Centerboards

[Phil Alderson]

What sort of size are people using for centerboards these days?

[john_hamilton]

6ft ellipse with around 30cm chord if I remember correctly

[Phil Alderson]

wow 6 foot, that is almost as tall as me. I am thinking of an elipse either 1400 or 1500 mm with a 260mm chord.

Something like the attached.

[Stuberry]

The board on ronin is 1650mm. Which is as tall as me!

The new board I have made for strawberry is 1850mm, but I will probably reduce that. I haven't yet decided by how much.

My rough plan is to pull it up until I find the right point, then chop off the excess. I will probably keep it a bit over length to drop it all the way down when struggling for power and lift it when over-powered.

[phil_kirk]

Ours is about 1.5m long, 30cm cord and an eliptical plan form.  A foot of the board is in the case.  I have been thinking about going a bit longer and a bit shorter cord maintaining the same area.  John's started off about 1.8m but appeared to have got a bit shorter by the nationals.  Unlike Stu's approach John appeared to have shortened it at the bottom (accidentially i think)   . 

As a general rule I think the board should be about the same height as the crew.  Assuming that you are going to raise some board when you come in and want to be able to touch the bottom when you jump out).  There are pleanty of holes in this argument like being able to breath whilst touching the bottom.

[Phil Alderson]

Just re-measured my existing board and it is 1300mm and 330mm chord.

I have redone my design for 1550mm long with a 270mm chord the top 500mm is parallel, and elliptical below that.
The case is about 250mm deep so I can raise the board 250mm for windy weather and still have it fit the case at the bottom of the boat. As I have less sail area and righting moment than the twin string boats, I do not want quit as much area

I tend to jump out first so by phil's theory I could go for a 1.9m board. Given that I have a gnav the limiting factor for length would have to be max boat holding standing depth plus distance from waterline to the bottom of the boom, so there is potential for a 2.5m board

[dave_roe]

Just for a laugh I've measured Pasta's board.   320mm chord and 1220mm long. When you bear in mind the fact that the Pasta has an unusually deep board case there's only 900mm of board projecting out the bottom of the hull. Admittedly this was designed for one wire and not much crew weight so you might expect a larger board for a 2005 setup and heavier crews. However the addition of a t-foil has increased boat speeds upwind when powered. This makes the optimum board size relatively smaller. If any of you have been calculating the aspect ratio of your boards and its' effect on induced drag, bear in mind that the board is end plated so the effective aspect ratio is doubled.

Am I right in thinking that Kevin has done some detailed cad modeling of boards?

[JimC]

This makes the optimum board size relatively smaller.

Agreed but... On my Plus Plus, which had too small a board in its first iteration, I found that the limiting factor on board performance was not what happened at speed but the ability to get out of a tack without the board stalling, especially in lighter winds. The boat had a minimum wind speed below which it was almost impossible to sail. Iteration 2 had a bigger board with more of a low speed section, which meant I could tack more than seven times out of ten, but seemed to take a noticeable amount of sparkle out of the top end. I  suspect there's probably a case for lifting the board once the boat is up to speed, and dropping it again before tacking, but there are obvious practical challenges...

[dave_roe]

My rough plan is to pull it up until I find the right point, then chop off the excess. I will probably keep it a bit over length to drop it all the way down when struggling for power and lift it when over-powered.

A cunning plan Stu, I have a couple of concerns..
The 'right point' will be different each time you sail. The optimum area depends on boat speed and hence wind strength, sea state and (as Jim mentioned) whether you've just tacked etc.
During your experimental period your bowsprit will not properly retract and quite possibly your crew and boom won't be able to tack. Just to clarify though, if the boat is over-powered you'd want a smaller board. (When sailing the Pasta with a light crew I quite often leave the board up a bit in windy weather.)

The true optimum when up to speed is very small and is also unworkable in practice. This is because water is 1000 times the density of air and yet the boat must always be able to accelerate from stationary. The next factor to take account of is the ratio of boat speed to wind speed. In practice centreboards end up in the typical range 2-4% of sail area. Where 4% is only really applicable for quite slow craft. In a boat like the Cherub there is also an element of personal choice trading top speed against slow speed acceleration. Light versys heavy weather performance etc.

For comparison Pasta's centreboard is 2.07% of upwind sail area.
As an example of an extreme case, consider a slalom board (windsurfer). In wind speeds of (say) 15 knots with 6m^2 sail it can exceed the wind speed but can also beat quite effectively at lower speeds. The fin area is about 0.3% of the sail area.

In the particular case of Strawberry I'm also concerned that the centreboard case is quite shallow. There is a practical limit to what length of board would be structuraly sensible. (What case depth are the E5s?) My hunch would be to saw off 400mm and be done with.

[phil_kirk]

Dave,

The E5 case is about 300mm deep. A bit higher at the front and a bit lower at the back due to the deck sloping downwards towards the stern.  The real case depth on slippery was about 200mm although it actually extended 60mm above the floor and this section was stiffened to sort of give more support to the foil.  The board was approx 1.2m long total so about 940mm in the water.

Interesting to compare sail to board area. 

[dave_roe]

Thanks Phil, I thought as much. Pasta's case is a similar setup to Slippery and is 290mm deep. Case depth is something that Kevin and I used to discuss and have similar views on. For everyone considering board sizes - remember it's the bit in the water that counts.

Strawberry's case will be at most 200mm deep so I can remove 100mm compared to the E5s to start with. I'm certaily not comfortable with the idea of letting Stu 'try' 1650mm of projected board encastre in just 200mm of case.   

[phil_kirk]

Taking the area of our board that projects below the hull and our sail area I get a board to sail area ratio of 1.7%.
If I drop the sail area to that for a 97 rules boat I am just above the 2%.  This is not an accurate calculation of area just cord * length *0.7 (area coefficient)

We do struggle with tacking in the stronger winds or choppy waters mostly due to not going for it and getting stuck in irons but otherwise don't have the problems that Jim mentioned.

I was thinking about dropping the cord slightly to give a higher aspect ratio but it appears that I do not want to drop area anymore. 

[dave_roe]

That depends Phil. What are you hoping to achieve with the new board design?
What is the problem or weak link with the current one?

[Stuberry]

My rough plan is to pull it up until I find the right point, then chop off the excess. I will probably keep it a bit over length to drop it all the way down when struggling for power and lift it when over-powered.

My hunch would be to saw off 400mm and be done with.

That sounds like a sensible ball park figure. It will leave me with 1450mm of length. Using Phil K's rough calculation that gives a foil/sail area of about 2.4%. Assuming I could lift the board by 200mm I'd be left with 1250mm, pretty close to pasta, and giving 2.1%.

[Stuberry]

Oops, I seem to have forgotten to take into account the depth of case.