semi-trad kickup rudder design

[Thorne]

I'd vastly prefer to have a kickup rudder on my Chamberlain dory skiff, as I'll be solo sailing a lot of thin water.

The Mystic plans show a fixed very wide rudder. I like the look of it, and if I go for a fixed rudder I'll make one up.
http://www.luckhardt.com/rudderplan3.jpg

Gardner's plans for similar-sized boats show two designs for kickup rudders. One has a 3" lead plate to keep the board/blade down, the other doesn't show how it stays down.
http://www.luckhardt.com/rudderplan1.jpg

Does anyone have any experience with this style of rudder? Does it work well in a heavy swell, or does the blade of the rudder swing back and forth (even when weighted)?
http://www.luckhardt.com/rudderplan2.jpg

By preference I'll go for the longer thinner rudder, as it should balance well with the centerboard I'm building, which is 3' long and 9-10" wide.

The only swing-up rudder I've used was on a much larger boat, and had very heavy shock-cord strung on the leading edge to keep it down -- certainly don't want to go that route.

I'd also prefer to avoid a lock-bar on the aft end of the rudder, but if that is the best method to avoid having it sea-saw or swing in rough conditions, I'll install one. Don't have any plans for this type of bar, but will surf the web and see what I can find...

As usual, Thanks in advance!

[ 10-30-2005, 12:13 PM: Message edited by: Thorne ]

[Bob Smalser]

http://pic3.picturetrail.com/VOL12/1104763/8408305/116470953.jpg

I recently made a lightweight plywood rudder based on the Gardner Whitehall rudder you show. Instead of bronze plate for the cheeks, I used plywood, and I added a hiking stick to the tiller.

A length of 3/16" shock cord holds the blade down ILO lead, and not rigged yet is the dacron cord and jam cleat that'll hold it up. You can see the cheek block installed for it.

[ 10-30-2005, 12:47 PM: Message edited by: Bob Smalser ]

[ishmael]

Properly built and weighted(or lassoed) it will stay down just fine in a seaway. With such a small rudder the ply would work okay, though I've seen them snap off if they get a little wet around the through bolt, and it's not an easy place to keep sealed. Epoxy and a bronze bushing in the rudder seem to work pretty well. I've always thought solid mahogany a better choice for blades.

Getting the clearance right between the cheeks and the blade is a bit variable depending on if you have solid or ply, how you've got it sealed, etc.

For the waters you describe the kick-up is the way to go. Run hard aground with a fixed rudder and something's going to give. More than once I've seen the transom torn up; unhappy results.

A piece of galvanized steel for the blade might be worth considering.

Good luck.

[Pernicious Atavist]

i used a ballasted rudder on my sailing canoe; it worked great! just be sure to use enough lead. i used a lead insert and a leading-edge shoe of lead to protect the blade when grounding. a line to a jam cleat held it up. i also used lead inserts for the leeboards, and will do the same for those on the new boat. do consider lead for lower, leading edges.

[John Meachen]

Consider a clamcleat CL257 and a rope downhaul.

[Thorne]

Thanks, everyone!

I'll probably go with the Whitehall design like Bob's but try the lead instead of the shockcord -- we shall see how that works.

I have plenty of lead, much of it in flat plates 12" square. There is nothing like owning some small artillery pieces and several matchlock muskets to encourage lots of lead in the shop.

;- )

Shaped up and epoxyed the centerboard today, a surprising thrill to create the foil and shape it properly! Looking forward to doing the same with the blade of the rudder, which will be 3/4" white oak split down the middle and glued with the grain opposite.

[Dave R]

I'm probably too late but I thought I'd mention that I also have a ballasted rudder on my boat.

I used a router to route a recess on each side of the rudder leaving a very thin web of wood in between. It was thin enough that you could see light through it but that was fine.

I ran a dovetail bit around the edges of the recesses to undercut a little. I poured a little bit of epoxy into the recess, laid in strips of lead, and topped off the opening with more epoxy.

After the first side cured a bit, I flipped the rudder over and did the other side.

Might be faster to melt and pour the lead as is traditional but I didn't have to worry about fumes from molten lead. It's been several years and it is impossible to see where the lead is through the paint. It worked for me anyway.

[DougWilde]

A traditional-looking rudder that kicks-up, can be locked up, and does not require lead weight or downhauls and anything beyond the tiller.

http://dbwilde.home.comcast.net/BoatScans/rudder.jpg

Doug Wilde

[saltytheseadog]

I built the whitehall in gardners book this spring and incorporated his kickup rudder design.I built it out of 1in. dougfir.On the leading edge of the blade I bored a 3 in. hole,beveled the edges ,coated the wood around the hole with waterglass and poured in lead.
The cheeks are made from stainless plate,held together by a 1/2 ss bolt and nilock nut.
I drilled a hole in the stock to insert a pin to hold the blade down,but the friction of the plates on the stock (adjustable by tightening nut)seems to do the job and saves the hassle of hanging over the stern to remove pin.
A shockcord from the blade through a fairlead to a thumbcleat on the tiller hold the blade up.
I can't say how it would work in ocean swells as I sail on a lake mostly.

[bainbridgeisland]

Bob Smalser, I have had trouble in the past with torsional rigidity of rudders shaped like yours. Have you had problems? From my past experience, I use a minimum of 3/4" thick material for the upper section on a lightweight boat up to about 16'. I think a full 1" thickness is required for larger boats.

I found this problem 40-years ago. I never saw the problem in moderate conditions. One day, in a squall, under reduced canvas, the rudder blade was 20-degrees out-of-line with the tiller. This was on an 8' boat with 5/8 ply rudder.

I once saw a failed rudder of this design on a rather heavy 17-footer. It was 3/4" luan and failed in torsion.

[Thorne]

Bainbridge -

I take it you are talking about the head/top piece of the rudder twisting, not the blade/bottom part, right? Or is it the whole assembly?

The Gardner plans call for 3/4" thick head/top piece, and I've assumed that the blade is an equal thickness. I'll be using oak, so as long as the hardware holds up, the transom attachment should fail before the rudder twists very much.

BTW, I did some research on dinghy foils when I modified the trad design of a wide, short centerboard in favor of a longer narrow one (to fit under both front and center thwarts).

For dinghy sailing speeds of 3-6 knots, the recommended max. thickness of a 6" deep foil is aprox. 3/4", for a 10" deep foil it is 1.2". By deep the NA's seem to mean what us common mortals call "wide", i.e. the fore-to-aft measurement of the foil (CB or rudder blade). This is also called chord length.

There are some very cool pages covering this topic:

This one seemed the most useful -
http://www.paceship.org/how_to/rudder.htm

http://www.philsfoils.com/designTips.html#rudderthickness (http://www.philsfoils.com/designTips.html#rudderthickness)

http://www.vacantisw.com/foildesign.htm

[ 11-02-2005, 05:03 PM: Message edited by: Thorne ]

[bainbridgeisland]

Thorne, yes I am talking about the top piece of the rudder twisting.

Aaaaa... I am a Naval Architect and spend much of my time on hydrodynamics. I suspect the 3/4" thick by 6"-chord is simply a rule of thumb. At 8% thickness, it should work fine. However, a case could easily be made for anything from 6% to 12% thickness. Even a plate rudder or an 18% thickness rudder can be made to work, though performance would be noticeably reduced.

[Thorne]

Well the NA stuff is all geeky greek to me, but it sure is exciting when I can dimly begin to grasp some of the basic elements and actually apply them!

;- )

Yeah, the first two webpages listed go for the NACA 0012 foil as a general guideline, nothing more.

From the Paceship site:

"... After studying various foil shapes, I chose the NACA 0012 foil. This is a 12% foil shape meaning the maximum cord depth (rudder thickness) is 12 % of chord length (distance from the most forward point of the rudder to the most aft point).

The 12% foil was selected because it is the most efficient design in the speed range of 3 to 6 kts...the normal sailing speed range of most medium sized ballasted hull sailboats. A boat that ran faster would benefit from a lower % (thinner) foil (such as the 0010 or even 0008 foil) while a slower boat might best be served best with a larger % foil (such as the 0013 or 0014 foils)."

I *strongly* suspect that the only way I'll ever get my Chamberlain dory skiff over 6 knots will be under tow, falling over a wier, or surfing a Tsunami in SF Bay...

[ 11-02-2005, 05:14 PM: Message edited by: Thorne ]

[JimD]

I made one similar to the one Bob shows on the dingy and the stock couldn't take the stress and cracked. I'm in the process of building a new one with a thick three ply stock where the blade pivots in a slot in the stock, not a blade with cheeks pivoting over the (thin) stock. Probably doesn't matter much on a very light dinghy.

[ 11-02-2005, 05:58 PM: Message edited by: JimD ]

[Bob Smalser]

Originally posted by bainbridgeisland:
Bob Smalser, I have had trouble in the past with torsional rigidity of rudders shaped like yours. Have you had problems? From my past experience, I use a minimum of 3/4" thick material for the upper section on a lightweight boat up to about 16'. I think a full 1" thickness is required for larger boats.

http://pic3.picturetrail.com/VOL12/1104763/8408305/116470953.jpg

This one is 3/4" DF marine ply with half-inch ply cheeks. On a 70lb, 8' pram with a whopping 35sf of sail, I believe it'll stand the gaff. ;)

On a bigger boats, I go heavier...this stock is three layers of half-inch marine ply:

http://pic3.picturetrail.com/VOL12/1104763/3075025/81689827.jpg

[Thorne]

How carefully do you guys balance the relative sizes of center/daggerboard and rudder? Ditto for the shapes when not reproducing a traditional rudder from plans.

In other words, I'm fishing for assistance yet again.

;- ()

One of the linked foil-design sites above mentions that the rudder area should be roughly 1/3 of the center/daggerboard area. Another indicates that a high-aspect rudder (longer & thinner) can stall out at low speeds.

Can I go (very) wrong by making my rudder *blade* using the center set of plans above, i.e. slightly over 2' long by 10" wide? My centerboard is a slightly tapered 9 3/4" - 8 3/4" wide by 36" long.

Or should I make it a bit shorter and/or narrower (less chord depth) to be more in balance with the high-aspect centerboard?

I'm not racing the boat, so it may not matter that much -- plus most speeds should be low and so a bit more rudder to avoid stalling shouldn't hurt.

I know that doing all this without knowing the sail plan and/or CE can be moot, but I hope to use at least two sailing rigs and will move both fore and aft to find the best balance with the rest of the boat and foil construction.

I'll probably put up a sail from one of the many 8' racing dingy sails, either an el toro or sabot, on my 13' mahogany mast. This will be for stronger or gusty winds. I also hope to make or buy a proper canvas sail, maybe a spritsail or standing lug design, for use on (probably) the same mast for lighter airs.

[ 11-03-2005, 10:05 AM: Message edited by: Thorne ]

[bainbridgeisland]

Thorne, here is a practical way to size centerboards and rudders.

Choose a boat that you have sailed on that is about the size of the one you are working on with a similar rig. Did you like the way it sailed and balanced? If so, obtain a profile drawing that shows the sail as well as the underbody. Trace the underbody (with the centerboard and rudder) onto paper and cut it out. Fan-fold the underbody parallel to the waterline to make it stiff longitudinally. Balance it on a butter knife to find the longitudinal 'center of lateral plane'. Transfer this position your drawing. Find the 'lead' (pronounced like leed) by measuring the longitudinal distance the center of lateral plane is aft of the center of area of the sail plan and dividing it by the length of waterline.

Your new boat should have the same lead as the one you measured for a balanced helm.

You can figure the size of the centerboard and rudder by copying the example boat. Just divide the rudder area of the example boat by the sail area of the example boat and then multiply this times the sail area of the new boat. Do the same for the centerboard. The shape of the rudder and centerboard is not as important as the area.

There is no magic number for centerboards or rudders. If too small the the helm will be sluggish in some conditions. If too big the boat will not be as fast as it could be.

If you are worried, make the rudder and centerboard a little longer than you think you need to. You can trim the relative length to adjust your weather helm if needed. Most folks would not notice if the area af the centerboard and rudder were 25% too much.

[ 11-03-2005, 02:14 PM: Message edited by: bainbridgeisland ]

[Thorne]

I made up the rudder head yesterday, and carefully followed Gardner's plans on the design.

I'm interested if anyone else out there has used his "compound curve" method of keeping the rudder blade from hitting the skeg/transom.

Some designers use a regular curve/circle for the rudder head/blade joint and just let the weighted blade whang into the boat, others recommend a rubber block to take the "thump" out.

Are there any downsides to using this method? The rudder head and blade are 7/8" white oak, so they should be able to take the impact and/or force of the rudder blade against the flattened part of the curve.

My descriptions are poor, so here's the image from the book. Note that the curve formed by the 6 1/8" distance from the pin/bolt hole is only about half of the width of the rudder head, the rest of the curve being flattened out to form a stop.

http://www.luckhardt.com/rudderplan1.jpg

[Thorne]

Well, not much of a response on that question, so maybe it is moot -- I have found a few other rudder patterns where the head/blade curve ends so the blade doesn't hit the gudgeon/transom.

Here's another question -- what gage bronze to use for the cheekplates? I've contacted a supplier here in the SF Bay area for their silicon bronze 655 alloy, but am not sure what gage will be needed. I don't want to use ply on this boat, and would prefer not to drag the thick oak cheekpieces through the water unless necessary -- hence the desire to follow Gardner's plans and use sheet bronze cheekpieces.

Thanks!

[Philip Maynard]

This on my cerlew 15'-8". It's heavy but very strong with 1/2" oak cheeks. The blade is oak and is weighted, I have a jam cleat to hold it up. You can just see a down stop on the forward edge of the blade.
http://www.pmaynard.lunarpages.com/index_files/cerlew-rudder-up.JPG
http://www.pmaynard.lunarpages.com/index_files/cerlew-rudder-2.jpg
http://www.pmaynard.lunarpages.com/index_files/cerlew-rudder-1.JPG

[Thorne]

That looks nice, and also very solid.

I may have to go that route, as the bronze plates were over $100 each -- yeoch! I'll start checking around for 1/2" oak, as I don't have access to a planer to reduce all that 7/8" white oak I have lying around.

I'll also check out using stainless plates as per the plans and painting it, as I'd really like to keep most of the visible bolts /fasteners/hardware as bronze if possible.

[Philip Maynard]

I do not have a thickness planer but I just bought my 2nd makita hand power planer, I had replaced the bearings I think 3 times on the first one. I had to bend the stock pintels to accommodate the extra thickness but I was surprised how easy the stainless was to bend. I had to add the swivel hole, the other existing holes are through bolted to the cheeks with flathead bolts, I forget now but they probably have to be in place when the cheeks go on.