Sock it to me..........

[b_rodwell]

This is a preliminary design for a cruising catamaran. It is published to elicit constructive criticism (and maybe praise) from participants of this forum. Let me know what you like and what you don’t like. Let me know where I should do more study.

Thanks.

THE BRIEF

A fast world cruising catamaran for a husband and wife crew:

Spacious live aboard accommodation for two with lots of light and the ability to see out,

Some accommodation for occasional overnight guests,

Suitable for running day charters to supplement the cruising kitty,

Able to be handled by one person.


KEY FEATURES

· Ketch rig and aft extended cabin roof

The ketch rig sailplan is to split up the sail area so that the largest sail is smaller and easier to handle. It also allows a comparatively small working jib which can be doused completely as the wind increases. This leaves the boat sailing with two fully battened and more easily controlled sails.

The obvious extras for this rig are, from front to back, an asymmetric spinnaker, a roller furling genoa/reacher and a mizzen staysail.

The cabin roof is extended firstly to the location normally occupied by a targa bar and then some more. The additional length is to provide a surface to stand on when working on the mizzen sail. It would also be used to provide davits to hoist the tender.

· Raised working cockpit

This is a half bridgedeck height working cockpit and is only on one side of the boat. I am quite comfortable steering from a windward or leeward position and I don’t like steering from a position on the centreline of the boat. I can’t see the point of having 2 external steering stations on a cruising boat. The offset position of this cockpit also allows a more open connection between the cabin and the rear bridgedeck area.

It should not be difficult to lead all sail and engine controls to this area and perhaps even reef and unreef both main and mizzen from here. There are no ropes or working of the boat in the lower rear bridgedeck area. This makes this area more comfortable for people in that area.

As the design is developed this cockpit will get a small windshield and a removable softtop for better protection in inclement weather.

· Single centreboard and kick back rudder

I expect to go aground from time to time and would like to do as little damage as possible when this happens. The hulls will also have a shallow skeg. This skeg will also assist when drying out by using the tide.

There is only one centreboard and the rudder blades are retracting as well as kick back on grounding.

· Full width bridgedeck cabin:

This opens up the interior and facilitates the design of easily used stairs from the bridgedeck level to the lower level in the hulls. Note that there is one centreboard so no deck access for handling daggerboards is required.

· 270 degree plus vision from cabin

I just like lots of vision. It is noted that this will behave like a greenhouse in hot conditions. It will be mandatory to have awnings over the windows. These awnings will need to be able to be used at anchor or while sailing.

· Raised (1’) cabin table and seats so that sightlines are still available when seated

Also the rear bridgedeck area is all raised 1 foot.

· Stair access and door to forward deck from bridgedeck cabin

This gives an alternative way to get to the forward deck and facilitates cabin ventilation in tropical conditions.

· Canted hulls

Primarily because it gives the boat a more unified look. It ties the hulls to the bridgedeck cabin and therefore to each other. Also reduces weight to some extent.

· The hull to bridgedeck step:

Is 2 feet above the loaded waterline so should not contribute significantly to slamming.
Gives more flexibility in the layout in the hulls and more available space in the hulls.


Despite the 70’ waterline length, there has been no attempt to add additional berths. There is nothing in the ‘ends’.



A FEW STATISTICS……….


LOA at waterline 70 feet
LOA (prodder to end mizzen boom) 88 feet
BOA 34.8 feet
Hull width at W/L 4.4 feet
Bridgedeck Clearance 4 feet
Beam to Length Ratio 15.9
Draft Hulls 2 feet Rudders 3.7 feet Centre board (1) 8.7 feet
Height Main Mast Mizzen Mast 73/82 feet65/77 feet
Wetted Surface Area(exc CB & rudder) 704 sq ft
Loaded Displacement 14.9 tons
Prismatic Coefficient 0.594
Centre of Displacement 39.2 ft from cutwater51%
Centre of Waterplane 41.2 ft from cutwater54%
Sail Area – jib Main Mizzen Total 382 sq ft 1030 sq ft 776 sq ft 2188 sq ft
Bruce No 1.45



SOME KEY INFLUENCES IN APPROACH

These are supplied as an acknowledgment of the source of some of the features and so that the reviewer may, if desired, review the discussion for the features in more detail. None of these sources have contributed directly to the design at this stage.


DASHEW (Offshore Cruising Encyclopedia and www.setsail.com)

Longer boats can be easier to handle
Ketch rig with widely separated mainsail and mizzen
Large roach on main and mizzen


FARRIER http://www.f-boat.com/pages/catamarans/index.html

Rudder design
Building approach (later)


SHUTTLEWORTH http://www.steamradio.com/JSYD/Articles/S70.html

The importance of minimising sharp edges to reduce windage
Use of a single centreboard


CATANA CATAMARANS http://www.catana.com/ (and originally, I think, CROWTHER http://www.crowther.com.au/)

The canted hulls
The hull to bridgedeck step



SOME INTERETING NEXT STEPS

· Doing some analysis to see if:

I can avoid having a conventional front beam. This adds weight, windage and tripping resistance when submerged in extreme conditions,

It is cost effective to use diesel/electric power.



A FEW QUESTIONS

How deep can you make the forefoot compared to the maximum hull draft? What happens when it is ‘too’ deep?

What are ‘good’ values for prismatic coefficient, centre of displacement and centre of waterplane? Are there theoretical or pragmatic rules about the relationship between the centre of displacement and the centre of waterplane?

What are the calculations to work out required centreboard and rudder area (the areas shown are a simple scaling from a Kurt Hughes design)?

How can the cabin windows be made sufficiently strong? 2 feet of vertical height becomes 3 feet high at the slanted front.


DRAWING

A DWF drawing are loaded onto the immediately following message.


MORE PICTURES

Some additional renderings to clarify the proposed design have been loaded onto my members gallery (http://www.boatdesign.net/gallery/showgallery.php?thumb=1&stype=2&si=b_rodwell&cat=500&perpage=12&sort=1&user=338).

[b_rodwell]

and the drawing...........


_____________________________
Brian

[b_rodwell]

I was only able to view the drawing as published by first downloading the free WHIP! viewer from autodesk (http://www.autodesk.com/cgi-bin/whipreg.pl)
and saving the DWG file attached to the above message.

Clicking on the DWG file invokes WHIP! and allows zooming and panning. Otherwise it is not readable.

[BrettM]

Pics look OK to me, but I am concerned about the handling of the yacht with only one centreboard & rudder, Cats are hard enough to tack at any time and the contribution by these boards is significant. As far as damage goes. Most times I have run aground in my boats (Nacras etc) it has been on the one board.

Be careful with offshore boats and removing the forward beam. Remember the Team phillips incident...

Good luck
Brett

PS what cad program are you using.:)

[Willallison]

I've only 2 comments to make - I'm sure I could come up with more, but given my lack of expertise in all things with sticks up the middle of them (ie masts...) I'd be making the others up.

For a cruising boat - whether power or sail, I'd always incorporate an internal helm station. You may have already done so, but I can't tell from the CAD images. Every day doesn't bring sunshine and warm weather and when you're living aboard, the novelty of wind in the hair and spray in face would soon wear off. Whilst a canvas enclosure might help, there's no substitue for sitting inside with the heater going....

Also, I've noticed that most accomplished designers of cats tend to incorporate closer to 1 metre of bridgedeck clearance to avoid slamming. Again, given that it is a cruising boat, you're hardly likely to be flying one hull to often, and two foot waves aren't exactly rare.

Other than that, I like the images - an interesting and attractive boat.....

[b_rodwell]

Brett

There are two rudders but only one centreboard. I wonder whether going from two dagger/centreboards to one makes any difference in the ability to tack?

I concurr with your comments about hull strength and the Team Philips type failure. If there is no significant forebeam then one hull must be able to take all the force of dropping sideways off a wave. I haven't done the strength calculations yet but I have anticipated a very substantial "beam" which is shown in the structures rendering (at just the point where Team Philips failed).

The design was developed 3D in Rhino and the shadings and rendering were directly from Rhino. All the drawing lines were derived from the Rhino objects. I used silhouette function for the layouts and the contour function for the sections. In both cases the results were projected to a construction plane. The sections were rotated so that they were in the same 2D plane. The results were exported as DWG and taken up in Autocad where the text was added.

Will

An internal helm station is intended. I will probably move the navigation desk to face forward at the same time.

The bridgedeck clearance is mostly 4'; it is only on the hull/bridgedeck steps that it is 2'.

Have a look at Derek Kelsall's website www.kelsall.com.
He has no front crossbeams on stock designs up to 57ft.

[trouty]

The main limitation I see as a world cruising yacht - is.

It's not self righting - tip it over, the cruise and the yacht are history.

Cats have a LOT of benefits in terms of stability, space and reduced heel angle - BUT they sacrifice self righting to achieve it over a monohul.

If it was a world cruiser - I'd go mono for the factor of a rollover not being the end of everything!.

The ability to kedge it off any reefs / shoals sandbanks etc is a lot less with the Cat than the mono hull also.

Now - with that out the way the room for visitors and chartering for extra travel funds along the way.

Most states of Aus and many other nations have STRINGENT charter regulations, requiring a host of qualifcations on the part of the skipper, not to mention public liability cover and of course a fully Dept Of Transport surveyed & licensed vessel.

Unless your going to build this and put it thru full passenger survey (which will probably double it's construction cost) - then forget the charter idea....is my advice.

The fines for illegal charters?

$100K for first offence with forfeiture of vessel to the crown!

Those of us with Charter licenses have substantial investments in those vessels and licenses and can only protect those asset values by ensuring the relevant authorities get "information" fed to them about illegal charters and illegal charter operators.

It's just not worth the risk to illegal charter and the difficulties of obtaining the relevant licenses for every state and nation thru which you pass are logistically probably insurmountable and would cost more than the cat your contemplating building!

Many states "Charter industries" are now effectively "closed shops" with the only way to get a charter license to buy an existing license. I.e. you can't just apply to the relevant govt dept - pay a fee and presto - it don't work like that - you had to be in the industry prior to industry benchmarking date (Sept 97) and apply for and meet all the requirements to be granted one of the limited licenses available.

Charter licenses are in effect like Lobster Licenses - worth a LOT of $ and not likely to change hands unless there is substantial outlay of cold hard folding greenstuff!

Multiply that equation for every state of Oz and probably every other nation you travel thru - and the idea of a little extra travel funds from the occasional casual charter will dissapear out the window pretty quick.

If it's a world crusing yacht then be realistic and expect that in order to be able to cruise your way around the world, you'll have to be able to afford to pay your own way round the world.

I'm not trying to shatter a world cruise dream, just pointing out the cold hard facts so that the dreams not predicated upon a romantic notion of what is and isn't achievable.

Be a shame to build a cat like that and donate it to the govt or have to buy it back at a seizures auction - don't you think?

Cheers!

[BrettM]

Yes, I believe that that asymetry of only one centreboard will cause problems in a cat and differences between either tack. A reasonable amount of lift is derived from the boards during a tack. In the case of a cat tacking one hull is likely to be moving far slower than the other (or stopped). We all know that tacking without a centreboard is not real easy.

Is this thing to be built?

[lockhughes]

Originally posted by trouty
The main limitation I see as a world cruising yacht - is.
It's not self righting - tip it over, the cruise and the yacht are history. Cats have a LOT of benefits in terms of stability, space and reduced heel angle - BUT they sacrifice self righting to achieve it over a monohul. If it was a world cruiser - I'd go mono for the factor of a rollover not being the end of everything!.
Trouty. Multi's are even more stable upside down than they are right-side up. Going from the crash and dinn of a roaring blustery day to the calm of an overturned multi is a rare pleasure. They make excellent survival rafts, with built-in access to the hulls while up-side-down... They don't go to the bottom, which is why you may have heard some horrific survival stories - dead men tell no tales. You figure no method of self-righting a (stickless) multi?

Originally posted by trouty The ability to kedge it off any reefs / shoals sandbanks etc is a lot less with the Cat than the mono hull also.

Boy have I been misleading folks... my understanding (just odds and ends heard/read) that the one of the things the Carib charter industry learned about multis was that they *could* be towed off the reef and repaired. The monos tend to lay over on their sides (`cause they gots keels eh? ) and thrash themselves to death in the surf.

Interesting to hear about the state of the charter biz in oz

Originally posted by trouty If it's a world crusing yacht then be realistic and expect that in order to be able to cruise your way around the world, you'll have to be able to afford to pay your own way round the world.


I have heard that the Carib is littered with boats whose electrics are in various states of disrepair, lots of non-functioning stuff. Guess my point would be, that the successful cruisers I have read about are "on the cheap". I would hope the boat and her systems will be kept as simple as possible.

Lock

(a small boat sailor with almost zero cruising experience)

[lockhughes]

I would be remiss not to suggest that the boat have a MEG driving electric motors, for propulsion, etc <smile>
L

[lockhughes]

Originally posted by b_rodwell
This is a preliminary design for a CRUISING catamaran.

Brian. I added the emphasis to CRUISING in the quote... about the centerboard? You already have the skegs. Maybe go asymetrical with the hulls (a la Hobie14), but try and ditch the centerboard and trunk? Just more work. Would you really be giving up THAT much performance?

· Canted hulls

I'll have to go look this up <smile> no clue what this is/means!

It is cost effective to use diesel/electric power.

Please consider adding Solomon Technologies to your KEY INFLUENCES IN APPROACH.... check out the multi's on their web site that have "gone electric".

A FEW QUESTIONS
What are ‘good’ values for prismatic coefficient, centre of displacement and centre of waterplane? Are there theoretical or pragmatic rules about the relationship between the centre of displacement and the centre of waterplane?
holycrap-I know so little. anyway, sorry... *might* be able to help. Sounds like you might understand what these gentlemen are talking about:
http://www.maths.adelaide.edu.au/Applied/llazausk/hydro/multi.htm

Cheers
Lock
http://groups.yahoo.com/group/QCYCTender/

[trouty]

Hmmm,

OK. Self righting

My comments stand - a survival raft, isn't a "world cruise" IMHO :D
Even a self righted monohul dismasted in a rollover, can use a spar or something to jury rig a small sail - and cruise on...to the nearest port (albeit slowly) for repairs

Cats upside down are - just that - upside down and likely to stay that way - spending the rest of the supposed cruise climbin thru a hull trapdoor into an upside down water filled cabin waitin to be rescued doesn't sould like a lotta "world cruisin" fun to me. Sounds more like a raft...:eek: Oh sorry - you already said that! ;) :D :p

I'll take it we agree there then! :)


Kedging!

Ahh - color me dumb, buy how do you heel that multi to get the keels off a reef, Lock?

I can tell ya how to do it in a mono, coz I bin there n dun that - but methinks if the keels are stuck on a cat - well it's stuck and barring a VERY high tide or some explosive to shift the reef - thats where the multi is gunna stay.

With the mono, simply use ya zodiac, to run a anchor out to clear water, then come back, undo a mast stay wire from the gunnel end and join a long rope & attach the long end to the zodiac & use the zodiac, to heel the yacht (the bit a multi won't do) by leveraging from the top of the mast, and yes they heel right over - the keels now clear of the reef and the yachts windlass winches her to clear water where the anchor was placed. :idea: :p

Without a VERY large tow vessel, AND a hi tide, that muti's gunna still be parked on terra firma until waves do break her up or someone can get to you in a big enough vessel to help TOW it off probably taking the keels with it....and sinking it in the process.

I say it again, as a world cruiser a self righting single hull would be my preferred choice.

Not sayin cats aren't fun nor got a lot of room but as a SOLO world cruiser (ie not accompanied by other potential rescue read tow vessel) they are IMHO too big a risk....for world cruising.

On a world cruise more often than not, you need to be self reliant and in my experience multi's aren't when stuck.

It's not like when your aground at the Monte Bello's your gunna ring sea tow US and they will come get you.

You get yaself off, or your history, no second chance, either way in a multi - I'd say ya world cruise is likely all but over IMHO.

A MEG?...why not? :cool: :D :p

I dunno Lock - I'f I didn't know better.....well - you get the drift!

Cheers!

[lockhughes]

Originally posted by trouty
Cats upside down are - just that - upside down and likely to stay that way - spending the rest of the supposed cruise climbin thru a hull trapdoor into an upside down water filled cabin waitin to be rescued doesn't sould like a lotta "world cruisin" fun to me. Sounds more like a raft...:eek: Oh sorry - you already said that! ;) :D :p I'll take it we agree there then! :)
hehe.. ok,ok... Remember, this is a fairly lightweight vessel? The cabintops and general design can be optimized so that the boat floats very high on her lines *upside-down*... Tons of water surging around in the hulls - well, very bad, but I don't think the hulls need to be as badly flooded as you might think? Key thing (while we're designing RAFTS here) is to get the crew into shelter, and out of the water. Access ports and hammocks slung in the hulls might serve very well.

I have seen plans for righting multis which involve watertight bulkheads and pumping systems. Basically, flooding bow or stern sections of the hulls so the boat "stands up" on her bows/sterns, and then flipping her the rest of the way - things like a jury-rigged derrick to hoist a bag of water out of the sea, for leverage. Advantage is, makes no diff. if the boat is dismasted or not.


Kedging!
Ahh - color me dumb, buy how do you heel that multi to get the keels off a reef, Lock?
I can tell ya how to do it in a mono, coz I bin there n dun that - but methinks if the keels are stuck on a cat - well it's stuck and barring a VERY high tide or some explosive to shift the reef - thats where the multi is gunna stay.
yeah. tide good. might not have to be THAT high, depending on where it's at, at the time of grounding. I'd also look at deploying a couple of air bags under the bridgedeck, to float the boat a little higher before dragging her off. Extra flotation might buy the crew more time too, if she's holed and they need to effect repairs? The same air bags might be figured into some sort of self-righting scheme as well...

I agree with you totally re kedging, and the need for self- sufficiency generally.

Not sayin cats aren't fun nor got a lot of room but as a SOLO world cruiser (ie not accompanied by other potential rescue read tow vessel) they are IMHO too big a risk....for world cruising.


Another point maybe, on monomaran vs.multi? Multi's tend to move around faster... Multi-heads like to tout that a modern multi can outsail bad weather systems, or take better advantage of good. But I suspect that because things are often happening faster, that a skipper can sail themselves *into* trouble a lot faster... The more "responsive" a boat is, the more the crew has to react. A higher attention level can be a good think at any time, but it can also wear on the crew more too. So on this point, I would concede, that a "traditional" (ignoring the Polynesians for a moment) full-keeled, heavy displacement cruiser that's pretty much self-tending, would be easier on the crew, I suspect!!!

OTOH, one of the other points I heard about from the Carib charter industry, is that they found they had fewer passenger injuries on the multis, `cause folks weren't dealing with bouncy, wet, canted foredecks,etc. so much!

so, good and bad in everything I guess.

Lock

[Portager]

I agree with Trouty! The measure of a passagemaker is its self righting angle and less than 90 degrees is totally unacceptable! I haven't seen a multihull yet that meets this criteria without a buoyant mast and even then they will usually break their stick.

In my youth I was a big advocate of multihulls and I did a good job of overlooking their deficiencies. With experience, I realized that multihulls are generally uncomfortable in beam and quartering seas! If you go with a very shallow draft design they are uncomfortable ALL the time.

On the subject of world cruisers, I find that far too many cruisers think that using sails gives then free propulsion. Well there isn't any free lunch. Sails and rigging cost money, require maintenance, hoisting and tending. There aren't many old rag baggers because they move on to trawlers and let the engine do the work. Why not skip a step and years of misery and go straight to the trawler?

Cheers;
Mike Schooley

[BrettM]

TROUTY,
First of all, why do you need to undo a stay from the gunwale. Surely you could use a halyard?

And Second, To get a multi off you could drag it off with a large boat and you could use some explosive or a shovel to dig it out but surely you could simply lift the centreboards?

Hey! I must be an ideas man.

[b_rodwell]

Trouty

MONO vs MULTI
I am not getting into this debate on this forum.

CHARTERING
Building a boat to survey is onerous and is required if you want to charter in Australia or USA. I don't know how important it is in SE Asia or the Pacific.

Building to survey usually adds to the total cost and, more seriously, frequently adds 'unnecessary' weight. I hate the idea of carrying around more weight than I need. I have been reviewing the USL codes and the relevant Australian Standards (AS4132.1&3). I intend to build in foam composite. I will be able to comply in strength and stiffness without a weight penalty. I am trying to work out how to comply with the penetration strength requirement. It seems more difficult and I have seen some boats where this requirement led to a significant increase in weight.

LOCKHUGHES

By canted I meant that from front on the hulls look like they are leaning inwards. See the front on rendering.

I am not convinced that assymetric hulls are an effective way to resist lateral motion. I think Hobie cats succeed despite having assymetric hulls. I do want the extra performance and I do want to be able to work to windward efficiently in storm conditions. I am told that centreboards will assist this.

[tspeer]

Originally posted by b_rodwell
...I am not convinced that assymetric hulls are an effective way to resist lateral motion. I think Hobie cats succeed despite having assymetric hulls. I do want the extra performance and I do want to be able to work to windward efficiently in storm conditions. I am told that centreboards will assist this. [/B]

I'm with you regarding asymmetrical hulls. The induced drag due to lift is inversely proportional to span^2, and the induced drag is doubled at the surface compared to lift produced at depth. So the deeper the lateral area, the better - this means some kind of board.

As for a combination of asymmetrical hull and board to generate "more lift", the total lift is determined by the loads applied by the sails. The boat will pick up leeway until the lift from the hydrodynamic side force equals that of the sail. Since the total is fixed by the rig, any side load carried by the hull is less load on the board. But pound for pound, the drag due to lift of the hull is greater than that from the board. So loading up the hull only hurts performance.

The main exception to this is that the load on the hull does extend the effective span of the board. The goal is to have the total spanload distribution of both hull and board add up to the optimum spanload distribution from the surface to the tip of the board. This optimum distribution can be calculated using lifting line theory (see http://www.tspeer.com/DesignTools/vortex95.xls). Also, see the research done by PACT, reported in CSYS #11.

So a small load on the hull can be benficial. But this may well be provided already by the symmetrical hullform.

I would be interested to see what the optimum toe angle is for asymmetrical hulls. It may turn out that by pointing the bows outward somewhat, the asymmetrical hull's bow would be more aligned with the leeway when going to weather, avoiding side force from the bow wave, and the toe-out could cancel the lift of the hull camber when moving straight ahead, thus avoiding a drag penalty due to the hulls fighting each other. This might be a more viable strategy for a trimaran ama, since the wrongly-cambered hull would be out of the water. However, it might even work with a catamaran because setting the hulls for neutral lift at zero leeway makes them comparable to symmetrical hulls (in the linear lift range) and when loaded, the leeward hull would come to dominate the boat's characteristics.

[Ric]

As to the old mono vs multihull debate. There are many pros and cons for both designs. But I don't agree with the cons mentioned. So far as running up on a reef, the shallow draft of a multi allows one to explore areas that never ever reached by a mono with 3 to 5 times the draft of a multihull.

Running up on a reef with a multihull and centerboards allows one to pull up the boards and get off. For this reason I would use two centerboards to protect both hulls. But there is no doubt that one centerboard can be made to work and is adequate in a cruiser (see John Shuttleworth website). There is a reduction of weight, complexity and an increase in space with a tradeoff off of a slight increase in leeway.

So far as flipping a cat. I would like to see the statistics of how many 70+ ft cats have actually turned over especially with a 30 plus beam and a Bruce number of only 1.45.

The canting hulls is not a big advantage in a cruiser especially in a large wide cruiser.

Windage is a concern on light and fast catamarans and should be taken into account when designing them. In fact in 20 knots of wind 60%+ of the drag of the boat to windward is from windage, not from the keel or wetted surface.

I would still have a steering station on the other side also, perhaps a small remote control station. On a large boat one needs that sometimes as one's view can be compromised by the sails. I find myself constantly walking from one side to another on cats with a single station just to ensure that there isn't someone to leeward of me.

I like the extended roof and for a cruiser the ketch sail is bonus.

Keep up the good ideas.

I think that cats make great world cruisers. The only problem may be the wide beam at marinas.

[Guest]

This may have already been covered:

The single centerboard will not be in the optimum fore and aft position on both tacks. A single leaward hull board should be more forward than a single windward hull board. Having 1 off center board will give you a different helm feel from one tack to the other.

I'm impressed with your skills at Rhino.

Paul

[brian eiland]

I guess I should have posted a message on this tread referencing my answer to B Rodwell's prelim cat specs.

That answer is posted under the tread titled "Big Cat, alt CB's & sail rigs"

And 'guest' please note that this 'single CB location (twin asymentrical boards) will work equally on either tack.

[Max Avagliano]

A truely inspired rendering of a Sydney Harbour cat cruiser