Resistance vs. bottom design, small boat, slow speeds

I recently did what you plan to do. My boat is flat-bottomed with a little rocker, smaller than your planned boat (10') but beamier (4') and probably similar displacement. I was very surprised by how effortless it is to row, and I also made a sailing rig which I plan to try out next year.

It came out at only 60 lb with a lot of (probably excessive) built-in buoyancy, but I did not glass and epoxy it to save weight. For rowing you don’t really need built-in buoyancy.

It is easy to get on the roof of my minivan using a very similar method to the Eide boat loader, but my method is a lot cheaper. I rest the bottom of the transom on a small box to prevent gravel scratching the paint before I lift the bow, I rest it against the van, then lift the transom and push it onto the roof rack. The box makes it easier to get my fingers under the transom. I bolt a couple of 2 x 4's across the roof rack to provide a smooth sliding surface. The boat has a small foredeck and a slightly raised transom top edge which overlap the 2 x 4's to prevent it from sliding and a couple of rachet tie-downs secures it from driving. I attach a safety line from the van’s bumper to the painter eyelet; takes about 10 minutes each way.

My boat was designed by Par, also a member here, although I modified it slightly to suit my purposes and my own construction method. One of the things I changed was to use a constant flare angle which gave the sheer development straight edges; not a big deal but it simplified cutting and did not change the appearance a great deal.

You may prefer more freeboard for rowing; I suggest you make a mockup of the rowing position before you build, which will also tell you if you need more beam or outriggers.

Your last design with deadrise and rocker looks like a nice shape for rowing but. If you build the flat-bottom version with zero rocker it’s going to be a slow and noisy dog, like an earlier boat of mine. I recommend you take lewisboats advice and add enough rocker to get the transom clear of the water.

You should get less drag from moving the maximum beam aft, and a wider stern might improve sailing qualities should you wish to try that later.

Is that a Serpent? if you can make that a boat should be child's play for you.

Good luck!

 

Russ
One thing that you have not mentioned is stability. You will find that Delftship provides the KMT at various displacements. This is the initial stability and an important parameter for any boat but particularly for a rowing boat. It is another factor when considering the merit of the various shapes.

Your target speed of 3kts is quite modest for a 12ft boat. You would be quite surprised at the shape of hull that would give reasonable stability but have minimum drag to do 3kts. In the circumstances of rowing it will be more about how it tracks and responds to waves than the actual smooth water drag.

Flat bottom and hard chine results in a low drag form as you have determined. However any dead flat panel needs to be stiff or have added stiffness to avoid flexing. If the panel has its own initial curvature it has inherent stiffness so avoid dead flat panels. You can still have the hard chines with a single piece bottom but build in a little rocker so the bottom is not dead flat. I expect the hull will not track as well as one with some deadrise but a shallow keel can correct this.

The in-built buoyancy that Terry talked about in his boat also has some structural virtue because the chambers form torque tubes between stern and bow. With an open boat you rely more on the gunwale rails carrying the torque. Building a little model will give insight into this.

The fast rowboat thread might be of interest if you are thinking about rowing reasonable distance:
http://www.boatdesign.net/forums/boat-design/designing-fast-rowboat-14250.html
It will give some insight into boats and set-ups that move easily using oars.

Rick W

 

Russ, you're attempting to design without all the skills and understanding necessary. This is a noble effort and preformed by many, but not an especially effective, efficient or timely method.

Without the education, you have little choice but two possibilities: one is to just give it a whack and see what happens. This is a time honored method and one I attempted when I was a young man trying to learn this stuff. It really helps if you're independently wealthy or have reallllllly understanding parents. The other option is to very closely pattern you first efforts after known designs. Since you're interested in a pulling craft, then might I suggest John Gardner's book "Building Classic Small Craft". It has dozens of complete plans and instruction details, making the 20 bucks or so you have to pay for it well worth the price.

I mention this simply because Terry's (ancient kayaker) little boat performs well, because I put the rocker in the bottom where it needed to be. Additionally, I knew how much rocker to use and what shapes to employ in it.

You can fool around with free software all you want, but without the fundamentals, you're just drawing pretty pictures. You see, it will not tell you if the rocker you've selected is any good (etc., etc., etc.). Since you seem serious about getting a design finalized and maybe built for spring, sticking closely to "know quantities" is the obvious way to go for the novice. As you gain experience, you can make slight changes and see what happens. Eventually, you'll become an old experienced fart with a good eye for these sort of things, but at the moment, you'd be best advised to follow the lead (and shapes) of well established designs.

 

Par,
You speak the truth. I was never going to attempt this without some foundation for my design. I own John Gardner's Book and two others, "Ultrasimple Boatbuilidng" by Gavin Atkin, and "Instant Boatbuilding" with Dynamite Payson. I have had John's book for about a year and the other two for several weeks and have read them all cover to cover.

These books are great, the designs are proven, but lack the type of performance characteristics that programs like Delftship can churn out. My little experiment was looking at bottom types and trying to keep everything else constant to see how the performance varied. I was going to use this information to decided what boat from the books to base mine upon because to be perfectly honest none of them is exactly what I want.

I probably should have stated this more clearly at the start, but I am not married to the design we have been batting around. Of all the designs from the books, I like the cartopper the best. Also, a reason for putting things back into a program like Carene or Free!ship is that at work I have access to a 36" plotter and AutoCAD and I can print full size patterns from the DXF files.

The bottom line is that I am considering boat designs with variations of all these bottom characteristics and I was trying to figure out if any of them were going to be terrible for a boat that will be rowed leisurely on calm, lake water. I was seeing very little difference in the performance on paper and wondered why. Talking to you guys will help steer my in the right direction and that will be invaluable.

 

I wouldn't trust the prediction elements of free software. Establish a clear performance envelope and look at actual designs in this range for clues on shapes. I haven't seen any of the low cost or freeware offerings have anything that could be trusted in regard to performance evaluation. Some rough overall guides, but mostly just obvious stuff that you really don't need a program for.

 

Russ
Your requirements are quite modest. You obviously have an enquiring mind and a considered approach.

Many people have built more complex boats that satisfied their needs with much less thought than you are applying here.

I get the impression the reason for your approach is more than just to build a boat. It is as much a learning exercise. Arguably the best way to learn is to get stuck into the detail and have a go at it. After you have finished you will be able to list a myriad of things you might do differently next time.

In the very unlikely event that the boat ends up as fire wood you will still have got a lot of pleasure from building and testing it - maybe a tinge of disappointment after it was launched. A small price for further education. If it works close to your expectations it will be tremendously satisfying that it was your creation and you took a risk to do something a bit different than using a stock plan.

I would not recommend relying on the Delftship drag data in absolute terms but on a merit order basis is is more than likely correct since you are looking at small variations from one boat to the next. The lower drag on the flat bottom is not unexpected.

The reason I suggested above that you look at the fast rowboat thread is to see the sort of things ocean rowers get into regarding how they set up the oars, rowlocks and seating. The efficiency of rowing relies on applying force to the water and this is just as important as the boat resistance to moving through the water. Mocking up the rowing position and working out where the seat needs to be to get the required trim is something you have do.

Rick W

 

- and those are the ones you will learn the most from. I speak from experience - my first 2 canoes (long before building the sailboat) ended up like that, but the lessons learned gave me the knowledge I needed to create really nice ones!

 

PAR,
It seems to me to optimize the rocker part of the hull form on a flat bottomed boat typical of the Atkin designs one has four choices.
1 Make a bottom that is fair and linear with the same radius curve fwd to aft.
2 Make a bottom that has all the curve of the rocker in the center third of the hull and the outboard ends straight and flat to the ends. The bottom of both ends would appear (as viewed from the side) as a straight line for the fwd and aft thirds of the hull.
3 Make a bottom whereas the rocker is all aft with the fwd end running straight fwd to the stem. As in a Swede Form hull as viewed from above.
4 Make a bottom same as #3 but with the straight part aft and the rocker fwd. AS A Fish Form hull.

It would seem to me that #4 would be the fastest under power. #1 would be the fastest with a strong oarsman. #2 would be the easiest to row. #3 would be almost as fast as #1 but be easier to maintain moderate speeds. If I was to build a rowboat with a flat bottom I'd go with #2 or #3. I just thought of another variation. The flat part midships and rocker fore and aft. Should be the fastest of all 5. How does your knowledge and experience react to these variables on a flat bottomed boat?

Easy Rider

 

Yes, my experience reacts to your variables, which is the point I think, though I disagree with all of your conclusions. For example you last bottom amalgamation wouldn't be especially fast at all.

 

Flat all the way works for canoes ... not very manouverable though

 

Well I'm surprised you disagree w ALL my conclusions. I don't consider this a learning experience. Perhaps others will respond.
AK, Yes, most canoes don't have much rocker and long WLL. Most of the bottom is flat but I consider canoes straight more than flat. I have a "Front Rower" rowing machine and can go very fast w it in a canoe. Even w oars it dosn't turn very well and it's not very stable (directionally) when going straight. I have two canoes. A Wenona Aderondak (15') and an 18' square stern (stern is quite narrow) Clipper in Kevlar. The Clipper is a cut-off 20' double end canoe w just the right rocker and arch bottom. As a handler the Mad River Explorer is excellent but the Clipper is even better. I'd like to get a smaller Clipper and use the Wenona for rivers. Our rivers are rocky.

Easy Rider

 

The "all" part was a typo I just caught Easy, I meant to say I don't disagree with all of your conclusions, but . . .

 

The shape of a hull is an endlessly fascinating subject. My small (10') sailboat moves quietly and effortlessly across the water on its 4' wide flat bottom with a few inches of rocker and is remarkably stable. The first canoe I built several years ago, also with a flat bottom but much narrower, was unstable, slow and noisy to boot. I thought the flat bottom would be the most stable configuration but in that case I was totally wrong. As I began to understand what was going on I began to build what I call 5-plank canoes, flat narrow bottoms with bilge planks angling up to meet the sheer planks just below or at the waterline. Much better in every way; but it might not work well for a small, light sailboat.

The canoes you have sound like fine boats. I don't have to deal with rocks or fast water so I can build my boats very light. This is good for me as I often have a long carry to the water. I think I have the best hobby in the World!

 

Russ,
I also read and very seldom write.
Several of my reading places specialize in small, wood, cartopable boats:
http://www.jemwatercraft.com/forum/
http://www.neilbank.com/phpBB3/
Both small forums, but full of honest characters and small boats. I do turn my ******** sifter on high when reading, but they are fun and I learn other ways of designing and building boats.
Rich S.