I'm thinking about putting a lead weight into my rudder to eliminate the down-haul.
My question is - how much lead would I need to put into a Weekender rudder to do that?
If the rudder has not a line or a sort of clamping, how can you be sure that it doesn't touch the outboard propeller?
Gianluigi
To sink a rudder blade you calculate the surface area in square feet, then multiply by the thickness. This gives you the cubic volume of the blade. Naturally, this will be less than a single cubic foot of volume (which is 62 pounds in fresh or 64 pounds in salt) so you figure out what percentage of a cubic foot you have in the blade and this is the weight you'll need to sink it with authority. Technically, you should subtract the blade's weight from the amount you need to sink it, but this isn't as important as sinking it.
The other way is to remove your rudder blade , climb into a buddies pool and add weights to the blade until it sinks. If you add weight to the end of the blade it will sink sooner and faster, but this generally isn't what you want. I almost always apply weight to the leading edge of the blade (well just behind it anyway).
This is the last ballasted blade I made and the client is holding it. As you can see it's not small and on the right side of the blade is the lead weight, aligned to the leading edge. It's about 18 pounds of lead, which this board needed to sink it. You'll note the board is strip planked, which is the only good way to make one in wood. You can also see where the bearing died, on the router I was using to shape the NACA profile in the face of the board. I've filled it with thickened epoxy and it's ready for final shaping.
As to keeping an outboard prop off the rudder, you have two choices,: one is to locate the outboard far enough to one side to prevent this from occurring, while the other is to "link" the rudder to the outboard with a rod and a couple of eyes. This is the wise option as you can thrust steer with the wheel or tiller.
Thanks Paul - I was hoping to skip the math but it looks like I'll have to sharpen my pencil since any swimming pools around here are still frozen.Â
I'd never thought that the position and shape of the weight would make any difference but now that you've said it, it certainly seems to make sense. also I think putting it in that shape would be more structurally sound than a large blob in the middle of the board.
I've got about 100lbs of lead that I cast into "pigs" a number of years ago so there's no shortage and I made a spare rudder last year so a mistake won't be fatal.
Andrew,
I think you don't need to sink the blade, just weight it and you have approximately the weight of the lead you need, because wood specific weight is about 0.5 (but timbers used for rudders are havier) so you need the other 50% to reach water specific weight (1.0). Consider also that the blade is not completely wet so you are sure to have extra ballast. Anyway this extra ballast will define rudder dymamics as for its location.
Paul, the rod between the rudder and the outboard is a very good idea! I will check if I can do this in my installation.
Gianluigi
Quote:I think you don't need to sink the blade
Of course you need to sink the blade, anything less then this weight and the blade will float or become neutral buoyant, neither of which is helpful.
Quote:this extra ballast will define rudder dymamics as for its location
Other then sinking, the blade dynamics will not change.
The only two considerations are weight location in relation to the pivot and the immersed volume of the blade, with consideration given to the specific species natural tenancy to float. You can skip all the math and just apply weights to the board until it sinks. Make sure it sinks with authority or it can come up easily.
Most folks don't need a ballasted blade and just make the cheeks snug enough to hold the board down. If it hits something like this then it'll be forced up, at which point it can be just shoved back down with a boat hook, once you're clear of the obstruction.
Ballasted blades with a positive up haul are necessary on rudders where access is difficult, like under slung arrangements. Most others that can be reached from the cockpit can get away with snug cheeks.
(03-08-2011, 03:57 PM)Paul Riccelli PE,NA link Wrote: Gianluigi_Iafrat Wrote:I think you don't need to sink the blade
Of course you need to sink the blade, anything less then this weight and the blade will float or become neutral buoyant, neither of which is helpful.
I meant that Andrew have not to brake ice in his pool to make sinking tests to find lead weight, but he can reasonably find it with a scale.
(03-08-2011, 03:57 PM)Paul Riccelli PE,NA link Wrote:Quote:this extra ballast will define rudder dynamics as for its location
Other then sinking, the blade dynamics will not change.
With dynamics I mean the rudder behaviour when it hit an obstacle, so impact energy and raising speed.
Gianluigi
A bath tub is almost as effective as a pool.
Damage to the blade will be mostly governed by the speed the impact occurs, since the stiffness of the leading edge will remain the same and the blade isn't fixed in location, plus the velocity quite low, so will damage be.
Another option to Pauls measure cubic inches and calculate is to simply weigh the darn thing. If you know what wod species you have you can get a pretty good idea of the density of the rudder from that and the weight. And you can then figure out cubic feet from that as well.
Asa general rule with most woods we are using you wouldn't go wrong figuring 30% density.
IE if the board weighs 10 lbs then the bouyancy is roughly 20 lbs give or take.
Adding 25 lbs of lead should do you nicely. 30 lbs if you want to hedge your bets.
Greetings all,
Anybody ever use Tungsten to weight their rudder?
Even though Lead is further down the periodic table then Tungsten and Lead has a higher atomic number then Tungsten, it turns out that molecular Tungsten and its alloys can be manufactured to achieve densities 1.7 times greater than Lead. This can be handy when designing small weight and size critical components for things like Cub Scout Pinewood Derby cars, where the exact placement of the center of gravity, and the concentration of mass for low rotational moments and maximized stored potential energy while still maintaining a razor thin streamlined shape, contribute to the difference between trophies and also-rans.
Powered Tungsten can be mixed with epoxy-like polymers to achieve densities anywhere between 1 and 11 grams per cubic centimeter. Commercially available Lead weighs about 9 grams per cubic centimeter. So with Tungsten/polymers you can simulate the weight and density of Lead, but with the spreadable consistency of void-filling shapable peanut butter that hardens up like any other epoxy filler you have worked with, and without any of the toxicity problems of Lead, or the fuss and danger of trying to pour molten Lead. The only down side is that Tungsten is a lot more expensive than Lead.
An example:
http://www.tungsten-heavy-powder.com/Tun..._poly.html
Any thoughts?
Cheers,
Tom