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Old 11-08-2016, 06:23   #1
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Propulsion question

Hi all, we have just bought a 38' James Wharram cat. The previous owners have installed 2 x 25hp four stroke Yamaha outboards but are unable to tell us any range of cruising speed to assist us in planning our maiden voyage home. Can anyone help with a rough estimate of what speeds we can expect? TIA
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Old 11-08-2016, 08:22   #2
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Re: Propulsion question

A couple questions will help the forum, are the motors the workboat versions with 3-1 reduction and the big prop or standard 2-1 high speed motors? How much gas do you want to use? Economy might be up to 6 knots, average might be 7, in a hurry 8+ depending on props, reduction, boat weight and state of the underwater surfaces.
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Old 11-08-2016, 16:04   #3
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Re: Propulsion question

a spin around the bay will give you some indication of boat speed using gps. Speed also depends on current, tides, wind speed and direction. fuel consumption is also inflenced by these factors. budget 4 litres an hour for diesel but carry some jerry cans.
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Old 11-08-2016, 16:27   #4
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Re: Propulsion question

There yamaha ft25bet high thrust.i understand there are a lot of factors effecting this question,I'm just trying to organise a bit in advance.i have a 200nm trip and I no @ max rpm (5500) they use 9.3lph (petrol).so from everyone's helpful input I think 5-6kts and around 4lph is a good start.we have 10m tides so getting it on the outgoing in the bay should assist nicely for the first leg.
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Old 11-08-2016, 16:32   #5
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Re: Propulsion question

You just need to find the fuel burn chart for the engines and graph that relative to the boat speed in a strait line on a windless current less day.

For a cat you also need to run the boat with both engines at matched rpm, and with only one engine in the water.
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Old 11-08-2016, 19:35   #6
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Re: Propulsion question

I'm a bit of a half empty sort when it comes to these guesstimates. i would consider a worst case scenario: high winds head, on big waves, one motor overheats etc etc. Remember too that a 200 nm trip is more than that because you will not travel in a straight line no matter how hard you try to. I would budget an average of 4 knots and a minimum consumption of 6lph. That's 300 litres for the trip. It might be less if you can sail or motorsail. If you have a fuel guage, iI would look to see how far you've travelled when the tanks are half full. If it's not more than half way, go back. But fuel guages are not the most accurate of devices.

This might sound like overkill, but think of the worst case and the consequences. Are there not places you can stop on the way? Remember too that you might need to travel overnight (2 nights) and that adds another layer of complexity.
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