Bubble Trouble

Maintenance Q & A — June 2005
By Capt. Ken Kreisler

Bubble Trouble
The causes of prop cavitation, V-belt maintenance, a varnishing tip, and more.
 More of this Feature
• Prop cavitation, and more
• A varnishing tip, and more
• PMY Tries... Interlux Boatcare

 Related Resources
• Maintenance Q&A Index

During a recent haul out, I noticed pitting on the edges of my prop blades. What is causing this? C.E., via e-mail
Cavitation is the culprit, and what you are seeing is known as cavitation burn. To help understand why this is happening, you have to understand the relationship between atmospheric pressure and the boiling point of water. At sea level, water boils at 212ºF. As atmospheric pressure increases, so does the boiling point, and the opposite is true when the pressure decreases. In fact, given a low enough atmospheric pressure, water can boil while at room temperatures.

Now consider what happens during normal prop operation. As the boat is being propelled through the water, low pressure forms on the back of the blades, but not low enough to cause any damage. However, if a blade is damaged, corroded, or poorly designed, it can significantly reduce the pressure on the back of the blade. During high-speed operation, bubbles will form there as the water begins to boil. As the boiling water seeks higher pressure, it moves up over the prop edges, where the boiling ceases and the bubbles collapse, releasing energy with such force that it can erode the blade surface, creating the pitting you noticed.

To prevent this condition, repair even the smallest nick or dent in your props. To be even safer, have your props looked at by one of the high-tech prop shops like Prop Scan. Visit the Web site at www.propscanusa.com for more information.

We have some mold on the cabin interior. What solution would you recommend we use to clean it up? G.D., via e-mail
One of the most efficient and least expensive ways of removing mold and mildew is chlorine bleach. A solution of one-half to three-fourths cup of bleach to one gallon of water will get the job done. Clean the affected surfaces prior to using the bleach solution, and never mix bleach with ammonia or household cleansers containing ammonia; the resulting fumes are poisonous. Use a scrub brush for both cleanings; since the bleach solution can discolor fabrics, you may want to remove any in the area that you are working in.

To make sure you kill the growth, keep the solution on the affected area for ten to 15 minutes. When cleaning with a chlorine-bleach solution, always wear rubber gloves, long sleeves, and pants to protect any exposed skin. Also wear safety goggles, not just glasses, to keep the solution away from your eyes.

If you are not comfortable with using the bleach solution, there are many spray-on, wipe-off household products on the market that will also get the job done. Make sure you read the directions carefully, and take the necessary precautions when using these products. Remember that mold is a living organism, so using a product that merely cleans the surface area without killing the organism is a waste of time.

Creating a dry, relatively low-moisture environment is the key to combating the growth. If you are away from your boat for extended periods of time, maintain a constant flow of air with a small fan, and open up interior spaces to enhance air circulation and thereby reduce moisture. Another good idea is to use a dehumidifier and/or dehumidifying agents in bilge and stowage areas and closets. Dehumidifiers must be checked and emptied regularly, and agents will need to be discarded, as accumulations of water can reintroduce moisture into the enclosed environment.

Next page > Part 2: A varnishing tip, v-belt maintenance, and more > Page 1, 2, 3

This article originally appeared in the May 2005 issue of Power & Motoryacht magazine.

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