Bridgedeck centreboard why don't they work???

you want Christmas in Easter? I'm not sure either.

 

Forward Rake endoresment

Every time I go back to that other subject thread I find something interesting,... i.e, this from Tom Speer who I highly regard...

 

yes
symmetrical at the water to suppress ventilation and asymmetrical under the water to develop the resistance.

 

Hi Brian,
when I re-read the text of my reply which you have quoted, I realized that I had made an important error due to distraction. I had modified the original reply this morning, so please take a note of the corrected reply: http://www.boatdesign.net/forums/boat-design/rudder-configuration-33622.html#post797972

Also, don't forget that so far all the arguments about raking and ventilation have been purely theoretical. Once the ventilation starts to propagate downwards along the foil, it modifies the flow field (and hence the pressure distribution) around it. And I suspect that the flow field of a ventilated foil is radically different from the field around a clean foil. Hence we don't know whether the theoretical results about hydrodynamic loading of fwd and backwards swept-wings are still valid. Even the CFD results should be treated with caution, due to unknowns related to the boundary conditions of the problem. IMO, only tests and trials can tell the truth.

Cheers

 

Only if I still get presents in December.

 

sure, why not.
My explanation of tacking makes sense?

 

Windsurfer Skeg Experiments

Wonder if we could learn anything from wonderful old windsurfer skeg experiments? I just ran across this old memory lane....

http://www.seabreeze.com.au/forums/Windsurfing/Gps/Sailboard-Dinosaur-Fins?page=1

Wonder what a slot up behind the leading edge of the board at the waterline would do to stall off ventilation initiation at that venerable point?...and/or keep that ventilation propagation from traveling down the leading edge of the foil??










...just brainstorming

 

Hi all,

I was referred to this thread by Brian, who left a comment on one of my YouTube videos. I've been reading some interest, as ventilation has been my research topic for several years.

There honestly isn't a whole lot for me to contribute at this point. Daiquiri hit upon almost all of the salient aspects of ventilation. For suppression, minimize adverse pressure gradients (and thus suction peaks) near the free surface.

Fences can work, but some of the work by Swales, et al in the 1970's showed that fences can increase the tendency toward ventilation in many circumstances. (http://journals.sagepub.com/doi/abs/10.1243/JMES_JOUR_1974_016_005_02)

Daiquiri also mentioned the fact that ventilation itself modifies the flow, and I want to add my agreement. All it takes for the inception of ventilation is a tiny region of separated (i.e. stagnated) flow and pressures low enough to pull air into that region. The fact that the free surface cannot sustain any chord-wise pressure gradients (in most cases) means that the requisite flow separation never occurs AT the free surface, but rather just below it. If something (waves, debris, etc...) perturbs that thin attached "seal" at the free surface, air may enter into the ventilation-ready regions residing just beneath and "blow out" the flow. The ventilation doesn't remain confined to the separated region, but will create it's own flow separation as it propagates across the foil.

In the attached picture, you're looking at the paint-streak flow pattern on the suction side of a strut run at 5-degree yaw angle (AoA). Only the first column of paint dots are swept forward, indicating that flow separation is confined to less than 5% of the chord length.

However, if you look at the linked video, you'll see the foil running at the same speed and same yaw angle. When the free surface is disturbed (in this case by blowing a puff of air upstream of the leading edge), the ventilation quickly grows beyond the original bounds of flow separation.

https://youtu.be/j-1aFyjJ1NU

 

first off, many thanks for your work and welcome to the community.
I am sure that the experts will have many questions but the first one is
would the use of a full aerofoil profile like NACA 2415 (no massive squared off TE) greatly raise the speed/AOA required to make ventilation more likely? Or only slightly, or not at all?
Next question, what type of beer do you prefer?

 

Wecome

Welcome to the forums CMHarwood.

I believe I understand that you know very little about sailboats? In the briefest terms we power our boats with sails, the forces of which aerodynamically power us forward and push us sideways.

Sailing downwind can be pretty straight forward, just let the sails push us downwind.

Sailing across the wind (wind on our beam) is our most efficient and pleasureful point of sail. We can elect to, or not, try to eliminate the side pushing forces by means of our foil shaped daggerboards / centerboards.

Sailing upwind is our most challenging point of sail. We generally have to sail off about a 35-45 degree course from directly into the wind, and tack over to the opposite point to continue to direct windward. We really need some sort of sideways prevention devices such as daggerboards, or centerboards that develop lift by their foil shapes to counter the side ways forces of the sailing rig.

Most often these daggerboards or centerboards are located in each hull of a catamaran vessel, or in the central hull of a trimaran vessel. Those hulls provide an upper end-plate for those boards.

In this particular discussion we are seeking ways to place those sideway reducing foils in the center of a catamaran vessel,...thus they are operating with a free-surface at their upper ends,..makes life a little more difficult.

From MANY of the discussions here, one would almost come to the conclusion that a free surface foil will just NOT be able to develop this sideways reducing lift force by any appreciable manner, over any appreciable time,....it will just succumb to ventilation, and all lift will virtually disappear.

Now if this is true, then explain to me how this catamaran can be sailing to windward utilizing TWO free-surface boards,....one dagger board and one steering board (rudder).

https://www.youtube.com/watch?v=ZggxS6ZrVOw


https://www.youtube.com/watch?v=KH3Q-5vHLCs

How can that free-surface steering board continue to operate if this ventilation problem is as bad as it appears??

What can we really do to make free-surface foils work better,....that is the challenge !!

 

Casey,

Thank you very much for joining in. Given your research looks highly focused on surface piercing hydrofoils, I would expect your research to find more and more applicability to sailing in the next few years. Everything from 30m maxi trimarans to 11' Moth's are spending more and more of their lives now out of the water on foils, and the number of boats that are foiling is growing by leaps and bounds. And about the only major adjustment is that sailboats will always have some leeway angle (normally up to 7.5% max) that needs to be taken into account, or taken advantage of.

 

The use of a fully-streamlined section will help mitigate ventilation, yes. However, I think the greatest benefit would come from the leading edge, rather than from the trailing edge. In my tests, air was never observed to travel upstream from the TE, but rather always started out near the suction peak -- near the leading edge. My leading edge was sharp (intentionally so, as I wanted a well-defined separation point for my studies), but a rounded LE would reduce the local streamline curvature and mitigate the flow separation. I'd wager that by choosing a medium-to-thick airfoil section (to delay stall) near the free surface would help you out.

The thing to remember is that the ventilation makes its own "home" in the flow. Once you cross that boundary from wetted flow ventilated flow, it can be difficult to return to wetted flow, regardless of the section shape you're using. Breslin and Skalak put together a NASA report in 1958 that (in my opinion) remains one of the most complete and eloquent tech reports on ventilation. They used geometries with sharp leading edges and a cambered NACA section. The takeaway (if I recall the report correctly) is that the NACA section was more resistant to stall, and thus to ventilation, than the sharp-nosed section, but ventilation persisted town to near zero AOA for both sections once initiated.

Oh, and I'm a whiskey man.

Hard to say for sure, but it could be a combination of a few factors.

At angles of attack below around 10 degrees, it's entirely possible to maintain wetted flow. I found that below 15 degrees, even my bizarre-o section was able to maintain wetted flow. However, it was also possible to cause ventilation above about 2.5 degrees. Between those two, ventilation was possible, but not unavoidable. So, it could be just luck.

They may also (by distributing some of the load across two foils) be operating at even lower angles of attack.

I don't know the aspect ratio of the foils, but if the span is especially long, then ventilation which does occur may remain confined to the shallow sections (and may be periodically washed out by the orbital wave velocities).

I'm sure I'll think of more possible reasons. Gee, it's almost like hydrodynamics is tougher in the real world than in a big laboratory.

Anyhow, here's a useful little map I pieced together. The X-axis is angle of attack / yaw angle. The Y-axis is the depth-based Froude number, or Fnh=Velocity/sqrt(submerged span * gravity). Different colored regions indicate where types of flow were observed. As you can see, at moderate angles, across almost all speeds, wetted and ventilated flow were both possible. The little insets indicate that flow separation were secondary to the fact that SOME flow separation preceded ventilation.

"Kill that flow separation, kill the ventilation," is what I say.

 

Whiskey it is. Always give a man who has the knowledge what he wants.
Thanks for the detailed reply.
Gives us a lot hope that "things can get done". One thing about sailing is that it is never ever static (except when you are up on stands ).
The waves and wind usually require constant course adjustments so the rudder is always varying its AoA. Good seamanship requires that the rudder is always swung through/around the AoA of zero degrees by proper sail setting. Probably there is a lot more ventilation than people realize, but 20 seconds later the AoA vanishes or reversed. Rudders traditionally have very thick profiles with very rounded LE: for strength and for stall avoidance.

The centerboard is another matter and its AoA should remain fairly constant for large periods. Here is where ventilation is going hurt us most. I'm hopeful that for moderate (10m) to larger catamarans the AoA will not exceed 10 degrees for long periods. The use of a non-lifting segment that spans the water surface to delay ventilation onset with a lifting span below seems to have promise.
We may have to learn to "fall off" if ventilation becomes serious which will bring the AoA temporarily to zero.
There is a long way to go, but with good fundamental research and public access to the results/conclusions, we can make some progress.

edit* just watched your video https://youtu.be/j-1aFyjJ1NUQ scary to see a free vertical water surface, with waves, under the water. The manner in which the separation spreads so rapidly and covers, what, 60% of the foil, is sobering.

 

On the video the board seems to be running quite shallow ?

Wouldn't a board on a yacht be like 3-4 times deeper ?

 

https://youtu.be/j-1aFyjJ1NUQ ??
believe we are looking at just a pure research configuration designed to trigger the onset of ventilation.
for a real world board you're right, board span of at least 2.5 times the chord. 3.5 times to allow for wave action? this needs a lot more discussion.