is closed as a normal part of shut-down at sea. A drain in the lowest part of the exhaust line completes the protection by allowing the complete removal of water from the line prior to start-up. If you also shut off a valve on the raw water inlet, the engine should be completely protected from water ingress - unless a gasket fails somewhere.

If you do get water in one or more cylinders, it is a great help to have decompressors on your engine. With the cylinders decompressed, any water can be blown out of the cylinders. Decompressors are also useful for starting an engine when the battery is low. Unfortunately, in their wisdom, the vast majority of engine manufacturers have done away with decompressors - another great advance in technology.



We have repeatedly heard owners complain, "I rev my engine right up and don't seem to get any speed or power from it - and when I go into reverse nothing happens." This is particularly true of boats with relatively new or recent engine installations. We have become convinced that marine engineers routinely err on the side of under-propping boats. Their primary concern seems to be to ensure that the engine can reach its theoretical peak revs and therefore run at optimum temperature. Unfortunately, whatever the theory, this results in poor performance at the lower revs at which most cruisers routinely run their engines in order to avoid excessive fuel consumption.

We think that for long distance cruisers it is preferable to run a propeller that is slightly over size and/or over pitched. This can't be taken to extremes or the engine will over-heat and there will be excessive strains on the gear train. However, a little over-propping gives good cruising speed at lower revs with lower fuel consumption, as well as some real bite in reverse.

In a world where the number of marine hardware manufacturers has been whittled down to only two or three major firms, there is an amazing variety of propellers available and they are fully represented among cruisers, with various degrees of satisfaction. One of the most popular of these is the Maxprop. This is an expensive, very sophisticated feathering unit. The blades are very thin and thus have very low drag. Because it feathers, it also gives good performance in both forward and reverse. From our observations its one major disadvantage is that it appears to suffer routinely from severe electrolytic corrosion. Whatever the type of bronze used it is clearly very easily affected compared to most other propellers.

Our own propeller is a Flex-o-Fold, two-bladed, geared folder (17x13). We have been extremely happy with it. 'Sunstone' displaces about 11 tons in cruising trim. With this prop turned by our 27 HP Yanmar 3GM30F diesel we can achieve 7 knots in smooth water and no wind at max revs. At about two/thirds max revs (2100) we achieve about 5.3 knots, using 1.5 litres/hour.



Next Tips, etc.