NACA Sections on Thin Foils

[Songololo]

Greetings WB forumites, this is my first post, so here goes...

I recently finished a 3m folding pram dinghy (similar to a Mirror dinghy) from a marine plywood kit. The rudder blade is currently un-profiled (in section)and has the following dimensions (sorry, having pic posting problems ):

Height: 400mm
Chord at water line: 240mm
Chord at base: 340mm
Thickness: 12mm

These chord and thickness dimensions would theoretically allow me to create a maximum of approximately a NACA0002 section.Correct?

Questions:

1. Is there anything to be gained from profiling such a thin blade using a NACA0002 section when typical rudder blade profiles are more in the region of at least NACA0010?

2. Would rounding off the edges with a roundover bit not produce a good enough result?

Lance

[Thorne]

Welcome!

Check out some of the similar threads here using the Search function. Here's one just a few days old -
http://www.woodenboat-ubb.com/vbulle...ad.php?t=59289


There are a number of online resouces for foil issues -
http://www.philsfoils.com/designTips.html
http://www.boat-links.com/foilfaq.html
http://www.duckworksmagazine.com/06/...oils/index.htm

I'm sure that some more experienced folks will know more, but it seems to me that the rudder thickness is WAY too small. How thick is the centerboard or daggerboard? What does the plans call for?

For most small boats under 16 feet that aren't racers or multihulls, a very rough guide seems to be that the Centerboard or Daggerboard (NACA0008) is 1" thick (25.4mm), and the Rudder is significantly thicker at 1.5" thick or more (NACA0012).

Here's what Phils foils says about this -

"The thickness of a rudder (as a percentage of its chord) will be greater than the thickness of a centerboard. Why? Because thin foil sections stall at lower angles of attack than thick sections. Centerboards may be thin because they always travel in the same direction through the water as the boat hull. But rudders sweep through the water as they steer the boat."

[mmd]

Welcome to the Forum, Lance. I'm sure you'll find plenty of opinions to choose from. I'll go first:

There is precious little to gain from shaping a dinghy rudder to a thin NACA foil. You can, but the advantage would be almost impossible to measure on such a small boat. It may, however, give you a psychological advantage and your competitors a psych disadvantage.

I would round-over the leading edge with a 6mm router bit on both sides, making the leading edge round. I would cut the tip of the rudder into an ellipse (in profile view). I would use a plane or a belt sander to evenly taper the aft half of the blade down to a 3mm trailing edge, leaving the aft edges sharp. Then fair the front of the tip into the aft edge. The important thing is to have the sides exactly symmetrical from port to starboard. Then seal, paint, & polish.

That's my opinion, and I'm sticking to it!

(Edit to add: OOPS! I guess I didn't type fast enough to be first. Boo-hoo.)

[Thorne]

Poor Lance -- gets two conflicting opinons right off the bat!

;- )

MMD is a Naval Architech and so knows his stuff. As such I'll ask him a question or two that might relate to Lance's post --

If thin foils are not really useful on slower-sailing dinghies, do you recommend the use of metal centerboards / daggerboards and rudders?

Or do you think that foils made of correct thickness (i.e. the dimensions all meet the recommended NACA numbers) are good but thin ones not good?

It sure seems that either galvanized steel or stainless would work really well for these centerboards / daggerboards / rudders if the chord shapes are not really important...so why use wood at all?

My friend's newly-restored Drascombe Lugger has a squared metal centerboard and rudder, which seems the best use of the metal as it puts the weight down where it will do the most good when sailing.

[JimD]

It sure seems that either galvanized steel or stainless would work really well for these centerboards / daggerboards / rudders if the chord shapes are not really important...so why use wood at all?

Wood is easier to make. Wood is much lighter than steel. My CB is half inch plate steel, galvanized. Adds 120 #s of ballast, is relatively indestructable, doesn't swell and warp and get stuck in the trunk when it gets wet.

[mmd]

Thorne; I made some assumptions about Lance's boat and sailing ambitions when I answered his questions. (No offense intended, Lance.) I assumed that he was not looking to squeeze the last milli-micro-knot of performance out of his boat. If so, he would have a carbon-fiber-kevlar-spaceage-resin go-faster boat, not a Mirror-like dinghy. Second, I assumed that he wanted to modify his existing rudder to merely get the best performance out of it without going to superhuman efforts. So, I told him how to make a piece of 1/2" ply into a servicable rudder.

On the other hand, the information you have posted is pretty much on the money. Fat NACA foils seem to do best in go-faster boats, and the rudder should be fatter than the daggerboard. How fat is determined by the expected boat speed (among other considerations).

Metal dagger/centreboards are a compromise between wanting ballast down low and shallow draft. So you give up some hydrodynamic efficiency to be able to sail in thin water. Do you lose much speed or windward ability by this? Yes, but so little that it really doesn't matter to most recreational sailors.

Finally, you ask, "why use wood"? Becaiuse it is still relatively cheap and much easier to work than metal.

[Thorne]

One of the reasons I am interested in this issue is that I had to make the decision between a wood CB in a proper foil shape and a metal CB.

As you can see I went with the wood CB and it has been good so far. I do quite a bit of rowing, so the extra weight would not have helped for that, plus when rowing most of the weight of the CB is well above the bottom of the boat.

But the wife works for Seaport Stainless in Richmond CA and I coulda gotten a solid SS centerboard reelly cheap...

;0 )

(Probably would have laid up two thiner SS CB-shapes, then epoxied them together and glassed the outside into a rough foil shape -- and hope I didn't hit anything hard enough to bend the whole shebang.)

[Boatmik]

Generally a foil thinner than 8% precludes efficient use of the NACA sections. They just start getting too pointy at the front and the flow tends to separate as soon as the angle of attack gets a bit wider - like in choppy water.

At any rate the Mirror used to have a restriction on how much of the foil could be shaped I think. That you were able to only shape a certain amount in from the edge.

There were a series of articles by an aerodynamicist, Neil Pollock, here in Australian who wrote some search programs to find optimum solutions.

For thin foils the optimum solution was to use a shaped leading edge and a shaped trailing edge with a flat in between.

Here are some pics of such a foil being made
Short thread
http://www.woodworkforums.ubeaut.com...ad.php?t=41486
Long thread
http://www.woodworkforums.ubeaut.com...ad.php?t=28107

I don't agree with the statements above that an optimal foil will make little or no difference.

Here is a tale from my youth when I used to hand shape foils by eye and then finally coughed the dough and bought an accurate professionally made set.

If you are interested enough to study how to set sails you should look at optimising the foils as well. If you notice the difference with sails you will notice them with foils as well. After all they are dealing with almost exactly the same forces but in opposite directions!

But if you just cruise around and don't worry about your sails that much then there's little reason to worry about foils as well.

MIK

Anyway ... to the story

Once apon a time when I was starting off in racing sailboats I bought a second hand NS14
– at that time a family style boat that had lots of performance. Over the first season I did
OK finishing about midway in my local fleet.

I had already owned a couple of wooden boats so I was used to doing all my repairs and
maintenance myself.
On this NS14 the centreboard and rudder were around the normal length but the boards
were a bit wide. So I decided to modify them myself. I sawed the back off both the rudder
and centreboard and reshaped them in line with more modern theory – moving the point of
maximum width back slightly and in the end producing a pair of really smooth shiny foils.
As a result of this the boat was a little bit faster and my results improved slightly.

A year later I felt that I had got the hang of the boat and was now looking to do everything
possible to make it faster. A new mast, new sails the fittings reorganised so the boat
would work like clockwork.
It improved a bit more.

Finally I decided to get rid of my homemade foils and buy the best ones available.
The first time I raced the boat was a revelation. Suddenly I was in amongst the top few
boats in speed. But more importantly the boat felt completely different.
It would tack and gybe faultlessly coming out of the manouvres with heaps of speed rather
than having to get the boat moving again.. When there were big waves and lots of wind it
would sail smoothly – before it used to stagger and stall. When sailing in tight conditions
with lots of boats around (like milling around before the start) it gave me the confidence to
go in close and pick out a good spot without getting into tangles with other boats.
Qualitatively and quantitatively the boat was much better – safer and more fun.

The difference between my hand hewn foils and the manufactured ones is not really all
that great – almost the same amount of labour – but the manufactured foils were built
carefully to accurately reflect the correct airfoil (wing) section.
They used templates to get that accuracy. With this knowledge I have never needed topay
someone for first class foils. About 50 dollars worth of materials and a few hours of light
labour (for small boats anyhow) and I can make a set the equal of what I could buy for 600
to 800 dollars.

[Tom Lathrop]

In a practical case, I agree with MMD on foils for a very small and low performance sailboat like a Mirror dinghy. Sure, a hydrodynamically correct foil is at least theoretically better but it is not possible to make one within your dimensions. A sharp leading edge should be avoided like the plague since it will stall earliest and a trailing edge like MMD offers is not hard to get. Adding weight to a small sailboat with metal foils is all wrong. Nothing wrong with metal but the weight is bad for performance. The rudder should be fatter than the centerboard and its leading edge should be even more blunt so it will delay stalls in sharp steering movements.

One class that I have made many daggerboards for has a very high aspect ratio board that is 3/4" (19mm) thick and allows shaping of only the edge 1/1/2" (38mm). This class has recently doubled the allowed distance of shaping of these boards to 3". I have just finished making a daggerboard to the new dimensions. It will be interesting to see if there is a significant performance difference.

[Thorne]

Well, we are (as usual) faced with the difference between answering the exact question that was asked, and discussing the general concept and offering advice. MMD has answered the exact question of what to do with a rudder that narrow.

The bigger question is this -- why is the rudder so thin? Is this the way the plans show it, and the kit provides it? Or was this a decision the builder made?

Either way, it seems that Lance could get a much better-performing rudder by taking another 1/2" thick (or thicker) piece of marine ply and gluing up a thicker rudder.

If the rudder hardware is sized for the 1/2" thick wood, then two pieces of marine ply could be glued to either side of the existing rudder, then the whole thing shaped to the proper NACA foil shape.

I've seen racing dinghy rudders with small shaped 'cheekpieces' added to the sides of a 1" thick piece of wood to make the correct foil shape, so this option has been used before.

[Wild Wassa]

Thorne, just after I download some Mirror foils to Image Station, you want to get serious again. Well I'm posting the shots anyway.

Foils from the two fastest Mirrors in the country. There isn't anything flash about these shapes, 12mm thick, a basic rounded leading edge and foot and then the final 8 - 10 cms is shaped to 2 - 3mm of trailing edge, not flash and certainly not fast.













Warren.

[Boatmik]

I'm pretty sure that the Mirrors had a restriction on the shaping and that it couldn't extend very far from either the leading or trailing edges. So more fairing is better - but how much.

Neil Pollock is the only one who has done work in this area as far as I know. He came up with an optimal shape for just this situation.

So some more detailed information about the shaping

A bit more shaping is possible for a better result. The optimum from Pollock's CFD analysis seemed to be shaping for around 5 times the foil thickness from both the leading and trailing edges.

This leaves a flat area in the middle of the foil - but it is an actual tested section.

The templates we use look a little like this (an image will open in a new window or tab)
http://www.woodworkforums.ubeaut.com...8&d=1140343497
They look a little strange because the datum is the table surface underneath and the foils are shaped until the template is resting on both the foil surface and the table.

Here is the trailing edge template in use. Not the sharp corners
http://www.woodworkforums.ubeaut.com...1&d=1140343497

Finally here are the near finished foils
http://www.woodworkforums.ubeaut.com...2&d=1140343497

So what shape?

A bit more detail on the computer optimisation.

The analysis would recommend a parallel sided section in the middle with something like a half ellipse as a leading edge - shaped for around 5 thicknesses of the foil from the leading edge.

Elliptical is not optimum but reasonably close. I use a computer to print out a full size template in the course of my design work.

In general for thin foils...

Around 5 times the foil thickness seems to be the right amount for shaping.

Trailing edge is not so critical - thats what the extensive computer analysis indicated. But shaping should be for about 5 foil thicknesses from the trailing edge

Trailing edge needs to finish with a flat on the back. If no rudder attached it will end up being as thin as you can sensibly make it - I'd normally go for about 1/4" or 3/8" for a workboat or 1/8th or 1/12" for a more performance oriented craft. If there is a rudder attached to the back of the shallow keel then it will have the same thickness as the rudder.

It can be marked for a template by clamping a batten along the edgeline of the foil at 5 thicknesses from the trailing edge and the free end bent in until it meets the desired width at the trailing edge without disturbing the clamped area..

Basically you want no hard points until you get to the trailing edge itself and then you want really sharp corners - no rounding.

Best Regards
Michael Storer

[Wild Wassa]

"NACA sections on thin foils."





This is a thin foil at about 6mm thickness, it is too fat nowadays for some Mosquito sailors. The new foils, I'm told, are only 2mm thick (at the thickest part of the blade below the water). I nearly cut my finger off when I slid it along the trailing edge of this fat blade. Doing the same on the trailing edge of a 2mm blade ... I could lose the entire hand.

If you are flying a hull and slip or pitchpole and come down on one of those blades, I imagine you could nac'a yourself ... well and truley.

A stunning thin blade.

[Songololo]

Thorne,

Thanks for these references. The third one (duckworks) I had not come across before. The following were also useful:

Wikipedia - http://en.wikipedia.org/wiki/Foil_%28fluid_mechanics%29
DCA UK bulletin: http://www.dca.uk.com/DCA_Bulletin_188.pdf p20
Vacanti page: http://www.vacantisw.com/tutorial.htm

"why is the rudder blade so thin..." - the dinghy kit came with the foils as 12mm plywood sheet, according to the plans. The dinghy has specs similar to those of a Mirror dinghy which has class rules limiting the dagger board and rudder thickness to a maximum of 14mm. These specs probably still reflect pre NACA foil days and class rules that have not been updated to take these modern foil shapes into account? I don't however intend to restrict my foil shapes because of these rules as the dinghy that I have is not a true Mirror. This page http://www.boat-links.com/foils.html has information regarding shapes for thin and parallel sided foils.

When I started this project, I was not aware of NACA foil shapes and just followed the building instructions. I have learnt a lot from this project (and this forum!). Early days still in the adventures of dabbling in boat building and sailing ...

MMD,

Good assumptions! I shall follow your suggestions for improving the existing dagger board and rudder blade and then also make a new thicker rudder blade with a NACA section - something to keep me busy during the winter months...

MIK,

Nice site. Good foil shaping instructions and pics. How did you arrive at the specs for the rudder i.e. the aspect ratio (and size)? Any rule of thumb for aspect ratio? Could one use the Portsmouth Yardstick number (Mirror dinghy - 113) as an indication of a foil's aspect ratio. Craig O'Donnell http://www.boat-links.com/foilfaq.html suggests an aspect ratio of 1 - 2 for small sail boats.

Thanks for all the feedback...

Next question: What would be a recommended aspect ratio for the new rudder blade, retaining the wet area of 0.11m2, using a NACA0015 section? An AR of 2 seems to be a good starting point...


Lance

[Tom Robb]

Perhaps I misunderstand (not unlikely) but I thought fat foils were better at low speeds and thin ones at high speeds. As a generality, is that so?

[Boatmik]

The dinghy has specs similar to those of a Mirror dinghy which has class rules limiting the dagger board and rudder thickness to a maximum of 14mm. These specs probably still reflect pre NACA foil days and class rules that have not been updated to take these modern foil shapes into account?

With the Mirror the rule to prevent much shaping in from the edges was to keep the boat simple.

I think you can only shape up to an inch or so from the leading and trailing edges.

Pollock's work shows that this is far too short to be optimal.

His work shows that NACA just doesn't work well under thicknesses of 8%.

If you go thicker than what is in the plan then you can improve things quite a bit. MMD's advice is good there.

MIK,

Nice site. Good foil shaping instructions and pics. How did you arrive at the specs for the rudder i.e. the aspect ratio (and size)? Any rule of thumb for aspect ratio? Could one use the Portsmouth Yardstick number (Mirror dinghy - 113) as an indication of a foil's aspect ratio. Craig O'Donnell http://www.boat-links.com/foilfaq.html suggests an aspect ratio of 1 - 2 for small sail boats.

Thanks for all the feedback...

Next question: What would be a recommended aspect ratio for the new rudder blade, retaining the wet area of 0.11m2, using a NACA0015 section? An AR of 2 seems to be a good starting point...

I don't know how Craig O'Donnell comes to the conclusion about aspect ratio. Generally higher is better for performance within the limits of the water you sail in.

His article has some good general advice but misses the point in a few places too.

I wouldn't ever consider going much below 3 for a centreboard and would try to get more if I could - it makes a lot of difference - which is why Craig's comments are way out of line with theory and practical experience.

But to work well any foil has to be accurately shaped using templates. Eye is not good enough (I wrote about my experiences above).

Generally there is a lot of performance and safety available from making good foils - similar to getting good sails.

There are some "guidelines for centreboard/keel size that wander around the internet - but they are a complete waste of time. They are based on keelboat figures - and old keelboats at that - with the end result that dinghies based on those figures are really poor performers.

In fact I would happily make the generalisation that if someone has built a "character boat" and it has poor performance compared to modern boats it goes almost inevitably back to an undersize centreboard (and maybe rudder). Lots of designers specify undersize ones because they haven't enough sailing experience in different types of boats to know the difference it makes.

Like my BETH sailing canoe was a pretty useless performer upwind with the original 2ft 4" of centreboard in the water. I increased it to just a shade over 3ft and it was transformed - I can race Lasers upwind and beat some of them - despite having a lug ketch rig. It really is that critical to performance.

Many designers also have inadequate sail area but the deficit is nothing compared to their shortage in the foil department.

My general rule of thumb is that I want a centreboard on boats between 8 and 16ft to be close to 3ft deep and the width would vary a bit according to the speed of the boat, its stability and the sail area.

With the OZ PDRacer - which is a basic introductory boat for adults- a punt under 8ft in length we used a foil width of around 10 inches to go with a sail area of 82 sq ft.

The Goat Island Skiff is a 15ft 6" boat for two people with 105 square ft. It has the same length board but it is a little wider because the boat generates more power with two adults aboard so its width is around 11.5ins.

Beth the Sailing canoe is 15ft 6" and sail of 85sq ft also but has a narrow beam of 32 inches and is sailed on up - so cannot develop a lot of power because of beam and crew weight- so the centreboard is not highly loaded - and the low power means that drag needs to be reduced. So the foil is quite narrow at around 8". AS it is a narrowish foil it is a 10" NACA section which is rare on my boats.

All these boats look traditional but have modern performance for a number of reasons - not the least being foil size and shape.

Foil shape/thickness Considerations
NACA - For thickness to chord of over 8% the NACA shapes become useful. The general area that boats with undrestricted centreboard and rudder shapes seem to stick to are:

Centreboard - 10% for faster boats. 12% for slower boats or boats that get knocked around badly by waves or if the centreboard is a bit undersize.

Rudders are generally around the 12% mark with some moving up a bit fatter if the boat has control issues.

Leading and Trailing edges with a parallel midsection
This is a really useful class of foils. Often NACA foils can become very thick as the boat gets bigger so it can get expensive in terms of timber and labour.

So I use a 22mm foil width for most of my smaller boats. This allows economic shaping from 25mm (1") stock with a great saving of weight, timber cost and reduction of the size of the centreboard aperture in the bottom of the hull - which is always drag producing.

The way they work is that they effectively have the leading and trailing edge profiles similar to an NACA foil (Pollock's optimised shapes are somewhat better) but the parallel mid section gives them quite a bit of extra area.

Though length is the most important determinant of foil performance area is important if the boat is likely to be slowed badly by waves, strong winds, having a lot of weight etc.

The leading and trailing edge shaping also makes tapering the foil very easy as the parallel mid section can just be reduced in width without any changes to the edge profiles.

To taper an NACA foil is very tricky requiring a number of different templates and some guesswork about what happens in between the template points, but the Pollock sections (or the approximations I discussed above) allow the template to be moved to any point along the leading and trailing edges respectively.

The basic shaping goes like this.

So the analysis would recommend a parallel sided section in the middle with something like a half ellipse as a leading edge - shaped for around 5 thicknesses of the foil from the leading edge.

Elliptical is not optimum but very close. I use a computer to print out a full size template in the course of my design work.

In general for thin foils...

Around 5 times the foil thickness seems to be the right amount for shaping.

Trailing edge is not so critical - thats what the extensive computer analysis indicated. But shaping should be for about 5 foil thicknesses from the trailing edge

Trailing edge needs to finish with a flat on the back. If no rudder attached it will end up being as thin as you can sensibly make it - I'd normally go for about 1/4" or 3/8" for a workboat or 1/8th or 1/12" for a more performance oriented craft. If there is a rudder attached to the back of the shallow keel then it will have the same thickness as the rudder.

It can be marked for a template by clamping a batten along the edgeline of the foil at 5 thicknesses from the trailing edge and the free end bent in until it meets the desired width at the trailing edge without disturbing the clamped area..

Basically you want no hard points until you get to the trailing edge itself.

Michael

[Boatmik]

The dinghy has specs similar to those of a Mirror dinghy which has class rules limiting the dagger board and rudder thickness to a maximum of 14mm. These specs probably still reflect pre NACA foil days and class rules that have not been updated to take these modern foil shapes into account?

With the Mirror the rule to prevent much shaping in from the edges was to keep the boat simple.

I think you can only shape up to an inch or so from the leading and trailing edges.

Pollock's work shows that this is far too short to be optimal.

His work shows that NACA just doesn't work well under thicknesses of 8%.

If you go thicker than what is in the plan then you can improve things quite a bit. MMD's advice is good there.

MIK,

Nice site. Good foil shaping instructions and pics. How did you arrive at the specs for the rudder i.e. the aspect ratio (and size)? Any rule of thumb for aspect ratio? Could one use the Portsmouth Yardstick number (Mirror dinghy - 113) as an indication of a foil's aspect ratio. Craig O'Donnell http://www.boat-links.com/foilfaq.html suggests an aspect ratio of 1 - 2 for small sail boats.

Thanks for all the feedback...

Next question: What would be a recommended aspect ratio for the new rudder blade, retaining the wet area of 0.11m2, using a NACA0015 section? An AR of 2 seems to be a good starting point...

I don't know how Craig O'Donnell comes to the conclusion about aspect ratio. Generally higher is better for performance within the limits of the water you sail in.

His article has some good general advice but misses the point in a few places too. There is little reason for specifying an NACA foil much over 15%

I wouldn't ever consider going much below 3 in aspcet ratio for a centreboard and would try to get more if I could - it makes a lot of difference - which is why Craig's comments are way out of line with theory and practical experience.

But to work well any foil has to be accurately shaped using templates. Eye is not good enough (I wrote about my experiences above).

Generally there is a lot of performance and safety available from making good foils - similar to getting good sails.

There are some "guidelines for centreboard/keel size that wander around the internet - but they are a complete waste of time. They are based on keelboat figures - and old keelboats at that - with the end result that dinghies based on those figures are really poor performers.

In fact I would happily make the generalisation that if someone has built a "character boat" and it has poor performance compared to modern boats it goes almost inevitably back to an undersize centreboard (and maybe rudder). Lots of designers specify undersize ones because they haven't enough sailing experience in different types of boats to know the difference it makes.

Like my BETH sailing canoe was a pretty useless performer upwind with the original 2ft 4" of centreboard in the water. I increased it to just a shade over 3ft and it was transformed - I can race Lasers upwind and beat some of them - despite having a lug ketch rig. It really is that critical to performance.

Many designers also have inadequate sail area but the deficit is nothing compared to their shortage in the foil department.

My general rule of thumb is that I want a centreboard on boats between 8 and 16ft to be close to 3ft deep and the width would vary a bit according to the speed of the boat, its stability and the sail area.

With the OZ PDRacer - which is a basic introductory boat for adults- a punt under 8ft in length we used a foil width of around 10 inches to go with a sail area of 82 sq ft.

The Goat Island Skiff is a 15ft 6" boat for two people with 105 square ft. It has the same length board but it is a little wider because the boat generates more power with two adults aboard so its width is around 11.5ins.

Beth the Sailing canoe is 15ft 6" and sail of 85sq ft also but has a narrow beam of 32 inches and is sailed on up - so cannot develop a lot of power because of beam and crew weight- so the centreboard is not highly loaded - and the low power means that drag needs to be reduced. So the foil is quite narrow at around 8". AS it is a narrowish foil it is a 10" NACA section which is rare on my boats.

All these boats look traditional but have modern performance for a number of reasons - not the least being foil size and shape.

Foil shape/thickness Considerations
NACA - For thickness to chord of over 8% the NACA shapes become useful. The general area that boats with undrestricted centreboard and rudder shapes seem to stick to are:

Centreboard - 10% for faster boats. 12% for slower boats or boats that get knocked around badly by waves or if the centreboard is a bit undersize.

Rudders are generally around the 12% mark with some moving up a bit fatter if the boat has control issues.

Leading and Trailing edges with a parallel midsection
This is a really useful class of foils. Often NACA foils can become very thick as the boat gets bigger so it can get expensive in terms of timber and labour.

So I use a 22mm foil width for most of my smaller boats. This allows economic shaping from 25mm (1") stock with a great saving of weight, timber cost and reduction of the size of the centreboard aperture in the bottom of the hull - which is always drag producing.

The way they work is that they effectively have the leading and trailing edge profiles similar to an NACA foil (Pollock's optimised shapes are somewhat better) but the parallel mid section gives them quite a bit of extra area.

Though length is the most important determinant of foil performance area is important if the boat is likely to be slowed badly by waves, strong winds, having a lot of weight etc.

The leading and trailing edge shaping also makes tapering the foil very easy as the parallel mid section can just be reduced in width without any changes to the edge profiles.

To taper an NACA foil is very tricky requiring a number of different templates and some guesswork about what happens in between the template points, but the Pollock sections (or the approximations I discussed above) allow the template to be moved to any point along the leading and trailing edges respectively.

Michael

[Cuyahoga Chuck]

The boat in my avatar is an 8' D4 by Jacques Mertens. This design and all his other small sailing prams/dinghies have foil shaped daggers and rudders. Not NACA but foil shaped. The fat part of the foil is supposed to be set back from the leading edge 1/3 of the width of the blade. I think the rational is that these boats are, basically, row boats with optional sail set-ups so anything that will help get you back to shore in time for dinner is worth doing. The only complaint I have is that planing a foil into a piece of plywood exposes all those junky inner verneers which makes for a rather ugly blade.
http://209.190.4.227/gallery/display...bum=343&pos=14
Foils? Yes.
NACA foils? I wouldn't go that far.

[Boatmik]

I did notice that the WEST System website has a section on revisiting some of the boats that were built in the early days of their business to see how they were going.

Mostly boats that are much bigger than we are talking about here.

One thing they found was that boats with plywood rudders and other foils had problems with breakages.

And that made me realise that every foil that I've seen broken in my 35 years of racing and around 20 years of building, repairing and selling boat building and repair materials had been plywood. I've never actually seen a broken foil made of real wood.

I don't mean that I've seen lots of them broken either - but each one has been ply.

Just remembering - I've seen a lot of broken foam/glass or carbon boards over the years too - but they were on boats where there was lots of load.

But let's stick to wood.

Ply boards are half the strength and stiffness of course. But my guess about what is happening is that the veneer layers on the outside are being crimped by leaning up against the sharp edge of the centreboard case or rudder box.

This also happens to boards layered with fibreglass or carbon.

So a ply board is half the strength already then then if the outer veneers are crimped by leaning against that hard edge - the most important ones from stiffness/strength are creased by contact with the edge of the case or rudderbox - then strength has to be down to very roughly around a quarter or a sixth of a board made up of glued timber.

The solution is really simple for the carbon and glass boards and would work for plywood as well.

A tiny amount is taken off the edge of the case or rudderbox. Normally not much more than a single run with a plane to remove a single shaving.

The shaving needs only be a sixteenth of an inch or less and extend up into the case about half an inch. Don't round the edge of the case/rudderbox - but that tiny cut means that the load on the board is distributed over a narrow band rather than being a point load as it is with a perfectly square case - or sometimes with glue, and subesequent coatings there can be a little bit of a bump on the inside of the case that is much worse still.

MIK