How are your kickup rudders held down?

[Chotu]

My designer uses a system of underwater block and lines to hold the rudder in place. I never quite followed this part of the plans and lines under water with marine growth didn’t quite appeal to me anyway.

I’m looking for a better way.

I had tried “fuses” but they came off because I tried an adhesive instead of epoxy. I jammed some soft wedges in between the cassette and the hull as a temporary measure and they have worked pretty well.

I’m going to try fuses again, but am unsure as to the right material to use so they hold the rudder down, but also will break when enough force is applied that the rudder should kick up.

Also, they should be easily removed so that you can pull the rudder up when you need to, such as when you are at anchor or at a dock for a while.

I was thinking some kind of brackets and a 1x2 on its flat end?

How is yours done?

[Benz]

On one tri I knew (Finn), there was a loaded hydraulic ram with a line spliced to it (I spliced the line) just trying to pull up the rudder. You had to relieve that, force down the rudder, and install the fuse. The guy had a whole bag of fuses, but I haven't a clue what they were made of. Whatever they were, they worked a treat.

[Chotu]

Originally Posted by Benz

On one tri I knew (Finn), there was a loaded hydraulic ram with a line spliced to it (I spliced the line) just trying to pull up the rudder. You had to relieve that, force down the rudder, and install the fuse. The guy had a whole bag of fuses, but I haven't a clue what they were made of. Whatever they were, they worked a treat.

Very nice. Elegant. Mine will probably be a bit more utilitarian. Ha ha.

Any idea the dimensions of the fuses? Then maybe I can guess some materials.

[rslifkin]

I'd be inclined to use a stainless bolt or something as a shear pin. Easy enough to adjust the size until you find something that breaks free before the rudder suffers a damaging amount of force but won't break in normal usage. And you'll always be able to get your hands on spares if needed.

You'd also have to give some thought to whether you want the rudder to just move freely or if you want it to be pulled up when a shear pin breaks.

[Chotu]

I should probably put up some pictures. A bolt is too strong for a fuse.

See the prominent circle in the top right area in the rudder plan? That’s where it pivots.

See the extreme stern of the boat in the second pic? The 1x2 fuse would go across the stern, becoming the aft most part of the boat, corraling the rudder cassette into place and breaking when you hit something. Or being removed when you want to pull the rudder up.

[rslifkin]

Originally Posted by Chotu

I should probably put up some pictures. A bolt is too strong for a fuse.

See the prominent circle in the top right area in the rudder plan? That’s where it pivots.

See the extreme stern of the boat in the second pic? The 1x2 fuse would go across the stern, becoming the aft most part of the boat, corraling the rudder cassette into place and breaking when you hit something. Or being removed when you want to pull the rudder up.

A bolt would make sense as a fuse in that design and should be easy to implement, it's just a question of finding the right size. I'm thinking it would be pretty small (likely less than 1/4 inch to have it shear easily enough). Do you have an idea of how much force you want the fuse to hold before breaking?

[Benz]

Originally Posted by Chotu

Very nice. Elegant. Mine will probably be a bit more utilitarian. Ha ha.

Any idea the dimensions of the fuses? Then maybe I can guess some materials.

The fuses were about 3/4" diameter composite rods. What they were composed of I don't know, but the fuse was sensitive enough to snap if a lobster trap was snagged.

[Chotu]

Originally Posted by rslifkin

A bolt would make sense as a fuse in that design and should be easy to implement, it's just a question of finding the right size. I'm thinking it would be pretty small (likely less than 1/4 inch to have it shear easily enough). Do you have an idea of how much force you want the fuse to hold before breaking?

Literally none. No idea at all. I can’t even bring to guess. It’s a very dynamic situation. I don’t see how the substrate holding the bolt (cored fiberglass) could manage to not be torn up before the bolt sheers, also can’t see how you’d set a bolt up be a sheer pin.

I definitely can’t cut into the cassette. Just need to hold it down into place.

[rslifkin]

As a note on strength of shear pin materials, a little searching indicates that a basic hardware store grade #10 stainless machine screw should break with around 750 - 800 of force in shear. Having the bolt go all the way across the rudder and attach at both sides would put it in double shear and increase that, requiring a smaller bolt for a given shear force.

Going down to a #6 machine screw would get you down to about 400 lbs of force to shear it (mounted in single shear).

For mounting, you'd basically want a tab on top of the rudder and one on top of the hull. You bolt them together, so as the rudder tries to kick up, it's trying to shear the bolt.

[Chotu]

Originally Posted by Benz

The fuses were about 3/4" diameter composite rods. What they were composed of I don't know, but the fuse was sensitive enough to snap if a lobster trap was snagged.

Cool. The dimensions say I’m at least on the right track with 1”. I’m not sure I’d want mine that sensitive. I could just remove the fuse if I snagged a lobster pot, pull up the rudder and de tangle.

[Chotu]

Originally Posted by rslifkin

As a note on strength of shear pin materials, a little searching indicates that a basic hardware store grade #10 stainless machine screw should break with around 750 - 800 of force in shear. Having the bolt go all the way across the rudder and attach at both sides would put it in double shear and increase that, requiring a smaller bolt for a given shear force.

Going down to a #6 machine screw would get you down to about 400 lbs of force to shear it (mounted in single shear).

For mounting, you'd basically want a tab on top of the rudder and one on top of the hull. You bolt them together, so as the rudder tries to kick up, it's trying to shear the bolt.

Hmm. I see what you’re getting at now.

Maybe the thing could bend out of the holders in that case. But still, that would dig into the rudder cassette without some sort of additional buffer material.

[rslifkin]

Originally Posted by Chotu

Hmm. I see what you’re getting at now.

Maybe the thing could bend out of the holders in that case. But still, that would dig into the rudder cassette without some sort of additional buffer material.

If you let the bracket on the rudder cassette overhang the deck very slightly (and move the deck bracket over accordingly), the shear point would be clear of the rudder cassette so nothing should hit it once the pin shears and the rudder moves up. You could probably just attach pieces of stainless angle with an appropriate hole drilled in them for the shear bolt.



If you were to put a setup like that on each side of the cassette (2 pins) you'd have the option of removing one to reduce the kick-up force if you're in more debris prone water (where you might prefer a nuisance kick-up in exchange for lower risk of a snag or damage).

[Breaking Waves]

Originally Posted by Chotu

I had tried “fuses”

I have run across two types of fuses.

One is a lashing/winding of small diamater line. This is easy to make, inexpensive and very easy to calibrate to a target load. Used on a lot of French boats (like the Vendee's) to make sure the fuse breaks before the boat when they are pushing hard.

The other is the shear pin - very traditional in machinery vulnerable to jams.

Most often the lashing/winding is used to protect linear loads and the pin used to protect rotational loads - but in theory either could be used either way. For rudders I have seen both used - usually a pin if the rudder is 'latched' down or a fuse if the rudder is pulled/held down with textile.

[Matt Johnson]

The Schionnings I've seen use a wood dowel of about 3/8" thick across the top for the fuse. Spent some time on a 55' Peter Wormwood design that also had the wood dowel holding it down. Seems to be the common approach with the surprisingly thin wood stock.

The pull down block arrangement actually looks pretty good to me. Instead of a lower bearing block - which will get fouled - I'd just make a slick surface for the rope to bend around like a fiberglass rod. Dyneema seems to shed growth well due to being so slippery.

Matt