Sinking a rudder

[J. Dillon]

I'm redesigning a kick up rudder. I need to know just how much lead to put into it, to comfortably hold it down considering some wave action.:confused: It measures 24"horizontal X 15" deep x 1-1/8" ,thick, made of doug fir.

Just enough please, ;)not overdone estimates .:eek:

Thanks :)

JD

[J. Dillon]

That involves math. Something I lack any skills in. My approach and probably the easiest is to take the board to the water immerse it with say 2 then 3 then 5 lb weights attached and see which one works or sinks the board. I'm guessing the 3 lb weight will sink it but maybe 2.5 is better. I do not want to over do it.

JD

[Osborne Russell]

Could be analogous to the scantlings rule: if it broke, you made it too thin; if it didn't break, you made it too thick.

[huisjen]

I figure your rudder has a volume of .234 cubic feet. At 32#/cubic foot that's 7.5#, but it displaces 14.5# of water, giving it an underwater weight of 7 pounds.

Lead weighs 711#/cubic foot. About .012 cubic feet, or about 20 cubic inches, should do the job. Given the 1 1/8" thickness, a 4 3/4" circle or a 4" x 4.5" rectangle, cut out of the rudder and filled with lead, should do.

Somebody check my numbers. I don't know if just getting past neutral buoyancy is good enough. It will work when adrift, but what when making way? Maybe increase the lead to 30 cubic inches?

Dan

[J. Dillon]

Thanks

I'm always amazed by those who can handle math like that. It's like watching an accomplished musician doing his or her thing.:D :cool:

But , how many lbs Will a piece of lead weigh that is 4" X 4.5"X1 1/8" weigh ?;)

JD

[Peerie Maa]

I figure your rudder has a volume of .234 cubic feet. At 32#/cubic foot that's 7.5#, but it displaces 14.5# of water, giving it an underwater weight of 7 pounds.

Lead weighs 711#/cubic foot. About .012 cubic feet, or about 20 cubic inches, should do the job. Given the 1 1/8" thickness, a 4 3/4" circle or a 4" x 4.5" rectangle, cut out of the rudder and filled with lead, should do.

Somebody check my numbers. I don't know if just getting past neutral buoyancy is good enough. It will work when adrift, but what when making way? Maybe increase the lead to 30 cubic inches?

Dan

Dan,
You have not allowed for any shape, rounding the corners nor chamfering to an aerofoil shape. So your sums are correct but an overestimate.

I'd make the rudder and put enough varnish on to keep it dry. Then duck tape lead sheet disks on until it sinks, you can then cut out a recess for the lead and epoxy it in.
Dan's lump of lead weighs about 8 lbs, but I think that you will need less than that.

[MiddleAgesMan]

My seat of the pants estimate is about 5 pounds. No math involved, however, unless I have a heretofore untapped aptitude for calculating such weights without effort with an unconscious mind... in about 2 seconds. ;)

[robm]

Why sink it with lead? Use a pull down to hold it down, and let it float up when released. Works great on Lasers, and plenty of other small craft.

Rob.

[John Meachen]

Why sink it with lead? Use a pull down to hold it down, and let it float up when released. Works great on Lasers, and plenty of other small craft.

Rob.

An approach I have advocated on this forum many times....A hole drilled through the rudder blade and a sheave sited to lead a downhaul line along the underside of the tiller to a Clamcleat CL257 will work beautifully and you won't have the weight of the lead wearing out the gudgeons and pintles.

[Rob27]

Thanks

But , how many lbs Will a piece of lead weigh that is 4" X 4.5"X1 1/8" weigh ?;)

JD

Based on the density of lead from matweb.com, lead weighs .4097 lb/in^3. Your volume is 20.25 in^3. These two multiplied together is 8.296 pounds.:)

[paladin]

although I agree with the basic math presented above, I also would not weight it...if you accidentally lost it, it would go to the bottom. I also would advocate the hold down using a small piece of line, perhaps 20 lb test, to hold it down. The line would act like a fuse if you hit something underwater. I used a similar arrangement on my 31 tri.
Another method would be to place the rudder in a down position, and drill a 3/16ths or perhaps 1/4 inch hole through the rudder cheeks and the rudder blade, and attach a dowell on a captive string, and insert it to pin down the rudder...if it strikes something hard enough to shear the dowell perhaps you won't have to build a new rudder.

[Pernicious Atavist]

And I prefer ballasting, if only because I don't trust hold-down-quick-release gizmos. I know they work, but....

I have cast lead into a blade, and cut out a hole to fit a lead piece, but the best method I've used--if epoxy is not an issue--is what I did with my leeboards: cut out a hole large enough to accommodate the amount of lead you need. Buy that weight in small bird shot--smaller the better. Apply a backing to one side of the blade--I used several layers of clear packing tape backed with a piece of plywood clamped in place. Add the lead shot and shake it until you have completely filed the hole. Pour in epoxy to fill. Done.

[J. Dillon]

Thanks for the ideas. I like the releasing cam cleat idea the best. Is there a USA distributor for this item ? It doesn't show up in Wast or Defender's catalog.

I do need the continuous use of a kick up rudder as the marina I keep the boat has thin water at low tide. So the kick up feature is an all time use thing.

http://img11.imageshack.us/img11/7145/kickuprudder5.jpg

BTW In the image I show two rudders one painted and one not The painted one is my original rudder. It works fine but now with thin water I need a kick up one. The other rudder is rudder # 2 with lead weights. It was a failure as it induced weather helm not experienced with rudder # 1. Rudder # 3 is under construction it features the shape of rudder # 1 but will kick up. So the deep rudder supposedly the better design, in my case not so.

JD

[TimmS]

I like the way that Iain Oughtred has designed the kick up feature on the Caledonia yawl -the pivot is located aft of the center of buoyancy of the rudder....it actually wants to float up and forward, keeping itself down. But when it touches the bottom, it easily pops up out of harms way. There is a little line that you tug to pull the blade back down again. A small camcleat allows the rudder to be set partway down and will release when you hit bottom.

[Captain Intrepid]

Thanks

I'm always amazed by those who can handle math like that. It's like watching an accomplished musician doing his or her thing.:D :cool:

But , how many lbs Will a piece of lead weigh that is 4" X 4.5"X1 1/8" weigh ?;)

JD

Well, after translating the units into a sane system, and rounding to two decimals...

10.16 cm x 11.43 cm x 2.86 cm = 332.13 cm^2

Lead has a density of 11.3, so...

332.13cm^2 x 11.35g/cm^2 = 3 769.68 grams, or 3.77 kilos.

Back into the outdated imperial system, that's 8.3 pounds, roughly.

[Todd Bradshaw]

You can also take a plastic Clamcleat and drill a fore-and-aft level (horizontal) hole through it, slightly larger than your line diameter and down at the bottom of the gully between the two toothy sides. The teeth hold the line, but at a certain level of tension, it slides down through them into the hole and runs free. The centerboard and rudder on our trimaran had these and they worked quite well.

There is also an adjustable quick-release Clamcleat available that is made specifically for this type of use.

http://www.duckworksbbs.com/hardware/cleats/sd002570/index.htm

http://www.duckworksbbs.com/hardware/cleats/sd002570/GroupAutoRel.jpg

[Ben Fuller]

Of course there is the KISS rudder, which works fine on a small boat. Bolt going through the cheeks with appropriate washers. Use an aircraft style nut and set it up so that the rudder can kick up when it has to. Give it a push back down when clear of the skinny stuff. You could do a pull down as well I suppose.

[J. Dillon]

Todd, Thanks for the link on how the cleat works.:cool:

It looks like one can improvise it by mounting the cleat on a brass hinge and incoporate the adjustable cam tension device your self from other stuff in ones "junk box".;):D

JD

[Hwyl]

You can also take a plastic Clamcleat and drill a fore-and-aft level (horizontal) hole through it, slightly larger than your line diameter and down at the bottom of the gully between the two toothy sides. The teeth hold the line, but at a certain level of tension, it slides down through them into the hole and runs free.

That's what I would have said too, but Todd has a more cogent description. I don't know why clamcleat don't make them like that

[J. Dillon]

but at a certain level of tension, it slides down through them into the hole and runs free.


Finding that level of tension could be at the cost of a pintle, gudgeon or even the rudder:(

JD

[Hwyl]

No, it's easy. It's more likely that the rope will slip into the escape hole to easily.


What did that old chap from Bristol, or possibly that old chap from Cowes: "If you want weight, buy a steamroller"

[J. Dillon]

OK worth a try.

I'm surprised no one has picked up on post # 13.

The deeper rudder gave much greater weather helm especially when heeled :eek: , while the old one, just the right feel regardless of angle of heel.:)

JD

[JimD]

OK worth a try.



The deeper rudder gave much greater weather helm especially when heeled :eek: , while the old one, just the right feel regardless of angle of heel.:)

JD

Any idea why?

[Hwyl]

The center of effort of #2 is further away from the pintles (just guessing)

[bucheron]

Just a couple of thoughts -
I would say the severe weather helm is caused by the centre of area of the rudder being a long way behind the pivot line.
I have discussed this at

http://ianprl.wikispaces.com/Cruising_Rudder

About bouyancy - remember that part of the blade is above the waterline when the boat is floating, that part will not need to be taken into account if you are trying to achieve neutral bouyancy. Another approach is to have a hollow rudder, which partly fills with water, to make it easier to get under water, but which would still float if detached from the boat. Perhaps a rudder could be ballasted in such a way that it floated vertically in the water, making it easier to find.

About swinging up - this means the rudder must be able to swing down. This must be easy, positive and secure. I do not believe enough attention is given to these factors. Lots of light dinghies take off under little or no control, with the sailor desperately trying to get a rudder blade down. If a rudder swings up after contact with the bottom, will it still be effective as a rudder?

[Thorne]

As mentioned above there are numerous forces on the rudder blade -- forward motion, weight of blade, friction with rudder head cheeks/bolt, flotation of the blade materials, etc.

My inclination (after guessing too light twice for a CB and rudder) would be to go for the 8lbs of lead mentioned above and see how it works. You can always drill out some of the lead and replace it with thickened epoxy if it is too heavy.

[tprice]

You mentioned that you had increased weather helm with the #2 rudder. There's all kind of stuff going on with your new rudder #2 that now has a large gap between the skeg and rudder blade! By estimating the area aft of the rudder post, I'm not sure you actually have more weather helm, more that now you can actually feel it with a blade operating in the stream, not just in the flow off the skeg. I'd bet it tacks a lot better.
Have you tried just moving the pivot bolt to move the blade closer to the rudder post? Or even raking the blade more forward?
You are bound to catch stuff on the blade now that you have that gap between skeg and rudder. How about sticking closer to the original concept and having your pivoting blade be just aft of the skeg when down?
Not that this helps with the original sinking question....

[Daniel Noyes]

http://img11.imageshack.us/img11/7145/kickuprudder5.jpg[/IMG]



JD

someone mentioned the KISS principle, simplest (and most elegant solution) is a shallow draft solid state rudder.

As if sailboats dont have enough finicky little bits already the kickup rudder is a cranky little contraption hanging off the stern.

kick up rudder owners can go on and on about pivot points, jamcleat tension, lanyards, bungees etc!

the #2 rudder is deep and may require the kickup feature, but the #1 rudder is quite shallow, is the marina that shallow, if you bump that rudder your practically on the beach.
certainly the boat won't sail in water where the #1 would ground out. not sure if a outboard could opperate that shallow either.

any photos of the whole rig?
Dan
http://dansdories.googlepages.com

[J. Dillon]

I am going with Todd's idea. The jam cleat with the enlarge slot is the simplest solution. The rudder will kick up when it engages rocks, bars, reefs, etc and will still be able to be re set. Kind of like a sheer pin as Chuck mentioned.

When docked I can raise the rudder so it will not touch bottom when the tider runs out . Right now there is just enough water to float the boat at low spring tides but in the fall when the tides are even lower I sit on the mud.

Here is an image not taken in the fall my slip is not in the picture, but this is typical.

http://img193.imageshack.us/img193/867/farmriver10.jpg

http://img6.imageshack.us/img6/2807/carrianneatdocksview09.jpg

This image is for Dan to get an idea of the boat.

Presently there is just enough water to float at low tide . In the fall she sits high and dry on mud. (clams too);)

JD

[P.I. Stazzer-Newt]

The center of effort of #2 is further away from the pintles (just guessing)

http://img11.imageshack.us/img11/7145/kickuprudder5.jpg
If that's in the down position, the centre of effort is a LONG way from the pivot axis.

The worst production example of this effect I know is on the Man Form Cowes's best known keelboat - and the result isn't realy bad weather helm, just a brutally ineffective lever to control it - it's like understeer in a car, safer but not quicker.

[Daniel Noyes]

beautiful country, nice looking boat too :)

sorry I didnt realize we were talkin grounding, I was thinking shallow water operation.

I anchor the Alpha dory in the marsh at about half tide height, I remove the rudder and drop it in the stern, steering lines stay attached. The rudder is one of the more fragile parts of the boat and I like that it is safe inside incase we get a storm with wave action, the rudder wont be straining. I notice a pin in your tiller can it be easily removed?

a shallow solid rudder idea might be taken from Bolger and his shallow endplate rudders. Think about reducing rudder #1 by half it's depth and adding an end plate.
Dan
http://dansdories.googlepages.com

ps I like anchoring shallow, with the boat dry half the tide I don't need antifouling paint.