Reduce resistance or increase propulsive efficiency

To improve top speed it is better to reduce the total resistance or increase the propulsive efficiency....?


If my boat runs parallel to water I will have more drag but at the same time I will have better propulsive efficiency because the inclination of thrust is near 0 deg....

 

YES!

 

Ranchi,

The simple answer is; both.

But, I suspect you want something more quantifiable, so, I'd start by looking up the efficiency of the propeller you're using in a specific case, and then figure out where, and how much of the energy used, is spent, and where the easiest gains are to be had.

That approach might be of some help in figuring out where the lowest-hanging fruit is, so to speak.

Yoke.

Yoke.

 

Resistance has a limited amount of return after the usual suspects are nailed down, leaving you with your answer.

 

It ain't that simple

Formula One boats decrease total resistance by increasing aerodynamic drag -- i.e., flying the boat.

 

There are craft that have taken "ground effects" to the ultimate in literally flying over the water, after reaching takeoff speed and piling up enough pressure under it's reverse dihedral wings. Truly floating above the water, not enough lift to fly, but enough to float over the ground effect under it. The comment still applies, resistance has a limited return after a point, and propulsion is the answer when talking about increasing top speed. Besides, would you want to be flying your boat when you trip over a wake you didn't see? Personally I'd rather chew my steak, then suck it through a straw.

 

Efficiency

Thats a very big headache for all boat designers because almost always, there is a compromise.

Yes, you have to do both.

On paddle power, say a canoe, reduce resistance by having the smallest surface area. Translates to a narrow beam.

When you paddle faster, say in a competition, reduce the wavemaking property of the boat by using a longer boat.

When powered by an engine(going faster) Stay within the limit of Hull Speed or Speed to Length ratio. Use the Biggest prop and Deep gear ratio for maximum efficiency. Dont overdo it because you will have all prop sticking out the bottom.

Still going faster and beyond hull speed, use semi displacement hull with low wavemaking characteristics. Propulsion efficiency will go down a little.

Want to go faster on a small boat, use a planing hull. The bow will rise up a little. You need a little angle to gain lift. The thrust angle helps to lift the bow up so it is beneficial.

Use "thin ships" or catamaran to reduce wave making and less hull resistance. For moderate speed.

Use SWATH hull at moderate to higher speed. Less wave resistance.

Use a submarine. they are very efficient at high speed. (not At low speed)