When a boat owner considers upgrading their boat to make it safer and get more performance, often the first thought is for new sails which are expensive, but a cheap hack is to improve the foils (centreboard or leeboard) which might take little more than a couple of weekends and cost around 50 bucks.
How much sailing Performance Improvement from foils?
#1 Cruising Sailor – Chuck Pierce TX
Chuck Pierce is a well known Texas Adventure Sailor. He was interested to improve the performance of his Mayfly 14 to make the boat more capable for cruising.
Chuck wrote:
The leeboard was initially roughly shaped according to Jim’s instructions. I tried to get both sides the same, but didn’t spend a whole lot of time on it. I had a whole boat to build, and had never done any of this boat building stuff before. It worked ok. I sailed the Mayfly for about 3 years with the board like that.
In 2014 I got some fairing compound and made a template (sized up appropriately) from the OzGoose plans that you were nice enough to send me, and spent a couple of hours fairing it to the foil shape you use, so that I could use it on ECDuck in the Tx200 that year.
It did quite well on that trip. I was able to go places (upwind) that some of the others could not. This was useful. :)
When I put it back on the Mayfly, it made that boat sail even better. She comes about smartly, and seems to have less leeway going upwind.
How much sailing Performance Improvement from foils?
#2 Cruising Sailor – Robin (Sydney)
Robin is a old friend, pilot, builder of sailboats and aircraft. He probably rolls his eyes when I talk about the difference that accurate foil shapes make to the handling and performance of a boat. At a tiny fraction of the cost of a new set of sails he improved the performance of his Ness Boat.
Robin wrote: Applying Mik’s Viola foil templates to the centreboard of an Oughtred Ness Boat.
First impressions under sail: less heeling and easy speed to windward where it used to take a fair bit more concentration (non performance sailor point of view).
On my sail last Sunday I just found myself having to pay much less attention to getting through and acceleration after each tack.
Definitely worth a few weekends work.
So how was Robin’s boat before – Why make the effort?
From Robin:
I had of course read your stories on storerboatplans.com, particularly the Orange Boat one, and I’d bought several of your plans with the template included.
The boat can work to windward but I think not as well as it could (from reading) and I usually had problems tacking, decelerating quickly, getting caught in irons and having to back wind the main to come about.
Even “Scaring Ourselves” because of trickier handling than some Ness Boats.
When the boat was starting the new tack it felt ‘mushy’,
I’d bear away to try picking up speed and lose some of what I’d gained on the previous tack.
Every now and then it could get into a groove which was startlingly different but I had no idea how to reliably and repeatedly get into it. Putting on my flying hat I thought of the narrow WW1 airfoils and the nasty stall characteristics, and that it was quite possible that my centreboard, during construction or in the intervening 15 years of groundings and trailer dents, was doing something similar.
4 Part Series – Homemade foils for better Performance and Safer Handling
How Robin reshaped his foils
First Robin chose a section. As above he decided to use our foil section in our plans which is derived from the optimisation work by Neil Pollock.
The simplest way to get hold of the template is to buy our Oz Goose Plans for $36. There is a lot more than just that section that will be useful. It is really a boatbuilding course in a book. We have a scaling method in the plans or you can blow up the section on a photocopier.
And the whole building process is covered step by step.
In general the leading edge is 30% of the foil width. The trailing edge around 50% of the foil. The extra approximate 20% width is a flat area that prevents the foil from rocking around as you shape it which increases accuracy enormously. The 20% extra can be cut off the back of the foil so it goes down to zero to provide an accurate taper if desired.
Both methods rely on the template and foil sitting on a flat surface which becomes a reference surface. The thickness of the foil also has to accurately match the max height of the template. When the base of the template hits the table and the highest part of the template matches the thickest part of the foil … you are done!
Template type 1
Robin has a bandsaw so printed, scaled and glued the paper copy to the timber he was going to use as a template. Use a non water based glue or the paper will change shape.
Robin planned to glue his sandpaper to the template with hot melt glue and release with a hot air gun. He found this a bit slow.
One point .. there should be no sandpaper on the area of the template that runs along the high point of the foil
Template type 2 – most common method
The more common type of template is just ply. And you work from top to bottom of the foil marking the high spots. Then remove them using a plane intially. Then mark the high spots using the template again. Getting closer change over to a sandpaper block. When the last high spot is just gone, the foil is shaped!
The other way to get the foil shape – plot the original Pollock foil curves
I strongly believe that section accuracy almost trumps section choice. In the NS14 class “Laminar flow” sections are popular despite the boats spending most of their time far below the Reynold’s numbers where the laminar section has any advantage.
Pollock Leading Edge.
y=Tmax*( 8*SQRT(x)/(3*SQRT(XLE)) – 2*x/XLE + x^2/(3*XLE^2) ) where:
y is the thickness at a given value of x
Tmax is the maximum thickness as a fraction of the chord
x is the position along the chord from 0 to 1
XLE is the length of the tapered leading edge.
(the LE is upper case because a small L could look like a 1 or I.
Pollock Trailing Edge.
y=Tmax*(1 – (3*x^2)/(2*Xte^2) + x^3/(2*Xte^3)) where:
y is the thickness at a given value of x Tmax is the maximum thickness as a fraction of the chord x is the position along the chord
x is 0 at the beginning of the taper and x increases to Xte at the trailing edge of the foil
Xte is the length of the tapered trailing edge
How much sailing Performance
#3 Racing Sailor (me in a past life)
We know the 10% figure from the NS14 class which recorded the times of boats involved in races. They found a 4 minute time improvement. As well and the handling and safety improvements.
This might not sound like much, but it only increases upwind speed. Upwind time was around 35 to 50 mins.
In the 1970s I bought a second hand NS14 Dinghy (one before the one above). The centreboard and rudder were around the normal length but the boards were a bit wide. So I decided to modify them myself reshaping in line with more modern theory – moving the point of maximum width back slightly and producing a pair of really smooth shiny foils.
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.
Racing Revelation
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.
Exactly as Robin experienced with his quite different boat it would now tack and gybe faultlessly coming out of the manoeuvres with heaps of speed rather than having to get the boat moving again. (just like Robin) 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 to pay someone for first class foils. About 150 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 around 1000 bucks.
Oz Goose Plans with foilmaking instructions and template with scaling method. (Can also scale using the printing scaling on your computer.
https://www.duckworks.com/product-p/oz-goose-id.htm