Fuel usage 135hp natural Perkins or Lehman vs.

Fuel usage 135hp natural Perkins or Lehman vs.200hp engines possibly turbo.

Newbie, I am going to retire and buy in the next 6 months, want to explore. I am going to Fl. at X-mas to look around. Looking for ~40' sun deck Trawler ~ 20,000# in the 1980's price range. I have read enough that if you cruise at hull speed , about 7kts, you can get best mileage, but twin 200 hp or larger are not are really not efficient at that speed. I am open to all ideals. Also can you remove Turbos and make the engine a lower HP? .Open to any advice.

Comparing the Ford, Perkins 120-135 nats with the 210 Cummins turbo- all should give near identical fuel burns if making the same hp typical of trawler service. No benefit in pulling a turbo, if it is not making boost, it just spins at low speed with little drag on the engine.

Some of the older Perkins and Ford higher output engines, around 200-250hp are rather troublesome due to aging on the various coolers.

I would be thinking of cruising at 6kts rather than 7kts. The difference on my Perkins 6.354 is 1.4 gph at 1500 rpm or 2.0 gph at 1700 rpm.

Tom

We have a Kadey Krogen 42' with Lehman 135. This summer we did 1,039 n mi, 183 hr on the Lehman and averaged 1.55 gal/hr. Between 6 & 7 knots most of the time. The fuel consumption includes the generator use too.

That I can live with, 6 kts is fine, I plan to take it easy, but go far. I think the Krogen is a single, don't know about Mia Terra. Most 40' boats are twins.

Mia Terra is a single engine. We've logged about 4600 miles over the last two years mainly between 6 and 7 kts. Average fuel economy based on distance and fuel consumed is about 4.8 mpg.

Tom

tpbrady said:

Mia Terra is a single engine. We've logged about 4600 miles over the last two years mainly between 6 and 7 kts. Average fuel economy based on distance and fuel consumed is about 4.8 mpg.

Tom

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nice Tom

To expand a bit on Ski's post above, it isn't surprising that these three engines: Cummins 6BT, Perkins 6.354 and the Lehman 135 all have the same fuel burn at low hp loadings.

They all are approximately the same displacement and all require about the same "parasitic" hp to turn over the machinery. And at low rpms the turbocharged Cummins isn't producing any boost to speak of, so operates similarly to the other two NA engines.

My 370 hp Yanmar 6LY at 5.3 liters will also deliver about the same fuel economy at low hp loadings at 1,500-1,800 rpms.

So, the point is, there is no fuel consumption penalty to having a high hp, turbo charged engine on a boat if you only want to run it at trawler speeds.

David

engine size

I read in other posts a formula for HP for hull speed. It seems to say its less than 100hp to move a 20,000# at hull speed, ~ 6-7kts. Best engine would be smaller, less heat, less fuel, probably mid range. saying that, if you are right, I could expand my search to larger engines, just want the best economy I can get.

The best engine would be smaller to get the best mpg, but that may be hard to find in a 80's 40' cruiser. Many boats then were powered with larger engines that could put it up to planing speed or close. Lots of those on the market. Even though the engines are much larger than needed for hull speed, they can still get decent mpg, but not as good as one designed for slow speed and powered appropriately.

A 40' planing style cruiser with 320hp engines can get 2-3 mpg at 6-7kts.

A 40' displacement hull trawler with 120hp single can get 4-5 mpg at 6-7kts.

Only 1 or 2 mpg difference, but can be 50-100% more fuel to make the same trip.

Full displacement trawlers command a higher price than planing cruisers, so you have to weigh cost of purchase against cost of cruising fuel. And weigh another gazzilion considerations!!!!

p.s. Florida boats can be sun baked and corroded due to the clime. Consider expanding your search northward, including Great Lakes. Those boats hardly age at all. And you can take it down the river to warmth!!

single engines

I don't see many single engines. I think you are right many "trawlers" got too big of engines, don't think 700+HP 40' trawlers are selling well. I know you are right about Fl. boats, but we want to look around. Most big boats here are under cover and up north some are put in heated storage!!

SteveandZoila said:

I don't see many single engines. I think you are right many "trawlers" got too big of engines, don't think 700+HP 40' trawlers are selling well. I know you are right about Fl. boats, but we want to look around. Most big boats here are under cover and up north some are put in heated storage!!

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Most Great Lakes boats of that size are in heated storage 7 months of the year. The boating season is typically 5 months and that translates to less sun exposure and usually less engine hours. Salt isn't a problem either if they have always been on the Great Lakes. It is amazing how long a boat and its equipment will hold up on Lake Michigan. But then, I am biased.

If you want the absolute best low speed fuel economy (and I am not saying that that is a good decision) then look for a full displacement (round bottom for the most part) hull. There aren't many of these on the market: Nordhavns, Kady Krogens, Willards, Diesel Duck to name a few, compared to semi-displacement hulls.

There is probably more fuel efficiency to be gained by the efficient full displacement hull type than to be gained by the smaller engines in them, but taken together they will give you the best fuel economy.

It will be a rare semi-displacement hull that has just enough hp to get to hull speed plus a small margin. As Ski said, they usually have enough hp to partially plane.

And finally fuel usage is not the biggest cost of boat ownership. I spend about $2,000 per year fueling my high hp, semi displacement boat, probably less now thanks to fracking.

David

Thanks

David, I think from what I have seen, a semi-displacement hull is the target. 38-44 sundeck, aft cabin. Yes from what I read fuel is not the only cost, and for many not the biggest. Just I dream of a lot of traveling and just don't want fuel cost to slow me down. ie 20gals/hr!! Steve

Steve:

20 gph of diesel makes about 350 hp. You only need 2-3 gph to go 7 kts almost irrespective of which engines it has in it!!!

David

Just look at it this way. No matter what Diesel motor 40-500hp turbo or not they all burn about one gallon per hour for an output of 20 hp. Many full displacement boats don't need more than 30-40hp to attain one knot shy of their hull speed. One two or three motors will need only the same amount of hp to push boat as a single and burn about one gallon per 20 hp between them. What is lost with twins is the extra fuel to idle or run alternators and other add ons but when moving along not that big a deal.Weight of the boat and water line length and beam are very important factors. A long light narrow boat will be best for fuel burn but not for accommodation or comfort. Many sailboats under power meet the criteria for very efficient full displacement craft.

>they all burn about one gallon per hour for an output of 20<

The IDEAL is about 20hp from a gal of diesel in a Properly loaded engine.

With modest underloading (like a claimed 120 hp tractor motor at 35HP), you would be lucky to see 15

With a 300HP lucky to see 12gp/gal at idle.

Many sailboats about 40 ft have smaller harder working (more efficient ) engines and operate at or under 1 gph at 6.5K.

Similar weight , to a 40 ft TT

Ski and DJ are right on target. My semi planning 38,000# aft cabin with twin 400 HP Cummins produces about 2-3 Nmpg at , 8kts, trawler speeds. Even better if I go slower .


Fuel difference from boat to boat turns out not to be as big a deal as people make out. The creature comforts of a wide, stable hull and the ability to go fast when I want all make us happy with the semi planning high HP design.


There are Prius and Tesla buyers then there is everyone else who wants a decent compromise. Those larger boats you mention as not selling to well is true. Mostly because people just don't understand fuel used is mostly determined by the operator ( speed), weight, hull form then down the list to installed HP.


If the maker called the boat a trawler back when it was made it has a higher price than if the maker called it an aft cabin motor yacht. So in the upside down world of used boats today with nobody able to really define what a trawler is larger more comfortable boats are cheaper and produce similar results. Go figure???


What everyone is nibbling around is an engine efficiency measurement called Brake Specific Fuel Consumption. (BSFC). That data is hard to find for each engine make and model but a web search will show that within engine classes (2 stroke, 4 stroke, turbo, etc) the fuel consumption of all engines in that class is similar. For 4 stroke diesels that is about 18 or so hP produced for one hour per gallon of fuel. That is HP used not MAX HP of the engine. So the difference between two same model boats with different engine sizes run at 7 KTs will be small but the one with more power will have the option of going faster.


I recommend forgetting what the boat model is called and getting the best value boat for your intended use in terms of creature comforts, stability, condition, equipment, and don't obsess over engine size. The results may surprise you, it did us and I would make the same choice again.

RE; fuel burn. How many hours/year are you going to be underway? A very large % of boats don't get the kind of hours use where 20% or even 50% more fuel makes a significant difference considering other fixed costs. Many "TRAWLER "type boats get sold after 10 years use with less than 1,000 hours on engines some considerably less. I can see where a owner of a 30+ mile/hour boat that sucks over 60gal/hr might get upset with fuel burn if used >100 hrs year but even then the other costs of purchasing and owning that boat may eclipse the fuel. I agree with the good advise of others here look to the boat first and the fuel burn only if it is excessive and off the curve.

Thanks, I am listening.

Bayview, and others, Thanks for all the advice, you have put alot of thought into helping me, I am processing it. A few more questions: I have come across some different boats, wanted your opinion on fuel usage with in the 6-7 kts we have been discussing. Here's some of the boats:1970s 53' Hatteras with GM8v71s, a 1981 43' Viking 34,000# 275hp Cats, 1980 43' Viking 310hp DD 671N. I know all these are planing hulls, does that throw out the fuel usage we have discussed? I gotta say the picture of the Hatteras makes me want it, even if is a 70s boat, kind of like women. Did I just type that! Steve

The GM engines are two stroke, and burn a bit more fuel for the hp they create. That Hatt is a BIG boat- I've worked on a few of the 53's. The GM's are very reliable if they start out in decent shape.

The Cats at 275 are very nice engines. Fairly good economy at hull speed, and are quieter than the GM's.

Regarding hp per gph, the 20 figure is about as high as it gets for our size engines. My Cummins will get 20 at cruise of 1800rpm on the prop load curve, making about 165hp and a little over 8gph, about 17kts. My boat happens to load the engine almost exactly along the 2.7exponent prop load curve from around 1800 to 2100 where I have good fuel burn data. That makes it easy to calc.

But at slow cruise of 7.7kts at 950rpm, it is doing much worse, probably about 12-14hp per gph, but I have no hard data to calculate that. The reason is the engine uses some fuel to just spin itself and pump fluids and air. At lower power settings, this becomes a larger percentage. Called "parasitic" hp loss. edit- see below.

It is probably fair to say that the parasitic losses are close to proportional to engine displacement. At least in our hull speed context.

Just hard to find the right boat that has the right engine loaded to the right spot on the BSFC map.

Edit: Just being bored I looked at the Cummins fuel map for my engine and it does go down to 1000 and 800rpm on the prop load curve. At 1000 it is making 34hp at 1.9gph, which nets 17.9hp/gph. At 800, it is making 18.6hp at 1.1gph, for 16.9hp/gph. Not too shabby for running hull speed. At 2000 it is 21.3hp/gph.

The Cat 275 is probably going to be similar.

Bsfc

Good info, although I will have to learn what is BSFC and how you cal.it. Also mentioned in another post 1.34 factor or multipler, I have some nautical math to learn. Steve

As noted above two stroke Detroits use more fuel than four stroke Cats, so what?
Heavier boats use more fuel than light boats.


I suggest rereading the above in reference to planing hulls.

does anybody know if common rail engines do better fuel wise at lower loading than standard injection. It is my foggy understanding that a big part of the common rail SX is that it can meter the amount of fuel needed much more accurately based on load factors and will not over fuel at lower loads( thus no smoke and bad emissions). Would that also lead to better efficiency at the lower loads?? It sounds that way to me if you are not over fueling would you not see a better hp/fuel ratio??

Common Rail vs Mechanical

Forum member Ski in NC in a moment of boredom did an analysis of common rail vs mechanical engine (QSB 380 vs 6BTA 370) fuel consumption and posted it on boatdiesel. Both engines have similar turbos, intercoolers and boost levels, so the main difference is the fuel injection.

Perhaps we could prevail on him to repost it here because it was interesting. As I recall there was little difference between common rail and mechanical diesel fuel economy at low or high power loads.

And it is not an over fueling issue at low loads. Over fueling can happen at high load where there isn't enough air to burn all of the fuel being delivered, particularly with mechanical injection which doesn't have the precise fuel mapping of common rail. At low load there is plenty of air to burn the little diesel being injected.

Diesels whether mechanical or common rail produce very little CO and hydrocarbon emissions which tells you that the fuel is all getting burned efficiently meaning that the fuel consumption should be the same. They can produce lots of NOx and that is what electronic common rail is good for- precisely timed pulses and in many case multiple pulses will keep the flame temperature down and limit NOx production.

David

All of the following is probably old news to many of the trawler fans here but no one has laid out the data in this forum for this potential new diesel operator.

Most engine manufactures provide BSFC curves or at least fuel used per hour and the horsepower delivered. From the latter you can calculate BSFC. The interesting thing about this is how much the BSFC varies based on engine RPM.

For example on a Cat TA (435 HP) the following are representative numbers:

1500 RPM 67 HP .381 pounds diesel per bhp per hour (BSFC)
2000 RPM 159 HP .340
2200 RPM 211 HP .339
2600 RPM 348 HP .357

You can see that at max efficiency the 3208 TA is 12% more efficient that at a low trawler speed of 1500 RPM.

The Lugger 6108 is another example where you can see very detailed BSFC numbers and their significant variation based on RPM. The continuous power curve (300 HP) shows the following BSFC numbers:

1400 RPM 220 HP .379 (less than 80 prop HP)
1800 RPM 280 HP .360 (120 prop HP)
2000 RPM 292 HP .358 (160 prop HP)
2300 RPM 300 HP .368 (295 prop HP)

But, as shown for the Lugger – the prop curve is really where you need to be looking at the power and BSFC. So the Lugger is 5.8% less fuel efficient at low RPM than it is at 2000 RPM which is the minimum BSFC.

A little research will show you similar data for almost any available marine diesel.

GoldenDawn said:

We have a Kadey Krogen 42' with Lehman 135. This summer we did 1,039 n mi, 183 hr on the Lehman and averaged 1.55 gal/hr. Between 6 & 7 knots most of the time. The fuel consumption includes the generator use too.

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Yep, that's about right for me too.
I think I've posted numbers for every 100 rpm from 1400 to 2000 rpms.

And though people say it, I've yet to see a twin setup that is as efficient as a single, or a 200 hp motor that is as efficient as a 135 on the 6-8 kt range.

I'd stay away from ANY old turbo engine. To much opportunity for abuse.

good numbers

What the weight of your boat? Thanks for the info, I am going to modify my search. I was just looking at an twin 375hp 47 aft and cockpit, but I am going to target singles or twins no bigger than 135hp. Thanks, Steve

Kadey Krogen 42s are designed at 39,000 lb displacement. Likely heavier when fully loaded. We usually run between 1600 & 1700 rpm on the Lehman 135.

The hassle is the modern electric 4 squirt injector may be more efficient at minor loads BUT

A mechanical injector like for a DD is a $40-$50 rebuild Cat about $65 locally.

The electric injector are about a grand each.
And a spare set should be aboard in case you ever get near a thunderstorm with lightning.

A lot of hassle and bother for 5% or even 10% better fuel burn on a pleasure boat,
if you run 2000 hours a year , thats different.