A propeller on your boat is the equivalent of a transmission in your vehicle (albeit a single speed transmission).
If you try to accelerate away from a stop light in 3rd gear, it will take a long time for the vehicle to reach speed. If you're towing your boat (additional load), it will take even longer. Conversely, if you try to drive around in first gear, you'll have a hard time reaching the legal speed limit.
In a boat, the difficult part is finding a single effective gear ratio that works well for low speed pulling, but doesn't completely kill your top speed. Most boats are shipped with a propeller that is better suited for mid range and high speed pulling power (a higher gear in your vehicles transmission), so when you add a lot of ballast, the engine is overwhelmed and the boats acceleration suffers.
To find the appropriate application for your boat, check out our propeller selector guide