Twin Centerboards

[tgom]

I've been sailing for a while, repaired a wreck, built a small "kit" type boat but now I'm hoping to start my first design&build.

The intended use is for camping adventures in shallow, protected waters. Right now I'm leaning toward an open deck sharpie based on draft and ease of construction. My reservations include (but are not limited to) excessive leeway, lack of ballast and having to choose between leeboards or a huge centerboard in a trunk.

My questions are (after much rambling):

Has anyone had any experience with twin centerboards?

Would twin centerboards reduce leeway better/worse than a single centerboard?

Would ballast in these twin centerboards be practical?

Thanks in advance.


Tim

[Thorne]

Welcome to the Forum!

We'll need more info. By "twin centerboards" do you mean two centerboards fore and aft, or bilge keels, or bilge boards (like centerboards or daggerboards but off the centerline), or ??

Bilge keels -
http://www.powertosail.com/boatimages/Westerly%20Konsort%20Bilge%20keels.JPG

bilge boards -
http://www.geoss.com.au/eun_mara/bilgeboards_and_cases.html



Really, the easiest way to build a trad design is to build the trad design -- those old guys knew what they were doing.

[Steve Paskey]

I can't answer his questions, but I assume he means twin centerboards side by side (rather than fore and aft), as on Iain Oughtred's Eun Mara.

[Steve Paskey]

Really, the easiest way to build a trad design is to build the trad design -- those old guys knew what they were doing.

Well, more or less. If you believe Phil Bolger, BLACK SKIMMER can sail rings around Munroe's EGRET, yet leeboards weren't traditional for the type.

[tgom]

Thank you for the welcome. And I apologize for being vague.

My question was concerning two swinging centerboards mounted in trunks side by side. The same configuration as shown in Thorne's picture above (not sure if they swing).

In my mind I see improved lateral resistance, sufficient low-hung ballast with boards which are still light enough to handle and a wide open cockpit floor.

I'm looking for a reality check with respect to performance and unforeseen complications.

[DGentry]

Ian Oughtred's Haiku. An Egret derivative, with an updated rig, etc and twin centerboards leaving the cabin open.

http://www.smallboatforum.com/SBF%20Image%20File/sharpie3.gif

Pesonally, I like the idea of fore and aft centerboards, allowing for lots of adjustment to completely balance the boat in many situations. More or less leaves the living space open, as well.

Reuel Parker is currently building a big sharpie with a ballasted centerboard. Check it out here: http://parker-marine.com/brokerage.htm

[KAIROS]

I doubt anyone would recommend side-by-side centerboards. Bilge keels, like in Thorn's photo above, keep the boat upright on the bottom at low tide. For this benefit, they sacrifice some sailing performance. You wouldn't get that benefit with centerboards, and would double the drag caused by the turbulence around the slots.

[David G]

tgom,

Is that you, Dutch?

OK -- for the moment, I'll assume it's not. So... my next question is: what do you know about designing a sailboat? It's hard to know where to start without knowing what level you're at. What have you read? What designers work have you studied? When we know more, we can answer more pointedly.

However... to baldly answer your initial question: there are a variety of reasons that a twin centerboard is not a common thing. I'll hazard a guess that designing in such an arrangement - especially in such a basic boat as you have described - would only be attempted by the very inexperienced... or the very sophisticated designer.

[cvdukes]

I sail a 1975 Chrysler Lone Star (fiberglass) that I rebuilt a few years ago. It has the swinging twin bilgeboards (centerboards) and I'm quite happy with the arrangement.
Honestly, the boat "feels" like a much larger boat. A lot of its sailing characteristics are more like a 22 footer with a swing keel I used to maintain. The size and feel of the cockpit are similiar and it doesn't have tippy feel of centerboard dinghies its same size. I suspect if you enjoy hiking out to keep your craft afloat, bilge keels will be too sedate for you, but otherwise I think they are great.

Advantages: The boat is extremelly stable and when the boat heels, the lee side board digs deeper into the water. May be my imagination, but it seems the deeper bite helps with stability, especially in waves.
The Cockpit is wide open. Tacking is easy for the crew as you don't have to clamber over the centerboard trunk.
The boat points up as close as any other ...maybe even a point or two higher. I can carry more sail in moderate winds and can catch and pass comparable centerboard craft.
In theory, I could drop both boards down for an increase in stability by getting more weight down low, but by that time I should reef sail anyway.

Disadvantages: The extra board slot adds a little drag as it creates a vacuum effect when the windward board is retracted...I probably should fit a gasket tape on the boat's bottom over each slot to seal when the board is pivoted up, but I'm not too concerned about the extra little speed boost. I do get a little water coming up in the retracted board slot into the cockpit but that is because I need to figure out a better gasket seal around its pivot point.
This particular design has a lever mounted onto each bailge board. When coming about, crew has to remember to raise the windward board and lower the leeward one. Two boards down creates drag, so it becomes a trick to figure the right time to shift them. Ideally, I would figure out some ropes and blocks that would raise one when I lower the other one, but that would create problems with raising both (for downwind and trailering).

The bilge boards on my boat are metal and have some heft to them...let's say, they are heavy enough to break toes when dropped from a few inches. Unlike centerboards that have a symetrical foil shape, the bilge boards are flat on one side and are only slightly convex on the other. The convex side mounts to the inside of the boat and helps to create lift that helps the boat point higher. If I were building the boat from scratch, I would make the boards a little thicker and make the convex side more like a shallow airfoil as I think there could be a slight increase in lift .

[rbgarr]

The purpose of ballast in centerboards is to get them to drop in their slots and remain in place without sliding back up under sail... not provide the righting moment of a ballast keel. If that is the purpose then they are called lifting keels and require much more weight and lifting gear/tackle to raise and lower, a procedure you're not likely to do on each tack, especially with twin (bilge) boards.

How large a boat are you considering?

[P.I. Stazzer-Newt]

The Alan Vines / Uffa Fox design - the Fairey Atalanta used twin boards.

http://www.hanseyachts.co.uk/boatview.asp?article=126 << good description.

[obscured by clouds]

a little bit of toe in on the assymetric bilgeboards will be enough to hoist the boat to windward on a close beat.

[Steve Paskey]

... my next question is: what do you know about designing a sailboat? It's hard to know where to start without knowing what level you're at. What have you read? What designers work have you studied? When we know more, we can answer more pointedly.

However... to baldly answer your initial question: there are a variety of reasons that a twin centerboard is not a common thing. I'll hazard a guess that designing in such an arrangement - especially in such a basic boat as you have described - would only be attempted by the very inexperienced... or the very sophisticated designer.

I doubt that it's really that complicated at all. The reason why dual centerboards (aka bilgeboards) isn't common are really rather simple: The ONLY advantage is interior room. The drawbacks are more work (building two trunks and two boards), and a bit more drag than a single board.

If someone's willing to build it, it shouldn't really be that hard ... it's simply a matter of keeping the CLR in the same location, and making sure that the area of the two boards is big enough. It's not rocket science, and it doesn't require great study.

[kenjamin]

To create acceptable twin centerboards may not take much study but to achieve optimum advantage of their use would probably involve some experimentation on someone's part. For example, why not make the boards' foil cross sections assymmetrical? This could make each board perform better on the tack where one was dropped the deepest and the other was completely withdrawn. Or would time be better spent coming up with boards which fill the open slot somewhat by not being withdrawn completely? This is the kind of stuff I would like to know about. Maybe a Haiku with easily replaceable (from the top) boards would be a fun research ship. Here in Florida when you tired of experimentation, you could hop over to the nearest island for a much deserved break. ;)

[JimD]

The purpose of ballast in centerboards is to get them to drop in their slots and remain in place without sliding back up under sail... not provide the righting moment of a ballast keel. If that is the purpose then they are called lifting keels and require much more weight and lifting gear/tackle to raise and lower, a procedure you're not likely to do on each tack, especially with twin (bilge) boards.

How large a boat are you considering?

I can't quite agree with that. There are many successful designs home built in great numbers that use steel plate boards . Call them swing keels if you like but the designers call them centerboards. I have one. Yes you need an inexpensive winch to raise/lower them but boards of a couple hundred pounds each are almost effortless to crank. To the best of my knowledge all designs with two boards by highly regarded designers like Oughtred are intended to have both boards down when sailing and tacking.

[JimD]

I doubt that it's really that complicated at all. The reason why dual centerboards (aka bilgeboards) isn't common are really rather simple: The ONLY advantage is interior room. The drawbacks are more work (building two trunks and two boards), and a bit more drag than a single board.

If someone's willing to build it, it shouldn't really be that hard ... it's simply a matter of keeping the CLR in the same location, and making sure that the area of the two boards is big enough. It's not rocket science, and it doesn't require great study.

Agreed.

[kenjamin]

To the best of my knowledge all designs with two boards by highly regarded designers like Oughtred are intended to have both boards down when sailing and tacking.

Perhaps a letter to Iain Oughtred on this subject would be one worth writing. It is my impression that the small more ideally placed offset board along with the hard chine of Haiku was enough to get the job done with the weather board withdrawn. It would be interesting to hear from Mr. Oughtred about this. Any Haiku plans owners out there?

[JimD]

Perhaps a letter to Iain Oughtred on this subject would be one worth writing. It is my impression that the small more ideally placed offset board along with the hard chine of Haiku was enough to get the job done with the weather board withdrawn. It would be interesting to hear from Mr. Oughtred about this. Any Haiku plans owners out there?

From the drawing it looks like the boards are controlled from inside the cabin. That would make rasing and lowering them while underway a royal pain.

[BarnacleGrim]

Stevenson's catboat uses bilge boards. Gerd Müller's steel yawl has a pair of daggerboards. The latter should be easy enough to adjust from the cockpit using blocks in the mast.

[Canoeyawl]

A couple of things…
The weight of the board must be large, say 500lbs, before it would create a larger righting moment than moving your butt a foot to windward.
Heavy centerboards (heavy enough to be ballast) must be a powerful mechanism (expensive) as they have enough force to rip the bottom out of most small boats.
The ballast issue becomes more complex when the weight is raised into the case because it is now higher up in the vessel (another problem).

A functioning foil (centerboard) actually increases heeling moment if it is working.

[rbgarr]

I can't quite agree with that. There are many successful designs home built in great numbers that use steel plate boards . Call them swing keels if you like but the designers call them centerboards. I have one. Yes you need an inexpensive winch to raise/lower them but boards of a couple hundred pounds each are almost effortless to crank. To the best of my knowledge all designs with two boards by highly regarded designers like Oughtred are intended to have both boards down when sailing and tacking.

I wasn't considering things like steel plates used as centerboards as being 'ballasted'. They are indeed heavy but the weight is evenly distributed across the plate. I interpret the poster's question about ballast as asking about additional weight on the end of the center/daggerboard (beyond what's needed to keep them lowered) and specifically intended for adding to righting moment.

[StevenBauer]

From the drawing it looks like the boards are controlled from inside the cabin. That would make rasing and lowering them while underway a royal pain.


No. The bilgeboards are controlled from the cockpit.




Steven

[David G]

I doubt that it's really that complicated at all. The reason why dual centerboards (aka bilgeboards) isn't common are really rather simple: The ONLY advantage is interior room. The drawbacks are more work (building two trunks and two boards), and a bit more drag than a single board.

If someone's willing to build it, it shouldn't really be that hard ... it's simply a matter of keeping the CLR in the same location, and making sure that the area of the two boards is big enough. It's not rocket science, and it doesn't require great study.

Steve - You quote me to make the point about it not being all that complicated. I wasn't saying it was dauntingly complex. I was saying - as you did - that it'd be more work. Also - there are few situations where it would be the best solution. A beginner might not know that by going that route, he was making more work for himself, and gaining no great advantage. An advanced designer would know enough to recognize the situations or design briefs where the drawbacks were sufficiently offset by the advantages.

But, that said, I would also argue that making double centerboards adds some complexity. Not just added work, but also more know-how. It may look basic from the vantage point of your vast experience ;), but I doubt that's the case for OP. One example - the area of the centerboard. It will take a bit of study (admittedly not a lot) to know how to determine the area of a centerboard. It will take some more digging to understand how to calculate the area of two boards. That info is far less commonly available. One can't simply take the original (one-board) immersed area and divide it by two. I'd certainly have to do some reading myself to know how to calculate. Does the OP know where to find such info? Will he even know that it's an issue? Why bother?


tgom - I will add that you don't need to create two centerboards in order to clear the center of the cockpit. You can simply offset your centerboard. Then you don't have two c'board cases to build, two boards to build, two (potentially leaky) holes to cut into the bottom, and two boards to handle while sailing, docking, or beaching. Oh... and... we are talking about centerboards (attached to case, swing/pivot up and down)? Not daggerboards (seperate, loose, and insert into a vertical-ish slot)? One nifty trick (if it works within your overall plan) is to have a fore & aft thwarts Port & Starboard with vertical faces. The vertical face becomes part of the rigidifying structure of the boat... and serves as part of the structure of the c'board case.

Cheers,

[BobW]

I'm not a boat designer and have never played one on TV. :)

Jim Michalak uses what looks like a single lee board (but is really a centerboard) mounted outside the hull. This is probably the extreme in an off-center centerboard but with minimal structure and smaller holes in the hull. Here's a link to some comments on the design.

http://marina.fortunecity.com/breakwater/274/2004/0715/index.htm#Leeboard%20Issues

Michalak's Ladybug and other designs use this type of centerboard.

http://www.duckworksbbs.com/plans/jim/ladybug/index.htm

Bob

[Thorne]

I dunno, Bob - looks like the second link refers to it as a leeboard in the article.

No matter if it swings/pivots like a leeboard or centerboard, or just drops down like a daggerboard, I suspect that any foil mounted on the outside of the hull will be considered by most to be a leeboard.

And the original poster has made it clear he's considering twin bilgeboards, or two centerboards side by side with slots through the bottom of the hull. Since he's working on designing his own boat, perhaps we need to direct him towards some books on design?

[David G]

Books on designing a sailboat. Good idea. There are some suggestions on this thread... as well as some other comments that might be relevant:

http://www.woodenboat.com/forum/showthread.php?t=105989

[BobW]

Thorne - you couldn't figure out the key element of my post? Ya know, the one I didn't state...?

I guess what I forgot to say is that a single board apparently works, and the structure needn't be complicated.

Personally, I've never cared for lee boards. This fair and reasonable (to me, anyway) position was based on absolutely zero personal experience and was not influenced in any way by reading up on the topic. I still have not embraced lee boards, but seeing the wide-open cockpit possibilities (as illustrated by Ladybug) offered, they are worth a look

Something like the Ladybug could be reasonable open boat camp cruiser in the Delta... There aren't that many places with beach camping (at least up in the northern reaches), so room in the boat to stretch out would be nice... maybe...

Bob

[tgom]

@ David C: I'm not your friend Dutch (but I'm curious as to why you thought I might be). And to answer your question as to my experience I am a utter novice to boat building. As a mechanical engineer I have a pretty fair background in fluid mechanics but only enough knowledge on this topic to be dangerous...

@ rbgarr: I'm looking to build something in the 22' to 24' range. Currently I sail a 20'ish fixed keel production boat having outgrown the Teal. My motivation is to go a bit larger to allow me to more easily carry passengers but at the same time draft is a concern because where I sail there is considerable submerged "stuff" to hit. I was thinking "skinned" steel plate all the way just for durability and a bit of weight down low.

@ kenjamin: Your point that experimentation aspect intrigues me a bit. Gun to my head I realize that performance-wise a often-built design would be the best choice. Sorta like how I'm willing to admit that my homemade wine, while lovingly crafted, isn't as good as the store bought stuff.

@ David G (again): I very much like your suggestion to simply offset a single board. The trick you mention about building thwarts is really interesting. Keeping it simple and using a single structure for several purposes is almost always worth the effort.

@ BobW: I agree that the leeboards just seem too uncontrolled I guess and I too believe this for no good reason :-) I do like your suggestion of an externally mounted rigid swinging board. The thought of having no drag inducing, happy to leak slots built into the hull sounds like a good idea. The structure might be a bit advanced for me at this point, however.

@ David ( again again): I skimmed through the thread you included. A few of the books I already have, a number I've been considering and a few are now on my list.

I thank you all again for the conversation as it has given me much to think about. It's nice to have a place to ask such questions and have access to so much experience (and humor for that matter).


Tim

[Primitive Pete]

Cape Cod Boats makes a glass 16' twin centerboard sloop called the Gemini. I've sailed one a bunch.
The twin centerboards help the performance of the boat beating and reaching, but only when the windward board is retracted. The Gemini is much faster than a Daysailer or Bullseye when it's sailed properly.
Now the downsides. Working the boards is a pain when singlehanded, less so with crew. More seriously, the boats have a severe tendency to quickly rudder stall when loaded up. This can lead to a crash tack or a sudden blowout to windward, often resulting in a capsize. This might be because of the offset between the foil and the rudder or maybe because the boat carries a pretty large mainsail. I'm not sure which, but it results in a boat that ain't exactly cruise-friendly.
A heavier, less rounded hull design might be more stable, but from personal experience I don't think the performance gains are worth the pain in the neck factor unless you plan on racing.

[JimD]

No. The bilgeboards are controlled from the cockpit.




Steven

Good to know. Thanks, Steven. Personally, I would not want a boat with twin boards of any description that required raising one and lowering the other for every tack. No idea if this is the case with Haiku.

[JimD]

I wasn't considering things like steel plates used as centerboards as being 'ballasted'. They are indeed heavy but the weight is evenly distributed across the plate. I interpret the poster's question about ballast as asking about additional weight on the end of the center/daggerboard (beyond what's needed to keep them lowered) and specifically intended for adding to righting moment.

Understood. Nevertheless the purpose of the steel plate board is to act as ballast to lower the CG and increase righting moment. Otherwise there would be no point in using steel. Easier to make them out of wood.

[kenjamin]

I think on Haiku the boards are close enough together that most times you could get by with using just one but still curious about how Mr. Oughtred envisioned their use. Anybody know????

For use one at a time, asymmetrical foil cross sections would seem to make a lot of sense. Another overlooked advantage of having two would be if one jammed or broke you'd still have the other.