Propeller Blade Area
A few times every year a client will ring my office about putting a larger engine in their sailboat. It’s almost always for the same reasons. The boat doesn’t have any oomph. It won’t push into a head sea. Stopping and reversing during docking is sluggish. Embarrassing stories of minor dockside crashes are likely to follow.
Contrary to my clients’ intuition, the engine is seldom the culprit; it usually has more than enough horsepower. In fact, the perpetrator is the propeller and that all-too-often forgotten propeller measurement, blade area.
Let’s say our boat Third Wheel is a 44-foot cutter, displacing 8.6 tons on a 38ft 10in waterline. The thrust required to drive Third Wheel at speed is about 865 pounds. If Third Wheel’s propeller is a 19in-diameter, two-bladed sailing model, it will have a total blade area of about 72in2. Dividing total load by this blade area yields a blade loading of 12 psi.
To be effective, a propeller’s blade area has to be large enough to keep the blade loading at acceptable levels. Blade loading that’s too high leads to cavitation, loss of thrust and poor responsiveness in low-speed maneuvering. Allowable blade loading varies depending on boatspeed and propeller depth, so designers must calculate it for each installation. Under 6 psi is optimal for most average sailboats—assuming other considerations allow. Unfortunately, the ideal blade loading for Third Wheel is 5.8 psi or lower. At 12 psi, it is more than double this, so it’s no wonder the prop isn’t generating much oomph.
Of course, it’s not surprising that sailboats often have small propellers. Sailing along with a standard three- or four-blade powerboat prop is like dragging a bucket—not good for boatspeed. Still, it’s important to achieve the correct balance if you want to avoid problems under power.
Note that designers also look at blade area as percentage of the total area of a circle with the same diameter—the disk-area ratio (DAR). You can see in the drawings the huge difference in blade area between a four-blade standard powerboat prop, a three-blade powerboat prop, a three-blade sailor prop and the two-blade sailor prop on Third Wheel, all with the same 19in diameter. Indeed, the blade loading on the four-blade standard would be just 4.1 psi, even lower than needed, while the three-blade standard powerboat prop would have a blade loading of 5.4 psi—spot on. necessarily
What can we do in the real world to improve Third Wheel’s performance under power? The first thing would be to go to a three-blade sailor prop of the same diameter. This would increase blade area by 50 percent, provide a much higher DAR and reduce loading from 12 psi to 8 psi. The result would be a noticeable improvement.
Even better would be to go with a three-blade sailor and also increase the propeller diameter to 21 inches. This would increase the blade area by 83 percent, reducing blade loading to 6.5 psi. Twelve percent to 15 percent over-loading is a pretty good target for a sailboat. It’s not ideal, but would give reasonable performance under power while minimizing drag under sail.
Of course, this is where folding and feathering propellers come in. In the case of Third Wheel, a large-diameter three-blade folding or feathering prop will actually create less drag when not in use than the fixed two-blade propeller she came with. At last, the ideal sailboat propeller—good performance under power and sail!
Alas, there’s a caveat to the above: it’s not uncommon to find sailboats, even from top designers and builders, with their gear configured in such a way that it’s impossible to install the propeller you want. Either the larger blades won’t clear the hull or fit the aperture, or the shaft rpm is too high to handle a larger prop. Larger-diameter propellers require more torque, even when delivering the same power as a smaller propeller, so they must be slower-turning to prevent overloading the engine. You can’t just increase diameter if the reduction gear won’t generate enough torque at lower shaft rpms to swing the bigger wheel.
In instances like these, there may be no practical solution that doesn’t require changing the reduction gear, shaft diameter, stuffing box, strut and prop—the entire engine installation and shaft line.