Bottom Hull Shape for a Tri, Cat or Proa?

Discussion in 'Boat Design' started by bjdbowman, Nov 23, 2018.

  1. bjdbowman
    Joined: Apr 2017
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    Location: Florida

    bjdbowman Junior Member

    Hi all, any help or input would be great.

    I am toying with the question of a small (18'-24') light beach/coastal cruiser design. Skin on Frame or Fiberglass over frame... no wood to speak of. Sail and small gas/electric drive. I need a few hundred miles of range from the engine and stores for a few weeks.

    I see some great plans out there, but I am only looking a benefits/problems of the hull shape in this thread.

    I want to use this for adventure coastal cruising around Florida and a shallow draft beach-able lightweight design is critical. With that said, looking around I see many Hull shapes for this type of project.

    1) FLAT BOTTOM - most logical
    2) Multi-Chine
    3) Round
    4) Vee
    5) Other?

    I want easy construction and speed of construction is also key, but I want the best bang for my buck and the best of both worlds given the best performance, price and ease of construction. I would also like as much of the weight (stores, fuel, battery, engine etc.) as low as possible in the hulls.

    I am planing on using solar/batteries and a small generator to power two electric drives. The idea is to travel around the coast with minimal stops for provisions and fuel.

    Let me know if you have any Hull shape Ideas... I like the small proa Mbuli type but I'm not sure at this point about the functionality of this design?

    Mbuli: Ultra-light Pacific Proa Beach Cruiser by John Harris https://www.clcboats.com/modules/catalog/boat.php?category_qn=wooden-sailboat-kits&subcat_qn=proa&code=mbuli-pacific-proa-beach-cruiser-plans

    Thanks,
     
  2. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    In order to arrive at a conclusion, you need to 'design' the boat. That means draw up a general arrangement and then work out the weight of the boat.
    Why?
    Because the length-displacement ratio will dictate to you which will be the best choice for you. And the 'displacement' part mena...you need to do a detailed weight estimate. Without which, you're just guess and not making an informed choice based upon facts.
     
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  3. messabout
    Joined: Jan 2006
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    messabout Senior Member

    Leave the proa design notion to the polynesians of old. A proa can be a fun boat for play but is not the most practical for cruising around the Florida coast. . There are many proven designs for cats and tris of the size that you suggest. Our member Richard Woods has several designs that might do well. There are a lot more multi designs that are worth considerations............going all the way back in time to to Arthur Piver's Nugget., a tri that was, at the time, on the leading edge of design, or at least Piver thought so. That boat was about as easy a build as you could hope for. Never mind that he somehow disappeared on one of his boats in the Pacific, never to be heard from again.

    Florida has an abundance of safe coastal harbors except in the Big Bend area. You would not need a full on Transat capable boat to manage that excursion. That last bit has a proviso. You need to be a competent sailor and a savvy navigator and weather observer to survive without undue danger.

    I leave it to my other forum brethren to make further suggestions.

    Why do you favor a multi for such an adventure? Monohull boats that can do what you want to do will most likely be less expensive and somewhat more comfortable as a cruiser. The used market is littered with decent monohull boats that can be had for less than the construction costs of a multi.
     
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  4. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    You should take into consideration that beaching your boat will be illegal in most populated areas or private beaches.
     
  5. bjdbowman
    Joined: Apr 2017
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    Location: Florida

    bjdbowman Junior Member


    Thanks, but I have just started to look at designs... not sure about tri vs cat even.
     
  6. bjdbowman
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    Location: Florida

    bjdbowman Junior Member


    Thanks... I do understand this and there are many places that we can go that are legal, and many shallow areas that I would like to explore on the cheap...
     
  7. bjdbowman
    Joined: Apr 2017
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    Location: Florida

    bjdbowman Junior Member

    Thanks for the reply... the monohull issue is out of the question... my wife cannot swim and she refuses to let me build one. She will however feel better with a larger beam stable boat with less heal and we are looking at a comfy cruiser/camper for the water to go exploring. We like to camp and having a wide deck or tramp will allow us to use a tent on those hot summer nights here in Florida... where we can get some air.

    I see many designs out there and many threads on this site that I have been reading. I just wanted to understand the benifits and drawbacks of the different hull shapes in the small light weight form that we are looking for.

    The other issue looking at tweaking some designs to incorporate what we want... we want light <500 lbs and capacity of >500 lbs to start with.

    The more I look the more I'm thinking tri... there are some great ones out there in this range...

    Thanks again...
     
  8. bjdbowman
    Joined: Apr 2017
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    bjdbowman Junior Member

    I am leaning towards a Tri with the capacity to row with a sliding seat aft of the aft beam... or forward of the forward beam?

    The tri has many features/pros which I like:
    1) The outriggers can be moved (hinge) along side of the main hull to allow simple marina fees and trailering.
    2) One rudder and dagger board (simple)
    3) the noisy drives could be moved to the outriggers to lesson the vibration and sound into the main hull/cockpit. And still steered with the central rudder.
    4) the mast and sails could be moved to the outriggers in a twin configuration which leaves the main hull deck open for tenting etc.

    I am sure that there are a lot more pros and a ton of cons as well.

    But the whole purpose of this thread is to find out input about the bottom of the hull design. I am not picking a design or a boat, but rather I would like to understand the differences, pros and cons, of the different hull shapes as to understand why some designs use one over the other.

    For someone to tell me that I'm an idiot for worrying about hull shapes is not the reason I am posting. I did not find much information about this subject on a small beaching coastal cruiser. I can build in any hull shape, but I want to understand why I should favor one type over another.

    If a flat bottom with larger rocker is better than I want to understand how this effects the overall project design.

    Simply looking for the experts to donate some hindsight and provide some assistance on helping me make some discussions.

    Thanks again for the help...

    And "YES" to those whom have EC experience because the plan is for me to single handle in the EC, not to win, but to participate and complete safely in a reasonable time-frame. This is why I want it light... and I like to stick with skin on frame (not a pun as i will build with plastic/composite) if possible even if i need to add fiberglass or carbon to the skin add strength and durability.

    I would prefer to purchase plans and I am still looking, but I want to understand the bottom up design issue first.
     
  9. rwatson
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    rwatson Senior Member

    The "problem" with Trimarans, is that you have to build 3 hulls, and two of them are really, really bad at storing things in, and very wet under sail.

    Messabouts comment "Leave the proa design notion to the polynesians of old." has some merit in it, but there is one variation of the Proa that beats the problems of that design.
    I first saw the concept in a book by Joseph Norwood, and it has found practical expression in the HarryProa.

    You could get a formal design done, or you could just utilize the concept yourself http://harryproa.com/wp-content/uploads/2015/04/Arrangement.jpg

    The advantages of the layout, with the Leeward hull carrying the sails, and the windward hull carrying the cargo would suit a sailor with a nervous partner.
    The idea of righting moment being improved by adding the weight of crew and the accomodation to a sheltered windward hull is so obvious when you think about it, but the other "traditional" Proa layouts have a really big gap in their logic.

    You get the stability of two hulls, but the ease of sail handling that unstayed masts provide ( no decapitating boom, failed tacks, scary gybes)
    The daggerboards are also the rudders, so less hardware underwater.

    Rob Denney has done a lot of work on optimal hull shapes, both from performance and load carrying, and the flat bottom has proved extremely viable.
    Some more development story here THE PROA HISTORY – Harryproa http://harryproa.com/?p=1881

    "Harry proas have the minimum possible amount of structure. Cats and tris both tack. Therefore they see loads from both directions, and have to be built to withstand these. On any given tack, both are carting around a lot of extra boat, solely so that, on the other tack, they will work. Eliminating all the extra bits of boat results in a substantial weight loss. This means that a proa can have a far smaller rig for a given power to weight ratio, which further reduces the loads."
     
  10. MindanaoTech
    Joined: Nov 2018
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    Location: Nevada

    MindanaoTech Junior Member

    Research design called FROG. I think it was a catamaran but you can convert it to a proper(lol) trimaran. It was 21 or 24 ft(just use 1/2), and don’t call the outriggers hulls. Just use 6inch thin wall PVC pipe. If I’m not mixed up FROG’s hulls were three flat panels(two chines -although no angle change - 90 degrees all around), minimal framing and the “flat bottom” had some rocker.
    Get 2 horizontal 5hsp gas lawnmower engines and install one with a shaft and prop( the other onboard only for emergency replacement). Also two used windsurfer rigs. All in $1200 bucks and 100 hours labor... roughly 500 lbs with 1000 lbs payload( just add a couple inches width to the design or maybe even a few feet in length) You might see 12 knots under sail and definitely 7 knots under power. Less than 1200 if you trust used engines ($15-$40) each. Good luck.
     
  11. MindanaoTech
    Joined: Nov 2018
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    Location: Nevada

    MindanaoTech Junior Member

    Harry Proa is great also, especially if you want to sail more. But 5 hsp disposable engine gets you 20plus miles per gallon. And if you have never built and want to look at the frog!
     
  12. Niclas Vestman
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    Location: Malmoe, Sweden

    Niclas Vestman Junior Member

  13. Ilan Voyager
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    Ilan Voyager Senior Member

    Whatever the shape of the bottom, I see a problem: "I need a few hundred miles of range from the engine and stores for a few weeks." That's a lot of weight on a "small (18'-24') light beach/coastal cruiser design" plus the complication of a "small gas/electric drive". It's even more complicated "I am planing on using solar/batteries and a small generator to power two electric drives." to put in a 18-24 feet multi. Add over that a wife who wants a comfy cruiser. In short words a comfy 18 feet multihull motorsailer with innovative electric drives with hundred of pounds of batteries, a big solar panel, a gas generator and autonomy for a few hundred miles, so a big bunch of gas. Add the water storage and the food for a few weeks. Plus change of clothes.

    Sorry but I have to say that it's an impossible task. There is serious problem with the SOR. You are wanting the features of an oceanic cruiser of at least 40 feet.
    Adventure may be fun, but it's surely not comfy on a 18-24 feet multi. It's sporty, always wet , often or too hot or too cold, and asks for some stamina and resilience but it's gratifying. I can compare it to hiking in the mountains, and long distance horse riding. The legs and feet hurt or the *** is aching, you sleep on the ground, you'll be wet and cold if it rains, you'll eat rations and drink bad coffee but some fools like me are enough masochist to be totally happy in conditions that make cry any urban guy.

    So if your SOR is more in tune with the sad and stubborn reality, your program can be done cheaply, with a reasonable comfort with a used 18 feet beach catamaran. There threads on the subject. i have written recently on a 20 feet raid catamaran.

    Forget hundreds of miles of electric power, use a lot of sail and if no wind a yuloh. Forget weeks of autonomy, 48 hours of food and drinks will be largely enough. You'll need to stop soon in a civilized place simply for washing the clothes and take a shower of fresh water....

    Forget proas, fun but impractical, and trimarans complicated and expensive to build with no amenities in the small sizes.

    And, I know that you do not like it, in reality if you want something a bit comfy, with autonomy a 24 feet monohull, dinghy with centerboard, will fit a lot of your requirements, at the cost of speed, no so important.
     
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  14. Ad Hoc
    Joined: Oct 2008
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    Ad Hoc Naval Architect

    Sums it up perfectly :)

    Good no nonsense sound advice...but usually the hard dose of reality is never what an OP wants to hear. Go figure..
     
    Last edited: Nov 30, 2018
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  15. messabout
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    messabout Senior Member

    Ilan V has been there and done that. He has a firm grip on the reality of the cruising life....even the casual week end cruising life. Ad hoc is in agreement and the number of experienced travelers will increase while offering the same general advice.

    Sorry to be a spoil sport but better to know what you are getting into before you get into it.
     
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