8mph 8mph 8mph

MYTraveler · Apr 1, 2018

My cruise speed is typically between 8.5 and 8.7 knots. Sometimes, I will bump it up to 10 knots. My waterline length is sufficient to make that all possible at efficient displacement speeds. I never travel (cruise or otherwise) at XX mph.

OldDan1943 · Apr 1, 2018

markpierce said:
Hull speed is no-way near efficiency. Slowing down a notch can easily reduced fuel consumption by 50% and more.

Published data for AT34, 1400 RPM, 8 knots, 2.1gph. Mathematically, the sweet spot is about 7.5knots. BUT that is with 1/2 filled fuel tanks, 200gal and empty boat. Mine is a live aboard with lots of additional stuff on board so, fuel usage will be much greater at the sweet spot and far different from the published data.
Of course, this will also be effected by wind and waves and currents.

So 'hull speed' is just theory but a place to start.

FF · Apr 2, 2018

"Of course, this will also be effected by wind and waves and currents."

These will effect the boats speed over the bottom more than speed they the water , although a big headwind will require more RPM to overcome its effect on the boat.

Nomad Willy · Apr 2, 2018

Old Dan,
The main reason your AT burns more fuel is that it’s a SD hull whereas Mark’s Coot is FD. Big difference.

OldDan1943 · Apr 2, 2018

Nomad Willy said:
Old Dan,
The main reason your AT burns more fuel is that it’s a SD hull whereas Mark’s Coot is FD. Big difference.

Sorry forgot to mention that.
But I still believe the load is equally important.

Nomad Willy · Apr 2, 2018

Dan oh yes the load ...
But that’s mostly a function of the prop setup WOT rpm and all that stuff.
But the hull is what the prop pushes against.

FF · Apr 3, 2018

"But I still believe the load is equally important."

Important , but not too big a deal.

Most boats need 3 hp per ton (2240 lbs) at displacement cruise so a ton load of beer would only cost 1/5 of a gallon per hour extra.

8mph 8mph 8mph

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