Propping yacht outboards

As owners of smaller keelboats know, there are many benefits of using an outboard auxiliary instead of an inboard engine, not the least being initial purchase price. Outboards are also easier and cheaper to service, can be stowed ashore when you don't intend using the boat for some time and, because they can be tilted out of the water under sail, create no drag when not in use.

But the trouble with most outboards used on yachts is that they are incorrectly propped, simply because the dealers who sell them don't understand the loads imposed on these motors compared to those on an aluminium dinghy.

Apart from engines specifically intended for displacement hulls, such as the four-stroke Yamaha F9.9D, most engines up to 18hp have the wrong blade design for yacht auxiliaries. The props are also sometimes raked aft (notably with Tohatsu models from 6-18hp) to reduce propeller ventilation through high-speed turns and consequently lose a lot of thrust efficiency when operated at displacement speeds.

Many small-motor planing hull props are also semi-weedless, with blades that curve rearwards out to the tips to reduce the possibility of snagging weed in shallow water. But displacement-hull props for twin-cylinder motors should have the largest possible diameter that will fit under the anti-ventilation plate, plus a relatively fine pitch and broad blades, with no rake, to effectively convert engine torque to thrust.

An outboard's exhaust gases either exit from beneath the anti-ventilation plate or through the prop hub and when going astern these gases end up on the blade faces, significantly reducing thrust. However, due to their size, fat blades have more of a chance to spin in clean (non-aerated) water and the well-rounded tips operate almost as efficiently astern as they do ahead compared to planing-hull props.

The accompanying photo shows two OMC/Bombardier 'high thrust' props designed for displacement hulls. The larger is for the twin-cylinder Johnson 6/8hp and has a 9.25in diameter and 6.5in pitch, while the smaller is for the twin-cylinder Johnson 4 and measures 7.5 x 6.5in.

Because the extra-long shaft Johnson 8 has exhaust outlets sandwiched below its two anti-ventilation plates, when going astern the gases rarely reach the prop, which continues to spin in clean water. The 4 has only a clutch, but can be swung 180° for full power astern.

To get around the gas problem with through-hub props, both OMC/Bombardier and Yamaha have developed 'High Reverse Thrust' and 'Dual Thrust' props, which have two hubs, with the blades attached to the outer ring. When going astern, the gases exit the inner hub and divert 180° to pass between the rings, leaving the props to spin in clean water.

Mercury Marine gets around this problem by suggesting Mariner and Mercury owners drill a hole through the specially-marked indentations in the lower unit, directly above the anti-ventilation plate. When idling or going astern, the gases exit these holes, leaving the water reasonably clean for the prop blades. However, the high-thrust props for the four-stroke Bigfoot 9.9/15 have too coarse a pitch for yachts, so I recommend a four-bladed Solas alloy prop.

Honda is lagging behind in the development of high-thrust props and only its BF8 and BF10 models are available in 'Power Thrust' models, where the gases exit above the anti-ventilation plate. The four-bladed props fitted to these motors also reduce vibration compared to the normal three-bladers.

Tohatsu has no high-thrust props in its range, but Solas has developed four-bladed models specifically for the 9.9, 15 and 18hp models. Exhaust gases in these models still exit through the prop hub, so power astern won't be as effective as the direct OMC/Bombardier and Yamaha competition.

As single-cylinder two-stroke outboards tend to lose power very quickly under load compared to twin-cylinder motors, which develop significantly more torque per cubic centimetre, they should not be fitted with high-thrust props. In this case, the answer is a semi-weedless design that has greater 'slip' under load and so allows the motor to rev freely.

Alternatively, plastic props are better than alloy props for motors under 4hp because, under high loads, the blades actually 'flatten' and reduce pitch.

Of course, these semi-weedless props are virtually useless astern, but most single-cylinder outboards can be spun 180° for full power astern, if needed. The four-stroke Mariner/Mercury and Tohatsu 4-6hp range can only pivot 150°, so power astern is limited, especially when using the standard semi-weedless props.

Make sure your motor revs out to where it should. There's no substitute for ensuring the engine is operating within the recommended WOT (Wide Open Throttle) rev range. All motors - two and four-stroke - must reach this rev range to develop maximum power and prevent powerhead damage from pre-detonation in the combustion chambers due to overloading. And the only way to do this is to borrow a portable tachometer from your local outboard dealer and try it on your motor.

OMC/Bombardier makes a tacho known as a 'Shoptach' that attaches, via a sensor, to a high tension (spark plug) lead and measures revs from zero to 10,000rpm in increments of only 10. I've used one for the past 11 years and have found it to be an invaluable tool.

Other manufacturers make their own versions. Otherwise, I suggest buying a tacho and wiring it up to your motor's alternator (if fitted).

Once the yacht has been loaded with all normal equipment, including the maximum number of adults that can sleep aboard, take your re-propped outboard on a test run. If you have an adjustable-height transom bracket, use a setting that will position the motor's anti-ventilation plate about 8cm below the waterline at rest.

Otherwise, when underway the stern is likely to squat and the prop will be running too deep. Combined with the additional leg and lower unit drag, this will put unnecessary load on the motor.

At WOT, the motor should reach at least a couple of hundred revs above the bottom of the recommended WOT range and preferably in the middle to upper end. For example, if the manufacturer's recommended WOT range is 4500-5500rpm, the motor develops maximum power at 5000rpm and pushing the normal cruising load aboard the yacht the motor should reach at least 4700rpm and preferably 5200 at hull speed.

When throttled back to normal cruising revs at two-thirds throttle, the engine load will halve. This will ensure good fuel economy and a long engine lifespan, particularly with four-strokes, which deliver their best fuel efficiency below two-thirds throttle.

For more information on a suitable Solas prop for your motor, contact Steve Evans on (07) 5479 4898, or email: sales@solas.com.au.