Couple of things.
1.) Possible spun prop. There is a rubber hub material that turns the propeller, but it designed to break away or slip if the propeller hits a log, rock or whatever. This feature was designed to protect the propeller shaft and drive shaft from being damaged. (Things to remedy that is a new prop or get the old prop rehubbed. (bring it to a local marina and they will know someone who does that sort of thing.
Here is a link to tell you how to evaluate if the prop is spun.
https://www.tinboats.net/forum/viewtopic.php?f=4&t=13786
2.) Make sure your motor is in the correct tilt position on the boat. There will be four or five rows of hole along the transom bracket. Plowing is when your motor is causing the back of the boat to lift up some and also causes the front to plow through the water. This creates a lot of resistance and very poor speed and cruising. Porpoising is when the back of the boat is pushed down some and the front of the boat is lifted in the air kind of like popping a wheelie on a bike. But much more subtle. It make it very hard to control the boat.
Make sure you have the correct tilt and trim of your motor. It is adjust manually. If you motor's transom bracket is in the first hole position, then adjust to second tilt position and see if your planing and ride is improved. Advance or decrease the tilt of the motor to find your smoothest ride and best performance.
You want to make sure your boat is neither plowing or porpoising.
3.) Is your motor a long shaft or short shaft. Also what is the height of your transom. Measure from the bottom of the boat to the top. Short shaft will go on a 15inch or so transom. A long shaft with go on a transom that is about 20 inches in height.
The anti-cavitation plate should be about even or an roughly an inch or so above or below the bottom of the boat for proper planing.
4.) What is the size of your boat and weight. Your motor may be undersized for your boat.
5.) Always check that you have spark on both cylinder, because those little motors will idle ok and run ok on one cylinder but will not have any power. It will run about half power. Always
Hope this helps you out.
