Mike

Michlet does not give dynamic lift.  

Michlet gives wave drag for the trim you nominate using an analytical method that is accurate for slender hulls - the default is level trim.  Likewise Michlet gives viscous drag in level trim using your choice of two empirical methods.  

Leo Lazauskas also has Flotilla freely available that iterates the trim condition of a hull due to the wave pressure that the hull creates.  It then determines the drag using the same method as Michlet for the trim.   

Leo also has a dynamic lift calculator for flat panels that uses an analytical method.  It gives quite different results to the flat panel empirical formula that NACA produced decades ago. I find the NACA formula is closer to what I see in practice.

What I have found is that Michlet will be close in most cases for giving the drag results until dynamic lift is a significant portion of the total displacement.  Even then Michlet can be used for the viscous and wave drag providing the hull is lifted and trimmed to suit the planing condition.  There are other real world factors that are more significant to a lw proa hull than hull drag in calm water.  The most significant are appendage drag, windage and ambient waves.  

You can use Michlet in conjunction with simple geometry to get reasonably close result for the condition where the stem is just clear of the surface.  Taking the hull in this condition you can determine the trim angle and the buoyancy; hence the dynamic lift required to achieve this.  Use Michlet to determine the displacement drag in this condition.  The dynamic drag is the lift times the tangent of the trim angle.  The unknown is the lift coefficient for the hull bottom meaning you cannot determine what speed the hull achieves the nominated trim.  However you will be able to determine that the increased drag associated with the dynamic pressure to achieve the new trim is less than the reduction in viscous drag and wave drag between the static trim and the planing trim for all speeds above a certain value.  This condition is also the worst case L/D, or close to it, for the planing forces because, as the centre of lift moves aft, the hull will level out so the L/D increases as the aft lifts.   

Rick

On 25/09/2011, at 3:16 AM, Michael Gehl wrote:

 

Rick, thanks for the reply. I still had this impression of planing (which to me = deflection) for bow trim causing more drag than the viscous drag of surface area in the water. 

I guess to completely understand this I need to model viscous vs. wavemaking drag at a specific speeds. Do Michlet/Godzilla model planing or dynamic lift behavior reasonably accurately?

Mike

On Sep 23, 2011, at 9:38 PM, Rick Willoughby wrote:

Mike

The dynamic lift on such a long hull actually reduces total drag.  The angle of inclination will only get to 1 to 2 degrees before the bow is out of the water.  The L/D ratio at such low angle of incidence is very high.  As the hull lifts there is a reduction in wetted surface so the viscous drag drops away.

There is almost universal misconception that hard chines are out of place on a sailing boat but essential for a runabout intended to plane.  I find it hard to comprehend the lack of logic with this belief.  

A long slender hull is not the ideal platform for high speed planing in terms of overall efficiency but a flat bottom will plane more readily than a round bottom.  The benefit of the long slender flat bottom over something wider and shorter is the the transition to planing is seamless.  There is no noticeable wave hump.  The hull just frees up as the bow lifts and there is a reduction in wetted surface.   

The wave drag on the the lw hull of a typical harryproa will be less than 10% of the total hull drag.  Viscous drag dominates.  Any hull that reduces wetted surface as speed increases will have a big advantage in outright speed.

Rick 

On 24/09/2011, at 10:59 AM, Michael Gehl wrote:

Mike (Crawford), 

   Given the long lever arm the leeward hull provides, might it not be a good idea to move easily shiftable mass to the end of that hull to combat bow-down trim? I nominate the water supply for that role; a tank at each end with a pump between - your choice of manual or electric.

I understand there may be some structural considerations at play, but can't help but th ink that dynamic lift carries a drag penalty. So does weight, but as long as you're carrying it, may as well put it to work.

Mike

On Sep 23, 2011, at 11:11 AM, Mike Crawford wrote:

Ben,

  I hear what you're saying.  Not only do you want the benefits of a proa, you want the benefits of a non-proa, too.  Fair enough.  I don't judge this because I want the same thing.  I won't get 100% of it, but that's no reason to not aim high.

  But for the sake of discussion, I'll play devil's advocate with regards to wei ght aft.  

---

  When compared to an equivalent trimaran in terms of weight and price, the proa is going to have a longer leeward hull.  Yes, there will be more bow-down trim than if you move everyone to the transom, but all that extra waterline length will mean a lot.  

  Take a Corsair F31, at 3,800 pounds, with a waterline length of 30', for $140k.  If you go with flat-panel construction, you could easily build a 50' stretched Harry (halfway between the Harry and the Visionarry) with a lower weight and lower cost.

  When the weather gets rough, or when pushing the boat in a race, I'd rather be on that 50' Harry than on the 31' Corsair with my crew in back.

  Particularly because the Harry can be made with long, wave-piercing bows that that easily shed water.  They'll be much more likely to slice thr ough a wave, or bury and then rise gracefully, than the Corsair's main hull.  No amount of weight aft is going to solve the problem of burying non-wave-piercing bows after surfing down a long wave face.  

  And since the proa bows are so far out in front of the windward hull bows, the boat will be less likely to trip over them.  

  Finally, with the right design, hydrodynamic lift will help with keeping the bow up, or at least not down as much as it otherwise would be.

---

  So I'd say weight location is an issue, but a secondary one given the design. 

  Of course, you could always build a proa with splayed beams, like the Expeditionarry or the Wantoo, and put crew further aft that way.  

  That said, I look forward to your new idea for shifting weight.  If we can have a long waterline *and* be able to easily shift weight, so much the better.

        - Mike



bjarthur123 wrote:

no, i disagree. i mean, yes, lengthening the hull is easy, but it does NOT result in bow up trim. merely less bow down. a very important distinction which i don't think many here or elsewhere appreciate.

having only been sailing for 13 years and never ventured out of sight of land, i don't have the experience that many others on this forum have. but the times which i've been in waves (1m on a dinghy, 3m on a yacht) it has been tremendously comforting to move all crew weight aft when off the wind. several degrees of positive bow up trim results and the hydrodynamic lift is very effective at stopping the bow from stuffing.

crew weight shift is difficult on a proa due to both ends being pointy. water ballast is possible but adds weight. pantographing doesn't do it because as mal (?) pointed out it moves the COB too. one could hinge the stern to reduce the buoyancy aft as i suggested on the proa forum a few weeks ago. i have one other out-of-the-box idea which i'll post soon once i get a chance to draw a picture so everyone can understand.

ben

--- In harryproa@yahoogroups.com.au, Rob Denney <harryproa@...> wrote:
>
> Probably not elegant enough to justify giving away your realm, but the
> easiest way is to lengthen the lee hull. Adds less weight, cost and
> complexity than hinges/pantographs/water ballast and increases top speed.
> Easy enough to make it removable or hinged if loa is a factor for
> trailering or marinas.
> 
> On Thu, Sep 22, 2011 at 11:46 PM, bjarthur123 <bjarthur123@...> wrote:
> 
> > **
> >
> >
> > 30:1 length:beam, okay, i can see how that wouldn't be stiff at all. and
> > true, a full circle has more surface area than a half circle for the same
> > volume, so more drag.
> >
> > a key difference with the weta is that the main hull is longer than the
> > floats, and has plenty of reserve buoyancy for waves. i've seen 1m waves
> > once and though exciting, wasn't a problem. the sa n fran guys routinely see
> > bigger i bet.
> >
> > another key difference is that it's possible to shift weight aft to get a
> > massive bow up trim. my entire kingdom for an elegant way to do this on a
> > proa...
> >
> > ben

Rick Willoughby

rickwill@bigpond.net.au

Rick Willoughby

rickwill@bigpond.net.au