Catamaran Evolution

Hello Rick,

My current rudder design is 20 knots at full perpendicular regardless if the rudder spins on axis to a lesser angle. I would be worried to balance the rudder beyond 15% area...it could cause a lot of fluttering. I don't think I'm brave enough to beach her on the rudders. I won't mind getting a little wet walking in 2 feet at low tide to keep her from touching bottom.

Do you have any knowlege of anyone having pinned inboard rudders so that they can kick up? I think I read something...but just don't recollect.

I thought that the 15% foil would have been the better foil after all the looking I did, but you blew that out of the water in 51 seconds, so I'm not going to prolong the pain. I drew it out yesterday on paper and it was very thick for the chord. Remarkably thick. Yep...I will need the ordinates, so I can draft it up in FreeShip.

Wanted to ask you a question about the optimized hull. Is the program able to generate the most optimized hull for any draft and displacement or does it just use one and the other is the result? For example...if I wanted a 34 foot waterline with a displacement of 8000#'s could I get it at any draft or only at the draft that the program considers optimal?

I ask because I was thinking that maybe the hull can carry the same optimum calculations a couple of inches above the waterline for payload without losing its optimized characteristics.

Thank's
J

 

In terms of priority, I place rudders very high on my list. Solid buoyancy is my #1 priority. Rudders are not far behind.

My view is to have them simple and solid.

I have experience with stern hung tilt-up and drop-in rudders but I have not seen through-hull rudders with a tilting pivot.

You can provide taper on the bottom leading edge of the blade so it will ride up on something solid. If each rudder has a solid 1.5" shaft they will survive a decent hit. They wil be more than capable of lifting the hull.

Again JavaFoil can produce a text file for the rudder coordinates.

The optimised hull shape for extra displacement will be different. You need to decide on constraints and have Godzilla do its business. You then check the hull for different load cases. Godzilla can do compromises for two speeds but not for different displacement.

Rick W.

 

yes i like that stick idea and yes you read me well but had it allready figgered out, keep going and venting

 

Hello Rick,

Really??? Riding up something solid and capable of lifting the hull???

The pivot point is more than 16 feet away in a hull displacing 3.5 tons with significant bouyancy countering submersion at the other end. Holy mackerel. I'm not sure I can design enough stiffening to keep the rudders from punching through the hull. Wouldn't it be better to design the rudders so that they sheer off instead?

Oooohhhhh...too brave for me. I'll walk onto the beach.

J

 

Thanks Yipster...

Motivation = locomotion. I will keep going and venting and asking.

Has anyone used my idea about the fighter stick and the functions? Anybody have any cons they can identify? What about with the cable system? Too crude and brute?

J

 

If the rudders are below the line of the hull then my view is that they need to be designed for the boat to be beached on hard ground with each rudder supporting about 25% of the total displacement. This is not a big ask when it should at least handle a side moment of around 1200ftlbf.

As I was writing this I heard this news report:
http://news.smh.com.au/aussie-transtasman-kayakers-in-danger/20071218-1hw0.html
People often take rudders for granted. Boats are next to useless without them. It is not that difficult to build the required strength around the rudder shaft.

Rick W.

 

They need to pull in closer to anchor, take the slack and tie off on the cleat and then cut away the loop to eliminate the strain on the rudder. In the morning...clear the fouled rudder. Ahem...what happened to the back-up anchor?

J

 

They have been try to get closer to land for about 10 days now!!!!
http://www.crossingtheditch.com.au/
I don't think many small boats carry more than one sea anchor.

The point I was making is that exposed appendages have a habit of collecting things. They need to be designed to take a bump. You do not want to have rudders that shear off or fold away when they are needed most.

Rick W.

 

Rick, if they fold away you can just use the engines until you have repositioned the rudder(s) ?

 

Richard
The sea can be unforgiving. Things tend to go wrong when stressed and this is often at the worst possible time. You do not want to be thinking about starting engines to regain control.

I am sure the things could be engineered but it adds complexity and cost. Just make robust rudders and mounts.

Rick W.

 

If I was sailing in conditions like that, and my rudder hit a rock, I would jump overboard and cling to the rock

 

Richard you are missing the point. It is in rough to extreme conditions when loads go up. If you have puny rudders with folding mechanisms they are most likely to break when doing 20kts down the face of a wave - just the time when you need maximum control. If you broach in the trough of a steep wave you will be very likely to roll.

Running downwind in the conditions in the earlier photo would see a light, easily driven cat hitting 20kts under mast alone.

This may seem far fetched but it can happen more readily than you might think. For example if you need to cross a bar to take shelter in a quiet harbour. Even a relatively deep water entry to a harbour will have steeper waves that will push an easly driven boat over 20kts. Here you could be in a narrow channel with no room or time to allow for steering failure.

The rudders need to be strong. Supporting the weight of the boat is unlikely to be the most demanding design case. Ultimately you want the shaft to fail before they rip the hull apart so the shaft tube needs to be a solid part of the hull.

The 1.5" shaft I suggested to JCD, and supported by Richard Woods, is a substantial shaft and it needs to be. It will not collapse under 25% of the boat weight.

Have a look at the history of open water yacht races and see how many boats have withdrawn through rudder problems. I am certain it is a substantial number indicating that it is an area that is underdesigned. Googling "yacht rudder failure" gave 33,000 hits. I am confident it is not the same story repeated that many times. As a matter of interest it might surprise you how often ships lose steering for one reason or another. Many harbour incidents are due to steering failures.

One other thing is that the steering force produced by an outboard motor mounted in each hull of a cat is substantially lower than the force that a decent rudder will generate once the boat has any speed. Unless you design the boat as a motor-sailor forget the motors as a reliable means of steering or even making way. They are there for when there is no wind or you want to get into a tight mooring.

Rick W.

 

One other thing I have known to happen is the top edge of a rudder punching through the hull when the rudder hit a log and the shaft bent backwards at the hull. It was only a small boat but it highlights another issue to consider.

Rick W.

 

Thanks Rick. I've had no experience sailing anything larger than 15 feet (and I have only ever sailed around a set of buoys). The points you raised are things I haven't thought of.
I sailed a P class when I was young. They tend to nose dive running in strong winds, and when the rudder comes out....it's all over. Most of the time I could manage it by stacking out over the stern.
I guess I have to lose some of my "she'll be right" attitude if I am going to sail a boat that won't give me a second chance.
I'd like to think that I am good at navigating around rocks and sand bars etc.....but...
the thought of flying down a big wave and 'flick' there goes the rudders.....yikes.....no thanks!!
I'll go for sturdy rudders

Thanks again

- Richard

 

Richard
Big boats behave much the same as smaller boats just that the tolerable range of conditions is wider and when they come to grief it is often more spectacular and way more consequences.

Rick W.