Leeboard question

The leeboards on my little trimaran have a symmetrical naca type shape to them. Would lift to weather be enhanced if the outside was curved and the inside flat? Or would the drag from the flat side outweigh the benefit? Thanks

 

leeboards

This is probably a Tom question but it seems to me that if you have two leeboards, one on each side of the boat and if they are individually retractactable that an asymetrical shape with the flat on the OUTSIDE of the
lee leeboard might be a little like having a gybing board( see that discussion under Boat Design). As I understand it an asymetrical board could dvelop about the same lift at 0° angle of incidence(parallel to the cl) as a symetrical board would develop at 2-2.5 ° leeway angle more or less. It's a bit more complicated than this and maybe Tom Speer will contribute his expertise...
At any rate, the flat(the section to do this job best might not have a truly flat side) would be on the OUTSIDE not the inside and this assumes that if one board is retracted there is still enough area for the lateral resistance required by the boat.
What kind/design trimaran do you have?

 

Sea Pearl Tri Sport: a Sea Pearl monohull that was converted to a trimaran. I am trying to equal or better the performance of the mono by reducing weight and trying some different concepts.

 

leeboards

Can you post a pix? Are the boards individually retractable?

 

The boards are retracted/deployed each tack. Sea Pearls are unique craft with freestanding rotating masts: main and mizzen. The boat has a flat bottom with a tombstone stern and is 21' long, 5'6" main hull beam, 14' overall beam for the tri.

 

leeboards

For the sake of others that may be following this I'll repeat some of what I said in my e-mail response to you.
Since you already raise and lower the leeboards each tack(seems like a lot of work) you could make the change to an asy board with -probably- some benefit. You could also -possibly- change the installation of your sym boards to have a 2.5- 3° angle of incidence(leading edge closer to centerline) which might be simpler than making new boards.
One thing I don't understand is why have two leeboards on a tri? Does the boat heel enough that the windward board would come too far out if you used just one? If you're not racing and want to simplify things one board would be enough depending on the angle of heel when the board is on the windward side.

 

The lift to weather won't change - see the thread on gybing centerboards. But you could get a small reduction in drag by using a properly designed, and built, cambered section. But probably the biggest gain in performance would come from making the boards longer.

 

Leeboards

In their current configuration the leeboards are on a swivel hinge. The swivel allows the boards to be lifted and dropped and allows for kick up in shallow water. The hinge is probably there for the same reason: allow the board more freedom in extra shallow water. When one tacks, unless one pulls the old, lee board up, it planes away from the hull. There has to be two boards because there is no structure to hold the off board against the boat going to windward. I believe that both boards are on centerline with the hull without any angle of incidence.
As far as I know, all Sea Pearls, regardless or type (mono or tri) or sail area (136,166,186), have been built with the same lead shod leeboard. One of my performance criteria is to reduce weight and having lead footed leeboards on a tri seems redundant given the righting moment. So, new boards are in order and if new ones, then lets get the right shape, size and length. Published draft is 2'8", max sail area 172-186

 

If i recall correctly, the Sea Pearl leeboards are quite high aspect ratio and have an fairly sophisticated foil shape, i.e. are not just flat plates. I doubt that there is much performace to be gained by going to an asymmetrical board. A little toe-in would be as good. Bolger suggests about 1 degree of toe-in at most (general remark, not specific for Sea Pearl). Since I believe you would be lucky to get the boards lined up with the centerline as well as one degree (I'm a pessimist about mechanical things), I would forget about it.

If you want to re-design the area/aspect ration/profile from scratch, then some researh and thinking is in order. Quite possibly, the section could be much thinner.

I agree that gettng the lead out makes sense. I don't think you will notice a performance difference, but the boards will be easier to operate if they are lighter. Remember that they need enough weight to sink, and with a fattish section, that could make the quite weighty.

If I wanted to make a Sea Pearl a lot faster, I'd spend more effort on the rig than the leeboards.

 

I have an aquaintance with a Pearl in Idaho who wants Idasailor to build him a new rudder. He lives a couple of hours away from them so I asked him to take a board along too to get their take on it. Maybe an alternative means of deploying could render the weight moot.
The boards weigh a little over 50lbs each, maybe we could save 60lbs between them. My floorboards weigh 40lbs, the floor, liner and unused ballast tanks/storage areas could all be tossed along with the outboard and mounting bracket. Maybe I could save 175-200 lbs.
I'd really like to try a two piece carbon mast: a wing section at the sail stubbed into regular round tubing to retain the furling around the mast reefing feature.
Save weight, incorporate new ideas

 

but righting moment can be variable...

Both here, and in the thread on gybing daggerboards, there seems to be the assumption that since the vector math of the sideforce components of the lift from the sails and lift from the foils has to zero out against the righting moment of the hull & ballast, then lift from the foils is essentially fixed when the boat is in equilibrium and the best you can do is to pick a section that will give reduced drag.

But of course different sections have different co-efficients of lift, or we'd all be sailing with flat plate foils. So WHY NOT pick a section that will optimize CL as well as CD? Especially with the unique opportunity of leeboards which can be assymetrical in section.

In many boats, from dinghies to the A-class scow (with leeboards!), or CBTF keelboats, righting moment is variable at will up to the point where you're fully powered up. Choosing a section with higher CL may just mean you're hiking sooner and harder (a good thing in light air).... of course this is only good if you've got the right tradeoff between lift and drag in your section of choice for the application.

Sorry this doesn't particularily apply to the trimaran thread, but its been nagging at me a bit. I'd be glad to hear arguements to the contrary.

My simplified view of the world anyhow,
Phil

 

assumptions

Phil, I didn't get that at all: the discussion was about the result of changing the angle of incidence of the board relative to the centerline of the boat and what happens then.
The righting moment was only part of the discussion insofar as the assumption was that ,for the examples given, there was adequate righting moment.
As I understood it Tom was saying that for any given side force generated by the sail the lateral resistance should produce an equal and opposite force regardless of the angle of incidence(angle relative to the centerline of the boat) of the board. That doesn't mean that the angle of incidence of a symetrical section board doesn't have the potential to improve vmg ,it does as shown by the fact that while the board is still at the same ANGLE OF ATTACK when gybed it is at a particular ANGLE OF INCIDENCE relative to the hull which means that the hull is now pointed slightly lower which, by allowing the sails to be slightly freed, will generate more speed etc. The same exact thing would work with an asymetrical section at ZERO ANGLE OF INCIDENCE: the boat would point slightly lower but have a potentially better VMG.
++++++++++++++++++++++++++++++++++++++++++++++
Where design comes in is in designing a sail that produces minimum side force for a given"lift" and a board that produces maximum "lift" for a minimum of wetted surface..
+++++++++++++++++++++++++++++++++++++++++++++++ Righting moment comes into it from the standpoint of power to carry sail and potential speed which will affect not only the choices of the rig planform but the area required to resist sideforce.The more power to carry sail the greater the potential speed and the more the board area will be affected.
Unless I completely misread you you seemed to be making a link between righting moment and the viability of a gybing board/asy board and the only link in the discussion I can see is the asumption that there is ENOUGH RM...

 

The original poster inquired as to lift to weather with an assymetric section. Followups then steered the conversation down the "angle of incidence" path.

I chimed in to note that I've seen a theme of discussing performance improvements in terms of reducing drag, rather than the other side of the coin of increasing lift. Yes the two are related, and yes reducing drag will increase velocity which will in turn lead to increased lift...

BUT if we get rid of the assumption that righting moment is fixed, then things become more interesting as with a given force on the sails you can increase lift from the leeboard if you also increase righting moment... the system still stays in equilibrium.


I'll go away and play with my tools now

 

One reason is that an optimum board is only optimum in one set of conditions. If the boat goes a little faster, or slower, or points a little higher, or runs a little free-er, then the "optimum" foil is different. Since the force (lift) goes up rapidly with boatspeed, most boats need foils that are larger than required for top speeds in order to sail well at low speeds. (You may have noticed that catamarans which sail faster on average have smaller foils, on average.)

I doubt that it is a big advantage to get so much lift from your foil that the hull is being pushed to windward of the centerline, and generating "lift" to leeward.

All of which is to say that absent a high competitive racing situation, it does not pay to cut it too fine.

 

That's true SeaDrive, but you can still optimize for "average" conditions. The point is that the demands placed on each leeboard are asymmetric, i.e. one tack only. So a symmetric foil's performance would be sub-optimal under ALL conditions, whereas a mildly asymmetric foil might crab a little to leeward at slow speeds, crab a little to windward at high speeds, and be "just right" at moderate speeds. So it should outperform the symmetric board in most circumstances, not to mention that most boats don't achieve excessive speeds anyway. I would agree that the issue might not be critical for casual cruising, but what the heck, it should help at least a little.