Help with power/sail cat hull design.

Discussion in 'Boat Design' started by conceptcat, Oct 17, 2014.

  1. conceptcat
    Joined: Oct 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: All over the place.

    conceptcat Junior Member

    Hi guys.
    This is my first post on the forum although I've read it many times.
    I'm getting close to build time on a power cat concept I've worked on for years. The basic idea is 45ft hulls joined together with two beams and curved arms to raise bridge deck height, with pod accommodation,outboard driven. My goal is a cruising speed of 10kn ,build brief is ultralight/minimalistic, reverse bows.

    The easiest way to visualize it is AC45 hulls joined together like trimaran ama's are to the main hull . The construction style is half frames with foam strip.
    My two questions are,
    -The AC45 hulls are almost exactly what I want to build however they are designed for sail power ,not outboard power. Would I be making a fundamental error by using sail designed hulls? Also the AC 45 hulls may not have enough buoyancy volume at cruising weight?
    -All of the design is free style except the hull shape, I'm not about to pretend I'm nearly knowledgable enough to guess hydrodynamics! all I really need is a CAD file to draft the frames for the stations. Any advice on where I could look to find this, anything open source or a particular designer drawing similar designs, I don't mind paying something for the file , but I can't justify purchasing a full set of stock plans.
    Any advice appreciated,
    Thanks
     

    Attached Files:

  2. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    Yes - you would


    Here's the "any advice". Please excuse the terse style, nothing personal, I just don't have the time /energy to pad it out diplomatically. Please accept it in a spirit of good advice.

    You are setting yourself up for a big fall.

    A novice like myself can do stations from a lines plan - no problem.

    Where you are going to come unstuck is the material specifications.

    Heres the calcs for a much smaller, much slower boat

    Its probably mostly 'greek' to you and me, but unless you want to have your hull(s) split in two in heavy seas, this is the kind of analysis you are going to need.

    Buying plans is the ONLY sensible way to go.

    The aim of "cruising speed of 10kn ,build brief is ultralight/minimalistic" makes the idea of not being able to 'afford' plans, ludicrous.

    have you ever purchased substantial carbon and epoxy supplies ? The price of plans will look like small change compared to the materials, motor and electronics. The plans will save 3 times the price you pay for them - guaranteed !

    You will get lots of other similar advice in this thread, to re-inforce the concept I am sure.
     

    Attached Files:

  3. TANSL
    Joined: Sep 2011
    Posts: 6,195
    Likes: 291, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    I totally agree with what Rwatson said, and I can offer my help to draw frames or calculate what you need for your boat. To dig deeper into this collaboration, if you would find it interesting, you can send me an e.mail.
     
  4. conceptcat
    Joined: Oct 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: All over the place.

    conceptcat Junior Member

    Thanks for the reply rwatson,a few more questions and clarifications.

    "Yes - you would"

    Why? I was hoping for some technical differences between the two designs.

    "You are setting yourself up for a big fall."

    ??

    "Where you are going to come unstuck is the material specifications."

    I once sailed halfway around Australia on a seawind 24 (no motor:cool:) these boats are a simple solid glass boat with alloy beams all held together basically with 8 SS bolts! As the boat I'm designing is missing the stresses of mast compression etc a sail boat receives I'm fairly confident I can "guess" the structural engineering.


    "Buying plans is the ONLY sensible way to go. "

    I am trying to buy plans!

    "The aim of "cruising speed of 10kn ,build brief is ultralight/minimalistic" makes the idea of not being able to 'afford' plans, ludicrous."

    From what I've seen an average price for 45ft cat plans are around $10000, that's a lot of epoxy and glass! what I was trying to put forward is that I only really need a plan of a specific hull shape , not a complete set. My design challenge is that I don't want to have living space in the hulls, the Crowther "Room With View" is a good example, however most hull designs like this are sail powered. A simple 18:1 stern powered, reverse bowed hull shape ,approximately 45'X 4X 2.5' is all I'm after .... surely not too much to ask.
    I excuse your terse style rwatson, I live in a family of Watsons.
    Thanks
    G Watson
     
  5. Nick_Sinev
    Joined: Aug 2014
    Posts: 63
    Likes: 0, Points: 0, Legacy Rep: 10
    Location: Sydney

    Nick_Sinev Junior Member

    I'm not a specialist.
    My personal opinion.
    There is nothing wrong about your idea. May be, except the price.
    Specially designed hulls for a power catamaran are cheaper than the hulls of a sailing catamaran.

    If you need to, I can try to find an I-net site I've seen recently. A shipyard is producing some kind of "kits": cаtamaran platforms with hydrodynamics opimized for low speed (displacement mode). I.e. 2 hulls + frame with hinges (to make the catamaran foldable => trailerable). The owner is supposed to install outboard engines and a cabin on the deck.
     
  6. rwatson
    Joined: Aug 2007
    Posts: 5,852
    Likes: 290, Points: 83, Legacy Rep: 1749
    Location: Tasmania,Australia

    rwatson Senior Member

    sure, best I can.



    Power hulls dont have chainplates, daggerboard holes and other specialist equipment strong points. Each hull can also can be expected to be out of the water from time to time during operation.

    Power hulls have to cater for heavier motors, and be capable of planing for fuel efficient operation. The shock loads are much greater at hiogher speeds

    ... because you dont know the difference between two types of hulls, the costs involved and expecting good results with no plans.


    here you go - you are going to base the engineering of an exotic lightweight power machines on a bog-ordinary small cruising cat ? "missing the stresses of mast compression etc" is replaced with "supporting and surviving high speed engine induced hull shocks and vibration.

    Now some more reality - have you considered the insurance problems associated with "guessing" scantlings ? They are going to laugh you out of the application office.


    You are ? You actually said ' I can't justify purchasing a full set of stock plans." and "really need a plan of a specific hull shape , not a complete set."

    yeah - well how much do you think the whole thing is going to cost, including labour ? $10,000 is going to be about what you pay for shed facilities for the project. Oh - you are going to build in the open ? I dont think so. Have you had thoughts on moisture control in laminate layups, dust control etc ? Do yourself a favour and visit a commercial building yard.

    What I am trying to put forward is that this is a poorly informed goal. You need to get that the shape is only a fraction of the problem The most useful shape in the world is a complete waste of time if you come apart in the ocean, or after bumping up against a pier for two weeks. Did I mention Insurance ?


    Yes, it is too much to ask for little money.
    1st, "I don't want to have living space in the hulls," - well, in a power cat, the planing nature of the hulls means you WILL have room. Dont' do yourself out of valuable reals estate.
    2nd " reverse bowed hull shape " means you just have this 'image' in mind without the information about operational problems. Why do you think power cats usually have positive rake - hint "logs and other rubbish in the water at 15 knots"

    Now, a few ideas - Derek Kelsall will design you an easy to build, lighweight reverse rake power cat "For custom designs we can prepare extended study plans which can contain materials list, specifications, 3D models, 2D plans (general arrangements"
    http://www.kelsall.com/Designs/KSSP30Tahiti.htm

    He can make the materials as exotic as you like, but my bet is you would be happy with cheap fibreglass with a sexy looking Carbon Fibre shim covering.
    Get a material rough cost off him for a start.
    I will guess .... $15,000 for the hull materials.

    Now, go and price the two, lets say 30HP outboards, and check out their cost say
    http://www.brisbaneyamaha.com.au/f30behtl-yamaha-4-stroke-30hp-long-shaft-efi-outboard-for-sale/
    say $6500 including fitting each. ( $13,000 )

    now you need to price in a trailer if applicable ( i cant see this machine floating around at a mooring or running up costs at a marina ) $15,000

    factor in your labour if you like.

    now ask yourself - "Am I REALLY in a position to do anything ? "


    Thank goodness, it saves so much re-editing :p
     
  7. Richard Woods
    Joined: Jun 2006
    Posts: 2,207
    Likes: 161, Points: 63, Legacy Rep: 1244
    Location: Back full time in the UK

    Richard Woods Woods Designs

    A sailing catamaran hull is very different from a powercat hull for many reasons.

    A sailing monohull hull is also very different from a speed boat hull.

    10 knots on a 45ft powercat is not very fast, probably twin 30hp outboards would do that

    Years ago I drew a sailing catamaran with a concept almost exactly what you describe. Egg shaped hulls, arching crossbeams, accommodation only in the central cabin. I still have the drawings available

    Richard Woods of Woods Designs

    www.sailingcatamarans.com
     
  8. Doug Lord
    Joined: May 2009
    Posts: 16,653
    Likes: 323, Points: 93, Legacy Rep: 1362
    Location: Cocoa, Florida

    Doug Lord Flight Ready

    Attached Files:

  9. gonzo
    Joined: Aug 2002
    Posts: 14,502
    Likes: 659, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    The hull shape is the easier part of the calculations. The structure holding them together and the attachment points are where the engineering is difficult and critical.
     
  10. TANSL
    Joined: Sep 2011
    Posts: 6,195
    Likes: 291, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    I can not understand why you say this. It is much more difficult to define the hull shape, with multiple parameters to be taken into account, than to calculate the needed section modulus (first and secon moments of area) of a beam subjected to some efforts that are clearly identified. Same for attachment points.
     
  11. gonzo
    Joined: Aug 2002
    Posts: 14,502
    Likes: 659, Points: 123, Legacy Rep: 2031
    Location: Milwaukee, WI

    gonzo Senior Member

    He could use, as intended, an existing hull. The attachment points need the calculations for all the forces the hulls will produce in the intended application. Historically, the attachment points and beams have been the failure points on multihulls.
     
  12. TANSL
    Joined: Sep 2011
    Posts: 6,195
    Likes: 291, Points: 93, Legacy Rep: 300
    Location: Spain

    TANSL Senior Member

    Of course, but that does not make true your statement: "The hull shape is the easier part of the calculations". That's not right. To consider it correct, you should give some explanation to clarify our ideas to everyone and especially to help the OP.
    Taking your reasoning to absurdity, if the OP buys a whole new boat the most difficult calculation will be neither the hull nor the attachement point but how to pay the boat.
    Cheers.
     
  13. conceptcat
    Joined: Oct 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: All over the place.

    conceptcat Junior Member

    Richard, love your your designs ,reading a lot of your older posts helped me in making decisions for this project. I would be very interested in seeing the plans of the boat you mentioned , you wouldn't by chance have any drawings of similar powercat hulls?
    I know a lot of members will laugh but I am hoping to power with 15hp Yamaha two strokes, excellent power to weight ratio,economic to run and buy ($1800) and from what I read can be modified to deliver 21hp. My estimate is 6/7 knots running one engine in calm weather 9/10 knots with both, 240nm/24hrs is enough for me.
    If I'm wrong (being known to happen:rolleyes:) I will bolt on more hp.

    Rwatson, thanks again for taking time to reply,
    I wasn't going too attempt planing , just keep in the magical semi displacement zone.
    The seawind 24 is a bog standard boat in today's world ,in its day a rotating rig,fully batten main,kick up rudders and center boards were pretty tricky!
    Did I mention I "axed" the bows, added a meter long sugar scoop on the rear then sailed it single handed 2500 nm Whitsundays to Broome ...without a motor, oh yes it's all crocodile infested waters.

    I have visited a commercial yard, in fact I worked as professional boat builder and doing refits for some years , I intend to mitigate costs by building in the Philippines, laminating will be done in a climate controlled building using vacuum bagging techniques, I can't justify infusion for this one off.

    I'm not sure that my goal is poorly informed , I think I have actually over considered and over thought this project and should have had a "crack" at it years ago, it's being 15 years since I first put pen to paper! I've noted four design aspects to the boat that I've never seen on any boat worldwide in my 67000 miles of sailing, or the internet for that matter. I'm not going to mention them as surely half will turn out to be ridiculous!

    Sure they look cool but I really do like the reverse bows! , I used to subscribe to the reserve buoyancy theory (which is possibly still relevant for the majority of designs) but in this circumstance with a lightweight and lightly powered vessel it would benefit greatly with the reduced pitching offered by this design, also the decrease in windage and wave action forward not to mention "less" boat to build and pay for makes it almost irresistible! :)
    Your correct hitting submerged objects and wet ride are the negatives.:(

    Thanks for the link to the Kelsell design it's one I hadn't seen before.
    As for exotics , I'm only considering carbon cloth or perhaps just longitudinal carbon rovings for the hulls interior for stiffness without extra weight as there's no abrasion or UV resistance required(did I get that right carbon degrade in UV?) And of course carbon in the hull to crossbeam arm structures. I don't consider Kevlar below the waterline to be worth the effort or cost.

    Sorry for rambling, I'm sitting here (for weeks) waiting for a distal radius fracture to heal , it's great to chat with informed people.
     
  14. conceptcat
    Joined: Oct 2014
    Posts: 10
    Likes: 0, Points: 1, Legacy Rep: 10
    Location: All over the place.

    conceptcat Junior Member

    Exactly! Thanks Doug Lord you've nailed it , I've seen the first boat before with the boxy style spacers for the cross beams but not the second boat ! Beatiful.
     

  15. Alik
    Joined: Jul 2003
    Posts: 2,900
    Likes: 237, Points: 73, Legacy Rep: 1306
    Location: Thailand

    Alik Senior Member

    Hull design starts from weight study of the craft. Secondly, for catamaran defining condition for hull shape could be the dissensioning of engine room for engine installation and access.

    So, have You done the homework?
     
Loading...
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.