Propeller Pragmatism

Jul 13, 2018 | Technical

A couple of months ago I mentioned that some of our members had replaced their engines with new diesels. Not only were the engines changed but also the shafts and props. Whereas with engines “big seems to be best” the perfect prop for a yacht is inevitably a compromise between optimum motoring performance verses optimum sailing performance. The best prop for motoring has the most drag when sailing. The best prop for sailing is the least effective for motoring. There is no perfect prop.

Two bladed versus three bladed props

Two bladed props are often used on sailing yachts to reduce drag while under sail. There is the advantage that the prop can be hidden behind the deadwood (the end of the keel where the prop shaft comes out) to further reduce drag while under sail. However while the right two bladed prop will power a sailing yacht to hull speed in calm water it will fall short in the thrust department if you are forced to power directly into brisk winds and lively seas. Two bladed props tend to vibrate more when motoring than a three bladed prop.

Three bladed props have more effective thrust for the same horse power and there is a view that the effect of drag can be minimised by allowing the prop to free wheel instead of leaving it in gear while sailing. However some gear box manufacturers do not advise leaving the prop to free wheel as this can cause mechanical problems within the box. A contrary view is that leaving the prop to free wheel can induce more drag and turbulence over the rudder blade. Either way the log will be the best judge of whether leaving the prop to free wheel has more or less drag.

Fixed versus folding and feathering props

One way of overcoming drag is to fit a folding or feathering prop. These props have blades that fold away or change orientation when sailing which reduces drag. The result can be up to another ½ knot of boat speed when sailing.

I haven’t seen any folding props fitted to an H28 – I guess because to allow the prop to fold would require cutting a lot of wood out of the rudder. Folding props sometimes have problems with the blades not opening properly and don’t have as good a thrust for reversing.

Feathering props can be easily fitted to an H28. As the blades change orientation when in reverse – the leading edge of the prop turns toward the direction the water is coming from. Hence these props have better reversing thrust than a fixed or folding prop.

Folding and feathering props are mechanically more complex than a fixed prop. There are usually gears and/or shafts that require regular maintenance and there is a risk that the blades may jam in the wrong position due to a bit of nylon, marine growth or corrosion jamming up the works.

Diameter and Prop Performance

The measure of prop diameter is the width, in inches, of the circle made by the blade tips. (Props still appear to be measured in imperial measurements)

In general for a displacement vessel like an H28 a large-diameter slow turning prop is the most effective way of converting engine horsepower into forward movement. These props are particularly effective in low-speed manoeuvring and in pushing heavy loads. However a large diameter 20-inch prop has more drag when sailing than a smaller faster turning prop. A smaller (14-inch) faster rotating prop is more often fitted to an H28.


Diameter is only one part of the prop equation.

Pitch is the term for the distance a propeller would move forward in one revolution if there were no slip.

Slippage or slip is the term for the difference between the distance that the propeller actually advances passing through water, compared to the distance a similarly pitched screw would advance passing through a solid medium. If it were screwing through wood, an 11-inch-pitch prop would travel forward 11 inches in one revolution due to the angle of the blades. In water, because of slippage, it might only go 8 inches due to the slip in the liquid medium. A slip of 20 to 30 percent is common for an H28.

Slip is needed to produce thrust. A propeller with no slip will generate no thrust.

As the vessel travels faster the less will be the slip.

The objective for any propeller installation is thus not to optimise slip but to optimise efficiency where efficiency is measured as the shaft thrust x forward speed (which equates to the power from the shaft) / power from the engine.

A very different and difficult equation in which slip will obviously be an important variable.

Props often have the diameter and pitch marked on them on the hub. A 14 x 11 prop has a 14-inch diameter and 11 inches of pitch.

The sailing prop

An alternative for reducing drag is to fit a sailing prop. These props have some of the blade area removed in a way that doesn’t affect the effective thrust from the prop too much. There are two and three bladed sailing props.

The effect of prop size on engine RPM

If the prop has too much pitch the engine will labour and not be able reach full speed. Hence the engine won’t be able to deliver the full horsepower when needed. An indication of an engine being over loaded is black smoke coming out the exhaust. This is unburned fuel and can result in damage to the exhaust valve(s).

If the prop is under pitched the boat won’t reach full speed.

When you really need the engine chances are the wind is on the nose and the hull is dirty. Under these conditions you want all the horsepower you can get to get you where you want as fast as possible. This means being able to get the engine going at full revs without black smoke.

The pitch of the prop can be changed by taking the prop into a “prop shop”. A rule of thumb is a change of 1 inch of pitch will result in a change of 200 rpm. So if your engine is not reaching full revs then its possible to take an inch of pitch off the prop. This will cost around $60 plus haul out fees.

Keeping it clean

No matter how good a prop is the efficiency of the prop reduces with fouling. All sorts of attempts have been made to reduce fouling including polishing, painting, coating with grease, and coating with lanolin. However as soon as the prop spins the coating starts to come off.

But wait all is not lost! Our technical co-ordinator Kerry Blaymires and Monty Python Skipper Richie Williams have tried coating their props with Teflon and it appears to work. It costs around $200 and the marine growth doesn’t stick as badly. However watch out for the travel lift operator with the big scraper as he may try to get all that black stuff off for you.

Typical prop sizes fitted to an H28.

An 18~20 hp engine installed on an H28 will typically be fitted with a 14 by 9 inch prop. The pitch and diameter will differ from engine to engine depending on the gearbox ratio and the maximum rated RPM. A 20-hp engine should be able to comfortable push along at six knots.

A 10~12 hp engine would typically have a 13 x 7 inch prop and should be able to reach 5 knots in moderate conditions. Again the pitch and diameter will differ from engine to engine depending on the gearbox ratio and the maximum rated RPM.

In my case I have a Yanmar YSM diesel rated at 12hp. Unlike most engines that use a left hand prop mine is the other way around. Some of the 12 horses in my engine are a bit sick so I have the prop pitched to 6 inches. I can comfortably reach 5 knots in moderate conditions.