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Old 07-24-2014, 04:46 PM   #1
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Underpowered situation - 120 hp vs. 35,000 lbs ?

Good afternoon.

I have my eye on a 45' full displacement trawler that weighs in at 35,000 pounds. She is powered by a single Lehman 120 hp. The seller reports that she cruises at 7 knots without specifying RPMs.

Does this seem like an underpowered situation to you experts out there? I'm not looking for a fast cruise. 7 knots is fine for me, and I prefer a single screw. But I'm concerned that there might be maneuverability compromises that are being made with an engine of this size on a 35,000 pound boat.

Thanks for your opinions !
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Old 07-24-2014, 04:59 PM   #2
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A single FL120 has more than adequate power for such a vessel. Any maneuverability issues will be more a function of your experience in handling a single engine vessel that the power of that engine.
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Old 07-24-2014, 05:36 PM   #3
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That boat is overpowered if planning to run at displacement speeds....the rudder would be the maneuverability issue or if the prop was WAYYYY undersized.

Big singles can be a handful if you are not used to adding a burst of throttle now and then.
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Old 07-24-2014, 05:37 PM   #4
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You're fine and some would say you're over powered. Assuming your water line is 41', you'll only need about 38 hp to push her at 7 knots and for hull speed (8.6 knots) ~70 hp.
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Old 07-24-2014, 05:51 PM   #5
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Eighty horsepower is plenty for my 28,000-pound FD boat and cruise at 1800 RPM (for one-knot under hull speed) out of a maximum of 2400.
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Old 07-24-2014, 05:56 PM   #6
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I've got a 39,000 lbs.Kadey Krogen 42 with 120 FL ... Plenty of power ... Docking is fun, too!
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Old 07-24-2014, 10:01 PM   #7
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Our 60,000lb DeFever 48 is way over powered with twin FL120s. When/if I take the plunge and convert her to a single then 120hp would be at the top of my range.
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Old 07-24-2014, 10:42 PM   #8
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As everyone has said you have plenty of power with a single 120 Lehman. If you are concerned about manoeuvrability factor in the cost of a thruster.

Check out the archives there have been plenty of discussions about this. Some prefer the bow thruster some the stern thruster.

Also, prices vary a lot depending whether you want it installed in the boat or as a bolt on unit.It seems they vary from about $5,000 to $10,000 roughly.

A lot of the more experienced skippers hold that you don't need a thruster as with deft handling of the throttles you can manouver the boat quite well. However, in my experience a thruster is a nice thing to have on a single.

Good luck.
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Old 07-25-2014, 06:41 AM   #9
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Your engine is plenty large , find out the LWL and use the square root of that as your cheapest cruise speed in K.

Happily most ICW miles are measured in lubber miles so 7 MPH will be easy to cruise at.
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Old 07-25-2014, 11:22 AM   #10
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Anybody here powered w 4hp per ton or less?

There was a boat in PMM quite some time ago that was powered at 2hp per ton. It was wood and and 65 to 85'.
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Old 07-25-2014, 01:49 PM   #11
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The Seahorse Marine Diesel Duck 462 specs out at 3.4 to 3.7 HP per ton.

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Old 07-25-2014, 02:50 PM   #12
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Quote:
Originally Posted by manyboats View Post
Anybody here powered w 4hp per ton or less?

There was a boat in PMM quite some time ago that was powered at 2hp per ton. It was wood and and 65 to 85'.

We lived and cruised on a sailboat with a modified fin keel (Stan Huntingford design) for years that had a 50 hp Perkins 4-108 with 32000lb displacement and 36' water line. We couldn't make hull speed (8 knots) under power in the best of conditions. Beating into any type of seas or winds would reduce the speed by 2 or 3 knots, sometimes more. I can't imagine intentionally powering a power-only vessel similarly. We did learn that a clean bottom and running gear were critical. We stopped mid ocean one time to clean everything and gained 1.5 knots of speed.
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Old 07-25-2014, 04:48 PM   #13
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Power is fine but it's the torque to swing the prop that counts, with a bit experience you will be grand but don't be afraid to ask a good skipper for tuition.
It's better to feel a fool for a moment and ask for help, rather than go on and confirm it.
I bought an ex demo 95kg bow thruster of ebay for 1750 pounds sterling and it works perfectly and makes berthing so simple.
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