Post Engine Envy - Propeller Pragmatism Part 1 of 2

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.

Part 2