Boat Craft Q&A: Shock Wave

by David G. Brown

Q: We recently moved from Lake Michigan to the Chesapeake Bay. This will be our first year boating in salt water, and we want to make sure our 24-foot sterndrive boat is ready. I’ve been told that I need to change my zincs. Why should I do this, and what should I purchase?
— Phil Beard, Baltimore, MD

A: ou’re right about needing the correct type of galvanic protection. Don’t be confused by the use of the word “zinc” to refer to all types of anodes. This goes back to the time when only zinc was used for that purpose. These days, sacrificial anodes are made
of zinc, aluminum and magnesium.

True zinc anodes are intended for boats used only in salt water. Aluminum is the better choice for boats used in brackish water (part fresh, part salt, as in tidal estuaries). Magnesium anodes are strictly for use in fresh water like Lake Michigan. The reason for using different materials is the rate at which the various metals react in water. It takes an active metal like magnesium to protect boats in fresh water. Salt water’s corrosive nature makes a more passive metal like zinc the obvious choice. However, don’t be surprised if your freshwater boat has true zinc anodes. Many boat builders install zinc anodes on all boats to make sure those that wind up in salt water get the maximum protection. If you’re positive you have true zinc anodes that are in good condition, leave them.

If uncertain, or if the old ones are in bad condition, replace the existing anodes with the proper metal. There are lots of places to look for anodes on an I/O boat: on the outdrive gimbal ring, under the anti-cavitation plate, in the lower unit’s shell, or as part of the propeller nut assembly. Traditional inboard boats will have anodes on the propeller shaft, strut and even the rudder. Also, don’t forget to replace the so-called “pencil zincs” in the engine block. These are often overlooked, because from the outside, they look like a cap screw holding the powerhead together. Check your owner’s manual for their locations on your particular engine. The rule of thumb is to replace old anodes when they’re two-thirds gone.

Q: My 1988 fishing boat still gets my family and me around. The only problem with the boat is that the deck hatch between the driver and passenger seats is so soft, you feel like you’ll fall through when you step on it. The fiberglass is also cracking. What can I do?
— Will Cubberlee, Hoffman Estates, IL

A: The hatches of your boat have a fiberglass top shell to match the deck. Inside, they contain a structural foam core for rigidity. Although extremely strong when new, every time somebody stepped on the hatch, it caused a little bit of flexing. Eventually, the friability of the foam allowed it to fracture at the bond with the fiberglass of the hatch. Now that bond is broken, even though the bulk of the foam remains intact. As a result, the original rigidity is lost, and the hatch flexes when you step on it. Fixing this problem is easy if you have the right power tools and skills to use them. First, cut away the fiberglass skin on the underside of the hatch. This exposes the old foam so it can be removed. I use a laminate trimmer with a straight cutting bit to remove the underside skin. The trimmer is really a miniature router that fits into tight corners. Another tool that does the job is a rotary cutter (e.g. RotoZip).

The secret is to use a small-diameter straight cutting bit, set so it goes only deep enough to cut through the underside skin and no deeper. You don’t want the bit to dig deep enough to damage the fiberglass shell that forms the top of the hatch. Never cut fiberglass without proper eye protection and a respirator. Gloves and long sleeves are also suggested. Dust-size pieces of glass get into unprotected human skin causing something known as “glass itch.” There’s no cure other than to let the glass work its way back out.

Cut fiberglass easily using a power tool like the RotoZip rotary cutter.

Removing the underside skin and the foam is a destructive process. Don’t try to save anything except for the top fiberglass shell. Use a scraper to get rid of most of the old foam core. The rotary cutter tool will leave a small lip of old inner skin around the edge of the hatch. The foam trapped between this lip and the hatch shell must be pulled out with a chisel or screwdriver tip. Finish the removal job by sanding the inside of the hatch shell smooth.

You can spend the time and effort to locate new structural foam for the core. An easier and cheaper alternative is a piece of marine plywood. You’ll need a piece twice the size of the hatch. The plywood should be one-half the thickness of the old foam core. Cut one piece of plywood large enough so it’ll fill into the empty groove between the lip and the front face of the hatch. Since plywood cannot be bent or folded, this piece must be cut into four pieces to be fitted in place. Cut off the left and right sides, and glue them into the shell first. The remaining center section is cut in half horizontally so it can be installed in two pieces.

Don’t worry about cutting this first layer of plywood. A second piece of plywood cut to size will cover the resulting joints and restore full strength to the hatch. Polyester resin can be used to make this repair. It’s easier to use and doesn’t give off noxious fumes. Read and follow the resin manufacturer’s instructions. Always wear eye and skin protection. The four pieces of the first plywood layer are bonded into the hatch shell using resin thickened to a peanut butter consistency with colloidal silica. Fill any gaps between the wood and the old underside shell’s lip with plenty of thickened resin.

Place the hatch facedown on a flat surface covered with polyethylene sheeting (a painter’s drop cloth). Once the wood is glued in place, cover it with more clear plastic sheeting and weigh it down with a cement block. The plastic prevents bonding the cement block to the hatch or the hatch to your workbench. The block’s weight makes sure the hatch and plywood stay flat while the resin fully cures (at least eight hours). Cut the second piece of plywood to match the exposed area of wood inside the line of the lip. Round over the edge of this wood, and then glue it in place with more resin. It can be weighted with a block to cure, or you can screw it in place with #10 wood screws. The reason for using such large diameter screws is they can only be as long as the thickness of the double layer of plywood. Bigger threads hold better.

Screwing the second layer allows you to move directly to covering the underside of the hatch with a new layer of fiberglass cloth. You don’t have to wait for the glue holding the wood together to cure, since the screws are left in place. Finally, use resin and fiberglass cloth to cover the wood and waterproof the repair. Rounding over the second layer of plywood allows you to lay in the cloth with fewer wrinkles or air pockets. The cloth should overlap and bond to that lip, but not have contact with the outer hatch shell. Careful work here will allow the hatch to fit properly in the boat. A single layer of 6-ounce fabric is sufficient. Let the resin cure for several days before painting. If you use epoxy, wash off the amine blush with diluted white vinegar in warm water. Otherwise, the blush will prevent the paint from ever drying. Polyester resin that cures in open air contains wax that comes to the surface. Remove this wax with a solvent before painting. Wash all clothing exposed to fiberglass dust separately from other items to prevent cross contamination.

Q: My 22-footer has a 70-hp outboard from the 1990s. The engine is equipped with a Doel-Fin, which I installed several years ago and haven’t had any problems with until I recently moved and now use my boat in the Piscataqua River. There’s a strong current on outgoing and incoming tides, so on those days with extra high or low tides, I try to run against the tide or current, but my outboard cavitates and overheats. Could it be because I just changed the water pump last year?
— Forum Member

A: If the Doel-Fin — a hydrofoil designed to improve performance — worked properly before, it should work fine now, unless you changed something. The last repair you made was to the water pump, which suggests the fin may have been knocked out of alignment when the lower unit was removed. If the fin is still OK, then the water pump is next on your list of suspects. A weak pump would cause the motor to overheat only under a heavy load, such as when stemming a tide. Did you just replace the impeller? Or, did you replace the whole pump including the housing, impeller and plate? A scored impeller plate (the shiny metal plate under the impeller) can prevent the pump from delivering its full output. Without enough water, the engine naturally gets hot. Another consideration is the copper tube that takes water from the pump up to the power head. This tube has seals (usually O-rings) at both ends. If the seals are bad, they can allow water to spurt out when the pump is delivering full pressure. Just like a weak pump, the result is too little cooling water in the power head. Another likely suspect is the thermostat, which may not be opening fully. It may pass enough water to cool the motor when power demands are low to moderate. But, a partially stuck thermostat may not allow enough flow to handle the heat generated under full load conditions. I’m not absolutely convinced the overheating problem is related to what you perceive as “cavitation.” You’ve just recently come to an area with strong tidal currents and aren’t experienced with the normal laboring of a boat pushing against that current. However, there are other problems that may give the impression of cavitation while also causing overheating — incorrect ignition timing or spark advance, or fuel starvation. Both of these will result in odd engine sounds and performance. They have also been known to contribute to overheating.

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