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Negative rocker TT?

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sergei Scotland
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Negative rocker TT?

Postby sergei Scotland » Wed May 15, 2019 10:15 am

Why not? All the plane wings have negative rocker at the bottom to generate lift? Could be a board for 8 knots. Obviously the very front might need some positive rocker ski-style? Just a thought 😂
:thumb:

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Re: Negative rocker TT?

Postby rynhardt » Wed May 15, 2019 11:00 am

Nothing wrong with negative rocker (camber) in the middle of the board. You'll find this happens with very flexible boards by default.
The net effect is the center of lift is moved more towards the rear foot.

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Re: Negative rocker TT?

Postby tomtom » Wed May 15, 2019 12:30 pm

It just doesnt work this way

its not lifting surface as wing. Its planing not flying. And TT its almost not even planing its semi displacement planing
Not same principle apply here as in homogenous liquid as per hydrofoil in water or wing in air.
Its on water air boundary /with very specific rules here/ and imo any negative rocker its pure drag.

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Re: Negative rocker TT?

Postby RalfsB » 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.

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Re: Negative rocker TT?

Postby Matteo V » Wed May 15, 2019 2:57 pm

tomtom wrote:
Wed May 15, 2019 12:30 pm
And TT its almost not even planing its semi displacement planing
This, as I read it, is wrong.

As soon as water is moving off of the tail of a TT, that hull is planing and displacement is no longer a factor.


Displacement requires volume in the hull. The volume of water displaced is excluded from entry into the hull to the depth submerged. If a hole below the waterline is created in a typical (open) boat hull, water will enter and fill the hull. If not enough water is displaced by impenetrable internal flotation (such as foam blocks), water will continue to enter the boat until it sinks.

Planing occurs when only speed and a contact area, NOT displacement, is required to support the weight of the boat. When planing, volume can be eliminated, and only the planing surface contact area needs to be watertight.


If this is unclear to you, it is easy to do some thought experiments on your own such as:

Consider a small motor boat with a near vertical transom and a drain plug at the bottom of that transom. When the boat is moving at "displacement speeds", a hole in the bottom of the boat or pulling the drain plug will sink the boat. But at planing speeds, the boat moves fast enough for the water to not be in contact with the transom. In this situation, pulling the drain plug would not allow water into the hull. And if a hole smaller than the drain plug is present in the bottom of the boat, water entering there would run out the drain plug hole.

For those of us that used to windsurf, looking at the tail of the board when moving is a clear indication of whether you are at displacement or planing speeds. If the water is coming up along the back of the board and/or on top of the tail, the windsurfer hull is functioning in displacement mode. But when the water is passing under the tail of the board fast enough come straight out without being able to move up onto the top of the tail (any water on the tail runs off), then the windsurfer hull is functioning in planing mode.

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Re: Negative rocker TT?

Postby yomotha » Wed May 15, 2019 3:47 pm

Isn't an OR Mako negative rocker?

https://oceanrodeo.com/product/mako/

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Re: Negative rocker TT?

Postby tomtom » Wed May 15, 2019 4:28 pm

Matteo V wrote:
Wed May 15, 2019 2:57 pm
tomtom wrote:
Wed May 15, 2019 12:30 pm
And TT its almost not even planing its semi displacement planing
This, as I read it, is wrong.

As soon as water is moving off of the tail of a TT, that hull is planing and displacement is no longer a factor.


Displacement requires volume in the hull. The volume of water displaced is excluded from entry into the hull to the depth submerged. If a hole below the waterline is created in a typical (open) boat hull, water will enter and fill the hull. If not enough water is displaced by impenetrable internal flotation (such as foam blocks), water will continue to enter the boat until it sinks.

Planing occurs when only speed and a contact area, NOT displacement, is required to support the weight of the boat. When planing, volume can be eliminated, and only the planing surface contact area needs to be watertight.


If this is unclear to you, it is easy to do some thought experiments on your own such as:

Consider a small motor boat with a near vertical transom and a drain plug at the bottom of that transom. When the boat is moving at "displacement speeds", a hole in the bottom of the boat or pulling the drain plug will sink the boat. But at planing speeds, the boat moves fast enough for the water to not be in contact with the transom. In this situation, pulling the drain plug would not allow water into the hull. And if a hole smaller than the drain plug is present in the bottom of the boat, water entering there would run out the drain plug hole.

For those of us that used to windsurf, looking at the tail of the board when moving is a clear indication of whether you are at displacement or planing speeds. If the water is coming up along the back of the board and/or on top of the tail, the windsurfer hull is functioning in displacement mode. But when the water is passing under the tail of the board fast enough come straight out without being able to move up onto the top of the tail (any water on the tail runs off), then the windsurfer hull is functioning in planing mode.

TT is definately not clean plane. I think we havent exact name for this but its just no clean plane. I know you have history of argument exercise here - and i dont wont to go into long argumentation. I wrote semi displacement planing because no better term to describe it. But for me TT is like half of V shaped hull which have virtual volume and displacement. You can clearly see how how much water you displace be sheer amount of wake wave and spray. Clean plane is skipping to water surface with much smaller wake. Im not saying that TT is displacement - im saying its not clean plane. Or at least its like plane in constant hard turn which DISPLACE /see the word/ quite a lot of water. Im not saying that what you say isnt true. Im just trying to explain myself and this is end of my argumentation on this subject /in very not native English/ :)

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Re: Negative rocker TT?

Postby tomtom » Wed May 15, 2019 4:35 pm

yomotha wrote:
Wed May 15, 2019 3:47 pm
Isn't an OR Mako negative rocker?

https://oceanrodeo.com/product/mako/
No mako just have deep concave /in perpendicular to rocker line direction/.

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Re: Negative rocker TT?

Postby Matteo V » Wed May 15, 2019 7:40 pm

tomtom wrote:
Wed May 15, 2019 4:28 pm
TT is definately not clean plane. I think we havent exact name for this but its just no clean plane. I know you have history of argument exercise here - and i dont wont to go into long argumentation. I wrote semi displacement planing because no better term to describe it. But for me TT is like half of V shaped hull which have virtual volume and displacement. You can clearly see how how much water you displace be sheer amount of wake wave and spray. Clean plane is skipping to water surface with much smaller wake. Im not saying that TT is displacement - im saying its not clean plane. Or at least its like plane in constant hard turn which DISPLACE /see the word/ quite a lot of water. Im not saying that what you say isnt true. Im just trying to explain myself and this is end of my argumentation on this subject /in very not native English/ :)

I understand your point of view. Personally, I do not understand all of the mechanics myself. As far as the categories of planing vs displacement:

1. displacement - where you would sink with a hole in the transom of your boat.
2. low efficiency planing - nose up, deep wake with high amplitude wake (removal of volume, or the back of the boat does not cause water to enter the boat)
3. medium efficiency planing - nose trimmed down, lower amplitude wake
4. Full plane - where increase large increases in speed does not produce significantly less planing surface area or significant reduction in wake amplitude.

2-4 are still planing. Displacement is the wrong English term refer to them.

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Re: Negative rocker TT?

Postby tomtom » Wed May 15, 2019 9:23 pm

Agree. Anyway concave rocker is worst possible shape. There isn't any engineered planing hull with reverse rocker line. Analogy with boats is inflatable without solid bottom where wave you push shape concave in rocker line and you feel it as HUGE drag.


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