The flat bottom on the harryproa is a recent innovation.  I believe all operating harryproas have semi-circular sections on the lw hull and slightly flatter sections on the ww hull.  The flat bottom is something Rob is trialling with Bucket List following the demonstrated performance advantage of a hard chine lightweight hull at speed where dynamic lift becomes significant.  

The harryproas vary over quite a range of weight and sizes.  Even for a given length the weight can vary substantially.  The eventual displacement will depend on the materials of construction and what gets crammed into it.  More weight means slower.

Hence the starting point is a weight estimate or target weight.  With a cruising boat a hard chine hull will be unlikely to have enough power to motor beyond full displacement mode.  So no benefit when motoring.  I have given the drag components in a previous chart and drag equation for motoring.  Depending on the props chosen the conversion efficiency will be in the range 50 to 75%.  Power is a function of speed times drag.  So 800N at 4m/s (say 8kts) equates to a power input of 3200W.  At 60% efficiency that becomes 5.3kW.  That is only for the hulls at light displacement of 4000kg and smooth finish.  Adding the underwater appendages, the above water windage (even in calm conditions), less than ideal surface finish it can easily double or worse.

The condition under sail are somewhat different because the hulls are no longer loaded as they would be in static trim.  With the harryproa the lw hull bears a greater portion of the weight when sailing.  Taking an example of an 18m hull with a flat bottom carrying 3000kg the dynamic lift at 15kts could be 8% of total weight.  At 20kts 15% of the total and 24% at 25kts.

The dynamic lift to drag on a slender hull is very high.  So the benefit comes from reducing wetted surface as the hull is lifted thus reducing viscous drag more than the increase drag associated with the dynamic lift.  The reduction in drag with a flat bottom could be as high as 15% at high speed. 

Working up polars for a harryproa has to consider many factors.  I produce them for the 18m proa but they are time consuming.  This enables me to analyse the impact of the various ideas the owner contemplates.  I can assess their merit on a cost to benefit basis.  This is an example:

This polar is reasonable for the conditions it is based on - primarily calm seas.  Once wind is above 10kts the wave height can be detrimental depending on the fetch.  The boat has reached 16kts in similar strength wind but with a short fetch to the windward shore.  Wind strength above 17kts makes it difficult to control the shape of the rig due to mast flex so I doubt that 20kts is ever possible unless riding a wave.  

Performance factors that tend to get overlooked are hull fouling, wave clearance and windage.   A surface finish or fouling equivalent to 80 grit sandpaper increases hull drag by 50%.  There is nothing that can be done with the selection or size of rig that can offset that sort of increase in drag.  30 minutes spent removing slime before sailing will be equivalent to tens of thousands of dollars spent on improved rig.  Similarly attention to detail with aerodynamics of cabin, beams and hulls will improve windward ability also equivalent to tens of thousands spent on bigger and better rig.  Once anything other then hulls and underwater appanedages is being forced through the water drag rises dramatically.  In fully developed waves in 15 to 20kts of wind the 18m proa has quite a few surface contact points that clip the waves - another reason it is difficult to get it above 15kts.

• Privacy • Unsubscribe • Terms of Use