Catamaran vs Trimaran

Glad to hear tris are fastest for their size - I love those ORMA60 boats they are a thing of great beauty.

Hi Doug, the Windrider 17 is very well designed and constructed (I haven't got a bimini). I have it moored down on our mud flats and other than a bit of weed growing it is coping well. Its great to have the boat on the water with mast up with the gib roller reefed. The draft is very low, which means that I have about 3/4 of the tidal range available for sailing (compare that to the fat, heavy & slow old stub keel monohulls on the next moorings!) The keel protects the perminant rudder - so all I need to do to go sailing is padle out in my old canoe, chuck the hatch covers into the canoe, pull the boom & sail out of the centre hull, and sailings on.

For me it was always the time consuming rigging and derigging of racing boats that spoiled the fun, especially after getting back cold and tired.

Doug, I am very interested in your comments on foil assisted stability. The one thing missing from the Windrider 17 (and the hobie 16 for that matter) is a foil to give good pointing ability. If I'm out adventuring, or burning off other cruising craft, then I want to be able to make headway into the wind as good as possible. So I am designing some bolt on foils to give me this ability. While I'm at it I want to angle these inward from the amas at 45 angle so they contribute to both the pointing ability and to lateral stability. On the Windrider the amas have a much shorter water line length than the main hull, so they must add a lot of drag as they get buried. In my head I can see the leeward ama skimming the water with the foil assist, me sitting on the windward ama to do my bit there, and the vaka slicing through the water like a very narrow, but very stable, monohull.

So far the sites I have found to assist with this mission are:

Catri 24, performance foil assisted tri - http://www.ahoy-boats.info/big/speed5-bsk.htm

Small volume carbon foil manufacturer -
http://www.philsfoils.com/

Next I need to decide on the foil size & shape.

Regards
Mike

 

foil assist

Mike, you might consider contacting John Slattebo at Hydrovisions. He builds a neat little proa that uses a foil stability system. They may or may not be suitable for your application but are guaranteed to be the closest on the market to what you could use. They are "L" shaped so that the lateral resistance and lift functions are separate and if you used two you might have a good solution. I'm not sure he'll sell them separately but I am sure that he's a first class guy with whom I've had great conversations. Here's his website:
Hydrovisions Innovative High-Performance Watercraft Design
Address:http://www.hydrovisions.com/ Changed:5:55 PM on Friday, May 5, 2006
Let me know how it goes....

 

Hybrid catamaran configuration

I was always lead to believe that the tri configuration permitted a stiffer rig and a much stiffer forestay - leading to an efficient headsail, and hence more speed.

Readers might care to view the attached photos of 'DaddyLongLegs' - a one-off ( rotating ) foiler presently sailing at the UK Sailing Academy, Weymouth, Dorset - the site of the next Sailing Olympics. It will be seen that a centreline beam is used to mount the mast, distributing the compression loads between the two carbon crossbeams. This would appear to have some merit.

I'm personally interested in this configuration, and whether I can 'borrow' the carbon beam-end 'lugs and pins' arrangement, and make it so for the alu crossbeams I already have.

Would any structural engineers/sailors care to comment?

 

Structure

The Decision 35 has a similar structure for the same reason:
Décision 35 Challenge Julius Baer Course trimaran, Course catamaran
Address:http://www.challengejuliusbaer.ch/pages/decision35/concept.lbl?lang=en Changed:9:27 PM on Monday, October 9, 2006

 

Hybrid catamaran configuration

Thanks for that pointer, Doug Lord.

Folks *may* be interested to see the boat being rigged for its first sail of the year, just a couple of weeks before the AYRS 'Weymouth Speed Week'......

I'm not a 'fundamentalist' about foils, but this was a fascinating example of the guys' R&D. And, yes, they actually are rocket scientists!!!

 

Hybrid catamaran configuration

Thanks for that pointer, Doug Lord.

Folks *may* be interested to see the boat being rigged for its first sail of the year, just a couple of weeks before the AYRS 'Weymouth Speed Week'......

I'm not a 'fundamentalist' about foils, but this was a fascinating example of the guys' R&D. And, yes, they actually are rocket scientists!!!

 

Diamond Foil

B, is Doug Culnane involved in that project? He tried a "diamond foil" similar to the forward foils on the cat on a Moth monofoiler.

 

BEAM Design

To bilbobaggins:

Re: Beam Design.

One reason a Tri, generaly, will have a stiffer forestay is
because a beams resistance to bending goes up by
the sq.

i.e. 2 two x fours, side by side will be twice as strong as
a single two x four.
However, a SINGLE two x six will ALSO be twice as
strong as a single two x four.

- And, most important for multi - hull designers,
a single two x six is - obviously - going to be
significantly LIGHTER, than 2 two x fours.

So it follows:
With most cruising tris - the center hull ( as a beam ) will have
more resistance to bending, than a cruising cat can sustain
( of the same displacement ), as the center hull will - most likely
- be significantly deeper - than one of the cat's single hulls.
Especialy, if you includ the cabin stucture, as part of said beam.

Therefore:
The force that can be applied to a typical tri's forestay,
backstay AND mast - without making a ' banana ' out of the
vessel is, almost always, going to be greater than can be
applied to a cat of similer displacement.

`````````````````````````````````````````````

In respect to your present focus:

I am a bit concerned about HOW: all the beams shown,
are inter - connected.
It's all very well to resolve stresses within the beam itself.
( That look good, on a two - dimentional computer screen )
However, I always get nervous when I see a multitude
of right angle corners - with a minimum of tri - angulation.

The forces on a vessel are 3 dimentional - and come from
ALL directions.

So another " RULE of BEAM " to remember:

When a beam DEFLECTS as little as 2 to 3 degrees from a
vertical, downward force it loses as much as 90% of it's
ability to resist, said force.

Given the above and from what is visible:
If it was my boat I'd be inclined to releave the stresses
on your center beam - with some triangular elements.

Cheers !

 

Therefore Decision 35 will have less resistance to bending than a 'true' tri with a deeper centre hull and the cat above with it's not very deep at all centre longitudinal beam will have much less resistance to bending again.
The whole argument relates to stiffness to allow forestay loading.
I suggest that this is no longer relevant to the cat vs tri question as forestay stiffness is no longer very important with the modern emphasis on mainsails or wings. A tri will be faster (overall, range of conditions and courses) than a tri of the same length, not because it can load it's forestay more.

 

Beam theory and practice

To Cormoran....

Thanks to you, and others, for input.
I'm very interested in this approach, and am considering how/whether I can adapt it for my own use, on a 13.5 metre hull-set.

Quote : "Given the above and from what is visible: If it was my boat I'd be inclined to releave the stresses on your center beam - with some triangular elements."

I attach another shot showing the centreline beam is triangular in section, and has a convex topside curve. I don't know at present what is internal to that beam e.g. does it have an internal long vertical web; does it have mini-bulkheads? I certainly agree that the sea provides lots of unanticipated loads/forces and would want to come up with an arrangement that 'triangulated' wherever needed.

So how would you suggest a re-structure of the centreline beam and its fastenings, the better to address these loads? How would I calculate what I need to have there, to carry the loads reliably?

Would a pair of similar longitudinal beams, arranged in a 'V', better cope with torsional loads?

 

Longitudinal Beam & Headsails/Headstay Load

Longitudinal Discussion
I would like to point out that I have been looking at this longitudinal stiffening problem for a significant number of years, particularly since my mast-aft rig designs needs tight forestays. Here's some excerpts from several postings I made quite far back:

1)"...BUT, what you may not have noticed was my alternative to the CB's in each hull. Look at the attached drawing, (or the very bottom profile drwg that denotes 'asymmetrical CB's, nacelle mounted'. 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). Now on either side of this flat plate I propose to mount a centerboard, not a single, symmetrical one, but rather two asymmetrical ones; sort of like a single board split in half. The flat sides of these asymmetric boards would fit up against the flat plate nacelle......"
http://www.boatdesign.net/forums/showthread.php?t=2225

2)"....I am 'pressing' the CB loads onto the 'flat-plate nacelle board', that in turn can be stabilized athwartships via two diagonal support 'wires' (maybe spectra or PBO) that carry the bending load imparted to the flat plate to an athwartship frame member (sub-bulkhead). Thus, very little exposed structure to create drag (flat plate aligned with water flow and two wires attached to bottom edge). Further I would recommend VERY large diameter CB bearing(s) to spread the CB bending loads to the flat-plate nacelle."
http://www.boatdesign.net/forums/showpost.php?p=37041&postcount=41

3)"There is a requirement for a transverse bulkhead in the boat structure at this CB location to which diagonal 'wires' can be attached to offset the big side-loads of the CB's"
http://www.boatdesign.net/forums/showpost.php?p=36348&postcount=36

4)"...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."
http://www.boatdesign.net/forums/showpost.php?p=35993&postcount=30

I believe you can begin to understand my position with reference to this longitudinal discussion. In fact my 'faired' center nacelle would appear very similar in idea to that 'mini center hull' of Decision 35 and Alinghi

Headsail/Headstay Discussion
I would simply redirect you attention to the discussion of the development of this most advanced race boat Alinghi and the attached PDF document,
http://www.boatdesign.net/forums/showpost.php?p=98787&postcount=39
quotes..."We chose a rig with one extra foresail, a smaller mainsail, and progressively overlapping foresails", and this "the platform was heavily influenced by the overlapping headsails..."
So I discount your observation, "that forestay stiffness is no longer important..."

 

Quote2)"....I am 'pressing' the CB loads onto the 'flat-plate nacelle board', that in turn can be stabilized athwartships via two diagonal support 'wires' (maybe spectra or PBO) that carry the bending load imparted to the flat plate to an athwartship frame member (sub-bulkhead). Thus, very little exposed structure to create drag (flat plate aligned with water flow and two wires attached to bottom edge). Further I would recommend VERY large diameter CB bearing(s) to spread the CB bending loads to the flat-plate nacelle."

That's all very well.

However, in real-world sailing one frequently encounters at speed a heavy semi-submerged object - such as a shipping container, a whale, or a tree trunk. Experience shows that a catamaran's structure frequently comes off worst - 'Queen Anne's Battery Marina', 'Jet Services', and many others. Where there is a structure depending down into the water, such as a 'dolphin striker' or indeed a 'flat plate nacelle with wires attached', then catastrophic structural failure frequently results.

This is not an outcome that the hapless owner and crew - faced with walking home from halfway across Biscay or somewhere west of the Azores - would thank you, the designer.

IMHO, it is essential that yacht structures be designed and built to survive predictable hazards; hence crash bulkheads, positive flotation, rigging 'fuses', and surface piercing structures.

 

Suseptible to Damage

I rather doubt it is as 'frequent' as you imply, but granted it only takes one time to be memorable.

Let me correct one misconception if there is one. I did not mean to imply that this 'flat plate nacelle' extended down into the water, but rather just skims the surface in flat water. Granted this is still venerable to being crashed, but it might not result in the lost of the rig, as it is intended to take the really excessive extra loads of the rig rather than the primary standing ones.

Most vessels are not going to like coming into a speedy contact with a partially submerged container, but that goes for all portions of the vessel. I would site the RACE catamaran "Team Adventure" on its attempt at a transatlatic record hitting some submerged object up off the Nova Scotia Banks. She lost a portion of her bow (usually the front portion of the vessel is the first to contact a foating object), which damaged her bow crossbeam attachment, which affected her forestay thus mast integrity...almost lost her rig. But this lost of the rig would not endanger her crew's lives. The flooding of the hull might have.
Hitting the vessel's 'boards' at speed might well result in very quick flooding

There were a number of reasonings I looked at when considering this 'central board' location. Of course I saw the potential to incorporated the 'boards' onto the truss framing I was considering for the vessel's structure itself. But also I have always favored 'kick-up centerboards' over daggerboards on cruising vessels particularly. Should you hit an object in the ocean or in shallow areas, the CB-board and the hull are not as suseptable to damage as with a dagger arrangement. But there remains a 'hole' in the hull's intregrity to accomodate either. What if we could eliminate this 'big gash' in the hull? Seems to me we've lessened the building cost and the increased the water tight integrity of the hulls at the same time.

 

Reference Posting #63 above. I don't believe that this mounting of the mast on this aft position of the center beam has anything to do with distributing loads. Rather I believe it is to move the sailplan to a more aft position (mast-aft, dare I say it) and lessen the tendency of that mainsail to drive the bows under on a power reach downwind.

I 'm not excited about their beam attachment, as I would guess it will allow to much 'flex' under heavy loading. And done in alum it would be lucky to last one season before all of the pin holes elongated.

 

AP:
“The whole argument relates to stiffness to allow forestay loading.
I suggest that this is no longer relevant to the cat vs tri question as forestay stiffness is no longer very important with the modern emphasis on mainsails or wings. A tri will be faster (over a range of conditions and courses) than a cat of the same length, not because it can load it's forestay more.”
Brian:
“Headsail/Headstay Discussion
I would simply redirect you attention to the discussion of the development of this most advanced race boat Alinghi and the attached PDF document,
http://www.boatdesign.net/forums/showpost.php?p=98787&postcount=39
quotes..."We chose a rig with one extra foresail, a smaller mainsail, and progressively overlapping foresails", and this "the platform was heavily influenced by the overlapping headsails..."
So I discount your observation, "that forestay stiffness is no longer important..."”
AP:
I suggest that Alinghi is a modern anomaly and the future lies with unstayed rigs where of course there is no forestay. An intermediate phase is the stayed rig with no headsail, with a wing mainsail, again no need for a tight forestay.
I believe a tri will be faster (over a range of conditions and courses) than a cat of the same length and I don't believe the fact that the tri can achieve a straighter forestay is a significant factor in this.