15 foot sports recreational dinghy

a hull like that has tumble home, meaning if it comes up on edge when heeled it will completely flip over because there is no flair at the gunwale, and the resistance to roll over goes down as the hull heels over. If you eleiminate the tumble home design you would get more interior room, more deck space and eliminate the tumble home tendency if you put a hard angle where the sides of the hull meet the deck (the gunwale line). this will increase the windage some, but other benefits would outweigh disadvantage, and the tall center cabin has a lot of very high placed windage.

That hull shape would be more time consuming to build, no matter the method. Hard chins would save a lot of build time and have little effect on performance with careful multi-chine design. Just something to consider.

 

Here's a small sailboat with water ballast that has found favor with some (though not my cup-of-tea):
http://smallcraftadvisor.com/component/content/article/361
Now you have something to compare yours to that's in the same league.
An advantage of tumblehome in combination with a watertight trunk cabin is it's less stable upside down, so while stability at 90 or 100 degrees may be less, it may be greater (or less negative) at higher angles, making it easier to re-right and more likely a wave will re-right it. It does, however, reduce the crew's ability to move outboard, and it reduces the shroud base which may force you to use a heavier mast or add a set of spreaders.

A boat that is my cup of tea is this 18 footer from designer Sam Manuard:
http://www.youtube.com/watch?v=L4qsFGfekjU&feature=plcp

 

Thank you very much for the feedback. You wouldn’t imagine how valuable any feedback is for me. It is like a student who is trying to solve calculus problem and knowing nothing if the answer is correct. Thanks again.

Now I would like to reply to Petros.

If this exercise gets to be built, the boat would have both the iron keel and water ballast. The keel would be a simple cut out of heavy structural wide flange steel shape. It would resemble inverted “T”
The iron would be covered with a mix of sand and epoxy so I could shape it and then it would be covered with fiberglass. Also, two 200 liters (approx. 50 gallons) tanks would be placed on each side at center of flotation.
My intent of tumble home shape was to obtain self-righting ability in open water. The tall center cabin was meant for this purpose as well. It will not allow the boat to capsize and the ballast would pull it back afterwards.
My intent was to sacrifice convenience, beauty and common sense in order to get more safety. The cockpit would be self-bailing with one-way valves at the bottom. The companionway would be sealed not to allow water in case of capsize.
The windage area is not important if the wind is low. If the wind blows hard, the boat will heel and the cabin should be within footprint of leeward side. Actually, tumble home makes it aerodynamic. If you look at the heeled picture with 15 degree angle, the cabin is half-hidden. With more heel, say 20~25 degrees, it will be completely hidden from the oncoming wind. There is turbulence of course, but it is an order of magnitude less I hope.
The above are my thoughts, as I said a lot of stuff goes on inside my head but I have some difficulties to methodically expressing it.
As I said, the purpose of this exercise was a stable, open water capable, dry and self-righting sailboat with 6’ headroom. Comfort and looks are secondary. Efficiency of sailing and speed yielded to the above requirements. Unless there are gross errors, I could care less how it looks.
Again, thank you Petros for your sincere input.

Now I would like to answer to Mr. Ditmore:

I love this boat! It is my cap of tea as well. But I want to be able to cross large chunks of open water and have a sheltered cabin.

Again, all of the above are raw, rough and preliminary ideas. What I would like to sense if I am going in the right direction for the purposed I underlined.

Again, thank you for the help. Best regards...


Here is an idea about the standing rigging with foam filled fiberglass (or carbonfiber?) masts on each side to prevent capsizing:

 

There's not enough volume in those bi-pod masts to prevent a capsize, though maybe enough to prevent going turtle. That type of rig will have excessive windage, will limit sheeting angles and will place excessive weight aloft. It will also need to be very heavily supported, as the luff will try to pull the cabin roof off, when tensioned sufficiently enough, to hold a reasonable leading edge on the main. Other than a well dogged hatch, I've never seen a compainway hatch slide, that could prevent water from getting below in a knock down and they only work when they're closed and dogged, which most often isn't the case, underway.

 

Kvsg,

It is very poor design objective to compromise performance and utility for minor secondary considerations such as making it easier to self right. Except in the smallest dingys, recovery from capsizing is never to take president over capsize prevention. It does not happen that often and not exactly a selling point, nor even a major consideration for most boat owners. Design for good performance and function first, than make minor tweaks to address secondary concerns like recovery from a capsize.

 

Agreed, most folks can cast off sheets quickly enough to prevent a capsize. A knock down is a different story, which can happen fast enough that the spreaders touch, before the boat rights itself. This is where your focus should be, her ability to withstand a knock down and still get back on her feet. You can't prepare for every eventually, but you can make her really hard to "hold down", if you're caught with a sudden gust. A well founded design, can take what is dished out and stand back up and carry on.

 

Thank you for the good intended advice. I am thinking about well brought points.

 

I haven't seen any alternative sails for different weather conditions, or at least reefing provisions. If you are planning to sail in unsheltered waters this need will soon arise. Each set will need to be balanced, and the mast and rigging will need to permit that. This will add yet another dimension to the design task!

 

The bipod mast thing has been tried and it works, but isn't especially efficient and has a few problems to live with. The main could be roller reefed, though presents even more issues, certainly doable.

 

This hull has only 10% increase in wet surface when heeled and the hull keeps its submerged shape. In my experience fiddling with traditional designs the increase is in between 25%~35%

Windage area, sheeting angles, reefing, pullout of cabin roof and "top heavy" issues are bypassed as they are not detrimental and are off lesser magnitude of importance.

Happy New Year to everyone!
http://www.boatdesign.net/forums/at...ot-sports-recreational-dinghy-mokasine_01.jpg

http://www.boatdesign.net/forums/at...ot-sports-recreational-dinghy-mokasine_02.jpg

http://www.boatdesign.net/forums/at...ports-recreational-dinghy-mokasine_03_h15.jpg

http://www.boatdesign.net/forums/at...-recreational-dinghy-mokasine_03_gaussian.jpg

http://www.boatdesign.net/forums/at...eational-dinghy-mokasine_concept_dscn3422.jpg

 

15'6" gaff - rig catboat

Here is my next exercise with 15’6” catboat with gaff rig which I did in time of doing zilch during this New Year days off. Nothing in particular was though as the purpose of this boat, neither for stellar performance. I thought that this boat may be convenient for some traveling in the inland water and along the coast, maybe some overnight camping in warm weather. I think I could go a bit easier on the sails area, but I always try to reason myself that I can always reef it. This is the reason I used boom for the foresail. I think it could be reefed in this arrangement as well. It also would allow controlling it with just one line. I left out the deck, coaming and other small details.
Any constructive criticism? Thanks --Happy New Year!

 

My suggestion would be something between this hull and the one in post #70. I like the hull in post #70 best, though I think it needs more rocker to keep the transom from dragging.

While I grew up around Cape Cod catboats, I prefer the Long Island catboats of Gil Smith:
http://woodenboatworks.com/index.php/boats/custom/madigan
http://woodenboatworks.com/index.php/boats/custom/lorelei
(I root for the Giants when they play the Patriots, but remain neutral when the Red Sox play the Yankees.)

Sail area should be a function of stability, so my recommendation is to decide what people will typically sit where with how much heel going upwind in a stiff breeze, and calculate righting moment. Then calculate heeling moment with the rig as you've drawn it assuming 1 pound transverse pressure per square foot of sail (4.88 Kg / M^2 if I'm not mistaken). I suggest the two numbers (HM and RM using those assumptions) be matched (or at least close).

If you want to use Dellenbaugh Angle to accomplish the same thing, there's that, too.

I'm not clear on your answer about DelftShip - does it relate your 3D shape to the unwrapped panel expansions? What does it take to generate those expansions? That's of significant interest to me, and a considered answer could be very helpful. Thanks in advance.

I'm also working on a boat for Water Tribe competition - a trimaran that pushes the limits of class 3 (the amas can be stowed in the vaca, but just barely). So perhaps I'll see you on the water ...?

Happy New Year!

 

Delftship hard chine "unfolded" development tutorial

In the “Delftship” software there is a convenient feature allowing development of the model chine lines into flat panels suitable for stitch-and-glue boat building.

In order to have this program command be able to develop your 3D model in flat sheets, the model must be separated into layers. Each layer must contain one and only one chine panel for development.

In order to be objective I must mention that the same functions are available in free software “Freeship” from which Delftship has evolved.

In order to see if the model has good development properties, the software has special view mode. It allows each chine panel be developed into flat surface with minimal errors. Ctrl-D toggles this view. If everything is colored green, then the development panels will have high degree of precision when each chine panel is flattened. Red areas indicate that there are areas where the chine panel is not flat. It should be corrected by changing the hull geometry.

Here is a very short tutorial:

1. Create a quick stock new model in “Delftship”
2. Convert all chine line into “hard chine”
3. Move each chine panel into separate layers
4. Mark the checkbox “Developable” in the layers properties
5. See the panels develop after selecting develop command on tools tab

I hope this would help to millions of new boatbuilders worldwide

 

Ctrl-D toggles the same developability check view in FreeShip. However I wonder how reliable it is. For example I have a FreeShip file for a canoe design with fore-and-aft symmetry, but the developability check view shows one end green and the other mostly red.

It is a design (Rushton's Wee Lassie) that develops well so the red is incorrect and the two ends should be shown identically.

 

I am not sure about my speculation here, but I think that even slightest deviation from the flat surface will trigger red color in the software. In reality it could be 0.1" difference. In real life, panels could be forced to connect even if the pattern is slightly off. This would show in panel gap difference.

I checked this routine by exporting chines in two CAD software (Autocad/ Microstation) by developing identical flat panels using two independent triangulation routines. The convergence with the original developed chine was excellent, all flat panels were identical in shape.

So, I think the math behind "Freeship" and "Delftship" is correct. If you check your model, most likely you will notice some minor differences between the bow and aft sections.