A question about fuel economy

With the cost of gasoline going up, fuel economy has become a regular topic in the e-mails that I receive. A recent question dealt with the addition of a hydrofoil fin on a reader’s outboard motor. He wrote that he recently bought a 1997 model 19-foot bay boat with a V-6 150-horsepower engine. The person he bought the rig from had a whale tale installed on the engine, not because the boat had problems getting out of the water, but because he needed the boat to stay on plane at a slower speed while he was scouting for redfish.

The new owner went on to state that he has tested the boat both with and without the plastic plate, and he is concerned that the motor seems to be burning much more fuel with the fin on than it does without it installed.

Hydrofoils can offer several desirable effects for your boat, but there are also negatives that are associated with these accessories. Boat-building technology has evolved that allows manufacturers to build lighter yet stronger boats than those of days gone by. At the same time, outboard engines have become larger and heavier. The resulting combination is a boat that is tail heavy and squats deep in the water. Because the boat drafts more water, it takes longer to plane off, and many times once it is on plane the boat porpoises at high speeds. Hydrofoils will counteract this weight balance problem. The boat will plane off quicker, and the problem is eliminated.

Whale tails increase the surface of the engine’s anti-cavitation plate. With this increased surface area, the engine’s power trim can exert more force to control the running attitude of the boat. As the operator trims the engine down, the boat will plane off faster. The bow of the boat will be forced down so that porpoising is eliminated, and the boat can remain on plane at a slower speed.

Unfortunately, there are a great many boats that are equipped with these plates on their engines that do not need them. The manufacturers of these products have done a great job advertising and marketing them. Many of these products are sold just because the boat owner has seen so many others in use that he feels he has to have one. Some are a result of misinformation that is spread through the grapevine.

It is your responsibility as a boat owner to properly evaluate your rig to determine if you truly need such a device. Before you decide to add one of these hydrofoils to your motor, make notes of exactly how your boat performs without the foil. How fast does your boat plane off? What is the lowest RPM and speed that your boat will remain on plane? Does your boat have any problems with porpoising, and finally what is your boat’s max RPM and speed? All of these tests should be performed with a normal load in your boat.

Armed with all of the above information, you can now install the hydrofoil of your choosing on your engine and go back out to the lake to make comparisons. If your boat does not need a foil, you will usually experience several negative effects from this accessory.

Hydrofoils hold the bow of the boat down. This creates more drag and requires more power from the engine to push the bow of the boat through the water instead of allowing it to float above the waves. The increased drag and the additional power requirements will cause your engine to burn more fuel and, in many cases, will also cause you to sacrifice some of your top-end speed.

Keep in mind that many of the problems that hydrofoils are designed to counteract can also be cured by simply redistributing gear and passengers in your boat. Most of these problems are simply a case of too much weight being located in the rear of the boat. By moving ice chests and passengers forward, you may be able to eliminate the need for a hydrofoil.

If you have any questions about your rig, you can e-mail me at theboatdr@yahoo.com.

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