The propeller on a boat, whether an outboard or inboard, creates an effect often termed wheel-walk when it spins. As the boat moves through the water the total pressure of the water on the prop is less at the top than the bottom, causing a right handed prop--which rotates clockwise when looking at it from behind the boat--to swing the stern of the boat to starboard, resulting in the boat turning to port. This effect cannot be eliminated in a single prop boat; it can only be compensated for.
Things You'll Need
- Socket wrench
Tilt the motor and locate the small trim tab located just above the propeller. It will probably be mounted on a circular plate with a nut in the center locking it in place. Different manufacturers have slightly different arrangements, but all are simple and obvious. If you don't have a trim tab fitted, either it is an old motor that never had one, or the fitted one has broken or fallen off. You will have to buy a replacement from the dealer for lost or broken tabs, or buy a retrofit tab from a marine chandler such as basspro.com or thmarine.com in their boating/outboard motor accessories section.
Locate the mounting bolt(s) and get your socket wrench of the appropriate size ready. While you are still at the dock or on land, make sure the bolt is free and not rusted in place. Use a little WD40 if necessary until you can loosen and tighten the bolt easily.
Take the boat out and run it up to the speed you normally cruise at, on a calm day with a helper on the boat with you, heading directly into any waves so they don't push the boat off course. See which way the boat turns when you loosen your grip on the steering wheel. The swing of the boat will vary with its speed, so adjusting it to track straight at cruising speed makes the most sense.
Stop the boat and motor and tilt the motor. Make sure:1) You are in a clear area away from other boats or hazards 2), you are wearing your life-jacket and 3) you have a line attached to the socket wrench and your wrist.
Slacken the mounting bolt and turn the trim tab a little; then tighten the bolt. If you have a right-hand prop and the boat was turning left, turn the tab to the left. You will have to judge how much to turn the tab, and you probably won't get it exactly right the first time.
Run the boat up to speed again and see how much difference the adjustment made. If it still isn't running straight, repeat the above step, adjusting the tab appropriately.
Mount an add-on tab if necessary. Add-on trim tabs are designed to attach to the skeg of an outboard motor when no factory-installed tab is available. They are inexpensive, usually under $20, at 2010 prices, and can be purchased from most outboard motor dealers or marine chandlers. They come with instructions for mounting and adjusting. You will need more tools, including an electric drill, to mount the tab.
Tips & Warnings
- If you are uncomfortable adjusting the tab out on the water, bring the boat back to the dock and do it there. You are vulnerable to sudden waves or wakes from passing boats when leaning over the stern, so make sure the helper you have with you can grab you if you start to fall and is capable of running the boat and picking you up should you fall in. It's a good idea to attach yourself to the boat with a safety line while you are performing this adjustment. SHUT THE MOTOR OFF each time you make an adjustment--you are working right next to the propeller.
- Photo Credit toy outboard motor image by pearlguy from Fotolia.com
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