Fuel consumption on Lehman 120s

[RogerG]

I have been down to see a 38ft trawler yacht at a broker's over the weekend, which is powered by a pair of 6 cylinder 120hp Lehmans. I researched the engine and boat size and the general consensus is that a pair would consume about 1.5 to 2 gals per hour each. I asked the broker to ask the owner for his consumption figures and he is quoting 1 litre per hour for each engine at 1800rpm 8-9 knots! As this compares favourably with my Albin 25's 2 cylinder Volvo MDb-2 which is only 25hp it seems to be the bargain of the century!

Can I safely assume that this is total B*****S or is my research incorrect?

Any help appreciated,

Roger

 

[oldgit]

suspect he means about a gallon an hour per engine,bit optimistic perhaps but not that far out.
My 130hp perkins of similar size do about that on average. !

 

[RogerG]

Thanks for that info on the Perkins.

Ok, I emailed the broker to express my doubts, and he has just this minute come back with a reply, saying that he had misread the notes and it is actually 1 litre per nautical mile per engine, which ties in with my research for a heavy boat.

Maybe he hoped the first quoted figure would promote a quick sale

Roger

 

[longjohnsilver]

Thanks for that info on the Perkins.

Ok, I emailed the broker to express my doubts, and he has just this minute come back with a reply, saying that he had misread the notes and it is actually 1 litre per nautical mile per engine, which ties in with my research for a heavy boat.

Maybe he hoped the first quoted figure would promote a quick sale

Roger

Our 48' trawler yacht has Lehman 135s and I reckon on about 2 lts per mile per engine at 1800 rpm which gives about 7.5kts, so the brokers figures are in the ballpark although I suspect speed at those revs will be a couple of kts less than quoted. The max hull speed will be about 7.5 kts for that size of boat, expect to cruise at 6.5 - 7 kts. You will be hard pushed to get 8kts even at max revs.

PS what's the boat?

 

[Latestarter1]

MMMM...One gallon per hour per engine??

If we assume displacement around 15 tonnes, max cruise speed of 7 knots, theoretical hull speed widdy bit over 8 knots.

At 7 knots we are only pulling about 20 kW (at the flywheel) out of each engine. Remember propellers move boats not engines therefore at 7 knots goverenor will only be putting in enough fuel to generate 20 kW of power, if I am a little generous with BSFC comes out around 1.6 litres/Hr per engine.

Yep surprised me.

 

[RogerG]

Our 48' trawler yacht has Lehman 135s and I reckon on about 2 lts per mile per engine at 1800 rpm which gives about 7.5kts, so the brokers figures are in the ballpark although I suspect speed at those revs will be a couple of kts less than quoted. The max hull speed will be about 7.5 kts for that size of boat, expect to cruise at 6.5 - 7 kts. You will be hard pushed to get 8kts even at max revs.

PS what's the boat?

The boat is a C-Kip Trader 38 and the spec quotes 8-9 knots cruise and 12 knots max, which seems somewhat optimistic for a 38ft displacement hull with a long heavy keel.

Roger

 

[longjohnsilver]

MMMM...One gallon per hour per engine??

If we assume displacement around 15 tonnes, max cruise speed of 7 knots, theoretical hull speed widdy bit over 8 knots.

At 7 knots we are only pulling about 20 kW (at the flywheel) out of each engine. Remember propellers move boats not engines therefore at 7 knots goverenor will only be putting in enough fuel to generate 20 kW of power, if I am a little generous with BSFC comes out around 1.6 litres/Hr per engine.

Yep surprised me.

LS1 are you saying total ie both engines fuel consumption is less than 1gal per hour at 7 kts, or approx 10 miles per gallon?

 

[oldgit]

Perkins M135 HP .(ish )

Over last 12 months have logged 189 hours. Most of that was probably around 1100-1200 rpm and a fair bit up river at virtually tickover.SD boat weighs in at around 9/10 tons.
My log indicates we have used 1700L of fuel in total.
About 1 GPH per engine .
Even includes a dozen hours or so at 2500 RPM to get out of some ****ty weather.

 

[PCUK]

I'd say a couple of gallons an hour at sailing speed is about right and max speed of about 8 knots with a clean bottom.

 

[Paragon]

The boat is a C-Kip Trader 38 and the spec quotes 8-9 knots cruise and 12 knots max, which seems somewhat optimistic for a 38ft displacement hull with a long heavy keel.

Roger

Hi Roger

If it's similar to my own (see piccy / avatar!) which is a Marine Trader 40 Sedan. I have twin Ford 120's and can confirm the others that have quoted figures of around a gallon per hour per engine at cruise.

I would also say that the only way I would exceed about 8.5 knots through the water would be if I were being towed by something big and powerful!

PM me if I can help further!

John

 

[Gumpy]

I have a 120 in my barge, barge weighs 35+ton
800rpm 2 knots under a gallon an hour (canal speed)
11-1200 rpm 4knots a gallon an hour
15-1700 rpm 5+knots 2 gallons an hour
You can guess where I run my throttle, 4 knots is just fine by me!

 

[ccscott49]

I have two 135 mermaids, basically same as LJS, 48 ton boat though! 6.5-7 knots at 1300 rpm, 1.5 gals hours per engine. Which if there is any wind and its favoiurable, will drop to 1/2 gal/hr per engine, using one engine about 3/4 gal/hr, 6.5-7 knots. Benefits of a motorsailer. Consumption rises rapidly when pushing 10.5-11 knots!

 

[RogerG]

This is all great information guys

Just to put you in the picture, we had pretty well made up our minds that we were going for a 35-40ft motorsailer, when we buy in the late Spring early Summer, but as I have always liked the look of trawler yachts, we have kept our options open.

We looked at the C-Kip 38 and we were both staggered at the ammount of accommodation for the size of boat and the enormouse deck area for relaxing in the warmer climates. As we intend to spend time enjoying the areas that we visit, the value of comfortable spreading out space became immediately apparent. We are used to a lot of space as our home is a 57x10ft widebeam canal boat, but we are also used to motorsailing as we also have an Albin 25.

It really comes down to what is the most imortant to us, the ability to sail anywhere with minimum fuel costs, or big fuel costs (by our standards) more restricted cruising areas, but superb accommodation and facilities. No sailer or Motorsailer can come anywhere near the space of the C-Kip for the length, but I would hate to buy a boat that is stuck on a pontoon because we can't afford the fuel to go anywhere. With fuel continuing to rise in cost I can see that problem getting worse.

It has stopped us hard in our tracks though and made us look long and hard at how we would actually use a boat as a travelling liveaboard.

Roger