So... I'm still working on my F14 design and have had a good number of comments regarding hull rocker. When I look at F18s (Taipan, Nacra & AHCP especially) I see a boat with almost no rocker forward of the mast, and a rather lot of rocker going aft.

So... I drew a boat like that and recieved a lot of comments about how much drag it would create, and why was my rocker so far aft? (I drew the rocker rather centered under the mast beam, but with 6" of rocker aft and 3" of rocker forward).

So I looked at some other designs that seem to be doing well (F16 Mosquito), and at a recent drawing that was sent to me. The rocker was centered just forward of the mast, giving long straight lines aft that should "give a fast, clean path for the water". It also gives the bottom of the boat more of a foil-looking shape, and puts the greatest draft just ahead of the mast that would help reduce it's tendency to pitchpole.

So now I'm rather confused! I'm half tempted to build a starboard hull with aft rocker and a port hull with fore rocker and see which tack it sails best on! 'Cept I don't have a whole lot of time and money to expiriment with, and would rather get it "right" (at least mostly right) the first time.

Hi Brian,
Good to see you got my e-mail.
On the subject of the Mosquito, please bear in mind the hull design is from 1966/67, around the same year as the Tornado. (Some Mozzie sailors believe the Mozzie was first ! )
A lot has changed since then about the way designers think, although I have yet to see a 20ft boat with a "new" hull shape that shows itself to be far superior to the Tornado design, all other parameters being equal. If you took newer design hulls, and built the rest of the boat to Tornado class rules, equal weight, rig, platform width etc, I wonder what the outcome would be ? So this hull shape is a good starting point.
Looking at the newer F16 type designs or those similar to F16 specs, you`ll find that the designers have opted for "fatter" hulls with more bouyancy up front, plumb bows to extend the waterline to max. and revised rocker lines.
The taipan is essentially an "upgraded" Mosquito, with some 1980`s A-class design thinking. It`s still a fantastic design and damn hard to beat. It looks like the maximum draft has migrated a little aft compared to the Mozzie, but still in front of the main beam.
Now look at the Spitfire : Probably the boat that has most consistently shown the bigger isn`t always better, it has taken on the bigger boats & won on a regular basis in the UK. Again, max. draft is just forward of the main beam, with lots of bouyancy up front, to offset a larger than usual rig for a 16ft boat.
Now look at the Blade F16. From what I`ve seen, it has wave-piercing type bows with a fine entry, position of max. draft appears to have moved further aft compared to Spitfire, Taipan & Mosquito/Tornado. This seems to mirror some current A-class thinking. Modern A-class seem to have a lot of bouyancy behind the main beam, with very fine wave-piercing bows, but then they don`t have kites so have very different design criteria. They also have "teardrop" shaped bows when seen in section, so their bouyancy is placed as low as possible, this helps pitchpole prevention and also assists in "dive recovery" since there is no real deck forward of the main beam (A flat deck up front will slow dive recovery considerably, and can actually assist the boat to carry on diving.)
Unfortunately we haven`t seen any real comparative racing stats between Spitfire, Blade & Taipans to make any guess as to which hull shape is "faster", and even if we did it would be so difficult to prove that it was hull shape alone that made the difference. On paper they are all "equal" but I`m sure one design will be better suited to flat water, another might handle chop better, one might be faster downwind, the other faster upwind.
On the subject of rocker : The reason I`ve discussed position of max. draft is because esentially your rocker line is determined by this to a certain degree, assuming you design a boat without any drastic changes in hull shape. A boat with a lot of rocker aft is likely to be slower in flat water than a boat with a flatter rocker line, but it would probably handle chop better, up to a point. Because you want 14ft hulls with lots of bouyancy up front to cope with a large rig, you are likely to have a boat that hobby-horses if you`re not very careful about where you put the max. draft & rocker.
I`m not a boat designer, so anyone who feels like they could explain it better or correct me here, please feel free to do so.
Designing & building your own boat can be a lot of fun and a great acchievement, but having the best information before you start can prevent all those hours & dollars spent from going to waste.

Cheers
Steve

I have done what you are doing. My 14 is designed for a specific purpose Wed Night Racing in smooth protected water. Even so I am in the process of moving the front crossbeam back 18 ins and building a smaller higher front cross beam. It is just too painful in heavy chop.

I looked at the hobby horse issuing. I found deep bows (rocker forward) with a long striaght section aft (like a Tornado) maximizes stability and speed. If the boat starts to hobby horse, move aft.

Knowing what I know now, if I were building a general purpose boat, I would scale a Tornado hull, add plumb bows, and move the front crossbeam aft 6 inches. Attach the crossbeams like the C-Class boats. This would be a good starting point

One thing you may not yet realize, sailing technique will play as big a part as design. These boats are small and should be very light. Weight placement will make a huge difference and they are small enough to muscle around some. Sitting at the rear crossbeam and hiking hard like on a Laser I can actually torque the leeward bow out of the water on mine.

Profiles of some A-Class cats. Might help.
See Attachment

Steve- thank you very much for the sketch you sent. My final design may look very similar.

Carl- thank you for your insight. Is your 14 rigged with a spinnaker, reacher, or something similar? Where is your mast now, and how tall are your hulls?

Quiet1- those A-cat profiles are excellent! A very useful resource to everyone planning to design their own boat that won't deviate too much from the "norm".

The Freestyle 14 hulls will be a bit larger (taller) than is really needed, as I want the boat to be able to handle larger sailors and rough conditions.

Do you have any more measurements for us of these boats?
Is this available as an .dwg or dxf file?

Thanks,
Dave

Steve made many good points and I can think of one other factor to throw into the mix of trade offs to consider. As hulls move through the water they develop a boundary layer along the hull. Because of pressure variation along the hull the boundary layer eventually becomes unstable and seperates into a broad turbulent wake. Resistance can be minimized if this boundary layer seperation is delayed until as far aft as possible. The monohull strategy for doing this is to provide a asymetric waterline plane and aft rocker so the widest dimensions are aft of the midpoint. Sort of a pear shaped distribution of area. Of course, like everything else in boat design, if taken to extremes asymetric hulls perform worse than more moderate shapes.
As applied to catamaran shapes, it's more important to arrive at a shape that can go through chop and carry sail with little fuss than trying to minimize boundary layer friction. It's just another of a long list of things to keep in mind.

Kevin Cook

Dave,
I downloaded the image a couple of years ago.
I think it came from a French site. They were designing a new A-Class cat to compete with the Flyer, Mk5, etc.
I think the old site morphed into:
http://www.pulsar-performance.com/