Subject: Re: : Re: : Re: : Re: : Re: [harryproa] Re:: UptiP foils
From: "Rick Willoughby rickwill@bigpond.net.au [harryproa]" <harryproa@yahoogroups.com.au>
Date: 12/15/2014, 5:03 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

David

Physics of cavitation prevent a foil borne craft from going as fast as a wing borne craft.  Paul Larsen had to sort out his supercavitating foil design to get the boat much over 50kts.  And that foil is only holding him down and resisting leeway.   The wing provides the lift.

The NACA 16 series or 07 series are designed for flat pressure profiles and these will give better top end performance where cavitation becomes a consideration.  They are not suited to bi-directional operation.  The vortex shedding of the large radius edges would be unbearable. 

Cavitation is the driving influence on high speed hydrofoils and becomes increasingly more challenging above 20kts.  Conventional planing hulls will reach those speeds without any additional lift from foils or wing.  Slamming is not an issue with a long slender hull:
https://www.youtube.com/watch?v=aKUwK4NOUs8

The 18m proa I sail on has a bi-directional cambered swinging dagger board.  It is 2m deep and chord is 500mm.  It is fibreglass over an aluminium former. The nominal section is a NACA 6516-15 mirrored - edges are nominally 4mm radius.  There is no discernible vortex shedding.  The thickness is based on strength requirements and cavitation is not a consideration upwind in this boat.  The dagger board improved windward VMG by 10% to 15%.  Pointing ability by more than 10 degrees.  The board has a limited working range.  Once the boat is more than 60 degrees to the apparent wind the board starts pulling the boat to windward.  That slows the boat compared with just using the rudders.  This is a brief clip of the board looking down through the deck slot during testing before the lateral brace was faired:
https://www.dropbox.com/s/cjaexyps6wqgu2i/100_2307.MOV?dl=0

The significant advantage of the cambered board is that it only needs a very narrow operating range.  Meaning the edges can be "sharp" so there is no extra drag from blunt trailing edge and importantly no annoying hum from vortex shedding.  

The disadvantage of the cambered board is that it is producing lift AND drag whether it is needed or not.  That is unlike an uncambered board where the leeway creates the AoA and associated lift.  Of course the problem with the uncambered board is that it needs to work over a much wider range so needs much larger radius edges and is going to have added drag as well as severe vortex shedding.   


Foiling boats have a narrower operating range than planing hulls.  That is why I suggested you do a VPP.  You need to consider the range of operating conditions.  You can choose large foils to foil in moderate wind but not be ideal for strong wind or you choose small foils and only get foil borne in strong wind.  Unless the foils can be lifted clear of the water you have a lot of extra drag at displacement speed.   A good displacement hull will have L/D over 30 at low speed; well above what any foil will achieve.

You cannot think about the foils in isolation at one particular condition.  You need to cover their application on the boat for all points of sailing in varying wind conditions.   The analysis needs to include tests for foil cavitation. Also be mindful that there is not much validation of modelled data for blunt trailing edges and you can bet they are going to hum like a crying baby.

Rick
  
On 15/12/2014, at 6:37 PM, "cruisingfoiler@yahoo.com.au [harryproa]" <harryproa@yahoogroups.com.au> wrote:

<< The fastest sailing boat in the world is a planing proa>>

We're getting into semantics here: http://www.wired.com/2013/01/ff-paul-larsen-sailrocket/all/

" Despite the fact that his instrument cluster isn’t working, Larsen carves the turn perfectly. We accelerate the whole way. Except for the lack of engine noise, I can’t believe we’re not in a speedboat. SailRocket is starting to vibrate, hard. The boat’s three pods are sending up three Jet Ski-like rooster tails of spray. And then, suddenly, there are only two rooster tails—the wing has pulled the rear pod completely free of the water. I know this is what’s supposed to happen at 26 knots—the second crucial transition—but it’s alarming to feel the boat surge ahead once again. We’ve just shed a full third of our hull drag.

Larsen completes the turn somewhere between 30 and 40 knots, just in front of the sandbar. We’re out of the chop now, and the flat conditions give us another burst of speed. The zodiac, which had been chasing us, has now fallen far behind. Another few seconds, and a glance to the boat’s opposite side reveals that we’ve made the third crucial transition: the point very close to 45 knots when the leeward pod, the one under the wing, lifts free and takes to the air. SailRocket is now more plane than boat. We’re bombing along on just the front pod and the foil. There’s not much more wetted area than you’d find on a boogie board, and yet cantilevered behind that front pod is a massive machine—40 feet long and 40 feet wide."

 

Anyway, I don't accept the argument that the fastest things in the world don't (or merely incidentally) use foils therefore, by implication, we can find better ways.  Earlier in this thread I discussed at length (and drew attention to all that Tom Speer has said concerning) the merits of foils for higher speed, increased stability and seaworthiness, and for sea kindly motion - something I wouldn't describe planning craft as having.  So I'm not about to repeat those points.

These other matters aside, foiling is the proven way of increasing speed of sailing craft.  I don't need to go into what foiling has done for C-Fly, Moths, C Class and of course AC72.  Foiling is accessible.  It has its pitfalls, but these represent challenges that inspire me.  If you're interested in improving performance and motion, but not interested in hitting 40 knots then I can't see a better way.  I'm not saying there isn't one, but I can't see it.

I've already seen your proa video and the craft is impressive.  What bi-directional profiles do you advocate?

I'm very familiar with all things vector foil and canted rigs.  The vector foil is a great idea for pure speed but would not want to see it come out of the water (fouled or otherwise).

I'm presuming from your latest post, perhaps incorrectly, that you have no further critique of my foil exploration specifics and the uncertainties involved as I've articulated them?  Your critique has been helpful, even if we disagree concerning future directions.

I've compared my foils to Speer's H105 at Reynold's 4M and my foils maintain Cp > -0.6 through almost 6 degrees compared to less than 4 for the H105.

I'm drawing up some new ideas to see if I can further widen the pressure envelope.

 

David


__._,_.___
Posted by: Rick Willoughby <rickwill@bigpond.net.au>
Reply via web post • Reply to sender • Reply to group • Start a new topic • Messages in this topic (29)

.

__,_._,___