Hull Extension

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

 

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

 

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

 

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

 

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.

 

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

 

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.

 

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

 

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.

 

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

 

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

 

Transom Stabilization

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

 

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?


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.



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?


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

 

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

 

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.