The Concept Catamaran Project

Thanks Bob!

Hi Bob -- attached is the modification to the sail rig you suggested. I think it looks much better -- I just hope it would not be a problem in rough weather. Thanks!

 

Cat 5

I' am just curious..... I looked at the underside of your cat design and saw that the hulls are asymmetric. Nothing wrong with that - I just wonder why you created a venturi effect between both hulls. Could you clarify that for me? I am not a cat man, but I like them nontheless....
D'ARTOIS

 

duane,
i just got back from hawaii where i saw a cat with a unique rig. they were using a 150 jib on a roller furler as a mainsail. they also had another 100 jib out fron on a roller too. The rig worked rather well to my surprise. it eliminated a boom. The owner wanted a in mast roller bu could not afford to do it. the main mast is still there but the only real function is to keep the top of the roller furler of both the main and the jib. another configuration could be worked out such as an A frame to do the same thing... maybe better. the foot is free. the roller furler for the main is attached to the bottom of the mast near the deck. have you ever seen such a rig?
JIM JENSEN

 

Hi D'Artois -- Interesting question regarding the venturi effect between asymmetrical catamaran hulls...

My guess is that the more wetted surface option inside and underneath the catamaran would create a sucking effect that would draw the boat down into the water -- thus causing more drag. If the asymmetrical hulls had more wetted surface on their outside sides -- like the old sport Hobie catamarans, they should act like foils and make the boat rise during forward movement -- causing less drag.

The latest version of the Cat-Five now uses symmetrical hulls merely for simplicity reasons -- primarily for ease of construction reasons. But if I find more data that would support the need for asymmetrical hulls, I may change the design again. What do you think is the better choice for catamaran hull design -- asymmetrical or symmetrical? And if so -- why? Also, do you believe the venturi effect would help or hinder the design?

 

Hi Jim -- I believe I've seen the sail rig you suggest. The mast is significantly placed aft from center to support the large genoa and jib sails and uses stays. Apparently the design works -- but the real purpose of using a Lateen rig on the Cat-Five is the small mast without any stays.

 

Breaking rudders and auxiliary power

Hi duane

Sorry for th long absence. Been busy. I like your enlarged sail rig. I think it is getting close to the right size (if not already there). There's two other things I want to suggest.

One is the rudder design. Jim wharram's "Rongo" catamaran had pretty much the same appendage set up as you have drawn. Rongo had deep V hulls and deep high aspect ratio rudders. Even though your "concept 5" shallow V hulls, It also has long shallow keels. This, I reason, is simular enough to Wharram's design to make his experience applicable. Rongo worked out quite well on her first voyage accept for one problem. The rudders kept breaking. The problem was finaly fixed when he replaced the deep narrow rudders with shallow wide ones. I, myself, have had my own exprience with a deep. efficient rudder and shallow inefficient hull. One day I tried to motor my light beamy centerboarder with the centerboard retracted. I was taking my freinds out on a Summer ride down the river. Once the outboard was started, so was the trouble. the boat refused to respond to the helm and we ended up crashing into the river bank. Once I disentangled myself from the over growth, I lowered the board. From then on things went smoothly.
On another day, I was sailing on a small lake that was divided up into several lobes that were connected by narrow mouths. I was sailing down wind without an engine when I entered a lobe that was shallow, weedy, and infested with tree stumps. The board hit a few then got etangled in the weeds. Now I was trapped. I had to sail up wind, so I needed the board, but the bottom was such that I couldn't use it. After several attempts to knock the weeds off with a paddle and boat hook and thread my way through the tree stumps(which I couldn't see), I realized that I was making zero progress. After several attemps and much cussing, I raised the board and folded the rudder back as well. To my absolute surprise, I started making headway. Eventually I cleared the mouth and I was headed home .
I thought this very strange that I had been able to make windward progress with no keel and a very low aspect ratio rudder. I figured that the weeds had helped prevent leeway. I decided one day to try to sail upwind with the board retracted and the rudder folded back in deeper (weed free) water. To my surprise, she did it again. She went to windward. Not very well, mind you (about 120 deg. tacks) but sufficient to get there in a pinch. When I lowered the rudder, she sailed dead down wind. And down wind only.
What I surmised from all of this was that shallow keels and deep rudders (I mean rudders that are significantly deeper than their keels) don't go well together.

Two is your auxiliary power scheme. I don't think you have enough sail area, even with the larger sail, to to use the electric outboards as generators. Not only that, but I can imagine them seriously upsetting the ballance of your rig and hull to the point of making it sail down wind only if you try. Let me suggest an alternative plan. Why not use solar cells and some of your good old Florida sunshine to do the job. I hear very soon there will be some very inexpensive solar cell material being introduced to the market. It won't be very efficient (only 10 watts per square yard), but it will be much cheaper per watt than what is presently available.
To supplement that, let me suggest carrying along a five kwatt gen set. This (along with a healthy diode system) can be used to top off the batteries or even directly power the outboards. Although some may argue it would be more efficient to use the engine powering the gen set to run the propellers directly, I feel that the flexibility of using the power where you want it, (say for motoring at one time and microwaving at another) makes up for this short comming. I would also suggest carrying along a good supply of fuel (at least 150 gal.s).

Bob

 

Hi Bob.

Thanks for giving me suggestions regarding potential improvements to the Cat-Five Concept Catamaran project. Suggestions with serious numerical data is what the project needs -- and you have done just that.

Regarding the depth of the rudders -- I believe you are right. Case in point is the swing keel I have on my 20' Mirage ( http://ourworld.cs.com/duanekmccullough/sstory2.htm ). I've had the boat for about two decades and I only sail it with the swing keel in the up position -- not only because I sail in very shallow water here in the Florida Keys, but also because it seems to point as good if not better than with it down. The rudder is also a swing rudder -- so the boat can sail to windward in only 18" of water. However, with the rudder up, the helm is harder to steer.

Freya -- the old 37' Skipjack I used to live on ( http://ourworld.cs.com/duanekmccullough/sstory1.htm ), had a broad rudder design that seem to work fine because it was married to the stern keel area and helped control the parted water along the full keel from slipping away -- thus allowing the boat to point rather nicely. From the deep V bow, much of the parted water would swirl up against the shallow keel and gave a lengthwise low pressure force to help the boat point without a centerboard. Unlike your light beamy sharpie without a full keel or use of the centerboard, Freya could actually point to windward pretty good -- as long as the seas weren't too rough. She only started to tilt in winds greater than 15kts.

It's my guess that my Skipjack -- like your Sharpie, used the sharp chine of their similar design when the wind picked up to "dig in" and keep the boat from slipping sideways. Again, Freya -- like the Mirage, had only 18" of draft -- which is remarkable for a 37' sailboat.

Perhaps the only real problem with a shallow rudder design would be the lack on contact with the water in rough swells. If the seam of the rudder was tilted backwards like on Wharram catamarans or old classic monohull sailboats, that would give more water contact in rough swells and help keep control of the steering -- but that concept would suggest a slanted transom to the Cat-Five design and give less buoyancy in the stern. I'll play with some views using the Anim8or CAD program and see what looks good.

Regarding the auxiliary power scheme, the electric outboards would only be capable of generating electric power in off wind situations. Solar cells would definitely be part of the power package of any cruising sailboat -- but ask anybody who used them, they just do not have the power generating capability of pushing a large boat any significant distance. Even if there existed new solar cell designs with double the power that covered the cabin top, it would be hard to depend on them for primary auxiliary power.

A 5 kwatt gen set would be about right for powering the electric motors when batteries can't meet the needs. Trying to keep highly flammable fuels at the minimum volume to power the gen set is a design challenge that will take some testing. 150 gallons is a target -- but I would prefer much less flammable materials on board.

Anyway, thanks again Bob. Like I said, I'll play with some views and see what looks good.

Duane

 

Don't know that I follow your description of the rig, but did it have any similarities to my mast-aft, twin headsailed rig at
www.runningtideyachts.com ??

Do you have any pictures?

Were you thinking of something like this for your A frame version?

 

Yes Brian -- your rig and others like it seem to be very unique in that they use simple roller-reefing technology for most of the sail power. Your design is even more unique because of the wishbone sail set aft of the mast. Nice catamaran design as well. My C5 catamaran design is obviously much simpler than yours -- just rying to keep cost down while providing for a capable cruising catamaran.

 

Duane,
So far the concept seems sound,although the logic behind the smaller sail due to no loose roach has me puzzled.I keep seeing my delta wing stunt kite with both the trailling edges vibrating and causing significant noise and enough vibration to feel at the end of 200 foot lines.Certainly generates alot of power when put at a reaching angle to the wind,1.6m2 sail area will pick me off the ground for seconds at a time,Im an overwieght 230 pounds.
K4s

 

More comments

Hi again Duane

You're right, of course, about the solar power. the thought I had in mind is that most boats, when they make it to paradise, tend to stay there. I mean they spend a lot of time in harbor. With the nice tropical sunshine beating down on them all day long. I was thinking that with some modest performance expectations, you cat could spend at least half of its underway time under sail alone. And the solar cells could account for maybe a quarter of your powering needs. This would leave only about 3/8ths of your underway power to the 5kwt genset. I guestimate that your 7.5 hp diesel genset would run about two hours on each gallon of diesel, so 150 gal. would get you about 300hrs operating time at the (hopeful) speed of 6kts. This would give you a maximum theoretical range of say 1500 nm under the deisel alone. Now assuming that that is only 3/8ths of your propulsion needs, this should give you a maximum range of approximately 4,000 nm (I subtracted about 15% for efficiency losses). Of course, If you were to find a really good wind on your voyage, such as a reliable trade wind, You may be able to go farther and faster than that.

As for the rudders, I think that it must be pointed out that catamarans are much different than your usual monohull. With a monohull the rudder is usually on the centerline of the plan veiw. That means that when the boat heels the rudder tends to not only be at an unfavorable angle for steering but also tends to be lifted out of the water as well. With a catamaran, the lee hull is pushed deeper and deeper into the water as the weight of the entire vessel gets shifted to that side due to the press of sail. This pushes the lee rudder deeper into the water as well (and at a much smaller tilting angle), so the ventelation that often happens with a monohull's rudder is a lot less likely to accure. And broad shallow rudders are less likely to stall than narrow deep ones. T. F. Jones built a catamaran very simular to the Wharram designs but with verical transoms. The boat had a few problems, but steering wasn't one of them.

Bob

PS: Although I very much admire the type, I have never owned a sharpi. Or even sailed on one.

 

Hi K4s,

I understand your feel for the power of the "anti-roach" delta wing kite. I built my first one over three decades ago for $3 and could reach estimated speeds of over 60 mph -- after flying it into and out of the water. I built small gliders out of coke cans using a penny as the payload and could do perfect loops in the air -- and then pluck it out of midair as it hovered in a steady wind. The waterproof coke can glider could also be seen gliding through a particle cloud in water and would show no wing-tip vortices.

Regarding the vibration and the flapping areas near the wing-tips of your kite, I too had some vibration -- but if the fabric fit is right with the right angle, it may not be so bad. One problem I had was that the kite would exceed the pull of the lines and went into gliding stall until it crashed. My brother bought me a radio controlled kite a few years ago -- but I never got around to modify it and waterproof the electronics to test here in the Florida Keys.

Anyway, thanks for your input. The subject of kite rig designs are applicable in discussing sail rig designs.

 

Hi Bob,

Your numerical data regarding power needs is very useful -- it will be use as projected data for the C5 catamaran with credit due. A wind generator could also help with the power needs -- however, they can have certain problems like noise, radio static noise, vibration, dealing with storm winds -- and bird strikes.

It would be nice if a water cooled 7.5 hp diesel genset could be isolated near the stern somewhere and be used sparingly. Nothing can be so annoying as an air cooled genset running on a calm day at anchor. Perhaps a genset system could take the shape of an outboard motor and be placed at a site near the stern.

The rudder data you provided regarding how catamaran rudders need not be so deep like monohulls makes sense. I will create a set of wider/shallower rudders and see how they look.

Thanks again Bob for your help -- I will post some changes asap. I have several projects running and will try to work on the C5 more this week.

 

Cat Design / Venturi Effect

First of all a happy and prosperous 2005 to all forum members

Duane, you could consider the following effects:

Doubtless that the a-symmetrical hulls of your design create a venturi effect.
The result of thsi effect is that the compressed volume between the floats will try to separate the hulls, forces to that effect. Another effect is that you create a resistance-factor between the narrowest point of the hull. The full effect of this will only be measurable in a test-basin or in reality.
I think it's not a good idea. The flow of water between the two floats should not be interrupted or modified.
Another point might be the fact that it might - I say might - have an negative influence on the steering caracteristics.
What I am writing is just a feeling, I am not a catamaran man, a cat is totally useless in our waters, not because of seagoing capacities but for practical reasons.
Another point has brought up by Mr Leask, who came forward with some very practical subjects. A lot he says is true. If you have no intentions to stay for a long time in the North Atlantic area, a Cat will be definately no 1 choice. The Cat is the boat for the southern hemisphere.
D'ARTOIS

 

Hi D'Artois,

What you say regarding the performance of the asymmetrical hulls of the earlier version of the Cat-Five catamaran (C5) seems true. The latest version of the concept catamaran are symmetrical in shape and should not have any significant "expanding or contracting" aspect regarding the flow of water between the hulls.

Someday, certain useful data may support the idea of a catamaran design that uses asymmetrical hulls -- because, together with some minimal foil assistance within the hull design to help lift or plane the boat out of the water and achieve better speeds due to less wetted surfaces, the asymmetrical catamaran design may yet be created. But, as I said, the C5 now uses symmetrical hulls to minimize possible problems that may arise in using improperly designed asymmetrical hulls.

However, I am concerned as to where the bow wakes might meet and "conspire" under the bridge-deck cabin at high speeds -- if they are too high, then there could be some unexpected constant contact with the wake and create extra drag. Some real water testing of a C5 model may help out in answering that potential problem.

Regarding the idea that catamaran designs are not very safe in large North Atlantic waves -- well, you may have a point there. All boat designs have a capsize threshold -- but monohull boats can be easier to recover from a full capsize than catamaran boat.