its for science. l need help to do it

Think of how many times you would have to turn a small wheel to get the same result as a half turn with a large wheel. Get it?

besides Max is answer as well, back in the old days captains always stood, therefore they needed a large enough wheel for the captain to turn and use while standing upright, hence the need for a large wheel

Ever since the invention of power steering, it is been a matter of tradition. Before that, it was needed for leverage.

For mechanical advantage to move the large rudder.

The only reason a ship has a large wheel is the tradition of years ago. Any modern ship, built in the last 40 years or so, will not use a wheel for steering. Some boats still use a mechanical wheel and linkage for the purpose but even fishing boats in the 80 foot range use electric/ hydraulic systems that are controlled with a very small lever about 6'' long.

Tom has it right .Max has it all wrong . one turn on a huge wheel is 360 degrees , one turn on a small wheel is also 360 degrees. It is for mechanical advantage - leverage - to turn the rudder .. in the bad old days, the rudder was turned by a set of lines wrapped around an extension of the spindle of the of the wheel & then to the rudder quadrant at the top of the rudder . the rudder of large sailing vessels weighed hundreds & even thousands of pounds on the largest sailing boats & on early ships. They needed all the leverage they could get.

Old ships have large wheels, not new ones. The ships with large wheels use them for leverage. Newer vessels use electric/ hydrolic steering steering is done with short, horizontal joysticks. Never ran a boat with a big wheel.