excellent! thanks for the quantitative analysis rick!! my skills in this area only permit me to think qualitatively. it's very nice to see the numbers.

how would things change if dynamic lift from planing were included? if i understand correctly, your calculations included only hydrostatic effects. this really has been my main point all along, perhaps not well articulated.

on the 4.4m weta trimaran, it is common practice to sit directly on the transom when going downwind. in really heavy conditions one even hikes off the stern. i don't mean sitting on the side rail right next to the stern and hiking outward, but rather sitting on the transom right next to the side rail and hiking aft. the bow might pitch 10 degrees up. 85 kg crew on a 115 kg boat.

the effect is extraordinary. that big of an AOA combined with the weta's more-or-less flat planing hull permits you to surf straight down a one meter wave in 25 kts of wind and plough directly into the next one without worrying about the wife and children you would otherwise leave behind. green water comes up to the mast, but the boat miraculously pops back up and keeps going. downwind VMG is much faster of course if you steer sideways down the wave like a surfer. but you won't pitchpole if you momentarily lose focus.

a massive increase in dynamic lift, and the accompanying safety / performance benefits, is what i have been longing for in a pantographing proa. correct me if i'm wrong here but no matter how flat the bottom, if it's pitched down it's not going to generate lift.

but then it occurred to me this morning that proas, harry proas at least, don't deploy a spinnaker off the wind. (do russell brown's?) same sail area upwind as down then, unless it's light enough you can shake out a reef down wind. so if the overall sail plan is sized for close hauled work, then the lighter apparent wind on a broad reach means you're going to be underpowered. and so you don't need the extra righting moment, because you're not going to pitch pole anyway.

i imagine a really hard-core racer would want that spinnaker though...

ben

--- In harryproa@yahoogroups.com.au, Rick Willoughby <rickwill@...> wrote:
>
> Ben
> My comment about the trim from buoyancy alone was to make the point
> that there is already a tendency to lift the bow once the hull is
> moving that counters the driving moment forcing the bow down. As the
> hull is pressed into a wave there is added lift due to wave making
> until it is fully submerged.
>
> Comparisons between a cruising harrrproa and a big tri highlights why
> one is more likely to trip up than the other. As an example an ORMA
> 60 has an allowable mast height over water of 30m. For the same
> length the 18m proa has a mast height over water between 16 and 17m.
> So huge difference in height of the CoE for the length. A ballast
> tank aft in the trimaran is more than handy for even moderate
> conditions.
>
> An ORMA 60 is about 6tonne. The 18m proa is 4t. The ORMA 60 can
> carry 285sq.m of sail. The 18m proa 87sq.m.
>
> I will take a punt and say that the fine bow of an ORMA 60 ama would
> be submerged 6m to carry the entire displacement. The 18m proa lw
> hull about 4m.
>
> Assume the ORMA 60 has CoG 2m aft of centre giving righting moment
> for pitching of 48tm. The righting moment for the 18m proa is less
> at 28tm.
>
> Considering a slow motion pitchpole where the leeward bow is buried,
> speed drops and apparent wind moves aft, the flat panel Cd of the rig
> comes into play. Can take the Cd as say 1.2.
>
> Take CoE in both cases as 40% of mast height giving 12m for the ORMA
> 60; 7m for the proa. The rig drive required to induce pitchpole is
> therefore 4t for the ORMA 60 and also 4t for the proa.
>
> The ORMA 60 rig will develop this drive in 14kts of wind. By
> contrast the 18m proa will need to be in 25kts of wind.
>
> Lets now shift the CoG of the proa 2m aft so the pitching righting
> moment goes up to 36tm. Hence sail drive for pitchpole rises to
> 5.1t. Wind strength to induce pitchpole goes up to 28kts.
>
> Is the ability to safely handle 28kts significantly better than
> 25kts. Also is it a real gain because the ability to shift CoG will
> have some weight penalty. Then there will need to be more sail to
> get the same speed with the extra weight. That means higher CoE and
> more drive for given wind strength and so on around the design spiral
> - end result will be less than 28kts.
>
> The elegance with the harryproa is with its overall simplicity. The
> margin of safety is as much controlled by how the boat is sailed as
> any inherent features. Any lightweight sailing boat that can reach
> maybe 20kts in 20kts of wind has potential to pitchpole. On the
> other hand a boat like an ORMA 60 that can reach 40kts and sail
> better than twice windspeed has much more potential to pitchpole. In
> both cases the likelihood of the boat pitchpoling will be more to do
> with the mindset and skill of the operator than the inherent
> potential of the boat.
>
> Rick