Ask The Experts

Solutions from the BoatUS Tech Team

Here it is, your column, devoted to the technical challenges of boat ownership. Our top tech team is standing by to help with the answers to your questions.

Power, The Ultimate Aphrodisiac?

I'm on a trawler with a microwave oven powered by an inverter. My house bank is two Trojan golf-cart batteries. I notice that my battery monitor drops way low when I heat a cup of coffee or warm a rotisserie chicken — below 12 volts. Then it jumps right back up. Is this wrecking my batteries by making them think it's a full cycle? I know batteries only have so many cycles.

Don Casey: A 700-watt microwave seems like it should draw just under 6 amps at 120 volts, but due to reactance inherent in the microwave circuitry, the actual draw is closer to 10 amps. Microwave ovens also operate about 30-percent less efficiently on the square-wave power most small inverters supply, so now we're up to 13 amps. We calculate current draw by dividing watts by voltage, so when the supply voltage is 12 rather than 120, the current draw is 130 amps rather than 13. Add to that the inefficiencies of the inverter and your microwave is drawing close to 150 amps from the batteries when it's running.

Your two-Trojan bank is only rated at 225 amp-hours total, based on a 20-hour discharge rate — that is, a constant load of around 11 amps. It's no wonder that when you draw 150 amps from this bank, the voltage takes a dramatic dive. In answer to your question, yes, this is probably shortening the life of the batteries, not because of cycling, per se, but because these are thick-plate, deep-cycle batteries and not designed to provide high bursts of current. While your microwave is running, the current draw is similar to holding down the starter button on a non-starting engine for that long. About 45 minutes of microwave operation, less as the batteries age, will completely flatten this bank.

One solution is to run the engine while running the microwave so that your charging system at least partially absorbs the load. A second option is a larger battery bank. Dividing this load over four or six batteries, rather than two, reduces the demand on all of the batteries. A third possibility is to power your inverter/microwave combination from a regular thin-plate starting battery designed to handle this kind of high-amp draw. You still have to put back the amps with some type of recharge, but the battery will suffer less, assuming it's not left in a discharged state for long. Start batteries are designed to deliver a burst of current, then bask in the charge coming from the alternator. They're also cheaper to replace if you're less than diligent about recharging. You might just want to take to using the stove for most heating tasks — assuming it, too, is not electric.

Oil Is Oil, Right?

Is there any difference in 4-stroke marine oil and engine oil for cars? I'm having difficulty finding 4-stroke marine oil. I have a 2009, 115-hp Yamaha outboard.

John Adey: Yes. There are a ton of labels and certifications on engine oils that you should know about and pay close attention to. Approved and certified FC-W (Four-Stroke Cycle Water-Cooled) oil starts off with a heritage of American Petroleum Institute SM (Highest "grade" for passenger-car oil) performance quality better than your current passenger car oil.

It's then given a mix of additives and required to pass an array of additional tests including a salt-fog-rust test and a 100-hour performance test. If an oil formulation has passed all required tests, it's then certified as FC-W marine oil. Brands using these formulations are annually registered and listed on the National Marine Manufacturers Association Web site. Engine warranties are usually void if FC-W oil is not used. There's a huge amount of information on the NMMA Web site (; they certify the oil for the marine market. Click on the certification tab.

My opinion (from someone who likes to save money): ALWAYS use the manufacturer's recommended oil, never skimp on this crucial component.

Fuel-Tank Woes

On our 23-foot open fisherman, we're dealing with deterioration of the fiberglass fuel tank due to ethanol. Fiberglass debris passed into the fuel-delivery system, causing several problems. Mechanical repairs have been made but now we're uncertain of our next move. Do we replace the tank? If we remove the gas containing ethanol, thoroughly clean and vacuum the tank, and use non-ethanol gas going forward, will the deterioration of the tank stop? It hasn't deteriorated to the point that it's not usable. This would certainly be a cheaper option than replacing the entire tank.

Don Casey: I think you're going to have to bite the bullet and replace (or at least abandon) the existing tank. It will be hard to assess the seriousness of the damage, and a tank that surreptitiously unloads itself into the hull between uses could send you and yours to the next life. But even if the tank remains sound for the moment, I doubt that you can reliably get ethanol-free fuel now and/or in the future. The current emphasis is on introducing additional ethanol into the national fuel supply, so ethanol-free fuel may disappear altogether, or at least become ever more difficult to obtain. And because of the way gasoline is distributed, I'd have zero confidence that what is sold as ethanol-free can be reliably counted on to be ethanol-free. Safest, most convenient, and least detrimental to your future enjoyment of your boat is to install a new tank.

Prospecting For Oil

Last Saturday, I stopped by the boat to check on it. I noticed under the engine maybe a quart of oil in this aluminum lasagna tray that I had put under the engine to catch any drips. Some oil had leaked into the bilge. First I thought it may have been the oil pan that let go, but after looking it over, the oil seems to be dripping from above, aft of the oil pan. The other project is the exhaust odor in the cabin. I may try replacing the exhaust hose in case there are any leaks in the hose. What do you think?

Tom Neale: You've taken what's probably the best step, and sent the question to your mechanic. It's a problem that probably only can be diagnosed by being there and having hands-on access. Usually an oil leak, the source of which isn't obvious, must be traced back to that source. Frequently this involves having the engine running. But here's my two cents' worth — and take it only as that. From the engine drawing you sent, it appears that there's a possibility that the oil could be leaking from the aft seal and settling into the bell housing. I've had this happen before. At first you may not even know it's happening because it may be contained for a while within the bell housing. But eventually it begins to be slung out while running, or finds its way out bolt-holes, the starter connection, or other areas. A problem with this theory is that usually oil leaks past that aft seal only when the engine is running. But you could've had a lot of oil leaking out as you ran it before you put it to bed for the winter, and then that oil could've pooled in the housing, and eventually begun to seep out. You said you found around a quart. I assume your dipstick showed that loss of oil. Of course, this can only be a guess and hands-on work by a good qualified mechanic is the best course.

As to your exhaust smell, definitely get that diagnosed and repaired before you use the boat again. This could be a bad exhaust run, which could be lethal.

Baffled At The Beach

I have a 2-year-old Yanmar 30-hp with 600 hours on the clock. In neutral the engine revs up to 3,000 rpm; although in gear it only revs to 2,000. All filters are clean. I'm baffled.

John Adey: Congratulations for putting 600 hours on your engine in two years! That is time well spent! I can only assume that this is a new condition for this boat and that's what caused you to ask the question. Going on that assumption, here are a few things I'd look at:

Propeller condition

A fouled prop can do amazing things to speed and engine rpm. Excessive growth and/or fouling (like a line) can dramatically affect the engine's ability to effectively turn your prop. You didn't mention it but I assume you have also had a drop in your expected top speed. Taking a look yourself, at a nice anchorage somewhere, or hiring a diver is the only way to determine this.

Exhaust or intake obstruction

Your engine operates differently under load. You've ruled fuel out as a possible problem, so I suggest you take a look at air intake and exhaust. Could you have a blockage in the exhaust system, perhaps a kinked hose? How about your intake filter or screen? I worked on a boat where the alternator pulley was severely misaligned and created so much belt debris that it eventually clogged the intake screen to the point where there was not enough air to keep the engine running at normal rpm.

Carbon buildup

This is caused by the way we use our engines. Diesels love high-load, max-rpm running habits, which we don't do very often. This promotes carbon buildup in the fuel injectors and exhaust-mixing elbow where the raw water is injected into the dry exhaust. Rough idling is one symptom; low rpm under load can be another. If this is suspected, have the fuel injector spray pattern checked by a qualified Yanmar shop. You can inspect the injection elbow on your own with a flashlight and sometimes a small dental mirror.

Missing A Screw?

Can I use stainless-steel screws in place of bronze on an impeller housing that has a few screws missing? Will the stainless set up some sort of action with the bronze housing?

John Adey: Your concern is valid: Can there be some type of galvanic action between the two metals you'd like to use together? Going back to high-school chemistry class, let's look at the galvanic series table. I'm concerned about the "anodic index" in volts. Different metals have a different rating; take a look at this excerpt from an online engineering handbook: (

Metal TypeAnodic Index (V)
Brass & Bronze0.40
Stainless Steels (Type 316)0.50

In a harsh environment such as saltwater, we want to be no further apart than 0.15V or so. Looking at these numbers, we see that they'd be compatible in a saltwater environment. To confirm this, I can tell you that in the absence of bronze set screws, I used 316 stainless to hold a cutlass bearing into a propeller shaft strut and was able to remove them three years later!

Now that we've established that the galvanic action is compatible, my next concern is the difference in hardness between metals. You could easily strip the bronze threads with the stainless screw. This is a common access point for you and the maintenance of your boat; I'd hate to see you turn a $1.25 screw into a $300 water pump because it was over-torqued. I suggest you find a bronze screw through a good fastener store or supplier; Fastenal ( or McMaster-Carr ( come to mind.

Making Water

I own a 1982 32-foot Bayliner with twin gas Volvo AQ 140, four-cylinder inboards. I've been restoring the boat for the past two years. There is an issue in the raw-water system on the starboard engine that's causing water to leak from the screw cap above the heat exchanger. The cap allows access to the raw-water screen. The screen is clear. The discharge out the stern is about half the flow of the port engine and steam instantly comes out with exhaust when the engine is started, whereas the port engine discharges only water. What could be causing the flow restriction? Someone mentioned that mineral deposits can accumulate inside the system and that there may be a solution that can be added to dissolve them.

Tom Neale: There may be several different things going on. One possible cause is that there's a leak in your heat exchanger (usually in the tube stack), allowing raw water to migrate through the leak into the freshwater side. When you say "screen," I assume you're referring to the tube stack inside the heat exchanger component. This might cause the water to rise and push out of the freshwater fill cap. The decreased water flow at the stern exhaust normally would be caused by something else, such as a bad impeller in your engine's raw-water pump, or blockage, such as might be found at the injection nipple where water is injected into the exhaust. Pull the hose at that point and, with a small flashlight, check inside the nipple for blockage.

However, the "steam" with the exhaust implies other issues. It could be caused by the engine overheating, but I assume you would've noticed this from the temp gauge. Or it could be from a breach in your head gasket or in the casting, allowing combustion gases to get into the raw-water side. This sounds likely as you say the "steam" appears instantly. This happened to me once and I started producing a lot of "steam" (probably combustion gas) at the exhaust. This problem could mean the end of your engine if you don't get it corrected right away. A breach such as this could also dump water into one or more cylinders.

With the engine and its cooling system cold, remove the header tank cap (where you add fresh water to your system), start the engine, and check for bubbles rising from the water inside the header tank. If there are bubbles, this could signify a blown head gasket or a breach in the casting. Tiny bubbles at first, which soon stop, are not what I'm talking about. The type of bubbles to which I'm referring would continue and be significant. If you do this, take great care not to burn yourself. As the water heats, it will begin to overflow because it's expanding.

Yes, you could have deposit buildup on the heat exchanger; it happens over time, and the exchanger tubes should be periodically cleaned. But it's unlikely this could cause all the symptoms described.

Sound Advice

I'm looking at two systems for ultrasonic bottom cleaning. These are fairly new technologies and it's my understanding they'll replace the need for annual or frequent boat bottom painting. The basic idea is that the unit, which operates on a very small power drain, emits an ultrasonic beam that kills algae and barnacles. The unit is installed inside the hull and is fairly simple to install. When the boat moves through the water, algae and barnacles are removed from the boat. Are you familiar with this concept and any of the companies providing the product?

Don Casey: This isn't a new technology. I've periodically seen advertising for ultrasonic units like you describe for 30 years, and they probably pre-date my notice. In all that time, I've yet to see a single bit of documented evidence that ultrasonic "bottom cleaning" actually works. There is no question that you can discourage and even kill algae with sound waves, but expanding a laboratory concept to the real world of keeping a boat hull clean is a giant leap, one, it would seem, that no one has yet clearly made.

Here is the test for you. Before you spend money on such a system, find two people not associated with whatever company you're considering, who have such a system installed, have had it for more than two years — to rule out the effects of their existing antifouling paint — and who will recommend the product without reservation. My guess is that you simply cannot do that, and if not, that should tell you all you need to know. When this technology matures, if it ever does, there will be thousands of testimonials, from commercial shipping interests, from fishermen, from dock owners, and, of course, from pleasure boaters. Until then, I remain circumspect.

Father Knows Best

Back in the mid-1990s, I bought a 4-hp Yamaha outboard. I probably didn't put more than five hours on it. I started it at least once a year in a five-gallon bucket of fresh water just to make sure it was able to run. Last year when I ran it, I noticed the water pump witness stream, which comes out of the bottom of the engine block, was not steady. The flow was sputtering. The water stream, which mixes with the exhaust gases, always sputtered, as I would expect. After about five minutes of running, I noticed small puffs of steam intermixed with the sputtering water from the engine block, but the water was not so hot that I could not hold my hand in it. The other water stream, which was mixed with the exhaust gases in housing, remained cold. I suspected the water pump was bad. I just replaced the water pump assembly, even though the components looked good. The motor started right up.

After a few minutes of running, it exhibited the same conditions as described above with the sputtering stream and small puffs of steam. Could it be the engine block has deposits preventing the water from flowing cleanly through it? If so, could I add an automotive radiator-flushing agent to the cooling water in the bucket, and let it run through? Would that hurt the exterior finish or the lower unit seals and rubber impeller in the water pump? Or could it be the thermostat?

Tom Neale: The problem could be caused by many things, most of which you've already guessed at. But one thing you need to consider is that an outboard that old, particularly just sitting most of the time, could have corrosion in its water-jacket components, or even in the block and head. I assume that the block and water jacket are aluminum, which would be particularly susceptible to corrosion if the motor was used in saltwater. Corrosion could allow water and/or gases (which might manifest as "puffs of steam") to escape around gaskets. The puffs of steam may actually be combustion gases or exhaust from a cylinder — which could also cause the "sputtering." You may have leakage around the head gasket into your cooling water. If you see any white residue around your coolant system gaskets, this is a sign of possible corrosion where the surfaces meet. Having said that, I should note that I've seen outboards with water-indicator streams that are plumbed from the pump and don't actually go through the cooling plumbing in the block. Trace the hose back from the hole to be sure it comes from the engine itself before you get too far into this theory.

Also, as you surmised, corrosion could've clogged the passageways, but normally your engine would run hot if this were the case; from what you describe, it isn't. I assume you've run it for long enough to know that.

I'd also check the thermostat, although it doesn't sound like the cause. It's time to do this anyway, on an engine that old. Usually thermostats are easy to check and replace. Signs of corrosion in the thermostat housing would indicate corrosion elsewhere and that the thermostat may be impaired. You can pull it and put it in heated water and see if it opens at the temperature in the engine's specs. But a bad thermostat, again, would probably cause overheating and not the puffs of steam and sputtering that you describe.

I can't recommend any automotive flushing product other than whatever the manufacturer recommends for the engine. If the engine-cooling water passages, or water-jacket surfaces, are corroded badly enough, it's probably too late for a flushing agent. Automotive radiator flushing agents are usually designed to treat other types of buildup.

Check to be sure there are no wasps (or other bug) nests or bodies in your cooling passages. Gentle prodding with a wooden dowel, skewer, or similar tool, or back-flushing with water pressure through the weep hole may get them out. Also, vinegar in the water will sometimes dissolve minor corrosion.

Follow-up: Captain Tony later remembered a trick he'd learned from his father. He filled the five-gallon bucket with hot water, added two cups of Arm & Hammer baking soda, and ran the engine with it. He reported to Tom that this solved his problem. 

— Published: August/September 2011

Meet The Experts

Don Casey

Don Casey
He's been one of the most consulted experts on boat care and upgrading your boat for 30 years, and he has been on our BoatUS "Ask The Experts" website for the past decade. He and his wife cruise aboard 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", a bible among do-it-yourself boaters.

John Adey

John Adey
The VP/Technical Director for the American Boat & Yacht Council (ABYC), John grew up boating. He's been in the industry since 1990, with diverse 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 that he completely restored, including the hands-on installation of all systems. John is a trusted source for technical information for industry professionals.

Tom Neale

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


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