Catamaran Evolution

Good Morning...

Took a look at the picture. That is what I call Captain Stupidity. The loads that vessel are experiencing in not necessary whether he is in distress or not. Assuming the state of the sea is consistent, that Captain should be taking those crests and troughs on the forward quarter and not perpendicular to all that energy. That wave alone has to generate what...60 or 70 tons per square foot? Chances of testing if the vessel will function as a submarine on dive are pretty damn good when that picture was taken.

On the other hand...if the wave frequency is long, this could have just been a freaky one that he had no choice but to take on the nose. One thing is certain however, if that was me in there...it would have been WOT to the crest and full astern to the trough.

J

 

Hello all,

Did some work on the rudders and wanted to ask if things looked right. I think I have it forward enough so that she will have some bite even downwind and I think far enough aft behid the position of the prop for the outboard.

I used the 8 inch chord and 2 foot span on a 64021 rudder in Freeship. Does the size strike anyone as a bit small or do my eyes deceive me?

Should I still rattle my brain and try to figure out a way to try to get them to kick up? Maybe I can have them swing up into a cavity beneath the hull? I don't think it would be easy, but I think I can do it. Just a thought.

Thanks
J

 

I have seen where weak points are deliberately set to break in such extreme events and leave at least a third or so to limp home. Just a thought.

Seasons salutations all.

 

Proportions look OK. I had the impression that the hull rocker would make it look less exposed.

The 64021 section will have about 30% less peak steering force than the NACA0020 for almost identical in-line drag. I have attached the co-ordinates for a NACA0020 if you want them. There is not much point drawing it to great precision in FreeShip unless you want to produce mould sections using it.
Rick W.

 

Good Morning Rick,

Okay on the proportions. I thought that maybe it looked small. I gotta stop flying by the seat of my pants and trust more on the instruments.

I used the Freeship rudder because it was close and not to draw it in detail. Thanks for the coordinates but I think the file may have been corrupted when it uploaded because I only saw a couple of numbers with little squares in it. Can you re-send it? Thanks.

Would I get better steering from the NACA0020 if I removed the taper and made the tip chord the same as the root chord or will I just be adding extra wetted area for little gain in steering?

Would there be any benefit to designing a foil plate for the tip to increase lift by reducing vortices and to create a little more bearing area to spread the loads in case if I beach her instead of point loading the rudders?

Thanks,
J

 

JCD
I just opened the file that I posted and it opens OK. It is only a text file with 61 pairs. Way more than needed. It does not really matter with the FreeShip drawing because proportions are similar. I was making the point that foils are reasonably sensitive to shape. I am not sure what the 6-series foils were developed for but I have found the NACA four series and modified 4 series give good results.

You are reducing area a little by tapering the chord at the bottom but this makes it more efficient so you can compensate by make a fraction longer. Say 9" instead of 8".

There is nothing stopping you taking the shaft all the way down to the bottom of the rudder. You will need to taper it slightly to match the reducing thinkness. Most rudders of this size that I have seen have the shaft moulded in. There are tabs welded on the shaft to stop the shaft spinning. You can have a metal strip welded along the bottom of the shaft to act a wear strip. The whole of the bottom could be a metal plate on edge.

Here is an example of moulding a shaft into a rudder:
http://www.fincofab.com/rudder_making.htm
It looks like this has a tube with bolts through it. The shaft does not go all the way down but it is tappering at the bottom. I prefer solid shaft with tabs welded front and back.

Rick W.

 

Thanks again Rick,

Dammmnnn. Excellent post. Picked-up a lot today... here is my thinking.

Taking the cord to 9" takes her to 2.66 AR. What do you think about increasing depth also to take it back to 3? Performance issues?

Actually I wanted the shaft to be only as deep as the keel because I wanted to consider either designing a kick up system or have them sheer off. It turns out that you're right that there is a lot more exposed area due to the shallow rocker. If they sheer off at the keel, then there will be a lot of area still left to skadoole to port. Still, as a challenge I want to air some ideas about them kicking up. It may be worth something at least to me.

What do you mean by metal plate on edge? Like vertical fins or is it a horizontal plate or both? That link provided was good info.

Thanks again...

J

 

By reducing the chord at the end you have increased AR anyhow. You can determine AR as the blade area difided by the maximum chord so at 9" chord and 2.66sq.ft the AR is close to 3.

The rudder can be made from a shaft with metal blades welded down the front and trailing edges. This is filled with an epoxy/glass bubble or epoxy/glass fibre bog and then wrapped in glass. Basically the glass only provides a faired surface. All the strength is in the metal. And all metal ideally SS.

They are not huge blades so they will not be too heavy to handle. Just real solid. The shaft tube needs to be strong enough so the shaft bends before the hull is damaged.

Also you cannot have the top of the rudder exactly following the rising line of the hull because it will drag against the hull when turned. This means the top edge will have clearance to the hull when in-line. The rudders do not need to turn more the plus or minus 40 degrees.

Richard Woods said he had a fool-proof folding system on his web site. I have not looked at it yet but it would be worth a look. My personal preference is to make rudders small area, powerful section but strong.

Rick W.

 

Ahoy there...

I said I would try to air out some ideas about an inboard kick up rudder and I played around a little bit with Auto Cad to see if there may be any merit in continuing. Well...for me it looks like it's a toss. It didn't take long to draft something up.

I couldnt save it the way I wanted so I copied it on to excell and hope everyone can view it. Basiacally, release arms designed to a certain load will release the top of the shaft when struck, it will pivot on lateral bracing connected to the hulls allowing the shaft to swing back into centerline cavities. The cavities will be left open and the sides will be built to a point above the waterline.

Turning the rudder on axis is accomplished by a bearing on the shaft at the pivot point by cabling with springs attached to absorb shock.

The rudder skeleton will be split in two with the lower half below the keel waterline for sheering them off but this is not required for the kick-up. None of it is to scale or calculated...it is freehand to find feasibility only.

Any thoughts?

Rick, I did look at Woods rudders. Very simple, effective and nice. I think his kick-up. Anyway, I wanted to air this out first and see if maybe there is any empirical or similar data.

Thanks
J

 

Hello all,

I tried a different concept to the kick up inboard rudders to make things a bit more simpler since the cavity in the hull along with all the other associated hardware is somewhat outside my initial scope to keep it simple.

Noone gave comment on the prior concept. Here is another concept to consider and possibly compare? Comment?

Basically, it is a standard inboard rudder. However, the rudder below the keel line is hinged so that it swings back when enough force strikes it. The hinges will be recessed and the whole mechanism will be encased in the glass just as the rest of the rudder in order to not disturb the flow. Given enough force it would sheer it off completely in order to save the rest of the rudder.

Update...the daggerboards have been incorporated into the design. Once I have some things worked out to operate them...I will be disclosing secret weapon #2...highly classified.

Does anyone have any information on my secret weapon #1...flight stick for steering? Has anyone done it before?

Thanks
J

 

Well....all I can say is that I must be behind the times....cos that rudder on your boat looks like a tooth pick. I guess I need to accept that times have changed LOL

 

I gotta trust the numbers or perhaps I'll have to remajor the rudders after some trials and just increase their size. I was looking at the program Rick produced them in and it's pretty consistent...I guess that could mean accurate.

Which of the two would you consider a better design principle to explore? Kick or swing back rudders?

I did a lot of work this time trying to get it to final stage. I'll post final stage soon. Then I'll do a final streamline and take a break.

Thanks
J

 

from Richard Woods site but could not find a fool proof folding rudder
nor was i all the way with him on rigs and more but ok,

semi balanced rudders for light steering
boats with daggerboards have lifting rudders
a wheel offers more power than a tiller but its
on boats ove 30' where it becomes personal choice
there are pros and cons to wheel, tillers (and joystick)
also consider emergency steering
autopilots are simple to use, reliable, draw very little power
and when passage making are almost certain to be used

not that easy to find info on cat rudders but darn interesting
there is also a rudder build in to a hinging rear end
and come to think of that, how about, like the old chinese junks,
sailing downwind have that "trunk" filled with a following sea
emptying that compartment as the wave rides by not burrying the bow
dont peticular like your halfrudder hinge but who am i, i'll have a break ;-)

 

JCD
I looked at your rudder ideas and neither appeal to me. The curve shown with your swing up does not follow the keel line so it means you need a 12" (or so) wide slot in the hulls below the waterline. This will add drag and possibly allow the rudders to ventillate so you may as well make them 3X bigger and hang them off the stern. The hinge in the rudder will cause turbulence so you need to go bigger for some steering.

You know my preference concerning the rudders. Beyond making them solid and simple I would invest money in reliable charts to suit a GPS with course deviation alarm and a reliable depth sounder with alarm. These would give me greater peace of mind than knowing the tip of the rudder will fold away if I happen to hit something.

Rick

 

Good Morning...

Yipster...good reference. I'll give things more thought.

If you are referring to the transom steps acting like a huge scoop that might hold tons of following seas...I looked at it and you're right. I will look at that and decide whether to cut off the inboard sides of the steps and exposing them. Very good point.

(Edit: I looked at the steps after post and it wouldn't look right if I chopped out the side. The alternative is to carry strongly battened heavy canvas that I can attach to the steps while underway and in a worse case situation, I can just lose way by pulling up the drougue from the stern and drop a sea anchor from the bow to bring her bows to the waves until things settle down.)

Redundancy for all systems including emergency steering has always been a part of my design process. The steering in this case has 3 different back ups.

Rick...I agree. I will give more thought along those lines. I don't think that walking to the beach in 2 feet of water would be a problem...unless I'm in the Bay of Fundy!

Thanks
J