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Old 01-09-2010, 16:22   #1
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Matching Engine, Prop and Boat

For example, John Deere makes a diesel marine 84-horsepower engine model 4045D with a rated r.p.m. of 2400 with peak torque delivered at 1400 r.p.m.

In regards to a displacement hull, would the ideal be to match a propeller and boat with this engine so that cruising speed is reached at 1400 r.p.m.? What is the "rule-of-thumb" competent boat makers use in matching engine and boat to be operated at displacement speeds? Thanks.
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Old 01-09-2010, 17:20   #2
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For example, John Deere makes a diesel marine 84-horsepower engine model 4045D with a rated r.p.m. of 2400 with peak torque delivered at 1400 r.p.m.

In regards to a displacement hull, would the ideal be to match a propeller and boat with this engine so that cruising speed is reached at 1400 r.p.m.? What is the "rule-of-thumb" competent boat makers use in matching engine and boat to be operated at displacement speeds? Thanks.
you want to put full load on the engine at half RPM. I doubt JD will ok that warranty.
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Old 01-09-2010, 19:01   #3
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I think peak torque rpm is a good choice for cruising speed. You should still prop it so it can just get to the rated max rpm.
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Old 01-09-2010, 19:19   #4
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What is the "rule-of-thumb" competent boat makers use in matching engine and boat to be operated at displacement speeds?
One rough rule of thumb is 1 hp per 550 lbs of displacement.
Note: you should use fully loaded displacement, including all tanks full, max crew, stores, etc.

To get a more accurate figure for horsepower needed, calculate your boat's resistance at hull speed, divide by the prop efficiency, and add some reasonable reserve. You really need to get a copy of Dave Gerr propeller handbook, if you're serious.
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Old 01-09-2010, 21:02   #5
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35' Seahorse Coot?

I'm assuming you are posting about a 35' Seahorse Coot, 28,000lb and 13' beam.

I put a John Deere 4045D into Boracay, a Roberts Offshore 44 that's about the same weight as the Coot.

With a (from memory) 22"x15" prop I got a maximum of 1700rpm and 8 knots with everything clean. From memory 1400rpm gave me 5-6 knots with everything clean.

I had the prop repitched and cut down to 20"x12.5" and I can now get 8 knots and 2500rpm with everything clean. However the engine does not sound happy.

At the moment I'm using 1700rpm to cruise at 6 knots with good fuel consumption. With the boat half dirty I now only get 2300rpm and 7 knots.

My prop man said that if I brought a new prop he'd go for less pitch and slightly more diameter.

I'm thinking I'd like 10" pitch, but this is less than most "tables" suggest.
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Old 01-09-2010, 21:28   #6
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Thanks! I'm a self-admitted ignoramus concerning engines.

My philosophies for engine running are (1) don't operate beyond 66% of maximum engine rpm except for very-short-term emergency conditions, and (2) cruise between 40-50% of max rpm. Feedback?
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Old 02-09-2010, 00:32   #7
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OK on a John Deere...

What you're suggesting sounds OK for a John Deere 4045D. I have found that there is a power setting where the engine sort off "digs in", things stop rattling (Problem with DIY install...), and I'm sort of moving faster than other boats. Engine is loaded but not overloaded.

The higher reving engines may need a different technique.
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Old 02-09-2010, 04:35   #8
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Thanks! I'm a self-admitted ignoramus concerning engines.

My philosophies for engine running are (1) don't operate beyond 66% of maximum engine rpm except for very-short-term emergency conditions, and (2) cruise between 40-50% of max rpm. Feedback?
if you had a gas engine I could go with your running philosophies.
You have a high torque diesel. Running it like that will be killing it with kindness. Akin to running your 5 speed car in 5th gear all the time.
Prop for 110% of hull speed at max RPM. then you get what ever speed you get. AS you pull off the power.

Diesels are rated differently than gas engines. The will have a 1 hr rting. This is a power setting that can be run for 1 hr out of 12 hrs. Then they will have a slightly lower setting know as the continuous rating. You can run at the continuous rating until the cows come home. the engine won't care.
Over loading will exacerbate cylinder wall wear.
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Old 02-09-2010, 10:05   #9
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if you had a gas engine I could go with your running philosophies. ...
How perceptive! My only experience has been with gas engines. Thanks!
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