input and help needed with this design

Discussion in 'Boat Design' started by wayne nicol, Aug 8, 2011.

  1. wayne nicol
    Joined: Dec 2009
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    wayne nicol Senior Member

    i did list my intial thoughts in my first post- but things do get lost a bit- and my first post was a little longwinded- good idea to repost-so here they are( with newly acquired amendments!!)

    quote"so this was my brief:
    1. wood construction
    2.motor sailer( 50/50!)
    3.tabernacle mounted rig, that is easy to handle single handed
    4.accommodation for a small family- 2 adults- 2 kids on the floor( at a pinch)
    5.trailerable- without oversize permits( 8' wide approx 26 odd feet long)
    6.cockpit to suit fishing - self bailing- lockers etc
    7.enclosed wheelhouse
    8.i have always been partial to dory type designs( not absolute though!!)
    9. inboard or outboards in a well!??!!
    10. beachable
    11.comfortable motion at sea!!!!!!!!!

    thanks
    wayne
     
  2. BATAAN
    Joined: Apr 2010
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    BATAAN Senior Member

    The scandinavian work pram is a long-ignored hull shape that makes a fabulous boat. Up to 40' long, they were common in fisheries that required great load carrying in tide rips and bad weather. They motor well, sail well and row well, the construction is very quick and cheap and strong, they take well to beaches etc.
    Worth considering.
     
  3. Tad
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    Tad Boat Designer

    Beachable and trailerable both speak of shallow draft, while comfortable (and safe) motion at sea (IMO) would seem to indicate more depth of hull. A hull that sits on top of the water (in worst case a flat bottomed barge) exhibits very quick shifts of CB (Center of Buoyancy) thus quick and tiring motion. In a deeper hull CB shifts less quickly and righting motion is slower, she will roll farther but slower.

    So you have the seakeeping compromise......

    Then you have the drag problem.....

    An efficient higher speed hull will be fuller aft and have some transom immersion, whereas an efficient sailing hull will have no transom immersion.

    Then there's the stability under sail (which is really horsepower) issue....

    You want light weight (minimum fixed ballast) for trailering and shallow draft for beaching, but you want a hull that will stand up to her sails and go when it blows....thus form stability (wide beam)....which goes against comfort in a sea.....

    After these hull form issues you can address appendages, power, rig, and arrangement problems.....

    On hull form I will note that my TimberCoast Troller is a true double-ended full displacement (deep vee) form and will do a maximum of 7.3 knots which is a speed/ length ratio of 1.6. Yes, she is standing on her tail, but still manageable, and the owner likes it for bucking tides. This is possible because she is wildly overpowered (29 HP diesel) with approximately one HP for every 140 pounds of displacement. With proper appendages this form would make a fine sailing hull.......and with some adjustment of aft sections she could do much better speed under power.
     
  4. philSweet
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    philSweet Senior Member

    Cost of a small diesel doesn't have near the pucker factor when compared against two outboards. Mainly because I think you end up a bit overpowered with twin merc 9.9 bigfoot or prokicker or T9.9 Yamahas with dual throttles and double all the accessories. Here again I'm sticking to 25" shaft as necessary. If someone convinces you that you don't need a 25" shaft, that changes the packaging quite a bit. With twin motors, I'd want to be able to kick them up clear out of the water by 6". So most of the transom gets cut away. This is commonly done on all sorts of boats and isn't as bad style wise or performance wise as many people think. It certainly beats dragging two big wheels around. Run a loop of pipe around outboard of the raised motors and rudder to clear lines and prevent bashing at the dock and rearend collisions on the road. Can hang flip up steps on pipe for boarding.
     
  5. Tad
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    Tad Boat Designer

    Something like this, four strakes each side with twin pivoting bilge boards (gets them out of the standing area) and cat-schooner rig with lots of reefs. The longest possible waterline means highest speed and most comfort in any sea. This shows a small diesel down low just aft of the main bulkhead but outboards (one or two) in wells aft could be used......

    Motorsail26.jpg

    Note the pilothouse sole is 10" below DWL......
     
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  6. BATAAN
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    BATAAN Senior Member

    Tad, very nice design.
     
  7. philSweet
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    philSweet Senior Member

    Well that certainly fell into place nicely, Tad. I'm a fan of twin boards. I'd route the mechanism outside the cabin at the aft edge. I think you would want to raise the stern so that the forward edge of the outboard well is at the waterline if you were going with a well, and that will take off 0.2 knots at top speed.
     
  8. Milehog
    Joined: Aug 2006
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    Milehog Clever Quip


    Water ballast?
    The Macgregor 26 uses it to make a dual mode vessel.


    BTW, nice prototype.
     
  9. wayne nicol
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    wayne nicol Senior Member

    i am happy with water ballast- in fact i quite favour the principal- but have never used it before.
    yup nice drawing by Tad.
    twin boards sure open up some space!
    is that cockpit self bailing.
    what would the top speed be, and at what hp.
    could twin skegs be fitted to ensure upright beaching- without adversly effecting the performance.
    so the only real question that remains is twin outboards vs. inboard!!!!????
    would love to see a top view sketch
     
  10. Tad
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    Tad Boat Designer

    Water is not very dense, so the CG of a water ballast tank ends up pretty high in the boat if she is vee-bottomed (it's better in a square Bolger sharpie type). It's better than nothing but if you have the space in your weight budget, fixed lead will be lower and thus you can get away with less to gain the same benefit.

    Yes, the cockpit well would be self draining.

    If she could be held to design weight (say 3800 pounds) a 40HP outboard might get her up to roughly 12-13 knots...perhaps more with the rig folded down. A 20HP diesel might give 10-11 knots
     
  11. wayne nicol
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    wayne nicol Senior Member

    i hear you on the ballast- point taken.
    those figures sound really good so far.
    would really like to chat further.
    thanks
    wayne
     
  12. Milehog
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    Milehog Clever Quip

    Wayne, if symmetry is what you are after how's about twin rudders and a single outboard engine?
    The sails will provide the redundant propulsion.
     
  13. wayne nicol
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    wayne nicol Senior Member

    thanks milehog.
    what i want with the twin motors is a built in safety factor, with regards to having a spare motor, that one can still limp home with, but also a lower powered( economical!!) motor for trolling all day whilst fishing for the salmon etc.- we like to troll anywhere from 1.5kn to 3kn.
    but i do worry about the hugely increased drag for sailing, and the increased loss of bouyancy in the stern.
    and even a 25hp motor can be lifted out of the well a lot easier than a (50 or 40) and be worked on- a highly unlikely scenario- but prepared is alive i geuss!!
    i am just not sure if this is a good trade off though- loss of bouyancy and sailability vs a hypothetical safety issue.
    and i really dont want to hang a kicker off the stern for trolling- i always feel it detracts somewhat from a beautiful wooden boats appearance.
    just geuss its somethings i am going to have to work through.
     
  14. Milehog
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    Milehog Clever Quip


    Understood. The more I learn about small boat engines the more I like outboards even though they don't look right in some cases.
     

  15. Tad
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    Tad Boat Designer

    Smaller twin outboards have some attractions....frankly I would discount the reliability issue, modern 4-stroke outboards are (IME) completely reliable. Your backup propulsion are sails, and a big sweep can do a lot with a small boat like this. The attractive part would be the ability to motorsail with just one outboard running at a high idle....this would really improve windward performance. And you have the economical trolling.

    For the 40Hp I would have a inboard (forward of the actual transom) well on centerline with twin close set outboard rudders aft so that they partially blanket the outboard thrust. With twins I would have one centerline rudder aft of a double width well. Both outboard systems would be set to tilt completely out of the water for sailing and service.

    Looking at the outboard option, twin Yamaha 20HP 4-strokes are $4k each and weigh 114 pounds each. A single Yamaha 40HP 4-stroke is 205 pounds and $7160 MSRP. So the weight is a wash, money for the twins will be quite a bit more with 2 control/steering systems, electrical systems, and fuel systems.

    A single 20-25HP diesel will be in the $9-10k range but cost can double with the associated systems and installation.
     
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