Ask The Experts

Solutions from the BoatUS Tech Team

Purple boat prop paintingIllustration: Bill Roche

From The Latin Cavus, Meaning Pit

What is cavitation? What causes it?

Don Casey: Cavitation is the formation of bubbles — liquid-free cavities, really — caused by a partial vacuum. In the context of a cavitating boat propeller, the cause is negative pressure on the suction face of the blades, typically as a result spinning at a speed that develops significant amounts of slip. It can be altered by changing the shape, pitch, and speed of the blades.

The problem with cavitation isn't so much loss of thrust as it is vibration and damage to the prop. The tiny implosions of these bubbles literally suck metal from the prop, resulting in pitting and eventually imbalance. If you feel your prop cavitating often, you should take corrective measures.

Pump Ups

I had my boat pulled for the winter but had forgotten to empty the waste tank. Is there a way to empty the tank manually?

Tom Neale: Many yards and marinas have portable pumpout equipment. If yours does, get them to use it on your boat. The fact that the boat's been hauled shouldn't matter. Otherwise, depending on your boat and the tank, there are several things you can do. Many tanks have inspection/cleaning ports and you may, if the port is on top and won't leak waste when you open it, simply be able to go in and "withdraw your deposit," using a scoop or small bucket. Or you could suck the waste out using a Whale gusher-type pump with the right-sized pumpout fitting on the end of its intake hose. Or perhaps a shop vac would take it out, if you don't want to use the shop vac again. Maybe use a friend's shop vac, if you really don't like the friend.

If your boat has a macerator discharge for offshore, you could activate that with ample buckets under the thru-hull, but the problem isn't limited to getting the waste out of the tank. You must then properly dispose of it.

Fire Up The Fridge

The original DC/AC fridge on my Bayliner failed and I didn't want to spend the money on a replacement. I bought an AC-powered dorm fridge that would fit instead. When I turn on the breaker it trips right away. The breaker is rated for two amps. I don't just want to replace it with a larger breaker because I am unsure of the wires from the breaker to the fridge. At the breaker and at the AC plug receptacle, the wires appear to be sufficient gauge but it's what I can't see under the floor that concerns me. How could this ever have worked when trying to run on 110 AC when in port? Somehow the system knew when to switch from 12-volt DC to 110-volt AC.

John Adey: The DC/AC fridge you replaced should have had both a DC feed and an AC feed. Inside the unit would have been a DC-to-AC transfer switch where the unit sensed the power provided and operated accordingly. You did not indicate that you found two sets of wires so I wonder if there's an inverter somewhere on the boat.

You didn't mention the actual requirements or the size of the fridge. The dorm-sized fridges that I researched only drew 90 watts or .75 amps AC continuous while running. What they don't tell me in the spec sheets is the startup current to get the compressor going. This may well exceed your two-amp breaker, where on the OEM fridge it did not. If it were me, I would take it home, plug it into the house outlet and put a clamp-on amp meter on it, and see what it draws at startup.

As far as the wire size, if you have 14 AWG or larger, you are good to go. If you have 16 AWG, I would experiment with a 10-amp breaker first and see what happens. Please be cautious when doing this — all AC power must be unplugged. I will say if the replacement breaker also trips, then there is potentially a grounding issue somewhere as some household appliances have a different treatment of the AC neutral wire from marine appliances. That's one good reason to avoid the household fridge and buy the marine one.

Just Say No

I've recently found out a marine-fuel supplier near me has been dispensing E15 fuel at fuel docks and gas stations in my area. The pumps at the fuel dock are not marked with E15 stickers, there's only a sticky note behind the counter. Two different employees at the fuel dock verified this. My understanding of E15 regulations is that it has to be posted, and E15 cannot be used for marine applications. I have tried to contact EPA with no luck. Can I add fuel from another marina that contains no ethanol to "dilute" what I've added to my tank?

Beth Leonard: E15 is a gasoline blend with 15 percent ethanol, as opposed to E10, which is 10 percent ethanol and what most gas stations and fuel docks are pumping now. E10 caused significant problems for marine engines when it was first introduced, but manufacturers have redesigned their engines so that newer engines now play fairly well with E10. Not so with E15. Not only could E15 damage your engine, it would likely void your warranty. You're correct as to the regulations — pumps must be marked with E15 stickers, and it's technically illegal to put E15 in a marine engine. Your instinct is also correct — diluting the E15 with ethanol-free gasoline and avoiding those pumps until you're sure the situation is resolved is the best way to prevent any lasting damage.

I Can't Hear You Now

This past July we encountered a large thunderstorm with high winds, lightning, and whiteout conditions. Since the storm our VHF radio is barely receiving any transmissions, even when reducing the squelch control. Our transmissions seem to be OK. The problem is the same when using the base or remote, and the same when the remote is disconnected. Should I replace the masthead-mounted antenna?

Tom Neale: You could have electrical surge damage from all the lightning, even though you weren't directly hit, and this would require a qualified electronics repairman or new set. But I'd try some less expensive checking first. Also make a call to the radio maker's customer service department. They may have some helpful hints.

Take the radio to a friend's boat with a working radio, hook yours up, and see if it performs the same. If it does, the issue is within the radio. If your radio works well on the friend's boat, you'll need to check the cable and connections. Disconnect the antenna wire from the antenna, checking for signs of damage, corrosion, or moisture. Next, spray the connection with WD-40 or one of the marine-specific moisture-displacing sprays, then connect and disconnect the wire several times (to clean the connection), and see how it performs. While you're up your mast, look for antenna wire problems such as chafing, splitting of outer insulation, or bumps under the insulation. Moisture could have breached the isolation between your inner wire and its shield. Also check throughout the rest of the accessible antenna wiring and its connections for signs of trouble.

If what I suggested above doesn't help, before you go to the expense of a new set, consider replacing that antenna, particularly if it's an old one. They don't last forever, but you should be able to replace it yourself if you're able to go up the mast safely.

Diesels & Carbon Monoxide

I have a Catalina 34 with a diesel engine. Do I need a carbon monoxide detector aboard?

Beth Leonard: Carbon monoxide results when fuel is burned without enough oxygen to create carbon dioxide (two oxygen atoms instead of one). Diesel engines produce very little carbon monoxide compared to gasoline engines because there is excess air/oxygen in the combustion chamber even at full load, where gasoline engines run a mixture with more fuel in proportion to the oxygen, especially at high loads. The chances of an exhaust leak in a boat with a diesel engine causing serious harm are low enough that having a CO detector is not part of the American Boat and Yacht Council (ABYC) standards for such boats. That said, it is cheap insurance, especially if you have other potential sources of CO aboard like a propane stove.

Kicker Picker

I'm considering adding a kicker motor for trolling to my 31-foot Boston Whaler. Dry weight at birth was nearly 13,000 pounds. How large an outboard would it take? Or, conversely, how small can I get away with?

John Adey: From my previous life selling small outboard fishing boats, I can tell you that there's no hard and fast "formula" for what you are doing. Here are some key points to consider:

  • Many suggest a two-cylinder model from a vibration standpoint; this puts you in the 9.9- to 15-hp category, which is where I would've started as well.
  • You want a "high output/thrust" model to give you more torque rather than speed, which will allow you to run a prop with a higher pitch.
  • While off plane, your boat has a hull speed that it won't exceed no matter what you do. This could be around 6-10 mph, which is data I'm sure you can find from Whaler. Take this into account as well as your fishing style and location (e.g., are you bucking currents upriver?).
  • I am 90 percent sure you'll be looking at a 20-inch shaft to keep the important stuff in the water when it gets a little rough or all your crew decides to move forward!
  • Unless you want to be overboard running a tiller, you may want some type of remote option on your new outboard. There are many controllers that can dial in your speed in small increments; check this out as well as shift and steering. Some steering setups are simply tied to the main engine with a link bar.

In the end I think you'll find a high-output 9.9- or 15-hp will give you every bit of what you need for trolling and help you get home if need be. You'll most likely need a bracket, and consider having your new engine run off the same tank as the current engines; it saves some hassle but involves a bit of initial installation. If you're unsure how to do this, consult a qualified mechanic. Find one at

As an additional resource, dig into the owners' forums for your boat. You might find an owner who has experience with other options I haven't listed here. 

— Published: February/March 2015

Meet The Experts

Photo of Beth Leonard

Beth Leonard
BoatUS Magazine's technical editor, Beth Leonard, grew up powerboating, waterskiing, and fishing on Lake Ontario. Since 1992, she and her husband have completed two circumnavigations on two sailboats, doing all maintenance themselves. They also installed the systems on their 47-foot aluminum sloop. Beth has written The Voyager's Handbook, the how-to bible for offshore sailors, and hundreds of technical articles.

Photo of Tom Neale

Tom Neale
He's cruised long distance with his family for most of his adult life. He can take apart and fix almost every system aboard, has written two books, filmed a two-set DVD on East Coast cruising, written for top marine magazines, and has won nine first-place awards from Boating Writers International and many awards for his technical writing.

Photo of Don Casey

Don Casey
One of the most consulted experts on boat care and upgrades for 30 years. He and his wife cruise their 30-footer part of the year in the eastern Caribbean. His books include Don Casey's Complete Illustrated Sailboat Maintenance Manual, and the recently updated This Old Boat, the bible for do-it-yourself boaters.

Photo of John Adey

John Adey
The president of the American Boat & Yacht Council, John has been in the industry since 1990, with experience from a yacht brokerage and boatyard to owning a marine supply store. He and his family sail their classic 1976 Irwin ketch, a boat he completely restored.


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