Centerboard size?

[Peter G]

Hello!
I ´ve got a 4,2 m wooden boat with flat bottom and I´m gonna build a centerboard. I need help with the design of the centerboard. What I know, the centerboard should be about 4% af the sailing area, but i dont know how to calculate the width and length of the centerboard. To fit in the boat it should not be more than 35 cm in width. Could that be too smal? It is not a fast boat and only got one spritsail with about 5 m2 sail-area.

[JimD]

Welcome to the forum, Peter! May I ask what design your boat is and have you looked for similar designs and sail rigs to see what they are using?

[Thorne]

I just finished putting a centerboard and sailing rig into a traditional pulling boat, and discovered that unless the designer has specific plans available, there aren't any hard and fast ratios or calculations for this info that can be fully relied on.

Much depends on the hull design, specifically how much lateral resistance it has (v-hull, skeg), plus rudder design and size.

Is the mast fixed in place, and was the boat set up to sail without a centerboard? If the mast can be shifted, you can build a centerboard and rudder, then tweak the mast placement and rake to help balance the sailing properties and handling.

Photos would help! ;-0 )

[Peter G]

It is klink-build (massive pine) with flat plywood bottom. I think the boat is about 50 years old, but the bottom must have been changed, maybe because of som injury (can´t inagine that it would be plywood originally). There is a little keel. The keel is about 20-30 cm deep in the back of the boat at the rudder, but it soon get smaller and is only 5-10 cm in the rest of the boat. I have seen some boats that looks like this at the westcoast of Sweden, they are called "flatbonna". I have only one good picture of this kind of boat (flattbottoms of this type is not common in Sweden). That boat have a bigger width than deep of the centerboard, about 50-80 cm and only a deep of about 50 cm. In that boat they also have put the centerboard in the middle of the boat, but I have to put it right behind the mast because it would otherwise be were I want to put my feet when roowing.

I think the most lateral resistance is in the back of the boat, because when I am sailing and there suddenly come a wind, the front will go with the wind. The boat is nice to sail but it is impossible to sail to windward. The mast is fixed in place. I dont know if the boat originally was set upp for sailing. When I look at the boat it looks like it has been changed in the design and repaired at some places.

My biggest worry is that if 35 cm would be to narrow for the centerboard because I don´t want to cut up the bottom just to find that it would not work. If it is not to small maybe I should go with trial and error to find a the best area of the centerboard?

It is very uncommun with centerboard in woodenboats in Sweden so it is not easy to look at other boats except the pictures that is on internet (the fact that i live in the middle of Sweden where everyone have got ugly plastic boats with 2-stroke outboard engines make it difficult too).

I will try to take some photos and also put them on a site as soon I got the time.

/Peter

[JimD]

I think the most lateral resistance is in the back of the boat, because when I am sailing and there suddenly come a wind, the front will go with the wind. The boat is nice to sail but it is impossible to sail to windward. You already know this but if there is that much lee helm you really need to get the centerboard well forward or do you have a serious trim problem with far too much weight aft leaving the bow in the air?

[Peter G]

I don´t think it´s too much weight aft leaving the bow in the air because it was the same thing when I put my dad in the middle of the boat.

I was just thinking if I should put the centerboard just behind the mast or more aft. The frame make limits were it is possible to put it and I think I only can put it close to the mast. Like you say I should put the centerboard well forward, so I hope it will be ok to put it there. But if the center of lateral resistance get too much forward from the center of effort, could I only make a deeper rudderblade to compensate that?

[Thorne]

Peter -

A lot depends on the size of the sail, in effect where the Center of Effort(CE) is located.

Here is a graphic from a website to hopefully explain it all -- by the way, your English is SO much better than our Swedish!

;-0

http://www.sailnet.com/images/content/authors/colgate/052600sc_figure3.gif

Some boats have the mast directly in front of the centerboard -- here is a photo of the plans from the Mystic Seaport Museum for my Chamberlain dory skiff.

http://www.luckhardt.com/cb-design1.jpg

You will note that I have modified the plans, allowing the front thwart to remain and lengthening the centerboard to (hopefully) make up for the reduced width/height.

I've put a pin in my centerboard case that allows me to pin the board in three positions: fully up, halfway down, and fully down. Both my rudder and centerboard hand straight down when fully extended/released. So in certain conditions, I hope to be able to change the Center of Lateral Resistance (CLR) by pinning the centerboard halfway up.

As far as keeping the area free for rowing, as long as you can keep the height of the centerboard case below that of the rowing thwarts, you don't have to be limited as to the fore-versus-aft location -- put the centerboard where you wish and row with your feet on either side of the case.

Here is the page for my modifications -- I'm no expert, so take it all with a grain of salt, but it works for my boat which has less of a keel than yours -- I just have a short skeg in the rear 1/3 of the bottom.

http://www.luckhardt.com/dory1.html

[ 02-26-2006, 06:41 PM: Message edited by: Thorne ]

[Boatmik]

Hi,

35 cm width will be fine on a boat this size if the board is made long enough.

See our photo essay about foil making for the pair of puddleduck racers we are building on
http://www.woodworkforums.ubeaut.com.au/showt hread.php?t=28107&page=2 (http://www.woodworkforums.ubeaut.com.au/showthread.php?t=28107&page=2)

Posts numbered 29 and 30 have a bit of description and some photos that will make things clearer

Slow boats need more centreboard area than fast ones. The most common reason for boats not going to windward is a too small centreboard in terms of length and/or area.

With a width of 35cm, the board will probably need to extend about 900mm below the bottom of the hull for good windward performance. You could go for one a bit shorter if depth is a problem where you sail, but a 600mm board would be way too short to go to windward well.

Generally length is much more effective than area - so area comparisons are a bit "old hat" once length is taken into consideration.

Thickness should be a minimum of around 22mm for it to easily take the load of the crew standing on it to right after capsize. Recommend glassing with 6oz (200gsm) woven glass fabric and epoxy.

You don't need to shape the whole thing into a foil. Shape the leading edge back about 100mm (5 x thickness) and same for the trailing edge.

To take benefit from a full classical NACA airfoil shape the centreboard would need to be a minimum of about 30mm thick so would be heavier and more expensive - and a lot more work to shape.

I recommend a thinner foil for my designs with shaped leading and trailing edges and a parallel mid section as it saves time and money.

Take pains to make smooth transitions in shape between the leading, trailing edges and parallel mid section. No bumps or hard edges in the leading edge.

Trailing edge planed off to a width of about 4 to 5mm is durable and though not optimum from a performance perspective is still quite reasonable.

Making up a template for shaping out of plywood makes a huge difference to performance - consistency in profile will reward you with much better performance as will a good surface finish. With the above method you can make two templates. One for leading edge and one for trailing edge and simply move them up and down the foil to find out where material has to be removed.

The foils are the hardest working parts of the boat - some attention to detail has a great benefit. But if it seems like too much bother with the whole lot, any step toward the above will be of some benefit.

Best Regards

Michael Storer my boat page (http://members.ozemail.com.au/~storerm)

[ 02-27-2006, 09:11 PM: Message edited by: Boatmik ]

[Peter G]

Thanks for all help! Have now made a daggerboard with the size 0,35*0,9 m. Putted the boat into the water yesterday-no leaking (despite i dont use epoxi).

/Peter

[Thorne]

Peter -

How is the lee helm issue, and/or any other sailing rig problems?

[Jay Greer]

Here is a simple solution prior to making major modifications. It is possible to calculate the size and placement of the dagger board by mechanical means. Pick a shape you think will work. Lay it out on a piece of plywood of a thickness you plan to make the actual board and taper it to a foil shape with a power plane. Then lash or clamp it to one of the rails where you think it should go and take the boat out for a sail. You should know in a short while where the placement should be. The best part here is that you can shift posistion and change shapes till you are happy with it!
JG

[Peter G]

Well, I haven´t try to sail yet. I think I have to mix with the rig to get center of effert more forward. I calculated CE and center of lateral resistance theretical on a paper and I think I have ta use a jib to awoid the stern to blow away from the wind (I had to put the daggerboard right behind the mast to awoid having the DB-trunk between my legs). I hope I will have some time to try the sailing properties as soon as possible. Right now work and sleep take all the time.