Faster ferry?

[dboatsafrica]

Introducing safe marine transportation to Africa’s inland waters has been a major struggle but our first catamaran ferry is now carrying around 200 people/day from a Kenyan Island to the mainland.

While I am generally happy with what I have designed and built I am disappointed by the max speed of around 9 knots, I reckon it should do over 11 knots, and would greatly appreciate some input on what seems to be the black art of propeller sizing.

Vessel details: waterline length 13.14 metres, waterline beam 0.9 metres, moulded draft 0.530 metres, max displacement 9 tonnes, 2 x 42 hp @ 3000rpm Mitsubishi diesels, shaft angle 7 degrees.

The drivetrain was supplied by Vetus and they specified the original 4 blade props, 16” diameter x 18” pitch with Fa/F 0.69. Since the engines will only pull 1800 rpm I assumed that we are way over-propped and that the right size would give much better speed.

So Vetus went back to their prop sizing software and sent me 3 blade props, 15” diameter x 13” pitch with Fa/F 0.53. Performance comparison as follows:

Propeller type Engine rpm Boat speed Fuel Consumption
P4E 16” x 18” 1800 8.5 - 9 knots 6.2 l/h
P3B 15” x 13” 2800 8.5 - 9 knots 8.9 l/h

Given that the engines only produce around 28hp each at 1800 rpm and this is a 9 ton boat I think the performance indicates that the hull design and CoG to CoB relationship is ok. There is no stern squat and the shaft skeg is an efficient elliptical section.

You blokes, haven’t noticed any active bloke-ettes, have immense experience of real-world catamaran performance so I hope you can tell me:

A. The boat is simply under-powered - so I should just shut up and accept what the 4 blade props give me and be happy with the fuel consumption.
B. There is a prop size which will give me a big enough speed gain to compensate for the higher fuel consumption resulting from higher engine revs.

Thanks, Malcolm.