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Thread: Hull shapes for small catamaran

  1. #36
    Join Date
    Sep 2015
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    629

    Default Re: Hull shapes for small catamaran

    Quote Originally Posted by Joakimu View Post
    Hello Lugalong. Thank you. Yes, I have seen his designs and I know about the proa, but how difficult is it to sail a proa? I actually once bought the KD 860 plans from him.
    Sailing a proa is certainly different, if it is a shunter, so that is why the cataproa might suit a beginner. as an easy way to get a handle on the ropes and gear. Although my one 5 YO boy learned to sail on a very simple and cheap proa (a shunter) that we made using our flat bottom/boxy section canoe tender and his little V bottom plywood kayak as an ama. We stuck a sailboard mast through a midships located thwart in the vaka or katea and used the blade of a broken oar as a steerer. Stability was good, and at times ( maybe for a morning through afternoon daysail) we managed going with two adults and two boys. This was possible because the hull length(of the longer hull) was close to 4 M, and having a boxy section and roughly 7:1 L:B, was a fair carrier of weight.Also, the 1 sheet plywood ama/kayak, was about equal in terms of L:B ratio.
    Getting to the heart of the question about hull shape for a double hull craft, you might do well to begin thinking in terms of skinny hulls for performance and beamier ones for carrying, and since you are hoping to keep length to around 3M, there is no option but to decrease l:B ratio.This is fine if you mainly have fun and are happy to just get on the water and sailing at a low budget level. Performance and racing is going to be a step up to unavoidably greater cost, despite the way some catamaran designers have been punting the line that multihulls are "faster and cheaper"..... light and flimsy might sound like a shortcut to performance without much cost, but when things break it certainly adds to cost.

    For a long time Wharram's found a happy medium with a simple V section hull that was sturdy and durable, which could be driven faster by spending more on the sail rig.Performance of the V hulls semed to make sense up to the point that skinny boxy section hulls with equally expensive rigs and lateral area by means of high aspect foils (daggerboards/rudders) that offered reduced wetted area, could out perform them.
    Last edited by Lugalong; 08-08-2019 at 12:24 PM.

  2. #37
    Join Date
    Aug 2019
    Location
    Sweden
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    18

    Default Re: Hull shapes for small catamaran

    Quote Originally Posted by DeniseO30 View Post
    Thank you. That's about the size I'm thinking!

  3. #38
    Join Date
    Aug 2019
    Location
    Sweden
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    18

    Default Re: Hull shapes for small catamaran

    Quote Originally Posted by Lugalong View Post
    Sailing a proa is certainly different, if it is a shunter, so that is why the cataproa might suit a beginner. as an easy way to get a handle on the ropes and gear. Although my one 5 YO boy learned to sail on a very simple and cheap proa (a shunter) that we made using our flat bottom/boxy section canoe tender and his little V bottom plywood kayak as an ama. We stuck a sailboard mast through a midships located thwart in the vaka or katea and used the blade of a broken oar as a steerer. Stability was good, and at times ( maybe for a morning through afternoon daysail) we managed going with two adults and two boys. This was possible because the hull length(of the longer hull) was close to 4 M, and having a boxy section and roughly 7:1 L:B, was a fair carrier of weight.Also, the 1 sheet plywood ama/kayak, was about equal in terms of L:B ratio.
    Getting to the heart of the question about hull shape for a double hull craft, you might do well to begin thinking in terms of skinny hulls for performance and beamier ones for carrying, and since you are hoping to keep length to around 3M, there is no option but to decrease l:B ratio.This is fine if you mainly have fun and are happy to just get on the water and sailing at a low budget level. Performance and racing is going to be a step up to unavoidably greater cost, despite the way some catamaran designers have been punting the line that multihulls are "faster and cheaper"..... light and flimsy might sound like a shortcut to performance without much cost, but when things break it certainly adds to cost.

    For a long time Wharram's found a happy medium with a simple V section hull that was sturdy and durable, which could be driven faster by spending more on the sail rig.Performance of the V hulls semed to make sense up to the point that skinny boxy section hulls with equally expensive rigs and lateral area by means of high aspect foils (daggerboards/rudders) that offered reduced wetted area, could out perform them.
    Thank you so much for sharing your experience Robert. I appreciate it! Very useful. At first, I was actually looking at maybe building a simple plywood canoe, or two, and making some kind of cat configuration possible between them. And the proa's are very nice indeed. How long was you sons canoe (ama)? That sounds very cool! Well done! I love the look of the proa too.

    Yes, I can go bulkier and the L:B ratio will be less, but I was thinking I'd stick to at least 8:1 if I can, but it's an interesting point. I will think about it. I could go a little bit taller on the hulls though, right? However, I don't need to carry much, I just meant load-capacity in relation to performance is more important for me. The sail can also be a bit smaller.

    Maybe I wasn't clear in my answer to someone earlier when I said it's cheaper and faster to build. That was not meant as a general statement, for example, compared to a monohull or something else, I was only referring to a smaller catamaran is cheaper and faster to build than a bigger catamaran. That's all I meant.

    Can I ask you, what would be the big difference if I make a bit of rocker in the stern of the hull? And/or possibly the bow?

  4. #39
    Join Date
    Feb 2006
    Location
    Pennsylvania
    Posts
    8,578

    Default Re: Hull shapes for small catamaran

    I was only referring to a smaller catamaran is cheaper and faster to build than a bigger catamaran. That's all I meant.
    This is not always the case. Depending on the specific designs being compared, the longer boat may be much quicker and possibly cheaper to build. When multihulls are taken down to the shortest possible length, the designer has a very hard time drawing hulls with simple shapes that can be put together with a few pieces of plywood. You often wind up with complex curves that require more complex and time consuming construction. The biggest expense can easily be in the rig - sails, spars, shrouds and hardware. But there are lots of old dinghies to be had for free or next to nothing, and finding one with a usable rig may actually be a good starting point and a determinant of what size boat to put under that rig.
    -Dave

  5. #40
    Join Date
    May 2016
    Location
    Niagara, ONT Canada
    Posts
    438

    Default Re: Hull shapes for small catamaran

    Some small boats designed by some people who do big boats seem to me to be unnecessarily complicated. As in you build the boat, then you build the boat inside the boat, but because it's tiny, it's seeming like keyhole surgery or a ship in a bottle jobbie. In the bigger boat say 16ft up, you'd have ample room to climb into it and work inside, in the smaller boat, you've gotta be outside reaching in and bending over it. Some could use complete replanning so you're building up the inside as you go along. Some seem so enamored of the "it looks like a hull as soon as you tack/stitch/ziptie the panels together" approach that they make subsequent bulkhead and flotation chamber fitting needlessly awkward, when it could have all been jigged to a strongback in a third the time it takes to get it all squared up afterward. Seems philosophical, in being a build a hull, then fit it out to be a boat, approach, works for big boats, could use being more integrated for smaller ones.
    2019: returning from being sidelined with medical probs, crossing fingers worst is over, still in "armchair enthusiast" mode for time being.

  6. #41
    Join Date
    Sep 2015
    Location
    Whangarei New Zealand
    Posts
    629

    Default Re: Hull shapes for small catamaran

    Quote Originally Posted by Joakimu View Post
    Thank you so much for sharing your experience Robert. I appreciate it! Very useful. At first, I was actually looking at maybe building a simple plywood canoe, or two, and making some kind of cat configuration possible between them. And the proa's are very nice indeed. How long was you sons canoe (ama)? That sounds very cool! Well done! I love the look of the proa too.

    Yes, I can go bulkier and the L:B ratio will be less, but I was thinking I'd stick to at least 8:1 if I can, but it's an interesting point. I will think about it. I could go a little bit taller on the hulls though, right? However, I don't need to carry much, I just meant load-capacity in relation to performance is more important for me. The sail can also be a bit smaller.

    Maybe I wasn't clear in my answer to someone earlier when I said it's cheaper and faster to build. That was not meant as a general statement, for example, compared to a monohull or something else, I was only referring to a smaller catamaran is cheaper and faster to build than a bigger catamaran. That's all I meant.

    Can I ask you, what would be the big difference if I make a bit of rocker in the stern of the hull? And/or possibly the bow?
    Hi Joakimu, you seem to have been lucky guessing my name, because my middle name is Robert.
    Since shared experience is helpful, it is worth offering some more then?
    Going out on your own design and build venture may not be a bad thing if you have a strong creative drive and don't mind a lonely road, but it sure would be easier to rather go with the flow and listen to other advice offered here.
    Getting to your questions though........the kayak made for my son was as long as a sheet of plywood, being in fact slightly less than 2.4 M. It was meant to have a curve tortured into the bottom, made from half of the 1.220mm wide sheet(divided lengthways) but the 4mm ply was too stiff to bend without fracture, so I ended up splitting that in half again and joining these halves along the keel-line with some rocker put into the join -- this rocker was mostly rounded toward the ends, so did create a canoe or an amphidromic hull(identical double ended).
    At the same time, the bottom became a shallow V, which worked OK for tracking as a paddler(carrying a light payload). Then when used as an ama, it could be pressed to greater displacement and which was also OK because a shunter does not require rocker in the same way that a catamaran does.
    Bear in mind, shunting suits primitive craft that are limited in shape by being carved from straight-ish tree trunks. so that they do not pivot easily about their centres when required to come about. Changing direction is entirely dependant on the rig.This is why it is probably good to follow others advice on choosing a proven design, especially if you are going to use a ready made Bermudan rig.On the other hand, small or short hulls doubled to carry some burden, are theoretically going to be easier to match with a sail rig that is also more primitive or less critical as to where its effort centre is located.

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