G'day,
Yes and no. Lots of things have been designed without fea. It is
mostly used for fine tuning, not the basics. Beam theory, experience
and common sense are adequate for most work of this type. I have not
done any fea on the hinged rudders, but we will be building a hinged
unstayed mast for a 45' cat early next year, and will be doing it on
this.
If the support for the rudders is situated below the hinge (ie a strut
with a fuse in it from the shaft just above the water to the opposite
beam to take the fore and aft loads and struts to the beam it is
mounted on to take the sideways loads), then the loads on the hinge
are very small compared to having the support above it. If the
sideways struts are also hinged, then the loads are taken even when it
is kicked up.
Having said which, the hinged rudders have not been used on any of the
bigger boats, and won't be until they have been tested on one that I
am involved with and that the owner is aware of exactly what he is
getting into. Mike's advice to wait until this has been done should
be heeded.
The hinge shafts do not easily allow for lifting rudders.
The hinges are plastic outer shells with carbon/epoxy pins, all
wrapped in carbon tow. They will not be affected by salt or time.
regards,
Rob
On Tue, Dec 16, 2008 at 9:43 PM, Mike Crawford <jmichael@gwi.
>
> <<This lets the rudder shaft and attachments stay fixed when the rudder kicks up>>
>
> I have worries about this kick-up design.
>
> I'll admit that I'm one of the first to support Rob's general design of using the rudders for leeway resistance. But that's with either a solid rudder from top to bottom (blind date), or a solid rudder with a solid stock (new beam-mounted design). While the forces aren't going to be significantly different than the force on the rudder stock of a 60'+ trimaran, they are still going to be huge forces.
>
> Rapscallion and Rob are the only ones who really know what the forces are like, because they are the only ones who have had Finite Element Analysis done on the rudders. Rob is generally positive with respect to the new beam-mounted design. Raps is more skeptical, and kindly offers a "you've really got to look at the FEA before saying a design is a success" to the discussion now and then. He's right.
>
> To put a hinge in the middle of the system, though, noticeably increases those huge forces. Plus it adds the issue of those hinges needing to work well while exposed to salt water for weeks or months at a time. That's something I'd have trouble trusting over time.
>
> If you aren't planning on building this spring, I'd hold off on the rudder design for the contrarry and see how Rob's new design evolves. It looks as if it will be able to raise and lower (raising and lowering the stock), as well as kick up in both directions (massive bungie or weak link at the top and bottom of the bracket). Although it is beam-mounted, there's no reason why it can't be mounted close to the leeward hull. The only thing it won't do well is steer after kicking up, but one can only ask so many things of a rudder that is also a daggerboard.
>
> Alternately, have an engineer do some FEA on the hinged design. The numbers might pan out.
>
> But without that computer-based finite element analysis, there's no way to predict if the design will work or not.
>
> - Mike
>
>
>
> Gardner Pomper wrote:
>
> Doug,
>
> The missing lw hull middle section in the Contrarry layouts is so tht the ww hull cabins can overlap the lw hull when the beam is contracted for shipping in a container. That allows the boat to reduce its beam to 7.5'. And, yes, that idea is totally my fault. Rob hasn't really weighed in on it except for a general note of approval.
> In the rudder diagram, I am trying (unsuccessfully) to indicate that the rudder will raise and lower. It is shown in the fully down position. In the fully up position, the knuckle would be right under the beam. This lets the rudder shaft and attachments stay fixed when the rudder kicks up. I am definitely looking on more feedback on the whole rudder design for the harrys. That has always been my biggest area of concern with the implementation of the designs.
> I am surprised that I have not gotten comments before on the mast placement. They do seem pretty close to the ends of the boat, but I am not aware of problems in doing that. I have been hoping someone would pipe up and mention any issues that arise from that.
> I am not sure about the reference for your last paragraph. Is this related to the mast placement, or are you talking about the sail area? I am all in favor of reefing when the wind picks up. My philosophy has always been that if I think I should reef, then I reef. it is easy enough to shake a reef out. Alot harder to right a capsized multihull.
> Thanks for the input!
> - Gardner Pomper
> York, PA
>
>
>
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