Rarebird's power of 8kW to do 6kts certainly suggests a heavily fouled hull and low prop efficiency. Taking the value 6kts at 8kW then that corresponds to 101kW to do 14kts. Power for wave making will be negligible compared with the seriously high viscous drag and that will closely follow a cube relationship versus speed.
I can absolutely guarantee that 30kW will not push a 4500kg proa up to 14kts.
The 18m proa has managed 6kts with 4kW total battery drain but in perfect conditions with fresh anti-fouling. With full 10kW on each drive and slight tail wind in flat seas and fresh anti-fouling it has hit 8.5kts. That is with large diameter (520mm) low BAR props pitched and geared for that condition.
A planing calculator is meaningless for a slender round bottom hull. A slender flat bottom hull will produce some dynamic lift that could be significant but at best drag is only reduced by 20% - it can only ever be semi-planing because there are no steps and planing sleds. It could not have steps and sail in either direction. The calculator also lacks any speed cut-off where the hull would be in displacement or semi-displacement mode - there would be no way a 4500kg boat could fully plane at 14kts.
On 20/03/2015, at 5:53 AM, "mateo002@yahoo.ca [harryproa]" <harryproa@yahoogroups.com.au> wrote:
Oops, somehow missed your message Rick. Good info there about props, seems shopping around is in order. But your speed estimate for 30kW seems a little low compared to Rarebird's chart showing 6kts with fouled hulls using 8kw from twin Torqueedos. 14kts sounds better to me :)
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Posted by: Rick Willoughby <rickwill@bigpond.net.au>
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