Ben
That design I posted was a study for someone else and is still in development. The comments made about it so far have already provided some progression that would improve it. I was still playing around with the overall beam. As shown it was 3.5m centreline separation.
There has to be a starting point for the first iteration. I have produced some charts that I previously posted without much explanation. Here is a link to one for a proa:
These charts provide the principle hull dimensions for the minimum resistance displacement hulls for various speeds and displacement.
The particular rectangles drawn over the charts show the limit of operating regime for proa hulls for a 2t displacement with a 25/75 share of displacement under static trim. The overlaid ellipse is intended to show the most frequent region of operation for the two hulls.
My aim with these charts is to give an idea of how the hull would ideally morph to always be close to the minimum resistance. For this particular case you can see that a 16m LWL would suit the lw hull through quite a lot of the range. If the target was for higher speed then the length is going to come down. On the other hand the LWL of the ww hull should be reducing as speed increases; implying there is advantage in having rocker on the hull. If you decided you would like it to be really in the groove with 25% of the displacement on the ww hull then that hull could be less than 10m LWL.
The sensitivity of drag to dimensions may not be as significant as you expect and the hull coefficients also change considerably. They are based on round sections but not much different for rectangular sections.
Lighter hulls that can go fast planing will become a factor and these charts do not apply to that condition. The speeds to get a 2.5t slender hull planing are going to be in excess of 20kts but the unloaded ww hull with a flat bottom would have substantial planing at lower speed.
The charts are aimed to getting basic dimensions into the ballpark. There are other factors like method of construction, longitudinal trim, sea-keeping, useful space and so on that might be more significant in getting the best outcome than achieving minimum resistance hulls.
It does not take long to generate the shape of the minimum resistance hull for a single set of constraints like displacement and maximum length. It gets harder to develop a shape that might morph the right way. Producing polars of the final shape takes longer still and those polars only include the things that I consider significant. I can check if longitudinal trim is going to be an issue but so far the model does not automatically correct for it.
A reason I posted the original image was to make a point that the ww hull did not need to be always shorter. One of the sites that Todd referred to has a proa with identical hull shapes. This could be desirable for simplicity.
Rick
On 18/05/2011, at 7:43 AM, bjarthur123 wrote:
rick,
i very much like your starting point on hull design-- lw hull shape to minimize drag at high speed and high payload, ww hull shape to minimize drag at low speed and high payload.
i was surprised to find that with this design strategy, 250 kg hulls come out roughly the same length, just different shapes. this is about the displacement of elementarry. how would things scale up at 2500 kg, the size of visionarry? still the same length??
would you mind giving us details and the dimensions of your boat? LOA, BOA, etc??
as a scientist, who spends most of his time engineering new experiments, i very much appreciate the quantitative aspect you've brought to this discussion. thanks.
ben
--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
> The total displacement is 250kg. Static condition is 200kg on ww hull.
>
> The shape of the windward hull is the lowest drag displacement hull
> for 6kts carrying 200kg having a flat bottom.
>
> The ww [LW?] hull is the lowest drag displacement hull for 200kg at 16kts
> with a flat bottom although at that speed and load it will be planing
> so the displacement aspect is somewhat meaningless at that point but
> it has to get there and above.
>
> The ww hull ends up longer than the lw hull for this size boat based
> on my starting point.
>
> The shapes are distinctly different and highlights the advantage of
> the proa configuration over a catamaran. The fine entry and long
> waterline length on the ww hull reduces wave drag in displacement mode.
>
> The full bow and deep draft of the ww [LW?] hull works to reduce wetted
> surface when viscous drag dominates overall drag at high speed. The
> full bow gives high KMl for the waterline length and the wave
> pressure at the bow creates buoyant lift once under way. This nose-up
> attitude assists transition to planing.
>
> Both hulls have narrow flat bottoms that will plane at shallow angle
> of attack once speed gets over 10 to 12 knots. This means the lift
> to drag is high once in planing mode. There is no hump to planing
> just a gradual transition.
Rick Willoughby
03 9796 2415
0419 104 821
