As non-native english speaker I'm not sure if chop was the right word. What I was talking about was the steep waves typical for Baltic Sea in the shallow parts of the sea that can get very uncomfortable in any boat I've been to up to 100+ foot long. In those conditions all the boats I have been on were truly beating when upwind getting loud bangs on almost every wave. That is why I raised the question about flat bottoms and waves. However, I have not experienced wave piercing designs in those conditions so it may not be an issue with such designs. Hopefully that is the case since it would be a big plus.   Arto --- On Mon, 5/16/11, Rick Willoughby <rickwill@bigpond.net.au> wrote: From: Rick Willoughby <rickwill@bigpond.net.au> Subject: Re: [harryproa] Re: Design your proa To: harryproa@yahoogroups.com.au Date: Monday, May 16, 2011, 4:27 PM   Arto On your points: 1.  They are slender hulls.  I notice a bit of slap in small chop but that is about it.  It is more noise than felt impact.  I expect the flat ends also provide some pitch damping so reduces degree of harmonic pitching.  In bigger waves the bow cuts through the wave somewhat like a submarine.  It is nothing like the impact that occurs with a flat bottom planing hull.  The planing lift is not as significant as the buoyancy until speeds get high.  Something like 15kts for a light racing boat and above 20kts for a heavier cruising boat.  Even then it requires a large proportion of the waterline length so it will be rare that this all lifts and then hits the water at the same time.  Rob said he experienced this condition with a flat bottom ww hull upon releasing sheets as the hull was flying - it crashed down.  So can happen.    2.  I did some detail analysis on the buoyant lift for the 4T proa that Dennis Cox was/is developing.  I compared a round section hull and flat bottom hull that was 18m long.  Above 20kts the flat bottom could have the CoE 14m above the waterline and still trim bow up.  The round section was 7.5m at 20kts however it was 10m at 15kts compared with 4m for the flat bottom at 15kts.    So the hull shape and speed has a lot of influence in this regard.   I did not look at the self-trim  condition in the little proa design I posted recently because it could be trimmed with crew weight.  On larger boats it needs to be considered carefully because it will have some bearing on how much reserve buoyancy the ends need.  It is possible to calculate the trimming forces with some certainty. 3.  I notice he is using a single dipping rudder either end - proves the idea works on a proa as well as a pedal boat. Rick On 16/05/2011, at 10:04 PM, Arto Hakkarainen wrote:   Rick,   Thank you for very informative explanations. They make sense to me. Still every answer raises at least one more question :) From a sailor's point of view there are some things I keep wondering:   1) You have made very valid points for flat bottom design. However, having sailing experience mostly at Baltic Sea my worry is how the flat bottom would behave in waves and especially the choppy waves we see here. It could be quite uncomfortable and require bows to have deep V which could make the flat bottom not practical.   2) I have raised the question of bow immersion here before. Have you got any figures on the bow up forces of the flat bottom design vs. rig forces pushing the bow down? I think the question is most critical while running or broad reaching.   3) A comment, not a question: what you have said here seems to make sense to the traditional proa lee hull design with almost flat leeside and curved windward side like this Gary Dierking's Tarawa design: http://homepages.paradise.net.nz/garyd/tarawa.html .When working with no foils it must have been the most effective hull form.   Thanks for posting your analysis. Keep them coming.   Arto  A sailor and wannabe proa owner who has once sailed on Ono 43 years 2 months old with 42 years 8 months sailing experience to back up the theoretical questions   --- On Sat, 5/14/11, Rick Willoughby <rickwill@bigpond.net.au> wrote: From: Rick Willoughby <rickwill@bigpond.net.au> Subject: Re: [harryproa] Re: Design your proa To: harryproa@yahoogroups.com.au Date: Saturday, May 14, 2011, 8:50 AM   Arto There is some advantage to the cambered section over non-cambered section.  That is the reason most planes use cambered wings.  A NACA0012 of same aspect ratio as my proposed centreboard has a best L/D of 23.  This compares with 28 for the moderately cambered section I have.  I could get better ratio with more camber but it then becomes difficult to make it strong enough for the forces.  The performance difference is really so small that it would hardly alter top speed. There are a couple of reasons I do not like the idea of under-hull rudders doing the leeway prevention.  The main one is that the bow mounted rudder will see a lot of air as speed rises and this will cause it to suddenly lose grip at at a time when you want stable control.  The other is that the rudder has to work through a wider range of angle than the centreboard and making them bi-direction will result in them having more severe vortex shedding.  This would not be an issue if the rudders were set up to only go one way.  I also took a closer look at the ventilating rudders.  For comparison sake I looked at the performance difference using a ventilating leeboard with two small non-ventilating rudders as this is how my VPP is configured now.    This will give a slightly better result than two ventilating rudders.   With similar aspect ratio the ventilating board will have a best L/D of 8.  This would be noticeable on performance.  For the 15kt wind at 60 degrees true the boat speed at point of flying ww hull is 14.5kts versus 15.5 with the centreboard.    You can view the full polar comparison at 15kts wind here: http://www.rickwill.bigpondhosting.com/PN250_VPP_Boards.pdf In case you wonder why the polars converge when running it is because the modeling assumes the centreboard is lifted to just counter the leeway force,  So they are not adding to resistance when not needed.  Really the flat sided hull does a better job of preventing leeway than ventilating rudders above about 15kts.  The reason being that the hull is already being forced through the water so there is no extra viscous friction from using it to create the lift just the extra drag associated with the lift.  The best performance would come from lifting the leading rudder clear and adjusting the depth of the trailing rudder to give just enough control.  At slow speed there could be value in have more grip with the rudders as leeway will be high otherwise.  The situation would not be as clear if the hulls have round or "v" sections because these cannot generate as much lift for the area as a flat panel. Rick   That would be interesting to know. Ono has two independently steered rudders below hulls with NACA sections. I believe they are effective. Still Arttu decided to add daggerboards and said that the boat sails better. I wonder what kind of figures would we see if there were two independently steerable naca foils vs two smaller rudders and centerboard. But I guess figuring out the answer would take considerable amount of time.   Arto --- On Fri, 5/13/11, bjarthur123 <bjarthur123@yahoo.com> wrote: From: bjarthur123 <bjarthur123@yahoo.com> Subject: [harryproa] Re: Design your proa To: harryproa@yahoogroups.com.au Date: Friday, May 13, 2011, 9:14 PM   rick, could you please compare here more quantitatively the case of two large rudders vs. one big leeboard and a small rudder? you give numbers for the latter, but not the former. in rob's typical designs they ARE tweaked independently to prevent leeway. at least when going upwind. yes, they break the surface of the water and so could ventilate, but wouldn't necessarily do so unless the AOA were large. so yes they need to be large to keep the AOA small, which would create more drag. but how much more? specific numbers would be great. the tradeoff is that two big beam-mounted rudders are lighter (b/c the hull doesn't need reinforcing etc). lighter boat is faster (and cheaper). but does the lighter weight compensate for them being less hydrodynamically efficient? hard to tell without concrete numbers. rob-- do you have specific numbers on how much lighter? ben > The biggest benefit comes from the advantage of the high L/D possible > with a centreboard under the hull compared with the much lower L/D > that a slender hull can produce. > > For example taking the case of the design I have shown in 15kt wind > at 60 degrees true the boat is capable of flying the ww hull but, > assuming the wing is backed off to keep the ww hull on the water, > forces are: > Wing thrust is 626N > Wing healing force 1255N > Centreboard drag to balance this is 43N so L/D for the board is > almost 30 > > In the case of two large rudders, unless the twin rudders are > cambered or they are set up to be independently tweaked to balance > leeway the hull becomes the leeway preventer. I expect leeway will > be as high as 6 degrees for a shallow long slender hull even with its > hard chine. In this case the L/D is 10 so the extra hull drag > associated with resisting leeway is 125N. This is 3 times higher > than the cambered centreboard and an extra 13% drag on the 626N total > drag. > > The case for tweaking the twin rudders independently does not help > much if they are ventilating because they have to be very large and > draggy to generate the required lift. > > In practice the above may not be as significant as the more stable > control provided by a non-ventilating centre board compared with > ventilated rudders with varying degree of immersion. Rick Willoughby rickwill@bigpond.net.au 03 9796 2415 0419 104 821 Rick Willoughby rickwill@bigpond.net.au 03 9796 2415 0419 104 821