Newcomer on daggerboards

[SweBlueBox]

Hi!
I have an old cruiser-cat with a centerboard between the hulls. The upwind performance is ... bad. I intend to build a new board and have been playing around with the Profili 2 software based on the Xfoil simulator to learn more.

I have a few questions that I haven't found answers to anywhere else and I hope someone would like to help.

1. Xfoil computes a Cd/drag coefficient somewhere in the range 0.005-0.03 for normal angles of attack. What reference area is used to calculate the actual dragforce for the Cd reported by Xfoil?

2. NACA 00XX profiles seem to outperform almost any other profile in the database in the Reynoldsrange I am interested in. Is this due to a favorable coincidence between Xfoil analysis algorithms and the research done by NACA on profiles or could I expect the NACA 00XX to be the strongest candiate on sea?

3. Some of the designs I've seen around Stockholm use daggerbord profiles similar to Eppler 521 or symmetrical NACA 64XX that are slightly concave in the 75-85% part of the foil. But these profiles have few or no advantages compared to NACA 00XX according to Xfoil. Even the assymetrical boards on the 60foot Tri's I have seen have a concave area. What am I missing? Does the noncompressibility of water make Xfoil a bad choice of simulator for my application?

4. A question on planform too ... i have seen somwhere that the reduction on Cl/lift coefficient due to wingtip effects can be computed with the ratio AR/(AR+2) for an aircraft wing. Will this apply for a centerboard as well? I think the reduction should be less since there is only one wingtip. I addition the AR for the daggerboard is computed on only half an aircraft wing. What would be an appropriate Cl-reduction-factor for a centerboard?

If any other info needed ... please ask.

BR /T

[Raggi_Thor]

I think that naca 0010 to 0015 give the best overall performance.
Most other profiles try to optimize for certain conditions that are seldom met i real sailing.
Does your centerboard hang free in the water?
You double the effective aspect ratio if its under a hull.

Some useful stuff here, from David Vacanti,
www.proboat-digital.com/proboat/200506/
look at his article starting at page 76
(use the menu "Content")

This may also be interesting for you :-)
http://www.mboats.no/Hardware/hardware.htm
High quality naca profiles produced in extruded aluminium in Sweden...

[SweBlueBox]

Thanks for the useful advice, most of my thought are clarified.

Originally Posted by Raggi_Thor

I think that naca 0010 to 0015 give the best overall performance.
Most other profiles try to optimize for certain conditions that are seldom met i real sailing.
Does your centerboard hang free in the water?
You double the effective aspect ratio if its under a hull.

The existing centerboard case is only 30 mm wide and with a XX10 foil it leaves me with only 300 mm chord and a small area and strength. The case exit is appr 200 mm above the surface wich adds nonlift span and momentum.

Originally Posted by Raggi_Thor

Some useful stuff here, from David Vacanti,
www.proboat-digital.com/proboat/200506/
look at his article starting at page 76
(use the menu "Content")

Very interresting and useful article! Thanks! The part on foils conforms with my observations from my "playsession" with Profili/Xfoil. I can't figure out if D.Vacanti's s/w is based on the same algorithms as Xfoil wich leaves us in the dark on question 2. But also in good company!

Originally Posted by Raggi_Thor

This may also be interesting for you :-)
http://www.mboats.no/Hardware/hardware.htm
High quality naca profiles produced in extruded aluminium in Sweden...

Good to know, thanks for the tip, even if their products won't fit in my (cb)case.

Seems as if Q1 and part of Q4 are still looking for answers although the article clarified a lot. Anyone?

BR /T

[Raggi_Thor]

Think I have to tell, it's my products in a way, I don't make them, but I sell them (the alu profiles).

:-)

[tspeer]

Originally Posted by SweBlueBox

...
1. Xfoil computes a Cd/drag coefficient somewhere in the range 0.005-0.03 for normal angles of attack. What reference area is used to calculate the actual dragforce for the Cd reported by Xfoil?

The planform area is the reference for the nondimensional coefficients from XFOIL.

2. NACA 00XX profiles seem to outperform almost any other profile in the database in the Reynoldsrange I am interested in. Is this due to a favorable coincidence between Xfoil analysis algorithms and the research done by NACA on profiles or could I expect the NACA 00XX to be the strongest candiate on sea?

The NACA 4-digit sections are good all-round sections. You can do better, but probably not dramatically better unless you have very specific requirements.

You may want to decrease the exponent used to predict laminar to turbulent transition (ncrit). The default value of 9 is good for air, but something more like 3 seems to give results that match experimental data better for water.

3. Some of the designs I've seen around Stockholm use daggerbord profiles similar to Eppler 521 or symmetrical NACA 64XX that are slightly concave in the 75-85% part of the foil. But these profiles have few or no advantages compared to NACA 00XX according to Xfoil. Even the assymetrical boards on the 60foot Tri's I have seen have a concave area. What am I missing? Does the noncompressibility of water make Xfoil a bad choice of simulator for my application?

You're not missing anything. The NACA 6-series sections have a lower minimum drag, but you are likely to be operating outside the drag bucket the majority of the time with a multihull, since one doesn't sail dead downwind.

XFOIL is very good for incompressible flow. Except for maximum lift, its predictions are probably about as accurate as much of the published experimental data, especially the early NACA results.

A cambered section like is used for a daggerboard can make better use of a laminar flow section because the camber allows the drag bucket to be positioned over the operating range of lift coefficients.

Probably the most important thing you can do to reduce the drag of your board is to make it deep.

4. A question on planform too ... i have seen somwhere that the reduction on Cl/lift coefficient due to wingtip effects can be computed with the ratio AR/(AR+2) for an aircraft wing. Will this apply for a centerboard as well? I think the reduction should be less since there is only one wingtip. I addition the AR for the daggerboard is computed on only half an aircraft wing. What would be an appropriate Cl-reduction-factor for a centerboard?

It's going to depend on how much lift is carried over to the hull and how fast the boat is going. At high speeds, the induced drag at the water's surface is doubled, while it's halved at very low speeds. So a reasonable engineering approach might be to ignore any benefit from the hull and let that conservatism help offset any additional source of drag for which you've not accounted.

This spreadsheet will allow you to calculate the effect of planform shape and size, and assumption about the surface effects. The hull can be represented as part of the planform, possibly with a reduced lift curve slope and/or lateral area if you want to take credit for its end-plate effect.

[SweBlueBox]

This really helps, Thank you, Tom!

It is really wonderful to be able to get in touch with the best knowledge available in this matter, beeing an ordinary world citizen. Thanks again, Tom.

This spreadsheet will help me to take the next step in understanding how planform interacts with the basic lift/drag characteristics from 2D analysis. Now I "only" have to sharpen my english and dig in to understanding what the figures really mean.

This is probably way beyond the scope of my application for today (cb to an old 23-foot cruise ply-cat) but nevertheless fun and engouraging to further design challenges.

All the best! /T