Author Topic: Maximum Displacement Hull Speed  (Read 16041 times)

Admin

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Re: Maximum Displacement Hull Speed
« Reply #45 on: March 31, 2023, 03:05:50 AM »
Yes.  For surface paddling at low speeds, less paddling force will be required on an otherwise similar round bottomed, displacement board than on these narrow, flat bottomed, planing boards with hard angled chines.  The smooth flow of water around the displacment hull is exactly what makes the round bottomed design efficient in this limited lower speed, surface bound setting.  They do however, sacrifce stability, and I imagine they would be awful to pump off of the surface as a SUP Foilboard. The flat bottomed boards (which force water downards and outwards) have been narrowed and lengthened now to the point where they are much faster paddling than their short, wide, predecessors but as a paddling only solution this would not be the most efficient choice.  Fotunately, that is not what they were designed to do.

 
« Last Edit: March 31, 2023, 03:43:47 AM by Admin »

SUPeter

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Re: Maximum Displacement Hull Speed
« Reply #46 on: March 31, 2023, 04:10:01 AM »
There is a ton of interesting content in this thread for a design newb like me (thank you to the contributors). 

One thing that I'm trying to better understand is how to compare hard chines vs a more rounded hull on a pintail downwind board.  I generally understand the pintail, but more designs have very hard chines in the rear portion of the hull leading into the pintail (Baracuda, FFB Dagger, Sultan, Axis, Etc...).  There are some exceptions with more rounded features (Dale Chapman and Sunova), but they are not the norm.

Wouldn't the harder chines create eddys and drag?  wouldn't softer chines allow for better water release and perhaps less wetted area?



Please help a newb understand the hull design philosophy!  Thanks again.

Just the reverse-  water likes to stay attached to curved surfaces as it flows. Not only is water moving longitudinally along the length of the board but, it is also moving laterally as the board slaps down onto the waters surface as we pump and hop. These hard chines allow this lateral moving water to relaease .  When hulls are curved, the water will ride up along that curved hull trying very hard to stay attached to the hull.  All this added water weight is detrimental to an easy and clean separation of hull from water. I hope this helps.

Thanks for the reply - this is helpful.  If I understand correctly, my assumption that the hard chines were disturbing the flow, wasn't wrong, but it is an oversimplification (assuming linear flow).  My oversimplification didn't take into account the priority of releasing the hull from the water surface. 

So basically if the board weren't on a foil, the rounded chines would be faster, but because the board is on a foil and needs to release from the surface, the hard chines are a beneficial compromise.  But in that case, why do foiling monohull sailboats generally have rounded hull features (AC36)?

Taking this one step further, why do these boards then have soft chines towards the nose?  On a touchdown, isn't release a priority again (in which case harder chines would be beneficial)?

Am I understanding this correctly?

Racing sailboats have almost unlimited power so all that is needed is/are hulls with a high hull speed. No hopping, pumping required. Paddlers have relatively very little power which requires the use of hopping and pumping to engage the foil for additional speed as well as to break free of the water's surface.  The reason the very hard edges generally do not go all the way to the nose is just a compromise to make a hull a bit more forgiving on high-speed touchdowns, as far as I can tell. Some makers bring the hard edges all the way forward and some totally round off the front end. They all work to greater or lesser degrees. Again, compromise is the order of the day.



JohnnyTsunami

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Re: Maximum Displacement Hull Speed
« Reply #47 on: April 03, 2023, 09:04:33 PM »
Just wanted to comment on the "bob" that is done with SUP foils and is likely not factored into anyone's historic equations. The video of the 1st page with the SUP racer you can actually see the tail sucking water up the back just like the scientific a/b test of the canoe and the planing hull. The SUP sucks the water for only a portion of the stroke because the board sort of gets out of the water in conjunction with the riders momentum and seems to almost plane for a moment.

These race SUPS have hard a vertical stern section to allow for this - I believe - as do the barracudas.

I have watched a world class SUP foiler get his sup up in flat water. Bobbing was a major factor. In fact, he mounted the same foil on a friends board +20L with a wider tail. The tail produced so much lift that he couldn't get it to sink for a pump and couldn't "bob" the board and pump the foil. Something to consider.

As with all things foiling I think there is a golden area. Bigger is safer and "easier" to not fall on, but actually harder in every other way.

I have a 4'8" wing board with no bottom contours and a vertical tail rail. The thing indeed won't go above 3mph slogging (I have been passed easily by beginners slogging on their 6' 130L boards!) But, I can pump the s*it out of the thing and unstick it and get it going 8-10mph required to take off on <600cm foils easier than a bigger board with rounded tails.

I'm building a SUP DW board, 7'6" x 19" 100L. I have it shaped with zero hard chines. I'm hoping it will get me up to speeds where I can bob the foil and catch big swell. Inspiration from KT's board for Kane. Maybe something there in ditching the chines. Obviously they would be good for clearing water at planing speeds, but just cause drag below those speeds?

instagram.com/p/Cosy8HiuTaL/

https://www.instagram.com/p/Cosy8HiuTaL/


StellaBlu

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Re: Maximum Displacement Hull Speed
« Reply #48 on: April 04, 2023, 05:23:36 AM »
There is a similar chat going on at one of the other forums and there are a few firsthand comparisons between harder chine boards (barracuda) vs softer chine displacement hulls (Appletree, Chapman, Gong) and several people say the speed of the softer chine board makes it easier to takeoff than the hard chine.  Perhaps designs like the barracuda  overstating the impact of “release” and compromising speed too much?  Early days here…

Solent Foiler

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Re: Maximum Displacement Hull Speed
« Reply #49 on: April 04, 2023, 05:52:03 AM »
I'm currently waiting for the build of my DW board to be finished, and the tail design was a real headache. I opted for a Gong/Casey inspired, more displacement style tail with the hard chines ending just behind the box. The original drawing from my shaper had the hard chines finishing in front of the box, but I worried it might be too sticky on touchdowns.

The main reason for going for a more displacement style tail was I felt that as this was my first DW board I'll need all the help I can get in getting up, so a planing release edge wasn't going to be the answer there. Also, all the 3 boards I borrowed were pin style tails, and I wanted to pull in the max width as far as possible but keep that width out wide along the rail for as long as possible for stability which is the opposite to a pin tail. Volume is relatively aggressive for a first DW board with no SUP background being +20 my wet weight.

Board dims are 6'10 x 20 x 90L. Have another 10 days or so to wait to see how many design mistakes I made. Haha!
I'm 5'10", 66kg riding:
Swift Foil Boards custom 4'10 x 19.5" 35L
Gong Lethal 4'6 65L
Axis ART 799, 899, 1099, HPS 880 US & CS Adv fuse, 85cm mast
Gong Fluid L-S, XXL-S on 85cm and 65cm mast
Takuma RS 5.1, 4.3, 3.5

StellaBlu

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Re: Maximum Displacement Hull Speed
« Reply #50 on: April 04, 2023, 06:26:48 AM »
I'm currently waiting for the build of my DW board to be finished, and the tail design was a real headache. I opted for a Gong/Casey inspired, more displacement style tail with the hard chines ending just behind the box. The original drawing from my shaper had the hard chines finishing in front of the box, but I worried it might be too sticky on touchdowns.

The main reason for going for a more displacement style tail was I felt that as this was my first DW board I'll need all the help I can get in getting up, so a planing release edge wasn't going to be the answer there. Also, all the 3 boards I borrowed were pin style tails, and I wanted to pull in the max width as far as possible but keep that width out wide along the rail for as long as possible for stability which is the opposite to a pin tail. Volume is relatively aggressive for a first DW board with no SUP background being +20 my wet weight.

Board dims are 6'10 x 20 x 90L. Have another 10 days or so to wait to see how many design mistakes I made. Haha!

Care to share any pics of the design?  Sounds interesting.

Im getting a downwind inspired wingboard made, and am struggling with a few of the features.  I was thinking hard chines and V Tail, but now I'm second guessing it.  The purpose will be for (a) lightwind winging, and (b) downwind winging allowing me to use smaller wing sizes.  I don't intend to paddle up on it, and will probably have more power than a paddle (with the wing), but the design theories are largely overlapping.  Im probably going 6' x 19.5 / 80 Liters or so (Im 80L and daily driver wing board is 60L which I can get up in 8-10 knots). 

JohnnyTsunami

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Re: Maximum Displacement Hull Speed
« Reply #51 on: April 04, 2023, 08:01:15 PM »
There http://is a similar chat going on at one of the other forums and there are a few firsthand comparisons between harder chine boards (barracuda) vs softer chine displacement hulls (Appletree, Chapman, Gong) and several people say the speed of the softer chine board makes it easier to takeoff than the hard chine.  Perhaps designs like the barracuda  overstating the impact of “release” and compromising speed too much?  Early days here…

That was my hope! Thanks for the info. The more I thought about it the more like an Olympic sprint single canoe I was thinking of.

https://www.plastexboats.com/pliki/Hydromechanics-of-Sprint-canoes-ver-final+.pdf
« Last Edit: April 04, 2023, 08:04:29 PM by JohnnyTsunami »

 


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