Installing Thru-Hull Exhaust: A Step-By-Step Boat Guide

Installing a thru-hull exhaust system on a boat can improve its performance and increase its engine's life expectancy. Boat owners need to consider the various benefits and drawbacks of such an upgrade. For example, while a thru-hull exhaust system can increase performance, it may also decrease the longevity of the engine. There are also local noise laws to consider. The installation process itself can be complex, with boat owners needing to carefully measure and cut holes in the hull of the boat.

Cutting holes in the hull

Before you start cutting, it is crucial to select the right location for the thru-hull exhaust. Choose a spot that provides good access to the thru-hull and ensures that the seacock can be reached quickly in case of an emergency. Make sure there are no obstructions like bulkheads or frames that restrict the operation of the seacock handle. It is also recommended to avoid placing the thru-hull in front of a depth transducer or knot log, as they can be affected by the turbulence generated.

Once you have identified the ideal location, mark the centre of the thru-hull and drill a small pilot hole from inside the boat. This step allows you to double-check the position on the outside of the hull before proceeding. Many technicians recommend using tape when drilling into fibreglass to prevent chipping.

Select a hole saw that matches the diameter of your thru-hull or is slightly larger. Ensure the hole saw has a pilot drill bit to prevent the tool from walking during drilling. Start drilling from the outside of the hull, using the pilot hole as a guide, and maintain the drill perpendicular to the hull surface. Drill until the pilot bit penetrates the hull, then finish drilling from the inside. If space constraints don't allow drilling from the inside, continue drilling from the outside, being cautious to control the bit once it goes through the hull. Smooth any rough edges around the hole with sandpaper.

If your boat has a cored hull, you will need to excavate the coring around the hole. Remove the coring to a depth that covers an area larger than the thru-hull or seacock flange. Fill this void with thickened epoxy and let it cure before moving on. This epoxy compression ring provides a sturdy base for the thru-hull and prevents water infiltration into the core.

It is advisable to use a backing plate to provide additional support to the hull and create a flat mounting surface for the thru-hull nut or seacock flange. A fibreglass backing plate glassed into the hull is ideal, but marine-grade plywood is also commonly used and acceptable. The backing plate should be 2 to 3 inches (5.08 cm to 7.6 cm) larger in diameter than the thru-hull nut or seacock flange. Cut a circular backing plate from 3/4-inch (1.91 cm) marine plywood and drill a central hole using the same hole saw you used for the thru-hull. Shape the backing plate with a sander or wood rasp to match the hull's curvature, and mark it for proper orientation later.

Remember to seal the exposed wood with a wood sealing product if your transom is constructed from wood. Additionally, if you plan to plug and leave the existing Y-pipe, cut a plug from a standard 2x4 piece of wood, seal it with resin, and then install it.

Sealing exposed wood

First, it is important to prepare the wood surface. Use sandpaper to rub off any previous finish or residue on the wood. You can use different grits of sandpaper, starting with a coarse grit to remove any stubborn residue, and then moving on to a finer grit to create a smooth, uniform finish. This step ensures that the sealer will adhere properly to the wood.

Once the wood is prepared, it is time to apply the sealer. Choose a sealer that is specifically designed for marine use, as it will be able to withstand the harsh conditions of the marine environment, including UV rays, water, and humidity. Follow the manufacturer's instructions for applying the sealer. Typically, you will use a paintbrush to apply a uniform coat to the wood, ensuring that you get into any nooks and crannies. Allow the sealer to dry completely according to the instructions.

After the sealer has dried, it is important to let the wood cure. Curing allows the sealer to fully harden and create a durable barrier against the elements. Even if the sealer seems dry to the touch, give it some extra time to ensure that it is fully cured. This will help prevent any water or humidity from seeping into the wood.

Finally, remember that sealing is not a one-time process. To maintain the protection of your boat, you should inspect the wood and sealer annually for any signs of wear, water damage, or UV damage. If necessary, re-apply the sealer to keep your boat in top condition. For extra protection and a longer-lasting seal, you can also apply a topcoat that is compatible with your sealer.

Measuring for space

Boat Hull Measurement:

Before installing a thru-hull exhaust system, it is essential to measure your boat hull to ensure you have the required space. This measurement will determine if you can install a 'REAR EXIT' thru-hull exhaust or if you need to opt for a SIDE EXIT due to space constraints, such as the presence of a swim platform.

Swim Platform Measurement:

If your boat has a built-in or bolt-on swim platform and does not already have a thru-hull exhaust, you'll need to take the following measurements to assess the feasibility of installing a standard system or the need for a custom one:

• Measurement A: Determine the distance from the top of the drive (gimbal) housing to the underside of the swim platform.
• Measurement B: Measure the distance from the vertical center line of the drive (gimbal) housing to the upper corner of the drive cavity. For bolt-on swim platforms, measure to the first mounting bracket.

Exhaust Hole Measurement:

If your boat already has a thru-hull exhaust, the following measurements will help determine if the exhaust holes are in the standard location or if a custom system is needed:

• Measurement X: Measure the distance from the top of the drive (gimbal) housing to the horizontal center line of the exhaust hole.
• Measurement Y: Measure the distance from the vertical center line of the drive (gimbal) housing to the vertical center line of the exhaust hole.

Diameter Measurement:

When measuring for thru-hull diameters, it is important to note that pipe measurements are typically based on the inside diameter. If you are replacing a thru-hull, measure the outside of the threads and select a hole saw of the same size. If you are unsure, it is recommended to test the fit by drilling a 1/4-inch hole in a piece of plywood and trying the thru-hull fitting.

Additionally, when ordering thru-hull exhaust components, be prepared to provide the following information: boat year, make and model, engine size, drive model, and whether it is an EFI or carbureted engine.

Selecting the right exhaust tubing

Exhaust Tubing Diameter:

The diameter of the exhaust tubing is important to ensure proper airflow. The most common size mentioned in forums is 4" exhaust tubing, which seems to be a standard size for many boats. However, it's important to measure and confirm the correct diameter for your specific boat and engine.

Materials:

Choose exhaust tubing that is made of high-quality, corrosion-resistant materials such as stainless steel. This will ensure the longevity of the tubing in the marine environment.

Mufflers and Flappers:

Consider adding mufflers and flappers to your exhaust system. Mufflers can help reduce noise levels, especially if you plan to operate your boat in areas with noise restrictions. Flappers, also known as exhaust valves, prevent water from entering the exhaust pipes and causing problems. Look for stainless steel flappers that can be sourced from suppliers such as Eddie Marine or CP Performance.

Silent Choice Exhaust:

If noise is a concern, you may want to explore the option of a silent choice exhaust system. This setup allows you to switch between open and silent exhaust modes, providing the best of both worlds in terms of performance and noise reduction. However, keep in mind that some camshaft changes may not be compatible with silent choice setups.

Hole Placement:

Before installing the exhaust tubing, carefully measure and mark the transom for hole placement. Take into account the angle of the exhaust risers and the distance to the waterline. Remember to "measure twice, cut once" to avoid mistakes.

Blocking the Y-Pipe:

When installing a thru-hull exhaust system, don't forget to block the Y-pipe hole. You can do this by cutting a plug from a piece of wood (such as a standard 2x4) and sealing it with resin before installing it.

By carefully considering these factors and choosing the right exhaust tubing, you can ensure optimal performance, noise reduction, and the long-term health of your boat's engine.

Installing a plug

However, if you are looking for a more affordable option, you can create a drain tube by installing a PVC pipe that allows a standard 1" marine plug. To secure the inner pipe, use all-purpose cement for PVC, and seal the fitting from the outside water with automotive goop. This method is suitable for smaller boats. If you are installing a plug on a small dinghy or tender, you can also purchase a boat bung online, which is easy to install and fairly inexpensive.

Now, let's walk through the installation process. First, you need to find a suitable location for your plug. Make sure it is easily accessible and does not interfere with any other components of the boat, such as a depth transducer or knot log. Mark the centre of the plug location and drill a small pilot hole from the inside of the vessel. This will allow you to check the position from the outside of the hull before cutting a larger hole.

When you are ready, use a hole saw to drill the hole for the plug. The hole saw should be the same diameter or slightly larger than your plug, and it should have a pilot drill bit to prevent the tool from walking. Start drilling from the outside of the hull, maintaining a perpendicular position to the hull surface. Drill until the pilot bit penetrates the hull, then finish drilling from the inside. Smooth any rough edges of the hole with sandpaper.

If your boat has a cored hull, you will need to excavate the coring around the hole and fill the void with thickened epoxy. This will create a solid base for the plug and prevent water from entering the core. Allow the epoxy to cure completely before proceeding.

Finally, install the plug according to the manufacturer's instructions, ensuring a watertight seal.

Frequently asked questions