For a litany of reasons, you may have to consider replacing a boat’s engine. Sometimes a lack of maintenance from a previous owner finally catches up and there is a catastrophic failure. Sometimes the water pump fails, the head gasket blows, the exhaust melts and the engine seizes. Perhaps you feel like the boat is underpowered or losing power.
There’s a lot to consider when changing out an engine. Here are some things to think about to ensure that your repower is a smooth one.
Which Engine?
In the event of an unexpected failure, the best-case scenario is that there is still a direct replacement engine available, or at least a close update. This scenario will require the fewest changes in your engine room, assuming everything else is in good shape.
If you are increasing horsepower or using a different engine, we’ll have to do quite a bit of measuring and calculating. Substantial weight or horsepower changes might also merit engaging a naval architect to avoid problems.
Make a list of the positives and negatives of each possible replacement engine. Horsepower, weight, physical dimensions, fuel efficiency and parts availability are all important considerations. Some engines are short and squat, while others are long and tall. One may not fit without drastic changes to the space, while the other may minimize extra work.
Also include dealer responsiveness in your thinking. If it takes 10 phone calls and emails to answer a question about an engine, or even to get a price on it, look elsewhere.
Changing Gears
As you compare possible replacement engines, you’ll need to know the current engine’s transmission ratios and transmission angle or offset. These can sometimes be a challenge to locate on the engine or transmission, but if the manufacturer’s number plate is legible, you can usually find the information online or from the manufacturer.
Transmission ratios are expressed in a format such as 2:1. The larger the ratio, the slower the shaft speed. A larger prop diameter (assuming it will fit) will give more thrust and boat speed. The smaller the ratio, the faster the shaft speed. Typically, a smaller propeller diameter will give less thrust and boat speed.
The transmission angle will be expressed as degrees (different from the crankshaft) that can sometimes allow an engine to sit closer to parallel to the waterline. A transmission offset could drop the output shaft lower in the boat, allowing a prop shaft to exit the boat closer to parallel to the waterline. The greater the shaft angle, the more thrust is wasted pointing down toward the seafloor instead of pointing aft to propel the boat.
Staying In Line
Typically, you’re not going to want to change the shaft angle on a repower. It’s important to make sure your new engine will line up with the shaft as it exits the shaft log.
As you measure the dimensions of the new and old engines, it may be necessary to shorten the current shaft or have a new shaft made to accommodate a longer or shorter engine. The shaft coupling may also need replacement if the transmission coupling is a different size. Any new coupling should be fitted and faced by a machine shop on a lathe, which confirms that it is perpendicular and centered on the shaft.
If horsepower is increasing, then the shaft diameter will need to be assessed to be sure it doesn’t need to be larger, which would, in turn, likely require a new shaft log. (Replacing or resetting shaft logs and struts can make a repower significantly harder.)
Out with the Old
Getting the old engine out of the boat and the new engine in can be as difficult as remembering the virtues of our enemies.
Many engines are set in place before the boat’s deck is installed at the factory. Often, there is a hatch over the engine, but no opening that’s large enough to remove the engine in one piece. Breaking down an old engine for removal isn’t so bad, but no one likes to disassemble a brand-new engine. Of course, it can be done, and every removed part must be kept pristine and reassembled to the manufacturer’s torque specs with fresh gaskets.
In some cases, it may be better to cut a temporary hole in the cabin top or remove a window to facilitate the swap.
In preparation for the engine removal, install protective moving blankets, plywood or other cushioning material to any soles or woodwork. Avoid applying tape to interior varnish, as it can remove the varnish when the tape is pulled. In some cases, you may need an A-frame with a chain fall or hoist to lift the old engine into the cabin, and then plywood ramps or rollers to slide the engine to a place where a forklift or crane can reach it.
Drain the coolant and seawater from the engine. Label, disconnect and plug fuel lines. Remove the exhaust. Label the wiring, and then disconnect and cover the terminals with electrical tape to prevent arcing. It may be necessary to temporarily connect removed DC negatives if other DC loads on the boat will be used during the replacement.
On removal day, have enough people ready to ensure safety. Engines are heavy and awkward. Discuss the plan with everyone, including the forklift or crane operator. Have a pallet and blocking set up to receive the old engine, as engines typically should not rest on their oil pans.
It is always satisfying to successfully place the old engine on the ground outside the boat with everyone injury-free. If the old engine has been causing you stress for a while, here’s your chance to give it a kick (but don’t hurt your foot).
Time to Clean
With the engine removed and the compartment cleaned, repainted or re-gelcoated, now is the time to satisfy the requirements of the new engine. Study its installation guide for the diameters of plumbing for the fuel, raw water intake and exhaust (a new engine cannot be certified for the manufacturer’s warranty if any of them are undersized). If the hoses in the boat are more than a decade old, replace them.
Note that water-cooled exhausts have strict guidelines regarding back pressure, and particular requirements related to the height of exhaust elbows and water-lift mufflers relative to the waterline. Failing to meet these requirements can void a warranty, or can flood and ruin an engine via seawater traveling backwards in the exhaust, draining into the valve train and pistons.
On boats with dry exhausts, inspect the current exhaust stack and confirm that the diameter and heat insulation/air gaps are adequate for the new engine. All keel coolers should be assessed for the proper amount of heat transfer for the new engine. Access to these components will likely never be better, so replace anything that is in question.
Engines must pull their combustion air from somewhere, and typically work better with positive pressure in the engine room. Usually, this is from vents on the hull or cabin side, and from blower arrangements. The original boat designer may have incorporated the vents holistically into the profile. These vents must be the proper volume to handle the engine air intake, as well as have some baffling to prevent water ingress. Proper airflow also helps mitigate excess heat. If your new engine requires more volume, the vents (and the blowers) may need to be enlarged.
A New Bed
Next, prepare the engine beds. If it’s necessary to change the motor mounts’ location on the beds, then old holes will need to be filled, and the new area will have to be checked to be sure it can receive fasteners. In some cases, beds will need to be cut down in height or relocated.
Engine beds can be made in a few ways. They may be fiberglass-
covered but cored with hardwood, solid fiberglass board, Coosa board or foam. The coring may or may not hold a fastener. In some cases, metal plates are glassed over in the construction and tapped to take a fastener or brackets that may have through-bolts. Some beds have pockets to accept the fasteners. New metal brackets may need to be custom-fabricated.
When the motor mounts are set up correctly and the prop shaft is aligned, the motor mount feet and height adjustment screws will be centered in their adjustment. This setup allows for future engine alignments as motor mount rubber sags, cutlass bearings wear and the boat’s shape changes. Remember: A properly aligned shaft coupling to transmission coupling only permits four thousandths of an inch of misalignment. It doesn’t take much movement to exceed this.
Planning for proper alignment can be challenging without the new engine in place, unless there is a close engine template. Some manufacturers have lightweight template engines for rent or borrow. A good carpenter can make up a wood template based on the drawings or measurements from the actual engine.
Check the cold cranking rating for the engine battery to be sure your current battery will satisfy the engine’s starting requirements. Inspect the cabling and replace anything undersized or corroded. Think about the charging requirements of your battery bank. Now may be the time to install a larger alternator with a more sophisticated external regulator.
If the engine is not a drop-in replacement, chances are the engine gauge package will also change. Most new engines are digital and come with a variety of displays or the ability to share information on a multifunction display. This change will require replacing the engine wiring harness. Switching from older, individual gauges can slim down the harness substantially (it is also satisfying to remove obsolete wiring in a jam-packed chase).
While we’re pulling wires to the helm, note that newer engines may also be suitable for digital throttle and transmission controls. This means the stiff control cables may be removed as well.
You may also need to find space on an engine-room bulkhead or under the helm for a remote engine computer or two. Since the gear at the helm may substantially change, give some thought to the layout so you can see the information and comfortably reach the controls. You might need to cover the old holes and refresh the area.
In with the New
Now that the engine room is refreshed and the engine beds are ready to accept the new engine, it’s time to lower the engine into place. If you measured twice, the motor mounts can be fine-adjusted to match the shaft coupling and bolted down. Continue around the engine, connecting raw water, exhaust and fuel.
If you replaced fuel lines, an oil-removal vacuum pump works great to pull fuel through the lines to pre-bleed the system.
Connect the electrical cables and wires, the engine harness, and the shift and throttle. Then, check every connection again. Check all hose clamps. Make sure fuel valves and battery switches are turned on. Open the seacock. Check the oil, transmission oil and coolant levels. Most engines come with a pre-start checklist and procedure. The point is to be sure everything is ready to go before you make an expensive mistake with your new engine.
Once you are sure, start the engine in the slip and let it idle while you inspect again for leaks. Go over the engine with a touchless pyrometer to check for hot spots. It is not unusual to have air pockets in the freshly replaced coolant; this may not cause an issue until the engine is running faster under load.
When everything looks OK, it’s time for the trial run. Again, most engines come with a procedure to start the warranty. You’ll be looking for proper oil pressure, no signs of overheating, proper exhaust back pressure, and a host of other measurements. Electronic engines will track most of these parameters automatically.
One of the most important things to check is that the engine turns up to its rated rpm, but not over. If something different occurs, then the problem will need to be tracked down. The most likely issue is improper propeller pitch.
Trouble No More
Reliability is the name of the game. It is comforting to know that your engine will start when you need it, and will carry you to your destination without a hiccup. Getting a few extra horsepower doesn’t hurt either for punching into a head sea or making it through an inlet before a storm.
After reveling in your newfound power and dependability, turn your attention to maintenance so you can keep your new engine running smoothly.
This article was originally published in the September 2023 issue.
