Review: Mercury 3.5hp outboard motor

I thought I would write a review of our Mercury 3.5 hp outboard motor that we use to power our 8’2″ high pressure floor inflatable dinghy. While this motor is not new to us, we have used it for the last 6 seasons, I am in the process of deciding if I want to purchase a larger outboard or even a larger dinghy. Check out my YouTube channel for a “5 minute review” of this motor where I start it for the first time of the season.

First the specifics. The engine is sold by Mercury Marine but manufactured by Tohatsu who also sells it under their brand and several others as well. The engine develops 3.5 Hp at 5,000 rpm, is a four stroke, single cylinder, and liquid cooled. The engine is carbureted and manual start and runs on regular 87 octane gas. It weighs 41 lbs with a 15″ shaft, a 20″ shaft is also available. Additionally, the engine has a forward and neutral shift but no reverse. Reverse is obtained by turning the motor 180 degrees, so the thrust is aimed forward pushing the boat backward. This reverse set up is not ideal but for a small motor and the weight savings, it is workable. The light weight of the motor makes it easy to move on and off the boat and transport in the car.

Gas for the engine is carried onboard in tank at the rear of the engine and holds about a quart. While this engine as expected is fairly thrifty on gas, not being able to connect a remote tank is a drawback. The engine can run for about 45 minutes at full throttle on one tank of gas. Throttling down to 3/4, extends run time to about an hour. I have run out of gas and had to be towed back to calypso which was over a mile away. To avoid this problem, I carry a jerry can of gas with us most of the time. This is also sub optimal because refueling in conditions other then flat calm risks spilling gas into the water. Interestingly enough, the 4 hp motor does have a connection for a remote tank. This engine however is larger and not based on the same block.

As far as pushing our inflatable dinghy, the 3.5 hp will not plane the boat with 2 or 3 people in it but does push it along respectably. With one person, the dinghy scoots along. If you are interested in speed this is not the set up for you. The motor is reasonably quiet and starts on the first or second pull. These engines do need to be maintained to keep them running well. Clearing the gas out if the carburetor when you are done for the day is the best way to prevent moisture in the fuel as a result of using E-85 from clogging the carburetor jets. In the 6 years I have owned the engine, I have had to replace the carburetor. I also replaced the internal fuel tank which cracked for some reason.

Other regular maintenance includes changing the oil annually which is very easy and the gear lube about every two or three seasons. This is also easy to do. The raw water impeller should be changed about every 50 hours of run time or three seasons. Changing the impeller is not particularly difficult but putting the lower unit back into the shaft and synching the shaft splines takes patience and practice. Other than that, the engine requires very little other than change the spark plug when you change the impeller. This motor has been reliable and easy to use. If you are looking for a low-cost engine to power a dinghy and speed is not important, then put the Mercury 3.5 hp on your list. It’s worth noting that this size motor does compete with the current electric outboards which also should be considered.