Rudder Design Questions

I have a small 1965 11' Whistler MFG sailboat with 18' mast, main sail and jib. Everything was original on the boat except the rudder which is currently laying at the bottom of Atwood Lake in Ohio.

I've made a new tiller out of quartersawn white oak and have made two 1/2" thick rudders out of Dupont Corian. The first one seemed to not have quite enough resistance in the water which gave a little less control from what I had previously so I decided to make one longer and wider near the bottom. (I was just going from memory as to the size of the old one.)

Unfortunately, I didn't realize the amount of resistance or pressure put on the rudder in high winds and this larger rudder snapped and scared the hell out me causing the boat to spin in a circle and be towed back to the ramp.

I've concluded that I either need to live with a little less control using the smaller rudder or possibly make the rudder out of wood and make it out of 3/4" thick stock and try again with the larger size. I've attached the specs for what I've made already with the one that broke on the right.

Any help would be appreciated.

 

Just the wrong material I think. Go for 3/4 inch plywood as that's reasonably conventional. The more work you put into shaping it to approximate to a proper streamline section then the nicer it will behave on the water.

 

Welcome to the forum.

Plywood is the least acceptable material you can use, unless you select something brittle, like Corian. It was a good idea and worth a shot, but a better product would have been HDPE, which is similar to Corian, but not as brittle.

Plywood works on really small boats of which yours is borderline. Simply put the veneers tend to fail in "rolling shear" within the plywood panel. You can mitigate this with a plywood sandwich, to make up the thickness, say two layers of 3/8" plywood, glued together to make the required 3/4" thickness, but the same thing eventually develops (rolling shear failure).

There are lots of ways to make a rudder, strip planked, composite, inert materials (metals, plastics, etc.) sandwich construction, solid laminate, etc. West System has a PDF about foil making that you should look up and download.

You basic problem is you've increased the rudder loading with the reshaped blade. From what I can see, you've nearly doubled the original's area and placed a significant bulk of this extra area as far away from the "fulcrum" as possible, further increasing loading.

Make the same blade as the original as best as you can remember. If you want it to be deeper, then continue the leading edge along it's same path, without adding more than would come naturally as the depth is increased. This will provide you a blade that is smaller at the tip, but still offers more area.

I'm not sure I fully understand the "didn't have enough resistance" comment, but I'll assume you felt it wasn't responsive enough and the helm felt sluggish? Most of the time rig tune and boat trim will cure these things, rather then a newly shaped blade.

Below is attached a few different "plan forms" to consider. The farthest left is a balanced blade with the same leading edge rake as the stock one, but with a higher aspect ratio. This blade will ease helm pressure and be more responsive. The next one is similar, but the leading edge has been "stood up" a bit and the balance removed. The third one is a constant chord blade, where the plan form is simply a parallelogram. This is the easiest to make if using foil sections. All add about 20% more area to the original blade and you can always cut it down if you find it's "to much". Your boat doesn't really need NACA foil sections. A "slab sided" foil will do and even a flat plate with rounded leading edge will work too.

 

Guys, thank you for your replies. PAR, thank you for the very detailed reply.

I'm going to attempt to duplicate your far left design and will try to find a source for the HDPE.

My thoughts with the Corian were that it would hold up well to the water although it would be on the heavy side. I just never dreamed there would be as much pressure put on it as it had. I think the HDPE is probably lighter and would work much better as long as I can find a source. My option "B" would be to continue with the quarter sawn white oak.

Obviously, I hadn't duplicated the kick-up style that looks like what the original may have had. I'm going to try and keep it simple since my knowledge here is not that great.

Again, I greatly appreciate your detailed reply.

 

HDPE is costly stuff, though relatively neural buoyant and inert. Have a look at:

http://mothboat.tripod.com/CMBA/Building/foils.htm

An edited version is shown below, skipping how to draw and cut NACA foil section. You can also skip over the carbon reinforcement section to, on this rudder.

How to Build Rudder Blades & Centerboards
By J.R. Watson

When the centerboard of my Searunner trimaran broke in the middle of a windy race around the Black Hole, the question I kept asking was "why now, after working fine all of this time, and when we were leading the race?"

"Guess it just wore out" was my excuse to myself. This centerboard was built of laminated layers of plywood, resulting in a thickness of 2". It was then covered with two lavers of 6-oz woven fiberglass fabric. It was a deep and wide board with a lot of area, and like any rudder or centerboard on a boat that is sailed hard, it was exposed to a fair amount of stress.

The answer to "why now - while leading the race?" could have been fate. But there is a more scientific answer. Extensive laboratory testing at Gougeon Brothers, Inc. defines why the centerboard failed. Understanding why can help us design and construct components that will perform more efficiently and last much longer.

The plywood centerboard did, in fact, wear out - or more accurately - it failed from rolling shear fatigue. Fatigue cracks in a material result from repeated (cyclic) stress. Fatigue is a reality of all structures and materials, and eventually culminates in structural failure. Repeated loading and unloading or even worse, loading one way and then the ' other(reverse axial), rapidly reduces a material's physical integrity and accelerates degradation. The higher the load is as a percentage of the material's ultimate strength, the more rapid is the deterioration.

Materials
Some materials have a greater fatigue life than others. Ounce per ounce, wood is capable of operating at a much higher percentage of its ultimate stress level than most other materials. That is why such wonderfully efficient structures can be built with wood. However, plywood is not a good choice for cantilevered structures such as rudder blades and centerboards. This is because plywood is susceptible to rolling shear, shearing forces that roll the structural fibers across the grain.. Plywood's unidirectional wood fibers are laid in alternating layers, approximately half of them are oriented 90 degrees to the axis of the loads. Like a bundle of soda straws, which resist bending moments quite well one way, they simply lack cross-grain strength laterally and can roll against one another and fail under relatively low stress, especially in a cyclic environment. Therefore, when anticipated loads are primarily unidirectional, it is ideal to use a material with good unidirectional strength. Since only half of plywood's wood fiber is used to advantage, a plywood rudder blade or centerboard going from tack to tack (reverse axial loads) will fatigue much more rapidly than one built as described in this article.

If you were to look at the end of the board, say a fish's view of a centerboard or rudder blade, you'd view its cross section. A section that has a faired airfoil shape is preferred over one that is flat with parallel sides. This is because the airfoil shape produces lift when moving through the water, thereby counteracting the sideward forces exerted by the sail rig. A flat section produces less lift and at a great expense of drag, slowing the boat and making it more difficult to steer.

Selection of a proper camber and section can be a subject of great theoretical debate. One can become intimidated with technical terms such as thickness distribution, Reynolds number, boundary layer, and so on. These terms do relate to the subject, however, for the builder/ sailor whose boat floats forlornly in need of rudder blade the following will do just fine. In fact, the best designers and builders will be hard-pressed to do better. An excellent choice for most craft, is a realistically accurate and fair NACA (National Advisory Committee for Aeronautics) 0012 airfoil, where maximum board thickness is 12% of the fore/aft length (chord length). Maximum thickness is located about 30% of the chord length measured from the leading edge (see sketch). The dimensions used to establish a specific shape (called offsets) are given in the appendix of The Theory of Wing Sections*. See the foil-owing article, How to loft Airfoil Sections. From offsets make a good drawing of half the section on transfer paper.



Western red cedar and redwood are good choices of wood to use for rudder blades and centerboards for boats up to 25 feet. Both of these woods bond very well, are generally clear and straight grained, have good dimensional stability, are easily worked and affordable. Cedar is just a little heavier than the foams used for rudders, is much stiffer and has far greater shear strength values. On larger craft, a higher-density material like African mahogany is a better choice. Oak is not a good choice.

Construction
Buy flat-grained 2'x6s or 2'x8s, and then rip them to the designed board thickness. Turn every other ripping end-for-end to neutralize the effects of any grain that does not run exactly parallel to the blank, and to reduce tendencies to warp or twist (see sketch). Rotating the rippings 90 degrees to expose vertical grain will permit easier shaping with a plane. The last trick is to rip the end pieces of the nose and tail in half. Bonding with a couple of layers of glass tape between keeps the fine edge of the tail from splitting too easily and offers a precise centerline.

Bond the ripping with a slurry of epoxy and 404 High-Density filler. Plastic strips prevent inadvertent bonding to leveled sawhorses (see sketch). With both sawhorses leveled, you're positive no twist exists in the laminated blank. Bar clamps should be snugged r2b.jpg until excess glue squeezes from the joints. Over tightening only stresses joints and tends to squeeze all the adhesive from them. When the laminate is cured, a light planing to clean the surfaces is all that is needed before shaping begins.



First, tack the 1/8"-thick plywood template that describes the cross section shape to the blank's ends. This is sawn from the impression made when traced with the transfer paper you originally drew it on. The key to producing an accurate and symmetrical board is maintaining of a systematic removal of material from one side, then from the other. To do this, mark the shape to be removed, stick to straight-line shapes (see sketch). Use a smoothing plane to remove the wood. After planing to the guide lines on one side, flip the blank over and plane the same shape on the other side. The procedure is similar to producing a round shape from a square by first forming an octagon, and then flattening the resulting eight comers to produce a 16-sided shape and refining that until very minute flat surfaces. Fifty-grit sandpaper bonded with 3M brand feathering disc adhesive to a 1/2"-thick by ll'x4.5"-wide plywood sanding block is a good tool to use for fairing this out.

Reinforcing the blade

Now you should decide if the board needs reinforcement. Your board requires reinforcement if the chord thickness is at or below 4/o of the unsupported span. The unsupported span of a dagerboard or centerboard is that measurement from where it exits the hull, to its tip when fully lowered. The unsupported span of the rudder blade is that distance from the rudder case to the tip. If it is a non-retracting blade, measure from the waterline to the tip. So, if the board extends 48" below the bottom of the hull and is 2" thick, .04", it should be reinforced for strength and stiffness.

If the board needs reinforcement, graphite fibers are a good choice as the strain-to-failure values of wood and graphite fiber are quite similar, hence they enhance each others performance. The high-modulus qualities of the graphite fibers provide stiffness. The addition of graphite will efficiently increase stiffness and ultimate strength. Don't be intimidated by the high-tech qualities of graphite fibers, they are easy to work with.

The amount of reinforcement needed is usually figured at 101/o chord thickness. Using the same board for our example, the board is 2" thick, then 10% equals .20" total reinforcement,.10" per side. Graphite fiber tows are .01" thick, so 10 tows per side should give the necessary reinforcement to do the job.

The graphite fibers will be laid into a channel that is routed into the shaped board (see sketch). The specific depth of the channel is determined by the above rule. Make the channel a little deeper than what's required (1/16") so you won't be sanding the graphite fibers.



The profile of the channel is similar on all boards. The centerline of the channel is usually located at the point of maximum chord thickness (about 30% from the leading edge). The widest point of the channel is where the board exits the bull when completely lowered. The channel width at this point should be about 16% of chord length. Toward the ends of the board, the width of the channel narrows by about one third that of the widest dimensions Keeping this in mind, more graphite can be laid in that area, a little above and more below that point that exits the bull. Maintain a consistent channel depth throughout.

Take a one-inch-square stick to serve as a router guide. It's best to bevel the edge of the channel to reduce stress concentration. A rabbet plane serves best for this task. A layer of 6-oz fiberglass cloth is laid in the channel first (@ serves as an interface between the wood and graphite fiber), followed by the schedule of graphite. You can complete the entire bonding operation for a side in one session. Try to do the other side the next day. Finally, fair the reinforcement area with WEST SYSTEM brand epoxy and a low-density filler.

A layer of 6-oz woven-glass fabric should then be bonded to the faired board to improve the cross-grain strength and abrasion resistance. The radius of the leading edge should be about a 1% radius of the chord length, and may not permit the fiberglass fabric to lie flat around the radius. In that event, cut a strip of woven glass fabric on the bias (which will lie around a tighter radius) and bond it around the leading edge.

It is better to leave the trailing edge slightly squared rather than razor sharp. This will cause less drag and the centerboard will be less vulnerable to damage. Flatten the trailing edge to 1/16 or 1/8 of an inch on small boards, and closer to 1/4 of an inch on larger boards.

Axle installation
Any board, no matter how stiff, will deflect. To prevent the axle hole that the centerboard pivots on from binding when deflection occurs, make the hole somewhat larger than the pin diameter. The perimeter of the axle hole should be thoroughly protected with fiberglass, as exposed end grain can absorb moisture.



Abrasion of the axle against the axle hole dictates that you should bond fiberglass into hole's perimeter. To do that, wrap fiberglass tape around a waxed (use auto paste wax) metal rod that is about 10 to 15% larger in diameter than the actual axle pin. The hole should be heavily chamfered on each side, so when the wet layup is placed in the hole and the nuts tightened, the fiberglass is pressed by the large washers into the chamfers on both sides of the board (see sketch). The same procedure may be used on retractable rudder blades; but the tolerance between axle hole diameter and the diameter of the axle pin should be closer.

You can bond control lines for centerboards and rudders-in-place by wetting a slightly oversized hole (about 1.5" to 2" deep) with epoxy/404 @gh-Density filler mixture. It helps to mark the hole's depth on the rope with vinyl electricians tape to serve as a guide. Then, after soaking that end of the rope to be bonded in epoxy for a minute or so, shove it in the full depth of the hole.

Centerboards and rudder blades are often overlooked components that are of vital impedance to a boat's performance. Built correctly, they will reliably operate with the efficiency of a fish's fin, and you should note a measurable improvement in the quality of pointing and steering of your wind ship.

References:

1. Jozset Bodig, Ph.D., Benjamin A Jayne Ph.D., Mechanics of Wood and Wood Composites, Van Nostrand Reinhold Co., New York (1982)

2. Johnston, Ken, Some 7houghts on Rudder Sections, Multihulls Magazine (Jan /Feb 1980)

3. Eck Bransford, Everything You Ever Wanted To Know About 505 Fins.

4. Lindsay, Mark, Centerboards and Rudders, Yacht Racing/Cruising Magazine (April 1981)

5. Abbott and Doenhoff, Theory of Wing Sections, Dover Publications, Inc. New York (1959). n

Durable Edges for Centerboards & Flip Up Rudders
By Jim Derck

When centerboards and flip up rudders drag across the bottom, the first fiberglass to abrade away is usually the leading edge at the bottom. This exposes the end grain of the wood, allowing water to be absorbed the length of the centerboard or rudder. The wood then expands, cracking the fiberglass along the leading edge and causing more problems. When it is time to repair the tip, it usually takes a long time to dry the wood for an effective repair.

To isolate the end grain before applying the fiberglass, cut diagonally from the leading edge to the bottom, apply several coats of epoxy to both pieces and bond the tip back on. In the future, if the fiberglass cloth abrades away and the wood gets wet, it is quick to dry out the short length of end grain prior to making the repair.

Here is a technique for fiberglass covering that has proved over the years to provide a long lasting trailing edge. When rebuilding an existing blade or before applying fiberglass cloth to a new blade, plane a flat on the trailing edge about 1/8"-wide (wider for large rudders and centerboards). When applying the fiberglass cloth, position the rudder or centerboard horizontally so the leading edge is up. Drape the fiberglass over the foil and trim so that it extends 1/2" past the trailing edge. If you make a full-scale drawing of the trailing edge of your board or rudder, you will get a better idea of exactly how much fiberglass cloth to leave.

After the fabric is wet out, use an #807 syringe to apply epoxy thickened to a non-sag mix with 406 Colloidal Silica to fill the gap between the two layers of fiberglass cloth.

Squeegee out excess epoxy and align the trailing edge so that it is straight. If necessary, clamp a plastic covered straight edge in place to make the fabric conform to the shape of the trailing edge. After the epoxy cures, do the final shaping with a sander or sandpaper and a block of wood. Use caution, the edge can be very sharp!

 

I've attached two more drawings. One shows what I had when I purchased the boat. (From memory)

The other is what I'm going to do.

One other note, I did notice with the Corian rudder that broke that there was some vibration on the tiller that I didn't have previously. I'm thinking with the new shape that it should probably be more balanced and not have that problem.

(So much for my quarter sawn white oak idea.)

 

One more question PAR, I've found some reasonably priced HDPE on Ebay. Would 1/2" thickness be sufficient or should it be a little thicker?

 

It should be thicker, 3/4" would be the least I'd use. If you could find a local CNC shop, you could get them to mill out a deadnuts (technical term) foil section, for not much more then tool time. On this boat, you really don't need a NACA sectioned rudder, but it does help somewhat. A slab sided foil or flat plate will do nearly as well, considering this boat's performance envelop.

HDPE is easy to work, though can be hard to make smooth afterward. Scraping, polishing and heat treatment are the usual choices. Most folks find working with this plastic easy, until it's time to smooth it. This is why wood and foam cored blades are more popular, as most can cope with the various difficulties and techniques employed with these.

 

Ye gods, this is OTT. There are umpteen thousand larger boats than 11ft out there with 3/4 inch ply rudders, 20,000 Enterprises just for a start. For a more sophisticated boat then sure I would build a glass or carbon coated board with a blank made up from cedar strips, but we're talking a little family boat here. I wouldn't even bother to glass coat it.

The substantially similar Gull dinghy I used to sail as a youngster actually specifies 7/16 ply for the rudder, but plywood was better then and I think that's a bit light. Also with the thicker material you can afford to shape the edges better. You can use the stripe effect from the ply to help you sand it down to an aero shape, but I'd leave something like half the chord of the ply flat for strength. Oh, and make sure the grain on the outside layers of ply is top to bottom: they're the ones that will do the work.

 

You can use plywood and I'd recommend two layers of 3/8" (9 mm) glued together at a 12 degree surface grain cant to each other. Again, plywood can fail as the fibers roll in repeated bending loads. I agree it's easy to shape a plywood board, as ggg notes, the glue lines between the veneers help guide you toward the shape. Also as he noted, the plywood needs to remain fairly unmolested, to retain most of it's stiffness. If I was making a plywood rudder for this boat, I might consider making it thick enough to shape into something usable, maybe a 1/4" (6 mm) layer in the middle of the two 3/8", knowing much of the trailing edges of the 3/8" on each side will get sanded off in the shaping process.

This type of board will need a 'glass sheathing, just to protect it from sucking up moisture through the extensively exposed end grain. A single layer of 4 - 12 ounce will do, but consider two layers of 6 or one layer of 6 and another of Xynole or Dynel, which are much tougher than regular glass cloth.

I've replaced enough plywood blades that I wouldn't intentionally make another, unless the customer insisted, but it is an easy route to go and the low loads on this boat's rudder will be relatively kind to it's long term durability.

 

True and a lot of them are in little pieces now...

I've made quite a few replacement blades from Cadets (12mm thick) upwards as plywood will not hack it. Especially on newer boats with higher rig tensions and more power transferred through the whole boat. The only ply c/boards I've seen that last, are the phenolic resin pressure layups with sub 1.0mm veneers throughout. Effectively they are fully resin impregnated. Must have seen literally tens of ply blades and c/boards broken over the years, compared to only a few solid/laminated/foam core glass/carbon ones. Actually I don't agree that 'plywood was better then', you can still get very good ply today and a lot of the old 3 ply (5 and 6mm 3/16" and 1/4") is very poor indeed. To their credit, most of the Bell kits for many boat designs had very good ply.

This little 11''er will merrily sail on with a 20mm thick stacked blade, epoxy sheathed. It doesn't need glass, but does require a good fit to the stock and enough 'grip' around the head area along with a sensible pivot position.
If you really want it to last, q/sawn Doug Fir with a Sapele leading edge would be nigh on indestructible. It can be sectioned easily without weakening it. I've been using an unsheathed 20mm (max chord) fixed rudder that is over 20 years old in winds in excess of 37 knots on the sea (in the last 2 years) without any problems. On a more powerful racing boat than this little'un. Not only that but the blanks (Sitka Spruce modest grain tightness) are glued up with Aerolite 306 not epoxy. It is sheathed with epoxy, 3 coats and painted (2k) white to reflect heat and show weed.

Personally I'd aim for a vertical leading edge in side profile. PAR is completely right, aim at a slightly bigger (deeper is better generally) blade. If the boat is unbalanced, move the rig slightly ie rake aft for more weather helm or forward to relieve. Or move mast heel for same effect. A well sectioned blade should not vibrate at any speed so any humming usually means an out of balance section.

A timber blade will be far stiffer than HDPE, but if you do use the HDPE and it breaks, just bends more likely, at least you wil spot it floating about....

 

This post clearly illustrates that there is not much of a dinghy sailing culture in the USA.The Enterprises of the past actually did have better ply for their foils as Thames Marine Ply made a special ply for the purpose which was called anti-fracture board.The veneers were all makore and very few were orientated across the sheet-if memory serves 7 out of 11 veneers were longitudinal.It was very tough to work,but not quite in the league of the phenolic resin impregnated ply of today which was all beech in the types I have encountered.

I would be equally happy to use a ply rudder for this boat or to laminate a timber blade.For such a small boat it seems like a lot of work to glass sheathe the blade given that it is only likely to spend an hour or two at a time in the water.A good paint job should be more than adequate.A full NACA section is probably a bit over the top for a recreational boat and a two inch chamfer would probably be enough of a gesture to efficiency.
The other comment I have is that the rudder would work just as well and put less strain on the transom fittings if the leading edge were vertical,rather than raked aft.

 

Regrettably when Docklands was developed the UK lost the last decent marine ply manufacturers, along with quite a few veneer merchants. However you can still make your own ply for specific applications. There is still one good veneer merchant in London, I used to go to them quite a lot when based in Shoreditch. Whatever you fancy as to layup, face veneer? rosewood? which one of the six types Sir?
However the imported (Italian and French) ply made from gaboon (with optional facing) is of high quality though only 90 deg layups. Having just replaced a complete Enterprise transom (mid 18k No), which was very poor quality 3/8" 7 ply, and built in the 70s' I was not impressed with it. But to be fair, even then you got what you paid for. In fact custom ply was only a tiny bit more expensive, I've seen and owned boats with obeche used as part of the core make up, circa late 80s'. Also made ply with custom angles for specific jobs.

Thanks for that information wet feet, maybe those Thames ply board boats just aren't sailed much anymore. There a couple of old Ents locally 1k and 2k Nos, the latter being amateur built. I had to repair it a couple of years ago after a T bone with a Laser. Pretty good still, and made from 5 ply, glued with Aerolite thank goodness not the dreaded C*******, in fact checked it over yesterday with the owner - all well.

It would be interesting to see if manufacturing marine ply in the UK is at all viable. However most of the timber trade is done by pretty large companies which have amalgamated a lot of the smaller specialist merchants in the last 15 years. Now for me, buying Sitka is a 400 mile trip instead of 100. Well I could buy it unseen but not prepared to chance that, and won't deal with merchants who will not let you pick from the stock.

Funny how the Oppie foils (replacements on training boats) I've built out of stacked Doug Fir haven't broken compared to at least 8 ply ones...... Actually I could make custom ply for them but it is more work for no real benefit over the stacked timber option. Might even have to start making rudders for the things if a few more go snap, done about 4 or 5 tillers...

If I HAD to use ply for this beast for the OP's boat, my gut feel would be to cap each edge with solid and shape those parts. Benefit of sealing and slightly easier (at least on the plane and spokeshave) to tool edges. Not too bad an option but not as stiff as can be obtained with a proper laminate.