Inflatable Hulls

Any solid craft displaces water even when its skimming and the impact of every ripple against a solid hull adds to the drag.
So...... Im testing a new design with a solid deck, ultra thin rails and a hull made of 70 Denier ripstop fabric.
Initial testing shows that low inflation gives a smooth ride and the pliability of the fabric lets the hull slip over the ripples rather than impact into them. I havent done any GPS runs yet.
I initially made a few bodyboards to test the idea in the surf and on some waves it builds up to a certain speed and then takes off with a great burst of speed so thats why Im trying it on a windsurfer.Could be barking up the wrong tree but its worth a try.
Anyone else tried this ?

This video isnt me but the last minute shows how fast an unpowered inflatable hull can be.
Check out from 10:26. No sails, no fins, I think its faster than a solid hull could go on the same wave.

I dont think its the power increase but its the drag reduction that makes it so fast.
What do you think Decrepit ?

I've had daydreams about doing this as well.

A couple of years ago at corros I sailed a mate's board that was very delaminated on the bottom, and it was like a magic carpet ride -- very smooth, felt amazing! I didn't notice any increase in speed but as I hadn't sailed the board pre-delam I wouldn't be able to say whether or not it's quicker.

Sounds like a good idea to me anyway.

Good luck with it

The bodyboard prototypes show promise and I want to be fairly sure Im heading in the right direction before I take a jigsaw to an old windsurfer.
From a bit of reading on the net Ive worked out that an inflatable hull minimises the contact area and dissipates the friction, it conforms to the water surface and because of its high bouyancy and low weight it doesnt need any more than mimimal nose rocker. Because of this it can be a lower profile shape that will lower form drag from wind resistance.
High inflation pressure gives rigidity to a surf mat but with the solid windsurfer deck and rails the board will be totally rigid but I can use a softer inflation pressure for the hull, it that will be faster across the water surface due to its pliability. The soft inflation will smooth out the chop.
Im going to do a few more test boards and then its on to the full size WS. The plan is to cut out the bottom skin and scoop out most of the EPS and then Vacbag / Epoxy glass the exposed foam and then add the fabric skin.
Heres a few of the proto bodyboard designs.

A carbon fibre twin pontoon hull version.










And an EPS foam / Innegra Perimeter design with fabric deck and hull.







yoyo, different idea behind a RIB with its displacement vee hull and rear power source Vs my idea with a planing hull and forward power thru the mast/sail.
Its easier to power a rigid craft from behind but easier to pull an inflatable from the front.

I have worked out that these two things contradict each other. How can you minimise contact area whilst conforming to the water surface. Surely this increases contact area, in turn increasing drag.

I am a big fan of new ideas and re-engineered concepts but cannot see how it can work, to go fast you need less board touching the water which causes less drag surely.......unless my logic is totally wrong. Please explain in more detail how it will work.



The very last thing you say is interesting...about pulling from the front.

I am thinking that the material used must have a high something of something..
Like a superball bounces higher because it does not dissipate much energy.
Like a formula board goes faster when sailed with stiff legs- no energy lost in knee joint flex.
More experimentation first- air inside is efficient but the base material probably needs unidirectional flex, ie up and down but not front to back.
Does a dolphin bounce well when dropped from a great height?

I sailed a yacht that was designed in a similar way. Jim Ingliss built his yacht with the same concept. 38ft and a flexible plywood bottom. When planning downwind it would flex and "conform" as such to the waves. Couldn't say if it was faster or not than a stiff hull like the norm, but it went like sh## of a shovel. That was about 20 years ago. Haven't seen anything like it since.
I think it could have merrit to some degree. Let us know how it goes.
Fez

When I'm at speed in a bit of chop, the rear entry point is just in front of my back foot, the front entry just in front of the front foot. If there's no chop, it's in between my feet.

So if you're only interested in high speed, half way between mast and front foot strap would be the maximum you'd need.
However blending the front and rear planing surfaces could be a problem, and any irregularity here would impinge on early planning ability.

Like redsurfbuss, I still have lots of doubts about the whole idea.
A planing hull works best when the lift to drag ratio is high, although you may reduce drag, I think you'll definitely reduce lift, and probably reduce the lift to drag ratio.

There is a chance though, that because of a smoother ride there will be a gain in control, in certain conditions that may mean faster.

Hi guys, Ive spent the last week testing the mini board and its going well, I usually ride a standard bodyboard in the surf so its a familiar environment for comparsion with the inflatable version.
The speed is equal or higher than a standard solid board, def not slower and apart from bursting the inflation valves in the surf Im keen to move onto the next version.
Ive also towed behind a tinny and theres a speed point where the board feels to be skimming on the tips of the ripples and leaving almost zero wake and Im 80 kgs.
Theres a lot of theoretical thinking to be done and reading to see if there are applicable examples to consider and youre input helps too...

redbussurf,
I think an inflatable hull at the right pressure has pliability to absorb minimal chop like wheat in a field yeilding to the wind and at speed it would have less resistance to acceleration and at a higher speed where the lift required in minimal, a flexi hull reduces the impact energy from running into thousands of little water peaks.
Im not clear what you mean about the "air being forced to the back of the board" but I think a fairly high pressure and a lightweight material will be the best combo.
Thank you for the offer of an article but my ideas change after each test run and I need to learn more before I open my mouth too far!

Decrepit,
I agree that the area to convert wont be the whole hull. Last week I thought it might be, but not now after the runs behind the tinny because I now know I can plane my 80 kg on 650 sq in of inflated fabric and its flying. So maybe just the back 3 foot or so ? and the air cushion only needs to be about 2 or 3 cms thick rather than a totally hollow board.

YoYo,
Im using PVC from an inflatable boat factory, its about 600 Denier, I was going to use Hypalon but I figured that a lighter material would be inherently more pliable. Ive got some 200 denier fabric from my local CLARK Rubber for the next board.
The hard deck part is about 3/4 in thick EPS so I laminated it with 3 layers of glass/ innegra and Carbon fibre for strength so Im sure its air tight.
The glues are from the boat factory too but I found their distributor in Sydney so I can source it cheaper. The inflatable hulls are a soft ride for sure and quieter.

Heres some pics of the new inflation valves...



Blue Nipples !



These valves are better because I can easily adjust the inflation while being towed to see how the ride is affected.

The new boards 1/2 finished and should be ready in a week or so but if I make a full size speed version I'd probably minimalise the board to reduce aerodynamic form drag too.

Well if you're serious about speed sailing, it's a no brainer, the Locosystems GT31

Always looking for more info on board dynamics at speed..







Taken without permission from...

Looks like the chop is throwing the boards around and its not such a choppy day but at speed hes hitting them at a greater rate. If it wasnt for the chop how much faster could he go ? Now its clear why they build up a small wall on the windward side of speed strips !

More design thinking suggests that the solid part of the board doesnt need to be completely hollow , just enough of a concave on the underside for the flexible hull to move. The latest board under construction has a 15 mm concave.



Old board , new board.

Good to meet you this afternoon Brett.

Tony

So the latest one is almost ready, its glassed and I put in a simple inflation valve. I was going to get all techno and install a high pressure valve but the operating pressures are so low it only needs something simple.




The black lines are for deck contours on the next board.







Just needs a sanding and put the hull on. .. .... ....

Obviously red, because it goes faster

nebbian of course youre right !!! It has to be Red !
yoyo, its 200 denier, weave on one side, smooth on the other. Its sold as "nylon" but it also had a sticker indicating its 'Ripstop'.
IMHO the previous grey PVC was a bit stiff to be considered for later construction.
This red/blue material feels about a 1/4 the thickness of the grey.
How did your inflatable builds go ? What did you find out ?

Sanding done.



The undersides got glue lines / thickness and weight notations on it.



Fabric ready but I need less humid weather to do the glueing.
Theres always something to slow things down.