Hull Extension

Discussion in 'Boat Design' started by tomherrick, Jul 20, 2008.

  1. tomherrick
    Joined: Sep 2006
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    Location: Versailles, Kentucky

    tomherrick Junior Member

    I’ve got a 1984 22-foot C-Dory and I’m considering adding to its length. I have a long list of reasons, but the short one is this:

    • Primarily, I want to clear the cockpit of the engine splashwell, batteries and fuel
    • Increased buoyancy and space aft to carry increased fuel load (total of 58 gallons)
    • Mount for new kicker (Honda 8 or 9.9HP); one that is remotely steered, controlled, and tilted
    • Space for a proper propane locker in extension
    • Increased space for starting and house battery bank storage with better protection, mounting surfaces and ventilation.
    • Even with the Honda 90 (385 lbs.) engine trimmed in all the way, the bow has been bouncy and seems to ride higher than I'd like when planing; I expect/hope that the increased length will help provide a better ride in heavier water.

    The idea to adding length started after I found the transom core rotten. I removed the splashwell and found I liked the increased open space in the cockpit – a lot. The splashwell, however, provides a lot of structural stability for the outboard engine, so the new addition will have to incorporate similar stabilizing elements for the new engine mounting surface. I began fitting 1/8-inch strips for a vacuum clamped epoxy lamination on the curved transom, but that's on hold until I decide what I'm going to do with the extension.
    [​IMG]
    The cockpit sole core was wet but not rotting; I removed the inner cockpit fiberglass to speed the process; it is now nearly dried with no readings above 15%.
    [​IMG]
    Below are some preliminary sketches of the proposed hull extension to give you some idea of the project scope. The top sketch is my existing boat profile; those below are what I'd end up with. I'd continue the curve of the gunwale and the straight (parallel) lines of the chine. The new transom will essentially mimic C-Dory's current design: straight abeam instead of the Classic curve; just a couple of inches narrower at the gunwale than the new boats. We plan to use the boat for what might be termed 'spartan cruising'; week-long or more noodling along the Chesapeake or Puget Sound shorelines. The additional space in the cockpit seems as though it'll be plenty of elbow room for the two of us, and I'll appreciate the more secure and properly vented storage of gasoline, batteries and propane.
    [​IMG]
    Construction would be of plywood and epoxy laminated fiberglass. Most of my epoxy experience is with West System for old home restoration work, so I'll probably stick with the product I know.

    I'd like to be able to get insurance for the boat when I'm finished which will probably require a looksie by a marine surveyor. I'd also like to have my plans reviewed before I start construction so that I don't get disappointing news from the marine surveyor when I thought I was done...

    Guidance from experienced hands regarding the process for plan review will be much appreciated; I only want to do this once on this boat...

    Tom Herrick
     

    Attached Files:

    Last edited: Jul 20, 2008
  2. Knut Sand
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    Knut Sand Senior Member

    What about considering GRP as the boat is probably made of?

    My personal idea, I've seen it done on larger boats...;

    Place the boat al level as possible....
    Wax the middle section of the boat....
    Make a mold of the 4 feet behind the steering house....
    Bolt in 2 horizontal beams for alignment, with overlap, 2 on each side, zero vertical tolerance, one boltet aft, the other bolted front...
    Cut the hull 1-2 feet behind the steering house..., do not cut the beams for alignment.
    Pull 2-3 ft apart...
    Lineup, and bolt with beams to align firmly... use laser....
    Degrease and grind thoroughly, taper edges 1:10/ or 1:8....
    Fiberglass, to build up a little more thickness than originally...
    Remove lineup beams....
    Paint/ spray the hull with 2K paint or thinned topcoat....(epoxy for water proofing?)

    If you need more stiffness in the hull, for peace of mind, longitudinal(?) stiffeners can be added, use some foamcore material that do not suck water.. (Divinycell or others), all areas where you intend to work/ add resin or glass should be degreased, grinded and all topcoat should be grinded away, tapered, if needed...

    On the boat I saw this done on (pretty large fishing vessel), they used a middle section of the boat, where the "planking" were paralell, makes fairing easier...
     
  3. diwebb
    Joined: Jun 2008
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    diwebb Senior Member

    Hi Tom,
    Knut has a good idea, I have seen it done before several times, however I am not shure it is right for your boat. Have you contacted the manufacturere or the original designer of the boat? They may have some useful input on the project.
    My personal feeling is that you are on the right track with your original concept except that I would regard the plywood as formwork only and continue the hull extension in fiberglass of a similar layup to the original hull. I would pit in stringers bolted and fiberglassed to the inside of the existing hull, then add thin plywood panels then glass over with a taper back in to the original laminate as suggested by Knut. The fact that your existing transom is rotten means that cutting it up to construct the new extension should not involve any extra work. The design you show with a bulkhead in the location of the existing transom looks fine. The framing for the fuel and batteries also looks good except that if her bow is a bit buoyant at the moment you may wnt to move some of this weight forward to counteract that. Possibly seats at the forward end of the cockpit housing these items may serve the purpose. A consultation with a local naval architect may be in order when covering your bases for insurance purposes.
     
  4. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    Tom,

    I think your idea will work but there are a few things to keep in mind.

    The sheer and chine lines aft are not parallel on the C Dory so Knut's idea will make for a lot of unfairness in the inserted section. The only place you can reasonably cut a hull apart to add length is in the widest center part and I'm sure you don't want to do that.

    With your method, the transom will be narrower than the original because the extended lines will project inward.

    I assume you will want to re-use the existing deck parts for the transom area to lessen the work.

    Like Webb, I suggest that you use plywood mainly as a form and lay heavy glass on both sides to long tapers over the existing hull panels. The best glass schedule will include non-woven biaxial such as 18-8oz.

    C Dorys tend to have a nose high attitude and most people put heavy anchor chain forward to help hold the nose down. The reason for this attitude is not a buoyant bow but a narrow aft bottom with low buoyancy. Your extension should do a better job of that without the penalty of more weight.
     
  5. tomherrick
    Joined: Sep 2006
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    Location: Versailles, Kentucky

    tomherrick Junior Member

    Thanks all for your responses. Lots to consider as I move forward from the concept to the design phase.

    I'm dead-set on adding to the existing boat, even knowing that most boat-lengthening operations take place in the center - fore and aft. Once I get the transom sheer line worked out I'll know if there will be enough room abeam to mount the main and kicker engines as I would like. I don't think I'll lose more than about four inches, but we'll see...

    I'm planning on using a double ply of 3/4-inch plywood in the sole (as does the original) and the same in the new transom; all glassed on all sides with the same number of glass plys as the original.

    What's going to be a real trick is that I want to mount a 58-gallon fuel tank on the sole of this extension. That's not hard; what's going to be difficult is that I want to have heavy plywood supports running from the old transom to the new one, and tied into the new sole to stabilize the whole structure. Cutouts for the tank would defeat the purpose, or at least the effectiveness; placing them at the outside of the tank length (tank length oriented abeam) would provide marginal support right where I need it at the engine mount. Anyway, this'll be interesting...

    Back to Teale's design book...

    Thanks,

    Tom
     
  6. diwebb
    Joined: Jun 2008
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    Location: New Zealand

    diwebb Senior Member

    Hi Tom,
    I would seriously reconsider the location of the fuel tank. The fuel when the tank is full will weigh over 400 pounds and this weight right at the aft end of the boat could be a real problem. I would suggest that a location between the middle and forward end of the existing cockpit would be a better option as it will not affect the trim of the boat and the trim will change very little as the fuel is used. Having all that weight aft could seriously affect the performance of the boat. I helped install tanks in a similar location in a 50 ft boat here in NZ and was concerned about the same thing, but the owner refused to reconsider. When the tanks were filled the swim platform was under water and the boat could not get on the plane. They can only use about 30 percent capacity on the tanks. There was space in the boat to mount the tanks much further forward which would have prevented the problem.
    One thing you could do is build as you suggest but allow for a second location as I suggest and then see how the boat performs and use the solution that works best.
    If you want to keep a nice open cockpit then consider installing narrow tall tanks under the gunwhales both sides, in this way the cockpit floor will be kept clear.
    All the best with the project.
    David.
     
  7. Ike
    Joined: Apr 2006
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    Ike Senior Member

    I hate being a wet blanket but I can tell you several horror stories about boats that were lengthened, and that was by professionals. They did not work out well. (One manufacturer had to recall 260 boats and replace them)
    May I suggest an alternative that will give you some of what you want without all of the expense and work of an extension.

    Simply fix that transom, make it structurally a little stronger and then add a Gil bracket to the transom to mount the engines. It gives you a few feet of extra cockpit space, gets the engines out of the boat, moves them a few feet back to cleaner water and reduces noise transmitted through the hull. Some are also adjustable in height so you can raise and lower the engien like a jack plate. You can get an open bracket,which is light in weight, or a closed one that is essentially a box, which under heavy loads adds buoyancy to the boat. It's a lot simpler and easier to do and doesn't get you into all of the structural and hydrodynamic problems related to a hull extension.

    Anyway, just a suggestion for what it's worth.
     
  8. tomherrick
    Joined: Sep 2006
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    tomherrick Junior Member

    Ike,

    "Don't tell me that I can't. Tell me how I can!";) LOL... Your concern is not unfounded regarding the extension; there are lots of horror stories and I've read more than a few out here on the Internet. That's why I plan on taking my best thought-out plans to a naval architect before any construction begins, and to have the final work inspected by an accredited marine surveyor. Although I currently (hopefully, temporarily) live in Kentucky, this won't be a slap sumpin' on 'ere and call it good projek. Whatever comes out of the final design should be a long-term improvement.

    I did spend quite some time looking into engine brackets of all sorts. The primary problem with a commercially available engine bracket for me is that my transom is curved. I'd have to find a semi-custom unit or build my own. There's a guy in Alaska I've communicated with who built an engine bracket for his C-Dory; over a year later he's still thrilled with the overall performance. The issue for me, however, isn't the proximity of the engine to the cockpit or cabin; boat handling, storage, and usable space in the cockpit are the issues compelling me to explore this avenue.

    David,

    I'm new at this... ...like it's not obvious.:rolleyes: I calc the weight of the fuel and tank to around 402.5 lbs. I was discussing the concept with a graduate of the Northwest School of Wooden Boat Building; he was telling me that the additional 11.25 cu.ft. below waterline would add around 708 lbs. of additional buoyancy; this based on the expected displacement. Sounded to me like I'd end up with 300 lbs to spare... If you can confirm my assumption or dash it squarely I'd appreciate knowing either way; and why...

    Thanks all,

    Tom
     
  9. Ike
    Joined: Apr 2006
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    Ike Senior Member

    Well, it sounds like you have put a lot of thought into this, and getting the numbers crunched by a NA is the way to go. The NA needs to do a weight and moment calculation because all of this changes not only your buoyancy, but your displacement, Center of gravity, center of buoyancy, your stability, and other parameters. Sounds like a lot for a small boat but adding length can significantly affect those factors, and not always in a good way. There was a recent thread where the question was, "if I lengthen my boat without changing beam how does that affect my stability?" After looking at it the boat was actually less stable. So do the numbers.

    The reason I shot off my big mouth is, that many times boats are extended by just adding a few feet to the stern without thought to how that affects the strength of the structure. Moving the engines aft a few feet can make a major change in the moment of inertia, and stress on the hull. The boat I mentioned above did just that. They added 2 feet without doing the stress calcs and the sterns fell off off those boats. So you have to look at not only doing the math but the financial math as well. Is it going to be worth the cost, or should you just move up to a bigger boat?

    The other consideration is performance. The aft 1/3 of a planing hulled boat is extremely important to the overall performance. Are the buttocks going to be straight in the aft portion of the hull or is this going to introduce a hook or hump. Someone mentioned the bow up attitude of the C-Dory. This will push the bow down. That may affect overall drag at displacement speeds and steering. More bow in the water could result in bow steering, that is veering off to one side or the other.

    All I'm saying is, if you take into account all the factors, and it works, then go for it. But if you haven't well maybe you shouldn't. Let me know how it turns out.
     
  10. tomherrick
    Joined: Sep 2006
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    tomherrick Junior Member

    Ike,

    I'll do my best...

    T
     
  11. tomherrick
    Joined: Sep 2006
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    tomherrick Junior Member

    Ike,

    Oh, and thanks for shooting off your big mouth. That's what keeps us uninitiated folks thinking about the right stuff.

    T
     
  12. Village_Idiot
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    Village_Idiot Senior Member

  13. diwebb
    Joined: Jun 2008
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    diwebb Senior Member

    Hi Tom,
    you are not talking of just the fuel in the stern. You mention in the original post that you are adding an 8 to 10 hp kicker, this is an addition 100 pounds. The main motor is moved back by two feet, increasing its moment and helping to move the CG aft. You are moving the batteries to the aft lockers, another 150 pounds (depending on number and weight). The new hull structure will weigh in at a couple of hundred pounds extra as well. Something has to counteract all of this weight movement to the stern.
    If we assume that the existing center of gravity is 10 feet forward of the existing transom then the moment of the existing engine fuel (assume 20 gal) and batteries (assuming the batteries and fuel are directly forward of the transom) is approximately 6,845 foot pounds. With the extended transom, additional engine and fuel, this becomes approximately 14,755 foot pounds (10,355 without the fuel). The positive moment of the additional bouyancy at the designed waterline is approximately 7,700 foot pounds. This means that you have a negative moment burying the stern of 7,055 foot pounds with the fuel, and a negative moment of 2,655 foot pounds without the fuel. So to maintain the existing trim you will need to reduce the negative moment to zero. This means moving the fuel to the forward end of the cockpit at the very least. If any other weights can be moved forward that would also help.
    These figures are very rough, but they do seem to indicate that you will have a problem with the weights in the locations that you are suggesting.
    I hope that this helps.
    David.
     
  14. tomherrick
    Joined: Sep 2006
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    tomherrick Junior Member

    David,

    Alas, I've not grokked the use of the term 'moment' yet as related to engineering. I think I get the point, however, that too many negative moments can add up to a really bad day.

    I've got to find a basic, really basic, book on boat design that speaks to the total novice if I'm going to participate in this forum...

    Thanks,

    Tom
     

  15. tom28571
    Joined: Dec 2001
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    Location: Oriental, NC

    tom28571 Senior Member

    Tom,

    No, you don't want to add a bracket on the existing transom. That will only make a stern heavy boat worse and lift the bow even more. It is already hard enough to see where you are going, as I'm sure you are aware of with that high bow. Yes, you do need to go slow and work out the balance of any hull extension. Figure out the added buoyancy and figure out the new CG. I know of a C Dory with 2 Honda 35's which weigh at least 415# so a small kicker can be accomodated. it's true that the CG will move aft but it's also true that it will move forward relative to the transom. The balance will probably be better than now but needs calculation. Saddle fuel tanks similar to the current ones may be best since there is no cockpit sole to put tanks under.
     
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