THROUGH TRANSOM

QUESTION: I have a MerCruiser 7.4-liter 454 Magnum engine in a 1996 Formula boat. I want to convert the exhaust to a through-transom setup. I am unable to find anyone around my area who knows much about this.

It looks self-explanatory, but just for comfort I am trying to find out proper installation instructions. Obviously I do not want to end up with water in my engine or with an exhaust system that may be getting too hot. Any information would be very much be appreciated.

ANSWER: It should be easy to add through-transom exhaust to your Formula. You will need to purchase a pair of stainless-steel 4-inch exhaust tips with flappers, about 4 feet of 4-inch rubber exhaust hose, some No. 72 all-stainless-steel hose clamps, and a few other items depending on how you are going to do the modification.

Currently, you have a pair of elbows that are clamped into place with exhaust hoses on either side that connect your exhaust manifold risers to the exhaust "Y" (sometimes referred to as a "bullhorn") that directs the exhaust out through your drive and prop hub. It is nearly impossible to remove the bullhorn and cap the drive exhaust passage without removing the engine from the boat. Instead, you can accomplish the conversion a couple of ways.

The inlets to the bullhorn are elliptical. A pair of stainless-steel tubes that are flattened slightly to match the shape of the inlets with a cap welded on one end can be clamped into place using the original connection hoses to serve as block-offs. If the existing bullhorn is not blocked off, or incorporated into the proposed through-transom exhaust system, water will pour into the boat through the openings.

In lieu of blocking off the existing through-hub system, you can install a pair of stainless Y-pieces that allow exhaust to exit through the hub and transom simultaneously. You may also take this opportunity to install a switchable system allowing you to choose from through-transom to through-hub exhaust with a switch at the helm. Mercury Silent Choice, CORSA Captain's Call and CMI Internal Sound Choice are a few of the systems that are common.

The transom tips are installed by drilling 4-inch holes in the transom in-line side-to-side and slightly lower than the exhaust riser outlets on your engine. The exhaust outlet hose should angle down at 10 to 12 degrees to ensure that water flows out and not in. The holes cut in the transom should be sealed with resin and the tips need to be sealed with a good grade of RTV silicone when installed. A combination of internal and external flappers are best to prevent water surging up the exhaust system from waves and wakes from the stern. External flappers are desirable since they are easy to inspect and service.

TRANSOM REPLACEMENT

QUESTION: Would you please recommend the correct material to use to rebuild the transom on my cabin cruiser? It has a 350 MerCruiser engine with a Bravo Two drive.

ANSWER: Unless you are a fiberglass and woodworking expert, I would think twice about tackling this job without help. In order to properly replace a rotten transom, you must first derig the boat including removing the drive, transom assembly, trim tabs, exhaust through-hulls and so on.

Usually, the fiberglass and wood are removed from the inside, leaving the outside layer of fiberglass and gelcoat intact. Once the transom is cleaned and prepared, layers of three-quarter-inch marine plywood are cut to a shape to fit the
transom area and are laminated into place. The plywood is clamped in place in order to assure adhesion to the existing transom. The next step includes applying layers of fiberglass to strengthen the installation. The inside surface and the outside surface of the transom must be parallel in order to facilitate a proper installation of the transom assembly and engine package.

—Need some guidance from Bob Teague, Powerboat's lead test driver and owner of Teague Custom Marine? To send him an email click here.

If you like what you're reading, get more by subscribing to Powerboat magazine here.