Subject: Re: [harryproa] Rudder Ventilation
From: Rick Willoughby
Date: 8/15/2010, 1:45 AM
To: harryproa@yahoogroups.com.au
Reply-to:
harryproa@yahoogroups.com.au

 

Rob

One of your earlier comments about the vortices coming off the square chine got my interest.  If you are using a fixed symmetrical rudder at the bow then you have to suffer leeway in the hull unless it is set with some AoA.  The sideways component will be the cause of the vortices.  The hull is not a very efficient means of generating lift so this will add quite a lot to the hull drag.

Also your comment about seeing the ventilation on the Tom Speer asymmetrical section means it would be giving lift.  If you are not seeing the same thing with your symmetrical rudders then you are relying on the hull to provide lift.

Any rudder that is moving at 18kts and giving substantial lift and have surface exposed without fencing will be  ventilating.

The performance difference I gave for the two aspect ratios is related to the non-ventilating condition and getting all the required lift from the rudders.  If you are relying on the hull to provide some lift then this will degrade performance substantially more than I have estimated against the high aspect rudders providing all the lift.

The boundary layer thickness along the hull is not very thick.  On Dennis's test mule at 15kts it will be about 30mm at the bow rudder and 90mm at the one aft.  And this will still be working but at reduced effectiveness.  Actual reduction in effective area would equate to reducing the depth by 20mm and 60mm respectively.

I agree there are some engineering challenges to setting up the drop-in blade that can rotate.  It just seems a good use of space in a hull that is otherwise empty.

Rick
On 15/08/2010, at 2:15 PM, Rob Denney wrote:

 

Thanks, 5% is livable with, at this stage. 

I understand the difference between the end plate and no end plate, and why ventilation is more likely with a surface piercer, although I have hit 18 knots on El and Rare Bird, with no signs of it happening.  Neither set of foils is perfect 0012 in shape or finish.

I don't understand  why surface piercing is different to end plated, as the flow cannot get from one side to the other around the end any easier than with an end plate. 


Other comments for consideration with in hull and out of hull rudders.

Hydrodynamically, the top of the in hull blade is working in the turbulence attached to and caused by the hull, plus that of the slot and the box if these are not perfect fits. The out of hull rudder is not affected by this.

Generally,
You can see and clear weed and plastic from an out of hull rudder
It is less prone to damage, leaks and growth.
You can see the flow over the rudders and tell when they are stalled.  This will be pretty critical for a 100mm wide foil, I think.

For a balls to the wall racer kept out of the water, with a long skinny foil, the in hull is best.  For a cruiser, out of hull.  Need to decide where you fit.

I like the beam mounted, but hope you go with the in hull, so we have some actual points of comparison.  Either way, put as much of  the fitout required for the in hull rudders in before you put the sides on or you will be working in a very small space.

rob


On Sat, Aug 14, 2010 at 8:23 AM, Rick Willoughby <rickwill@bigpond.net.au> wrote:
 

Dennis

I need to make significant qualifying remarks on the comparison with under-hull and surface exposed rudders.  

At very low speed where the pressures are low they are essentially the same because the gravity waves formed around the blade are not very large and do not affect much of the blade.  The rise and fall of the surface around something moving through the water reflects the water pressure.  

At higher speed the pressure, as indicated by the height of the water level around the blade, becomes significant relative to the size of the blade.  The hole on the low pressure side gets bigger to become full ventilation.  

The AR correction used in JavaFoil is based on a wing with elliptical plan form.  So it has tip losses on both sides.  With the rudder against the hull it is as you describe with only one  end suffering losses. So AR correction in JavaFoil is going to be half of the for a rudder under a hull or effectively fenced.  Hence if it has elliptical plan form you should double the AR value. If not elliptical the AR used should be a little lower.   

Rob
The comparison for the 350 deep by 200 long rudder to the one with AR 6  is not as bad as I calculated.  The respective corrections should be based on AR of 12 and AR of 3.5.  For these cases the L/D for the same lift is 46 versus 22.  (If not an elliptical blade form it will be a bit less but not much)  The speed difference will be 4 to 5% depending on the blade shape.


Rick
On 13/08/2010, at 11:07 PM, Rick Willoughby wrote:

 

Dennis

As far as carbon goes it means that you could get away with something that small and handle the loads if you need to. It would pay to confirm shapes first with whatever is cheap and easy to make.  At 15kts the forces are halved to what would be there at 20kts.

You can make the slot a little bigger to allow for say 150mm chord.  This makes a big difference in the blade area for same AR or you can reduce the stresses significantly by going bigger chord with lower  AR for same area.

Hobie have done a wonderful job of marketing the flappers.  There is no doubt it is a clever system.  I have used oscillating foil drives but found rotating foils (propellers) to be a long way ahead.  I have a paper on the Hobie foil I did with a guy who owns one.  At normal cruising speed in the Adventure hull they are around 36% efficient and they get to 53% at energetic level.  My props have efficiency in the mid 80s.  This video shows the sales pitch:
How many times did you hear efficient?  If you want to see an efficient pedal boat then take a look at this:
Or this one at more sustainable power level:
This is one my sister took of me on the V14 boat while she was moving along in her flapping Hobie.  

The rudder against the hull is the same as the surface apart from the fact that the air can get down the blade easier when the blade cuts the surface than when it is under the hull.  The reason for the higher aspect being more efficient is that the flow from the pressure side to the suction side around the bottom of the blade is a smaller proportion of the total flow over the blade than with the low AR.  The flow is not 2D but rather 3D.  Some water is curling around the bottom from high to low pressure creating tip vortices.  This cannot happen at the top of the blade if it is against the hull or continues up through the surface.

The main reason to suggest the under-hull rudders was to eliminate ventilation but it should also make the whole set up more rigid.  You do not need to do the detail engineering for the actual blade but having a well built into either end of the lw hull now that will accept a 6" long rudder would make it easy to try it out.  Then there is the difficulty of engineering something in the beams or off the side of the hull that can take the loads without causing extra drag.  I am reminded of the issues that Todd spoke about with his linkage.  For best effect you want the rudders linked with some easily adjusted cam set up to make small corrections to relative blade angle.  I am suggesting a trim range of about 3 degrees of one blade relative to the other.

Rick
y You may need something a big bigger at slower speed because the 
On 13/08/2010, at 9:51 PM, Dennis Cox wrote:

 

I don't want to leave a pretty rock unturned... so let us talk this through.  Being an engineer, I tend to be pessimistic in my evaluations... but even with that, the numbers I'm running are really swaying me.  So here we go... cons first then pros.  (want to leave on an optimistic point).
 
Con - Carbon - At first I almost stopped at the mention of carbon.  If I haven't said it, its an oversight on my part.  I am not going to put carbon or anything that is remotely expensive on My Little Mule (MLM).  If MLM doesn't break-up and sink on the first outing... we'll talk about replacing parts with carbon.  Right now... out-of-pocket is projected to be around $200 total... in the water.  However, I did do some napkin analyses anyway... so read further.
 
Con - Rudder Strike - A very big issue.  I've demolished a daggerboard on an unmarked old bridge pier before.  I can and will find that needle in the haystack.  Its a curse I have.  Due to the high loads, I believe trying to make it pivot out of the way may potentially add too much flexibility to the issue.  Using a plug system like you have... I could just make extras and let them be sacrificial.  When I run out of extras... I go home.
 
Aside - Rick, that is one beautiful piece of hardware.  I've seen aerospace engineering prototypes that didn't look that good!  Makes perfect sense... build all the complexity (and expense) into one unit and you can pull it from one of your prototypes to another.  Hell, with a few modifications, you could pull it from one boat and put it in another boat in seconds for instant real-world trade studies.  Real COOL!  BTW - have you looked into the Hobie dolphin flipper thing?  Its unit is self contained and they at least market at its fluid dynamic superiority.  Out of my field, but I'm curious to your assessment.

 

Pro - Wing Symmetry -  This is the HUGE pro... so correct me if I'm wrong... because I depend on it below.  It is my understanding that because its on the bottom of the hull, the wing acts like its on a wall of symmetry (as compared one that has to go through the water surface).  In my simplistic mind... the ventilation can not get to the rudder!  THUS - Its aspect ratio is effectively doubled.  If I am correct, then the rudder can be make with an AR = 2 (benefiting structural) yet still act like an AR = 4 (benefiting fluid-dynamic).    ???????????????

 

Pro - Structural - So here is the back of the napkin analyis using fiberglass tow.  I'd probably do a finite element analysis.  Besides strength, I'd want to check resonant frequencies of the structure (including the boat).  Wouldn't want it to flutter itself to death at 30 knots!    

  1. Rudder area - I'm not sure if I'm at the point of determining proper rudder area needed to efficiently support lateral loads AND have some extra lift available to actually turn the boat.  So I'm assuming your numbers below already do that.  At AR=4, you have:  0.7m * 0.7m/4 = 0.12 m^2  (190 in^2)
  2. Assume rectangular plan view.  Although some kind of eliptical or tapered plan view would benefit both structural and fluid-dynmic issues.
  3. New rudder dimensions @ AR=2.  Length = 0.5m (20 in) Chord = 0.25m (10 in) Thickness = 0.025m (1 in)
  4. Deflection using simple beam analysis.  I get a lateral deflection of 8 mm (0.33 in) at the rudder COE.  The tip will be more than double that.  Too lazy to calculate it... even if it is... say 25 mm, that doesn't seem like too much deflection to me.
  5. Bending strength safety factor is nearly 6!

Pro - Fiberglass - I happen to have a spool of fiberglass that I don't consider out-of-pocket. 

 

 

 

From: willoughby_rick <rickwill@bigpond.net.au>
To: harryproa@yahoogroups.com.au
Sent: Fri, August 13, 2010 3:01:27 AM
Subject: [harryproa] Rudder Ventilation

 

Dennis
It is poets day here right now and my thoughts are already onto the weekend activities.

I have been thinking about the difficulty of managing rudder ventilation. Fencing is one method and I have another idea to try without using fences but with the talk about mast bearings I started to think about through-hull rudders.

If you use rudders 0.6 to 0.7m deep they should only need to be about 0.1m chord length to develop the required power. They will certainly turn you if they cannot completely cancel leeway. With that in mind I was thinking you could build a well into either end of the hull that goes from bottom to deck that can accept a drop-in rudder bearing block.

I designed something similar to this for a drop-in pedal drive leg although in this case the well was within the cabin:
http://picasaweb.google.com/adventuresofgreg/ExpeditionBoatBuilding?feat=flashalbum#5378098100766812562
The block that takes the shaft tube is moulded in chockfast that sits inside a matching CF well that is higher than the waterline. This shows how neatly the block matched the hull:
http://picasaweb.google.com/adventuresofgreg/ExpeditionBoatBuilding?feat=flashalbum#5377035108790569554
And from inside the cabin:
http://picasaweb.google.com/adventuresofgreg/ExpeditionBoatBuilding?feat=flashalbum#5378098468295589570

You would make rudders as blades that slide down into a slot within a swiveling tube. That tube would be supported by bearings in a drop-in block that mates to the well.

A rudder that is ventilating has about 30% of the lift of one that is not ventilating. So there is big benefits in avoiding ventilation.

A well in the hull may actually simplify the rudder mounting and reduce the bits that need to hang off some mounting point. It reduced the forces at the bearings because they are as close to the blade as you can possibly get.

The forces on even a small blade like this are not trivial. If you ever manage to get to 20kts each of them will be capable of generating 2400N. The force will ultimately be limited by your righting moment but it still ends up being significant at the rudders Maximum lift occurs at 6 degrees so not much room for wobbling rudders with floppy supports.

You may never get to 20 knots but if you do not design for it in the first place then you could break lots of things in a thrilling way before it all gets robust enough.

Rick



Rick Willoughby
03 9796 2415
0419 104 821



Rick Willoughby
03 9796 2415
0419 104 821




Rick Willoughby
03 9796 2415
0419 104 821

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