Walnut - a 7-foot dingy

Again, without any practical or empirical knowledge (let alone and education), you haven't a real chance at making that rig work. For example, how do you expect to get reasonably sheeting angles on that thing? Also, your stretched version with a 2:1 B/L ratio?

Small boats are some of the hardest to design, because you don't have much margin for error. There are literally thousands of small boat plans, most quite low cost, some free. Avail yourself of the engineering, then doctor it up as you desire, knowing you're not guessing at any of the important stuff, like needing a 6' long boomkin just to handle the damn sheet. Maybe a square shaped, double boomkin so you can mount a traveler too.

 

you should be able to make that work okay, you do not show a keel or rudder, which is part of the control issue. So the illustration is incomplete for guessing at control issues.

Also, the fact the mast is in the back does not make the rig easier to control. All sails are controls and adjusted from the lower/aft end, no matter the mast location. That large unsupported surface of the sail will be difficult to control, it will want to flog, and not hold its shape very well. The ability to point will also be limited I suspect, and not a particularly efficient sail shape. This is important when you are trying to make headway in low wind conditions, or if you are being blown into a lee shore.

The easiest rig to build and control is the fully battened junk rig. It will not flog, it keeps it shape, and forces on the sheets are much lower so rigging is simple and light. It hoists and drops quickly without trouble or fouling, and is popular for single hand operation. With a cat rig all you need to control and adjust the sail, including raise and lowering it, is right in the cockpit. That is why these rigs are so popular. And there are reason the aft mast is not.

before you choose a sail rig you need to go out on a local gathering of boat builders and bum some rides. Take some sailing lessons if you can, learn how to use a convetion rig first, and than experiment from there. If you do not have a feel for the sailing a conventional boat I think you will get very frustrated with an alternative and unusual rig.

I have built 14 or 15 small boats, have owned a number of others, sailed in all types of sailboats large and small, including experimenting with some alternative rigs I built myself (I am an engineer myself, and like to experiment with new ideas). I do not think I would attempt to sail a configuration like you have on your boat except perhaps in the most limited of situation (near shore fair weather play). It would not make a good all around cruising rig. The junk rig is very easy to make work on the first try without a lot effort, even a conventional sloop rig always has some issues to work out the first few times you take it out on the water.

I highly recommend that you design it with a conventional rig that will be easy to sort out and get working well, and than try out the unconventional configurations so you have a better understanding of how it would compare with a conventional rig. At least this way you can get some time in your new design hull without the frustration of learning to control with an unconventional sailing rig at the same time you are learning to sail the boat. If the unconventional rig does not work out, you can always switch it back to enjoy some sailing while you rethink and sort the other sail.

It will be real fun to see you get out on the water with your fun little micro cruiser. But if your first time out in it with an unproven rig, chances are good you will get very frustrated, get wet and go for swim, and than not want to spend anymore time sorting it out. Take it one step at a time, new hull design, go with simple conventional sailing rig, and than experiment with it later.

 

Thanks for any help

Thank you for the pointer for which I am so desperately seeking for from more experienced people. I will definitely take on the suggested reading. Perhaps I will learn the lingo so I could understand the language.

I looked at some designs designs, they look very nice and easy to build. But I want my boat. If someone would help me to go through the motions, it would be nice. If not -- I would go through it alone with more pains.

Here is my side of the story.

The aft mounting of the sail resembles to me just another head sail. I wonder how head sail is being manipulated with just one sheet? The center of the applied force in any triangle is exactly at the intersection of medians where centroid is. It just happens to be just over the middle of the boat, slightly either to the port or starboard.

After that all moments and forces are derived from that particular location. The centroid of the genoa shown just happens to coincide with the location of a "normal mast" of any mast center mount rig. That is why this aft location should have worked in my mind and it does work in reality. I gathered some pics to show. It may be slightly less effective, more difficult to adjust the sail or something similar, but it does work. It may look unconventional, perhaps it is the reason people are scared to use it.

In my opinion, it is immaterial where the driving force comes to the hull from the point of view of mechanics. Then applied force travels from the aft and bow in my case to the hull, or it is transferred from a concentrated location in the middle to the same hull with a little help of the head sail mount. Actually, it is better to split the force in two, applying it at the aft and bow, rather than applying it at a single location. Believe me, this I know.

Please forgive me for my forceful essay. I think I was carried away. I thank you again for your input and criticism. Regardless of its nature, any reply is constructive and it does suggest where to start, even if expressed in the negative intonation.

I forgot to mention that I do have about ten years of small craft sailing, albeit in protected waters. I also did not show neither ruder nor keel because I do not know how. I will try to draw it by hand. I don't plan any keel I think that oversized ruder will stop the boat from moving sideways.

Thanks again for any help.

 

Yep, you really need an education before you start buying building materials. After you muddle though a few texts, come back and look at your drawings and statements here and you'll see why we're "concerned".

 

?!?!?! you will not be able to control the boat at all. A statement like indicates to me that you are almost clueless as to how a sailboat moves forward through the water. Stop right now and do not attempt to build a boat this way, it will be uncontrollable. It will only work straight down wind, and even then it will skid and not be very responsive to rudder inputs, even with a large rudder. You wanted to be stopped if you had a bad idea, well friend, that statement right there is a show stopper. STOP NOW!

The wind puts lateral force on the sail/boat combination, the keel resists that lateral force. The forward component of the lift vector off the sail drives the boat forward, the keel resist the lateral component of the sideways portion of the lift off the sail. And you have to use your body weight to counter the couple that the sail/keel combination create.

Think of the sail as an airplane wing sticking straight up from the center line of the boat. The centroid of the lateral force has to coincide with the centroid of the lateral pressure on the keel (call the "center or lateral resistance" on a boat). That will include the whole underwater shape but on a fairly flat bottom boat you can just take it as the keel profile. The rudder just allows you to steer and trim the balances of forces on the sail vs. keel. And because we are talking about fluid mechanics, it is not exactly the centroid of the projected area of sail and keel (though it is a reasonable approximation, but you will still have to make adjustments to rig and rudder trim).

YOu would have to put the rudder directly under the center of the sail to make it perform both functions, and you would have very large lateral forces on the center rudder. and it would behave very badly if you were in effect twisting the keel to steer the boat. The most efficient modern hull designs have high aspect ratio separate keels or dagger boards, the an aft mounted foil shaped rudder.

There is way too much to explain on this forum here if you do not have a clear concept of how a modern sail boat works. If you are content to just go down wind than by all means build the boat the way you want. You need to learn some basic aero/hydro-dyanics of how a sail boat operates from some good basic text books on sailing and not impose on this list to teach you everything you need to know to make something of your own design that will work.

 

Slowly drifting, but where?

Thank you for the reply, but with respect I must disagree. I have seen many boats without keel or centerboard, with ruders only. Hobbycat is one example, as it has dual ruders and no keel or centerboard at all. I sailed them and other small craft just fine. There are many more examples, I am just tired to surf the web for pictures. How come those boats do behave? With this I must admit that having a centerboard does increase the speed because there is no need to compensate for uneven force equilibrium.

The scheme without the keel or centerboard can live on the water, however it has a disadvantage because ruders must compensate for the forces equilibrium. Pointing ruders slightly off to either side would generate enough twist about vertical axis at the expense of additional drag. It would generate a force-couple between the ruder and the centroid of the applied wind force. Oversized ruders would provide enough area of lateral resistance to avoid excessive drift. I can live with it. No one could convince me that this would not work. I think it would, but the boat would be slower. That is all.

As I said I have about ten years of small craft sailing experince, sometimes in pretty choppy waters with 3-foot short waves (that was scary!) It was not as often as I would want, but I think I have gotten the hand of it.

At the same time I humbly thank you for your help and friendly advice. I sure do study some books as I spend some time chatting on this board. I am bit slow in picking up "the naval lingo." Besides ruders and keel are a ***** in Defltship

Well, thaks again for the education. Maybe someone would comment on just the hull itself (I kinda like the oval shape)? If someone would add a ruder and keel to the model and post it here it would be really nice!

In no way I try to argue and prove anything, it just hurts when I am being dunk into the real thing. Thank you all for spending your time mentoring me, I deeply appreciate it. Peace!

 

you design would probably fit the requirements of the survival boat thread. http://www.boatdesign.net/forums/boat-design/survival-boat-41407.html