Ask The Experts

Solutions from the BoatUS Tech Team

Keeper Of The Gate?

My boat has a gate valve at the thru-hull water intake for the engine. Is there a problem with this?

Don Casey: For every boat that sinks underway, four sink in their slips, and that about 90 percent of those involve failing or improper plumbing. No matter how bulletproof your hull, what really keeps the ocean out is the thin wall of a rubber or plastic hose and the grasp of a narrow strap of perforated steel. The cautious boat owner will close the valves on thru-hulls when leaving the boat unattended, and herein lies the first of two major problems with gate valves. When you turn the wheel on a gate valve until it stops, you won't know that the valve is closed. Debris inside the valve can and often does block the gate partially open, changing the potential consequences of an all-too-common plumbing failure from inconsequential to catastrophic.

The second fault is that gate valves are invariably constructed of dissimilar metals, so they suffer from internal corrosion. This is made worse if you use your boat in brackish or seawater. It's a question of when, not if, the stem on the valve will fail. The valve is there only to allow you to stop the flooding should some plumbing failure occur, and the design and construction of a gate valve means it cannot be counted upon to do that. Imagine an inflatable life jacket made airtight with a coating that dissolves in humid air and you'll get the idea.

Replace all gate valves with true seacocks at your earliest opportunity. Seacocks close with a 90-degree movement of the handle, and the position of the handle shows positively that the valve is closed. Seacocks are also constructed of either all bronze or, my preference, an extremely durable polymer that eliminates the risk of corrosion. Meanwhile, immediately exercise your gate valve vigorously, then, with it in the fully closed position, cautiously disconnect a hose to determine if it does close. If no water continues to flow from the open hose, mark the wheel so you'll know in the future if it stops short of full closure, but this is not an alternative to replacement.

Proper Propeller?

I have a 1986 Sea Ray 410 aft-cabin with two Mercruiser 454, 340-hp gasoline engines, Borg Warner transmission 2.91:1, and boat weight of about 25,000 lbs. It goes 8 mph at 1,700 rpm and 12 mph at (wide open) 3,200 rpm. I know the last owner put new three-blade props on and new shafts in but don't know the prop size. We're planning an extensive cruise next year and I'd like to go at least 12 to 14 mph at 1,700 rpm. Is this possible and what size props size and number of blades would I need? Also do you have any idea how much fuel is consumed per hour at 1,700 rpm and 3,200 rpm?

John Adey: Find the prop pitch and diameter you have now. Usually it's stamped on the hub between the blades or any good prop shop can put it on a pitch block and get you the pitch and diameter. Your wide-open throttle rpm range seems a bit low for those engines, specs I've seen online call for 4,600 to 5,000, which means your pitch may be too large. Mercury says 1 inch of pitch reduced will increase WOT (wide open throttle) RPM by 150-200 rpm ( Fuel consumption is a moving target. Depending on boat weight, loading, bottom condition, sea state, etc., it could be very different. You can start with advertised fuel consumption for your engine. You can also install a fuel-flow meter to help you with finding the most economical cruising speed for you (

Do some comparison shopping. You may find some similar boats for sale that list the top speed at WOT, a good way to easily compare. You can also use a great online resource through Michigan Wheel ( This tool can help you even if you don't know the current pitch and diameter.

Hot Spot Astern

I have a black burn mark on my fiberglass transom about six by six inches. How should I repair it? It went through the gelcoat and I can feel fiberglass strands. It's a cream-color hull.

Tom Neale: You may have more than just a "burn mark." The structural quality of the fiberglass hull may be impaired from the heat. What you do will depend on how serious the burn damage is. It may be advisable to sand down into the laminate to see what's there, and repair using whatever method is best suited for the type of glass hull you have and the location and degree of burn damage. For example, if the hull is cored in that area, you'll need to determine whether the coring has been affected. If it's in an area of high stress you need to be especially concerned about the integrity of the laminate. Possibly you should replace some of the laminate as well as gelcoat in the burned area. Maybe you just need to replace the gelcoat, but it's safer to start with the assumption that you may have more than a cosmetic problem.

If it's just a gelcoat smudge you can probably repair it yourself, if you're reasonably handy, with one of the kits readily available from West System and others. They have color-matching kits. While we don't recommend any one company over another because we haven't used them all, the website has information that you may find helpful once you assess your problem. If it's any more than a superficial gelcoat burn consider having a qualified professional take a look because of potential safety issues.

Belt And Suspenders

I've read that a common practice for electric bilge pumps in sailboats is to have two — a small one deep in the bilge to handle small amounts of water, and a larger one higher up to handle larger amounts of water. My question is: How do you install the larger, higher-up pump and not have it air bound. It's not sitting in water all the time, so how does it get "primed" when water reaches it?


Don Casey: As boat manufacturers scrambled in the 1980s to grapple with the issue of blistering hulls, a conclusion of a study conducted by the University of Rhode Island was that wet bilges dramatically increased the likelihood of destructive blistering. It has been my observation that a pump of sufficient size to deal with flooding cannot pump a deep bilge dry, simply because the column of water in the large diameter discharge hose drains back into the bilge when the pump shuts off. In the original edition of This Old Boat I proposed the two-pump solution — the smallest automatic pump available discharging through 1/2-inch hose to take care of rain, ice-box melt, and stuffing-box drips, and a "real" bilge pump mounted higher to deal with an emergency. An unintended benefit of this configuration is that it extends the life of the large pump to near immortality because this pump sits high and dry and never sees use except for testing or a real emergency. The benefits are so compelling that this arrangement has indeed become common practice.

As to your concern, the type of pumps we're talking about are centrifugal pumps, which are self priming, so as soon as the water level submerges the impeller, the pump will be primed. You can put the pump into full-flow operation with the flip of a switch, either manual or automatic. Because an automatic pump will mask a serious leak on an unattended boat and will deplete the batteries in the process, if you connect the large pump to an automatic switch, you should also incorporate a loud high-water alarm or some other means to make sure someone knows instantly that this pump is running.

Statically Speaking

My Icom 504 works great, until I turn on my Garmin GPSmap 215. At that point, reception is all static. The excess cables aren't coiled and wrapped in foil. The position interface is connected. Both power leads and grounds are off the same bars. How can I use both units as intended?

John Adey: First, consider the location of GPS antennae to VHF antenna. Move them as far apart as possible, try again. Second, invest in an automotive radio-noise filter on the GPS power supply. They're cheap (under $5 for a 10-amp) and are marketed to reduce alternator noise. I've recommended these over the years with surprising results.

When You Wonder Where the Yellow Went

Is there any way that I can clean the black mold/mildew spots and streaks out of my yellow Marinco 50-amp power cord? White vinegar doesn't work.

Tom Neale: First, it's always a good idea to ask the manufacturer for recommendations. If your cable is new enough and still supple with an otherwise good and intact surface, try one of the several power-cable cleaners on the market. We use Starbrite's Power Cable Cleaner (, although we can't recommend any one product over the other because we haven't tried them all (thank goodness). Try a product intended for this purpose before you use anything more harsh. This product did remove most of the dark stuff and mildew on our old cable.

We've also used bleach mixed with boat soap and water and a scrub brush to remove the dark stuff. I've observed other people use Soft Scrub (with bleach) and a stiff brush for this purpose, with some success. You should unplug the cable and remove it from the boat and place in a safe spot for cleaning with any of these items, away from anything that would be harmed by bleach or other chemicals. Wear heavy rubber gloves to protect your hands and follow all instructions on the packaging of the product. If you use any other cleaner besides the Starbrite Power Cable Cleaner, it may be a good idea to give it a final cleaning with this product to take advantage of the protective polymer coating it gives.

Clear Instructions

I need to install a new windshield on the command bridge of a 28-foot Bayliner. How can I drill holes in the new Plexiglas windshield without breaking it?

Don Casey: You'll need bits intended specifically for drilling plastic. These have a shallower point angle, typically 90 degrees. Regular twist drills generally have a 118-degree point angle, which cuts faster but risks chipping and even cracking plastic. I say bits, plural, because it will also be essential to drill pilot holes, then enlarge them. This is necessary for two reasons. First, you need to be deadly accurate with the location of the holes. If you are a little off with a hole in wood, the fastener shoulders itself left or right to fit, crushing the wood on one side of the drilled hole. With plastic, if the fastener is not in the center of the hole but pressed hard against one side, the plastic will not compress and cracking is likely. So you must mark the centers of the holes accurately, then drill a small pilot hole centered exactly on each center mark.

You can drill the hole to size with your second drill bit, but enlarging the holes in steps, perhaps 1/16 inch larger with each pass, will both help to keep your hole from relocating slightly to one side and reduce the risk of chipping. If the acrylic is thick, spritzing a little water on the bit and into the pilot hole will both lubricate the bit and cool the plastic to prevent melting and binding. Be sure to have solid backing beneath the hole location so the exiting bit drills through rather than breaks through the bottom.

The final hole must be at least 1/16 inch larger than the fastener (if the fasteners are 1/4-inch, the holes must be 5/16-inch or larger). This is because the plastic expands and contracts, so the distance between the holes will change with temperature. If the fasteners are a tight fit, this movement will crack the plastic, but if the holes are oversize, the fastener does not stress the plastic.

I always chamfer both edges of the final hole slightly by gently applying a countersink bit. This relieves potential point stress concentration at the hole edges and it provides extra surface area for sealant. If your fasteners have flat or oval heads, they must never rest against the plastic. You need finishing washers to cradle the screw heads and transfer the loads to the plastic around the hole. I've never had acrylic crack on me at drilled holes. Use the right bits, take your time, and you can expect the same result. 

— Published: April/May 2011

Meet the Experts

Photo of Don Casey

Don Casey
He's been one of the most consulted experts on boat care and upgrades for 30 years, and a panelist on our "Ask The Experts" website for a 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, the bible for do-it-yourself boaters.

Photo of John Adey

John Adey
The president of the American Boat & Yacht Council (ABYC), John grew up boating. He's 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. John is a trusted source for technical information for industry professionals.

Photo of 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, written 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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