The Concept Catamaran Project

Lateral resistance

Hi Duane.

I think it is OK to plan on using the engine(s) to help come about. You can always wear ship (jibe) if that fails.

However, depending on the engine(s) to make up for lack of lateral resistance is, I think, a bad plan. That would mean that you would not be able to make any windward headway at all without your power. I can see where this can lead to trouble in fighting off a lee shore. Imagine your deisel quitting first then your batteries running out an hour or so later. Then you would be on the beach. And I hope its sandy.

It is my opinion that an under powered boat (less than 5hp per ton sustainable and maybe more with this one, considering its high sides and ample windage) must be able to fight its way to windward under sail alone in a blow. To do this, I estimate that you need at least 12sft of effective lateral area. And that means an aspect ratio of at least 1 to 1 (H to W). You can go to a lower aspect ratio (less H to W) only by increasing the lateral area substantially. How much so, I really don't know, but I've made some guesses.

I think you need either atleast one effective board or two effective shoal keels with this particular hull shape.
the alternative is to go with deeper 'V' hulls (at least 1 H to 1 W) such as with Jim Wharram's cats.
The main advantage to this is that the boat would sink deeper and therefore have less windage. The down side to this is that the lateral area would be even less effective than your present design with even shoal keels. But it may be good enough. My estimate is that if you went with this plan, your boat would sink down to a draft of almost 3ft and each hull would have a 3ft WLB. This would cost you considerable internal voume in the hulls unless you went with a step in your hull section.
If you went with the shoal keel idea and extended the keels the entire waterline length of 42ft (based on my guessed formula), you would need 6in aditional draft for a total of 2ft 2 in. draft. This would let you keep your 5ft 1in WLB and alow you to stow heavier stores lower down giving you maybe a lower center of gravity as well as considerably more living space.

Bob

 

Version 22 of the C5

Hi Bob,

You are so right about the necessary ability for a sailboat to climb to windward off a lee shore in a blow -- so I have tried to accommodate your suggestions by compromising the depth and angle of the hull bottoms. A 51' catamaran that draws almost 3 feet with nearly a full keel should perform much better than what was.

The hulls are the same length as before -- I just included the overall length that includes the rudder attachments. I lowered the keel frame and added another layer to the keel shoe. I also trimmed the keel shoe so that the boat could rotate better during a tack. A full keel and shoe would, I believe, restrict the rotation -- but I'm not sure. If you have any input as to how you would trim the keel shoe, I'm open for more suggestions.

As the included illustration states, the C5 now draws 2.9' (about 30") with almost the same 5.1' width of each hull. Basically, as in most planning hulls, the bow parts the water vertically and foils it to the stern where it departs horizontally. Most of the hull top and bottom midsection surface areas are nearly parallel to accommodate the strait lines of the bridgedeck and keel frame.

If the C5 were built out of fiberglass, there would be a smooth fairing between hull and keel, but if made from flat forming material like plywood & lumber, or other similar material, then the surfaces of the hull & keel would have more of a sharper seam -- particularly near the stern area where the keel would act like a fin against leeway movement.

To keep the impedance problem down between the batteries and the electric outboard motors, the primary batteries would be located low in the hulls where the weight would be better -- while a smaller battery station just above the motors would make a shorter distance to the workload and be more efficient I believe.

Anyway Bob, if I misunderstood something you suggested in improving the C5 design -- please feel free to tell me. You seem to have a lot of sea sense and your aeronautical knowledge is impressive.

Soon, this project will be ready to launch in the Virtual Sailor program and we all can see how the C5 may look in virtual seas. Check out my VS link at:
http://ourworld.cs.com/duanekmccullough/vs1.htm

 

inproved

when I first saw the vessel in the virtual sailor support forum it was a screenshot he asked what do people think i urged him to build it and i tested in the simulation the boat handled really well on reading the threads some comments were pretty sharp tongued but these have been very beneficial and this boat wouldnt have improved without them i hope to see her built in real life :!:

 

Redesign estimates

Hi Duane

I've just got back from my plyboats program.

After looking at your new graphic, I estimated that you added about six inches to the deadrise (the 'V' part of the hull section) which sinks the boat down the same amount giving you an estimated draft of 2ft for just the hull.
In your text, you said the new hull would have a total draft of 2.9ft. That leaves 0.9ft or 10.8in for the keel which is very close to 11in.

If the keels are half as long as the waterline, which is what they appear to be in your graphic, that should be close enough (according to my spread sheet).

By increasing the deadrise, though, you have increased the design displacement. The old displacement was approx. 6mt. The new displacement is in the order of 7.5mt (total of both hulls) which I think is OK. 6mt seems to be a bit aneorexic for a 50 footer any way, even for a multihull.

It is intersting to note that the Cat 5 is simular in displacement to its spiritual ancester, John C. Hanna's 'Tahiti Ketch', which weighed in at around 19000 lbs or 8.5mt. Even though John's boat was 18ft shorter and a mono, it had pretty much the same design goals; limited sail area and long range under power.

These are the new design specs as close as I can determin them:

Length 47ft (w/o rudder)
Beam Hull 5.08ft
Draft 2.9ft
Displ. 17100lbs
D/L 104
SA (approx.) 300sft
SD (approx.) 7.2

Bob

 

Thanks a lot Bob -- your math work and comments are very helpful in this project. I believe you have helped me and others better understand important aspects in hull design.

I will add a couple of more vertical divisions in the model hull stern area that should better define the curvature of the lines like what was done recently near the bow area.
Trying to make nice curves with strait lines is a real challenge.

The basics of the new design specs are good for this phase of the project -- now for some basic work within the bridgedeck and hulls. I will use your "Length -- 47' (w/o rudder)" statement for the project because it seems more appropriate than the 51' overall value I mentioned earlier. Anything to save a few dollars in docking fees at the marina.

In fact, one of the main concepts of the C5 catamaran project is the option of saving money when compared to other similar catamaran designs. Using a smaller simple efficient sail rig is just one affordable aspect of this project that could help make the C5 a reality someday. Going safe and relatively slow for a multihull is the way to go.

BTW -- I've been aboard a Tahiti Ketch and it did look somewhat underpowered by sail, but very seaworthy in stature. The double ended wooden ketch seemed heavy for its size and was capable of offshore long distance cruising -- in fact, the owner "Tahiti Mike" commissioned me to do a painting on driftwood of the boat back in 1980. I will have to read about John C. Hanna's adventures aboard his boat when I can.

Thanks Bob!

 

v23

Hi Bob,

OK -- a bit of change to the C5 today...

After studying the profile, I realized that the length of the sail booms measured nearly 34 feet -- which is not the easy sail rig one person could handle in rough weather. So, by dividing the total sail area into two sail stations, the sail energy should not only be easier to handle, but it gave the opportunity to increased the total sail area from 300 sqft to 375 sqft -- or 25% more energy.

The main sail booms now measure at about 26' or 75% of the original length -- and the rear sail booms measure at about 17' or 50% of the original main boom. This sail plan design change also gave the C5 the ability to throttle sail energy -- in very rough weather the rear sail could be raised on the main mast.

The only trick I need to do now is to create a support station for the main mast forward of the bridgedeck cabin. I'm working on it...

 

your problem

you probebly should put two beams stretching from each hull they should be situated were the sails are and placed diagonaly

 

Sail design change

Hi Duane

I am particularly pleased that you increased the sail area. With the boat getting ever more 'healthy' the 300sft sail area seemed to be getting more and more anemic.

There are, however, (as usual ) some suggestions that I would like to make.

1.)That you move the larger sail aft. The reason I suggest this is because multis seem to like their Center of Efforts (CEs) further aft. The reason most cited for this by experienced multi designers is that, along with a forward push, there is usually a downward thrust from the sails (at least on the lee hull). This downward thrust tends to push the relatively narrow bow down and thereby move the the Center of Lateral Area (CLA) forward. This in return can cause the boat to round up into the wind. On a mono, this is not as bad as on a multi. On a multi that is sailing fast, this can lead to disasterous increases in apparent wind which can lead to capsize. In a mono, if you are overwhelmed by a squall, the usual tactic is to round up into the wind. In a multi, the usual tactic is to do the opposite; for the reason cited above.

Also, on a mono, as the boat heals, the CE moves to the lee while the CLA moves to windward. This causes a huge turning moment into the wind. The design cure is to move either the CE forward or the CLA aft or some combination of the two. What this does is create a 'leed' where the CE is forward, and sometimes considerably so (up to 15% of the WL), of the CLA. The idea is to create a counter turning moment that will cancel out the windward one and lead to a 'ballanced boat'.

Because a multi heels so little (less than 1/3rd as much, in the Cat 5 case) there is very little movement of the CE to leeward. And because, as a multi heels, its lee hull goes deeper, the CLA moves to leeward (even further) as well. For this reason, there is practically no windward turning moment to cancel out, so there is practically no need for any leed.

I know these parragraghs seem to contradict one another, but I am assured that they are the fruit of long and sometimes bitter experience.

For a boat with as small of sail area as the Cat 5, the harm in having the CE further forward than usual is probably cosiderably less and maybe even negligable. That being the case, It would still be necessary to move the CLA forward for the boat to properly ballance. And that would mean moving your keels forward (which could be bad for running with bare poles in a survival situation)

2.) That you use bipod masts. There are two reasons I suggest this.

The first being that the lateral thrust would be downward on the lee side of the mast and that the mast would be more in compression than in bending. And the compression could be on or near the lee hull. For this reason, you could dispence with heavy cross beams in way of the masts (you would need a cross tie, though to keep the hulls from spreading apart under the masts).

The second being that that is a way to both avoid chafe (the sail rubbing against the mast) and to move the CE of the sail closer to the pivot point. The latter would be useful in avoiding damage due to panic jibes which are very likely to be needed from time to time with a boat such as this.

Bipod masts tend to be avoided in conventional (gaff and jib headed rigs) because, on those, the mast(s) extend to the top of the sails and sometimes considerably beyond. Being that a bipod is actually two masts side by side, the weight penalty aloft tends to be prohibitive.

On the Cat 5, the masts don't go any where near as high. And there is yet another advantage. That is that it would be much easier to design these masts so they can be lowered (even in strong cross winds) to cut down on windage, top hamper, and get under low bridges. Even if the bridge cabin prevents them frome being lowered all the way.

I hope these comments prove helpful.

Bob

 

changes...

Hi Bob,

Your comments are helpful in this catamaran project and I'm sure others are learning important design concepts as we explore the options.

I may have discovered even more sail area available -- based on your original estimate of approximately 300 sqft, I assume you made a grid of the sail profile as imaged in my drawings. However, the sail image is illustrated under a load -- which if imaged without a load fully sideways, more sail area should be calculated.

After splitting the original sail rig into two rigs and making some adjustments the square foot value was increased to 375 sqft -- but, based on new data views, one can add about 15% more value and take it to about 431 sqft.

Regarding the idea of moving the main sail rig station more aft, your reasoning makes sense and I moved the main mast back as far as possible without conflicting with the aft sail rig station -- which is only about two feet. If you think it needs further placement aft, I will have to make some value changes to the sail rigs or, like you suggested, move the keels a little forward.

Regarding the bipod mast arrangement you suggested, it seems like you are suggesting a sail rig similar to the old Aqua-Cat -- which is not as simple or efficient as the Lateen Rig as chosen for the C5. The bipole mast and top boom of the Aqua-Cat sail rig are fixed with the loose-footed sail that does chafe against the A-frame bipod mast in off-wind travel.

I have played with the idea of using two mast on each hull with a cross bar that would synchronize boom movement. This concept would add much to the sail area -- but would be more complicated than the simple mono mast arrangement. I like to keep the design as simple as possible -- so I created, as illustrated in the attachment, a version of the mast base that uses support brackets which could allow for a tabernacle arrangement. The idea is to build a strong support base between the hulls for a Lateen Rig.

The bipole main mast concept is not out of the question -- I just haven't found a good working version that beats the simple Lateen Rig.

While searching the Internet about John's Tahiti Ketch story you recently mentioned in a post, I came across the following link with several statements regarding the history of the Lateen Rig -- check it out when you can.

http://www.seaways.com/subarc99/spar.htm

Thanks for you helpful comments.

Duane

 

'Blow of the eye' sail area calculation

Hi Duane

Read your last post. In answer to your question, I calculated the spar lengths by comparing their length to the length of the profile. I then guestimated the angle between the boom and the yard. Then I went to ny note book Where I had drawn boomed lateen sails of various boom yard angles. Each one has a coefficient which I call the LSS or the longest spar squared. By squareing the length of the spar on a proposed design then multiplying by that coefficient, I get the approx. area of the sail. I guessed that that your spar lengths were 31ft and that your boom yard angle was 45deg. I also asumed that the spars were of equal length. Given this data, I multiplied it by 0.35 which is my LSS coefficient for a 45deg. lateen with equal length spars. That gave me an approx. area of 336sft. Having done that, I then guestimated axial width of the leach cut out. It looked like 30% of the axial line (or the imaginary line which starts at the boom and yard junction and ends at the leach at such an angle ast to divide the sail into two equal parts) I then divided that 30% by two and got 15%. I then subtracted that 15% from 100% to get 85%. I then multiplied my original area amount by that number to get an estimated area of 286sft.

Now that you have given the boom yard lengths, I want to ask you what the boom yard angle is.
I also want to know the width of the triangle that you cut out of the leach. And are the two sails just different size versions with the same proportions?

With this imformation, I will not only be able to better calculate the sail area, but be able to better guestimate the proper fore and aft keel placement. The keels may not need to be moved forward as far as I once thought they would.

By the way, Your mast step looks strong enough side to side but looks very weak fore and aft. What's it made of?

Hope to see your next post soon.

My best.

Bob

 

New sail data

Hello Bob,

Well, I discovered some new data regarding the sail area of the C5. After creating a visual sail grid based on the square foot measurement, a much smaller value of about 315 square feet is all I can squeeze out of the sail profile of the C5 -- which is, as you said, somewhat anemic for a 47' sailboat.

However, if a unique sail rig like the turbo-lift Lateen sail rig of the C5 can create a special torque energy while sailing upwind based on a concept of foiling wind energy without making atmospheric vortex wakes that cause drag, perhaps it may make enough energy to compensate for its small size. I could show a version of the C5 with a double set of Lateen sail rigs -- one for each hull, if you like which would double the sail area. But, like I said earlier, that version seems complicated and I would prefer the simple rig as is.

The following attachment should answer some of your questions -- such as the 60 degree yard angel between booms and the 61.8 value of the mizzen sail rig as compared to the mainsail rig. The mizzen sail rig has the same proportions as the main -- but measures at 61.8 percent the value of the main. The number 61.8 represents the unique mathematical "Golden Section" value found in nature -- check into this concept when you can.

Your question as to what the mast step is made of is a good one -- in fact, it leads to the question as to what the whole C5 is made of. To be honest -- I do not know yet. The mast step could be made from metal -- such as aluminum or some kind of laminate metal/plastic bonded by heat or glue. Through bolts would tie it together.

I have tried to form the C5 design so that it could be assembled from flat laminated material such as wood, metal or plastic sheets. The thickness has yet to be determined -- but should be thick enough to be shaped by a small amount of bending and yet not too heavy as a building material. Low maintenance, strength and cost should be considered when choosing this material. Compatibility between building materials are also important. Any suggestions would be welcomed.

Stay tuned for more C5 data -- and thanks again for your help.

Duane

 

Cat 5 rig particulars

Hi Duane

I just got done drawing a representation of the Cat 5 sail. Here is what I found:

LSS = 0.35
VCE = 0.23
LCE = 0.46

Which means:

Your main is: 26*26*0.35 or 236 sft
Your Mizz. is: 17*17*0.35 or 101 sft
for a total of 337sft total

The LCE of your rig is 18.7ft

from the front of the main boom with 6in between your main and mizzen booms.

The VCE of your rig 5.37ft

LSS = Longest Spar Squared
VCE = Vertical Center of Effort
LCE = Longitudal Center of Effort

Hopefully, I've given you enough information , so you can play with your rig a little bit by changing the spar lengths and by placing the rig on the boat.

I stand by my original recomendation of moving the larger sail aft. If you did that your rig LCE would be: 23ft

Or 4.3ft further aft which means you could step the masts 4.3ft further forward on the deck. That way, if you lined up the front of the forsail boom with front of the waterline, the rig LCE would end up about 1ft aft the half Waterline Length which I think would be good and not leave you with a lot of boom hanging over your transoms.

I also got a brainstorm on how to step the masts. The idea was to step the masts in the hulls with each mast steped toward the inside side of each hull.

As for hull construction material, I would suggest good old plywood sheathed in epoxy saturated fiberglass.

By for now.

Bob

 

C5v25

Hi Bob,

I moved the main mast & sail back to the front edge of the Bridgedeck and it does look better now. I also created a forward deck walkway to the anchor post and moved the electric outboards further aft a little. I'm still shuffling some rear deck objects to better organize the aft area.

And yes, perhaps the W.E.S.T. system with plywood and SS fasteners are a good choice of building material for the C5. See the attached image for more details and comments.

Again -- thanks for your help!

Duane

 

C5v26

Just a quick change to the bridgedeck sides from last night's update.

 

Hi Duane.

I see you have moved th rig back quite a bit. It looks like the main boom-yard juncture is about 15ft aft the bow tops. By 'blow of the eye' (with absolutely no mathematical data to back it up ) it looks about right. I can see why you wanted the smaller sail aft. With it the other way, a Mast would have to be sticking right through the cabin.

The masts, however, will have to be much more substancial in their girth. The Cat 5 will have about forty ton feet of max righting moment. My best guestimate of the VCE is about 16ft. So the load on the rig would be about 2.5 tons at its center of effort. Dividing that by the sail area of 337sft gives you a little more than 16lbs per sft. The good news is your sails will probably blow out before your boat capsizes. And thats only if the spars are strong enough to out hold the sails. Most likely, the Cat 5 will rarely or never be reefed and the spars will all have to relect that fact.

The Masts are so short that weight will not be so much an issue with them. They could be heavy aluminum tubes. The yard and booms would, I guess, be made out of carbon fiber. Their diameters I can't even guess right now.

On another note, I once took my Super Snark out on a windy day on my dad's small lake. The wind must have been blowing at least in the mid twenties if not the low thirties. The first course, of course was down wind. The boat was going so fast, it was throwing a rooster tail. The tiller bent in my hand (and is to this day, a degree or three off). Talk about a wild ride .
The 'fun' part was getting back. The 55sft sail had no reef points. However, it was cut absolutely flat. And I took full advantage of that fact. I feathered it all the way back home. And never capsized. (but did ship a lot of water). After numerous 'bailing breaks' and more tacks than I can count, I finally got her back home.
Through the whole experience, the sails did a lot of fluttering, but nothing compared to a more conventional rig.

Perhaps the boomed lateen, flat cut, is the 'perfect sail' for an under rigged boat.

My Best

Bob