Hi, I have in mind to build my own cat. I grew up sailing Hobies 16 and A-Class cats and I would like to design an 18 or 20 ft. cat. Having spent some time in the boating industry, I have a reasonable understanding of watercraft design and building.
That said, I would like to start from a good basis and modify a known design based on my needs and construction method. Question, can anyone suggest where I could find basic hull plans for something like the Nacra Infusion or Nacra 20C, as well as data such as center of buoyancy, Cp and basic dimensions. Alternatively if a member of the community owns one of these designs (or a similar alternative), I could come to visit to study their hull and take a few pictures for inspiration. I live in central NJ. Thank you very much for any help or good piece of advice.
Formula 18 and Formula 20 Hull Design / Plans
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Posted: Nov 21, 2022 - 12:05 PM
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Posted: Nov 21, 2022 - 09:53 PM
One obvious idea that comes to my mind is a Taipan 4.9. They were originally garage built boats and I think you can still get plans. Granted though this boat is a bit smaller that what you seem to be asking for... Maybe scaling everything up 20% would be okay. -
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Posted: Nov 22, 2022 - 05:08 AM
How about a wooden Tornado?
https://content.instructa…0/FBWB03O9SMEP282LA0.pdf
Edited by jack108136 on Nov 22, 2022 - 06:11 AM. -
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Posted: Nov 22, 2022 - 01:22 PM
If you absolutely want to waste time and money, so please go ahead!
The 5.5 hull will do nicely if you want to build a pair of F18 hulls, in need of some modernizing if you are up to it.
https://www.catsailor.com…re-18-sq-fleet#Post28052
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Brgds
Lars
Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler
https://www.facebook.com/groups/1192604934176635
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Posted: Nov 22, 2022 - 03:06 PM
I did buy plans for the Tornado for some initial analysis. I have also sailed one few years back. While I think the Tornado is absolutely amazing and possibly still the Queen of beach cats. Given the age of the design and that race courses were focused more on upwind legs and that gennakers were not around, my concern with that design is that the forward buoyancy is limited when compared to more modern designs. That is why I was looking to get more information on more modern designs, and I particularly like the Nacra Infusion and I think the scaling it to a 20 wouldn't bee too much of a stretch. Any ideas as to where I could find those plans? -
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Posted: Nov 22, 2022 - 07:54 PM
Do you want to build a boat?
Or
Do you want to sail?
The hulls are "easy". The rest of the boat is HARD. -
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Posted: Nov 27, 2022 - 04:09 PM
I really want to do both. Building a boat is like therapy, except cheaper. So I enjoy the process and the reward you get from sailing the craft at the end. About the "rest" being hard, my idea is to use off-the shelf components that you can acquire in the aftermarket from Nacra of a few other builders (Mast, Rudder, daggerboards, beams, etc. -
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Posted: Nov 28, 2022 - 03:31 PM
An idea I have thrown around is the Tornado general hull shape but with inverted wave-piercing bows and Nacra 6.0 daggerboards.
For scaling the Infusion to 20', just find one and enter the dimensions into AutoCAD and you can manipulate numbers from there.
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Scott
Prindle Fleet 2
TCDYC
Prindle 18-2 Mod "FrankenKitty"
Tornado Classic "Fast Furniture"
Prindle 19 "Mr. Wiggly"
Nacra 5.8 "De ja vu"
Nacra 5.0
Nacra 5.8
Tornadoes (Reg White)
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Posted: Nov 30, 2022 - 11:17 AM
Texastuma - I like the idea of the tornado general hull shape with inverted wave-piercing bows. About the scaling of an Infusion - I am looking for a boat somewhere in my vicinity and thought the blog could help me on that! About the AutoCAD, I am better at using MaxSurf (although I haven't used it now for a few years), so I will use that for the hull design, coefficients and stability calcs. Tell me more about the Inverted Tornado idea?
Thank you in advance. Best, Ben -
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Posted: Nov 30, 2022 - 01:51 PM
Hey Ben, the cheapest way for you to get all the hardware needed for a Formula, is to buy a used Infusion and dump the hulls. Plus you have got the chance to study the hulls closely before dumping them
.
For me, your hull building ideas you have said it is therapeutic, is there something else you want to achieve?
Contrary to Jack108136´s opinion, the fact is, that the hard thing to build, is the hulls not the hardware.
If you persevere and start your build, go for at least 20ft with 10-12ft beam. One idea could be to buy an Inter20 and widen it.
As texastuma I am a Frankencat/tri builder, using hulls from existing beachcats.
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Brgds
Lars
Frankentri 5.8/5.5/Inter20
Aerow trimaran foiler
https://www.facebook.com/groups/1192604934176635
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Posted: Nov 30, 2022 - 03:34 PM
For the inverted bow Nader, I have it as a concept in my head; no real buoyancy calcs, just experience of what could to be improved. I did see a H21 with inverted bows added.. I haven't sailed it, but would be willing to. Before you start designing, I would recommend you sail a few of these suggestions to get some experience of what can be improved. And, I mean sail them hard... aim to hit speeds over 25 knots. I originally envisioned a tear-drop hull shape similar to the Nacra 5.8 or 6.0 with the bottom third of the hull the widest point. But, having a sharp bow helps in the high speeds. I would look at making the deck rounder forward of the front beam; it eliminates some weight cutting the edges off but it also would help in the cases it does go under at speed. I would look at flattening the exit at the stern waterline. In medium speeds, the stern may skid around a bit with waves, but I think it would hit second gear sooner.
This is all concept in my head as I rather use what I can find and get on the water.
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Scott
Prindle Fleet 2
TCDYC
Prindle 18-2 Mod "FrankenKitty"
Tornado Classic "Fast Furniture"
Prindle 19 "Mr. Wiggly"
Nacra 5.8 "De ja vu"
Nacra 5.0
Nacra 5.8
Tornadoes (Reg White)
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Posted: Jan 06, 2024 - 10:29 PM
Just came across this thread. Have you decided?
The hull designs you are considering, Tornado vs modern (F18 style) have very different design philosophies. The tornado has a deep forefoot, and it gets its forward buoyancy from being tall. Deeper submergence, more boyancy. The modern inverted bows are designed to be practically out of the water. These boats are designed to get their buoyancy from a “belly” drooping down a bit, underneath the centre of the hull. The fronts of the hulls are wider than the Tornado, but that doesn’t matter, because they are barely in the water. When forward buoyancy is needed, that’s when submergence of the bow provides lift.
You might want to consider this if you want to just put an inverted bow on a Tornado, even if you extend it. Also, if you extend the tornado hull, do you do a lot of tacking? Because with a longer, deeper bow under water, it’s not going to turn well at all.
The deeper belly of the modern F-18 style cats gives it much more flotation in the centre of the boat, keeping bow and stern shallower, making turning much easier. It pivots around the centre.
A couple thoughts.
K
Edited by anopheles on Jan 06, 2024 - 10:33 PM. -
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Posted: Jan 25, 2024 - 12:45 PM
Nothing more than a thought and conversation over a cocktail... To adapt a Tornado, you'd have to redo the entire section forward of the crossbeam. Drop the freeboard and reshape the decks.. If I had a Nader to chop, it could be done... I got the opportunity to sail an Infusion and was beyond impressed with the wave handling... I agree with your assessment of the bouyancy... Having a flat section aft of the front beam would be the ticket... just like the Cirrus 901.
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Scott
Prindle Fleet 2
TCDYC
Prindle 18-2 Mod "FrankenKitty"
Tornado Classic "Fast Furniture"
Prindle 19 "Mr. Wiggly"
Nacra 5.8 "De ja vu"
Nacra 5.0
Nacra 5.8
Tornadoes (Reg White)
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