How long should my dagger boards be?

[Privilege]

I have a 44' Privilege with empty trunks where the boards used to be so I'm going to make my own. I have contacted Privilege but they have been of no use with providing design specs or drawings so I'm making it up as I go along.

With the shallow keels, my boat draws 4'4". The trunks from deck level to bottom exit point are about 7' tall. They exit the hull to the outboard side of the keel about 3' below water level.

If I make the board say 14', that will allow me to lower the top of the board to deck level and have 7' protruding below the keel or even let the board sit 1' below deck level and have an 8' board sticking out the bottom. When the board is raised, if I leave it sticking about 4' above deck level, it would leave 3' sticking out the bottom, about level or slightly higher than the keels. Is that acceptable practice?

Does anyone have any input to my thinking? I have never used boards and am just making educated guesses as to what seems right.

[GRIT]

Mine will protrude 6 feet from the bottom of the hulls, but I believe the plans originally called for 5 feet.

The longer you go, the thicker you'll have to have them. Mine are near 4" thick, and are naca 0012.

Good luck.
Paul.

[Jimmathieson]

PRIVILEGE 435 sailboat specifications and details on sailboatdata.com

If the boards exit 3' (1m) below the W/L then could you not have a shorter board ?

Interesting , someone will follow with answer ,my boards for a 33' 9.9m Cat we are building is about 9' long ...

[Nicholson58]

There are third party companies that specialize in boards and rudders. They often have access to plans and specifications we mortals can’t get

[Jimmathieson]

Have you contacted the designer Marc Lombard ?

https://marclombard.com/nous-contacter/

[mikereed100]

When I was building my boards I came across a source that stated that the underwater appendages should be equal in area to 2.5% of the sail area. Your chord is already determined by the daggerboard cases so you might try using this formula to determine the depth of the boards necessary to achieve (along with the rudders) 2.5% of the sail area. Otherwise, just make them as long as you practically can. In reality, as cruisers, not racers, we only rarely extend our boards to their full depth, usually just allowing them to float.

[44'cruisingcat]

Originally Posted by Privilege

If I make the board say 14', that will allow me to lower the top of the board to deck level and have 7' protruding below the keel or even let the board sit 1' below deck level and have an 8' board sticking out the bottom.

If you plan to lower the boards so that the top is below deck level, make sure the case is reinforced to suit. The deck provides great support to the case, going below that needs a strong case.

[catsketcher]

I would reverse engineer the thing.

Start with the case dimensions - this will give you a possible profile - NACA 0009 or maybe up to 0012. I like thicker boards for cruising.

If you can go 0012 what will you build them from? I would recommend two ways.

1 - get a sheet metal place to fold some 1mm sheet steel with a nice radiused 90 degree fold up the middle of the huge sheet. Lay your glass skins onto this. Remove Vee like skins. Put some hardwood at the max width and bend skins around. Glass top, bottom and trailing edge. This is how Tennant made his boards.

2 - use cedar - glue large pieces together and get to know your power plane. Put unis on recesses at max width. Double bias all over.

If you go too long the boards will break. So I would use a simple cantilever formula to work out (use a CAD program to calculate section moments) the failure load. You are designing for fatigue not strength so you need to back off the loading. Or pull the boards up a bit in heavy winds.

Or wander around a boatyard with a tape measure - find similar boats and measure their board dimensions. I would do both approaches because I like the engineering.

Like 44C said - don't put the board below the deck unless the boat was designed for it. You could punch the top through the case side.

cheers

Phil

[Jeannius]

Morning David, hope the hurricane season is treating you well?

Interesting to read that your draft is 4'4". That is exactly what mine was with the standard keels! Seems like adding boards is going to add drag and complication for likely very little performance increase.

Perhaps your keels are shorter than standard in the fore/aft direction?

[Privilege]

Thank you to everyone for your input. It sounds as though it would be normal for the top of the dagger boards to sit level with the deck so I should keep it like that. I think I'll also reach out to Mark Lombard to see if he can offer any guidance as well. Mike, I measured my draft using a very scientific method. I measured it in the water with a spirit level and a broom handle. If the standard keels are 4'4", and they certainly look different to mine, it makes me doubt my measurements. I'll get wet again.

[Privilege]

Jim, thanks very much for the contact details for Mark Lombard. I reached out to him and he was kind enough to send me the original plans for my boat including the dagger boards and trunks.

It turns out that the dagger boards are 10'6" long and they protrude 8' below the waterline. Only the top 4' of the board remains inside the boat when fully deployed.

Thanks again to everyone.

[44'cruisingcat]

If you can bear the extra height aesthetically, I'd make the boards longer, so they only drop to flush with the deck anyway.

With 8' of exposed board leveraging against 4' of buried board, that's going to call for quite a strongly (heavily) built board and case. It's also going to add complexity to the downhaul arrangement.

I've got a daggerboard building job coming up in a few weeks, I'll post pics once I get started.

[mikereed100]

Originally Posted by 44'cruisingcat

If you can bear the extra height aesthetically, I'd make the boards longer, so they only drop to flush with the deck anyway.

With 8' of exposed board leveraging against 4' of buried board, that's going to call for quite a strongly (heavily) built board and case. It's also going to add complexity to the downhaul arrangement.

I've got a daggerboard building job coming up in a few weeks, I'll post pics once I get started.

Not to be pedantic (actually, precisely to be pedantic) he wrote 8' 'below the waterline'. Goes on to say that the top 4' of 10'6" remain in the hull, so roughly 6'6'' are exposed? Looking forward to your pics, I'm always curious to see how those with more experience do things.

[44'cruisingcat]

Yep, I didn't read it carefully enough. Still a bit of leverage there though.

[GRIT]

Yes. I'd also look forward to seeing some pictures of your daggerboard build, 44C.

Cheers.
Paul.