Load Carrying Ability

Discussion in 'Multihulls' started by Richard Woods, Jul 24, 2009.

  1. Richard Woods
    Joined: Jun 2006
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    Richard Woods Woods Designs

    "Catamarans don't have enough load carrying capacity for serious cruising"

    Like many other people I have also made that statement many times. But recently I have been wondering whether it is actually true.

    There are three main reasons why people should be concerned about load carrying on a catamaran.

    1) Performance: All boats sail slower the more they are loaded. Clearly the bigger and heavier the boat the less influence an extra ton of gear will have. Thus small, light multihulls will sail significantly slower when loaded, whereas a heavy monohull will always sail slowly, irrespective of how much gear is on board.

    2) Comfort in a seaway: A heavier boat responds slower to waves, and pitches more. So it is a wetter, less comfortable boat to sail. On a catamaran excessive bridgedeck slamming can make life on board extremely unpleasant; it's noisy for starters, while loose gear - even food, gets thrown around or bounced off tables and worktops.

    3) Strength issues. This is more of a problem with open deck cats (as the cross beams have a finite strength), thus it is rare to have a problem on a conventional production cruising cat. But even so there is a limit to the load any boat can support.

    OK, we can all agree on that; what is probably harder to quantify is what you actually carry on board as a long term cruiser (and here I am talking about a couple living on board for a year or more)

    So let us compare what we had on our lightweight 32ft Eclipse performance catamaran and a heavy displacement motor-sailer monohull, the Maple Leaf 48. I chose this monohull as it was one that we lived on and sailed for a few weeks from San Diego USA to Cabo St Lucas, Mexico.

    First let's compare the useable space on board:

    Both have one shower compartment, the Maple Leaf sleeps four in two cabins (one double, two singles) without using the saloon. Eclipse has three double bunks so can sleep six without using the saloon. Both boats had five guests for a Christmas dinner, the galleys are a similar size and both have an oven big enough for a turkey. True, the freezer on the Maple Leaf is a bit bigger but Eclipse can still carry frozen food for two weeks in its water-cooled fridge/freezer.

    Both boats motor at 5.5-6 knots. Eclipse can do that with a lightweight, easily serviced 9.9hp outboard engine. Whereas the Maple Leaf needs a big diesel engine. A complex bit of machinery which also needs lots of big heavy spares and big heavy tools to repair and service it. Both boats do around 10mpg and have a cruising range under power of around 200 miles.

    I won't discuss sailing performance or handling except to say that Eclipse was faster on all points of sail and FAR easier to handle than the Maple Leaf.

    Both have a portable generator and solar panels for extra generating capacity. Although Eclipse has smaller water tanks than the Maple Leaf it also has a watermaker, (but Eclipse can still carry water for three people for 25 days without making any water). Eclipse also has a solid fuel stove, the Maple Leaf has no form of heating. Both had large chart tables, with room for an unfolded chart and lots of pilot book stowage.

    Eclipse keeps a rigid dinghy in davits, lacking these the Maple Leaf has to pump up an inflatable. Eclipse carries eight sails, the Maple Leaf four. Both are suitably equipped with safety gear.

    Both boats have hundreds of books and CD's. And dozens of DVD's, plus a computer (Eclipse actually carries three) to view them on. Nowadays an Ipod, movies on a SD card and books on a Kindle means this weight (and space required) is much reduced. LED lights use less power, so even Eclipse's battery capacity can run all this electronics.

    Eclipse carried a sewing machine which the Maple Leaf didn't.

    Both boats carried food for three on an ocean passage plus staples for several months.

    Thus the two boats are very similar, despite their size difference. And it is hard to see what else you need on board that Eclipse couldn't carry. Even when fully loaded Eclipse is still fast, comfortable and strong. Yet most cruisers would consider it small and light, so clearly a bigger, pure cruising catamaran would be an even better load carrier.

    A final comment: Most 40-45ft charter cats are comfortable with 8 people on board, say 750kgs including their holiday gear. A cruising couple would be say 200kgs. So that is a gain of around 500Kgs (1200lbs) carrying capacity.

    Another myth busted

    Richard Woods of Woods Designs

  2. Ad Hoc
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    Ad Hoc Naval Architect

    It does depend upon the L/B ratio or spacing of the hulls.

    If you have say two 15m hulls at say 2m apart, this is very different from the same but being 10m apart.

    The hull spacing affects the global loads imposed and in turn affects how much structure is required to withstand the loads. The loads are also a function of the full load displacement and the range, ie coastal or ocean. The wider the hulls, the greater the structure required.

    So, for a "fixed" payload for 'serious cruising' (Books, PC, DVDs etc etc) one can image a graph with weight on the X-axis.

    As the hull separation increases, the structure weight increases and the curve from left to right increases (either linear or non-linear, but always increasing).

    Whereas range, if the Y-axis is then distance, this would be in a decreasing fashion. Since if the payload is fixed (for cruising luxuries etc), with all variables remaining the same, except the structure, the range is proportional the structural strength. A thin very flexible flimsy structure wont go far as a stiff rigid robust one, which of course weighs more. So 'payload' + structure = total performance "envelope".

    So there then also comes a point where the two curves cross...is this the "optimum" to satisfy all scenarios, but would it satisfy every customer? Too much compromise?

    In other words, the 2 "drivers" are very much dependent upon each other and hence influence the design characteristics. If you want "all that excess" the penalty is less structure weight for the same overall displacement, hence the overall performance or range is compromised. Something has to give..

    This is true in the commercial field of multihulls in which i design. Whether this is true in the leisure world, I don't know. But i suspect there would be very little between them in terms of final design meeting the SOR.

    As a final note, like everything that is designed, cost is an issue. So, that same thin, but light flimsy structure instead of being GRP were a high performance composite, would the statement still be valid?...ignore the cost aspects?...ah, but that is the point...each design has its own unique solution, isn't that the the real story behind the statement of "Catamarans don't have enough load carrying capacity for serious cruising"
  3. catsketcher
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    catsketcher Senior Member

    Get out of Canada

    Hey Richard -

    The war is over here in Oz. It has been years since any monohuller said anything nasty about my cat. Indeed when we were cruising I heard a group looking at a cluster of cats and going

    "There are lots of cats now days"
    "Yeah - you would have to build one if you were to get a new boat"

    Then there are the times you say "Why don't you go to ......." and the guys look at you and say

    "Oh but you have a cat - we only have a mono" In the end I start the conversation by saying how great monos are.

    So if those Canadians get you down then come over and cruise the Queensland coast - you won't find enough negative opinions to crease a forehead.

    On the topic of load carrying. Our 38 footers sistership had her sterns in the water and couldn't tack without backing the jib with no cruising gear. Kankama never has had her sterns in the water at rest and can tack with no jib. The reason - excessive fitout in the sistership. We can put 3 months shopping - not 3 months fruit and veg - 500 litres of water and all our living stuff and have our sterns kissing the water. It is just a matter of being incredibly honest with your designer about what you want to bring along and forceful with the builder about keeping to plan.


  4. yipster
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    yipster designer

    interesting discussion, myth busted is optimistic but have been thinking bout the same
    apart from named i was seeking a cats light weight speed claim mainly in loaded having more wsa
  5. Fanie
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    Fanie Fanie

    I was wading through a local 'plastic' shop a couple of days ago to look at platic lightweight bins for storage. I was amazed to find some of these bins were actually friggin heavy compared to others.

    May sound like a simple example, but weight is like paying for something, it adds up incredibly fast.
  6. rayaldridge
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    rayaldridge Senior Member

    There's little doubt that a monohull can carry more weight than a comparable multi without it affecting the monohull's performance. But on the other hand, the monohull's performance probably isn't that great to begin with.

    There's an impulse to collect crap and keep it, common among almost all humans. If you can't resist that impulse, or at least keep it under control, then maybe a multihull isn't for you. It seems to me that multihulls are better suited to mindful cruisers.
  7. Manie B
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    Manie B Senior Member

  8. rayaldridge
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    rayaldridge Senior Member

    Fanie makes an excellent point. Coming from a lifelong interest in backpacking, I can describe several categories of backpackers. Most of us don't go on really long hikes-- I've never hiked the AT from end to end. For a week, most of us can handle a 40 pound pack, some of us can handle 60 pounds. But there are those who consider themselves ultralight backpackers, and can get their packs down to 20 pounds, and whenever I'd pass one of those guys on the trail (or, more likely, he'd pass me) I'd suffer a sudden attack of envy.

    Anyway, the difference between folks who hike with 40 pounds and those who hike with 20 are primarily mental. Part of it is pruning away all unnecessary weight-- those guys cut the handles off their toothbrushes. But part of it was being willing to camp in a less luxurious manner.

    I think those who want to cruise aboard multihulls of modest size just have to be willing to let some of the luxuries go, in exchange for all the benefits that multihulls provide.
  9. apex1

    apex1 Guest

    That was a nice deep insight! There are people willing to sacrifice luxury for performance.

    This comment was not related to a recent debate about multihulls and /or manners! And it was´nt meant to be sarcastic.
  10. Fanie
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    Fanie Fanie

    I'm sure the cat can handle the toothbrush full handle length :D but it really depends on how many toothbrushes you want to take ;)

    I was thinking about a manual water maker off late. If you cut standard fresh water to halve and manually make water you save a lot of weight and get some exersize as well. Maybe if one dump most of the water you may be able to keep up with Manie's fast mono :D

    Does large water reservoirs resort under luxuries ?
  11. Fanie
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    Fanie Fanie

    OK I'll be honest. The pumping water to brush teeth with is probably the only pump I'll get.
  12. marshmat
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    marshmat Senior Member

    From what I've heard, you get quite a lot of exercise trying to make any significant volume with those manual watermakers.... they really seem more like an emergency backup, to me, than something to use all the time.

    Still, I think Ray's point about having to let go of some of the luxuries we're accustomed to is an important one. We've all seen those 50' charter cats in the magazines, with a penthouse-like saloon, four cabins, three heads, electric dishwasher, etc. In the end-on shots, the bridgedeck is usually something like a foot or two above the water, the sterns are immersed.... this is the price you pay for trying to make your boat into a house.

    But what luxuries do we have to let go of? You can bring a 7-pound laptop instead of a full-tower desktop computer, same functionality. You can fit a manual head instead of a power vacuum head, again, it does pretty much the same job despite being lighter and cheaper. Do you really need three heads and two showers? Most families can get by just fine with one of each.

    I think it's more about looking at your lifestyle and figuring out what you really need and what's just superfluous, rather than about giving up the comforts we're accustomed to.
  13. yipster
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    yipster designer

    i rather compare a bunch of graphs first before minimising on SD cards, toothbrush handles or warm water for that matter
  14. Fanie
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    Fanie Fanie

    I'm with you on the hot water. I said to the neighbour's wife the other day in the shower I refuse to be bathed in cold water.

    I think it's a matter of what is the most important for you on the boat. If you spend a lot of time in the head then don't compromise it too much :D

    If you do a lot of PC work then by all means have the double top tower with flashing coloured neons and double suck and blow fans to keep Win doze cool.

    I think one can rig a boat fairly comfy and remain modest on the weight. Not everyone is like that. I think their money gets the better of their judgement and the have the heavy imported teak drawers instead of the light chinese el cheapo plastic ones. You look at them only once a week when you take fresh underpants out, but this seems to be important to some...

  15. Fanie
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    Fanie Fanie

    Win doze (one word) is a banned word ? :confused:
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