disappointed with twin screw fuel economy

My BW Velvet Drive Manual limits speed to "trolling speed" with one engine unused. Locked off, I suppose speed would not matter.
If you have a "get home" set up, a separate shaft/prop makes sense,as it covers drive train as well as engine failure. A saildrive unit, like an outdrive or leg, could be easier than a second shaft, though outdrives need maintenance. Mounting the gearbox underwater has always seemed odd.

If I had twins, I'd run them at the same time. If I wanted to operate with a single engine, I'd acquire a boat with a single engine, which I did.

You folks might want to check out the Chris Craft forum for this thread:

Chris Craft Constellation 500s

Some pretty good numbers and comments regarding the handling of the reduction gears while single shaft running with twins.

Al


"The NSA, the only part of Goverment that actually listens to you"

obthomas said:

My present trawler is 40 feet goes 7 knots on two 165 HP volvos and uses about 6 gallons per hour.
.

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OB

You need to get a bigger boat! My vessel with twin 225TIs gets about 4.3 - 4.6 gph (including genset) at 7.7 knots and 1700 RPM. It has been locked on these numbers since new with calibrated tank fills and hour meters the method for calculation.

Bay Pelican, a single engine Krogen 42, is set up with a separate Yanmar 27 hp get home engine with its own shaft and prop. The prop is a Max Prop for minimum drag when not being used. The engine is offset to starboard.

This is a typical arrangement for most of the the smaller trawlers (less than 60 feet /18 meters) in the Eastern Caribbean.

I use this engine on a regular basis when in harbors just to exercise it as a get home system which is not used will freeze up.

The Yanmar is also used for docking (very infrequent for us) and as a back up generator since being a sail boat engine I was able to put on a 150 amp alternator.

In 15 years and thousands of miles my main engine has only failed once. That was due to air in the fuel lines and the air shut down the auxiliary engine as well.

Marty

Engines are not usually specified by a NA , only a power size .

The purchasing folks or add guys have far more say than the boats designer about the product.

Thats how folks get Volvo (very inexpensive for OEM) and way too large engines.

seattleboatguy said:

Moonstruck
120 hp Lehman, single screw
7 knots, 1.5 gph

SomeSailor
120 hp Lehman, twin screw
8 knots, 4 gph total, or 2 gph per engine

So, SomeSailor has twice as many horses in the engine room, drinking twice as much diesel, but cruises at about the same speed as Moonstruck. Why don't trawler skippers just shut down one of their engines, once they are clear of the marina?

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So others have said, but I'll pile on: 8 knots is not 7 knots.

And LWL matters; it's not apparent whether these two boats are the same length or not.

And "hull speed" as usually used in common discussion is not the most efficient speed for all boats; instead, that usually means a boat's maximum theoretical full displacement speed (the 1.34... formula) and the most fuel efficient speed is less that than. Beebe's book has some discussion of it...

In these two example boats, it might have been that the most fuel efficient speed is 5 or 6 knots... and it may well have been that they both burn about the same amount -- or only slightly different amounts -- at their respective fuel efficient speeds.

Ref shutting down one engine... as mentioned, the current Chris Craft discussion touches on several points. It's not out of the question, but has pros and cons, and could depend on specific products in a specific boat.

-Chris

Gentlemen,

The real question regarding singles or duals should be - 15 or 18 year single Malts?

OBThomas

Is that you in Blue Dolphin?

It is.....pier 5

sunchaser said:

OB You need to get a bigger boat! My vessel with twin 225TIs gets about 4.3 - 4.6 gph (including genset) at 7.7 knots and 1700 RPM. It has been locked on these numbers since new with calibrated tank fills and hour meters the method for calculation.

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No.....I can't afford a bigger boat. Do you know that costs go up exponentially by the foot?

My numbers were validated last July, I cruised from Clear Lake TX to Lake Charles LA; 125 miles by GPS track. I had the bottom scraped and zincs changed the day before leaving.
I filled to the brim before leaving and refilled in Lake Charles. and then again in Clear Lake. At 107 gallons each way I get 1.17 miles per gallon including generator. My average speed was 7 knots again from GPS track. I'm a live aboard and unfortunately from time to time I notice that the waterline is edging into the boot stripe, and it is time to carry unneeded junk to the dumpster. Another possibility is that I could back off the throttles and find a better sweet spot. It is possible that a 7th knot is costing me a lot of fuel mileage.

OB

Hull shape plays a huge role too. A KK 42 like Dauntless is very slippery whereas a square a$$ed SD vessel not necessarily so. But your Labelle is a lot less money than a KK42 so it all balances out.

Too much fretting takes place over fuel burn numbers. The guys really saving the money don't own boats!

seattleboatguy said:

I was very interested to read the feedback from my earlier post "2 trawlers, same engine, different fuel consumption???". The great posts that were contributed on fuel consumption (many thanks to all who contributed) left me very surprised and disappointed with the performance of twin screw trawlers.

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Don't be disappointed as your data set is flawed. There are advantages to running twins as singles but fuel efficiency is not one of them. On a forty hour trip you can arrive with only twenty additional hours on your motors, how cool is that? We'll your ambient engine room temp will be 10 to 20 degrees cooler. Think what that means to your batteries and alternator charging curve. Spare parts, get home engine and redundancy are all a plus. Cost? If you run using one or both engines you'll use about 10% more fuel than your single engine sister ship. If you add 10% more fuel you can have the same range and better handling in following seas. If you slow down you can make it up as well. 30 kt headwinds, 7' seas and foul bottoms will all cost more than that 10% twin engine penalty, and these are all things under your control. All boats are a compromise. Prioritize to optimize your dollars and your time.

Via iPad using Trawler

ranger42c said:

So others have said, but I'll pile on: 8 knots is not 7 knots.

And LWL matters; it's not apparent whether these two boats are the same length or not.

And "hull speed" as usually used in common discussion is not the most efficient speed for all boats; instead, that usually means a boat's maximum theoretical full displacement speed (the 1.34... formula) and the most fuel efficient speed is less that than. Beebe's book has some discussion of it...

In these two example boats, it might have been that the most fuel efficient speed is 5 or 6 knots... and it may well have been that they both burn about the same amount -- or only slightly different amounts -- at their respective fuel efficient speeds.

Ref shutting down one engine... as mentioned, the current Chris Craft discussion touches on several points. It's not out of the question, but has pros and cons, and could depend on specific products in a specific boat.

-Chris

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This is probably the best post on this thread. "Hull speed" is viewed by most as some kind of magic point where all great things come together to some point of perfection. Not so at all for SD craft. And it's definitely NOT the perfect speed for a FD boat. Hull speed isn't much more than just another speed on SD boats and the most efficient speed for any non planing boat is basically just as slow as one can stand to go. So there's no "best speed" or "most efficient" speed. The most economical speed for SD boats is slower. And once you've gone slower the best economical speed is the same ... slower. So eventually one will be at or very close to an idle.

For a pleasure boat how long it takes to get where you want to go, noise and vibration (there's always been lots of "sweet spot" talk), engine loading, fuel being burned, sea state and many other variables will dictate what the skipper does. Some Sailor dosn't pic a very economical speed to run to Bremerton but it's a good choice for him on his boat and it takes into account many variables. If he needed more economy he'd go slower.

So we all have different best speeds and none of us are cruising at the best speed for fuel economy.

And re the comment about trawler twins being over powered applies only to those skippers that bought a boat w too much power for their needs. Sunchaser and Marin are not in that category nor are many others here. There was a GB 36 twin on YW a while back that had been re-powered w two 55hp Yanmars. That would be the perfect boat for many trawler skippers and it didn't last long on the market.

Years ago I saw a well done "boat test" of single vs: twin engined Gulfstar's. It clearly showed a 10-12% difference in MPG across the entire speed range (full displacement hulls). Both had Perkins engines, 2-4.108 vs: 1- 6.354 Seemed like a very fair test with pros & cons of each installation listed. As a side note, one of the pros of the twin installation was less rolling because, they claimed, weights were not as centered athwartship as the single installation...

Yes Brooksie,

Albin of Sweden recommended to add weight/ballast to the hull where the deck and topside meet (under the gunn'ls) the object being to ballast as far away from CG as practical.

What is odd about the Gulfstar comparison is that the single has more power than the twin. As bad as the usual twin/single set up is this is even worse in a way. Twins are usually considered to be a luxury high end and better set up ... an expensive option. Just odd.

I have twin 6.354 NA. At 1300 RPM I can burn 1 GPH per engine. But I'm not efficient at 50% of total RPM range and I can only crank 6 kts.

I see no ballast benefit. I can be rolly.

Gallons per hour or Miles per gallon? Apples and oranges. 20 gph sounds like alot of fuel. But if you're going 20 mph you're getting 1 mile per gallon - the same as someone burning 10 gallons per hour going 10 mph. If I'm cruising around on a Sunday afternoon I may be interested in gallons per hour. If I'm going somewhere I want miles per gallon. I think most on this forum are going places.

With my twin FL 120's on a Grand Banks 36 I get 2.8 miles per gallon traveling at 1500 rpms at about 8 mph.

captrigney said:

Gallons per hour or Miles per gallon? Apples and oranges. 20 gph sounds like alot of fuel. But if you're going 20 mph you're getting 1 mile per gallon - the same as someone burning 10 gallons per hour going 10 mph. If I'm cruising around on a Sunday afternoon I may be interested in gallons per hour. If I'm going somewhere I want miles per gallon. I think most on this forum are going places.

With my twin FL 120's on a Grand Banks 36 I get 2.8 miles per gallon traveling at 1500 rpms at about 8 mph.

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I have followed this whole thread and no one is going twenty mph. This is a trawler forum so everyone is going about 8 knots. It divides out and GPH is generally meaningful to this group so its not so much apples and oranges.

obthomas said:

I have followed this whole thread and no one is going twenty mph. This is a trawler forum so everyone is going about 8 knots. It divides out and GPH is generally meaningful to this group so its not so much apples and oranges.

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As a newcomer to the trawler world, when I look at 40' trawlers in yachtworld, I'm not always sure if the gal/hr figure they give for twin screw vessels refers to the total of both engines, or just refers to one of the engines. Any suggestions on how I can correctly interpret the number they give?

obthomas said:

I have followed this whole thread and no one is going twenty mph. This is a trawler forum so everyone is going about 8 knots. It divides out and GPH is generally meaningful to this group so its not so much apples and oranges.

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We sometimes run at 20 or even 22 kts

Not always by my preference, but The Admiral picked the short-list boat with more interior room -- trumping my last trawler choice -- so what choice did I have?

That said, we do prefer slower speeds, so we still often run like we did with our first trawler... when we can. Even though we're basically slow-speed folks, our current hull form isn't always comfortable at those speeds in various seat states, so we get up on plane and deal with it as best we can.

And of course that means I do have to pay attention to MPG.

-Chris

seattleboatguy said:

As a newcomer to the trawler world, when I look at 40' trawlers in yachtworld, I'm not always sure if the gal/hr figure they give for twin screw vessels refers to the total of both engines, or just refers to one of the engines. Any suggestions on how I can correctly interpret the number they give?

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I can't help you with your question but as to fuel economy, there's an awful lot of BS out there! Guys reporting fuel flows who have no real means of measuring and are guessing at best.

SeaHorse II said:

I can't help you with your question but as to fuel economy, there's an awful lot of BS out there! Guys reporting fuel flows who have no real means of measuring and are guessing at best.

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And those who believe their Floscans are infallible.

SeaHorse II said:

I can't help you with your question but as to fuel economy, there's an awful lot of BS out there! Guys reporting fuel flows who have no real means of measuring and are guessing at best.

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I would tend to agree....sure you can estimate based on a series of trips...and in the long run for a cruiser..that's what you want anyhow....

because it's rare to ever have a run where the sea or current are exactly the same, the time waiting for locks and bridges is exactly the same, many of us pull generator and or fuel from the same tank...etc...etc

so I would rather have an approximate of what my boat can do over the miles as opposed to what it can do over a measured mile...but I do envy those with some electronic form of instrumentation that can ACCURATELY tell what their fuel burn is and hooked to a GPS ACCURATELY tell wht their GPM is...

I set up my fuel manifold so I can gravity feed from a small graduated tank...maybe this winter's trip I'll run some from a graduated source and get numbers that do rival the Floscans..et al...

ranger42c said:

In these two example boats, it might have been that the most fuel efficient speed is 5 or 6 knots... and it may well have been that they both burn about the same amount -- or only slightly different amounts -- at their respective fuel efficient speeds.

Click to expand...

I idle at about that speed I'd imagine that would be the best efficiency.

Moonstruck is a much more modern engine as well, which I'd imagine is more efficient than my poor old gal.

But honestly, fuel economy is the least of my concerns. I pay more for electricity at my dock than I do in fuel burn.

SomeSailor said:

But honestly, fuel economy is the least of my concerns. I pay more for electricity at my dock than I do in fuel burn.

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I'll second that. . . . I could save more by switching off the dock power when I leave the boat.

I actually work harder at reducing that too. I bought LED lamp bulbs and all that is left is the fridge and chargers.

ranger42c said:

We sometimes run at 20 or even 22 kts

Not always by my preference, but The Admiral picked the short-list boat with more interior room -- trumping my last trawler choice -- so what choice did I have?

That said, we do prefer slower speeds, so we still often run like we did with our first trawler... when we can. Even though we're basically slow-speed folks, our current hull form isn't always comfortable at those speeds in various seat states, so we get up on plane and deal with it as best we can.

And of course that means I do have to pay attention to MPG.

-Chris

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I'm going to challenge you that you are a psuedo trawler and not like most of us that are patterned after fishing boats with displacement hulls.

You are describing yourself as a cruiser, maybe a long range cruiser. Planing and so fassssst so un-trawler like .

SeaHorse II said:

I can't help you with your question but as to fuel economy, there's an awful lot of BS out there! Guys reporting fuel flows who have no real means of measuring and are guessing at best.

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Mine is worked on math formula and chart that built. No BS.

SomeSailor said:

I idle at about that speed I'd imagine that would be the best efficiency.

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You must hate passing through no-wake/5 mph zones.