America's Definitive Marine Engine Magazine

marine engine digest logo

Testing the Performance of a Doel Fin Hydrofoil for Outboards and Sterndrives

Doel Fin top view

Doel Fin Improves Performance On Outboard Motors and Sterndrives. And Helps Sailboats With Outboard Motor Auxiliaries

Learn how attaching a Doel Fin hydrofoil to an outboard or sterndrive's anti-ventilation plate speeds time to plane, improves mileage, improves handling and increases top speed.

Some boats that are painfully slow to plane. But the good news is their lackluster performance can be perked up by the simple DIY installation of a hydrofoil. Hydrofoils work their magic thanks to airplane wing like lift. Lift elevates the stern, which in turn helps the boat climb over its bow wave and onto plane. As a result, time to plane is shortened, which pays big dividends. Visibility is improved during those critical few moments after hard acceleration. Mileage and handling improve as well. Besides stronger acceleration, hydrofoils also help a boat maintain lower speeds before falling off plane. Hydrofoils are appropriate for outboards and stern drives, including ski boats, sport boats, pontoon boats, and especially older inflatables with heavier four-stroke outboards. Installation is simple. All you‘ll need is a couple of hours, a socket set, a 3/8” electric drill and bit, and sometimes a tap to thread a ¼” hole. Begin by locating the lower unit’s anti ventilation plate, sometimes erroneously referred to as an anti cavitation plate. It’s the horizontal structure right above the propeller. Hydrofoil manufacturers include a paper template that shows exactly where to drill the four or five mounting holes. Mark them with s Sharpie. Then center punch the holes so the drill bit digs into the little dimple with the first turn of the bit. Otherwise the bit can wander, never getting a good bite, or worse, beginning the hole off center. If you let that happens the hydrofoil holes won’t line up with the ventilation plate holes. You’ll have to drill again. So do it tight the first time. For the same reason use a sharp drill bit. While the bit is spinning, streaming off little flakes of aluminum, periodically lubricate the bit with a drop or two of oil.

With the holes drilled, and detritus brushed off the surface, lay the hydrofoil in place. Make sure the holes line-up perfectly. The wing may attach by means of stainless steel bolts, washers and nuts. Or it may be necessary to tap threads into the holes. Either way, it’s a good idea to protect the bare aluminum bore against corrosion. Zinc chromate primer, or its equivalent, can be sprayed on. But you’ll have to mask off the lower unit so over spray doesn’t spoil the finish. You could coat the threads with anti size compound, but in time it will wash away.

An aeronautical engineer designed Doel-Fin to improve overall performance on boats powered by an outboard motor or a stern drive units. Advertised benefits of this hydrofoil include: Better stability, consistent planing at lower RPM, faster out-of-the hole performance, better gas mileage, steadier turns, higher top-end speeds and lower bow rise at all speeds.

So does Doel Fin deliver on its promises? To find out I tested Doel Fin on two different boats.The first one was an Avon 310 RIB rigged with a Mercury 9.9 horsepower, four-stroke outboard motor. The second boat was a 30-foot sailboat rigged with a Mercury Big Foot 9.9 four-stroke outboard motor.

Doel Fin: out of the box, two foils and four bolts

Let's begin at the beginning. Doel Fin is molded in high-strength plastic. Attachment is as simple as drilling four holes on the outboard motor or stern drive lower unit's anti-ventilation plate. Know that some amateur boaters and professionals alike improperly call it a cavitation plate. It is not. It is called an anti-ventilation plate because it prevents the propeller from sucking air down from the water's surface. With the prop pushing mostly air engine load decreases, rpm race and no thrust is generated. More on that later on in the story when we talk about sail motors (outboard motor auxiliary engines on a sailboat).

Meanwhile, back at the the 3/8 inch drill, with all four holes drilled, position the plates and bolt them down tight. Attachment does not violate the factory warranty protection.

Doel Fin hydrofoil improves time to plane, tracking and reduce bow rise.

As for test results, on the Avon 310 RIB I noted a number of immediate improvements. With two people on board it was sluggish to plane and there was an objectionable duration (time) of bow rise. With Doel Fin time-to-plane quickened and bow rise virtually disappeared. The Avon RIB rides on a semi-V hull so control was already good. With a motor that small (9.9 horsepower) it was hard to measure any increase in fuel economy. Though on a larger boat it may improve.

As for the sailboat: Outboard auxiliaries have limited abilities because much more than a ripple on the water hobby horses the hull form enough so that the propeller comes close enough to the surface to suck down air. The engines over-revs, the over-rev limiter cuts rpm, the prop submerges into undisturbed water and rpm climbs. The cycle continues over and over all day long. Installing Doel Fin did not entirely alleviate the problem, it did reduce the problem. Which is to say, the waves and hobby horsing could be a little more extreme before the over-rev limited went to work.

So what's the bottom line? Doel Fin takes minutes to install and pays big dividends on dinghy motors and sail motors. To my way of thinking it is a worthy investment.