BYYB Forums

Okay, Here goes. I know I'll probably get a mixed response and maybe even ruffle a few feathers but here's a few shots of the design modifications I came up with based on my idea of an aft cockpit Vacationer. I wanted something similar to Chapman's BERMUDA SLOOP. She's also based on the lines of MEDIATOR from Chapelle's Search for Speed Under Sail. I started with the Vacationer lines and began tweaking until I got what what I wanted.


http://byyb.org/gallery/albums/userpics/...s_Low2.jpg

http://byyb.org/gallery/albums/userpics/...esplan.jpg

Any input is appreciated.
Brett

Could you post a bigger picture?

Sorry Keith,

I'll try this. Having trouble getting the pics to re-size. They're also in the member's gallery.


http://farm3.static.flickr.com/2754/4305...d95f_o.jpg

http://farm5.static.flickr.com/4020/4305...c5bc_b.jpg

Brett

The nice thing about these new free or low cost yacht design software packages is, you can make some really pretty pictures. The problem remains that these programs don't tell you what shapes are good or bad, nor why they're good or bad. These types of programs are modeling software with volumetric subroutines. They offer a lot of information about the shape, that most users don't really understand, but little else.

What are your design parameters and general goals for this sampan like craft?

Hi Paul,

My goal was a relatively easy and low cost trailerable wooden boat based on an 18th Century sloop that I could use for both pleasure and educational purposes. I wanted to take the best of modern construction methods and apply them to as close to an 18th C. design as I could. I know, I could have gone with a traditional displacement hull and planking but that would have exceeded the cost and ease of building parameters. I live in the Midwest,  so blue-water capability is not really a concern. A cold molded radius chine was also a consideration but, again, ease and cost. The Vacationer was as good a starting point as I have found, both in terms of cost (of plans and materials) and easy construction. Incidentally, all the performance and stability numbers I was able to figure out were comparable to my Hunter 23 with the exception of weight. That number, too, may approach parity once I figure out how much ballast I'll need to hit my DWL.
Are there any design parameters in particular you were curious about?

Again, any input is appreciated.

Brett

Greetings,

Stripper hull?  Plywood won't bend to that shape!

Cheers,
Tom

Multi-chine

Yes, one of the strip plank methods would be an obvious choice for these types of shapes. Brett, can you post a "developability check" image of an inverted 3/4 view of the hull? Also have the sections, WL's and buttocks turned on so the shape is more defined.

From what I can see, she looks like she'd actually sail better in reverse then moving forward, but the shapes are difficult to judge from FreeShip (I'm assuming) images.

Having a target displacement is one of the very first things I have in mind when developing a new hull. Ballast ratio is another that I have an idea about and it's gets refined as you go around the design spiral a few times.

As for the planking, one of the dozen of so strip plank methods (including one I've invented), multi chine (though this shape looks prohibitively difficult in the ends for this), or split plank (a new form of taped seam), maybe one of the half a dozen or so molded or Ashcroft methods, of course the cored methods, plus one of the more traditional methods, such as carvel or lapstrake could be employed. This isn't all of them, but most.

Planking is a fairly small consideration, unless it's a monocoque structure. If it is (a wise choice if you want to keep it light) then you can use some very general guide lines (which don't produce the lightest hull BTW) or you can actually figure out load paths, which there is software for, but unfortunately not free, nor particularly intuitive to handle. If you elect to "flesh her out" with frames, stringers, structural floors, etc. then you'll to perform a center of masses calculation (oh, what fun). I personally think everyone that wants to design a boat should have to perform this task, by hand (using a calculator of course) a few times, but I digress.

I have to admit it's an interesting shape and more importantly an interesting idea. I've drawn up a few "down sized" packets over the years, but again this appears to have some fundamental flaws, that can cause a multitude of building and sailing issues.

The first image is an example, though considerably larger then your model of one of my down sized hull forms, though typical of 18th century craft, quite a bit lighter. The next two are 25' LOD Friendship sloops, one is mine, the other Owens and typical of late 19th century. The last image is a 22' LOD, 20th century interpretation of a 19th century schooner, by Atkins in single chine.

All have some common design elements, that the above design seems to be missing, despite the great disparity in age of design. I'm not digging at you, but I am suggesting that it's often much wiser to adhere closely to known well balanced hull forms when beginning to design boats. The reason is if you venture too far out on the design limb, you can't tell what element(s) of the design may need a closer look. In general, we find it's a whole lot easier to "sneak up on it" then to drive it out of the park on the first few swings.

Here you go, Paul:





Yes, I know the developability check shows red at the bow, but this is because I imported the offsets into DelftShip Free as chines. So, it tries to interpret everything as a straight line and there's some corners there that should be curves.  :-[ It is a work in progress. BTW, for those who would extol the virtues of "Making Sawdust," I am working on a scale model of the design so that I can test it before I spend numerous hours and dollars on something that may or may not work. If it does, I will begin making full-sized sawdust in the spring.

What fundamental flaws do you refer to, Paul?

The common elements I see in the designs you posted are that the draft appears almost equal to the freeboard, they are semi- or displacement hulls, and all but one (the first) have a very pronounced drag. I experimented with an angled bottom (a la Atkins) and amazingly, even though it greatly increased the square footage below DWL, it also reduced the resistance significantly. It also reduced the metacentric height significantly. I could never find a straight answer either in the books I have access to or online as to how the metacentric height affected the stability. Maybe you could shed some light. As it stands, it's at 7.5 ft.

Brett

The reason I posted the images above was to show that they had a lot more boat in the water then out. This is common of high D/L craft, packets and bulk carriers. The 18th century commercial vessel (about 60') was typical of the era and used the "cod's head" design principles. The primary consideration of this type of vessel was a S/L of about 1 and as much hold volume and you could carry. Your design has some elements of this concept. Of course it was tossed out the window once the world (well the British and French navies anyway) saw the "fine ended models" introduced by the "colonial designers" at the end of the 1700's and early 1800's.

This trend was slow to take up on the commercial side, but in war ships, it was used to great excess in some examples. To some degree American craft were "pressed" into service as prizes, not because they'd done anything wrong, but because they'd run across a much larger war ship and which wanted to steal the technology used in the hull form. The Baltimore pilot models were highly sought after.

This caused displacement and hull volume to become more centralized in general and the belly of the displacement curve moved aft. This tended to ease the entry and improve the Cp, permitting higher speeds without excessive lee bow surge wave making resistance. Heeled WL's also flattened out a bit, so the buttocks could be "laid down" for more "bearing area" too.

These are just some of the highlights and a primary reason the yacht America stomped on the best of the British in the mid 19th century. The British were still well entranced with cod's head designs as well as plank on edge hull forms. These deep, narrow designs did well up wind, but the drag associated with their under bellies and wave trains just prevented them from "breaking free".

Keel drag has had a number of uses. In antique craft it was as much a matter of necessity as it was a hydrodynamic consideration. You must remember these craft didn't have the ease of a travel lift at their local marina. They had to drag their ships up a set of ways and this usually meant a carriage and an inclined ramp. They parked her in position over the carriage at high tide, let the tide go out and the vessel settled, then she was literally dragged up the ramp, typically with ox, horses or possible with a one big ass capstan and a handful of poor guys schlepping their backs off. With some drag to the keel, they'd ride the carriage reasonable level.

Hoving off and lying too a hull was common and often used tactics (still is) to survive storms. In a day where you couldn't start an engine, you had the ship and it's inherent design attributes and that's it, to get you through. Anyone that's attempted to lie a hull in a modern fin keeler can attest, that keel drag works. The are several other considerations for drag as well, but as you can see this is a poor format for a dissertation on yacht design.

As for your hull, you'll never make that bow from a conical section. It'll have to be split or double planked. The hull form lacks bearing area aft, though this is often rig optimized for best result. I think the CB is too far forward, judging by appearance, her entry is quite bluff so will probably pound badly and make a huge wave train, limiting potential. I could go on, but these are things that I don't need to point out so much as things you need to recognize.

When designing your first hull forms, success is built on success. In another recent post is one of my very early design efforts. The lines drawing clearly shows a fairly conventional hull form. This is because I'd had a small amount of experience with it, incorporated some slight changes to address known issues and hoped these would produce a better version of a similar form. I got lucky and some minor mast rake trimming was all that was necessary to get a 3 degree helm in 10 knots.

If you don't venture far into "la la land" with the design concepts, then you have a reasonable chance of success. If an issue crops up, it'll be easy to identify and correct. If you start jumping up and down on the design limb you've shimmied out on, you'll likely break it or at the very least have no idea which element to address as they all interact with each other.

The shapes you've employed are a big leap of faith, on the skinny branches in the design tree of life. If you're a "small boned" guy, you might get lucky, but with this level of "uniqueness" you can bet on the contrary.

Besides, you don't want a boat that sails, handles or maneuvers anything like what you're attempting to style this after, trust me. They may have looked cool, but they sailed like crap. A quick look in my gallery and a few images of a design I'm working on for Craig, shows what appears to be a full up 19th century sloop. Of course below the LWL is a modern shoal draft 3/4 length keel with separate skeg hung rudder, because looking like it is one thing, but suffering the maneuverability and handling issues, plain isn't necessary with modern concepts.