Author Topic: Maximum Displacement Hull Speed  (Read 11452 times)

Beasho

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Maximum Displacement Hull Speed
« on: March 12, 2023, 01:11:46 PM »
Researching speed profiles for the next SUP Foil board build I came accross the formula for Maxiumum Displacement Hull Speed:

1.34 x (Waterline in Feet) ^ 0.5 = Water Speed (knots)

These formula have been around for 150 years. 

There is a caveat that a LONG THIN Shape can exceed this maximum, but this is a starting point, basically like shooting par on the golf course.
« Last Edit: March 12, 2023, 01:16:11 PM by Beasho »

Beasho

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Re: Maximum Displacement Hull Speed
« Reply #1 on: March 12, 2023, 01:13:24 PM »
The more I study on this subject the more things suggest an EPIC Kayak shape, or 8-Man Rowing Scull for best performance.

Hence the Barracuda shape, and the importance of the slender tail to reduce the stern wake phenomena.

@ 1:20: "The boat is starting to climb its own bow wave . . and that is the point of diminishing returns."

https://www.youtube.com/watch?v=H8hvKZ73S0Q
« Last Edit: March 12, 2023, 01:16:50 PM by Beasho »

clay

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Re: Maximum Displacement Hull Speed
« Reply #2 on: March 12, 2023, 02:54:05 PM »
I appreciate you starting these threads and sharing your engineering knowledge.   And it's cool how the YT algorithm is great at recommending similar videos that could keep me learning for days.

I am wondering how to cheat ventilation?  I've already done the big foil big board thing, and ventilation in turns is what got me going smaller and smaller.

How do we get lift on demand or retractable lift or multi stage lift?  We use the bigger board to get flying on the smaller foil, how do we get flying on a bigger foil and then transition to smaller foil once up?
Aloha, I welcome and appreciate all responses of positivity and good feeling.

YouTube: https://www.youtube.com/channel/UCOIE6FWr1SpWvbPJIIiEgog

Dontsink

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Re: Maximum Displacement Hull Speed
« Reply #3 on: March 12, 2023, 03:32:19 PM »
This is interesting.
The longer=faster displacement hull is something i have been told since i started sailing as a kid.
This explanation is the best i have read:

https://guillemot-kayaks.com/sea-kayak-recreational-kayak/petrel-play-petrel/are-longer-kayaks-really-faster

I still have to chew trough it but longer=faster is not that simple it seems.

Lots of info and studies on kayak hulls,which are longer than ours but a lot closer than sailing boats in dimensions,weights and speeds.

http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm

Beasho

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Re: Maximum Displacement Hull Speed
« Reply #4 on: March 12, 2023, 06:39:15 PM »
This is interesting.
The longer=faster displacement hull is something i have been told since i started sailing as a kid.
This explanation is the best i have read:

https://guillemot-kayaks.com/sea-kayak-recreational-kayak/petrel-play-petrel/are-longer-kayaks-really-faster

I still have to chew trough it but longer=faster is not that simple it seems.

Lots of info and studies on kayak hulls,which are longer than ours but a lot closer than sailing boats in dimensions,weights and speeds.

http://www.oneoceankayaks.com/kayakpro/kayakgrid.htm

This is great stuff.  I like the following graphic.  It shows where a Barracuda shape (Yellow graph) ~ 8 feet X 19 inches maxes out at 4.5 mph.   At that point you better be porpoising and converting to hydro-foil lift. 

Admin

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Re: Maximum Displacement Hull Speed
« Reply #5 on: March 13, 2023, 03:24:11 AM »
Did you guys come across this vid?

https://www.youtube.com/watch?v=Ugx-jT6k-aY

Very interesting to see performance across the speed range, especially considering our range (slow) for foiling when we are still in the water.  I found myself watching the "dock" to estimate dock start speed.  :). The flatwater paddling speed is super interesting to me as an aspiring swim foiler on hard to catch bumps.  Bump riding speed (non foiling) is of course much, much faster, so that becomes the goal.  Initial paddling burst, catch the bump, foil.

Surfskis, OC1, Crew boats, Prone paddleboards, etc are typically narrow, round bottom, true long pins without a wider Barracuda triangle tail (not a pin at the waterline) or a windsurfing style "planing" flat (rail to rail) nose rocker.  Those are elements that I believe were added to the Barracudas for SUP stability.  If we are considering the Surfski etc as true displacement designs, then the Barracuda design would diverge from displacement.  Really more of a narrow planing design, with a narrow squared tail at the waterline.  That balance of features seems to work great for its purpose, but aside from meeting a balance requirement, this is not really a displacement design and is not likely the most efficient at our low speeds. 
« Last Edit: March 13, 2023, 10:25:57 AM by Admin »

Beasho

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Re: Maximum Displacement Hull Speed
« Reply #6 on: March 13, 2023, 09:25:11 AM »
Did you guys come across this vid?

https://www.youtube.com/watch?v=Ugx-jT6k-aY

A few observations about the video:

1) The vessels appear to be ~ 6 feet long.  As they progress the colors alternate yellow / blue in what look like 1 foot increments

2) The speed increases from 3 feet per second to 10 feet per second.  This is equivalent to 2.0 mph to 7.2 mph

3) The high speed view looks much faster than 7 mph.  BUT that is what it is - Just 7 MPH.

We could assume there is a some scaling effect, but given that the length of the models is ~ 6 feet, it is very close to the length of our Foil Boards.  I would add that anyone who thinks they are paddling at 7 mph, on this short of a board, is kidding themselves given the disturbance, and therefore drag resulting from the speeds in this video.

So what is the magical takeoff speed in flat water?

Is it 3 mph, or 4 mph?  When does the foil engage FAST enough for lift off and continued flight?

Of course it depends on the foil, but that may be the next question:  What is the absolute minimum takeoff speed by FOIL per Rider weight?

Beasho

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Re: Maximum Displacement Hull Speed
« Reply #7 on: March 13, 2023, 10:39:31 AM »
Here is the upper limit of Displacement Hull SUP Paddle speed:

Connor Baxter - 200 Meters - 53.12 seconds = 8.42 mph. 

Note:  This is 45% faster than Maximum Displacement Hull Speed Calculation 14^0.5 x 1.34 (Knots) x 1.15 (mph/knots) = 5.76 mph

https://www.youtube.com/watch?v=IyZ3MPpPWgE
« Last Edit: March 13, 2023, 10:43:19 AM by Beasho »

SUPeter

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Re: Maximum Displacement Hull Speed
« Reply #8 on: March 13, 2023, 12:05:14 PM »
Interesting topic!  It seems to me that there is a hell of a lot more going on here than just dealing with maximum hull speeds. The shape of these boards and the way they are being propelled takes advantage of hull speed, foil induced speed and lift, and planing performance.  Greater hull speed is definitely tops when it comes to getting a board to fly. The additional speed and lift of the foil when pumping gives boards with pronounced bevels (ie. Barracuda rails) produces a narrower width as the board rises. Hence, even greater hull speed. Getting the board to completely leave the water (if only for a split second) , narrows the board a little more and then takes advantage of its planing performance. Hence the very flat bottom and hard release edges for most of its length.  For a flatwater pop up, All this happens in the span of maybe 30-60 ft. So all aspects of board speed become necessary, even planing.
« Last Edit: March 13, 2023, 12:21:14 PM by SUPeter »

Hdip

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Re: Maximum Displacement Hull Speed
« Reply #9 on: March 13, 2023, 12:10:36 PM »
Here is the upper limit of Displacement Hull SUP Paddle speed:

Connor Baxter - 200 Meters - 53.12 seconds = 8.42 mph. 



That math doesn't take into account him starting from a dead stop. Or switching hands. Does anyone have actual GPS data of paddle speed?

jondrums

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Re: Maximum Displacement Hull Speed
« Reply #10 on: March 13, 2023, 12:45:02 PM »
There is a caveat that a LONG THIN Shape can exceed this maximum

You explained Connor's speed in your original post.  The board he is paddling doesn't follow the rules because it is long and thin.  I believe this exception is quite relevant to the barracuda shape as well.

SUPeter

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Re: Maximum Displacement Hull Speed
« Reply #11 on: March 13, 2023, 02:23:31 PM »
All these boards take advantage of their partial planing hulls, if even only for brief moments. The more powerful the paddler, the longer and more frequent these brief moments of planing become, making displacement hull rules and regulations unworkable.

PonoBill

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Re: Maximum Displacement Hull Speed
« Reply #12 on: March 13, 2023, 11:13:51 PM »
Sheesh, welcome back to 2007 or thereabouts. I took a lot of shit explaining this stuff here back then. The first time I brought it up I got all kinds of folks explaining that hull length was meaningless, and it was just all about the paddler--which is almost true. If a paddler has enough power to overcome skin friction and hit the limitations of wave drag, then going to a longer hull makes them faster. If not, it doesn't. But that requirement fits a lot of paddlers.

Most of the serious work regarding boat hulls happened about 100 years ago--very sophisticated work. The MIT hull testing tank dates from about 1910, and a huge testing tank was built in England about the same time. If you really want to understand this stuff you mostly have to dust off some very old books.

I would never call SUP hulls of any kind planing hulls or even semi-planing (which is a hull type I've never heard of before). At best they are semi-displacement, meaning a hull that could plane if you applied a lot of power to it. And humans can't do that. The reason Connor can go 8.42 mph is that he can stand on a 20-inch wide board and still paddle hard enough to overcome skin friction and push over a mostly-flat bow wave. He's got a length-to-width ratio of 8.4, which is close enough to the magical 10.0 to mostly eliminate the stern wave, which makes the period of the bow wave much longer and therefore less steep which makes it easier to push over the bow wave if he's strong enough. Change that board width with the exact same hull design to 30" wide and he'd never get there.

Some definitions of planing refer to pushing over the bow wave but ignore the fact that it really means escaping the displacement limit entirely, moving to a lower drag domain, and maintaining that low drag through copious applications of power. A planing windsurfer has two third or more of the hull out of the water (and a sail that provides a few horsepower, compared to the puny one horsepower a tour de France rider can output with an ergonomically superior device that optimizes the output of their entire body). I've watched Connor paddle a lot, and I've never seen him do that. No paddle-powered hull can plane, and certainly, no SUP with its ergonomically disastrous long paddle and moderately braced standing position. No one talks about surfskis or OC1s planing unless they are downwinding and dropping into swells. Because they don't. But they are generally faster than SUPs because the paddler has ergonomic advantages and the length-to-width ratio is even better. No one talks about sculls planing, though their length-to-width ratio is even higher and the ergonomics of the sliding seat and long leverage for the oars is far superior. K1's do 10 to 12 MPH, nobody claims they plane.
« Last Edit: March 13, 2023, 11:35:32 PM by PonoBill »
Foote 10'4X34", SIC 17.5 V1 hollow and an EPS one in Hood River. Foote 9'0" x 31", L41 8'8", 18' Speedboard, etc. etc.

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Re: Maximum Displacement Hull Speed
« Reply #13 on: March 14, 2023, 04:16:10 AM »
Again, off on my own tangent but, burst speed performance sans pumping interests me the most.  The world's fastest swimmers can hit 5.3 mph without a board.  These guys could likely body surf Gorge swell.  I would be happy to burst to that speed swiming on an optimized board (hah!).  I think it is a very interesting question for foiling regardless of the final design for an individual purpose.  Aside from tap-downs once already foiling, all "planing" elements are actually being used for balance and pump initiation.  Without pumping and eventually foiling, those are slowing elements.  Same is true of the triangle tail.  At the lower side of the range it will slow paddling speed.  I think the video gives you a great idea of where each design gives way to the other.  I only wish they had posted an active speedometer, but I think we have some pretty good clues.  Also, very interesting to see the pitch on the "displacement" hull at speed.

« Last Edit: March 14, 2023, 04:49:13 AM by Admin »

SUPeter

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Re: Maximum Displacement Hull Speed
« Reply #14 on: March 14, 2023, 07:02:50 AM »
I'm not an engineer but does nothing take into account the apparent weight of the supposed displacement/planing  hull and just how much water that hull is actually displacing once the foil begins to apply a considerable amount of lift.  When the foil lifts 90% of the board and rider weight, could not the very features designed for planing then become more advantageous versus the displacement portions of the same hull. Having spent considerable time doing flatwater pop-ups, the hopping/slapping phase, just before lift off, consists only of the very flat bottom of board temporarily touching the water's surface .  If the apparent weight of this board is only a fraction of its actual weight(from foil lift), could not planing occur during each, ever accelerating, touch down? just wondering.

 


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