By Bob Currie, Recreational Boating Safety Specialist
U. S. Coast Guard Auxiliary Base Galveston Flotilla
This column’s topic is something that is specific for boat operators with outboard motors. The subject is jack plates. A jack plate is a device that is used to raise and lower your outboard motor. No, I am not talking about the power tilt. An outboard motor without a jack plate attaches directly to the transom. If the outboard motor has power tilt, the engine can be tilted to raise or lower the propeller in the water. When a jack plate is used, the jack plate is attached to the transom and then the outboard motor is attached to the jack plate. The power tilt still works when a jack plate is used, but changes in engine height are much better for operations than changing the engine tilt.

Types of Jack Plates
There are three different types of jack plates: (1) fixed, (2), manual, and (3) hydraulic.

Fixed Jack Plate (L), Manual Jack Plate (M), Hydraulic Jack Plate (R)

Fixed jack plates, which are usually a pair of brackets that offset the engine from the transom, are the cheapest. They cannot be adjusted without removing the outboard engine. By offsetting the engine from the transom, you get it farther away from the turbulent water created by the boat moving through the water. The propeller can then bite better and is less likely to cavitate.

Manual jack plates are quite adjustable, but they cannot be adjusted while you are moving. Like tuning a fixed jack plate, the idea is to find the overall best height for the propeller and then lock that height in. Manual jack plates, like fixed jack plates, are usually a pair of brackets that are attached to the transom.

Hydraulic jack plates can be adjusted up or down at any time, including when the boat is underway. Hydraulic jack plates offer the best and most advantages over fixed and manual jack plates, but their downside is their extra cost. While fixed and manual jack plates may be had for costs in the hundreds of dollars, hydraulic jack plates run into the thousands of dollars.

Jack Plate Advantages
Jack plates offer several advantages and benefits. Here is the short list:

• Increased Efficiency: You can observe the efficiency in two ways: Your boat will go faster and your boat will get better gas mileage.
• Access to Shallow Water: raising your outboard motor will allow you to access shallow water that you couldn’t access without a jack plate. Sure, you can use your power tilt to effectively raise your propeller, but doing so causes you to lose stability and you certainly can’t get on plane with the engine tilted up as the propeller wants to direct the water upward rather than to the rear.
• Ease of Planing: with a jack plate you can raise your outboard motor and get on plane in shallow water much quicker than if you had your engine trimmed up with the power tilt.
• Stability in Rough Water and Tight Turns: While you generally raise your jack plate to navigate shallow water, when the water is rough you can increase your stability and your ability to make tight turns by lowering the jack plate so that the propeller doesn’t cavitate as easily because the propeller remains totally immersed in the water. Cavitation occurs when the propeller pulls in air. Cavitation not only causes loss of propulsion, but it can also damage an engine that requires water for engine cooling when air is sucked into the cooling water intake.

Recommended Gauges
Although they are not necessary, there are a couple of gauges that can greatly assist you in adjusting your hydraulic jack plate. The first gauge I recommend is the water pressure gauge. If your engine is too high it could suck in air into the water intake, and that is very bad for the engine. The water intake provides cooling water for your outboard engine. Air does not work and can cause your motor to overheat and burn up. You want to see a normal water pressure all the time. If the water pressure drops, you probably have your engine raised too high. The second gauge is the jack plate height gauge. If you don’t have one, you must look back at your engine to see how high it is. This could lead to an accident due to being distracted. If you don’t have a jack plate height gauge, it can be helpful to make some inch marks on the jack plate that can be read from the helm.

Starting in Normal Water Depth
Finding the best hydraulic jack plate setting for starts requires some experimentation as well as an understanding of a couple of terms and how they go together. The two terms are slip and grip, and they refer to how the propeller acts in the water. Normally there is some slippage when applying full power to the engine. Slippage is important to a point, but too much slippage is detrimental. Grip refers to how well the propeller grips the water and is a function of propeller shape and pitch. Too much grip bogs the engine down, while too much slip lets the engine rev freely without biting the water. The key is to find the engine height that gives the smoothest and quickest start. You can find a general setting for your average load, but in reality, that optimum slip vs. grip setting changes when the weight in the boat (as well as its distribution) changes.

To find your optimum engine height, start with the engine tilt all the way down and the jack plate all the way down. If the engine bogs down and/or no rooster tail is generated, then the engine is too low in the water. Raise the jack plate by two inches or so and try again. You are wanting to see the engine rpm increase without being bogged down. If the engine bogged down with the jack plate all the way down, then you should see some improvement when you raise the engine in the form of a more rapid increase in engine rpm (indicating some slippage) as well as a rooster tail being generated. Keep inching the jack plate upwards until you get to a point of excessive slip and not enough grip, and then lower the jack plate until you find the sweet spot (some slip and a lot of grip). Make sure the level you end up with is one that shows good engine water pressure.

Normal Turns
Once you have found a jack plate level that affords you good quick starts, you need to make some normal turns at speed to ensure that the engine is not too high for the propeller to remain in the water column. If slip increases or water pressure drops when you turn, then the engine is too high. You may need to lower the jack plate in order to keep the propeller deep enough in the water to grip during a turn. The idea is to find an engine height that is balanced so that you maintain good maneuverability. While you may easily find an engine height that is best for straight line running, that position may be too high for making safe turns. It is better to sacrifice efficiency for safety rather than safety for efficiency. If you insist upon running at peak efficiency, then be sure to lower the engine and/or reduce speed before making a turn. If you do not lower the engine you run the risk of spinning out and losing control of the boat. You could end up in an end-swapping situation. End-swapping often results in passenger ejection. Ejected passengers are in danger of being run over by their own boat.

Quick or Sharp Turns
Quick turns should not be made too sharply, and sharp turns should not be made too quickly. If either of the above does happen, there is a high probability the boat operator will lose control of the boat, especially if the jack plate is raised for maximum efficiency. Before executing a quick or sharp turn, the prudent boat operator will lower the jack plate if it is raised for maximum efficiency. You should know how quickly your jack plate raises or lowers so that you do not have to look at the gauge or back at the jack plate to ascertain how much the jack plate has been lowered once you hit the Down button. If your jack late lowers at about an inch per second, you can use that fact to mentally count the number of inches you have lowered your jack plate.

Shallow Water Operation: How Low Can You Go
A hydraulic jack plate really shines when used to navigate shallow water. It allows the engine tilt to be lowered all the way to get the best attitude for starting in shallow water and helps keep the water intake below the surface so that the engine doesn’t suck air and the propeller doesn’t cavitate. A boat with a tunnel hull makes the most efficient use of a jack plate, as the tunnel provides a greater supply of non-turbulent water for the propeller to act upon. I have seen tunnel hulls with jack plates running in less than a foot of water. As long as the boat has enough water to float, a tunnel hull with a jack plate can run through shallow water. Such a combination of boat and jack plate can get up on plane where a boat equipped only with power tilt would merely chop up the water and never gain enough momentum to get up on plane. When operating in shallow water using a jack plate, one of the key caveats is to make sure that your water pickup is below the surface. Watch your water pressure gauge or make sure that water is coming out the tell-tale water indicator. An engine that is not pumping water through it will quickly sustain engine damage from being overheated.

Summary
The latest innovation in boat operation is the jack plate, which is used to find the optimum engine height for efficient operation, maneuverability, and shallow water operation. Even a fixed jack plate offers better efficiency and increased maneuverability due to the engine being set off from the transom. The top-of-the-line hydraulic jack plate allows the boat operator to raise or lower the engine on the fly. The jack plate height can be fine tuned as the boat moves through the water. The boat operator must also take into consideration the dangers of raising the engine too high, which can cause loss of control of the boat when making turns and can expose the engine water intake to the air, which can cause engine damage.

For more information on boating safety, please visit the Official Website of the U.S. Coast Guard’s Boating Safety Division at www.uscgboating.org. Questions about the US Coast Guard Auxiliary or our free Vessel Safety Check program may be directed to me at [email protected]. I am available to perform free Vessel Safety Checks in my area, and I will come to your location to perform them. SAFE BOATING!

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