Efficiency at hull speed - does weight matter?

[bayview]

It often helps to consider an extreme case. Suppose you loaded the boat so that it sat two feet deeper in the water. wouldn't you expect it to need more power to reach the same speed. Adding just a bit of weight also increases draft and requires more power.

You will gain better efficiency at any loading if you forget everything you ever read about hull speed and go at a speed that produces no bow wave.

 

[mcarthur]

The shape of the hull can determine the difference, as well as the speed you travel. If your hulls is in the water, and is entirely slad-sided above the waterline, then there's a linear relationship between added weight and immersion (thank you Archimedes). Different hulls have different immersion rates - ours is about 200kg/cm (I think!). So 10k# = 5,000kg = 25cm. That would add more than 20% to our underwater area. For low speeds (say a 1.1 multiplier on hull speed rather than 1.34), I doubt it would change the consumption much. Towards displacement hull speed (1.34x), it would have more effect on consumption but probably (IMO) not enormously more than it would normally be. I think going much over displacement hull speed is where the consumption would suffer. But I can certainly imagine a designer of a SD or planing vessel designing it so that the extra weight could still reach a plane. I doubt they actually do that in practice though...

 

[kapnd]

Drastic permanent weight gains will call for some propeller tweaking!

 

[AZ2Loop]

That 20% increase in weight will also make it harder to stop once it gets going. So your brakes will not last as long.

 

[BruceK]

Drastic permanent weight gains will call for some propeller tweaking!

No more donuts!

 

[ProMaritime]

Simply stated: Added weight increases draft and increased draft increases wetted surface which translates into additional drag.

 

[Nautica]

You can't get away from the simple fact that 10000# extra weight means 10000# extra water to be moved.
(At displacement speed)