Lee boards or centerboard?

[tapsnap]

This is the boat I am currently working on. It's my first, but I already have dreams for many others. When I started building it I thought of it just as a row boat, but now I am thinking of putting a sail on her. The only thing is I don't like the idea of lee-boards. I've been thinking of ways to change the design to a center-board. My concern is the mid thwart would be in the way unless I raised it, but then rowing may be awkward. What are your opinions on this. should I just leave the design alone and install lee boards or is a centerboard feasible in this design? I'm sure there was good reason for the designer to choose lee boards versus another design.
http://www.danieloates.com/dory/dory1.jpghttp://www.danieloates.com/dory/doryprofile.jpghttp://www.danieloates.com/dory/dorysection.jpg

[Dave Carnell]

The most frequent question I had in years of selling plans for NUTMEG was about using a CB instead of the single fixed leeboard. The CB would significantly increase the work and cost, limit the seating room and do nothing to improve the sailing performance. Build it as designed.

[James McMullen]

The CB would . . . . .do nothing to improve the sailing performance.

Actually, this is simply not true. This has been repeatedly and convincingly refuted through empirical experiments. A properly designed and shaped centerboard or daggerboard will undoubtedly improve your sailing performance--by a slight amount. Is this amount worth the structural modifications to your boat to fit in a case somewhere? You'll have to answer that yourself. If you are a dedicated string puller devoted to getting the utmost out of your sailing performance you'll benefit from a high aspect NACA section foil CB or DB. If you are a casual sailor you might be perfectly content with the leeboard.

Leeboards work okay enough that many people are satisfied with them and the entire package of features they offer, but don't delude yourself that they are every bit as good as an end-stopped, fully submerged foil for windward performance. They are a compromise, as are all features of boat design.
Each cat his own rat.

[tapsnap]

The most frequent question I had in years of selling plans for NUTMEG was about using a CB instead of the single fixed leeboard. The CB would significantly increase the work and cost, limit the seating room and do nothing to improve the sailing performance. Build it as designed.
I don't really care about the extra work or cost but the seating room and sailing and rowing performance would be an issue.

[Nicholas Scheuer]

That jull's slack bilges would ssem to lack "form stability" for stisfactory sailing. If she rocks to & fro under sail, and rewuires you to move around quickly in order to keep her up, don't blame it on the leeboards.

Most of the small boats I've had were fitted with leeboards. I vastly prefer them. We're cruising a E&D Shearwater yawl now, following many years in a Dovekie.

Moby Nick

[JimD]

If you find leeboards even a small fraction as ugly as I think they are you will find a way to make a centerboard or daggerboard work. The slot will create a small amount of drag but it won't be much and I would consider that a fair trade off to avoid leeboards.

[Ian McColgin]

The remarks about slack bilges are misplaced in this case. While the boat might be initially a bit tippy, especially when light, her final form stability with one rail a bit down is enormous and she'll sail easily and stabile. Like all dories, she's a bit narrow astern, lacking the bearing aft for real speed undersail, but she's designed with moderate SA anyway. Within her limits, a fine sailing boat and a finastkind all-around oar and sail sea boat.

Her form is utterly unsuitable for leeboards as you need a bit more verticle hull above the water line to give the boards enough bearing to hang on well.

Leeboards of the old traditional type are every bit as inefficient (though good enough in traditional boats) as old time centerboards, what with all the turbulence of the open trunk abaft where the board is when down. Modern leeboards as started by LFH and much advanced by Bolger have nice asymetric foils and work better, in the right hull, than a dagger board would work in the same hull. It's all in the balance.

[ishmael]

A pretty little dory. My inclination, as Ian said, would be to experiment with leeboards. They have the distinct advantage of not having to cut a huge hole in the hull.

All that said, you've got your hands on a fine little row boat. Rigging it to sail, yeah OK. A lot of added gear and rigmarole.

I built a Whisp, Steve Redmond's light skiff, a few years back. I rigged it for sail and was never very happy with it. Leeboard. It just never felt right with the rig in. Too tender. As a rowing skiff it was aces.

Good luck,

Jack

[goodbasil]

I've got to get a wider monitor. Say 2 metres.

[hokiefan]

A centerboard would take some real thought and creativity to fit in without messing up the rowing thwart. But you could probably put in a daggerboard quite nicely that just left a slot in the top of the thwart when the board is removed. You lose the kickup ability with a daggerboard, but its all a compromise in the end anyway. Chose the option that fits your sum total needs the best.

Cheers,

Bobby

[Ian McColgin]

Jack misread my post regarding the suitability of this design for leeboards.

Making the CB and case low enough for rowing is dirt simple and actually can result in a boat more open and more versitile for rowing stations. I made that modification for Leeward, my Chamberlain gunning dory, as I did not want the rise in the CB case Gardner drew.

First, the CB pin must key to the board itself. You can use an already keyed bit of stock - I took the end of a bit of discarded bronze prop shafting. The hole through the trunk can dead end on the port side in a removable plate. On the starboard side you want to shoulder down where the pin exits the trunk such that the pin goes through a plate with a smaller hole than the hole in the trunk. Once they are screwed in with a bit of packing grease, the pin won't shift athwartships and won't leak. Also, it will exert no inward bending effort on the CB case logs.

I like a Culler type CB with the forward end in a hemicircle and the pin right at the center, putting it a bit above load waterline. But whatever.

Keyed or bolted to the protruding pin end attach a lever at an angle that puts it just a slight rise before horizontal facing foreward when the board is fully up. For an unweighted board, I found a bit of a loop that sliped over the end of the lever to hold the board up and a bit of bungee to tie it at whatever down angle I wanted worked nicely. The bungee let me take the ground without damage. Unlike what happens with a dagger board.

I had a thwart at the foreward end as that included the mast partners but none aft and really none is needed if you finish the top of the trunk with a flat plank wide enough for comfort when rowing but narrow enough that when rowing the forward station, legs straddleing the CB trunk, it's still comfortable. If you carry that upper plank width down the front and back sides of the CB case, it will be braced enough for use.

G'luck

[Andrew Craig-Bennett]

I agree with Ian; this boat will sail well but needs a centreboard and Ian's method is the best one.

[donald branscom]

I don't like either one.

But what about trying just a kind of skeg with a small fin bolted to it and then experiment with different sizes and shapes of fins bolted to the skeg.

It would make the keel stronger and would not interfere with the accomodations.

You could even try a bendy type plastic fin sticking down about 12 inches.
That way you could still get close to the shore AND a lot of times the best performance is with a daggerboard down only half way. You don' need much.

[tapsnap]

Ian, I'm looking at the center board for Gardner's gunning dory. It looks considerably further forward than the leeboards are shown on my drawings. How critical is the placement of the CB in a boat like this. Could I place the raised section of the trunk directly behind thwart and incorporate the rest of it into the thwart?

[Ian McColgin]

I see that some of my assumption was wrong - my glance at the drawing made me think it was set up for a dagger board rather than leeboards. I think leeboards on a pin pivot, especially on a boat this size, is inconvenient at best. Which may be just fine for a boat not really meant for sailing to weather so much as long reaches, even if some are close-hauled.

Also, I am a very big advocate of being able to lower some sort of board while rowing to weather in a high wind. I not uncommonly have a viscious southeasterly to row home to my mooring against. With the board down I can point a couple points off dead into it and make good progress where trying to stem a Force 5 head on is way tiring. Leeboards are not so suitable for that sort of oar-tacking.

Oddly, such leeboards would be more work than the top-hinged model more common to larger boats that allow the new weather board to "broken-wing" atop the water after a tack before it's housed.

Anyway, whatever board, lee or center or dagger, needs to balance about the same. The normal centerboard sitting down like a wedge will have most of it's lateral resistance aft as well. It will also be longer and shallower for the same lateral plane and a hair less efficient to weather. With this rig, it hardley matters. She's not a Thistle or Fin or anything.

Leeboards as drawn would be less intrusive. Leeboards like LFH drew for the Meadowlark - essentially a hinge on the gunnel line to let the leeboard shift away from the boat and a pivot through that for the fore-and-aft raising would be better. The leeward leeboard snugs against the hull to work and if you've not raised the weather board, it just flutters on the surface, hense the "broken wing" metaphore.

If you go with a centerboard, calculate a CR to come at about the same fore and aft place as the design has the leeboard CR.

[Thorne]

Building a centerboard and case isn't hard, but getting the relative positions of the CLR (CB and rudder) and CE (mast and sails) can be quite difficult.

My suggestion is that if you can't build to plan, build and install the CB and rudder. Then build a temporary mast step and partner to get the thing sailing -- and see how it balances. Lots of lee helm = move the mast aft. Lots of weather helm = move the mast forward.

The other alternative is to add a jib, and possibly a bowsprit to fly it from, if you have excessive weather helm and don't want to reposition the mast. That is what I did on my Chamberlain dory skiff sail conversion.

[Andrew Craig-Bennett]

I think that the leeboards as shown on the drawing are mighty far aft. I really do. Espescially with the coble style rudder which provides some of the lateral resistance.

[tapsnap]

So I did a little photoshop work and pasted the John Gardener gunning dory centerboard into the design I'm working from to find out your comments. As it is, it raises the rowing thwart and moves the center board forward. I don't know how critical it's position is. Is there some rule of thumb or mathematical calculation designers use for the calculation of these proportions?http://www.danieloates.com/dory/centerboardpasteup.jpghttp://www.danieloates/dory/centerboardpasteup.jpg

[Canoeyawl]

If you really want to sail, the position is critical.
A few weeks ago I did a little informal survey here about board position and the consensus was that no one has built or sailed a boat with the board too far aft. (I'm certain of course that there is a limit, and it is a complex equation with the heeled hull form and the speed (waterline length) of the vessel important factors) Talk to an aeronautical engineer about wing placement, he’ll have something to say and it won’t be “Oh we’ll just move the tail forward a bit if it doesn’t fly…”
Empirical design is a good economy on small boats and leeboards are easy to move around, much easier than a centerboard if you are just guessing. When you get the placement right then build a centerboard. Modifying the rig is a poor remedy for improper balance.
In most of the old small boat designs that I have studied the board placement seems to be way too far forward. The recent one you pasted in included.
To my eye, the board as shown in the original drawing reflects modern thinking and I would be surprised if it did not sail well as drawn.

[Thorne]

(snip)
A few weeks ago I did a little informal survey here about board position and the consensus was that no one has built or sailed a boat with the board too far aft. (snip)
In most of the old small boat designs that I have studied the board placement seems to be way too far forward. The recent one you pasted in included.
(snip).

All too true! The 1860's plans from Mystic for my Chamberlain Dory Skiff have the CB very far forward, and I moved it a few inches further forward to narrow it down and lose the hump-back CB case design -- and save the forward thwart.

This resulted in savage weather helm, which required a bowsprit to correct. She sails very well now, but all the changes took money and time...

I LIKE having the space aft of the CB case open, and I LIKE having my forward thwart, don't get me wrong. But I had to do some serious mods to the plan to add a bowsprit, certainly not part of the dory design and not historically correct.

[tapsnap]

Thorne, do you have pictures of your chamberlain dory skiff that you could post?

[Andrew Craig-Bennett]

Tapsnap - that drawing as you now have it looks just right, to me. I've not built a dory, but I have built a couple of small pulling and sailing boats.

[Thorne]

Tapsnap -- you gotta be new around here, as everyone else has suffered thru my questions and restoration already and seen more about my CDS than they ever wanted to know...

;0 )

http://www.luckhardt.com/dory1.html
http://www.luckhardt.com/dory2.html

http://www.luckhardt.com/elk_mesailingweb.jpg

[tapsnap]

Thanks Thorne,
These photos look great. Nice renovation. I see the centerboard trunk is even further forward than where I have placed it. I would like to avoid the bow spirit option if possible. I will experiment a little with different CB trunk styles and placement to see if I can bring it further aft.

[Ian McColgin]

I wish I knew how to make pix. Looking at the gunning dory (post 20) here are my modifications.

The trunk is flat all the way back to front, at the seat riser level. The board has the foreward end rounded to a semi-circle radius 1/2 board width (not thickness) and the pivot is centered, not down near the bottom. This gives more room for the lever.

I'd origonally planned a set of lines coming out the upper front of the trunk as uphaul and downhaul pendents but like the lever better.

With the lever, I did not need to weight the board.

With a long board like this, you can move the CLR quite a bit fore and aft just by how deeply you set it. After all, it can go to verticle. So don't be afraid to put it a nudge further aft than might otherwise seem good.

G'luck

[tapsnap]

Thanks Ian I'll try that and post the image - maybe by tomorrow

[tapsnap]

Well I reworked the image a bit and put in a straight centerboard and moved it back. I'm not to keen on this version.It looks like it gets in the way too much and looks weak.http://www.danieloates.com/dory/centerboardpasteup2.jpg

[Ian McColgin]

Leeboards, especially for this hull on which they are unusual, are deceptive for those used to centerboards because the working plane of the leeboard extends up to the water surface, well above the molded bottom of the boat. This is higher than the load waterline as the boat will be heeled. So my eye-ball of the lee board's CE is a bit under a foot ahead of the obvious - possibly under the center thwart or even the foreward edge of that. I'd make a centerboard that started under the fore thwart and ended under the center. That way you could have some board a bit aft when it's down such that no slot is exposed and move progressivly foreward as you lower further. This gives some nice choises for rowing into or across or down wind as well as for trimming under sail.

Remember in this design that the board can be as much use rowing as sailing.

[Woxbox]

Where's the center of the sail plan in that drawing? The board in the revised plan looks way to far aft to my eye, but without seeing the sail there's no way to be sure.

And you can put a board too far aft, but at the same time there's no perfect location. What you can also do if you get it wrong is move the mast, which is a lot easier than moving the centerboard.

One other thought, I'd rather sail a boat with too much weather helm than any lee helm -- the one fights you but goes, the other just doesn't go at all.

[Ian McColgin]

Both weather and lee helm are trouble if excessive. With too much weather helm, as happens say with a Cape Dory under sail alone, she won't bear off to get under weigh and won't move enough to tack.

Another point, these small boats are fantasticly sensitive to trim, which really means crew location. I couldn't quite tack Leeward in the St Lawrence River Skiff manner. As you'll recall, the SLRS has no rudder and is tacked by trimming sail while racing to the bow. If the angular momentum is enough to tack, you race back aft easing the sheet as you go. To gybe get the sail way out, weight right aft, and rock the boat till the boom flys over.

Point is, back to Leeward, a gunning dory, I could trim her to a mighty near neutral helm on any point of sail if I trimmed the sail correctly and got my weight in the right fore-and-aft location.

It of course makes sense to find the CE of the sail and then (the ultimate rule of thumb, applies to drawings 1-1/2" to the foot) locate the CR of the hull, board down about 30 degrees about your thumb widths ahead of the sail's CE.

Again, this shape of narrow board moves the CR rather dramaticlly foreward as you lower it.

It could not possibly hurt to call the designer or consult a local.

G'luck

[tapsnap]

I was asking earlier in this thread if there was some rule of thumb in the mechanics of all this stuff and I just found this article that explains it all in simple terms that even I can understand.
http://www.thebeachcats.com/index.php?name=News&file=article&sid=15&theme=Printer
Now I'm a little clearer on the subject. Thanks for everyone's input I really appreciate it.

[Ian McColgin]

It's a start but the diagrams are incredibly simplified. They are abit like the economist Laffer's curve - a supposed mathematical demostration with no scale. In this case, no reason is given why the sail's center of effort or the boat's center of resistance are located where they are. In fact, they look wrong.

For a triangular sail, I just take the intersection of the bisected angles as center enough. For a gaff, divide it into to two triangles and then porportionatly average their centers. Same on a rig with a jib - porportionally combine the centers into one.

The underwater shape can be harder to center if you have wierd board or keel configurations, extra large rudder and skeg, etc.

Now how should the centers line up? It depends on the hull and how it's sailed, but note that the profile CE for the sail is no where near the real CE under weigh. The sail is out, shifting the CE outboard and foreward (the outboard part of the final vector contributing to weather helm, the shift foreward being minor contributes a tad of lee helm but not significant. Point is, this is why you need to factor in a bit of "lead" for the CE, and that's often a judgement call.

I can think of several LFH designs that benefitted from either rig or foil changes to improve the sailing behavior, and others (Nat Benjamine for example) have not been afraid to move a centerboard after a few of a given design were sailing. The fameous foreward rake to the Winanno Sr's mast is without doubt due to a refusal to more sensibly fix a dreadful weather helm.

The good news is, this CB design has huge fore and aft mobility depending on board angle so all you have to do is get close.

G'luck