Interesting problem today sailing with a buddy. My 18-2 had massive cavitation. So bad that I would completely lose the steering. Now, it took a bit to figure it. At first I thought I may have a weed dragging. What I found though was where we had our weight. When trapezing my buddy must have been too far forward. He weighs more than I do, so the bow was much further down than usual. I had him move back to bring the stern down a bit and it seemed to solve the problem. This was happening when we had some good gusts and the rudders were getting loaded up.

Does this make sense,or do I have to adjust something? This felt just like when I windsurf and I would place too much load on the fin. It would feel like the fin just snapped off.

Thx

Pete

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New Prindle 18-2 Owner
Former Prindle 18 Owner
Multiple Hobie 16s
Boylston Massachusetts
Webster Lake Indian Lake Narragnsett Bay in Rhode Island
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That's exactly what it was, you pretty much have it figured out.

--
Greenville SC

Offering sails and other go fast parts for A-class catamarans
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peterk123

What I found though was where we had our weight.

DING!
You are getting the hang of this.
Winter reading -
http://www.amazon.com/Cat…Rick-White/dp/1880871009
Not just for racers.



Edited by klozhald on Sep 05, 2014 - 09:33 PM.

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Sheet In!
Bob
_/)_____/)_/)____/)____/)_____/)/)__________/)__
Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
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Do you run the 1st or 2nd generation rudders ? The 2nd generation rudders will help with the cavitation even with the weight forward . Also when your heated up ! There is a picture of one for sale in cat parts category .

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Doug Klem
Pensacola , Fl.
Blade F16
Prindle 18-2 w/spin
Prindle 18-2 x 3
Prindle 19 MX
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2nd generation. I probably need to do some fairing as well to smooth out a few rough spots.

--
New Prindle 18-2 Owner
Former Prindle 18 Owner
Multiple Hobie 16s
Boylston Massachusetts
Webster Lake Indian Lake Narragnsett Bay in Rhode Island
--

peterk123

I probably need to do some fairing as well to smooth out a few rough spots.

Use this on your rudders and centerboards. Really strong, easy to fair and cures in the sun in 5 minutes. Watch the video on fixing surfboard dings to see how to use mylar to fix your foils.

http://solarez.com/produc…rez-extreme-3-5-oz-tube/

--
Sheet In!
Bob
_/)_____/)_/)____/)____/)_____/)/)__________/)__
Prindle 18-2 #244 "Wakizashi"
Prindle 16 #3690 "Pegasus" Sold (sigh)
AZ Multihull Fleet 42 member
(Way) Past Commodore of Prindle Fleet 14
Arizona, USA
--

Just wanted to share info

your problem is VENTILATION, not cavitation, unless you're doing more than 20kt or so:

http://books.google.com/books?id=Qppwp1Gt2_kC&pg=PA288&lpg=PA288&dq=rudder+ventilation&source=bl&ots=owwX3xCQ0e&sig=steYHtk2wB93_9fwQzDNyd9DUmU&hl=en&sa=X&ei=XY8ZVIWWGszqoAS8joC4BQ&ved=0CHEQ6AEwDQ#v=onepage&q=rudder%20ventilation&f=false

The old trick for Hobies was to bevel the trailing edge using a pair of scissors. The angle is similar, if ASCII will work, to:
_
/ \
| |


that is to say, not a fine edge along the very trailing edge, albeit that may work for other brands. The solution in the Farrier trimarans was to build a rudder 'fence' to prevent air from being drawn along the leading edge.



Edited by jaybird1111 on Sep 17, 2014 - 07:47 AM.

I have never heard that and i have been sailing cats for 15 years

so all cat sailors (many are engineers) have been calling it the wrong term ?

Quote

Just wanted to share info

your problem is VENTILATION, not cavitation, unless you're doing more than 20kt or so:

Edited by MN3 on Sep 17, 2014 - 08:31 AM.

MN3

so all cat sailors (many are engineers) have been calling it the wrong term ?

Seems like it. Most of the Google stuff on cavitation vs ventilation concerns powerboat props. Evidently they use the wrong term a lot too.

Here is a fairly clear definition pulled from an article on hydrofoils on the mid.edu site.

from http://lancet.mit.edu/decavitator/Basics.html

There are two particularly persistent problems faced by designers of hydrofoils: cavitation and ventilation.

Ventilation occurs when part of a hydrofoil pierces the surface of the water and air gets sucked down the lifting surface of the foil. Since air is much less dense than water, the foil generates much less lift and the boat crashes down. Ventilation can occur at any air-water interface.

Cavitation occurs when the water pressure is lowered to the point where the water starts to boil. This frequently happens with propellors. When a propellor is turned fast enough, the blades generate so much lift (i.e. the pressure on the lifting surface of the blades goes down) that the water flowing over the propellor blades begins to boil. When cavitation occurs, the foil no longer generates enough lift and the boat crashed down onto the water.

--
Damon Linkous
1992 Hobie 18
Memphis, TN

How To Create Your Signature

How To Create Your Own Cool Avatar

How To Display Pictures In The Forums.
--

Trust me when I say you can trust jaybird on this particular topic.

--
Jerome Vaughan
Hobie 16
Clinton, Mississippi
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Quote

Trust me when I say you can trust ..... "him"

hmmmmm,
no disrespect to anyone but my moto is ... trust no one, question everything until you get the real answer, ... fondle everything you can without breaking it or getting slapped...

but... seems legit. Jake on the other forum confirmed and explained it very well too. "Sucking air down from the surface along an edge of a foil is ventilation"

Quote

"Sucking air down from the surface along an edge of a foil is ventilation"

Oh no, here we go again...in the world of physics, nothing is "sucked", it gets pushed.
Many people use the words interchangeably, but it is always the higher pressure that tries to push its way into the lower pressure, thus equalizing.
In this case a partial vacumn,(low pressure) is created. The higher outside atmospheric pressure pushes its way down in an attempt to equalize the two areas.
The same principles apply to a wing in flight, it is the higher pressure below the wing that does the work of creating lift, the wing is not "sucked" upwards.
Even the simple act of "sucking" pop through a straw is actually pushing. You have created a lower pressure in the straw by expanding your rib cage, the larger volume means a lower pressure. It is the higher outside ambient atmospheric pressure that pushes on the pop, moving it up into the straw, again in an attempt to equalize. Stand on a chair, drop a 5' length of tube into a bottle on the floor, & try to "suck" pop up, you can't. Atmospheric pressure is roughly 15PSI at sea level. Once the vertical column of liquid exerts a pressure equal to the atmosphere trying to push it upwards, no amount of "sucking" will do anything.
If you don't believe it, grab a length of tubing, jump in the pool or ocean, & try to breath below about 16". Your diaphragm drops & your rib cage expands, creating a lower pressure in the snorkel. The higher atmospheric pressure tries to push into the snorkel. However, the water pressure is squeezing your body, & at about 16" it equals the atmospheric pressure, so no air movement occurs, & you come up gasping. This is the same reason divers get more minutes out of a tank at 20' than they can at 80'.
With cavitation, many of you may remember the high school physics demonstration where you lower the pressure til you get the beaker of water to boil at room temperature. The is why the technician hooks a vacuum pump to your cars air conditioner anytime a line was opened. They want ALL the moisture removed, & the easiest way is to drop the pressure so that any moisture in the system boils off.
Anyone who has camped in high mountain country has witnessed the frustration of trying to cook KD, it takes forever because the boiling temp of water is lower, due to the altitude, which results in a decreased atmospheric pressure. Conversely, increased pressure increases the boiling temp, that's how a pressure cooker works.
Many times, people who know better use the terminology of "sucking", mostly because it's easier when communicating with the non scientific crowd.

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

Alrighty.............. this was helpful :)

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New Prindle 18-2 Owner
Former Prindle 18 Owner
Multiple Hobie 16s
Boylston Massachusetts
Webster Lake Indian Lake Narragnsett Bay in Rhode Island
--

I think much of the error occurs because we have gotten used to using the word "suction", as in apply suction to drain a wound, or bleed brakes. What we should be sayiing is vacuum, & realizing it is the higher pressure behind that is doing the work.
We all know what they mean, so it slides.
The HVAC guys are generally taught proper terminology. You almost never hear an air conditioning tech talk about applying suction to dry out a system, they will say "vacuum it down", evaporate it, or "pump it til you get close to 29"Hg".

--
Hobie 18 Magnum
Dart 15
Mystere 6.0XL Sold Was a handful solo
Nacra 5.7
Nacra 5.0
Bombardier Invitation (Now officially DEAD)
Various other Dock cluttering WaterCrap
--

This thread sucks

--
Jeff
Houston TX
1986 Hobie 18 Sail# 13031
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I actually had a 30 minute conversation, yesterday, with a Mechanical Engineer PhD about this subject. I showed him the Farrier rudder fence, and he agreed that it would cut down on the ventilation.

Another idea we had was inducing turbulence on the rudder foil to help avoid flow detachment at high angles of attack (turning the rudder). This would be limited by class rules of course. Sanding a line vertically on the rudder at the thickest point might be enough to induce boundary layer turbulence and help the flow when the boat is turning.

--
Jeff
Houston TX
1986 Hobie 18 Sail# 13031
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peterk123

Alrighty.............. this was helpful :)

--
Tornado (80's Reg White)
Prindle 18-2 (sold)
Dart 16 (hired and hooked)
13 mtr steel cutter (sold)
Etap 22, unsinkable sailing pocket cruiser.

Amsterdam, the Netherlands
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Ventilation (beachcats):

https://www.youtube.com/watch?v=0J0jaMIRMR0&index=36&list=WL



Cavitation (propeller, and extremely high performance craft, such as the AC trimarans or l'Hydroptère)

https://www.youtube.com/watch?v=DijdU0rmDdc&index=12&list=FLCqf4CrtfoxFcfAyVh0wMkg


Thanks to Will Rottgering

Quote

Another idea we had was inducing turbulence on the rudder foil to help avoid flow detachment at high angles of attack (turning the rudder). This would be limited by class rules of course. Sanding a line vertically on the rudder at the thickest point might be enough to induce boundary layer turbulence and help the flow when the boat is turning.

Instead of sanding a vertical line on the rudder, I'd try a thin layer of tape, or some other object that would stick to the rudder and actually build up a line. When flying high performance sailplanes, many people would use a "trip strip" for the purpose of creating a boundary layer. I can't remember exactly how far back we put the strip, but I believe it was ~25% MAC. The strip was effective at keeping the boundary layer attached, especially at high AoA. Of course, not every foil benefits from this "fix".

If you try it, use something like pinstriping tape of about 1/8" width. Probably about 3 layers stacked would give you the desired trip thickness to affect the foil.

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Tim
81 Hobie 16
87 Nacra 5.7
Austin, TX
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