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    • UnderDawg

      A Few Simple Rules   05/22/2017

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sa666

Rudder stall or cavitation minimunisation

31 posts in this topic

  • Looking for suggestions 8 m sports boat
  • smooth helm movements, If I thought we were on ragged edge ,would understand but all seemed fine !
  • Conditions ,20 + knots ,moderate sea state Sailing free not pushing for height
  • Mast head A1 , boat speed low 20
  • Foils clean , excellent condition, gloss finish not nicks or damage

  • Would wet rub finish help keep flow attached

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Foils might be too smooth, sand with 400# works on Hobies

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Cavitation only really happens above about 40knots. Problem more likely to be ventilation - air dragged down the side of the foils. Section shape has a big impact on this.

Stall - read Bethwaite on finish, but basically the rougher the surface the sooner it stalls.

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The most important thing is the section shape - not too fat at the leading edge.

This needs expert advice and look at the rudders of similar boats to yours that are faster than you and compare and talk to them.

Other things to think of is to try different rakes on the rudder - sometimes it's good to have the tip further forward. You'll need to put packers under the gudgens to do this.

Is the boat balanced with rig position and sail sizes.

How good is the crew, the crew trimming the sails do more of the steering downwind than the skipper - rudder is more of a fine tune thing.

 

 

If none of this helps maybe be the rudder is too small for the job.

Go for a deeper rudder or a longer chord lenth or both.

So many boats don't have a big enough rudder on them.

 

Bigger rudders don't need to be turned as much so less chance of stall.

 

Some thoughts.

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  • Looking for suggestions 8 m sports boat
  • smooth helm movements, If I thought we were on ragged edge ,would understand but all seemed fine !
  • Conditions ,20 + knots ,moderate sea state Sailing free not pushing for height
  • Mast head A1 , boat speed low 20
  • Foils clean , excellent condition, gloss finish not nicks or damage

  • Would wet rub finish help keep flow attached

 

Gooday - triple6. See if you can find any record of all the work that Bob Miller (aka Ben Lexan). Also involved in most of that research were Cole, Payne, Crowther, Leverton & many others. We did 1000's of section trials in Sydney. All that info - must still be 'in records' somewhere. The answer is - while a tad complicated - it is in real terms - quite straight-forward. Foil section @ top & bottom are neither - the same nor in the same position - thus giving the rudder - that - force-fed - feeling - which is - 'pressured-up' water flow & thus much less tendency to either cavitate &/or stall.

 

You'll need to do a lot of digging - but it should all still be there. If not & if you're still looking & want - some opinions - PM me & I'll have a dig. Ciao, james

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Was the stalling quite random ie: not related to a gust, slight round up or a wave? This time of year there is lots of dead seaweed in the water and you may just have picked up a bit every now and then which could promote stalling.

 

I was watching you from the beach and you were smoking, you didn't look out of control at all (until just past the jetty that is). What happened?

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Put a cav plate on foil bout 200 mm under waterline it must stay underwater when healed it will stop air travel down foil causing loss of grip

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The most important thing is the section shape - not too fat at the leading edge.

This needs expert advice and look at the rudders of similar boats to yours that are faster than you and compare and talk to them.

[snip]

It's often the case that the section of the rudder as built doesn't match that as designed. We had a Farr designed / Cookson built rudder on a 40 footer painstakingly faired to the original templates a couple of years back (and there was a sizeable discrepancy and asymmetry), it made a huge difference to helm feel and stalling behaviour.

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1342436455[/url]' post='3787411']

Was the stalling quite random ie: not related to a gust, slight round up or a wave? This time of year there is lots of dead seaweed in the water and you may just have picked up a bit every now and then which could promote stalling.

 

I was watching you from the beach and you were smoking, you didn't look out of control at all (until just past the jetty that is). What happened?

 

Was a great ride high teens to 21.5 then rudder let go ,no rhine or reason , broached ,tack line released kite set backwards and bang Bit of a bugger was going to be fun day

 

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Looked like a long cold motor back to the club. That's where you need the emergency bottle of port

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Cavitation only really happens above about 40knots. Problem more likely to be ventilation - air dragged down the side of the foils. Section shape has a big impact on this.

Stall - read Bethwaite on finish, but basically the rougher the surface the sooner it stalls.

 

Cavilation. I invented it, it is when you don't know whether it is cavitation or ventilation. Or then it could be Ventitation... Hard to tell.

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1342436455[/url]' post='3787411']

Was the stalling quite random ie: not related to a gust, slight round up or a wave? This time of year there is lots of dead seaweed in the water and you may just have picked up a bit every now and then which could promote stalling.

 

I was watching you from the beach and you were smoking, you didn't look out of control at all (until just past the jetty that is). What happened?

 

Was a great ride high teens to 21.5 then rudder let go ,no rhine or reason , broached ,tack line released kite set backwards and bang Bit of a bugger was going to be fun day

 

Lets face it, its all a bit of a compromise. Its tough to design a rudder that feels good and reasonably easy to steer to going upwind at 6-7 knots and will also not stall out at over 20 kts. Obviously a bummer that it did stall out but a thin section that has good control at 20+knots will thus stall out at 6 knots. Have fun figuring this one out.

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I second the cavitation plate approach. Not because I've tried it on a boat but wrestled with it on sailboards. Tried everything you could in terms of fairing, symetry, flattening trailing edge and changing foil shape. Helped a bit but the loads and conditions (hopping chop with huge side loads) meant I needed a drastic upgrade. Found it first with fences. I mean DRASTIC change. Like it was on rails. Then got same benefit with perhaps less drag with a cutout shape, where trailing edge has big cut out at interface with hull where the venting progresses down the foil.

 

You could add the fence (Cavitation plate) to the same rudder and then load it up with confidence. Much better situation on a sportboat than on a sailboard.

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Done it before on multis big and small and it works a treat commonly known as ventilation plate you can tack it on till you get the best position and will cost next to nothing

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Why isn't this done on 18's, 16's VO70 IMOCA 60 (or is it?)

 

Can't be a new issue...

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Rake fwd just like on the Hobies

 

The most important thing is the section shape - not too fat at the leading edge.

This needs expert advice and look at the rudders of similar boats to yours that are faster than you and compare and talk to them.

Other things to think of is to try different rakes on the rudder - sometimes it's good to have the tip further forward. You'll need to put packers under the gudgens to do this.

Is the boat balanced with rig position and sail sizes.

How good is the crew, the crew trimming the sails do more of the steering downwind than the skipper - rudder is more of a fine tune thing.

 

 

If none of this helps maybe be the rudder is too small for the job.

Go for a deeper rudder or a longer chord lenth or both.

So many boats don't have a big enough rudder on them.

 

Bigger rudders don't need to be turned as much so less chance of stall.

 

Some thoughts.

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All boats mentioned need to be sailed really flat if not the same issues will occure , boats with rudders under the boat not transom hung are less prone as the top of the foil barely leaves the water when healed. Sailed a magic for a few seasons and had issues similar and the gate fixed instantly

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Cavitation only really happens above about 40knots. Problem more likely to be ventilation - air dragged down the side of the foils. Section shape has a big impact on this.

Stall - read Bethwaite on finish, but basically the rougher the surface the sooner it stalls.

 

Cavilation. I invented it, it is when you don't know whether it is cavitation or ventilation. Or then it could be Ventitation... Hard to tell.

 

 

i think jim is correct, cavitation is water boiling at low pressure,dont think this happens on sailboats.

try letting the main out , right out, if that doesnt work throw the hobbie rudders away and put a decent single rudder on, blokes world, peow peow, no limits and others use the same blade.

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1342524170[/url]' post='3788813']
1342481124[/url]' post='3788192']
1342382909[/url]' post='3786844']

Cavitation only really happens above about 40knots. Problem more likely to be ventilation - air dragged down the side of the foils. Section shape has a big impact on this.

Stall - read Bethwaite on finish, but basically the rougher the surface the sooner it stalls.

 

Cavilation. I invented it, it is when you don't know whether it is cavitation or ventilation. Or then it could be Ventitation... Hard to tell.

 

 

i think jim is correct, cavitation is water boiling at low pressure,dont think this happens on sailboats.

try letting the main out , right out, if that doesnt work throw the hobbie rudders away and put a decent single rudder on, blokes world, peow peow, no limits and others use the same blade.

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Hobie rudders workin just fine read the comment not our issue "fact hunt " maybe do as your name suggests

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1342524610[/url]' post='3788824']

Hobie rudders workin just fine read the comment not our issue "fact hunt " maybe do as your name suggests

 

Didn't blokes worlds rudder cost it a national title may be wrong

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I second the cavitation plate approach. Not because I've tried it on a boat but wrestled with it on sailboards. Tried everything you could in terms of fairing, symetry, flattening trailing edge and changing foil shape. Helped a bit but the loads and conditions (hopping chop with huge side loads) meant I needed a drastic upgrade. Found it first with fences. I mean DRASTIC change. Like it was on rails. Then got same benefit with perhaps less drag with a cutout shape, where trailing edge has big cut out at interface with hull where the venting progresses down the foil.

 

You could add the fence (Cavitation plate) to the same rudder and then load it up with confidence. Much better situation on a sportboat than on a sailboard.

 

are these what those little lumps on hyprotere's foils are?

I've been wondering

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Hobie rudders workin just fine read the comment not our issue "fact hunt " maybe do as your name suggests

 

Didn't blokes worlds rudder cost it a national title may be wrong

 

 

ok ,workin just fine, no problem then if its not broke dont fix it.

as for bw yes the rudder structure failed too light for those conditions,the rudder has no stalling issues. however i dont think you would get downwind with hobbie rudders in the same conditions.

 

twin rudders are also problematic in that the steering geometry needs to turn the inside rudder more accutely,they also run slight tow in, some tinkering with settup may improve things a little.

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As said problem isnt with kiss i was answering comment made in forum. Pierre has put a heep of effortt in to get it right and as far as were concerned it is setup well . Two rudders can help in heavy air as one is pressed well into water giving more control when pressed, hense why open 60 s and volvos adopt twin rudders and if one breaks we may have finished the race on one . Its all heresay anyway

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Cavitation foil: here in noumea we have it on more than ten boats:

 

first was on my elliott 10.5 pig hunter at about 30 degrees of angle rudder would ventilate it never happens after..

 

then farr 11.04 2

 

elliott 8.5

 

Whiting 1/2 tonner

 

surprise.

 

young11

 

young88

 

i make it like the one on betwhaite book, but some put it on the the back of rudder and it works too.

 

On most single rudder boat it must be quite deep as it must stay in the water all the time so many time like 60cm...

 

Guarantee you can have any profile quite bad it wont ventilate anymore.

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Foils Stalling?

Read this bras, and you'll quit sanding your foils smooth with 400-600 grit.

google

Effects of surface finish on aerodynamic performance of a sailboat centerboard.

D. S. Miklosovic, M. P. Schultz, and C. Esquivel. 2004.

Journal of Aircraft 41 (no. 5). American Institute of Aeronautics and Astronautics.

US Naval Academy

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are these what those little lumps on hyprotere's hydropteres' foils are?

I've been wondering

 

hydroptereDown.jpg

That thing could have had cavitation problems at 61 knots (70.15 mph) top end

 

End plate fences

 

image_foil_300.png

 

563480_415890008453682_1385233992_n.jpg

 

66356d1327373355-rudder-design-shape-high-speed-kite-sailer-hydroptere-rudder-foil-bulb-copy.jpg

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are these what those little lumps on hyprotere's hydropteres' foils are?

I've been wondering

 

hydroptereDown.jpg

That thing could have had cavitation problems at 61 knots (70.15 mph) top end

 

End plate fences

 

image_foil_300.png

 

563480_415890008453682_1385233992_n.jpg

 

66356d1327373355-rudder-design-shape-high-speed-kite-sailer-hydroptere-rudder-foil-bulb-copy.jpg

 

'Good fences make good neighbours'

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The Hobie 33 has a notoriously shallow rudder. When pressing hard, the boat is narrow and is a bit low on the righting moment so the rudder root gets exposed and voila, ventilation and stall. It happens fast with little warning sometimes - the wrong wave angle in a bit of chop, a puff and it's into recovery mode.

 

I've been thinking about a rudder upgrade for a few years - not an uncommon thing to do on H33s, and came across two interesting concepts. The tubercle design is intended to prevent ventilation - I've had some email conversations with the designer and builder of this rudder and read the white papers it was modelled after:

 

http://ptsail.org/20...rudder-designs/

 

It looks like the new Riptides will be featuring (standard or as an option I don't know) the tubercles/bumps on their rudders now based on the success they've had.

 

Separately, I've been talking with the Swiss designer of the ONYX (http://www.onyx-yach...ge.asp?seiid=14) who also modelled their rudder on a whale flipper to achieve the same result, but a different approach. The good pics of the rudder are on p48-49 of this PDF: (http://www.onyx-yach...X_Heft_2011.pdf) and if you read German you can get an idea but it's basically the same overall concept. The designer said he's been experimenting trial-and-error with rudder designs for years, his goal being to reduce abrupt stalling of the rudder on overpowered boats, and he claims his rudder achieves that.

 

Both designs do the same thing with different methods - as you run down the leading edge of the rudder it protrudes forward (either with several small bumps, or effectively one big one). This is what redirects the ventilation aft (it becomes a path of least resistance) rather than further down the rudder blade. The Swiss design looks cleaner to me, but until someone actually goes to a university and tank tests these things I guess we won't know.

 

People have been poo-pooing the tubercles for years, but testing in several research studies is pretty conclusive that this concept allows a higher angle of attack to avoid stalling at the cost of minor drag at low speeds. What does 'minor drag' mean? What is the optimal shape (eg # bumps and their size/shape, or the size of the forward leading edge, curve, angle, etc)? I don't think anyone has it figured out to the point that you plug in some boat dimensions and out pops a blade shape.

 

Here's some light reading for anyone interested: (http://upcommons.upc...le/2099.1/13814 )

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