Daggerboard Design for a Dinghy

Hi,

I'm looking for a design guidence for a daggerboard of 3.8mt/1.3mt dinghy having total 6.5m2 sail.
From web search ;
- Area should be more than %3 of sail area,
- Naca0010 suggested ( to be coupled with 0012 Rudder)
- Eliptic shape to reduce tip losses.

Questions ;
- Which range of drift (angle of attack) acceptable for a small dinghy ? 4 degrees to 10 degrees ? What is general performance range ?
- Is 30mm thickness too much to create drag forces or normal as expected? Naca0010 with 300mm chord results that thickness.

Waiting for your valuable feedbacks, thanks.
Serdar

 

- 3% of the sail area is a minimorum minimum. For this boat size, around 4% may be prefer if you're willing to be the first at the pub, rather than spending your day, tacking over and over again.

Design point is 5 degree. Range is 4 degree to 7 degree. 30mm sounds far too much. 22mm maybe a good option. I don't know the details of your daggerboard well, but 220mm for a chord sounds good. The more aspect-ratio you have, the best lift to drag performance you will have. Diminishing the chord will increase the span of the daggerboard, giving you a better aspect ratio. Of course, if you have no draft requirement or rule to comply with. The limiting factor is the structural strenght of the material.

 

Thank you Alan, nice and clean answers..
There is no class rule needed, target is not to race , just fun and sail in safe.. I considered 300mm chord to reach 30mm thickness to be able to stand on it in case of capsize due to narrow beam of boat (130cm).

 

Whenever two or more sailboats meet, a race will ensue.

 

Thickness should be sufficient for required strength which its appears serdar has taken into account with his 30 mm thickness.

The NACA 00XX sections are scaled versions of each other. A NACA 0012 section is the same as a NACA 0010 section scaled in thickness by a factor 0f 1.2. There is nothing magical about a 0010 section compared to 0006 or 0016 sections. For 30 mm thickness a 0015 chord of 200 mm may be a better choice due to reduced wetted surface area as long as the target 3% or 4% of sail area is achieved.

 

Serdar
12% chord thickness ok at this scale.
Make it 20% longer than the theory and test at low speeds.

 

You are allowed to cheat and leave a flat side on some of the exposed daggerboard. Let's say the exposed board has a root cord of 325 mm and a tip chord of 250 mm and is about 1 meter deep; and you make it out of 30 mm stuff. You can fully profile the lower 2/3 of the board starting at a 0010 section. For the upper portion, develop the proper section leading edge and trailing edge and just let them run out leaving a smallish flat cheek area on the upper 1/3. This is perfectly fine and will not noticeable affect performance. It is better than trying to fine the entire profile down to 9%. You can also simply truncate the aft 2-3% of the profile so there is something there. Some foil offset generators let you have a finite trailing edge.

 

I think best choice is to use multiple daggerboards for different conditions We have 3-4 for different fins as well as several different boards and sails for windsurfing. No one tries to use single fin for all wheather conditions...

I lost my oppurtunity to sail Optimist/Laser when I was young unfortunately , that dinghy will be my first sail boat (except windsurfing). In another words I do not know sail with a boat Another choice could be to shorten underwater length as I saw Laser/Optimist users. In that case the taper trough tip may create some problems due to increase of gap between daggerboard & edge of hull slot.

 

Are rudders normally included in the 3-4% foil area?

I imagine the greater the efficiency of the foil(higher aspect ratio, etc) the smaller you can make the foil? How much difference is there between a flat plate and a true foil?

 

No, the rudder surface is not included in the 3-4% of foil area, because the a well balanced boat should should need no effort on the rudder. The rudder surface is calculated by the determination of a yawing moment, around a center that is roughly taken, from the aft perpendicular, at two third of the lwl.
Rather than the "balance" 12% for the rudder and 10% for the daggerboard, I generally come up with the opposite, ie a relative thickness of the rudder lower than the one for the daggerboard. First, the drag of the rudder @0° should be minimized, what is obtained with a thinner profile. Second, when seeking the highest aspect-ratio of of the daggerboard, it is, almost everytime, its structural strenght that governs the final thickness. Additionnally, one may consider the use of Eppler profiles for the daggerboard, instead of the commonly used Naca profile. The l/d ratio of the Eppler is a little greater, and the same goes for its stability. For the rudder, you can also look for good candidates among the Wortmann series.

The difference between a flat plate and a curved profile is to be appreciated whether you're seeking lift of L/D ratio. A flat plate is not necessary a bad choice, especially regarding its building simplicity. Thiese simulations have been made at low Re, but the tendency is there. (attached document)

 

You want the boat to balance with the rudder operating at about 1/2 coefficient of lift as the dagger board. This means if the rudder is too big, there is a lot of extra surface area - which is bad. So when calculating the area ratio, you can use half the rudder area if you design the boat to balance with the rudder at half the coefficient of lift of the dagger board. Note that in practice, this will result in the rudder being on the centerline when sailing because the downwash off the dagger board is going to change the inflow to the rudder by about half the angle of attack of dagger board. For boats this small, it is often convenient have the rudder about 50% - 60% as big as the dagger board. This lets you push the board forward in the hull to get more room in the cockpit. So one option would be a dagger at 3% area and a rudder at 1.8% area which you score as 0.9%.

The answer to your second question depends on the design point and what you really mean by a flat plate. If you take a thinner board and put a 3:1 elliptical leading edge and a 7 degree taper on the back edge, you're probably about 90% of the way to the full foil performance from flat plate performance. But even a theoretical infinitely thin plate isn't terrible by comparison because most of the time the board should be operating at a fairly low coefficient of lift. It would be most noticeable when trying to tack in light air. It would stall easier and not want to regain bite on the water as you power up on the opposite tack.

One comment on aspect ratio of the foil. The aspect ratio doesn't matter. That's right, it doesn't matter. You have already spec'ed the hull and the rig, thus all the loads are fixed. So we are designing a foil to a fixed lift load regardless of it's shape. This is not the way it is normally presented in textbooks. It turns out that if you increase the cord while keeping the span the same (and thus lower the aspect ratio), the induced drag will be exactly the same. The only thing that matters for the induced drag term is draft. Increasing the cord affects friction, of course, but not the lift-induced drag. So unless you are racing, aspect ratio is the absolute last thing to worry about. Just get a decently deep dagger. It doesn't matter if the aspect ratio is 2 or 10, the induced drag will be the same. This is why I really don't like the "naked" requirement of lateral area ratio. A 4% ratio with an AR of 3 and a 3% ratio with an AR of 4 (which will yield the same draft) will sail nearly identical 90% of the time. A pro could eek out a victory in the 3% boat, but most of us would fair better in the 4% boat. The most popular board classes of all time have aspect ratios around 1. Several use flat plate boards. So you needn't get too hung up with foil optimization. A chunk of plywood, a belt sander, and an eye to smooth the exposed ply lines will do just fine.

 

Thank you all for your kind and helpfull replies.

Dear Alan,
I decided to go with your idea . Checked several foils and found that E836 has some advantages around 16-26 deg. of attack angles over naca0012 /63012a, which they lost major lift around those angles. I assume those angles will be important while tack&gybing ?
Due to %12.6 thickness ratio I can reduce the chord to 250mm, tip 170mm and increased span to 1150mm. By this way I can achieve 490N lift with 16N total drag with 5.8knt low speed, I hope that will be enough lift to sail Total thickness will be 31.5mm and glass/carbon layered pine core will have enough resistance to stand on in to turn hull.

The only concern will be the production quality of trailing edge, ı need to cut-off some part to have square edged form there for safety, because its like sharpy knife , no need bloody sailing

Which Wortmann series do you suggest for rudder , fx76100 ?? That foil also have lift losses around 19-26 deg range, instead another Eppler may be better ?

 

Dear Alan,
I decided to go with your idea . Checked several foils and found that E836 has some advantages around 16-26 deg. of attack angles over naca0012 /63012a, which they lost major lift around those angles. I assume those angles will be important while tack&gybing ?
Due to %12.6 thickness ratio I can reduce the chord to 250mm, tip 170mm and increased span to 1150mm. By this way I can achieve 490N lift with 16N total drag with 5.8knt low speed, I hope that will be enough lift to sail Total thickness will be 31.5mm and glass/carbon layered pine core will have enough resistance to stand on in to turn hull.

The only concern will be the production quality of trailing edge, ı need to cut-off some part to have square edged form there for safety, because its like sharpy knife , no need bloody sailing

Which Wortmann series do you suggest for rudder , fx76100 ?? That foil also have lift losses around 19-26 deg range, instead another Eppler may be better ?

 

Yes, indeed, tacks1gybes are more progressive and smoother with an Eppler.

The trailing edge can be a monolithic carbon stock, easier to cut with a dremel, limiting the weakening of the resin due to the saw vibrations. As you seems to already know, it is important for the foil to have at least one sharp edge on the trailing edge.

FX76100 are interessant if you don't want the effort on the helm at high angle. If you wish not too loose power at these high angles, the FX79100 are better. A better compromise would certainly be another Eppler, that's right.

It's very fun to built his own appendages, so I wish you a lot of pleasure in your work.

 

You are very good teacher, thank you for your contrubutions, i'll share my sea trials .