# Pocket tunnels, running angles



## Brett (Jul 16, 2008)

Anybody in the forum running a hull with a pocket tunnel?

Also:

With all the small skiffs at this site, what is your average hull
running angle at speed, on plane? From all the images I have
viewed on line, it appears that most small boats are running
at 4 to 7 degrees from horizontal. I actually place a protractor
on my monitor screen and measure the hull angles from the
image's horizon.

And: what does your hull draft at the transom when on plane?

I'm trying to understand the hydrodynamics of wake, hull trim angle
and pocket tunnels. I need a little more raw data. The tech sites
are too involved with formulae and graphic representations of
turbulence, friction and lift to get raw information from.


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## costefishnt (Dec 11, 2006)

cheese and rice brett. i aint smart enough to know answers like that.


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## Brett (Jul 16, 2008)

> Posted by: costefishnt: cheese and rice brett. i aint smart enough to know answers like that.


Sure you are Curtis, you've just never given serious,
multiple cold ones, type thinking about what's going on at the
back end of the boat. Or maybe you have, you did install
a jack plate and that requires the same style thinking to get it right.
That running angle is what makes the jack plate so useful.
The further you get away from the transom the higher you can
jack the motor. That's because the water of the wake is bouncing
back from where your hull has pushed it down.










I'm just trying to figure out how effective pocket tunnels are on a flat
bottom hull.


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## snooknreds2 (May 5, 2008)

I think he is looking for some one who has a tunnel hull just to say some numbers.  

Brett I am pretty sure you realize that to figure it out for your boat you need to include the wake because it is a non-lamiar (or turbulent) flow.  Turbulent flows create more frictional skin drag which also must be accounted for.  In addition the angle must be accounted for because that also creates a drag force.  Basically the water can not turn sharp corners easily and it creates a mixing layer where turbulent and laminar flows are mixed.  This is called a boundary layer.  The larger your boundary layer the more drag you have.  (actually in some rare cases they induce this turbulent flow to help speed up the molecules right next to the boundary wall.  A good example of this is on an airplane wing.  Next time you fly notice the little nubs on the wing, those are to induce a turbulent flow which helps mix the fast moving air located away from the wing and the slower moving air right next to the wing).  Anyways, the tunnel hull thing is allot like an airplane wing.  When you are moving fast through the water it is more about lift (from propulsion planning your boat) and drag than it is about displacement at that point. With out picking my brain too much and referring some of my old books that is about as far in depth as I would like to go. Hope that helps you understand the "fluid mechanics" of a tunnel hull a little bit more. Now you see why I dont want to do mine. When I started trying to think about it based on what I know from fluid mechanics it was just too much for my brain to try and comprehend :-[


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## Brett (Jul 16, 2008)

Yep, played with the friction resultants and due to the shape of the
pocket, a rectangle on my toy, without creating a vacuum the
pocket acts as a jackplate that puts the prop 5 feet behind the
laminar shear point. I'm just attempting to get some hard data
to check my calculations. I may have to wait until I get the Slipper
in the water to verify my theory.










It looks like all a pocket tunnel really does is compensate for
the hull running angle. 4 to 7 degrees from horizontal works
out to about 1" of rise per foot of run. At that angle, the beginning
of the tunnel is at the same level as the top rear of the 4" tall
tunnel pocket. The problem is that the cavity created by the tunnel
decreases the bouyancy of the hull in the stern, so load location
in the hull is critical. The Slipper is going to need to be balanced
perfectly to get the best use of the pocket.


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## costefishnt (Dec 11, 2006)

dang it. what i do know is just enough to get me in trouble. I was blessed with ton's of common sense, and mechanical understanding. I however did not pay any attention in school as I was too busy getting high, drinking, chasing girls, and fighting the system...which only 2 things have changed. I dont smoke weed any more (not that I am opposed to the fact) and my wife says i cant chase ladies anymore.

hydrodynamics are a bit out of my league although they bear a very close resemblence to aerodynamics. many factors come into play when trying to determine actual measurments, espeacially while underway. Now, I have posted this pic many times. I would like you to examine by what ever means you have, and then I will get another very soon of my boat running especially since a few modifications have been made....










now understanding that the waters surfaces rises1/2" for every 3, or 6" (I can not remember the exact measurment) and with the fact that my motor weighs in at 179 I didnt really have much choice other than a jack plate. My decision was more or less made for me. 

now as to how to get a running measurement while running , this seems like a pretty interesting task. Please enlighten me as I would really like to do this, not only on my boat, but other Gheenoes that I am lucky enough to test. setting up an inclinometer to get the actual angles that the hull rides out would be rather easy so I might can get something for you soon on this.

on a side note, I really would like for you to take a ride south for the rally. I would love to sit and talk with you, and share a cold one. If not I am going to need an address as I will gladly make a trip up to chat somethime soon. you are one interesting feller. I think thats why i like that weird guy deerfly!


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## Brett (Jul 16, 2008)

easy inclinometer:

Take a 1/4 oz sinker and attach it to 18" of fishing line.
Attach it to the side of your console so it hangs along side the console.
12" above the sinker tape the line to the console.
While your boat is on the trailer level your hull fore and aft using
a carpenters level.
Where the sinker hangs is the zero point.
Mark the location with a pencil.
Go out on the water on a calm morning and run.
Where the sinker hangs at speed is the running angle.
Mark this location with a pencil.
The distance at 12" from the tape, measuring from the zero mark
to the running mark will probably be about 1".

And measuring the image I get a 4 degree running angle.


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## costefishnt (Dec 11, 2006)

i didnt want to let on to the fact that I have a nice digital inclinometer that i use for pointing VSATs while in the field. but hell, I like your enthusiasm. Old school is the real school. ;D


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## Brett (Jul 16, 2008)

You and Dave, always with the acronyms.
I have to go look them up to be able to
understand the reference.


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## costefishnt (Dec 11, 2006)

Very Small Aperture Terminal. Satellite dishes 3 meters and smaller. and no it aint for TV. (TV= Television)


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## snooknreds2 (May 5, 2008)

Brett I would love to look over your calculations and see what formulas you actually used, also what kind of assumptions you had to make to assume a ideal situation. I am just a curious Dork and would like to be able to discuss the real world results you get with the theoretical results. I dont think that this is a very well developed area of knowledge and am interested in learning more.

When and where is this rally at? I would not mind trying to come and meet all of you and learn some more info while enjoying a cold one.



Costfish I am confused by what you say here. What location of the waters surface are you referring to? after it sheds off the aft of the boat?

"now understanding that the waters surfaces rises1/2" for every 3, or 6""


Also Hydrodynamics and aerodynamics are governed by the same fundamental equations if I recall correctly from my days of chasing girls..LOL so if you know aero you can easily do hydro


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## costefishnt (Dec 11, 2006)

Snook, the water rises from the transome. the water is being pushed down with the forward movement, and displacement of the boat, the waters reaction after clearing the transomes bottom edge is to fill in the hole greated by the boats displacement. being that water is fluid, if tends to rise just over its natural heighth before coming back to its normal state. the 1/2" for every 3 or 6" comes from this, and figuring out the formula used can be left to you smart guys


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## snooknreds2 (May 5, 2008)

Ok that is where I thought you were talking about. You kinda sorta have the theory. Your bow wake and the fact that the transom drops down actually has more to do with changing pressures than displacement of your hull at that point. Like I said earlier the displacement still plays a role but at that point it is more about drag and lift. That is what planning is. You are basically lifting out of the water, kinda like hydra foil just not quite as much lift.


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## Brett (Jul 16, 2008)

The problem I was looking at was not friction,
or lift. I had made the problem too complicated
by adding too many variables. For speeds less than
25 knots the only variable to worry about is hull angle.
The only critical location is the water-hull boundary shear point.
On a normal flat bottom hull that point is the bottom rear
edge of the transom. At this point at planing speed the water
is ripped away from from the bottom of the hull. Turbulence
shatters the boundary layer immediately after the transom passes.
The surface of the water begins to push back to its normal level.
With an engine bolted directly to the transom, the cavitation plate
of the engine is level with the bottom of the hull. By adding a jackplate,
and a cupped propeller you can raise the engine up so that the cavitation
plate is above the bottom of the hull. As long as the cupped prop
is not ventilating and the engine maintains water pressure
no problems. You are not raising the top of the prop above the level
of the shear point or the bottom of the hull unless you install a
low water pickup and a surface piercing prop.
A pocket tunnel takes advantage of running angle and moves the
water-hull boundary shear point farther forward and as a result the 
center of hull wake surface upwards. The bottom of the hull
is still running at basically the same depth but the cavitation plate
is at or slightly above the level of the front edge of the pocket tunnel.
Pocket height should not exceed stationary hull draft.
The design draft on the Slipper is 4", the pocket height is 4".
A 6 degree running angle produces a 1 inch rise per foot of run.
To place the bottom front of the pocket at the same level
as the top rear of the pocket, when running on plane with
a hull angle of 4 to 6 degrees, the tunnel length would be 4 feet
for a 4" draft hull. I calculated the location of the front of
my tunnel at 4-1/2 feet forward of the driveshaft of my engine.
That way I kept as much bouyancy as I could and still lifted my engine
4 to 5 inces above a normal hull setup. 
Did that make sense?

Here's an image that shows what and where:


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## deerfly (Mar 10, 2007)

at 27" of depth ground effects should start to become a factor too.


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## beavis (Dec 11, 2006)

> I think he is looking for some one who has a tunnel hull just to say some numbers.
> 
> Brett I am pretty sure you realize that to figure it out for your boat you need to include the wake because it is a non-lamiar (or turbulent) flow.  Turbulent flows create more frictional skin drag which also must be accounted for.  In addition the angle must be accounted for because that also creates a drag force.  Basically the water can not turn sharp corners easily and it creates a mixing layer where turbulent and laminar flows are mixed.  This is called a boundary layer.  The larger your boundary layer the more drag you have.  (actually in some rare cases they induce this turbulent flow to help speed up the molecules right next to the boundary wall.  A good example of this is on an airplane wing.  Next time you fly notice the little nubs on the wing, those are to induce a turbulent flow which helps mix the fast moving air located away from the wing and the slower moving air right next to the wing).  Anyways, the tunnel hull thing is allot like an airplane wing.  When you are moving fast through the water it is more about lift (from propulsion planning your boat) and drag than it is about displacement at that point.  With out picking my brain too much and referring some of my old books that is about as far in depth as I would like to go.  Hope that helps you understand the "fluid mechanics" of a tunnel hull a little bit more.  Now you see why I dont want to do mine.  When I started trying to think about it based on what I know from fluid mechanics it was just too much for my brain to try and comprehend :-[




Interesting,

But laminar flows create more friction not turbulent.  Think of a shark's skin.  It is like sand paper.   When you get down to the molecuar level as in molecules of water vs. molecules of your hull, you want the same molecules in contact as little as possible.  The longer they are touching each other, the more friction is created.  Look at bigger tankers.  They have that big bulge in the bow that is underneath the water.  If you look at a point in the water as the tanker goes passed. That is there to create turbulence and get the water moving as the hull goes by a prodducing less drag by the water.   To your plane wing example, the fast moving air is actually going over the top of the wing creating the low pressure region allowing for the lift from the higher pressure below. That faster moving air is next to the surface not away from it. 

In terms of a flat hull with a pocket tunnel, look at the XF20 on the bateau webiste.  That is a 20 footer with a pocket. You should be able to get some dimensions off of that for similarities.  Water coming off the back also has variables of shape of that back hull/transom intersection point you are calling a shear point.  If that is more rounded or a harder square angle, or just slightly angled up, that will all affect how the water releases off the stern.  Also speed has influence on that.  But we are refering to once the boat is on plane and those will be within a narrow range in terms of all of our boats and they are all planing hull anyways so that is just about a wash on that one. On that one think of shooting a bullet horizontally and dropping a bullet with both being at the same height. They will hit the ground at the same time just quite a ways apart.

The water will come up higher than normal level as it releases off the back of the baot.  That is where the jackplate setup and/or the tunnel take advantage  thanks to our hulls displacing water.  It is an undulating affect that takes a series of a few wave cycles to balance out.  Watch any boat as it goes by to see this.  Especially bigger boats.  The tunnel just makes this take affect sooner and allows the water to go higher.  If you really want enhance that, you could even shape your tunnel and create a venturi nozzle effect.


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## snooknreds2 (May 5, 2008)

I am confused as to your frame of reference for the air plane wing example.  I am thinking of it as the wing is stationary and the air is moving at some known speed 'X'.  (same thing as the air being still and the wing moving just easier to picture what is going on in my head)  You say that the faster moving air is next to the surface not away from it.  But if the wing was not there the air would be moving at X MPH.  So now the wing is put in place and the air has to go around the wing in order to return to its normal flow direction and speed.  But the wing is in the way...it slows the air down.  The air 20 feet from the wing is still going X MPH,  the air that is going around the wing but is still in a laminar flow is going X MPH, but the air that is near the wing is going X-skin drag effects MPH, Furthermore the molecules right next to the wing are theoretically not moving



I dont have time to go into depth right now but basically what is called "form drag" is greater the longer it takes for the flow to go back to a smooth laminar flow.  That is why a rain drop is pointed on the trailing edge.  By having a larger turbulant boundry layer you create more skin drag (the individual molucules moving by the hull of your boat.  Theorertically the molucules right next to the hull dont move and have a zero velocity.)  So by mixing the layers of flow together it helps speed up the molecules closer to the hull and they dont get "left behind".  Now they are not trailing the rest of the flow and can more easily return to a lamiar flow once they leave the vessel body.  As for the ship 

It has been awhile since I learned all of this so forgive me if I am not very clear in my explanations as I am trying to remember the info plus figure out how to portray it to you guys.  By the way You are assuming a high remolds number when using the theory of mixing the boundary layer.


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## deerfly (Mar 10, 2007)

snooknreds2, you're not thinking about the problem correctly, Google "Bernoulli effect" and you'll find plenty of equations and examples to keep you from getting anything done on your project boats. 

IMHO Bret is really struggling with drag as a consequence of the hulls inclination angle and less about the effects of various tunnel configurations and geometry. Put another way, trimming and balancing the load in the Slipper will have more profound effects than the variations in efficiency between different tunnel designs. However, a bad tunnel design will contribute greatly to the "this sucks" effect.


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## snooknreds2 (May 5, 2008)

Thank you but I am already very well versed in the Bernoulli equations and have studied them in depth for a year straight while getting my degree in Ocean Engineering, including how to derive them from the momentum equation.  I guess my problem is I am thinking way too in depth probably along with an after effect of too much alcohol while going through college.

By the way there are several MAJOR assumptions that must be true in order for the simplified Bernoulli equations to hold true. If my memory serves me correctly F=ma is actually derived from the same governing equation as the Bernoulli equations are. Just by different assumptions different terms go to zero and drop out. 
......It is I just looked


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## iMacattack (Dec 11, 2006)

more HP!


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## Brett (Jul 16, 2008)

Gentleman, again, you're making this way too complicated. So was I.
At speeds LESS than 25 knots, on a flat bottom skiff,
friction and lift and drag have no major effect on the water path
through a RECTANGULAR pocket tunnel. My top speed will be between
12 and 14 knots. The only effect my pocket tunnel has, is to take
advantage of the incline created by the running angle of 4 to 6 degrees
to move the surface of my wake up 4" from the bottom of
the transom of my hull. Look at the last image I posted. It is
drawn to scale. The angles are drawn correctly. The pocket is drawn
correctly. At planing speeds, the surface of the water only comes back
up a small amount, in the time it takes for the boat to travel the
distance from the front of the pocket to the rear of the pocket.
In order to force water up higher than this, a funnel or tapering
shaped pocket would be necessary to squeeze the water in and up.
Such a shape would be too much stress for the hull I'm playing with.
I just want to float and run shallow. I don't plan on chopping seagrass
or oysters. Or setting any speed records.
All that a pocket tunnel does for a small lightweight boat,
is to eliminate the need for a jackplate. When properly designed,
The pocket tunnel is the jackplate, built into the hull.
My whole interest was proving the running angle in the real
world, not measuring off images on my monitor. As I don't
have a powered skiff at this time, I figured I'd ask here.


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## snooknreds2 (May 5, 2008)

> Gentleman, again, you're making this way too complicated.....
> My whole interest was proving the running angle in the real
> world, not measuring off images on my monitor. As I don't
> have a powered skiff at this time, I figured I'd ask here.



As far as the running angle I guess you just need some one with a working skiff to go for a ride and bring one of those things they put on sail boats to measure the roll angle???

Brett wikipedia the Bernoulli equations. scroll down until you get to the derivations. Check out the derivation and the picture that is shown there, that might get with the theory side of it. I did not read thru it but I think one assumption that is made for the Bernoulli equations is that you have laminar flow. So really, we know there is turbulant flow around the boundary of a boat so my guess is that Bernoulli equations dont apply any more. Thats why the next class is called hydrodynamics II/ ship design


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## Brett (Jul 16, 2008)

Yep, the tech sites hide their real world data, and bury the results
in very pretty graphs. For high performance hulls, the graphs
are useful. But at speeds below 25 knots, there's not enough variables
involved to make it worth their while to publish.
I just wanted real world verification of a digital extrapolation.
(boy, that even sounds nice, say it out loud...)


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## deerfly (Mar 10, 2007)

welcome aboard, I'll take a vodka on the rock's with a twist of lime. Oops, wrong thread, sorry.


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## Brett (Jul 16, 2008)

Eric, your "sucks" comment is entirely accurate.
I am building a vent tube into the tunnel, once I flip
the hull, to alleviate what little pressure differential
my top speed might create.


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## deerfly (Mar 10, 2007)

well, personally I don't think a tunnel on such a small, low powered boat is worth it. I would go with a manual jackplate and preserve as much static displacement as I could, which I think has more value more often with the way you use a small boat like the slipper. Otherwise, I think you're potentially subjecting yourself to a lot of trial and error with the tunnel for nominal gain. Its a great topic for study though.


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## Brett (Jul 16, 2008)

You're probably correct DF. A tunnel on a light weight skiff
is not going to produce any great difference in running depth.
The fact that I'll build it, and report on how it works, may
be helpful in someone else's decision in their purchase or
construction of a hull. In which case my experiences will save
them some time and money.

For me, it's all entertainment, and very educational.

Where else, except on an engineering forum, are you gonna'
find people spending their lunch hour arguing laminar vs turbular
and referencing Bernoulli in self-defense?
                                                             

real world verification of a digital extrapolation...I'm proud of that one!


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## aaronshore (Aug 24, 2007)

You guys are once again blowing my mind. 

I'm gonna go drink a beer with Curtis and we're going to make fun of you guys for having brains too big for your own good ;D


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## costefishnt (Dec 11, 2006)

> You guys are once again blowing my mind.
> 
> I'm gonna go drink a beer with Curtis and we're going to make fun of you guys for having brains too big for your own good ;D



lol


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## deerfly (Mar 10, 2007)

being smart isn't nearly as important as being smart enough to know when you're not smart enough.


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## snooknreds2 (May 5, 2008)

Just like what you know and your ability to communicate what you know are two completely different things. 
I hear what you are saying though... I could not figure this problem out on my own thats for sure!!!! Never said I could though, just trying to share a little bit of what I do know that is relevant. Not trying to argue with any one!!!!


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## beavis (Dec 11, 2006)

> My top speed will be between
> 12 and 14 knots. The only effect my pocket tunnel has, is to take
> advantage of the incline created by the running angle of 4 to 6 degrees
> to move the surface of my wake up 4" from the bottom of
> ...



The hull displacement is what is going to dictate your wake and the pocket tunnel won't have anything to do with that. Every boat I have seen with a tunnel had a jackplate. The jackplate is going to let you fine tune performance and it is going to let you run even shallower because you are taking more advantage of the water coming off the transom at a higher level in relation to the non tunnel portion of the transom. Most pockets usually have an angle to allow the water coming out of the tunnel to rise faster than off of a square intersection so the motor can be raised even higher. In a few cases, the prop is almost completely above the bottom of the boat. You would have to raise the transom to get the full effect. Just throw a jackplate on your without the pocket. I have had 2 boats with jackplates and no pocket and got great results. 

You keep talking about a smaller type type and a pocket will take away from your at rest draft some. Not a major amount but the percentage would be dictated by what draft you are looking for.


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## beavis (Dec 11, 2006)

> I am confused as to your frame of reference for the air plane wing example.  I am thinking of it as the wing is stationary and the air is moving at some known speed 'X'.  (same thing as the air being still and the wing moving just easier to picture what is going on in my head)  You say that the faster moving air is next to the surface not away from it.  But if the wing was not there the air would be moving at X MPH.  So now the wing is put in place and the air has to go around the wing in order to return to its normal flow direction and speed.  But the wing is in the way...it slows the air down.  The air 20 feet from the wing is still going X MPH,  the air that is going around the wing but is still in a laminar flow is going X MPH, but the air that is near the wing is going X-skin drag effects MPH, Furthermore the molecules right next to the wing are theoretically not moving
> 
> 
> 
> ...



i will start a thread in the off topic for you and I to [smiley=1-boxing1.gif] this one out so we don't bore people here.


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## Brett (Jul 16, 2008)

Morning beavis. Take a look at the images on the link
in the post "start from scratch". See the height of the transom?
I've already designed higher than I need so I can cut down.


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## beavis (Dec 11, 2006)

its afternoon for me.

Very nice build! Is that the engine you are going to be running on it?


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## Brett (Jul 16, 2008)

Yes, there's an explanation in the "start from scratch" post somewhere.


This is the design of the angled tunnel:










The problem with the image as shown on the web,
is that it shows the tunnel horizontal.
(bottom middle of page)

< http://www.bateau.com/studyplans/XF20_study.htm >

The typical skiff runs 4 to 6 degrees from horizontal as I've shown here.
The pocket doesn't make the water jump up much at all.
Just takes advantage of the running angle.
If you hold a straight edge to your monitor,
match the line indicating suface of water,
you can see how the true water level fits the pocket.


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