Dinghy Design: Open 60 influence?

--------------------
Note that it is the highly loaded lee foil that has the basic same pressure distribution as the reverse curve foil. And I guess the windward foil does too when and if it is pulling down.WRONG. See post 48 for the correction
-I thought it would be interesting to look at the immersed parts of the two foils together. You have to remember that the blue line at the top of the curved foil represents the waterline at a 6 degree angle of heel and that this concept can be optimized in a number of ways. At any rate, the top of both pictures is at or near the wl:

click on image:

 

Doug,

Correct me if I'm wrong, but AFAIK the Hobie foils are toed inwards, i.e.the opposite of your system.

 

====
Frankly, I'm lot positive but if you look at the foil below you'll see fences on one edge of the foil. I figured that the fences would be on the leading edge which would mean the foil is outboard or inboard. I'll look for a definitive picture.
UPDATE: Mal-I wasn't thinking-its too late.. That is the leading edge on the stbd foil; you are right the foils are pointed inboard. The pressure would be similar to the reverse curve foils on the windward side UNLESS that foil was pulling down.
click on image:

 

Toed in

They toe in.


Interesting topic. Took a while to get through all the banter though. Instead of complicated trunks, the foils could be mounted to a shaft and rotated as leeboards do. Could making testing easier. It seems to me that unless the crew is restricted to the centerline of this dinghy, their weight is going to be a significant piece of the RM puzzle. How much righting moment are we actually talking about with the horizontal piece of the curved board.

 

Open 60 influence

==============
Thanks for the picture! The foils can be designed with a widely varying approach where the design can be varied for more or less vertical lift, more or less lateral resistance etc. The curve can be changed and the athwhartship position of the top of the board could be adjusted in a manner like the curved foils on Steve Clarks Aethon. Angle of incidence of the lifting portion can be easily adjusted(like DNA A Class cats,for instance). The angle of incidence of the lateral resistance portion can be fixed parallel to the centerline, toed in relative to the cl or adjustable.
A very preliminary calculation shows that for a two person trapeze boat something like a 50% increase in SA would be possible for the same weight. That is about the same as adding a third crewperson but without the weight.
Lots of analysis left to do to see if the pressure issue can be resolved-the other issues are a piece of cake by comparison. If there really is a problem a fence might solve it if it can be made to work with a trunk(foldable?).
All in all a pretty interesting thing that is going to be quite a learning experience either way it goes.
Thanks again for your participation!
------------
I'm building a model trimaran that will be radio controlled( http://www.boatdesign.net/forums/mu...-self-righting-trimaran-test-model-36058.html ) and will be suitable to do preliminary testing of these foils. And maybe we'll get some more insight from other people.

 

Doug, why stick with the curved foil? Because it is fashionable, sexy, maybe? A straight, angled out foil with angled in lower sections is far simpler to build and operate; no complex, variable, subtle changes in angle of incidence leading and tailing edges, just a 3-4 degree in the upper and the same with the lower ... or you could make a tight radius curve between the two, a J but a reversed J. The simple arrangement I used to have on Groucho (see jpeg with foil lifted) did an okay job, KISS principle.

 

Reverse Curve Foils

=================
Thanks ,Gary. The reverse curved foil is easy to retract and adjust while offering a multitude of design variations but there is the nagging problem of the weird pressure distribution. I looked at the trifoiler "J" foils in a couple of previous posts. It was interesting to note that when I thought they were deployed outboard they had the same weird pressure distribution as the reverse curve foils on the lee foil and deployed inboard ,as they actually are ,the pressure distribution was the same on the windward foil but only until that foil begins to pull down! Resolving the pressure distribution problem in favor of the reverse curve foils will open a huge new box of design innovation that can be applied to dinghies, multihulls and keelboats. If the pressure distribution problem is shown to be real it will pretty much invalidate the concept though there may be solutions that I'm not aware of yet. It's pretty damn exciting to contemplate the potential of a successful resolution of the problem.
I should say that more and more evidence of a precedent in aerodynamics has been found with non-planar wings which is very encouraging as well as the trifoiler "J" foils and Jon Howes "Loop Keel". See the examples below including the "ring wing" glider project for kids from NASA!(last picture on lower left) http://www.boatdesign.net/forums/sailboats/radically-different-yacht-keel-loop-keel-16265.html
Here is a "summary" of the reverse curve foil illustrations so far starting with a comparison to an outboard "j" foil as well as some of the possible aerodynamic precedent evidence uncovered so far:

(click on image twice for greatest detail)

 

Reverse Curve Foils

This is an e-mail from a friend, Rick Willoughby; his point #3 pretty much nails it. This e-mail is posted with his permission:


"Doug
It is an interesting idea. My comments:


1. Probably make the boat more twitchy in gusts if it is being overpowered. The vertical lift reduces with heel so drag from the hull will increase as it heels.


2. The ability to power up should be higher due to the improved RM.


3. The L/D of the outer portion will be worse than curving the other way because there will be higher induced drag from the vertical lifting section. You are right that there is no induced drag for the middle section where there is zero lift but this means the aspect ratio of the outer portion is reduced thereby increasing the induced drag associated with this portion of the foil - meaning the pressure gradient is not as evenly distributed spanwise as it would be with inward curved foil. This is not a show stopper but you probably want to be thinking about an overall aspect ratio of 8 or so otherwise the L/D will be quite poor. This would mean the aspect ratio for the outer portion will be 4 say. Less than this and the foil L/D is going to be worse than planing surface. (I guess there is still the advantage of the higher RM from the vertical lift but it will be at the cost of extra drag)

4. It is more complex to make but nothing too difficult.


5. It extends the beam of the boat underwater. Make a bit more allowance on markers and other boats.

------------------
More info has been recieved and will , hopefully, be posted later.

 

Reverse Curve Foils

This is an e-mail I got from Greg Ketterman in response to the question:"Is there any special reason the trifoiler foils point inward?". It occurred to me that the Trifoiler case was similar to the reverse curve foils and since Greg has been very helpful to me I asked him. It should be noted that Greg actually tried outboard foils and sent pictures of them. I have his permission to post this here. Greg is also a member of the boatdesign.net community. I urge anyone with an interest in hydrofoil design and development to read the enclosed paper-page 14 and Figure 14a&b on page 32 are directly retated to this discussion. For those interested in model hydrofoils there are details of his model throught this paper including sail area, foil area and improvements warranted by testing. I'm grateful to Greg for taking the time to put this together and for his help:

(emphasis by DL)

Hi Doug,

Forgive me if I am too condescending but I am amazed at how many people do not get this. ***** argues that it would provide more righting moment. Imagine how ineffective a curved daggerboard would be if it curved out. I would have expected you to understand.
The boat in the attached photo did not perform well at all.

My senior project report explains it here; http://picasaweb.google.com/gketterman/SeniorProjectReportTriFoiler#slideshow/5075757835053693858

A simpler way to look at it is the aspect ratio of the horizontal foil adds to the aspect ratio of the vertical foil when the foil points in. Aspect ratio is everything with hydrofoil sailboats.

All the best;
Greg Ketterman

VP Engineering Hobie Cat Co.
------------------------------------
Hobie Trifoiler specs: http://www.hcana.hobieclass.com/sit...2004/hobies/hobietrifoiler_specifications.htm
------------------------------------
Click on the left photo to see the outboard foil; on the far right photo to get an illustration of the boat lift forces at high speed(14a):

 

At this point, it looks like the reverse curve foils may not work well but there are still questions I need answered and I will continue to look into it.
But I think that "normal" foils as used on Open 60's and multihulls may have a place on dinghies particularly since they are retractable and can develop a substantial amount of lift.
I designed and built an experimental monohull dinghy in 1975-77 that had molded in leeboards called "Tantra" She sailed well upwind-comparable to a US1 when tested with a US1 rig. She was also tested with "foil assist" foils.
It occurs to me that a variation of this design could be the perfect application of curved lifting foils. I'll do a couple of sketches when I can.
Here are some pictures of the boat just before her death in 2004(?).
The leeboards were molded with removable tips, the cockpit was circular with very comfortable seating, there was a large cooler molded in aft of the cockpit and a large storage area molded in forward of the cockpit. The traveler was a reverse curve that worked well and was done to ensure the mainsheet led from forward. I visualize the curved foils coming out of the deck just forward of the traveler. In this sort of application "normal" curved foils would probably increase RM a bit.

 

Dinghy Design: Open 60 influence? / Tantra II

Here are some rough sketches of an "update" of the original Tantra(shown in last post) with the inclusion of curved lifting foils. Because of the design of the molded in leeboards the boat lends itself perfectly to the use of curved foils to lift approx 80% of the boats weight. The design would use sliding bench seats and coupled with the outboard positioning of the curved foils would develop a lot of RM allowing approximately 150 sq.ft. of sail area singlehanded. Since the boat also includes a sealed cooler and dry stowage compartment performance daysailing with two would be possible.
One of the advantages of curved foils in this application is keeping the maximum draft to about 18".
This is just one of many possible uses of curved foils in dinghy design....

Tantra II
LOA-15.5'
Beam- 8.75'(pods fold to trailerable width)
Sail Area-150sq.ft.+ asy-spin
Draft-1.17' (14")
Weight- 175lb
Sliding Bench Seats
Retractable Foils
Retractable Rudder Gantry
Wing Tip Rig RJ(ribbon jib)
Reverse Curve Traveler




click on image:

 

Why?

Doug:

What do you see as the benefit of the reverse curve traveler?

I've always operated under the assumption that the traveler curve should follow the arc of the boom to reduce/minimize the need for a vang as the tensioned mainsheet would be pulling straight down, and once the main was cleated you could tack with only the traveler moving from side to side. Finns (and similar boats) trim the main on the traveler, using the vertical mainsheet to control leech tension much like a vang. The Devoti D-One also is designed to use a full arc traveler. Luca Devoti is a big fan of full boom arc travelers, and he is one seriously good dinghy sailor.

I can't see how there is any benefit to the stylish reverse curve other than allowing the very unique hot-tub style cockpit. Please let me know where my logic has failed me?

--
CutOnce

 

see post 55

 

Doug, that is poor mechanical design on your vang traveller. Not only is it too close to the mast, but the reverse curve track shape doesn't make any sense. Are you expecting to ease it as the boom goes out? You've got enough room to mount a decent curving main sheet traveller/track set directly below the clew and straddling the racks. Wangs are wankery.

 

Dinghy Design: Open 60 influence? / Tantra II

==================
Gary, the traveller(vangeller?) works and works well-I used it first with two different rigs
34 years ago. On the big rig there was a traveller track on the boom as well.
It's critical to me to have the mainsheet lead from forward and not have anything aft that can foul the tiller or hiking sticks.
The front of the circular cockpit and the aft face of dry stowage compartments served as structural members tied into the hull to give the center portion of the vangeller strength. The end of the traveller "bed" was tied directly into the sides of the hull.
----------
So you don't like the traveller-what about the foils and their shallow draft?