15 foot sports recreational dinghy

I hope I am not degrading

Well, here is yet another try to come up with a something. This is a 6.50 meter hull shape which supposedly should qualify for Mini Trans Atlantic race. Basic dimension boundaries are there, my concern is -- “what did I do wrong this time?” I did try to take in account all earlier suggestion, no matter how subtle they were. I hope to hear more good advice and maybe less shuns. For those liking to say that I should read books, I did read a few. For those who say I should sail more – I just came back from a full day of sailing dreaded “sunfish.” It was a miserable experience because of some chop. I was all wet and tired of bouncing waves. I hope this is a bit dryer boat. Thanks all for consideration and some time you might think was wasted by reading this post. Best regards.

 

much nicer lines on this go around. This hull will handle chop much better than a sunfish, and keep you dryer. The sunfish to me is something suitable for ponds or swimming pools.

I would make the rudder a little larger and reduce the aspect ratio (length to width), you will never regret having too much rudder authority. Keep the depth the same and increase the chord width of the rudder about 50 percent, than I would take a sailboat like that anywhere.

 

Not much preformance potential from that one, especially with it's Cp being what it is (.51) and lack of power aft. She'll be fairly dry, though her entry maybe too fine and might hunt around on down wind legs. Firm up the aft sections a bit, so she can carry her press. What's her wetted surface to sail area ratio?

 

mini transat 6.50 - for me to sail away

Well, here is a hull with “power” aft which supposedly qualifies as “racing machine” based on Mr.Eric W. Spongerg’s definition (http://www.sponbergyachtdesign.com/THE DESIGN RATIOS.pdf). I beefed-up water lines to have Cp in pretty large category both in flat and heeled arrangement. To me it looks a bit stretched, but I guess many people that are more knowledgeable would know better. Here are some coefficients for this particular hull:

Cp level = 0.67
Cp 12 degree heel = 0.57
DLR= 107
SA/D= 32
SA/WS= 2.5
S= 6.3 “Racing Machine”
MCR= 37

I am not that sure about the last one, but I think a heavy lead keel would help to make this hull stiff.
I attached two print captures in Imperial Units: one for level output and another for 12 degree heeled hull, they are all came from DELFTship.

I made a flash movie file of the rotating models, both level and heeled, but I don't know any decent hosting service. If someone would suggest me, I would be very thankful.

I would be most appreciative if this exercise ends in something reasonable for Mini Transat 6.5 category sailboat. Perhaps someone would critique this work?

Thank you for your help, best regards.

 

You really do not seem to have the slightest grasp of how boats are shaped and why they are shaped that way. If you want to design a 6.5, why not start by studying everything you can find out about existing ones? They work.

 

Thanks No Eye Deer. I appreciate your opinion and I would like to ask what exactly did you mean? Your comment gives me no spatial direction to aim my efforts. I did study 6.5 class and I hope did understand the principle behind it. Here are some pictures from my archive which I keep for my own purpose. I also attached a short descriptive article about a boat with different shape used by others which won the latest Transat 6.5 Race. Thanks for your opinion. Best regards.

http://www.yachtingworld.com/blogs/elaine-bunting/453787/pocket-rocket

 

I have to agree with EyeDeer and as I've previously stated, you need to study yacht design, before playing with what you might think are preformance shapes. I say this because there are several very obvious flaws in that design you just posted, as being a preformance model. Frankly, that boat would be one ill handling beast. It would never go upwind without a winch on the tiller and it would never plane off either. If you don't know why that particular design is so poorly suited, then you obviously need to hit the books a little more. Simply put a Catalina 22 would kill that thing on all points of sail and a Catalina 22 wouldn't be considered a preformance boat by any stretch of the imagination.

 

Thank you PAR for your honest opinion. It is invaluable to me because it forces me to study more. But I would like to ask if you would be so kind as to let me know which obvious flaws are so bad there? It would help me in narrowing the problem. Thank you for your help.

 

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Those guys are right, K. The most clearly wrong thing about the hull shape is the rounded transom corners. The transom should have sharp edges-sides and bottom- so the water can break free rather than cause a draggy suction that will dramatically slow the boat. Just look at the shapes of performance small boats. Go to the mini6.5 site and check them out. Above all study, and research ,research and most important of all SAIL!

 

It has problems everywhere, buttocks, diagonals, waterlines, volume distribution, coefficients, shapes employed, just everywhere you look are issues. Pointing out the flaws isn't much of a help, if you don't understand the concepts. The corners on the transom and aft buttock angles will make it all but imposable to hold a close hauled course. If you don't know this, you shouldn't even be drawing yet, you should be studying principles and concepts instead.

 

Thanks guys for your help. I will try to improve the lines later. Thanks again.

 

Don't touch the lines. Study the above book and when you've absorbed it's contents, pull out those drawings posted above, so you can have a good laugh. We've all gone through it, dreams and drawings of grossly unsuitable craft, before we knew what we where doing. I remember a 40' ocean cruiser I was playing with, many years ago, before I had a clue. It had a pilothouse offset to port, so I could place a dinghy to starboard of it, it must have displaced 50 tons on a 35' LWL, ridiculous rig, everything was wrong. Every so often I pull that drawing out just to remind myself where I once was.

 

Hello, it has been a few days since my last post. I did try to listen to friendly advice of people who were kind enough to help me with advice to read and educate myself. So, I am back with my studies of simple boat design and here I tried to come up with a cruising sailboat which may venture in open water once in a while. I tried to account for rugged conditions and possible knockdowns. I hope this is not blunt erroneous combination of hull shape, coaming, sail area/placement and keel design. If it is not useless work on my part, I would be most appreciative if someone with experience would suggest me where I made gross mistakes. Remarks like “sail area is a bit big,” or “mast is a bit forward,” or “keel is not deep enough,” or something similar would be greatly appreciated. I hope that my studies are advancing and I am not wasting mine and anyone's else time. Thank for viewing my work.

 

If you go to www.touchcad.com and download the demo software, there's a sample file that comes with it called solidexample.t3d. You can rotate it by clicking the the box icon on the left. I'd be tempted to push bmax forward slightly and make some other minor tweaks, but this 'solidexample' hull is a good starting point - a bit like a dinghy version of an Etchells.

Looking at your latest design; why do you feel the chine needs to hook in so radically as it approaches the transom? I'd prefer to see the curvature in your chine line closer to midship.

Regarding mast placement, you should consult the appropriate portion of Larsson & Eliasson. My eye says your mast placement is fine, but you should give it more rake, which will mean moving the step forward or the partners aft. Ideally you might want it straight up, like you've shown it, going downwind, but raked aft when sailing to weather. Also, it would ideally have a little bit of pre-bend, and bend more as the wind increases to help flatten the mainsail.

 

that keel is not very effective, it will not point very well into the wind. I would use a high aspect ratio keel with the weight in a bulb on the end of the keel, with a separate rudder.

And what is with the stern? Either add some length and make it a full double ender (with lines closing smoothly to the stern), or cut it off clean. that is not only a lot of drag, but as the flow separates off the hard curves, attaching and detaching, it will never stay on course but wander back and fourth. This will be very tiring to sail, puts a lot of work on the helmsman. Also the sail is way too far forward for that keel, if you completely loose the jib it might balance okay, since it will not point very well anyway.

Are you in love with that stern design? It is a bad shape for any boat, it is a blunt bluff body, bad for drag, reduces rudder effectiveness, hard to build, hard to control and keep in a straight line. You keep putting up drawings of boats with that stern, and everyone keeps telling you to change it, yet it shows up here again on a new design. Why? Please, save yourself some trouble and do not even think about building it.

Study successful designs of similar size boats, you will never see a stern like that. Also study the relationship between the sail location and the center of the keel (center of lateral resistance). Study the lines of hulls, the slower you can change the direction of the water as it flows over the surface, the less drag it will have. You might start with that nice hull that Stephen posted above.

Back to the drawing board (screen?)