Planing is dynamic lift which can only occur when there is a positive angle of attack of the surface, ie the boat must trim bow up if the bottom is normally horizontal. A slight curve to the bow in a pro means an equivalent slight curve to the stern. These will both tend to pitch the bow up and the stern down, which will counteract the sail forces working the opposite way. If the combine effect is some bow up, there can be dynamic lift with an upward resultant.
Cheers,
Peter
From: harryproa@yahoogroups.com.au [mailto:harryproa@yahoogroups.com.au]
Sent: 30 May 2018 14:25
To: Björn bjornmail@gmail.com [harryproa]
Subject: Re: [harryproa] Length to displacement ratio and Bucketlist
Just as a guess, it looks like it might. I think Rob said he could see something. Or that the bows were not pushing down on Bucket.
A very light and long boat like bucket may not go bows down as much anyway.
On Wednesday, 30 May 2018 07:41:36 PM AWST, Björn bjornmail@gmail.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
At what speed will the dynamic lift be enough to provide any significant lift? And will it really come up completely and plane?
Michlet can't simulate dynamic lift.
On Wed, May 30, 2018 at 1:25 PM, doha720@yahoo.co.uk [harryproa] <harryproa@yahoogroups.com.au> wrote:
Remember that the hulls should now lift up and plane thanks to the upcurve at the bows.
Hopefully this shoukd happen and also stop the bow down-ness.
Though it may not matter significantly? It would be better not to have the flat bottom moving through water at a slightly inclined angle.
Much better to keep it level. And even better to also stay level and lift up on a plane.
How much improvement could it mean to ri se up a few extra inches. It would not dr aww much anyway lw hull. Suppose ww hull could also lift? Was that bow curve intended for both hulls?
On Wed, May 30, 2018 at 1:05 PM, Björn <bjornmail@gmail.com> wrote:
No, it doesn't work like an airfoil. I thought so too at first. Then I was corrected by knowledgeable people on the Boatdesign forums. Working on the interface between two mediums of different density/viscosity changes things a lot from foil theory due to the waves created.
Actually, I simulated a hull with a NACA0010 profile from top, with a half-circular cross section, and it it didn't perform well. Actually, it performed exactly the same running backwards, which was a huge surprise, but also backed by science, it turned out later, when knowledgeable people told me.
The best shape seems to be something which "fits" inside a wave or something like that (just my own theory). So that should be a sine or arc I guess. Michlet has a function which creates hulls based on math formulas, and you just input some constants which alters the shape. I used that function a lot, and I think it creates an arc profile..
Flat bottoms/rectangular cross section works well (in Michlet, I should add, but I believe the SW is good).. It's just that the wetted surface area is larger than a half circle. Or you can put it like the circumference is larger for the same cross section area. But it actually helps a lot to round the corners a little. The closer to the circular shape, the better. A rectangular cross section with nicely rounded corners seemed to have 15% more wetted area than the half circle. And I guess the rectangle should have 27% more (4/pi).
So I think the flattish bottoms of the latest Harryproas will work well. I think Rick W has done some simulations and reached the same conclusion. The increase in wetted area is compensated by the low weight of the Harryproa, as I see it. But you could get both I guess, if you are really looking for performance! But for me personally, I don't think it's worth the effort with the round bottom.
I also want to mention that a square profile seen from the side works for proas, but not for boats which need to be able to tack, because they cant rotate easily around the mast, so to say. That is something Rob has written about in the past. Its another advantage with the proa. The rectangular profile prevents pitching. With an arc profile its easy to imagine it will pitch more.
On Wed, May 30, 2018 at 12:08 PM, '..' eruttan@yahoo.com [harryproa] <harryproa@yahoogroups.com.au> wrote:
| Most of the difference was from the increase in surface area when moving from a half circle cross section to a semicircle/oval. But it
|had not much difference in wave making, which I guess is because an ncrease in beam (negative effect) leads to a decrease in draft (positive effect), if the displacement is kept constant.
So very interesting! It seems hull bottoms don't matter much?
Is it possible to see the effects of a flat bottom?
Can you do round corners vs square?
Different radius corners?
If cheap hull bottoms work well, then...
| What I found was that the most important parameter is that the profile of the waterline seen from top, is a smooth curve, like a semicircle or sine...
So, the ?profile at the waterline? behaves like a airfoil or 2 dimensional fluid dynamics problem?
You are looking for a low drag, double symmetric 0 camber foil of a given thickness %. That does well at your Reynolds number, Right? Drag bucket right where you want it.
If so, there should be lots of tools like xfoil to help.
Exciting!
But perhaps you all already knew this?
Posted by: "Peter Southwood" <peter.southwood@telkomsa..net>
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