Hull Extension

[tomherrick]

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.
http://www.boatdesign.net/forums/attachment.php?attachmentid=23560&d=1216576123
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%.
http://www.boatdesign.net/forums/attachment.php?attachmentid=23561&d=1216576123
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.
http://www.boatdesign.net/forums/attachment.php?attachmentid=23562&d=1216576123
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

[Knut Sand]

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...

[diwebb]

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.

[tom28571]

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.

[tomherrick]

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

[diwebb]

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.

[Ike]

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.

[tomherrick]

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

[Ike]

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.

[tomherrick]

Ike,

I'll do my best...

T

[tomherrick]

Ike,

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

T

[Village_Idiot]

Well, if worse comes to worst, you could always try a jetpac
http://swordmarine.com/OurProductsAndYou.asp

[diwebb]

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.

[tomherrick]

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

[tom28571]

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.

[tomherrick]

I definitely don't want to add a bracket, for several reasons. Even if I wanted to, it would be difficult at best since my transom is curved and all the commercially available engine brackets I've seen are for flat transoms.

A bracket won't address any of the reasons I want to extend the boat either.

Tom

[Westfield 11]

Have you asked this question over at the C-Brats site? I would think that they would have some useful input.

[tomherrick]

Yes. I did post there on the topic. The responses I got were not helpful.

[tomherrick]

Tom,

Very early on I looked into engine brackets, but they didn't address the real issues driving me to extend the boat; I also never found a 'store-bought' bracket made for a curved transom. The loads aft that I have now (main, kicker, propane tank, house and starting batteries, and about 75 lbs. of fuel will simply be shifted further aft with the extension. The primary weight addition will be about another 325 lbs of fuel; better add another 50 lbs. of spare engine parts, tools, and 'MISC.BS' that will be added to the lazarettes in the extension. Saddle tanks may end up being the way to go, but I'd rather use the space under the gunwales in the cockpit for slim storage cabinets; the ones I already built...

Thanks for the issues to consider. Back to the books.

Tom Herrick

[diwebb]

Hi Tom,
in order to keep on track with the weight distribution with your extension, it would help if you understood the basics with regards to calculating the center of gravity of a boat. The CG is calculated by starting with an assumed point, it can be anywhere but is usually the bow or stern location of the assumed waterline. You then multiply the weight of each item in the boat, everything including hull, decks, motors, batteries, tanks etc etc, by its distance from the assumed point. The sum of the weight times the distance is called a "moment". Add these "moments" together and divide by the total weight and this will give the distance of the CG from the assumed point.
With the rough calc I did for your proposal I assumed a CG ten feet forward of the transom and did a calc for the existing weights that you are going to move. I then did a calc for the weights in their new positions with the added kicker motor, fuel and hull weight for the extension, and subtracted the buoyancy of the addition to get a nett effect of the addition. In order to maintain the trim as existing there must be no difference between the old and new moment calculation. If there is a negative moment then the stern will be lower and if a positive moment then the stern will be higher. Basically the moment of the buoyancy of hull addition must be the same as the difference in moment of the revised weights This assumes that weights are treated as a negative and buoyancy as a positive number.
I hope the above helps you to understand where my comments are coming from.
All the best with the project.
David.

[tomherrick]

Thanks, David,

I'm hoping to achieve a bit of a positive moment to lower the bow a bit. It rides a bit high even with the hydrofoil-equipped outboard trimmed in all the way.

First order of business for me is to read "Understanding Boat Design" by Ted Brewer to get on board with the basic concepts and issues. Second will be to draft it all out; probably will do that by hand first, then try my hand with Free!Ship and see how it comes out. Once I have CB and CG numbers/location that I like, I'll email the files off to a naval architect to see if I'm on track.

Thanks, David, for your guidance.

Tom Herrick

[diwebb]

Hi Tom,
that sounds like a good approach. You can do rough calcs on the CG as I described to make shure that you are on the right track before completing your design and having to pay for the Naval Architects time. You want him to be the verification for insurance etc as far as possible, to save as much in fees as possible.
I hope that it all goes well for you.
David.

[tomherrick]

I'm preparing to measure the boat for a hull design software program. I've been trying to figure out a method that is simple, accurate, and easy to measure the boat. So far, I've only figured out methods that address one of these three attributes at a time; it's either simple or accurate or easy, not all three... I guess I'll stick with accurate.

So far, the best method I can devise, given my workshop and experience constraints, is to level the boat fore and aft and abeam near a plumb and flat wall dead parallel to the centerline; then to prepare a one-foot grid on the wall; take measurements from the wall to the hull at grid intersections using a physical guide that keeps the measurement dead perpendicular to the centerline/wall at the stations. By constructing a differential mathematical model of half the hull in one-foot grid increments, I should be able to convert those measurements into values based on the design program frame of reference, then mirror the hull to keep water out of the port side as well...

If anyone has a better methodology, I'm all ears...

I've downloaded a number of free boat design software programs; the one that gets high marks from the most people on this Web site is Free!Ship. The learning curve for that program appears to be a tad steep, but the other four do as well. I own and have experience with a versatile pro-level 3-D design program that I've used for house design that I can pretty easily enter the mathematical hull model, but it won't export in a format that Free!Ship recognizes and I've not found a third-party solution as yet. The process thus-far seems a bit like it must feel to slog through mats of bull-kelp at displacement speeds... I don't mind spending some money on a 'real' hull design program, but hundreds of dollars is not in the cards now. Suggestions are most welcome.

Best to all,

Tom Herrick

[Ike]

First you need a flat level surface. Concrete garage floor will do. With a chalk string lay down a straight line longer than the boat on that floor. That will be your centerline. level up the boat over the center line. Use a plumb bob dropped from the bow and stern to determine the boat is centered up. This will be an approximation because you will never find the exact center line on an existing boat. Use the plumb bob to also determine the over all length of the boat by dropping the plumb bob from the bow and stern to the line on the concrete, marking the spot at bow and stern and measuring the distance between them on the floor. That will give you the length accurate within +- 1/4 inch. Do the same for max transom width, max beam. You can do the waterline too if you know where it is on the boat. A big if.

Now draw another line parallel to the center line a known distance away from the center line on the concrete. This will be your baseline. (or you can use a wall that is parallel to the centerline.) Take all of your measurements from the baseline to the side of the boat. The distance from the baseline to the centerline minus the distance from the baseline to the point on the boat you are measuring is the width of the boat from the center line. You need to establish X, Y and Z coordinates of known points on the boat. X is the distance of a point aft of the bow (or some other abritary point you pick along the length of the boat) Y is the height of the point above the floor. Z is the breadth or distance of the point off the centerline. You can do this manually with a tape, but a laser measuring device makes it a lot easier. You can buy one at Home depot or any hardware store, that is very accurate. You just have to make sure you aim it so that the line of sight is either paralle to the centerline or 90 degrees form the centerline for breadths, and vertical for heights.

You need to divide the centerline and the baseline into equal length sections (stations) just like on a lines drawing, and measure distances at each station. By measuring at known heights above the floor, i.e. 1 foot, 2, feet, 3 feet, and so on you establish waterlines. You can do the same for buttocks but it is a lot more difficult and may not be as accurate. For breadths just getting the 3 coordinates should be enough.

You can then use the data to build an offset table.

[tomherrick]

Thanks, Ike.

That's the way to go if one has an unbroken flat, level floor; I don't, so I'm having to improvise using a vertical plane instead of a horizontal one. The last time I had the boat out, the river left a clear demarcation of the waterline on the hull; I sorta lucked out on that one, I guess...

Tom Herrick

[Ike]

Oh yeah, those scum lines do come in handy. LOL Well, we have to make do with what we have.

[tomherrick]

One of many issues I've got to deal with in the design of my C-Dory hull extension has to do with ensuring that the new transom is sufficiently stiff and stable to transfer its power to the boat without the transom flexing. It'll be constructed of 1-1/2 inches of plywood laminated with epoxy and fiberglass for a total thickness of 2-inches. The original transom had a splashwell that was 3/4-inch plywood glassed to the sole. Kinda hard to see now, but if you look at the pic below you can see where the old splashwell extended; it was about 20-inches forward of the transom.

So, what I'm trying to discern is: 1) whether the transom needs the stabilizing characteristics of the splashwell, or if it's purpose is solely to keep water out, and 2) if fore-aft stabilizing of the transom is necessary, how/where does one find guidelines for determining the type and design of that support.

The boat is a semi-dory planing hull, originally rated for a 90HP outboard at the top end. The primary reason for this question is that I'm considering mounting a new permanent fuel tank between the original transom and the new one which would certainly affect the design of any fore-aft stabilizers for the new transom. The proposed fuel tank location is shown in the second image below with the heavy dashed line between the new and old transom.

http://www.boatdesign.net/forums/attachment.php?attachmentid=23560&d=1216576123



http://www.boatdesign.net/forums/attachment.php?attachmentid=25655&d=1222393269

Thanks,

Tom Herrick

[tomherrick]

Think I got the transom cut-out dimensions worked out using the main and kicker 'mounted' side-by-side on a 2x6. There seems to be sufficient clearance between the units to work properly with a 48-inch cut-out at the 20-inch mounting elevation (it'll be much improved by replacing the tiller-handle model kicker with a remote start/tilt/steer model). That leaves a much more substantial amount of material on the transom from the mounting line to the gunwale. Does a four-foot transom cut-out seem a bit sparse to you experienced boaters? Anything I'm overlooking?
http://www.boatdesign.net/forums/attachment.php?attachmentid=26152&d=1223862221

Below you can see how the original chine at the sole went aft as a straight line. I had to bend it in to make the sheer line work without a wierd twist in the hull sides. I'm wondering how the slightly narrower planing base will affect the boat's ability to get up on plane as well as the hull's resistance. There's still lots of info to enter into Free!Ship in order to get reliable hydrostatic calculations.
http://www.boatdesign.net/forums/attachment.php?attachmentid=26153&d=1223862221


The original transom had a 10-degree angle for mounting the engine. Most of the recommendations I've read seem to call for a 12 to 13-degree angle. I wonder if that has anything to do with the high bow stance when I have the engine trimmed all the way in. I'm wondering if I should consider adding a coupla degrees to the angle on this new transom. Thoughts?
http://www.boatdesign.net/forums/attachment.php?attachmentid=26154&d=1223862221

Prairieboy is now back in the driveway waiting for me to finish making space in the shop. Gonna have to make a low dolly to wheel it in under the garage door; a bit of a project in itself. Always something...

Tom Herrick

[erik818]

I would set the transom to the angle recommended by the motor manufacturer, and not trust the original boat design. My guess is that 13 degrees is more correct than 10 degrees.

Erik

[marshmat]

12-13 degrees from the vertical would be a typical transom angle for an outboard or sterndrive boat (if it had a jet, it would probably be 5 degrees).

When you talk about 'transom stabilizers', I presume you are talking about the bracing members that will transfer loads from the new transom back to the hull. Dave Gerr's book "Elements of Boat Strength" would likely be a worthwhile purchase for you, if you're going ahead with this modification.

With outboards, it's reasonably easy to visualize where the loads are located. Thrust from the propeller pushes forward on the bottom of the drive leg; thus, the bottom of the mounting bracket tends to push into the transom and the top of the bracket tends to pull backwards. Engine weight acts similarly. Steering forces tend to twist the bracket side to side. If you try to visualize how a (hypothetical) very thin transom would bend in response to these loads, it's fairly easy to see where you need to add bracing to transfer those loads to solid parts of the hull.

I wouldn't consider the old transom to be a good solid part to attach to, without further splicing into the actual structure of the hull. I'd be looking to brace the new transom and engine mounts well forward, diagonally to the original hullsides and with a couple of wide, beefy webs glassed to the existing stringers with a substantial overlap.

[tom28571]

Most motors will give a horizontal prop shaft if the transom angle is 11to 12 degrees. Making it 13 degrees as suggested would allow more adjustment to help bring the bow down.