I want go over some of the most common mistakes I see in the field and how to avoid them. These don't only apply to DIY repairs and installs, I have run into them on boats fresh from the factory. Hopefully these will help you spot issues on your own boat or make your next project get done right.
Do
Use the right size pilot hole and countersink all screw holes in gelcoat and fiberglass. A quick tip for the most common size screws found on boats: For #6, #8, and #10 screws, I use a 7/64”, 1/8”, and 9/64” drill bits respectively for most pilot holes in fiberglass. The harder and thicker the material, the larger the pilot hole needs to be and vise versa for very thin material. Always start out small and go bigger if needed. Also you don’t need to bust out a separate countersink bit, just take your Phillips bit thats already in your drill/driver and run it in the hole for a second. This does two things: it puts a bevel in the gelcoat and prevents it from chipping and cracking around the hole, and creates an area for sealant to sit and create a gasket against water intrusion. This step is so easy, yet I rarely see it done on most production boats, and they all have ragged, chipped edges around hardware when you remove things.
Don’t
Use the wrong tool to crimp heat shrink connectors. Heat shrink connectors are designed to keep the water and salt air off the wire connections. This doesn’t work if the heat shrink coating is pierced or not properly shrunk down until the glue seals the wire. So don’t use the crimpers that have the plunger on one side, most crimpers have an area labeled INSUL (for insulated), use that and don’t crush it so hard that the insulation splits. Next, heat it evenly with a heat gun or small torch until you see the adhesive lining ooze out the ends and you're done.
Do
Secure bilge pump wiring and connections as high as possible. This ties into the above. The most common cause of bilge pump failure is corroded butt connectors and the adjacent wire caused by people making these connections, assuming they are waterproof when in reality they are not due to the above mistakes and letting them hang down where they can be splashed or even fully submerged in bilge water. Over time the connections take in water and the wire wicks it up sometimes up to many feet away, destroying the wire from the inside out. Even if these connections are done correctly, they should be secured up as high as possible. Float switches fail all the time, and water levels will occasionally get higher than expected. If your bilge pump or float switch quits on you, don’t rush out to buy a new one check your wiring first.
Don't
Use wing nuts on battery posts. It is against ABYC standards and a bad practice to rely on wing nuts to secure lugs to batteries; you simply cannot properly torque them. This is made even worse when multiple lugs are stacked on top of each other on one post. ABYC standards limit it to a maximum of four connections per post, but an even better practice is to install an unswitched positive and negative bus bar near the battery to land all 24-hour circuits.
Don’t Use undersized wire. Another thing I see all the time from DIY installs is not using the correct gauge of wire for a given circuit. Either the wire isn’t rated for the amount of amps required or the run is so long that voltage drop is excessive. If you are unsure just use one of the ABYC wire size calculators or tables available online, don’t guess. When making these calculations remember that cable run length is for both ways (double the length from the battery to the equipment) and always go to the next size up if it is borderline.
Do
Use flush cut nippers to cut tags off zip ties. When you use a pair of diagonal cutters, scissors, wire strippers or any other tool besides a small pair of flush cutters to snip off a zip tie end you leave a very sharp little nub that will cut you or the next guy who brushes up against it in the future. I have actually seen and received some impressive wounds from these little things. It’s a cheap tool that you will use all the time so go get one and keep your blood on the inside.
Well that’s enough for now, hope this short and definitely not all inclusive list helps you on your next boat project.