funny you should say that, there is one on eBay right now!

Ebay Aclass Capricorn

I have no idea how legitimate that is.

AHPC,

Quiet a few successfull A Class designs.

Taipan 5.7. Was originaly designed for a spinnaker and later adopted in it's class rule. Very good design and pretty quick. Was not quiet as succesfull sales wise due to the high price tag and only marketed in AUS which is a very small market.

Stephen,

By that line of thinking the Taipan 4.9 counts as well as it too was first launched (tried) with a spinnaker although that was quickly dropped after the first races with it. And it is a succesful F16 design although the newer designs are better.

The T4.9 hull is very similar to the Mark 3 and 4 AHPC A-cats. But I wouldn't not call either the T4.9 or T5.7 a dedicated spinnaker boat as the Formula boats and M20 etc are. Back in the days of the T4.9 and T5.7 the spinnaker was very much an after thought without making specific changes to the hull to carry a spinnaker better. Same with respect to the Tornado and Hurricanes. That pretty much changed with the introduction of the F18 class in 1994. From then onwards the spinnaker feature became a integral part of the hull design. But the designs began really to diverge a little after 2000 as is my original statement.

(To Matt) I'm really at pains to recognize the (strong) similarities between the A2/A3 and the Infusion, apart from both being of the same length. It may be that some distributed set of percentages relate the two hulls, but then the design part is only redefined as finding the right set of those. And where do we stop ? Some one commented that the Nacra 20 hulls look much like the old clippers of yore ! High V-shaped bow and fat and wide behind that. I imagine a similar relation exist between a super bike (road) and a dirt bike (of road). In the end both are motor cycles and there is only so much that can be different within such a framework. Still doesn't mean that the best frame for off the road is also the best frame for road races.

(To everybody) But I grant that that is a personal opinion.

What I can however state as fact is the following.

Using a single percentage to scale the Infusion hull to F16 size will produce a quite reasonable F16 hull without any modifications (and the factor is 90.58% or simply the ratio of hull lengths). This rescaled hull with have the exact same waterline when a crew of 138 kg is sailing it (against 150 kg on the Infusion). Doing the same to any A-cat hull (young or old) will result in awful F16 platform (will carry on a 6 kg crew on the same waterline ! yes less then 10 kg). You'll need at least several different scaling factors to even get an acceptable F16 hull that way. And we only know one of those factors, the other needs to be found somehow (experimentally)

Wouter

I think look at the under water shape and not the bits above the water. Since the F18 has to take bigger loads and weight, then this shape is different to the A2/A3.

Yes, I'm doing that. Although, the waterline is rarely in the same spot all the time. Obviously, there is a difference in hull attitude in light to strong winds but also during a dive the underwater body includes most of the bow and therefor any "above waterline" differences ARE important. I think this is also bore out in Greg's comment that for a spi boat bow height is a very important parameter, more so as bow volume.

I think it is the one feature that makes both the T4.9 and T5.7 reasonable spi boats as both have reasonably high bows while neither has much volume there. The modern A-cat designs have no bow height worth mentioning (less then 250 mm if I'm not mistaken against the 450 mm on my own boat).

For the remainder I refer to my eaxmple of scaling the Infusion hull by a single factor of 90.58% or any A-cat hull by 91.07%. When doing the latter a new scaling of 197.6% is needed for the bow height (when equalling 450 mm), but doing this all over results in something like 720 mm of mainbeam clearance which is FAAAAR too much. S0 we need a different scaling factor for at the mainbeam probably 139,9% (and just assume the rearbeam is now alright too). The so far rescaled hull will then still only carry 84 kg above the F16 class minimum of 107 kg when using the same water line in static wind conditions (no gusts). So the hull width needs to be scaled by 128.2 % if it is to take a 2-up F16 crew of 138 kg. For a 1-up F16 hull we need to find a different optimum between height scaling (bow and beams) to width scaling, but it is almost certain that these two factors will still not be the same.

Notice the large differences in scaling factors.

Now please do the following mind experiment. Take a ball and scale it by such different factors in the three main directions. Does it still look like sphere ? And do you think it will behave the same as the round ball ?

Now use the same scaling factor for all dimensions (as we can when scaling the F18 hull) and answer the same questions.

That is in a couple of phrases the measure of difference between any A-cat design and the F18/F16 designs. Irrespectively of whether they all share the same (cosmetic) raked back bow.

Wouter

Actually I think that there is a lot of comonality between the Infusion and the A-cat shown in the pictures here. Don't focus on scaling number, but on the principle:
* Having the volume in the bow low (inverted V), the "wave piercer" principle
* Reduced bow volume AND reduce stern volume, which leads to a pronounced rocker and a volume concentration around the front beam. I think that this is quite important to lift the bow for the downwind leg.


Some other comments:

you can't cut the upper part of the bow of an F18 or F16, because you want to attache the spi pole bridles

Bimare builds A-cats, F18Ht and F18, and apply the leasons learnt form the A (and the F18HT).

Agreed that you need different scaling factors for different dimensions, but I would still say that a Javelin 16 or X16 resembles the A-cats of their time.

I would prefer if the hulls are shaped to prevent nose diving instead of having a good recovery. Once the bows are under, you are slow even and you have to pick up speed again.

Cheers,

Klaus