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Old 04-19-2010, 10:04 AM   #1
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Hi,

Is anyone able to point me in the direction of a reliable resource that will help me work out how powerful an engine a displacement vessel will require?

I'm interested to work out what an appropriate replacement engine would be for a yacht (monohull) in terms of horsepower/kilowatts, given its LWL, beam and displacement. Going by the previous engine may not be a good option in case it was underpowered to start with (unless it is the original in a production yacht, then maybe it'd be ok).

Thanks!
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Old 04-20-2010, 01:33 AM   #2
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Originally Posted by Reaper' date='19 April 2010 - 05:04 PM View Post

Hi,

Is anyone able to point me in the direction of a reliable resource that will help me work out how powerful an engine a displacement vessel will require?

I'm interested to work out what an appropriate replacement engine would be for a yacht (monohull) in terms of horsepower/kilowatts, given its LWL, beam and displacement. Going by the previous engine may not be a good option in case it was underpowered to start with (unless it is the original in a production yacht, then maybe it'd be ok).

Thanks!
"(unless it is the original in a production yacht, then maybe it'd be ok)"

Taking the last point first, The new Leopard 38 Catamaran's specification has 2 x 19hp engines --- then in the options one can have 2 x 29hp. This technique is also understood to be used by the big production mono builders. The 'Bean Counters' method of maximizing profit. IE : use as small an HP as is necessary to achieve designed hull speed in good sea conditions.

Because there are enumerable formulae applicable to answering the question. I have attached naval architect Erik Sponberg's "The Design Ratios" in pdf format where in page 45 Erik provides the formula to establish 'The horsepower required to drive a vessel through the water' called Effective Horsepower, EHP

One other way of establishing HP would be to look at designs by Ted Brewer of boats for sale in Yachtworld USA, boats similar to the boat you have an interest in, and check what engine and HP is specified. Ted is renowned for his contributions on appropriate auxiliary HP for cruising sail boats
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Old 04-20-2010, 02:16 AM   #3
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PS to the above post :- My own experience with choosing an engine for a displacement cruising sail boat would be for a naturally aspirated engine which produced the required HP and torque at between 1500 and 1900 RPM. Many of the modern engines have to use a turbo charger to provide the HP. This means that one has to have at least 2500 RPM to drive the boat at designed hull speed - therefore noise and fuel consumption becomes a concern.
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Old 04-20-2010, 03:13 PM   #4
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Quote:
Originally Posted by Reaper' date='19 April 2010 - 12:04 PM View Post

Hi,

Is anyone able to point me in the direction of a reliable resource that will help me work out how powerful an engine a displacement vessel will require?

I'm interested to work out what an appropriate replacement engine would be for a yacht (monohull) in terms of horsepower/kilowatts, given its LWL, beam and displacement. Going by the previous engine may not be a good option in case it was underpowered to start with (unless it is the original in a production yacht, then maybe it'd be ok).

Thanks!
"In the case of displacement yachts, a motor performance of approximately. 2.5 kW pro 1 metric ton displacement is required, in order to be able to attain the theoretical hull speed. If this speed is to be exceeded by 5%, at least 25% more performance is required (approx. 3 kW / 1 t). 90% of the hull speed is achieved with a drive performance of 1.5 kW / 1t (41%). The performance requirement increases enormously, when reaching and exceeding the hull speed. It therefore does not make much sense to attain 100%, in the case of a sailing yacht using a motor, if 90% hull speed can be attained with 50% performance. If, in spite of this, a sailing yacht has a motor performance of approximately 3 to 4 kW / 1 t displacement, this is because the high performance is required in the lower speed range to be able to manoeuvre in bad weather. This can be designated as „extra manoeuvring power”."

I found this info*here under point 8. To my understanding these numbers apply to a traditional Diesel engine, even though this site is dealing with diesel-electrical engines.

Almost always older yachts are underpowered regarding today's expectations and needs as in older days the engines of sailing yachts were only auxiliary engines used for manoeuvering and in dead calms. That is different today - no matter what, today's yachts must reach hull speed also under conditions of beating against wind and sea following the philosophy "If the wind does not bring you home in time, the engine will. No matter what". But that is off topic here...

Uwe

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Old 04-24-2010, 10:53 AM   #5
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All great ....thank you for this info, Ive got the research info I need to review!

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Old 06-12-2014, 10:35 AM   #6
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Your help has been appreciated. Can you tell me the source of this info pl?
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Old 06-13-2014, 09:29 AM   #7
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Reaper, Have you considered going to www.boatdiesel.com and click on their calculators for calculating this info? George
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Old 06-19-2014, 10:23 PM   #8
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I think it would be wise to add in some other factors. For example, would the owners want a two blade or folding prop in order to enhance sailing performance or would they want a three blade prop with sufficient pitch to maneuver well in tight quarters under power.
Then too, a bow thruster would allow increased maneuverability without the more robust prop, but then also possibly present a sailing performance reduction.

My choice, aboard my already "non-performance" design, is to have a three blade prop with sufficient horsepower to kick my stern about in tight quarters.

Bottomline.- It's not all about matching Hp to dimensions & weight.
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