The Concept Catamaran Project
 

Hello fellow boat designers -- the following web link reveals an article I wrote entitled: The Concept Catamaran Project.

http://ourworld.cs.com/duanekmccullough/newcat.htm

Basically, I am seeking feedback on views presented in the WIP article. All constructive views are welcomed.

Comments or questions about the article should follow this post so everbody can read about any changes to the design. The attached image file is what the project looks like at this time -- but could change over time due to new concepts posted here at the forum.

Cat 5 update
 

Version 2 of the Concept Catamaran Project is ready for viewing. Using the Cat 5 design -- I have changed certain concepts from Version 1. The following points are the changes:

1. Better scaling. Version 1 sported a 36' design -- but after making proper headroom in the cabin, the bridgedeck cabin looked a little stubby -- so I stretched the whole catamaran to 48'. This design action not only allowed for more room for "visiting guest" on our liveaboard home, but the first 10' of the bows are designed to give better longitude stability and are basically empty of weight. The downside of a longer boat will cost more at dockside -- but true liveaboard sailors live "on the hook" where the mooring cost can be next to nothing.

2. Better bow freeboard to handle those big waves at high speed.

3. Material keel framework that attaches to the stern & stem -- and "deck keel". If the catamaran is made from wood or metal and not fiberglass, such a rigid frame would be necessary to give rigidity to the hulls. The widest distance between hull chine lines is no greater than 4' -- making the design composed from 4' by 8' sheets of marine plywood.

4. More detail to the kick-up rudder design. A steering shift cable can be used to move the tiller bar under a rear deck where a small tender craft (yet to be designed) could be stationed. An elevated steering station should be able to see a clear view of the desired heading -- and while standing, should be able to see the bow during docking.

5. A novel outboard electric motor design that could be raised when not in use. There is room for two of these motors that also could be used as water-generators to charge batteries.

6. Some forward support crossbeams that could hold a set of anchor stations and netting for what catamarans are known for -- lots of fun for laying or romping.

Not shown in the latest image is the bracket frame that binds the bridgedeck to the hulls. Interior layout is not fully designed at this time -- but should have a nice galley & at least two bath sites. A roomy center lounge and a computer/navigation station would be nice.

Feedback is welcomed.

Duane,

Is this a sailing cat or a power cat??? hard to tell, but it seems under-powered either way. Also, have a look at some of the authoritative texts on Aerodynamics, they may clarify a few points in you're article.

Tim B.

Thanks Tim for the feedback. The following response is almost the same I gave over at the SimFlight forum regarding reaction to my last post of this catamaran project.

The small sail is better for hard sea condition handling and stability. Perhaps the most common response I received from friends (and family) is that "the sail is too small" -- but there is a reason why it is "small" and I say so within the article.

If sail power were only measured by the square foot size -- yes, the Cat 5 Lateen sail plan is small when compared to conventional sloop or cutter rigs. However, because I believe the Lateen sail rig shapes the wind into a better torque driving device than a comparable conventional sail rig of the same size, the overall size can be made smaller than what may be assumed as the standard sail rig for a 48' sailboat.

I'm sure one could place a very large Lateen sail rig on the catamaran for racing purposes -- but long booms that measure over 24' would be hard to keep in shape in strong winds and over time they would probably also loose their integrity -- unless they are made from exotic material that could handle the changes.

A large sail rig is not fun to handle when it is very stormy. Many large rigs have sunk sailboats when they were not managed properly during "whitewater events".

I remember one day I took off on a tack in my 17' sailing canoe by myself with a Sunfish lateen rig and it was blowing over 20knts -- wow, I had to ride the entire tack to the other side of the sound because I could not come about without capsizing. The forces were just too strong -- so I built a smaller lateen rig with a "anti-roach" design in the sail and, over time, I notice that I could not only handle lots of wind, but never needed to reduce sail because the sail could not luff -- due to the shape of the sail. There is no sail area to luff in the arrow shape lateen rig like the one I designed.

So my point is that a sail that can't luff is a sail that doesn't need reefing -- as long as the sail plan is not too large for the boat in the first place.

I'm sure there would be light wind days when the skipper of a Cat 5 design wished he had the capability of flying more sail -- and I thought about adding a small liftable extension to the mast that could handle a jib or a small genoa sail, but the design started to get complicated and I decided not to go there.

Besides, on very light wind days the electric motor would help. And if the crew is on a time schedule and need to get somewhere fast on a low battery situation -- just run the onboard generator to power the electric motor.

BTW -- the concept of an electric outboard motor is somewhat experimental for a 48' catamaran. I'm not aware of one that large even exist -- but because I've used one on my 20' sailboat for over a decade with excellent results, perhaps the idea of creating a larger version for a larger boat is not too unrealistic.

Regarding your statement about the aerodynamics of my catamaran sailboat article, could you be more specific? Am I misunderstanding some aspect of wind-flow around a foil -- or something else?

Sail design of the Lateen sail
 

Over at the SimFlight forum a question came up regarding the sail design of the C5 catamaran. The following attachment is an explanation of the "anti-roach" concept and how it can improve the standard Lateen sail rig design.

keep going Duane i think thats looking sharp. your asking feedback so is that standing headroom in the deckhouse? perhaps rounding the edges a bit for streamline? having the house even bigger? like to see that bracket frame, what materials and more. anyway i like it, looks big enough, simple and quite atractive to my eye, ok, i first better read http://ourworld.cs.com/duanekmccullough/newcat.htm

headroom and more...
 

Thanks Yipster for the feedback -- the headroom in the deckhouse/bridgedeck is perhaps 6.5 feet according to the model. By creating several simple standing and sitting human models in the CAD program, I can adjust the interior to just the right height.

After all the main parameters are met, I will "smooth" the model with extra polygons and bevel some edges to make it less "edgy". That should streamline it some like you suggested.

The free Anim8or CAD program is a great tool for creating models and the Virtual Sailor watercraft simulator program is a wonderful program environment to water test the design -- check them out when you can.

Version 5.3
 

Well, here's another update on my Cat 5 concept catamaran project I started a few months ago. Changes are found in the following statements.

1. I raised the decks to give standing headroom in most of the hulls and to better allow for a greater clearance from wave action under the deckcabin that has a passageway to the hulls. This change added more freeboard/windage to the boat -- but I believe the benefits of a high dry deck in rough seas outweigh the cost of windage problems at anchor during a strong wind.

2. The deckcabin was modified by making it longer and now has two "keel frames" instead of one as before. "Keel frames" are support frames that help give longitude support -- both the deckcabin and the hulls have them and the design lends itself to the idea that the Cat 5 could be built out of marine plywood & boards.

3. The addition of a high steering station atop a rear deck bar frame that could use a modified tiller rod extension from the tiller bar to steer. A teleflex steering cable could also be attached in such a way as to allow steering from the high steering station.

4. Although not a change -- but just to note that the large adjustable electric outboard "trolling motor" could be doubled if needed -- and also could be used as a water-generator underway.

Interesting Stuff...
 

Hello...

At work just now - will blush when I get paid...

Very interesting 'stuff'...

For those interested see http://hangsim.com/vs/ ...

Reminds me of 'the good old days' of Interstate 76 and 82 by Activision - open code and lots of great idea developments...

What I would like to see is some monster deep vees in Virtual Sailor with rocket launchers and a couple of 20 mm cannons on them - that would be some fun...

Cheers...

SH.

center cockpit
 

i think your design could be improved if you would consider a center cokpit rather than the raised area at the rear. the only reason sailboats have a rear cockpit is the tiller... if you change to wheel steering the placement can be anywhere... a center or front cockpit puts the helmsman up front where he can see better, keeps the center of gravity lower, keeps the person near the rigging, and drier? maybe/ inside 2nd sreering station out of weather...
a rear area can be used for deck lounge and toys...
alos i recommend you widen the boat a bit... 48 ft long x 24 wide...
also the most recent issue of multhull has a article on electric generation using a sterling engine... have you read it?
jim

Thanks Jim
 

Hi Jim -- and thanks for your feedback.

Your suggestion about a center cockpit on a sailboat does have certain advantages. Dividing a catamaran bridge-deck into efficient working parts has many options -- however, not including the forward lightweight deck area, there seems to be three aspects found on modern catamaran designs.

First, the forward enclosed cabin area is followed by the second aspect of a main cockpit and steering station out of the wind & wave action with perhaps some stern space to keep a tender on davits. And third, a growing design aspect on some new catamarans is the secondary steering station atop of the main cockpit with better visibility.

A problem can arise in the third aspect if the space of the secondary steering station interferes with either the rigging movement or the easy access space into the forward cabin from the main cockpit. That's why I relocated the secondary steering station of the Cat 5 design atop the stern bar support, which has a good steering view in fair weather and does not consume access space from the main cockpit into the cabin -- or interfere with the rigging movement.

If the steering station was just a place to steer a boat, placing remote controlled steering wheels or even joysticks anywhere onboard that has a good view of the course would work -- but the steering station aboard a boat is also place to gather with friends around an area in fair or bad weather while controlling the boat.

Because of this concept -- and your suggestion, I will review the existing Cat 5 design and see if the upper rear steering station could be consolidated with the main steering station near the main cabin entrance with some possibility of enclosing the area in bad weather. The stern bar support could be left for electrical generation space like solar panels or wind generators.

BTW -- wind generators can be annoying and dangerous, but at anchor on a cloudy windy day, do perform necessary power generation needs on an electric liveaboard.

Regarding the sterling engine and its electric generation capabilities -- I'll have to read the article in Multihull Magazine you suggested. It's sounds interesting.

And regarding the width or beam of the Cat 5, like I've suggested in the article, the proportion formula of 5/8 is being followed. If this formula is not followed, perhaps the strength of the design may be compromised. For every foot gained in width, more cost and stress is added to the project.

Again, thanks for your input on this project.

C5 Update...
 

I helped assembled my brothers 27' Stiletto catamaran last week and learned a few interesting aspects that may help in the design of my Cat-5 concept catamaran project. The latest update includes the following:

1* Simplified bridge-deck cabin design that fits better to the hulls and allows for a forward trampoline area.

2* Steering station placed inside cabin near main hatch/door with rear deck cover.

3* Slightly larger sail rig should help on light wind days.

The boat is looking more like a motor-sailor with a pair of electric outboards. STI (Solomon Technologies Inc.) is working on a new 20hp electric motor design that may someday be placed within an outboard/shaft casing. If doubled, such a concept could give a serious 40hp push to a 48' catamaran.

Cat-Five update
 

I was hoping to have the Cat-Five design finished before Christmas -- but while trying to add and polish some details, the modeling has taken longer than I expected. Trying to design a 48' catamaran that could be made from a wide range of composite flat surface making materials is not as easy as I thought.

In fitting the major parts together, I feel like a CAD carpenter wherein the parts need to be interlaced in a way that needs to be simple and realistically cost effective during fabrication and assembly.

The following design changes and additions are the latest improvements.

1* The hulls have been rebuilt again and each half side is now symmetrical.

2* The Cabin-deck has been reconfigured again with more space and better support to the mast. Further work is needed to mount it to a bracket that links it to the hulls.

3* The addition of two bulkheads per hull now divide the hulls into three compartments.

4* Support for a dingy lifting station near the transom links to the rear bar and deck roof.

Perhaps next month the Cat-Five may be ready for some virtual sea trials in the Virtual Sailor program. Stay tuned...

cool cat
 

Hi duane

With my best guestimates, your proposed catamaran would displace at least 10,000 lbs or roughly 150 cubic feet. And my best guestimates on your rig indicate less than 140 square feet. This with your high house and hull windage may not be enough to make windward progress even in a strong wind. I would suggest increasing your spar lengths by about 30%. That would nean adding about six feet to both the boom and the yard. This would give you about 210 sft of sail. Still a modest amount of sail but, hopefully, enough to do the trick.

By doing this, you could also cut down on your engine power. Maybe you could get away with as little as 1 hp per ton. That would move you along at at least six knots in a dead calm. As it breased up, you would need the help of the sail. But, with a sail as modest and easy to deploy as this one, I think that would be little trouble except in a high wind dockin situation. How much fuel do you plan to carry?

Other than that, I think its a really neat design. What I like most about it is that it defies the trend of multihulls being designed primarily for speed. The rig has a classical look that would give a polynesian joy.

Bob

Thanks Bob -- it's feedback like yours that the Cat-Five project needs.

Regarding the sail size -- you are probably right about how hard it may be able to sail to windward with such a small rig and all that freeboard. I was hoping that a full keel with a little stern fin would help on a windward tack based on the experience of my old 37' Skipjack design wherein it drew only 18" -- but had a similar keel and could sail to windward pretty good. I will increase it to your specs asap and see how that looks.

I've avoided making the Lateen rig larger because a very large rig could be hard to handle in strong winds, but based on the unique "anti-roach" layout of the sail, the sail may be "self-reefing" -- which suggest even in a high wind docking situation, the sail should not luff significantly because there is no extra sail along the leech line to flap as the sail is aimed into the wind.

I'm sure the rig design would catch the eye of any sailor -- Polynesian sailor included. The Lateen rig is the very rig design that started windward sailing -- it has a lot of history in the concept. I could talk for sometime regarding why I chose it for the rig of the Cat-Five.

Regarding the motor needs of the Cat-Five, in choosing two electric outboard motors as propulsion and charging units (that haven't even been built yet) is a radical concept indeed. Any fuel onboard would be needed to run the generator system that would charge the battery bank during motoring on no wind days. The exact science in making this concept work efficiently is still in a state of flux -- but the trend is to go electric propulsion these days. In the meantime, a drop down fossil fuel outboard may have to do.

Anyway, thanks again Bob for your help. I will try to accommodate your suggestions.

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.

Cat Project
 

I am more curious what your relative short floats are going to do when you pick up speed. If you combine that with a choppy sea with waves of 5 - 6 ft you might sail them under from time to time. Your cat becomes a wavepiercer by then. Have you taken in consideration that shallow seas like the Carib, can be pretty choppy from time to time? If your floats are strong enough, ok, but your pilothouse, is that one sufficiently reinforced to take braking waves? I had a look at the design and the height of your deckhouse is considerable and relatively close to the bows, where you have very little lift. Your bows therefore, might go quickly under in shallow water.
A couple of years ago I raced an ex AC from one of the northern Islands to IJmuiden seaport. The boat went with 9 knots on a windward course head on in a number of fallovers caused by current-against-wind conditions. I knew that something was coming, that part of the Northsea is infamous for those seas, and I had everything closed and locked. The bow went under, I went under (Loa boat 45ft) and after what looked liked eternity, the bow rose slowly, although I thought it was not going to happen.
Just an example....... :?:

Hi D'Artois,

When I update the boat this week, I will slide the bridge-deck cabin back a little -- that may help ease off on the forward weight some. I believe there is sufficient buoyancy in the high freeboard hulls forward of center. It would be important not to load the forward hulls with weight for the reason you mentioned. I'm aware of the need for forward buoyancy -- I've only crashed a cat twice in over 30 years of sailing them. Once when a 14' Hobie Wave pich-poled in heavy seas while sailing very fast -- and while flying a hull on an old classic Hobie 14', I "fell backwards" after I ran into soft mud. But I have never capsized a catamaran. I have come very close and have rescued many who have capsized, but so far, I've been careful and/or lucky.

The bridge-deck cabin is not attached yet to the hulls pending the bracket design that would marry the three parts together. I'm still working on the bracket design -- it should allow for a strong flexible link between hulls and the bridge-deck that could be taken apart during overland transit or storage of the C5.

Like my brother's 27' Stiletto catamaran, there are two large telescoping cross beams located aft and center beam that the bridge-deck will enclose. A forward support beam should link the bows together. Working on it...

Asymmmetrical Hull Shapes
 

I would encourage anyone interested in asymmetrical hulls to go find a Prindle catamaran (P-16, or preferably the P-18 asymmetrical model), turn it upside down and do an inspection. You will find a VERY interesting hull shape, and particularly the exaggerated toed-in keel line. The P-18 had even better load carrying capabilities at her stern, and better anti-pitch characteristics.

Then keep in mind that these asymmetrical hull designs were really intended to operate in a one-hull-up attitude to gain their best performance.....not exactly a cruising vessel attitude. Aside, there was also a tunning method that permited the rudder blades themselves come into play as leeway reducers.




It's the really fat load carrying hulls that 'pile' up this peaky wave under the bridgedeck. Either stretch things out a bit to give a better slenderness ratio, and/or create a wave-splitter foil down the center. Wave splitter; First, imagine a flat plate, on edge, mounted down the centerline on the underside of the bridge deck. This flat plate will act as a rib to strengthen the fore-to-aft rigidity of the vessel, a somewhat weaker characteristic in a catamaran structure vs. a keeled monohull. If a tow bundle (rope, etc) of carbon fiber (kevlar, PBO, etc) was laid along the bottom edge of this flat plate, the rigidity could be even greater (sort of akin to a bottom truss structure, or a flange of an 'I' beam). The front of this nacelle/plate could be configured to act as a wave splitter to actually attack, up front, the formation of those peaky waves under the tramp areas that eventually slap at our bridge deck underside. We kind of slice those waves down a bit. A lightweight fairing might also be added to this 'flat plate nacelle' so it appears outwardly much more esthetically pleasing, as well as more curvature to shed those peaky waves."

C5 update
 

Hi Brian,

Interesting that you suggested a Wave Splitter stringer device under the bridgedeck cabin -- I put one as a cabin-floor stringer that meets the mast base yesterday. It could also have a small forward "shoe plate" shaped like an arrow "I-beam" to divide any oncoming wave or bow wake reaching the bridgedeck. Like most all catamaran designs, some kind of exterior cable support will be needed to strengthen the middle and forward crossbeams.

Regarding the asymmetrical hulls of the Prindle catamaran -- you are right, they can provide better load capabilities in their stern while flying a hull. Trying to achieve the same foil control while sailing flat is still a challenge in cruising catamarans. The C5 does have a nearly flat bottom at the stern that may help it to plane somewhat.

Anyway -- attached is the latest version of the C5. I do not show the bulkheads and there is much design work to do on the brackets that unite the hulls with the bridgedeck. Designing a small sizable waterproof entryway from the bridgedeck to the hulls is proving quite the challenge. If the C5 was the typical large fixed catamaran design, it would be easier to construct the crossbeam supports -- but I believe it would be better if the design accommodated the concept of overland transportability -- which needs to be modular and yet strong.

Hi Duane, have you considered employing chamfer panels between the bridge deck and inboard hull sides? They offer some nice benefits including strength. An interesting article on them can be found at http://www.graingerdesigns.com.au/considerations.php
They may help soften the inner hull to bridgedeck aesthetics. The ones in the sample pic attached only took a day to cut and fit to our 40ft cat.

Hi Craig,

The Australian Grainger catamarans look very nice -- years of building experience shows in their designs.

If I understand the chamfer panel concept right, it seems as if it represents a bevel bracket that mounts to both the bridgedeck and the hulls -- which is exactly what the C5 needs at this stage in development.

The article you posted suggested that two types where used in the Alfresco 1060's -- the straight bevel version and a curved version, with better performance from the straight version. If so, then a straight bevel bracket could be created and used that may have a forward angled extension to add angle strength to the concept and help carry the shape of the bridgedeck bottom to the hulls. The bridgedeck cabin topsides should link the top of the hull "keel frames" to the cabin frames as shown in the latest C5 image.

If each bracket were shaped like an "U" while looking forward, they could bolt to both the bridgedeck and the hulls with the nuts protected inside the cabin. The sides of the hulls facing each other would need to be flat to accept the strait longitudinal bracket -- perhaps special faring panels placed between the curvature of each hull and the bracket could make the concept fit snug enough to work properly. The fairing panels would look something like the chamfer panels you posted in your sample pic -- but would be slightly smaller and act more like a "washer" between the hull and the stronger bracket that bolts to the cabin frame.

Back in '81, as part of a crew while sailing a 62' Peter Spronk catamaran (http://ourworld.cs.com/duanekmccullough/sstory3.htm) from St. Maarten to the Bahamas, I noticed the boat overtook some big seas and the foam filler used to fare the link between the bridgedeck and hulls began to break out -- which suggested that the area does take serious stress and should be reinforced.

The C5 design could be made from several types of materials -- including lumber and plywood. I know fiberglass and other composite materials are preferred for large production boats because of the durability of the materials -- but it would be neat to see a large modern cruising catamaran design have the option of using wood or similar fiber material as a primary building material. Moreover, to have this design marketed as plans so that backyard boat builders around the world could afford to safely enter the fast growing cruising catamaran market would also be neat.

The aforementioned 62' Peter Spronk catamaran was "woven together" using African mahogany plywood and the W.E.S.T. system. I remember the veneers were crossed in a hex-system and were very strong. I wish I knew the whereabouts of Spronk's boat and could see how it has faired over time.

Anyway, thanks Craig for your input -- a modification of your chamfer panel concept in linking the bridgedeck with the hulls should help the C5 design.

Proportions of hulls?
 

Hi Duane

Me again. Just stopped by to ask you what the proportions of your hulls are.

1.) How wide are they? At the chine? At the shear?

2.) How high are they? From the keelson to the chine? From the chine to the shear?

The reason I ask is that I am intrigued enough with this design concept to want to run it though PLYBOATS c to see what kind of numbers I get displacement, wetted surface, hull surface, and bouancy wise.

My guess bestimates are that the displcement will be around six metric (or long) tons. I would really like to see how close to the mark they are.

Thanks

Bob

Hull data
 

Hi Bob,

Attached to this reply is the hull layout graphic data of the current C5. The free Anim8or CAD program is great -- but it doesn't have measurement data tools. No problem though -- I just created a reference grid to measure objects, making sure I kept the scale between the objects equal during the copy process.

I hope this data can help your PLYBOATS test -- I'm also curious as to the data. Thank you for helping out on this project.

The exact total width/beam measurement of the C5 may still fluctuate depending on what numerical data seems best. A wider beam value will help in banking stability -- but at a performance cost to pitching stability I believe.

Again -- thanks!

PLYBOATS c Cat 5
 

Hi Duane

Me again. I drew as best I could your Cat 5 hull design using Plyboats. Here's what I came up with:

Length 47ft
Beam 5ft 1in
draft 1ft 8in
Freeboard M 5ft 10in
Freeboard B 5ft 4in
Displ. 7,000lbs
PPI 793lbs
CP 0.56

My guess is that with proper scantlings each hull sould wiegh around 3,000lbs.
Your cross beams, wing deck, and cabin will probably come in at another 2,000 lbs, leaving you around 6,000lbs to play with for your engine(s), rig, fuel, water, and stores which is probably not a lot for a 47 footer.

I thought a little about your latteral area. Since the 'V's of your hulls are so shallow (1 to 4) they shouldn't be counted. I have written a spread sheet for this purpose. It takes into account that long shallow keels are not as efficient as short deep ones. It also takes into account profile area. According to my spread sheet, a 30ft long keel would have to project 11 inches below each hull. This, I hope will give you the abillity to sail 100 degree tacks (50 degrees off the wind). I would start this keel about 2.5ft forward of your transom.
It can be pointed out that such keels will add considerable wetted area to your boat. This is true. But it is also true that such keels will bring your draft to a little (1.5in) deeper than originally specified. And it would have no moving parts. And no cases to project into the hulls or wing cabin. I would further like to point out that such keels could be copper clad and not need bottom paint that can be scraped off during groundings. The rest of your bottom could then have bottom paint which would be easy to apply when the tide ran out because the entire bottom would be exposed and easy to get to.
Such keels could also protect your rudders and propellers from floatsom and groundings.
A center board case could be placed underneath your wing cabin to do the same job. And it wouldn't have to be above the water. But it would have to be long and well braced. It could easily add several hundred pounds to your weight and increase your sailing draft. And much of the wieght will be above the waterline (the case, the bracing, and at least 1/3rd of the board itself). And it would not protect your rudders or propellers. Even so, It may allow you to sail closer to the wind (45 dgrees rather than 50) and the wetted area would be gone once it was cranked up.

Such are the compromises of boat design ;).

I also thought about your mechanical propulsion. I thought of the idea of puting a single diesel in just one hull. It could turn a feathering propeller through use of a sail drive unit. That way the engine could be tucked away in the stern of a hull (the choice of which would be based on propeller rotation), close enough to get to, but far enough to be out of the way. It would be about 12hp and would usually run at 1/3rd to 1/2 throttle. It would be belted to a 90 to 150 amp alternator to provide most of your ship board electrical needs. Because of your small sail area, it would usually be running.

You originally showed 'electrical outboards' that resembled large trolling motors. I liked the idea but thought they looked vulnerable to wave damage and looked heavy as well. Also, in order to transmit electrical power over any distance, higher voltage is needed. Otherwise, the power losses due to impedance can be substntial.
With some ingenuity, I am sure the problems can be overcome. It might be an interesting idea to put an electric motor in the other hull. This way, short movements about the harbor (as well as changing tacks) could be done without starting the diesel.

Bob

Hi Bob,

First, thank you for your work in translating the graphical data into numerical data. I appreciate your participation in this Concept Catamaran Project.

Everything you suggested to improve the C5 makes sense -- particularly the idea of compromising the mechanical propulsion system from a total electrical outboard system to a split inboard diesel and electrical system. The inboard electrical motor could also have the option of being a generator under off wind sailing situations using the Electricwheel motor system from Solomon Technologies.

Some may say that the technology of going full electric propulsion is possible -- but since I have no real experience in working with large conceptual electric outboard motor generator and propulsion units as imaged in the current C5 project, I am open to your idea of changing the design. Perhaps on a smaller catamaran design, the idea of twin electric outboard motors with proper flexible shafts would be better. The C5 started out as a 36' catamaran, but grew another eleven feet to add offshore stability and headroom in each hull.

Regarding the modification to the keel -- I see no problem with making the changes you suggested. The original keel shoe is designed to add strips of 2" by 4" material like wood or other more durable material onto the keel frame. Coating these strips with copper should be easy enough. The only problem with full strait keels would be the lack of a pivot point from which to rotate while tacking to windward. Also -- without the ability to backwind a jibsail, coming about could be problematic on a large catamaran.

Speaking of pivot points from which to rotate while tacking to windward, perhaps a "wave splitter" centerboard design like in Brian's catamaran project could be created that would help the C5's ability to come about -- and help it point to windward better.
Brian has some great concepts in his designs -- the challenge is to project strength to the centerboard system without obtrusive spars or cables that could add drag in high wave action.

My brother's Stiletto uses stainless steel support spars that project to a support beam from which the daggerboard rest on. These support spars are apparently designed to collapse if the catamaran runs hard aground -- he just had them fixed during a recent refit. These support spars do cause drag in heavy seas, but also provide adequate support to the daggerboard under the bridgedeck. In the C5 case, if there was centerboard instead of a daggerboard, then perhaps the design could be better supported with angled supports to the pivot point of the board. Finding the proper size and placement of the centerboard underneath the bridgedeck on the C5 may need some aeronautical math -- or I could try some guesswork for now.

My 20' Mirage sloop uses a "swing keel" that when the board is down a small area of the board rises up into a small trunk -- perhaps a similar design can be used under the bridgedeck of the C5 project. I will play the concept this week and see what may look feasible.

I had to slightly flatten the sides of the basic symmetrical hull form of the C5 last week to accommodate a flat surface so a mounting bracket between the bridgedeck and hulls would fit better. If the bridgedeck were permanently mounted to the hulls, then gluing a simple triangular bracket would work -- but because the C5 is designed for transportability in mind, a removable bracket is needed instead. I will show the design change soon.

In fact, I will show all the above changes in the next update soon. Again -- thanks a lot Bob for your help.