Posts: 3508
Location: Elk River, Minnesota | You can mount a 20 inch shaft on a 16" transom, but with a 4 stroke, there is a major word of caution in doing so...
First and foremost, when the powerhead and oil reservoir is closer to the water, you run more risk of water intrusion to main engine block, which will ultimately end the life of the motor. This is why many of the larger boats out there are now going with a 25" transom as compared to the popular 20" and from years ago, the 15"
Secondly, make sure that the boat has been constructed with 4 stroke engines in mind. I am not sure about Nadens, but many boat manufacturers have added extra floatation in the rear of the boat in order to pass USCG requirements for level floatation in the event of swamping the boat.
Now..in your case with a 100 pound motor, floatation may not be an issue, but I can tell you this...mounting a 20" shaft motor on a 16" transom will be very lethargic on getting the boat up on plane, and will be more apt to hit underwater objects since the shaft is 4 inches lower in the water. The more lower unit in the water, the slower and more inefficient the motor will be...you will use more gas in order to run the boat because of the excess drag of the extra 4" of lower unit in the water.
Now..there are options, though, too. Many marine dealers will carry some sort of plate that will allow you to run a 20" shaft motor on a short shaft transom. They range from having a set-back of 4 inches or so from the transom, to no setback at all...esentially raising the motor straight up off the transom. For a Naden, my suggestion would be a plate to mount the motor straight up off the transom. Then, add wave wackers to help keep excess water at bay in the rear in the case you are backtrolling in some wind, or running with following seas.
Even though you are running a small boat with a small HP motor, I wouldn't be surprised if the rear of the boat "squats" a bit due to the extra weight of the 4 stroke. a 35 hp 2 stroke weighs about the same, though too...
Good luck with your choices.
Steve |
