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Old 11-03-2019, 11:41 PM   #1
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Minimum Hp for 48' SD Trawler

All,

I just learned that BOTH my Lehmans are shot (long story; wait for the book for all the details).

Current iron are twin Lehman SP 225s. Before both engines cratered, we ran at 1200 rpms which yielded at 6 -7kts. That speed was fine with us.

Have an opportunity to buy a pair of 80hp Lehmans which we could (hopefully) mate with our PRM/Newage transmissions and away we would go.

My question is: do you think we would be TOO underpowered with the 80hp Lehmans?

P.S. - If we were able to make this work, I don't think there would be any questions about fuel economy
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Old 11-03-2019, 11:52 PM   #2
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I think the boat would be underpowered...
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Old 11-04-2019, 12:31 AM   #3
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If you only run at 6-7 knots then the 80 hp Lehmans would better suited than the 225's.
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Old 11-04-2019, 05:32 AM   #4
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Minimum Hp for 48' SD Trawler

To be more exact, you need your LWL and Displacement, but a good indication would be your current fuel burn and then what the power curve for your engine indicates the hp generated at that speed (perhaps minus a bit for being old and worn out).

But obviously the 80’s are better suited. Running your old school engines at 1200 rpm isn’t healthy and they likely are fuel slobs as well at that loading.
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Old 11-04-2019, 05:57 AM   #5
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Greetings,
Mr. F. Well, THAT'S a real lick in the pants, for sure. Sorry to hear about it.


Bear with me here as my memory sucks and my results may not be applicable BUT we have 2 NA Lehman 120's that we usually cruise at 1750 RPM burning 2 GPH/per engine at about 8 knots. We are a 46' FD (42' WL) weighing 25 tons.



IIRC that's about 20 HP/gallon/engine so you should be fine with twin 80HP's.
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Old 11-04-2019, 06:43 AM   #6
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The usual rule of thumb is 3 HP per ton (2240lbs) of displacement.

Use your past fuel burn , 15 HP per gallon / per hour to figure current HP delivered.

Smaller engines may not make the HP required at low RPM to spin current tranny/prop combination.

A prop repitch could be required.

If the numbers look good , go for it!

The automotive folks sell EGT gauges that hook up with normal wiring, which would be good if attempting to buck large seas or headwind. About $125 each.
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Old 11-04-2019, 06:49 AM   #7
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Guessing that your boat weighs 40,000 lbs and is a semi displacement hull. My experience is that it takes 2 hp per thousand pounds to push a semi displacement hull to its hull speed. This is a little higher than FF's rule above because yours is a semi displacement hull and they are less efficient than displacement hulls.

For your boat that is 80 hp to reach almost 9 knots hull speed and at 7 kts it will take about half of that. Twin Lehman 80s will be fine for those speeds.

So go for it.

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Old 11-04-2019, 07:42 AM   #8
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Gut feel only - your boat has a fair amount of windage - 2x80hp might be a bit underpowered in adverse conditions (winds while docking, headseas, etc.). Why not consider 2x120hp? You may want to check on whether the FL80hp is as dependable as the 120hp. Will probably need to re-prop as unlikely 80hp engines could make RPM with existing props.
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Old 11-04-2019, 10:40 AM   #9
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Lets put this question into perspective.

A Nordhavn 475 has a single John Deere 180 hp engine. Yes it is a displacement hull which is more efficient than the OP's boat, but it probably weighs more at 62,000 lbs and is a couple of feet longer.

A Krogen 48 has a single Cat 210 hp engine and weighs 56,000 lbs also more than the OP's boat.

The OP's boat weighs 44,000 lbs and 160 hp is certainly in the ballpark for someone who never plans to exceed displacement speeds which is what the boats above are designed for.


I don't know if the FL 80 has the same footprint as the FL 120. If it does and two are available at a reasonable price, those engines would be a simple drop in replacement.


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Old 11-04-2019, 10:57 AM   #10
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The 80 is just a 120 with two fewer cylinders. Same footprint, but the front feet are about a foot closer to the back feet.

I'd rather have the 120 sixes just because they are smoother both at low rev and higher rev. But if you can get a sweet deal on the fours, then they should work.

With the 225's, what was your burn rate per hour at your happy cruise? The 225's would make about the same hp per gph as the 80's, in the 16-18 ballpark. That would give an idea of how hard the 80's would have to work. I would not want them to have to make more than 50hp each, or 3gph each. 2gph each and below would be a happy spot.

What went wrong with the 225's? Probably something in the turbo/aftercooler system? Some things can be fixed, and aftercooler can go away.
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Old 11-04-2019, 11:04 AM   #11
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Quote:
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Lets put this question into perspective.

The OP's boat weighs 44,000 lbs and 160 hp is certainly in the ballpark for someone who never plans to exceed displacement speeds which is what the boats above are designed for.
This boat was designed as a motoryacht with props and rudders matched to 225hp engines, hopefully with respect to the A/B ratio and Prismatic Coefficient of the hull form. The risk is there may be more to this than paper-based theory on hp vs speed based on mill-pond conditions. I'd be curious to see what the running gear looks like out of the water - try docking a Nordhavn or KK with a small rudder and see how it works out.
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Old 11-04-2019, 11:08 AM   #12
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Our President 41 has SP225s. They were built with twin 275s and twin 120s and I just found an owner that has a single 120. I think that the 46 in question would be fine with twin 80s. It certainly won’t be a speedy boat but he says he cruises at 6 to 7 knots anyway. I agree with Ski about the 6 cylinder engines being smoother but if he can get a deal on the 80s then ok.
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Old 11-04-2019, 11:11 AM   #13
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This boat was designed as a motoryacht with props and rudders matched to 225hp engines, hopefully with respect to the A/B ratio and Prismatic Coefficient of the hull form. The risk is there may be more to this than paper-based theory on hp vs speed based on mill-pond conditions.
If the OP was happy going 6-7 kts he will be just as happy doing it with smaller engines, irrespective of A/B ratios, Prismatic Coefficients and the existing rudder. He will have to reprop and maybe change the transmission ratios, move the engine mounts and maybe lengthen the prop shaft.


The latter two are not inconsequential and may point towards drop in FL120s depending on what kind of deal he can get on the FL80s.


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Old 11-04-2019, 11:18 AM   #14
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If the OP was happy going 6-7 kts he will be just as happy doing it with smaller engines, irrespective of A/B ratios, Prismatic Coefficients and the existing rudder.
Agreed for underway. I do not understand why no one is concerned about having reserve power in more challenging conditions such as docking with cross winds. I used to move boats around for boat shows in San Francisco where boats were routinely stuffed into small slips. The setup being recommended will have consequences in close-quarters. Not saying 225hp is the minimum, but twin 80hps may not get it done. I guess no one will know until sea trial.
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Old 11-04-2019, 11:30 AM   #15
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I can't think of a situation where you'd even use 80hp while docking. Have you ever had to bring an engine to WOT for a docking maneuver?
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Old 11-04-2019, 11:33 AM   #16
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See if you can find a GB42 owner w a single 120 FL. The GB is definitely smaller than your boat but there could be favorable comparisons to be made. You’ll have about 30% more power and if your boat is 30% heavier you’ll have an apples/apples comparison.

I think two 80’s will be fine at 6 knots perhaps in a 20 knot headwind but in a blow ??. Then again ask the 42GB owner about headwinds.

The biggest downside or risk will be at selling time. Even if the power is 99% of the time sufficient you’ll need to convince a buyer that it is. Few may bite. I’d say go for it if you think you can handle the resale.
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Old 11-04-2019, 11:34 AM   #17
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I can't think of a situation where you'd even use 80hp while docking. Have you ever had to bring an engine to WOT for a docking maneuver?
Think stopping before you crash the wood floats.
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Old 11-04-2019, 11:41 AM   #18
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I can't think of a situation where you'd even use 80hp while docking. Have you ever had to bring an engine to WOT for a docking maneuver?
Let's see.....headed down a fairway, returning to your slip in the afternoon when winds have kicked-up a 15-20kt cross wind on a boat with a ton of windage and tiny rudders designed for bigger engines. Suddenly, you realize someone has homesteaded your slip, so you have to swing the nose through the wind to turn around or back-out in a relatively straight line, both of which will need extra RPMs to counteract the crosswinds. How many hp do you need? Dunno - don't want to find out I have too few.

This conversation seems fixated on textbook calculations about hp vs displacement speed. There's a lot more to the design of running gear than this.
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Old 11-04-2019, 12:39 PM   #19
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Interesting discussion, glad it is not my decision to make.
I would also be concerned about potentially losing the ability to increase speed in adverse conditions such as larger following seas and wind. Many times I have been able to improve our ride considerably by increasing our speed (semi displacement). Having the extra horsepower to do this is a good thing in my opinion.. however, how much hp do you really need??
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Old 11-04-2019, 01:24 PM   #20
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For a reference point, my boat is 45', around 45,000 pounds, semi planing hull. Have a single 4045 John Deere 135 HP. I cruise 7 knots at 2 GPH, around 40 HP. 8 knots at 3.5 GPH around 75 HP.

While my engine is pretty good with counter balance shafts, I agree with Ski, the 6 cylinder Lehmans will be a lot smoother than their 4's.

Also, it's very likely if you went with the 4s, that you will need to change transmissions to a taller ratio and will likely need different props. Basically you will need to run at a substantially higher RPM and then reduce the prop RPM through taller transmission gears. Probably nowhere near enough HP 1,200 RPM out of the 4s.

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