Drag on planing flat hull.

Discussion in 'Boat Design' started by molebrat, Feb 25, 2011.

1. Joined: Feb 2011
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molebratNew Member

I am currently in the process of designing a towed hydrofoil craft. The craft will be an average sized freeride windsurf board (almost flat hull, a bit of nose rocker to avoid submarining through small waves) modified with a large rear foil for the majority of the lift and roll stability (likely to have a dihedral shape) and a smaller front foil for pitch control.

I've deduced a speed of 8 to 9 kts for the take off speed (when the craft will become foilborne) and calculated the drag on the hull at this speed relatively simply using the skin friction coefficient methods (modelling the board as a flat rectangle). I appreciate that the board is likely not to be on the plane at this point. I've also added the induced drag on the foils and struts and am happy with this.

However, I don't believe that this is an accurate enough way to determine the hull drag immediately before take off, when obviously drag is a maximum. So, i've been reading papers written by D. Savitsky (who appears to be very highly acclaimed in this field) but have found them a little hard to digest. The windsurf board has an almost entirely flat hull. Therefore the angle of deadrise is 0.

One paper that I have been focussing mainly on is 'Hydrodynamic design of planing hulls' which has improved my understanding but I don't know if it can be applied to entirely flat hulls. Papers to exist obviously purely on the subjects of flat hulled planing craft, but the ones I have found I have not been able to access.

Can anyone shed any light on the matter?

Cheers

2. Joined: May 2009
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Here is a paper by Jim Drake for calculating lift and drag on a sailboard:

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3. mark775Guest

I don't have anything concrete for you right now but float planes have struggled with your problem, and suction, from the beginning. I'd look there for some insight.

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Village_IdiotSenior Member

There is an article published in the scientific journal "Nature" sometime around 1993 (give or take a year, can't remember). The article chronicled the research of two scientists looking to reduce the drag of an object in water, or in this case, of water through an object. They used different treatments on the inner wall of a pipe and measured the flow velocity of water through the pipe.

They found that random protrusions on the surface created the least drag, and increased flow something on the order of 17 percent (again, I don't remember the exact numbers, I've slept since then). Uniform protrusions were much less efficient, and smooth pipe was pretty inefficient to begin with (it was used as the baseline).

The science behind the theory is that a boundary layer forms between the water and the surface it is flowing against, due to friction. This boundary layer creates vortices that tend to roll up (like rolling up a carpet along the floor), and once the vortices get large enough, they break away and disrupt flow. The protrusions act to break up the vortices before they become too large.

As an aside, from running many different flat-bottom planing hulls, I can tell you that they reach maximum speed with a slight chop on the water, as opposed to 'glass-smooth' water. There could be complications from prop behavior in smooth vs. choppy water, however.

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