she looks a lot lighter than her sister ( bobabout 1 ), is this just a “weight watchers” effect due to longer and narrower hull, or the displacement is really smaller ?
what about the performances with a bit more than 7 kts ? I am daily awaiting to see on youtube how she goes upwind ( and downwind … most obviously )
3 ) there are many different opinions about “critical” true wind speed for rig change ( from “A” to storm ) : 10 kts ? 12 kts ?.. more ?
it seems that you have spent a lot of time testing your models.
Have you reliable tws ( at least for bobabout2 ) ?
looking at your drawings I have noticed that keel profile is not anymore a flat plate like your previous designs… do you think that performance improvement using a streamlined section is remarkable ?
last but not least , are we going to see bobabout entering liverpool GP ?
Hi Flavio!
Thanks for the compliments.
1, Displacement is similar to Bob1,so yes “weight watchers” effect from a longer boat.
2, We sail here in all weather,unfortunately it is now dark before work and after so sailing is confined to weekends when not building boats or sailing big ones.Winter weather pattern is either gale or calm so nothing useful to show anyone lately.2 weekends ago we sailed in a very strong southerly gale with winds of more than 30knots and 2 foot high waves.These conditions showed some weakness to the boat when compared to an earlier design.however the conditions where extreme and Bob2 is a better alround boat than my earlier works.
3, yes I have spent considerable time systematically improving my designs over a 6 year period,much has been learned and continues to be learnt. Bob 2 cross over from A to B rig as designed is around 12knots TWS
4, Yes, I believe that a shaped foil is better than a flat plate.I use the HT 14 Drela section
and finally,No I will not be attending the Euro Footy GP,I do not know if any of my designs built and sailed by others will attend.
very interesting to know that such slim model is able to manage 30 kts and waves double her lenght
even more interesting your remark that such narrow boat is also a good allround design, not only a “fairweather” design
I have not built and tested 30 footys ( yet) ,but I have done a huge amount of calculations and my theoretical results -also- seem to say : “narrow is better”
based on these numbers I am now giving finish touch to my ITA-5 which will enter liverpool GP
++
about keel section :
I understand that drela HT 14 has been designed as an airfoil for model gliders tail
Due to the fact that this foil has been designed for minimum drag at zero angle of attack - not as a lifting wing - do you have carried out a real “two boat testing” on two sisterboats ( one with drela and one with a flat plate ) to confirm its good performances ?
Moreover, improvement is a matter of :
good tacking perfomances due to rounded and “softer” leading edge
and/ or better straight line speed too… ?
theory is nice, but reliable answers come only from sea trials
Flavio,
The Drela sections are chosen because it is claimed they will not stall at very low Re numbers
The biggest problem with footy keels appear at low speeds when the foil is not working,the boat that gets the flow attached to the fin first starts moving forward whilst the boats with flat sectioned fins are going sideways.
At high speed the Drela section is most likely not perfect,but appears no slower than others.
This was highlighted to me when we built the 507 kits last Christmas,the short chord(25mm) flat plate fin was useless until the boat gathered quite a bit of speed.Before the 507 gets moving boats with better shaped fins have sailed circles around them.
More testing of keel sections/lengths and areas is needed,I have concentrated on hull and rig development so far and often change so many things from model to model that it is not clear what was better and what was not.
Brett
I 2nd Brett’s comments on keel stalling. While sailing a Razor Footy with a flat keel in a swimming pool, the wind was swirling about due closeness of house & trees. I was amazed at the stalling and complete sidewise motion which got me thinking I needed more keel area such as Bill Hagerup now uses.
I used to race a dagger board dingy in the 70’s (Butterfly-12’ Scow) and always eased the sheets to get it going due to stalling. Then the Lasers came along with a foil design.
Flat was easy, but thanks to your comments, I am going airfoil shaped for next boat, a B-2.
From experience of Footys and big boats (i.e. 6 metres +), I agree that the aerofoil shpe is vitally importnt to stall - far more so than area. One vivid example was a Dubois designed 1/4 Tonner ‘Plum’ which was virtually uncontrollable to windwad in puffy conditions. We cured the problem very simply when we lifted the boat out and inspected the rudder. Ten minutes with a big rasp rounded off the almost knife-edge leading edge to something more sensible. This changed the stall angle from not much more than the steady state angle of attack to something with which you could usefully control bhe boat. After that she became quite nice to sail!
In my view the chord of a Footy fin should not go much below 37 mm and a thickness ratio of 9% is appropriate, although it is clrarly possible to make thinner foilss. For rudders, thickness ratios as high as 12% seem to me to be within the bounds of reason - much better to sail half the distance uner control but 1/4% slower than the other way round. If you can’t steer the brute, nothing else matters.
We havn’t been able to do much sailing recently,several weekends of light wind sailing however have shown both Bob2 and Comet to be very able.
Comet is especially good and I now regard her as my best boat to date.I wasn’t so sure a few weeks back but have tweaked the fin design and shifted the rig…is sailing on rails now.
(she was fast before but sailed on a knife edge of balance)
A mould will be built so I can build another more refined version.The original is now peppered with holes from modifications etc and has steadily gained weight (not that it was that light to start with.)
Comet has managed 4.00min around 3 laps of the internet course in light unsteady winds.
an aside…Looking for 180 degree or 360 degree lightweight servos (no heavier than 20g) I am hopeless when inside electronic parts so am looking for someone to do the dirty work for me,I will swap much fine gold,boat parts or whatever your Fooy heart desires for such modded servos.drop me a line anyone if you feel a swap can beneifit us both!!