Keel hung rudders?

Discussion in 'Sailboats' started by Pylasteki, Apr 7, 2008.

  1. Pylasteki
    Joined: Apr 2008
    Posts: 74
    Likes: 3, Points: 8, Legacy Rep: 42
    Location: North Carolina

    Pylasteki Junior Member

    Hi Guys,

    I'm working on a Pearson Triton, which has a rudder hung just off of the keel. I'm also at that point where I know just enough to be a menace to society, and not quite enough to understand whats going on with them.

    I'm looking for any tips and tricks to reduce the drag of this rudder. Currently it is a non tapered barn door, cut with a rounded shape on the back. I would like to build a "Constellation" (not quite sure why they are called that) with the squared profile, and glass in the prop-aperture.

    The original rudder:

    [​IMG]


    Albergs 2nd set of line drawings, though I don't believe Pearson ever used the rudder design in production.

    [​IMG]
    [​IMG]

    Are there any guidelines as far as the gap between the top of the rudder and the bottom of the hull, and the gap between the rudder post and the semi-circular channel in the aft end of the keel? I'd like to spend the time and get whatever drag reduction is possible by working through the details!

    I haven't found scaled plans that include this new rudder design, so I can only assume that folks who build them give a rough eyeball to what looks right. Do you guys have recommendations for books or computer programs that will spit out formulas that list the ratios for the surface area required to turn a boat this size?

    I've been reading about endplates a little bit, and notice that folks try to keep the bottom of the boat directly above the rudder as close to the rudder as possible. Is there a benefit in making this flat surface wider so it acts as an endplate at times when the tiller isn't lined up perfectly with the keel? Pylasteki experiences some weatherhelm, and I'm wondering if the increased efficiency would reduce the effort required to hold the tiller, or even let it stay closer to centerline while sailing!

    Is there a benefit to having a rudder that is neutrally buoyant, or is it alright if I build one a bit heavier? Intuitively it makes sense to me that a heavy rudder would ease the pressure on the tiller, as when healed your putting the tiller to weather or the high side... so the rudder is going down. I'm thinking that for the amount of weight added to the rudder (to a point) would reduce the pressure required to hold the tiller towards you. Do I have this all wrong?

    Any thoughts on the width of the blade? I'm thinking that when the rudder is inline with the keel it would be best for it to taper from the width of the keel down to the 1/4 inch wide square'd tip. I have found it exceedingly difficult to find any information about keel hung rudders! Every paper I have found discusses spade rudders hanging on their own! Does a skinny blade living in the lee of a keel have less drag than one that tapers down smoothly? Sort of like when I draft a truck on my bicycle?

    Thanks for your help guys!

    Zach
     
  2. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    Well, you have got a slow boat and one that was never designed to make much use of NACA sections and so forth. That said, the boat was designed by a real master and so it isn't lacking in other areas of performance such as a seakindly motion, great tracking, and comfort and safety in foul conditions.
    Tapering the rudder aft would make sense, as you say, which means a whole new rudder. How much keel area is enough is a matter of years of design experience because every hull is different and every rudder location is different. For instance, a spade rudder hung a few feet back does more per square foot than a keel hung rudder, and also has less wetted surface.
     
  3. PAR
    Joined: Nov 2003
    Posts: 19,133
    Likes: 472, Points: 93, Legacy Rep: 3967
    Location: Eustis, FL

    PAR Yacht Designer/Builder

    I wouldn't call it a slow boat, but it was designed to compete under CCA which tended to favor wholesome cruising hulls. The beam was drawn into the ends of the boat to provide interior volume and a full keel for directional stability and prop protection. Most of the competitive CCA era yachts converted to a yawl to take advantage of a glitch in the rules which didn't count the area of a mizzen mule. Once heeled over to about 17 degrees they start to firm up and the waterline increases dramatically, making them surprisingly fast for their designed waterline length.

    I think these boats are some of the finest cruisers available. High D/L ratios by today's standards, which makes a comfortable sea boat, full length, built down keels which permits bilge stowage, better shaft angles and good tracking. They can be pushed harder then contemporary canoe bodies, before having a reef, much more comfortable to live with in a heaping sea and have much more hull for the buck, all things a cruiser wants.

    The Triton does have a foil sectioned appendage, but is what it is, a well founded cruising hull. A gen set can fit in the bilge, along with all the tanks and ballast. Try that with a modern canoe body.

    In the end, whittling down the trailing edge of the rudder, will likely increase the weather helm, not decrease it on this boat. The trailing edge should be crisp, not rounded off, which may be what is contributing to the weather helm.

    I've sailed a few different Tritons, all balanced well, so I suspect your helm issue is not the rudder. Lots of things can affect the helm, older sails will be high up on the list of things I'd look at first, also mast rake and boat trim.

    If you really want a new rudder of similar configuration as above then any capable designer could pen one up for not much money. I have the original detail drawing for the original, round rudder, which includes the post gaps.
     
  4. MikeJohns
    Joined: Aug 2004
    Posts: 3,176
    Likes: 198, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Zach
    welcome to the forum. Sounds like you need a comprehensive tutorial. Designers are often aware of the compromises in their designs and you may find that efficiency was sacrificed somewhat to robustness. So no blame on the designer:

    The aperture is a big drag source and you will definately reduce drag if you eliminate it. The rudder should be tapered back to a blunt edge around a qaurter to half an inch thick. You want the flow to leave the rudder without encouraging vortices so a sharper transition is good at the trailing edge, but don't take it back to a knife edge .
    The rudders root gap has to be almost touching surfaces to make the effort worthwhile as the hull can be made to act as an end plate. But given the shape and angle of your rudder I would not bother, so you can make the gap large and square the top of the rudder and the bottom corner too. ( with the rudder top angle the designer makes a call on the most likely flow angle which will be somewhere between horizontal and the adjacent buttock lines).


    This gap may not be such a problem unless it is large and the rudder LE is not partly eclipsed by the keel TE when looking beam on .

    Yes, but you have the boat and you can make a call on the rudder area and rate of turn you currently achieve. Losing the aperture will allow a smaller rudder by 10-15% for the same efficiency.

    Sounds like you would achieve better performance gains in balancing the boat but you do want some weather helm, just not too much.

    Ideally the rudder should be neutral bouyancy. The force you experience when well underway is hydrodynamic and extra weight is not a good solution.

    Also a thought if you want better windward performance it is worth checking and altering the sheeting angles too.

    Performance is all about power to carry sail, direction of drive from the sails and just where that energy goes. The immersed components are Form drag, frictional drag (WSA) and eddy making (but you probably know all this) People often get hung up on one aspect of performance when another aspect is more important. So tabulalte all the wind speed-water speed-heading, hoped for improvements and do one of those cost benefit type studies :)

    That's my 10c worth
    Cheers
     
  5. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    "I wouldn't call it a slow boat..."

    Nor would I... unless I was attempting to convince someone that the boat was being approached as if competitive with lighter dinghy-bodied modern designs. The use of modern coring methods and materials and the concern for converting the design to incorporate modern methods/materials will not make the boat into a boat that could compete based on a contemporary idea of what "fast" is.
    In fact, I prefer boats like the Triton. I sail to relax and to me, speed is a relative term, more accurately "speed in safety and relative comfort in many varying conditions".
    I said "slow" to grab attention---- to the fact that the basic design is a very good one, and its speed potential is based on (to me) a definition long forgotten, but still quite valid, BUT the design can never take advantage of features and materials that produce a modern boat because modern designs define "fast" in ways that would support an argument that the Triton was "slow" in comparison.
    I'd take a Triton any day rather than a shallow-bodied and ugly (to me) modern cruiser. If akyone said it was slow, I'd just nod my head and move on. the advantage of such a design is that one DOESN'T have to worry about a few additional pounds, or what's been learned about ultimate speed in the last fifty years. Each advance comprimised something else, as far as I'm concerned.
     
  6. Pylasteki
    Joined: Apr 2008
    Posts: 74
    Likes: 3, Points: 8, Legacy Rep: 42
    Location: North Carolina

    Pylasteki Junior Member

    Thanks for your comments guys! Really my pursuit of efficiency is to improve averages over long distances, I'd like to get the daily averages up and have less drag for ghosting along catching ripples. For me on a race course the best skill I can utilize is my voice... "NOOOO INSURANCE!" :D I have to rebuild the rudder and go through the everything back there before being comfortable it won't drop off half way to Bermuda, may as well give her a massage while I'm at it. :D

    As far as the sails go, she's got a 170% laminated genoa, and a relatively new quantum mainsail. She's better equipped as a boat than I am at sailing her! All sorts of fancy running rigging going everywhere... (A lot of this is going back low-tech, rather buy shorter lines of less stretch that stop at the mast, than cheaper ones that lead to the cockpit. Cost is an issue...)

    Yeah, as far as weatherhelm goes shes really not bad until she's really heeled over and the GPS reads 7.5 knots. I tore my right rotator cuff a few years back, and after an hour or two of broad reaching on a starboard tack I end sitting on the low side pushing.

    Of course with a grin you can't wipe off... the need for an advil later on is justifiable. I just have a funny feeling that if the tiller pressure is reduced, or the rudder can remain on centerline through more points of sail it won't be like putting on the brakes.

    When it comes to testing drag, I'm wondering if a dynamometer would be successful... hook up a bow eye and go tow her around at slack tide on a windless day, and watch the needle? (Of course I have NO idea how much pressure there is on a 28 foot boat and how highly rated that fish scale would need to be.)
     
  7. Pylasteki
    Joined: Apr 2008
    Posts: 74
    Likes: 3, Points: 8, Legacy Rep: 42
    Location: North Carolina

    Pylasteki Junior Member

    Kind of piggy backing on the end plate questions...

    I've got a question about this little accent line running straight from the rudder to the transom. Fish don't have those, whats its purpose? It comes to a pretty decent pointy-ish rounded tip. Should underwater details follow the 1/4 inch wide squared off profile, or in the interest of speed (and easier bottom paint scraping) I fair this into something less prominent?
     
  8. Brent Swain
    Joined: Mar 2002
    Posts: 951
    Likes: 33, Points: 0, Legacy Rep: -12
    Location: British Columbia

    Brent Swain Member

    I singlehanded a boat like that to New Zealand in my early twenties . Downwind she was a disaster to try control. In New Zealand I scrapped the barn door rudder and put a separate rudder on a skeg, six feet further aft. Then I sailed to New Caledonia , Vanuatu and Fiji. The difference in balance going to windward was unoticeable.
    The improvement in downwind control and thus the amount of sail I could carry was huge.
    I made the skeg a keel cooler for the engine.
    I highly recommend anyone with such an underwater profile do the same . People who have were extremely happy with the results.It will be one of the best moves you ever made on your boat.
    They put rudders on airplanes at the back end instead of in the middle for a good reason.
    Brent
     
  9. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    How did the boat motor--- forward and reverse? Another thing, (and this is me) I wouldn't any lobger own a spade keel boat up here in Maine. where that boat was before (Falmouth, Me), Ive more than once snagged another's mooring with the prop, and had my share of lobster warps grab my rudder . Offshore, no problem, but inshore, no thanks!

    Alan
     
  10. Brent Swain
    Joined: Mar 2002
    Posts: 951
    Likes: 33, Points: 0, Legacy Rep: -12
    Location: British Columbia

    Brent Swain Member

    The boat motored well both foreward and reverse. Lots of leverage with the rudder so much further aft. Making the skeg low aspect ratio with a good slope to the leading edge makes lines simply slide off. Some have run a pipe from the bottom of the skeg to the keel horizontally. A line cutter on the prop solves the lines around the prop problem.
    I ran the leading edge pipe into the cockpit sole and the trailing edge pipe into the deck for good two point support.
     
  11. Pylasteki
    Joined: Apr 2008
    Posts: 74
    Likes: 3, Points: 8, Legacy Rep: 42
    Location: North Carolina

    Pylasteki Junior Member

    Hi Brent, thanks for the comments. Cool idea about using the skeg as a cooler for the engine.

    I'm not really keen on adding a skeg hung rudder, simply because her lines came from the drafting table of Alberg. I have no qualms with massaging here and there to improve efficiency with what I have to work with, but a drastic change of design isn't really what I like to do. My rudder shaft goes through the cockpit sole... I'd have to move the traveler to the cabintop, or switch out for a double mainsheet. Way more work than I want to do!)

    Now if I had a steel boat... :D

    Pylasteki has the best docking ability of any 9k lb boat I've been on. With a 6 horsepower outboard she'll spin around on a dime, back up straight... even into wind and current! Get a spring line on, and turn the outboard tiller over and push or suck her over to the dock.

    With the atomic four you could be in reverse for 3 seconds and already be walking to port. The throttle was on the side of the cockpit, and the shifter was a 5lbs of massive bronze that slipped into a slot in the cockpit sole. (You also pulled back for forward and pushed forward for reverse... neutral was somewhere in between. (A lot of fun, after a line or two is on and you realize its still trucking forward... easier to hop down the companionway and shut off the key than fiddle with the shifter.)

    All that... and the ability to yank the prop clear of the water under sail. I can't deny though there are moments when she makes a sailer out of me... stiff chop an outboard on the transom doesn't do much than make noise. Of course the atomic four spent more than a few days going 1.5knots into a steep chop and headwind cavitating...
     
  12. Pylasteki
    Joined: Apr 2008
    Posts: 74
    Likes: 3, Points: 8, Legacy Rep: 42
    Location: North Carolina

    Pylasteki Junior Member

    What I'm thinking for the aft end...

    [​IMG]

    Thats what the detail looks like on an Alberg 30. Production start 1962 on the 30 vs 1959 for the Triton. Probably not something that even matters on the grand scheme of things... but I'm a little obsessive (Ok a lot) about details. (the last two or three inches directly under the transom will remain original to keep that heart shaped transom.)
     
  13. Petros
    Joined: Oct 2007
    Posts: 2,936
    Likes: 139, Points: 63, Legacy Rep: 1593
    Location: Arlington, WA-USA

    Petros Senior Member

    It is always true that the smaller all gaps (both end and at the hinge line) the more efficient. I have not seen it done, but it seems there might even be benefit to sealing the gaps with a rubber blade seal as they often do on aircraft surfaces. The pressure difference bleeds through the gaps reducing the effectiveness and requiring more input for the same lateral force (increasing the drag).

    If you could close the trailing edge to a sharp knife-like TE it would have less drag but you risk damage and flutter, so putting a small squared off trailing edge is usually done as a compromise. The worst drag TE shape is rounded, with a squared off TE, it should have sharp corners.

    You will also increase the effectiveness by making the surfaces convex from the LE to the TE of the rudder surface (like the aft section of a symmetrical airfoil). The thicker the rudder blade the more effective or responsive it will be. But when the rudder becomes thicker than the aft edge of the keel the drag goes up (even with a smooth transition). If you put a skinny rudder aft of a thick keel, the effectiveness will go down and the drag will go up (when you draft a truck on your bicycler you are only considering the drag on you alone, not the whole system as for your keel/rudder assembly). You want the transition from the aft edge of the keel to the rudder to fair exactly at the same thickness, with as small a gap as possible.

    If you build the new rudder to make as small a gap as possible (which might require making sure there is no wear or sloppyness in the hinge or pivots), with convex surfaces smoothly fairing to a sharp trailing edge, there should be noticeable drag reduction and it will be more effective over the current rudder. The more effective the rudder, the smaller the trim angle you have to hold the tiller for the same lateral force, and the less induced and form drag you will have.

    You can find a lot of information on the effectiveness of a trailing edge rudder by studying "Theory of Wing Sections" by Abbott and Von Doenhoff. There are whole chapters on high lift devices, flaps, ailerons and similar devices that would apply directly to the rudder.

    Good luck
     
  14. alan white
    Joined: Mar 2007
    Posts: 3,731
    Likes: 121, Points: 0, Legacy Rep: 1404
    Location: maine

    alan white Senior Member

    The skeg is a good idea, providing a gudgeon at the bottom, and most importantly, if the angle is right, it allows the baot to slide over pot warps and so forth.
    The cutter device on the prop is useful, though fishermen might disagree. But I would rather tie a warp back together than dive in c-c-c-cold water with a knife in my teeth. Every time I ever got hung up it was on a falling tide at sunset ten feet from a big rock. My current boat could slide over a whole forest of ropes---- full shoal keel and transom-hung rudde.

    A.r
     

  15. MikeJohns
    Joined: Aug 2004
    Posts: 3,176
    Likes: 198, Points: 63, Legacy Rep: 2054
    Location: Australia

    MikeJohns Senior Member

    Petros

    I think you will find that the root gap minimisation is not very significant on a raked keel hung rudder of this planform, with such a small root . in fact adding a fairing to keep the gap only millimeters wide with 15 degrees either side can cause significant drag itself unless designed and added with care.

    I agree though if implemented that the gap should be as small as possible. A a root gap around 2-3mm is good but the advantages rapidly diminish. For example you get around a 2% increase in eddy specific rudder drag between 20mm and 300mm for an average AR rudder. However it is not that applicable to this rudder since it is not a low drag high efficiency foil but more of a trim tab particularly at that narrow top.


    references worth a look:
    Ship science report 1/78 Mollard (Uni of Sthampton; Root gap drag and lift).
    Gutelle Design of sailing yachts Rudder design-root gap P180.
     
Loading...
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.