Minimum cruising cat-size & cost

Discussion in 'Multihulls' started by Alex.A, Feb 24, 2010.

  1. Milan
    Joined: Apr 2005
    Posts: 317
    Likes: 24, Points: 18, Legacy Rep: 279
    Location: The Netherlands

    Milan Senior Member

    If money is tight, it really pays to research your options very carefully. As a budget is limited, boat is small and you are an amateur builder - it leaves you basically with two building options – plywood or strip-building. (Or some combination of these two). (Foam would be (much) more expensive, more work and more difficult and wouldn’t really be much lighter at that size)... Plywood is easy and fast to build. It is also already faired, saves a lot of boring sanding. You do need a lot of bulkheads and stringers that clutter interior and are difficult to keep clean. Hull skin is rather thin – termic and sound isolation is bad.

    Strip planking is probably a bit more work, but you don’t need stringers, just a few bulkheads so interior is les cluttered, cleaner. Hull skin is thicker, puncture resistance and isolation are better, boat is quieter.

    You do need more epoxy then with a ply and probably have to put glass cloth on the interior as well. – Calculate carefully what’s cheaper in your area.

    That $ 21 000 is with a 800 $ second hand rigging.

    Glenn’s boat is really bare to the bones. You family probably expects a bit more comfort.

    Richard provided material lists for his designs on his excellent site so it is easy to calculate costs.

    Wharram would sale study plans with materials lists.

    Multihulls are high performance boats. To be able to realise their potential they need very efficient sail plan. They are much more sensitive to proper aerodynamic shapes and good trim then slower boats. Because of their speed apparent wind moves to the front – good windward characteristics are necessary.

    In most countries the best way to save money with rigging and sails is to by second hand form the local racer classes. (Racers change sails annually to stay competitive and discard last year set although for cruising purposes it usually have a lot of life left. You have to find classes with sail plans that suite your boat of course).

    Cat is not really suitable for free standing masts.

    As of crab claws, if you want to experiment with them, then go all the way that route - cheap, basic, double canoe concept. Maybe first try on the small outrigger canoe to see how do you like it?
  2. ThomD
    Joined: Mar 2009
    Posts: 561
    Likes: 25, Points: 18, Legacy Rep: 111
    Location: TO

    ThomD Senior Member

    I don't know that boat, but my feeling is that the words ezi and fold should not be in the same sentence, from the boatbuilder perspective anyway.

    I have neighbours in the Port of P, who have cruising dreams and have bought a large mono, and are outfitting it, years worth. She is now seriously ill... I get that they want all the comforts of home. What modifies that for me is if the boat has to be home built. Then I feel like I need to be reasonable about the time and effort involved, not to mention the potential return.
  3. AndrewK
    Joined: Mar 2007
    Posts: 485
    Likes: 43, Points: 28, Legacy Rep: 344
    Location: Australia

    AndrewK Senior Member

    How much stability is required

    So far its been established that for coastal cruising its the personal comfort factor that will determine the minimum size boat one needs.
    But for ocean cruising I would want a catamaran that is capable of surviving a storm on its own as did the previously mentioned "RAMATHA".
    So what is the minimum amount of stability required to do this? and how do you asses if a specific design meets this?

  4. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 149, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    Chris White's writings, among other sources, suggest 30,000 (40 kN.m) static righting moment as a minimum, and 100,000 (135 kN.m) as ideal for a cat that is routinely expected to handle heavy weather. The figures are somewhat arbitrary, but there's a good statistical basis for them in terms of the wind and sea state needed to put a given boat in serious trouble.

    The former value corresponds to a boat of about two to three tonnes displacement, 16' to 18' overall beam and about 30' overall length.

    The latter, preferred value corresponds to a vessel of four to five tonnes displacement, 22' to 26' overall beam and about 40' to 44' overall length.

    I'm not saying these are set-in-stone values, but there is substantial evidence to suggest that smaller boats are significantly more likely to get into trouble (capsizing, etc.) in less severe storms. This is obviously not as big a concern for a coastal cruiser, but I'd certainly not ignore the benefits of plain old size for an offshore boat.
  5. sandy daugherty
    Joined: Jun 2008
    Posts: 132
    Likes: 4, Points: 0, Legacy Rep: 52
    Location: Annapolis, MD

    sandy daugherty Senior Member

    There can be no absolutes; the sea will defeat any design. For the security of a cave in OLD mountains, you need, er, a cave in old mountains.

    Sailing is an adventure, and a rational sailor sets his own level of excitement. What you are asking is not about the boat, it's about the sailor. How well does he understand weather in a particular area? Is he likely to button up and batten down when things turn sour? Is he prudent about venturing into high latitudes and bad seasons? Some one has or will survive a plunge over Niagara Falls in a padded barrel, thus making padded barrels safe for anyone to sail in, if you set your standards by anecdotal evidence.

    The strength of a vessel is of no value whatsoever if the skipper is naive. Rather, he should shop for a good supply of luck.
  6. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 174, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    The Recreational Craft Directive (RCD) is used to regulate boatbuilding in Europe. The stability standards that accompany these rules are up to change.

    Currently the standard calls for (approximately) a 4T displacement for ocean going vessels. However the standards will be changed next year.

    The new standards will be more sophisticated and will include a "bare poles" factor and a wave capsize resistance factor.

    The latter is in part based on the assumption that a boat is sensibly immune from wave capsize if the boat's beam is greater than the wave height. So wider boats (so not necessarily longer boats) are more stable.

    Having said that, the new standards are no more stringent than the old ones, just more accurate and more based on naval architecture.

    There is also a new section which defines the "uncapsizable multihull". Or at least when sailed in its design category. These boats that will not be required to have positive flotation, nor have a "viable means of escape" AKA escape hatches

    You can buy a copy of the current ISO standards, but the new ones won't be available to the general public until next year. However interested designers and boatbuilders can see drafts by contacting (I guess) the relevant national boatbuilders association (eg the BMF in the UK, FIN in France etc)

    Hope this helps

    Richard Woods of Woods Designs
  7. Alex.A
    Joined: Feb 2010
    Posts: 348
    Likes: 9, Points: 0, Legacy Rep: 108
    Location: South Africa

    Alex.A Senior Member

    Why are cats not suited to free standing masts - if they can be used in a proa?
    Lower mast and less sail area for a lighter boat?
    How will RCD changes affect home builders with this 4t rule? As i understand it - it is mainly for manufacturers - so can someone build and ocean sail whatever they want? Unsafe or not?!
    RE voyaging canoe type - is anyone except wharram designing similar?
    Still searching and learning..... please be patient.
  8. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 174, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    You are right, home builders are exempt from the RCD (fortunately, as the paperwork for a one off ocean going boat is very expensive)

    As Arthur Ransome famously wrote in the 1930's "if not duffers won't drown" (I paraphrase)

    Meaning if your boat is well designed and built you will be OK even if it doesn't have a CE mark on it.

    Richard Woods of Woods Designs
  9. marshmat
    Joined: Apr 2005
    Posts: 4,127
    Likes: 149, Points: 63, Legacy Rep: 2043
    Location: Ontario

    marshmat Senior Member

    There have been cats built with free-standing masts, and it can work. A cat's wide beam, though, allows a very wide staying base for a conventional or a rotating wire-stayed mast, thus offsetting some of the stayed mast's main disadvantages. As for lower mast / smaller sail area- well, I suppose that rig efficiency improvements could be used that way; still, in practice, a designer who finds a way to make a rig more efficient usually uses that to get more speed rather than a smaller rig.
    If you want to try a cat with an unstayed rig, I won't stop you. Just be aware that you're dealing with a platform whose initial stability is more akin to a concrete block on land than to a monohull, which heels with the gusts.
  10. Milan
    Joined: Apr 2005
    Posts: 317
    Likes: 24, Points: 18, Legacy Rep: 279
    Location: The Netherlands

    Milan Senior Member

    Wel, basically, it is anybodies guess.

    Plenty of small cats survived storms, but, fact is , all things being equal, (they never are), bigger boat will have much more stability then smaller one.
    Size is very important safety factor, (for multis much more important than for the monos). Different designers have various opinions about size – safety relation.

    For example, Gino Morelli thinks that about 45 feet are necessary for offshore cruising. He doesn’t consider even that as safe enough for higher latitudes. Above 40 degrees North and 40 degrees South he recommends 10 to 15 feet more for every additional 10 degrees of latitude.

    Although quite a few of his cats in 38 to 40 feet range did a lot of ocean sailing, Erik Le Rouge says that he personally wouldn’t feel safe in some of the storms he experienced in the Biscay Bay in any smaller cat then his 46 ft Freydies. But he wouldn’t go around Cape Horn in her.

    Tom Jones on the other hand, criss -crossed Atlantic many times in very small cats and tris, experienced his share of stormy weather in process, including a tornado. Yet, he wouldn’t go above 30 feet.

    Richard thinks that 28 feet is about minimum for offshore cruiser.

    So, in the end, I think everyone have to decide for himself. There are many other factors to consider, of course.

    Look good at different designs and sail as much as you can on different types, preferelably in rough weather to see what you like and to develop a feeling for them .

    A lot of weight and windage, low bridge clearance are bad things in the multihulls.

    Width should be a bit more then half of the length. High prismatics hulls, with enough buoyancy in the ends to prevent nose diving, weights concentrated in the middle, ends kept light, solid platform kept small, vertical centre of gravity and windage kept low are all good things.

    It is possible but it is not very practical and certainly not cheap. It can work very well on the monohull, where good results are possible even with a simple, inexpensive structures without exotic materials. On the monohull, free-standing mast is quite a low stressed structure because hull has a lot of depth to bury must in, deck and hull sides are easy to beef up for carrying stresses from the mast and if the mast is overpowered by sudden wind-gust, boat heels and release wind from the sails.

    Multihull on the other hand have enormous form stability and doesn’t heel, That means that rigging must take much larger forces then in monohull. One way to make must more rigid is to enlarge its diameter. That is acceptable in cruising mono, but not in the multi. Having by its nature high performances, multi would be hurt by thick mast.

    Other way is to use carbon – expensive and still inferior results to stayed mast.

    It is easier to put unstayed mast on the trimaran then on the catamaran. (On the tri, it can be can be buried in the main hull).

    On the cat is practically impossible to put un-stayed mast on the crossbeam. It can be only done in the hulls, one mast in each hull –bi-plane rig. These rigs brings a whole new set of specific problems with them. Sails blankets each other, boat is very difficult to keep under the control during the reefing.

    As your goal is economical, simple boat I would stay away from bi-plane.

    But if you like to experiment try it on something small and cheap:

    Attached Files:

  11. rayaldridge
    Joined: Jun 2006
    Posts: 581
    Likes: 26, Points: 0, Legacy Rep: 322
    Location: USA

    rayaldridge Senior Member

    Just to agree with what Matt and Milan had to say, the problem with cats is the lack of "bury" out on the mast beam, and the weight of freestanding masts. A proa will have the mast in one hull, where it can be well-supported.

    When I was first drawing my little cat, I wanted a freestanding rig desperately. I even built a model with a balanced lug and struts supporting the mast.


    But in the end, it just didn't make sense. It might well have worked, especially on such a small scale, where the forces involved were relatively manageable. But in order to have a freestanding mast, it would have had to be quite a lot heavier than a stayed mast, if built of non-exotic materials. Since the basic concept for the boat was a kind of volksbeachcruiser, which ruled out exotic materials, it didn't make sense to have an excessively heavy mast, because pitching is likely to be a severe problem in fine-hulled multis. That much weight aloft was the deal-killer for me.

    There are also problems with controlling sail shape in heavy air. Chris White used freestanding masts in his big tri Juniper, but eventually replaced them with a stayed rig, for that reason.
  12. redreuben
    Joined: Jan 2009
    Posts: 1,983
    Likes: 214, Points: 63, Legacy Rep: 349
    Location: South Lake Western Australia

    redreuben redreuben

  13. davefried
    Joined: Mar 2007
    Posts: 10
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Toronto

    davefried Junior Member


    There have been many cautions about excessive windage in this thread but no limits suggested. I think that I could move around in the space described below, would this be considered a high or low windage shape?

    A cat

    LOA 10.0 m
    BOA 6.1 m
    Disp 4500 kg​
    Standing headroom hull:

    Sole -45.0 cm (below waterline)
    Clear space 195.0 cm
    Hull top 2.5 cm​

    Freeboard 152.5 cm (5 ft)​

    Seated headroom on bridgedeck:

    Bridgedeck clearance 70.0 cm
    Floor 4.0 cm
    Clear space 167.5 cm (5.5 ft)
    Coachroof top 2.5 cm

    Total height 244.0 cm (8.0 ft)​

    This has been a very informative thread and has helped enormously in my understanding of what size cat to consider. Thanks to all.
  14. Richard Woods
    Joined: Jun 2006
    Posts: 2,209
    Likes: 174, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    I don't think excessive freeboard and cabin heights are too big a deal, especially when compared to the benefits of more interior room and a drier boat to sail

    My 10m Eclipse has 1.8m headroom on the bridgedeck and 2m in the hulls.

    My 9m Sagitta has 1.5m on the bridgedeck and 2m in the hulls

    Both of them sail extremely well to windward

    So the figures you give sound OK to me.

    Going back a few posts:

    I don't think canoe sterns are that great an idea. Early UK boats, Heavenly Twins and Prouts for example may have had canoe stern hulls, but they also had nacelles and low bridgedecks. So as far as the large breaking waves were concerned these boats had a very big "transom"

    Suppose a following wave breaks against the transom and pushes the boat forward. How fast will it then go?? Say a huge wave made the boat go 15 knots under bare poles?? That doesn't mean the bows will bury and cause a pitchpole. After all in normal sailing you might well do that sort of speed and want to keep upright.

    And of course if you are in those conditions you will probably be dragging warps or something to slow down.

    If you have canoe sterns the lack of buoyancy may well mean a breaking wave breaks over the boat (it certainly won't all go round the canoe sterns). Most boats are designed to have waves hit from the bows.That is why pooping has always been so dangerous.I prefer to keep water off decks and out of the boat

    I haven't seem any canoe sterned boat that was not trimmed down by the stern. And as I have said before, canoe sterned boats have very little room inside. And on a small boat you really need to maximise interior space and load carrying ability.

    Richard Woods of Woods Designs

  15. Alex.A
    Joined: Feb 2010
    Posts: 348
    Likes: 9, Points: 0, Legacy Rep: 108
    Location: South Africa

    Alex.A Senior Member

    I get that you dont like canoe sterns. But they are only narrow under and at ends - the top can be wider. Wouldn't have nacelle - or cockpit - rather a high and flat deck - more fwd than other cats, with cuddy (or similar) more centrally placed. Canoe sterns dont automatically follow with a deep v - like Tama Moana - it is wider, with shallow v centre, much like conventional cats?
    Even with a shortenned canoe stern, a 9m cat will have plenty space for 3. As to trimmed down sterns - the small outboard could easilly be mounted near(er) the centre of the cat and not necessarilly aft - like most over-powered condocats? Stores central and berths either side of that....
    Everyone on this site keeps on about overloading cats - i would keep it simple and light. My house is 48m sq and i dont use all that space - even with a wife and child!!

    As to the rig - a lot of traditional voyaging canoes and proa's didn't use stepped masts - but had 2 booms and a v over the beam - no stress connected to a hull- and not much on the beam either. Also allowed it to bend off if over powered! Even better - break if totally over powered - better to lose the rig than damage the boat? Being conservative i'd reef early though. Cheap n easy to fix and/or replace too.And minimal rigging. Going bi like this?
    Am going to try it on an outrigger and the turn outrigger into double canoe - can't decide on melanesia or one of Gary Dierkings designs...... Will get his book and study plans from wharram and then decide....
Forum posts represent the experience, opinion, and view of individual users. Boat Design Net does not necessarily endorse nor share the view of each individual post.
When making potentially dangerous or financial decisions, always employ and consult appropriate professionals. Your circumstances or experience may be different.