Kickup rudder critique

I'm an amateur building a 28' trimaran. The design included a kickup rudder option. I read an article by Eric Spohnberg on foils and he addresses rudders. From my understanding ideally the rudder stock is perpendicular to the waterline and the leading edge also vertical.
I provided a copy of the drawing. I'd really appreciate a review of the design. I'm thinking there's room for improvement after reading Eric's article.
Thanks.

 

I'm not sure how much having the rudder raked back effects efficiency, but that looks like a Marples design. He's a very underrated designer, so there's that.

 

Looks promising but I do have a couple of concerns;I can't see clearly how the rudder is held down and am guessing that the largely missing note at the lower right that begins 1/16" might relate to a piece of hardware for attaching a downhaul line.It isn't obvious where such a line would go.The other thing that might cause problems is the rudder blade projecting beneath the hull.When,not if, you trap a piece of debris in the gap it will quite likely jam the steering and at the very least you will have to raise the blade quite a long way to get the body or piece of rope clear.It might be a good move to make a guard to go just ahead of the rudder as a preventive measure.

 

Thank you both for the reply.
Wet Feet, there's a piece of alloy tubing that runs down and connects to the rudder with a friction lock at the top. I was thinking of using down/uphaul lines instead. I attached the article I was reading. Scroll down to the section with the heading 'Rudderpost Location'.
Thanks!

 

I can understand why there is an amount of rudder area ahead of the pivot axis.My concern is that the small gap,leading to a notch in the rudder, will attract debris.A small triangular metal guard,a bit like an inverted shark's dorsal fin would prevent rubbish getting into the gap.I think you are doing a good thing by considering a downhaul line rather than a piece of tubing as the tubing may tend to buckle if you scrape the bottom A line may take some releasing in the same situation so think carefully about how to release it under load.

 

The original Great Barrier Express cats had a transom with similar rake and the the rudder axis aligned with this. Most of them now have been modified by a longer top bracket to have an axis just beyond vertical that allows some of the blade to swing in front of the axis, or a vertical axis with the forward blade rake incorporated in the cassette. This gives a lighter better balanced helm. Sorry about the poor quality pics but you will get the idea.

I have a swing up rudder on my 25' Tri. It is mounted with a vertical axis and the blade swings forward of this. I had to modify the pivot (to get more blade forward of the axis when I switched from pinhead to square top. The blade has a 250kg breaking strain line to hold it down. If it hits anything (and it has a couple of times) it breaks and the blade swings back. Easy to replace and no damage to the blade so far.

 

Jamez,
I'm thinking about modifying the transom to make it vertical. This makes the rotation axis vertical. It opens up the option to maybe use a rudder from the f27 saving me the hassle of building one. I'm almost ready to laminate the outside of the main hull and now would be the time to make that change.
Would you have any pictures showing how you implemented the uphaul and downhaul on your rudder?
Thank you for the reply and pictures.

 

What boat design are you building? Can't help on the question and forgive my curiousity.

 

It's a Horstman 27-9

 

Yeah I thought it was a Horstman when I saw your picture.

On the one hand EH is an aeronautical engineer with tremendous experience. The 27-9 has been around forever with many built and used. On the other hand I never hear anything from Horstman owners, well almost nothing, so it's hard to be sure they are not without problems. The few owners I have spoken to never mentioned a problem with that rudder, which is common across the range.

There is a cleat that is extremely cheap that releases on impact so that could be a cheaper/easier option for strikes.

As you say thee raked transome does not lend itself to a vertical pivot. The real question is does this arrangement cause heavy tiller or drag. It is part of the looks of the boat.

What material are you building in ? If foam you might consider biax/double bias rather than woven cloth.

2c.

 

One thing I always appreciated about the design of the swing up rudder on my Farrier Tramp (pretty sure F27 is the same), was the ability for it to swing all the way back and up vertical so that it didn’t add greatly to the overall length of the boat when on land, in a slip or on mooring.

I however did not care for the concentrated load it put on the transom hung by two small gudgeons with small backing washers, especially when you consider the transom was only stapled and glued at the seam between the top and bottom half of the boat, and the boat was 40 years old.

I worried that if I did have a strike at speed and the rudder didn’t kick fast enough, or in situations when riding down swell with and applying strong input, that the rudder might damage the transom.

So I fabricated a stainless plate to spread the load outside with gusset brackets on the inside that tied the transom into the top deck by way of the central stringer. The rudder gudgeons then mounted to and bolted through the outside plate and inside brackets.

Have you considered a more modern setup that uses a rudder bracket/mount with a drop in foil (cassette style) that can be raised and lowered for depth adjustment? I don’t have any first hand experience, but imagine they must not have any way to handle a strike. The swing up was great for beaching, but a bit of a pain getting back off in surf and navigating shallow rivers with current.

 

Guzzi, I'm using foam. I did get an updated laminate schedule using biaxial cloth and epoxy I believe just eliminating the mat reduced the weight by 300 lbs. I'm nearly done futzing with the main hull and should be laminating the outside when my help is available.
The kickup design is only available for the 27-9 and smaller boats by my understanding .. everything else got a spade. I've also tried to run down any owners and pick their brain there's just not that much to find. There's one guy on SA with a 31' in Hawaii.
I modified the transom to be vertical .. if it doesn't work out it'll be a lot easier to trim it back than to add it on later. The Searunner construction manual also shows the transom mounted rudders plumb.
Thank you all again for the feedback.

 

No pics sorry, but a drawing attached which shows the general location of everything. Bearing in mind the blade floats horizontal when the controls are loose: the downhaul ties on the starboard side of the rudder near the top (dashed red) goes down through a stainless loop (in this case an old shackle epoxied in) on top/front of the rudder blade and up to the cleat on the port side of the rudder stock. Being all external its quick to replace if necessary. The uphaul is attached to the back of the blade and emerges through a slot on the port side of the stock and to a cleat. I think thats it.

If I was going to build another one I would make it a dagger rudder with a hinged cassette. That way you can still steer properly in shallow water with a partially lifted foil whereas the hinged rudder gets heavier to steer the more it is raised.

 

Great work. If you wanted to preserve the art deco lines you could also do an underslung kick up rudder as per Richard Woods, Ian Farrier I think and lots of others. I suspect EH added the raked aft faces to break up the otherwise boxy lines of his boats and give them some style.

I'd love to get my hands on some 27-9 plans some day and have a proper look over them. I think it could be made trailerable at 2.9 meters in the same way the 24 is. I would love to see your progress building the boat. I don't suppose you have a blog or something ?

The kick up daggerboard rudder is a great solution.

 

Jamez your setup looks like what I had in mind but with the downhaul on the tiller with one of the cam cleats that release if the rudder is struck. I did think about the cassette and hinged arrangement but I'm going to stick with just the kickup for simplicity. The bottom of your drawing is cut off what length is the chord and how wide? Ed's calls for 2.25" or 57mm wide and only 32"/812mm deep. I don't recall the chord but basically a naca 0012. Definitely going to make it deeper the owner of the 31' said he'd like an additional 6" on the spade.

Guzzi, Ed's designs are definitely a bit industrial looking .. function over form for sure. I didn't do a build thread but probably should have.