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Negative rocker TT?

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BWD
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Re: Negative rocker TT?

Postby BWD » Wed May 15, 2019 11:51 pm

Actually no.
Examples of engineered planing hulls with negative rocker (or similar):
Any planing or semiplaning motorboat with trimtabs -probably millions...
Many fiahing skiffs and some sportfisherman types...
Anything that rides bow high and gets a wedge added to the stern...
Stepped hull speedboats...

tomtom
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Re: Negative rocker TT?

Postby tomtom » Wed May 15, 2019 11:56 pm

Always learning new stuff :)

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Re: Negative rocker TT?

Postby sergei Scotland » Mon May 20, 2019 4:20 pm

RalfsB wrote:
Wed May 15, 2019 2:25 pm
TT already requires uneven foot pressure, more pressure on the back foot than on the front foot. If you do negative rocker, even more back foot pressure will be needed to compensate for the lift in that part. Sounds like uncomfortable riding.
May be 2 negative rocker areas then, one at the front and one at the back? To even out the maximum lift distribution. Just a thought. A bit like foil with 2 wings?
8 knots riding?

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rynhardt
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Re: Negative rocker TT?

Postby rynhardt » Mon May 20, 2019 5:50 pm

Maybe read up on Savitsky planing hull theory if you are really interested in answers.

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Re: Negative rocker TT?

Postby Matteo V » Mon May 20, 2019 7:15 pm

rynhardt wrote:
Mon May 20, 2019 5:50 pm
Maybe read up on Savitsky planing hull theory if you are really interested in answers.
Any chance you could give some insight or a primer of what to expect as far as the relationship of Savitsky theory's and flat planing hulls engaged at an angle (edged) with relatively continuous rocker and variable pitch (weight forward/aft dynamic) capabilities?

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Re: Negative rocker TT?

Postby RalfsB » Tue May 21, 2019 8:18 am

sergei Scotland wrote:
Mon May 20, 2019 4:20 pm
May be 2 negative rocker areas then, one at the front and one at the back? To even out the maximum lift distribution. Just a thought. A bit like foil with 2 wings?
8 knots riding?
In 2 places it might work but I am not sure about how much drag will be there, especially at low speeds.

It is not exactly about negative rocker, but I know Airush was experimenting with stepped bottom in their early Slayer models:

Image

but it looks like they later abandoned the idea.

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rynhardt
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Re: Negative rocker TT?

Postby rynhardt » Tue May 21, 2019 9:44 am

Matteo V wrote:
Mon May 20, 2019 7:15 pm
rynhardt wrote:
Mon May 20, 2019 5:50 pm
Maybe read up on Savitsky planing hull theory if you are really interested in answers.
Any chance you could give some insight or a primer of what to expect as far as the relationship of Savitsky theory's and flat planing hulls engaged at an angle (edged) with relatively continuous rocker and variable pitch (weight forward/aft dynamic) capabilities?
No chance 8)
But I may be able to comment qualitatively on the riding conditions that make up 80% (and by 80% I mean roughly speaking, the majority :lol: ) of my own riding.
This assumes fairly steady state conditions where the forces are close to being in equilibrium, within a time period of a minimum of 300ms (assuming 5m/s on a 1.5m board).
A further assumption is that I can approximate a typical board through the linear superposition of several flat surfaces with different angles of attack, negating transition effects.
If you are comfortable with those assumptions then I can comment, otherwise I can't. :o

The basis of my qualitative discussion can then make extensive use of the following diagram, with some references to Savitsky's 1964 publication
http://www.westlawn.edu/ReferenceInfo/S ... ls1964.pdf
Attachments
fig2.PNG
fig2.PNG (21.14 KiB) Viewed 377 times

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Re: Negative rocker TT?

Postby Matteo V » Tue May 21, 2019 12:50 pm

rynhardt wrote:
Tue May 21, 2019 9:44 am
If you are comfortable with those assumptions then I can comment, otherwise I can't. :o
Go for it. You have got me curious!

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Re: Negative rocker TT?

Postby trickcoach » Tue May 21, 2019 1:46 pm

Ronix went all out with camber boards a few years back , worse thing I ever rode ... were really crap ...

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Re: Negative rocker TT?

Postby rynhardt » Wed May 22, 2019 7:39 am

Matteo V wrote:
Tue May 21, 2019 12:50 pm
rynhardt wrote:
Tue May 21, 2019 9:44 am
If you are comfortable with those assumptions then I can comment, otherwise I can't. :o
Go for it. You have got me curious!
So.. under fairly steady state conditions, while under kite power, and edging on a typical reach/tack, the waterline under the board looks like the photo below.
The board is providing lift against the water through a combined angle of attack against both the direction of travel and the pull of the kite.
The waterline is probably a good indication of equal pressure, and is a visual confirmation of the spray root as per Savitsky fig 2.
We can transform the coordinate system a bit to reflect the pressure distribution, and the section in contact with the water can then be approximated by a series of flat planes orthogonal to the pressure distribution.
I drew some rectangles to indicate the series of flat planes - note that they are aligned to the pressure distribution and not aligned with the coordinate system one would typically assume for the board.
The red and orange rectangles are the areas of highest pressure (and lift, and drag), so I'll chat more about them in the next post.
approximate planes.png


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