Modify Sailboat to Semi planing

Discussion in 'Boat Design' started by Doug Meyer, Jun 14, 2013.

  1. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Another option would be to modify hull about 2ft forward and 2ft back of the transom with a reverse chine concave like the Whio end and the Shannon yacht SRD Hull. and or with wings like The Bartender.
     
  2. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    After deliberation I like PAR's option above (bare with me) but feel would like to err on the planing side rather than the displacement side. I would like the boat to move obviously as efficiently as possible and be able go upto a 40HP at a later date when I can afford it.I keep designing boats with the motor in hand and this can be detrimental to achieving the best boat possible. I will be finishing off the Tunnel Cat over the next three weeks then I can pull that out of the garage and start on this one. I will take a lot of pictures and post them up as I go through the mods so all can advise me angles etc.

    The motor I have at present is a Yamaha 20hp 2 stroke which is actually a de-tuned 25.I can remove the baffle and change jets to get it back to original 25hp.This obviuosly will be looked at after sea trials to see if worth doing or not. Thanks All.
     
  3. PAR
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    PAR Yacht Designer/Builder

    Gluing on big hunks of foam may be an option too. You can easily shape these, eventually covering in 'glass, once satisfied with a set of shapes. It's still a good idea to test stuff in cardboard or foam, at a reasonable scale, before hacking up the hull.
     
  4. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    I have manged to find a drawing of sorts at least the keel buttock profile. I am making up a rough engine mount for the transom and want to get on the water (in the next 2 weeks) to see new waterline with no keel and motor on the back. I will then measure the rest of the offsets to the new waterline.

    Also waiting for Hartley to get back to me with a price for the offsets.
     
  5. Doug Meyer
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    Doug Meyer Amateur Boat Designer

  6. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Hi all, The keel has been removed so should I put the motor on the back balance out roughly fuel water anchor battery etc then check waterline in the water and then do LCB calculations.Or must I do LCB calculations on the existing hull and waterline?
     
  7. Mr Efficiency
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    Mr Efficiency Senior Member

    You can't get any drawings of the boat ?
     
  8. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    No and they will not sell me the offsets can only buy the full set.

    I am just going to work it out and modify the hull.
     
  9. Easy Rider
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    Easy Rider Senior Member

    Doug,
    I am a fan of what you're try'in to do but in reverse. Converting semi planing trawler type boats to full displacement. This involves turning the bottom up to increase the buttock line/angle so the bottom meets the transom at the WL.
    Your mod can be MUCH simpler from a construction standpoint. Just cut thin pieces of plywood (tapered) arranged like a honeycomb or a bit like an egg carton. Fill in w foam and sheath w very thin plywood and finally FG. Would add a bit of weight aft that would be undesirable w 4 stroke OBs but a 15hp (or so) may get you 10 knots. The volume added under the stern should at least help support the weight of the OB. You did say semi disp as I recall.
    Of course if you add too much it will just be a planing hull. I'm think'in 50 to 65% of the way toward a straight run aft.
    I thought about doing this to an Albin 25 to up it's cruising speed to about 11 knots.
    Only reason to do this is the lack of good semi planing boats out there. There are boats to build from plans like Spira's boats or perhaps a Handy Billy.
    Good luck and don't get too discouraged from the beating you've taken here. I'm surprised at some of the "in the box" or "baugh humbug" comments here.
     
  10. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Hi Easy Rider,thanks for your encouragement I feel I have been hammered a bit but I have learn't a lot from the senior members and am now more confident about the project.The donor boat for the transplant was chosen as it is of light build in fibreglass (being a sailboat not subject to the heavy pounding) and multi chine.
    My plan is to cut a wedge out of the keel zero from amidships and 3" aft to allow the hull to drop to 2 degrees ( about 7 degrees at the moment).I will also cut the chines either side (from amidships aft) and warp the 2 bottoms to take the chines just below the waterline. The plan is to then fit hi gloss polished boards under each bottom and on the sides bringing the lines a further 2ft aft of existing transom running either side of the motor.

    Then epoxy glass internally the chines, keel and extensions to save on fairing and turning the boat over. I will also need to run some new foam core longitudinal uprights for the new berths forward and new self draining deck aft.

    This will bring the aft buttock lines below 2 degrees and give me a better waterline BL ratio. 17ft X 6ft 2.833.The helm and passenger positions are just behind the max beam with fuel and water below deck in between. The motor will hang on the original transom 2ft forward of the new aft sections either side.

    Sounds like a lot of work but I still think a lot less than stating from scratch.

    As above I liked PAR's option. The glass panels aft are flexible and will warp down.(I hope)

    Thanks again to you all for your thoughts advice and encouragement.I will start in about 4 weeks and will start a new project thread so that I can continue to get bashed as I go through the project.
     
  11. Mr Efficiency
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    Mr Efficiency Senior Member

    Beating ? hammering ? I must have missed something. I think it is a lot of work for a none-too-certain result. Easy to say ' go for it Doug, we applaud you, take no notice of knockers ', but it ain't your time and money potentially wasted !
     
  12. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Hi Mr Eff,Yep you are right "none to certain result"

    Maybe the term I used "Semi Planing" is wrong I am trying to get a low power low speed planing hull just want to get over the bow wave.Like the Ninigret,the Nina LB22, Little Hughey, the Top hat.All these hulls differ from one to the other but perform similarly. Max beam is forward amidships on the LB22 and aft amidships on the Ninigret mine is almost centre at present and will be 2ft forwars after the mods. Both have warped V hulls. I will do the same as these and be almost flat at the transom.Will add 2 strakes either side to stop the bum slopping around downwind but the boat is to be used in semi sheltered area's.I am hoping the worst scenario will be having to up the HP to a 40 cruising at 2 thirds throttle. I do understand that it would be unfair for Senior members to give a nod of approval to something that could turn ou pear shaped and end up a Lemon. I would appreciate any advice from experienced designers and builders through the build process.

    I look at the lines of this boat everyday and can see in my mind the finished lines and shape and want to get them there.I can feel the weight of the boat as I move it around I have also had one "see my Avatar" where I modified the cabintop kept the sails and the keel and tried to get a good motorsailer. Sailed fine but only went 6knots with a 8hp half throttle. From half to full throttle just fell in her hole with her nose in the air.This is where I hope to improve her speed and stability at rest as a fishing boat.
    May the force be with me.
     
  13. Easy Rider
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    Easy Rider Senior Member

    Doug I agree Mr Efficiency's comment is spot on "none to certain results". There's the structural considerations and small changes in the hull shape can change the speed where the boat performs well/best. Changing the QBBL from 7 degrees to 2 degrees would (I think) put you at the upper end of the semi-disp speed range I'm guessing about 15 knots or so. I thought you were looking for more like 11 or 12 knots. Probably OK but getting back to Mr Efficiency's comment what if it porpoises too much or squats in the stern because you failed to increase the transom width or something else. Having the wrong outcome will not be pleasing to you after all that work.

    http://www.spirainternational.com/hp_keyl.php

    Here is a very simple boat to build out of plywood. Probably as easy to build as your modification. I think it's a so so design but probably capable and VERY easy to build. One excellent feature for you is that this boat will probably perform acceptably well at speeds not far from disp speeds and also at 18 knots. Lots of flexibility. And before you hack up the sailboat it's sellable. You could buy some wood and screws. THEN you could tell me how the boat performed so perhaps I would build one. What think?
     
  14. Doug Meyer
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    Doug Meyer Amateur Boat Designer

    Hi Easy,I had a look at the site after you sent me your first post. Yes some nice boats.If I were going to build a dory I like the Bateau Harbour Master 19 and the Redwing 18 and Ninigret. There are plans for many great boats out there that will be better than mine but I am going to get it as close as possible within my time,finance and capability. Bare in mind I would still have to build a trailer. What I have paid for my boat and a good trailer is less than what a new trailer would cost to build.

    I have looked at the option of building from scratch and it is not the option I want to take. I am modifying this boat.

    Yes 2 degrees is low but guessing that will give me about a 50% immersed transom approx SL of 2.5. Still need to calculate power for 12knots can't do till I can get some accurate weight and displacement figures.
    As I have mentioned before Ist I need to get the waterline then measure up the lines then do the calc's. I will post this up for the force to have a look and take it from there.

    Remember I do know a "BIT" about hull shape this is what makes me dangerous.
     

  15. Easy Rider
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    Easy Rider Senior Member

    OK Doug full speed ahead.

    I'm strictly an amateur and know only enough to be a somewhat informed observer and have posted a few rather far out things and basically gotten away with it .... like you. These guys are great for helping us amateurs along and dropping a mild insult here and there when we deserve it. The beginners take a little flack most everywhere and we're very lucky there's many good articulate advanced designers and builders that actually take the time to help us along.

    I'll be looking for your progress.
     
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