Rudder Issues

[Ron Bowman]

Seems like many of the boats already in the water have had some kind of rudder issue. Is the consensus that the rudder mount in the plans doesn't adequately separate the pintles? Any ideas how best to correct this problem? Probably easier to correct on new builds than on boats already completed.

Ron
Lunatic Fringe #108

[Big_Dog]

Ron,
Looking at the i550, I was planning to put a stiffener on the upper pintle mount. I posted a reply to Tim (below) under the TTB's New Bum! thread on the .org site. The Viper had problem with the original boats and designed a new upper mount tower. This kit could be used to retrofit/repair the first boats. The Viper class did a similar kit offering when they upgraded the bulb design to increase bulb weight.

Tim,
Take a look at the viper rudder tower. It is a replacement since they have had some structural issues with the original. The damage was in the old tower and damage to the deck. forum.viper640.org/index.php?topic=511.0

Jon C

[slowpoke]

I agree that the rudder mount in the plans is inadequate when you factor in the length of the rudder. The depth or length of the rudder combined with the force of a gust of wind would impart huge amounts of force to the lower pintle area. I think in the end we will have to go with a combination of further separation of the pintles AND strengthening the pintles and gudgeons themselves. So far it seems people have tried one or the other, with mixed results. By the way, an example of the torque the lower pintle is under is the picture Shazza posted of his bent rudder pin; It was 5/16" stainless steel, and bent like a pretzel. And the carbon tape on his rudder was torn through, multiple layers of carbon that had been professionally vacuum bagged.
The idea of twin rudders is appealing for this reason: Twin rudders can be shorter, applying less force to the pintles, while still able to steer the boat adeqately while heeled. The problem with this system is drag. DBSS explained the problems he had on a thread on the .org site for me. Upwind you have great control with less drag, as the boat is heeled and one rudder is almost completely out of the water. But when you turn downwind, and both rudders are in the water, you have large amounts of drag. He attributed some of this drag to alignment problems. I wonder if there is a way to apply a kick-up on the rudders so you can get the leeward rudder out of the water?

[Shazza 273]

Separate the the pintles as per TTB !!.
I have repaired our rudder gudgeons on Shazza, no damage to the rudder itself at all, by going a full length gudgeon with two 1 inch slots cut out so the pintles can slot into those cut-outs in the gudgeon. Our mount on the back is now stainless rather than aluminium and is the same as the original in all dimensions. Look at my blog and you will see the original mount in place. Imagine a gudgeon that is the full distance in-between those two pintles plus 50 mm below and 75mm of gudgeon above and that's our new gudgeon--distance between the two pintles is 200mm.
It is Definitely strong enough for our intended use, which is to never intentionally sail in 25+ with kite up, anything less is never going to be an issue for us because we just wont go out to play.
I do think for those that have an option (we didn't cause I wasn't going to try and stretch our carbon rudder) that increasing the height of the rudder mount and bracing it forward into the cockpit some-what is definitely the way to go as I too have spoken to two local pro boat builders after our stack and looked at how to repair once and not have to do it again.

[Chad]

I got the impression from the photos I saw (I can't find them now- don't remember where I saw them?) that the rudder pin on Shazza bent first, and then levered its way out of the holes in the transom and rudder gudgeons. If that's the case, then as Josh says the best remedy is to ensure the pin is loaded more than once at the top and bottom, in "double shear" in engineer-speak. So using two bottom transom gudgeons, with the bottom rudder gudgeon between them, means the pin needs to bend twice to handle the same load AND those bends are acting in opposite directions so the rudder doesn't displace upward and increase the leverage and increase the pin load, etc. Never allow any significant gap to occur between the transom and rudder gudgeon sets- that gap is the lever acting to bend the pin, rather than load it more purely in shear.

ShazzasGudgeon.jpg

So with the hardware in this pic, I'd capture the lower transom gudgeon lug with two straps on the rudder, or use a beefy strap on the rudder at the bottom, and capture it with a second lug above the rudder's lower strap.

Here's the output of a spreadsheet I put together for another project, then adapted for the i550. It doesn't design your whole rudder for you, but it gives a pretty good idea of how the loads work.

Rudder design.pdf

[Shazza 273]

Yeah, what Chad said

[slowpoke]

Looksgood Josh, this time it shouldn't break!

[Tim Ford]

Along the lines of the Viper fix, I was playing around with the idea of modifying my transom configuration to provide a longer mounting dimension for my rudd attachment points (and effectively reducing lever-arm torque on them)

Think is is going to make any difference? Would it be legal? Is it worth it?

[admin]

The foam mock-up is not well executed or photographed....I would add another 3 sides, much like the Viper fix box, with a smaller inspection port. I left the measurements outside and am too lazy to get them, but I figured just another 3 inches would increase the mounting distance between pintles by approx 25-30% and take a tiny bit of wetted surface out of the water--might miss it in big breeze, but I will probably be headed for the dock then anyhow. Material would be laminated meranti with some 5/4 white oak or G10 backing up the aft face and some big-ass fender washers on the interior ends of the fasteners.

I wanted to keep weight out of the ends but I hear rudder failure and losing steerage can be sort of slow....

[Ron Bowman]

My pintles are almost exactly ten inches apart so that extra three inches should make a big difference.

Ron

[Chad]

Loads on the upper gudgeon get reduced by the difference in lever arm- twice as much separation means half the load to the upper gudgeon. The bottom gudgeon sees the full load of the foil added to the load going to the upper gudgeon. So increasing the lever arm doesn't have the same proportional load reduction to the lower gudgeon- there will be a point of diminishing returns.

Here's a look at the difference between 10" gudgeon spacing and 13" gudgeon spacing using the same rudder specs posted further up the thread:

10inch.jpg

13inch.jpg

One observation is that gudgeon spacing doesn't change the bending load that the rudder sees, just the compression load applied to the side of the foil by the strap assembly. From this I would think about making sure that the area under the lower straps is made of high compressive strength material, or make sure the straps distribute the compressive load to a large area of the core and sheathing.

[admin]

Thanks, Chad...great stuff. Probably add yet another couple, 2, 3, 4 layers of CF over the loaded strap areas.

[Tim Ford]

Ron. from the looks of your transom, if seems like you have pretty much maxed-out the vertical separation between gudgeons. Are you planning to do anything after the season? Sounds like you are tempted. I just hope you don't have to do anything BEFORE the season ends!

[M&S]

I see no reason to restrict the height of the rudder upper attachment point at all. The deck arch point probably might be close to 22 minus inches if a fair curve were described from sheer to sheer on the transom. If the connections were placed near these dimensions the structure itself might be tied in by a solid small beam from sheer to sheer.
Load at the top of the rudder will of course be pushing forward toward the bow. I am open to all ideas to strengthen this area.
an idea to consider is making overbored, filled and rebored compression columns in the rudder head and in transom to stop compression of the core materials of the rudder and the transom.

T

[Ron Bowman]

Wow, Chad, those are significant loads. No wonder people are having problems.

This is one of those "I wish I'da know'd then what I know now" issues. Lots easier to build than to rebuild but still easy enough to fix after the fact. I tried to spread my gudgeons out as wide as possible on the transom. Probably worth reworking but I'm hoping to defer that project at least through this season. It's tempting to crank on the tiller when the boat starts to spin out, though, which I think is the time when the most force is being applied to the gudgeons. I've had the tiller up under my chin a few times and am trying to repair some cracks under my bottom gudgeon. The rudder isn't cracked but it looks like the gudgeons need to be attached better perhaps spreading that force out across a wider area as suggested by Chad. Hopefully we can get through this season without having to modify the transom and rudder but it seems like that would be a good idea in the long run.

Ron

[Chad]

I guess I should point out (if it isn't perfectly obvious) that the numbers are much more useful for comparison than as absolutes. It's pretty arbitrary to pick 10 knots as the rudder design speed for instance, although I suspect nobody will have the tiller to their chin at speeds much higher than this. But I think it is handy to see just how the various choices impact the loads relative to other choices.

Being a sucker for this stuff, I've put the spreadsheet on "Google docs", which is accessible from the "some useful files" link on my blog. It'll be in the misc category. Viewing Google docs requires a google account, which is free and they don't spam you. Feel free to wiggle the various inputs, and see how things change. I find it amusing, maybe somebody else does!

[jerome]

Chad,

how do you change the values in the yellow cells ? İ have dowloaded both the pdf and the google doc format but İ cannot change the values of the yellow cells. İt seems like they are protected.

[Chad]

You should download the spreadsheet as an Excel file. I assume this should work- I don't own Excel so I can't check for you.

Or you can just change the values right there on the google docs webpage- I can change them back to a base setting easily enough if the web version gets thrashed.

[Kevin]

Very nice thread guys. I'm laying up my new cassette tonight and will definitely use this info. I already added something below the bottom pintle, I think I'll add something above the top one as well with 1 pin running the whole length.

I'll post a revised drawing and some pictures soon.

Kevin.

[Tim Ford]

Made a little headway on the rudder tower aka the "viper-fix"

see: http://www.nbayracing.com/i550Build.htm

Comments? stupid...ugly...poorly designed...poorly constructed...worthless? go ahead, have at it.

I readily admit I have no idea what I'm doing...

[Shazza 273]

Looks good to me Tim good height, good depth in the extra box. Any extra material under the cockpit floor for that box? (just a thought, I don't know either).

[Tim Ford]

Looks good to me Tim good height, good depth in the extra box. Any extra material under the cockpit floor for that box? (just a thought, I don't know either).

Josh, what's pathetic is I had to refer to my own bloggisms to remember what is on the underside...here's a pic.
Joe, thanks I think it's a decent idea but a lot of this is seat-of-the-pants!

[Shazza 273]

Hahaha, I know that feeling Tim. Looks like you have plenty of structure to support that Rudder mount, I reckon you have it in the bag--good work

[Tim Ford]

FWIW, here is what the "Viper-Fix" rudder tower is looking like...still needs some more tape and glass

IMG_0220.JPG

IMG_0222.JPG

glad I hadn't started building the cassette yet! BTW, anyone have any drawings of how they built their cassettes?

[Tim Ford]

Since it now rains about 98% of the time in Maryland, here is the new work set-up....man, I need a garage. Pix above from one of the 2 percenters.

IMG_0211.JPG