Mike
I can articulate more detail on flat sections here based on the fundamentals.
If the boat has potential to plane, meaning adequate power to weight, it will be faster with flat bottom than round. Refer evolution of A-class hulls and the amas on BMW-Oracle as just two examples.
In displacement mode there is not much difference between flat bottom and round bottom. When seeking a minimum drag hull with flat panel, a low speed to length hull will have more side flare than the high speed to length. Also the side flare will be reduced at the widest section of the hull to reduce maximum BWL.
The lowest wettest surface flat panel hull has a flare of 28 degrees off the vertical. This produces 5% more wetted surface than a semi-circular section whereas a 2X1 rectangle has 12% more wetted surface and a square section 20% more. However the situation is not solely dependent on wetted surface. When wave making is accounted for the wave energy is a function of beam squared. If wave making is a dominant feature of the hull due to relatively high speed to length then the sides tend to be near vertical at maximum beam and the draft to beam ratio is less than 2.
For my predominantly displacement speed pedal boats the difference in drag between optimised flat panel with variable flare and round section of same displacement is about 1% in favour of the round section. The difference is 3% with best constant flare flat panel. However the round section is about 20% longer and inevitably heavier so accounting for weight the flat panel is faster in displacement mode and gains at higher speed where there is some dynamic lift.
Another important factor with a high power to weight proa is the resistance to pitchpoling and this is best countered with length. So this consideration could result in a longer hull than that consistent with minimum resistance for a particular design speed.
I have been doing some wave response analysis of late trying to optimise the reserve buoyancy. Reducing the reserve buoyancy to reduce pitching causes the boat to ride lower with less pitching but it is more prone to pitchpoling. Flat panels improve pitch damping but do not reduce likelihood of pitchpoling. The most significant factor in reducing pitchpoling tendency other than length is lowering the CoE or vectoring the drive force to provide lift as well as drive - hence the reason for playing with the heavily cambered sail.
Rick
On 01/05/2013, at 10:43 PM, Mike Crawford wrote:
Actually, if you look at Rick Willoughby's calculations, there are times when a flat bottom is faster, and also times when the wetted surface area is nearly identical to a round hull. The choice is not as clear-cut as many would have you believe.
If you go to the Messages section of the group, click on "advanced search", enter "Rick" for the author and "Flat" for the message body, you'll get a semi-useful list. It's easy to see which messages actually apply in the search results. There's some particularly good data and discussion in July/August 2010 in the "rudder lift" and "my little mule" threads.
- Mike
LucD wrote:
I have read through some entries at boatdesign and it is clear now that regular beaching in Dutch like waters requires such strengthening of the hulls that one might as well stay with the local flat boat designs ... and miss out on speed potential. There is little compromise possible in a HP design I am afraid. But that is good to know and let that feature out of the SOR for a cruising HP.
Luc