15 foot sports recreational dinghy

The new name is "Almost"

Thank you my mentors. I think I started to understand where my culprits are laying. I think that I was fascinated with “smooth flowing lines” forgetting that in the “dinghy” mode the hull actually behaves more like a skipping stone thrown on the water. The second shortcoming is that I was all mixed up in my head with different boats types, modes of moving forward, heeled angles and so on...

I hope that understanding it would clear my mind and eventually bring me to better understanding what I was doing. For this, I thank not only you but also all people who know more and would like to help me.

Well, having said so, here I try to come back to the original topic of the “sports recreational” dinghy. I think I came closer to the understanding that both of these terms are excluding each other.

I sat down, took in account all what I was able to gather form different people and books, and I tried to implement all this in a “dinghy.”
Perhaps I offended a few die-hard people with my computer drafting, for what I am apologizing. For me it is easier to create simple line work on the screen.

I am attaching a simple screen snap, 180* fly-around and actual Deftship file for anyone who would want to view it. Of course, again, my gratitude is sincere and endless; I wish I could express it in better terms.

Thank you all for help and guidance.

P.S. In case I made mistakes again, it is my opinion that gentle pointing in the right direction would do more good compared to hints and grunts. Thanks again.

 

Decidedly better - I like it.
Minor stuff:

• The rudder looks small. A big rudder means more control and less stalling. If you're concerned about wetted surface consider reducing the daggerboard size (slightly).
• I don't love the sheer line in profile. A rule of thumb is the lowest point should be 3/4 of the total length aft. But if it's a straight line in body plan (front view) that means it is planar, so perhaps it looks better in 3D than in profile. I'm not sure if I should advise you change it. Perhaps if you take the 3D curve as is and tip it in profile 'til the low point is where I suggest.
• While there is merit to bringing the mainsail luff low on the mast, it raises a question what you'll do for a vang. Perhaps you could do a vang above the boom like the Vanguard Nomad: http://www.bana.com/nomad/support/nomad_promo_sailpan.pdf. A Leg-O-Mutton sprit or wishbone boom would be a possibility on a catboat, but on a sloop the jib will snag on it when coming about.
• I suggest your mainsail have battens and a roach as shown on the Nomad (link above).

I haven't taken the time to look at all your calculations, but if you haven't done it yet I'll suggest you pick a target heel angle (low for this boat, perhaps 15 degrees), decide how many people of what weight will sit where when the wind is blowing hard, calculate righting moment, then assume 1 pound wind pressure per square foot of sail area, calculate heeling moment, and see if they match.

 

BTW this boat should have watertight side decks, sometimes called 'tanks'. If water starts pouring in as soon as the gunwale is immersed it will be too easy to swamp. A cockpit sole that's above the load waterline, and scuppers in the transom, are also advisable to make it rapidly self-bailing. Making the scuppers one-way with some sort of flap is a good idea so the stern will not sink should a heavy person move aft momentarily.

 

Thank you - I am doing my homework

I didn't slack, I am just digesting the literature I read during past time. Here is the list so far:

1. "Principles of Yacht Design" Lars Larsson and Rolf Eliasson
2. "Practical Seamanship" and "Surviving the Storm" by Steve and Linda Dashew
3. "Basic Ship Theory" by K.J. Rawson and E.C. Tupper (skimmed important parts)
4. "Boatbuilding Manual" by Robert Steward
5. "Sailing Canoes" by O.S. Tyson
6. "How to Design a Boat" by John Teale

Not that I memorized all data, but I believe I grasped the essence and I know where to go in case of missing knowledge. Sailboat statics is not difficult for me, unfortunately, my blanks in fluids behavior is more tough to fill.

Thanks again and best regards.

 

"DIY" dinghy to participate in "Florida Challenge"

REF: http://watertribe.org/fl-ultimate-challenge/

Here is another take on a small dinghy, hopefully with good characteristics. I wonder if this is acceptable for the "Florida Challenge?" I tried to make the hull shape as simple as possible. Please would you be kind enough to give me your feedback -- thank you.

 

My gut reaction is that I'd:

• Increase beam just a little;
• Decrease freeboard just a little, but only if you plan to have watertight side decks;
• Make the transom, and along with it the turning axis of the rudder, a little more vertical;
• Increase the size of the main and decrease the size of the jib.
• If you can make the daggerboard a nice foil shape with a nicely rounded nose you can decrease its size slightly.

It's interesting you got that hollow in the waterline forward. If you're twisting plywood you'll wind up with convex sections forward. Indeed, you've shown the bottom part of station 1 slightly convex. Did you develop this conically or use a computer routine for developable surfaces? I'm guessing you'll be closer if you make that concave waterline closer to straight and allow the forward part of the bottom panel to become more convex in section. Also if you round the knuckle in the bow rather than drawing it as a hard corner.

 

Not a bad design. I agree with all points Stephen made, to elaborate; the canted transom is more difficult to build, buys you nothing, makes the rudder geometry less than ideal. With a lot of freeboard it will have a lot of windage, it will be difficult to control in high winds. Unless your plan is to have it so loaded there will be less above water line than the drawing suggests.

Also, please, make the rudder lower aspect ratio that the main dagger board. That high aspect ratio rudder will have a very narrow angle of effectiveness before it stalls, it will be easy to stall, and therefore not be very effective, especially in rough conditions. You will not regret a large, low aspect ratio rudder. You keep putting up pictures of long slim rudders and I keep pointing out the folly of it. Go ahead and build it that way and you will be very disappointed. You will end up making a lower aspect ratio rudder later anyway. It is always cheaper to build it only once.

one more point I just noticed, your mast is right over your dagger board, how is that going to work? Enlarging the main and reducing the jib will move the mast forward enough to clear the dagger board.

 

Aren't we there yet?

Thanks for constructive advice, Stephen and Petros. I did what you suggested with a few exceptions. I didn’t decrease freeboard height because I plan to use this boat in coastal sheltered, but choppy water. It also gives me an opportunity to load the boat beyond designed displacement if it is needed. Also, I don’t like wet sailing unless the water is really lukewarm and it is somewhere near the tropical beach …

I try to balance the rudder so there it wouldn’t be strenuous to hold and steer the tiller during long passages. The problem with low aspect ruder in my opinion is that when heeled the longitudinal balance does change a lot and there is not much rudder left in the water. All small dinghies I sailed have high aspect rudders (Sailfish, Laser, JY, Phantom, etc.) I never had any problem of stalling in these boats if all maneuvers were made under power. It is a small boat and it would be easy to beat the rudder a few times to catch the wind.

Regarding the curvature – when twisting flat surface, it is inherited function to have hyperbolic shaped projections. I attached a little picture for illustration. I tried to eliminate this twist.

Thanks again for your help.

 

OK. Now have a look at the optimum LCB graphs in Larsson & Eliasson. I like where you're prismatic coeff. is, but suggest pushing BMAX forward until LCB is near 53.33% (8/15 the waterline length). In addition to being closer to optimum in terms of drag I think the boat will have a more balanced helm with the BMAX farther forward. More beam amidship will also give you a wider shroud base for less compression on the mast. I'd keep the transom the same width so the chine aft becomes straighter.

The photo attached is of the new Baltic racing yacht Stig. Wide transom, but BMAX is forward with straight lines back to the stern.

For the purpose of calculating LCB and curve of areas I'd remove the foils and look at the hull alone. I see the resulting boat being somewhere between your most recent version and the version in post #61.

 

☺

Thanks, Stephen. I adjusted the model and think that it did improve both aesthetically and numerically. I hope that this iteration brings the model closer to "sports recreational dinghy." I just forgot about optimal LCB location.

If it works out, I would like to build it out of simple materials, laminate it with epoxy fiberglass and travel a bit. That is the reason for slightly high free-board. I also will consider of some sort of sleeping arrangement in the boat either with a boom tent or with a pop-up deck.

Again, I thank you very much l for your help -- best regards.

 

That design should serve you well, nice simple lines and fewer panels. should be a simple build. You will also learn a lot actually doing the build.

The rudder can be a bit deeper if you prefer, but also consider that dinghys do not sail with much heel since the crew's weight is used to counter balance the heeling moments.

post pictures of your build, lets see it finally take shape.

 

I'm liking it much better, too. My feeling about the rudder is that it can be high aspect ratio without stalling as long as:

• You use a conservative airfoil section, probably NACA 0012
• The overall area is generous

I rather like what you said about wanting it to be deep to stay in the water as the boat heels.

How are you liking your Delft software? It helped you get those panel expansions, I see.

You still have design work to do, but you have a nice hull here. Time to think about the rig, placement of interior frames and bulkheads, chainplates, mast step, other points where the rig will impart loads, etc. Are you building in plywood?

Well, let me back up. I just took another look at the hull and I do still have a couple questions. You're not showing much "rocker" (bottom curvature in profile) in this version. With any weight to speak of in the boat the transom will be dragging. Where you've calculated your hydrostatics, about 650 lbs, the transom will be dragging A LOT. If it were going to spend most of its time with an outboard motor on it, it'd be fine. But if it's going to spend most of its time sailing at displacement speeds shouldn't it have more rocker?

I'm also puzzled by the way the curve of areas overlays on the profile. Why does it extend beyond the aft end of the waterline (excluding the rudder)? Is this the curve of areas for the immersed part of the boat only?

The boat seems to have shrunk to under 14 feet. Any reason it couldn't be a little bigger? Allowing for a 4" scarf where you join two pieces of plywood, you should have about 15'-8" to work with for each panel, yes?

 

Mokasine

Hello, everybody, again

I was exercising with bits and pieces of fragmental knowledge and yet again with computer programs for a while. I am almost certain that simple dory and skiff lines somehow settled in my mind by now.

So naturally, I tremendously expanded the program and started to juggle with the multitude of grid points, contours, normal vectors, Gaussian surface curvatures, etc. etc. As the result of this frantic activity, I have arrived at the presented below “design.”

I do understand that these lines look somehow dramatically different of those shown most of the times elsewhere. But I did try in my earnest to arrive at all those right curves, coefficients and proportions.
Of course, I had the spiral in the works too. Its first turn is as follows:

(1) had some sort of design parameters so it doesn’t float as pine coffin on the water (works are in the progress)
(2) be able to withstand rough nature of short choppy seas
(3) have a standing room -- that is why there is a peculiar something in the middle of the deck
(4) survive *long* time in the open waters
(5) comfort and other parameters should yield to (1)-(2)-(3)-(4) above
(6) everything is negotiable

I plan to add a mast, sail and rudder with all other things, which should belong to a sailboat as my thought keeps on churning, hopefully in the right directions with everybody's help. I hope that 1.2 tonnes displacement would be sufficient for light hard foam /FG reinforced hull and minimal necessities. The model shown doesn't account for sail, keel and rudder.

I am indecisive about hard chine lines at the bottom at each side and I am looking for any advice about this design twist. I would be very grateful for any help in continuing to layout the most key-parts suitable for this sailboat. For example I am leaning toward catboat but I am not sure about it. Is this a good idea?

I would be most appreciative for someone knowledgeable let me know what errors I made this time. Thanks in advance.
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Here are the answers to the some questions from the previous post (to the best of my knowledge):

Delftship software is fine for green horns such as I am. It does allow dynamic manipulation of the curves with on-screen results display. For example, I could drag waterline or chine, or move the keel/rudder and see how all centers are changing. I find it very helpful for someone who is learning. The program also computes areas and center of efforts. Regarding the panels – it was only for pictorial purposes. I don’t use Delftship for this purpose. What I have done in the past is to export control curves after smoothing them. I export DXF file into any 3D CAD. I use Autocad and Microstation for this purpose. After that I can unfold patters much more precisely rather than scaling them off the Delftship printout.

No, I will build another boat in foam/firberglass sandwich. I am quite proficient in this technique. I would like to keep the construction as simple as possible. I will model the successful Delftship model in any of FEA software and account for high stress points such as standing rigging, etc. My goal to have an empty shell reinforced at critical points and capable of withstanding variety of static and dynamic loads. Roughly to explain it, the hull would withstand something like 2G in any direction, with more in key directions because of wind gusts. It sounds like an airplane frame design which I am familiar with as well. As soon as separate myself from my present employer at the end of my terms, I will start looking for a shop to build me a sailboat. I know I could purchase a used one, but I would like to build it with my hands.

I believe that Delftship would brutally process anything specified as a part meant for hydrostatic analysis. It appears that the curve reaches zero at transom, then it extends and goes upward as the ruder acts as an immersed body and starts to calculate cross sections again. I do tend to include rudder and keel in my hydrostatic approximation for centers of effort, but not the cross sectional properties.

At that time I didn’t think of exact material list for the boat. Indeed, if two or three 4’x8’ sheets are used, then the boat hull could be made a bit larger, to account for the curvature length. At this point I am trying to achieve fluency with understanding mechanics of a smooth immersed body.

Thank you for your subtle pointing in the right directions. Best regards and happy holidays!

 

Different, but really interesting. Will it be ballasted or unballasted? Good luck & post lots of photos!

 

Forgot to add Gaussian curvature pictorial to show that there are no concave pockets in the model. The boat will be ballasted with flat iron sheet encased in FG (approx. 50% or slightly less of displacement) and have 400 liters ( approx 100 gallons) of sea water ballast placed in two 200 liter cisterns on each side close to the midship with electrical and mechanical pumps.