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Old 14-03-2020, 11:13   #1
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Hull Speed/RPM Ford Lehman

I have a 1981 40 ft Ocean Alexander with a single 120 Ford Lehman Diesel. I cruise at around 1650 and travel at about 6 1/4- 6 1/2 knots fully loaded with fuel, water, stuff. With a WOT I can reach near the 1900 rpm mark at 7 knots. The manual states that a 120 should reach 2400 rpm? This is rebuild engine with 400 hours. Should I be going a little faster and more RPM? Thanks for any response. Ed
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Old 14-03-2020, 13:18   #2
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Re: Hull Speed/RPM Ford Lehman

How are you measuring engine speed? Confirm with a photo tach the accuracy of your present tach.

You may be over propped, this isnít uncommon.

How clean is the boats bottom and the prop? A surprising small amount of fouling can cause this as well.
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Old 14-03-2020, 14:15   #3
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Re: Hull Speed/RPM Ford Lehman

You should be able to reach hull speed of 8+ knots even over propped. I've run a number of over propped boats, and they all made hull speed.
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Old 14-03-2020, 15:01   #4
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Re: Hull Speed/RPM Ford Lehman

Sounds like you are over propped to me. At that low rpm you are probably half the 120 HP. (See HP curve)
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Old 14-03-2020, 15:18   #5
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Re: Hull Speed/RPM Ford Lehman

Quote:
Originally Posted by Rodeo44 View Post
fully loaded with fuel, water, stuff.
Ed, if you know what size the prop is you can run it through the calculators to give an approx size or confirmation. However, if she has fouling on the hull and particularly the prop, or there are any dings etc then you will suffer. The other killer is over loading a motorboat. Dump some stuff and try travelling with 1/4 - 1/2 fuel and water tanks if that is possible. You will notice the difference, even our yacht for day or over night trips we will have half tanks and fill up the water on arrival.

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Old 14-03-2020, 17:11   #6
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Re: Hull Speed/RPM Ford Lehman

SailMonkey, thanks for reply. Picture later as boat is 2 hours away. Boat bottom is clean. Prop into the shop to inspect and find out if it's is correct fit.
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Old 14-03-2020, 18:04   #7
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Re: Hull Speed/RPM Ford Lehman

V/ good motor ,was the same speed at same rpm before the engine rebuild ,also wot is the g/ box and ratio ,has this been re built or changed in any way ,is the shaft free to turn by hand ,if these are all good the motor should be producing much black smoke at full throttle if over loaded or over proped .these engines if normally aspirated ,need read like a large bore exhaust at least 3 inch I.d .if a long and winding exit go bigger ,make sure the exhaust ,water trap matches the hose .also check back pressure a sure killer of rpm . Hope this helps ,over load and carbon build up will eventually ruin a v/ engine . Searles
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Old 14-03-2020, 20:54   #8
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Re: Hull Speed/RPM Ford Lehman

Any chance you’re fuel starved? I had a similar situation with my Lehman 80, pretty much the same numbers right after I bought the boat—clean bottom, shiny prop. I assumed the builder intentionally overpropped her so she’d cruise at max torque at around 1600. Surprisingly though, after a bunch of sorely needed engine maintenance, including replacing all fuel filters, goo and all, and after running her for a few days, the next time I ran her up to WOT we had more like 2450.
Disclaimer, this is my first Diesel engine and I am in no way claiming expertise, just sharing an anecdote.
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Old 15-03-2020, 09:10   #9
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Re: Hull Speed/RPM Ford Lehman

I have an Albin 40 which has a 34 foot LWL. Hull speed is maybe 6.75 kts.

I have a Lehman 120 and do 6.3 knots at 1650 RPM....burn 1.9-2.0gph and get around 3.1 to 3.3 NMPG.

Hard to separate genset fuel and diesel heater but those numbers are pretty close after 3000+ hrs cruising.

I have run the boat at 8 knots and 2000+ RPM, but the wake is huge.

I have a 4 bladed 22L17 prop.

I think you are right on or close to the money...maybe over propped a bit like me but my numbers are close to other boaters I believe...not most of the numbers people put on the internet as they are much further than the factory engine curves than my numbers....unless their boats are actually that much different (like a 40 footer with a 38 foot waterline and handle chop better than my not very bow buoyant Albin).
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