Thanks for all the queries regarding the SK4. Here is the low down on the SK4, what it is, where its going.
Since I am on the Cherub forum, let me first say that the SK4 is not a boat intended to compete with the Cherub. It is for slightly heavier people who want (in particular) something more powered up with more upwind speed and generally more speed in less wind. It's a kind of mini 18 but, at 13 ft, is more like a Cherub than a yacht.
The answers below come from queries raised from youtube.
What are the boat's Vital statistics ?
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Design values:
1) LWL 4m
2) Beam WL 850mm
3) Ideal sailing weights 130 - 150kg
4) All up sailing weight - 70kg
5) Sail area - upwind 17m^2, asymmetric 30m^2
6)Provisional PN 760
Actual values
All upsailing weight 84kg
Sail area 16m^2, asymmetric around 26m^2
Club handicap PN=780
Why another class?
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I had 15 years out of dinghy sailing and primarily sailed boards. When you get back in a dinghy, it's just slow - particularly upwind.
I wanted to plane all the way around the course. This means achieving a speed:length ratio of >3 upwind. The S:L ratio is speed in knots / root LWL (in feet). This is >11kts. (Lots of boats claim to plane upwind, but they rarely achieve speed: length ratios in excess of 2 – 2.2, which is semi planing (eg around 8-9kts in an RS800).
The average wind in the UK is 8-14kts. So you need a boat that will sail close to true wind speed upwind.
Next, I worked out the hydrodynamic drag angle required to achieve this speed. This is controlled by the ratio (side force/resistance). The top term is basically leverage and the bottom term is essentially weight. So you you need a light slippery boat with loads of leverage - you canny change the laws of physics Jim, as Scotty said.
I then looked at any existing monohull classes that could achieve the required ratio. A foiler Moth, yes, an 18 yes.
A Cherub or a 12 foot skiff sailed by 2 heavy guys with a no 4 rig, yes gets a good ratio, but I’m not heavy, and we rarely sail in enough wind for a no. 4 rig.
So why a new class? – Quite simple: because there is nothing out there that comes even close to meeting my requirements.
Boats normally sail with the knuckle of the bows clipping the water. So are the bows out of the water upwind?
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In the youtube clip <object width="425" height="355"><param name="movie" value="http: we were making 13-14kts and occasionally hitting 15kts. If you go that fast, you are truly planing, so the bows should be out of the water. Only in semi planing mode (s:l <2.8 should the bows be in or just on the water).
Am I footing?
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No. Physics dictates that if you sail close to wind speed, best vmg will be at an angle of around 50 degrees. You will find this is true of all true high performance boats (eg foiler moth, Tornado cat). You are still sailing at an AWA of 23 degrees!
Is 75kg all up sailing weight heavy
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It could be lighter, I agree. But it is about the same as a 12 foot skiff with a no2 rig, and most UK cherubs. Few people seem to realise how heavy rigs and foils are – allow at least 20kg for a full carbon set up.
What's it like to sail
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An extra foot really does make a difference! The boat does feel longer than a Cherub, but the plus points are that it is nice to sail in light winds, is really smooth upwind in Solent chop and it doesn't nosedive. It sails a bit like an 18 footer.
How fast is it?
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The boat is a bit quicker than an RS800 in winds <6kts and a lot faster in anything above. In the low to mid teens of wind it is quicker than a pro sailed 49er. Upwind it points higher and goes faster. We managed to clock 14kts downwind on the GPS in only 7-8kts of breeze.
Our club PN is 780 and we win races if the wind is >=8kts. This is with the boat that is at least 10kg overweight and has 1m^2 less sail than designed. You get around 30-40 PN points improvement in speed for each 10kg reduction in weight, so there's loads of potential.
Testers responses
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'Awesome, why not enter it for the women's skiff ISAF trials?' - top Olympic Laser radial sailor
'Makes the 49er feel like a bus in comparison' - RYA 49er squad member
'Way better boat than a 14' RYA 4923 squad sailer and 14 helm
'Incredible, it's just so smooth and fast... not for a relaxing sail, though!'- RS800 fleet sailor
Where next??
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The current boat is a prototype. I hope to produce a production boat this year.
The prototype has proven well mannered and relatively easy to sail. A bit more sail and some more leverage can thus be accommodated.
The production version will be both lighter and wider winged and with a 2 rig option set on the same mast and boom. The big rig is for heavies, small for lights. This will increase the competitive weight range from 130 to around 170kg. It will be a one design hull, but with a box rule rig (a bit like the B14). The idea is to produce a relatively low cost, truly high performance boat which is fun to just sail. Fitness, agility and 'feel' to dominate over lard and brute strength. The design will achieve a PN of around 700 in anything over 10kts of wind, thus making it the fastest skiff except an 18 footer. It will sail at or even faster than the wind upwind.
If anyone is interested, please contact me on SK4skiff@rya_online.net
PS I am looking forward to a go in Will's SK4 influenced E5 Cherub.
