Trying to design my own cat.

Rick

If an optimised fully submerged foil, and an optimised surface piercing foil (for my cat) both have the same steering force/lift).....do you know their difference in drag as a rough percentage? I'm wondering how much drag I will have with 2 surface piercing rudders and 2 surface piercing dagger boards. I want to know if ventilation is a big problem, or insignifcant compared to the overall drag of my boat. I have been following your posts in J's forum....I was just hoping I could get some perspective without having to get more software just yet

Thanks

- Richard

 

Answered on the other thread where you asked the question first.

You will see from Brian's video of his cat that these optimised hulls are a bit like trams on rails. They are not inclined to turn easily. Not the best for racing around buoys. A liitle rocker makes them easier to steer but increases hull drag.

I have a very strong preference for non-ventilating rudders. Dagger boards are not so critical as you want them to operate at a low angle of attack so the pressure is not as high. With the long deep hulls they may not be even warranted.

Like I recommended to JCD you should learn to operate JavaFoil. It helps you understand the physics of critical bits of a boat.

Rick W.

 

sorry Rick, I didnt think you would be that quick. I deleted the question on J's thread cos I posted it there by accident.

Thanks for your help

- Richard

 

Hello Mr. Woods...

I apologize I missed your post.
Perhaps you would consider using Freeship just to view and critique, or give advise on some of the designs being generated here without using it for your more technical work. It's free.

I don't think I posted any PDF files but I'll go back and check and change them if it will allow you to read them.

A quick note to bring you up to speed without having to read my whole thread. The SR34 was one of 3 designs which was chosen to be optimized for a final design. That work on that design has already started and it is no longer the SR34. It is now The Green Lantern.

I'm not trying to polish my own shoes...but I believe the design has good merits and warrants further development. I would be dishonest if I didn't state that many in this forum contributed to the design in their own small way. Some unique ideas will be incorporated which I believe may be firsts but won't know until someone can point out otherwise.

I don't want to hijack this thread so I will not attach it here but I have attached the most recent work at my thread with some pictures if you would like to peruse it.

Thanks
J

 

I am sure you have been following the IDEC trip. If you go to the website and view the video of IDEC crossing the equator you'll see the spray coming off the rudder as the main hull flies. Proves there is a LOT of aeration from a surface piercing rudder.

That is in part because a surface piercing rudder acts as a hull and creates its own wave train. Obviously it is going VERY fast, ie has a very high speed/length ratio.

A 1 1/2in (37mm) thick wall hollow stock rudder is fine up to say a 10m catamaran. A useful rule of thumb is that the rudder stock diameter in inches is 1/20 of the length in feet. So a 40ft cat has a 2in stock, a 30ft cat has a 11/2in stock etc.

If you look at my FAQs page you'll see my very simple, yet 100% reliable lifting semi balanced rudder system. Don't forget that the faster you go the more the C of E of the rudder moves aft and this can overbalance the rudder when at speed even though it is fine at lower speeds. Also that you won't actually turn a corner at 20 knots. So the rudder loads are lower than you might think.

I am sailing in Mexico for 4 weeks after Christmas so won't read this forum often. Also, unlike many others who design for a hobby, I have to make money from my design work. So although Freeship is free I'm not(LOL)

Happy holidays

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

Thanks Jamez

....and

thanks Richard Woods

(this thread is LOADED with great advice!)


- Richard

 

Does anyone know the best way to make the bottom of my hulls more beach resistant? Like some kind of lamination?

 

Plastics like PE offers very high wear and impact resistance. I've never thought of using them on a boat's hull but it may well be possible. They could be a pest since nothing glues to it either, so may have to be shaped and screwed in place

 

Richard
It is not unusual to run a rib along the keel to give the hull the ability to ride up on hard ground. This is solid glass and eventually the gel coat will get worn off - just gets repainted when antifouled. Timber boats sometimes have a metal wear strip screwed to the keel timber.

Rick W.

 

Put and old chris craft cutwater over the stem and 10' down the keel. CC used bronze and cromed it. really looks nice. the easist way to do is wax up your stem and keel then put 2 layers of fiberglass over and fill with epoxy. make it at least 6" wide on each side of center. Once dry peel it off and you have a perfect mold for any sheetmetal shop to make you one 1/8" SS is more than enough and you can polish instead or crome. Stan

 

i was going to try 1 mm lexan on the bottom because i intend to park on the beach everyday (heat formable lexan film is now available, about $10 per M2, run it along the hull before epoxying, use a heat gun and form it into shape, take it off and stick it on last. lexan when surface prepared has the same adhesion as wood)

also having small mini keels - ribs maybe 5cm tall, 5 m long, on a 35 ft cat.

some stick 300gm csm as the outer layer to protect the uni, for rocks, dings, and stuff, but lexan is incredibly stronger, (play with a test strip!) and i may well end up doing this.

cheers,
mal.

 

Well, here it is. Unless there is something seriously wrong (and someone please tell me if there is), I believe my cat is almost complete. The final testing of models and simulations etc. should not result in a fundamental change from this design. Just some balancing of the rig and rudders hopefully. It would be nice if there is enough lateral resistance to do without the dagger boards.

The diagonal bracing from stern to aft crossbeam prevents racking and allows the bridgdeck to have no annoying diagonal bracing. This allows me to have 4 double bed mattresses which are supported by trampolines (instead of my earlier 'shark cage' idea). Each bed is supported down the middle by a rope to prevent people from rolling together. The final result is similar to 8 hanging hammocks, except you can walk on them without a foot falling through.
It makes an excellent shock absorber during bridgdeck pounding in heavy seas. No stress on the hulls.....just a little stress perhaps on the 'sleeping' passengers.

The hulls will have internal struts/trusses which will make them incredibly strong. I can do this because I don't have to worry about passenger thoroughfare.....just many rubber sealed ports for luggage access. Even the mini folding dome tents will have rubber seals so the hulls are watertight during a bad storm. Not having to worry about sufficating in the hulls is a big bonus to me.

The sleeping tents will be brightly coloured and will let the light come through the fabric. When you combine this effect with soft squishy mattresses, it think it will create a really nice atmosphere. The walls of the tents can be folded completely open to let the air through and preserve the panoramic views.

The box in the middle can be stored behind the rear seats

I have spent far more time trying to imagine the social and atmospheric effects on various designs....than I have on structural efficiency. It is just lucky that my preferences made me end up with a more simple design.

Oh yes, and I have opted for a plain old single outboard motor on the back. The twin props dragging through the water doesn't appeal to me.


Any criticism or ideas would be great.

Cheers
- Richard

 

Richard
If I was paying USD150K for a sailing cat I would be hoping to get some solid accommodation.

You seem confident that you will only be sailing in ligh air. I cannot see how this will be possible. I don't see your matresses standing up to any sort of weather.

There are some big trailerable cats around 30ft that have the hulls sliding on the bridge beams or easily assembled onto the beams They still have large trampoline area spanning the hulls and useful accommodation in the hulls. Here is an example:
http://www.reynoldssailing.com/en/pricing/rs33.asp
Bigger than you want and well within your budget.

The fact that these hulls are longer than your design will make them more easily driven at typical cruising speed. So if you do not like the big rig then just reduce the sails to the level of excitement you are prepared to accept.

Rick W.

 

Hello Richard,

I think it's great. I like the idea of the tramps fore and aft of the mast. I'm doing that. Mt aft tramps I'm going to make structural and the fore will just be regular tramps.

The pic shows that you may have room for Daggers, but if you don't want them, you can go to lee boards or put a keel guard of say 3 inches and you can get some pointing ability. Turns may be wider but you'll have grip.

I hope the caribbean accommodations is fully waterproofed from top to bottom and everywhere in between because exposure is significant. This is the best for daysailers or consecutive fair weather sails. I would recommend that you spare no expense when it comes to these and consider survival or storm ratings.

I would probably look at the cross bracing again and make sure you have good safety factors incorporated. My eyes could be deceiving, but it looks light...ppppfffff, my rudders are tiny!

Great job. Stuck to the design idea. Plan to put out any numbers to look at?
Material List?

Thanks
J

 

More great ideas pouring in....thanks people.
I like the idea of using something like lexan, as Mal suggests, and maybe screwing it in place as Fanie was saying. 'Replacable' sounds good.