difference 15/20 degree V

[Guillermo]

Originally Posted by Gilbert

Surprise indeed that a shape that generates less lift than another would achieve less resistance as the speed increases. Forgive my insubordination, but this sounds like a mathematical error to me. Please explain how this magic is made to happen.

Gilbert,
For a given deadrise angle there is an specific relationship between trim angle and lift coefficient which defines the inception of porpoising. Because of such porpoise considerations it is necessary to design the boat to operate at an unfavourable trim angle from the point of view of resistance, as the minimum drag-lift ratio occurs at an angle greater than the angle needed to avoid porpoising.

The trim angle required to avoid porpoising increases with increasing deadrise angle. Increasing the deadrise enables a planning surface to operate at trim angles more closely approaching those required for minimum drag.

So, it may happen a 20º deadrise brings better drag results than a 15º one at high speeds, if it risens the trim angle to a more efficient one from the point of view of the drag-lift ratio.

I attach the well known 'Savitsky 64' paper.

Cheers.

[Guest625101138]

Originally Posted by Jasper_ghost

.........My hope is to have a mpg in the range 1,5-2 but is this realistic for a 20 degree hull, 26feet, beam 8 and and total load 2500kg and a suzuki df300..........

Savitsky calculation indicates you need around 120HP for a boat with total weight of 2500kg doing 26kts and 60% overall propulsion efficiency. You could expect that performance will be similar to the boat shown here with your selected outboard:
http://www.suzukimarine.com/boat_bui..._268_cc/df300/

I estimate your top speed will be around 37kts so a bit slower than the hull tested. Likewise a little more fuel at the various speeds. The tested hull is wider so should be a little more efficient. It has 21 degree deadrise so similar in this respect.

Rick W.

[Gilbert]

Very good!
This is half the information I was hoping to see.
Now do we have similar data for a boat that has about 15 degree deadrise and similar length, beam, weight and the same type of motor?