Using single engine of dual engine boat?

[jbinbi]

I am coming from a sailing cat that had dual engines. When in the harbor for maneuvering, you use both engines and can do things I never dreamed I could do on previous monohulls. Having 2 engines 16' apart was just amazing.

However, when motoring on a passage, you normally just use a single engine. With both engines I could drive at say 8.5kts at WOT (2 gph). At about 7.2 at 2800 (1.5 gph). With a single engine I could do about 6kts at 2200. That was about 0.5 gph on my yanmar 30 hp. You just slowly pull back on one of the throttles over the course of a minute or 2 to let the AP adjust the rudders slightly for the single engine. So you use 1/3 the fuel to go from 7.2 to 6 kts.

On monohull powerboats with dual engines, can you do the same if trying to run at under displacement speeds?

 

[Pete Meisinger]

Not a good idea unless you have what is called a trolling transmission, it allows the transmission to freewheel. If you have a dripless seal on your boat shutting off one engine will cause that shaft to overheat.

Really nothing to be gained at trawler speeds anyway.

pete

 

[Jeff F]

Did you ever try running at 6 knots with both engines running? I'd bet you get equal or better FE using both than just one.

There are several threads here on the subject. The benefits are dubious IMO.

 

[jbinbi]

Did you ever try running at 6 knots with both engines running? I'd bet you get equal or better FE using both than just one.

There are several threads here on the subject. The benefits are dubious IMO.

Nope. On a sailboat, better FE with single engine for just cruising.non linear relationship between fuel used and speed as you get closer to hull speed.

 

[rgano]

Not a good idea unless you have what is called a trolling transmission, it allows the transmission to freewheel. If you have a dripless seal on your boat shutting off one engine will cause that shaft to overheat.

Really nothing to be gained at trawler speeds anyway.

pete

Never had any problem with my dripless packings over heating when trolling on one engine - just put my hand on the off side once in awhile. I imagine trying to to do so at a high enough speed could heat one up.

FYI, Arleigh Burke class destroyers, as a fuel saving measure, routinely shut down one shaft and run the ship at close to 15 knots for long transits, but they have controllable pitch propellers too.

 

[Jeff F]

Nope. On a sailboat, better FE with single engine for just cruising.non linear relationship between fuel used and speed as you get closer to hull speed.

I fully understand the power/speed/fuel usage curve. But I remain unconvinced that traveling at six knots with one engine running on your boat uses less fuel than going the same speed with both running. Maybe it does, maybe it doesn't. The big gain in FE that you cite comes from slowing down, not from shutting one engine off.

 

[mcarthur]

Not a good idea unless you have what is called a trolling transmission, it allows the transmission to freewheel. If you have a dripless seal on your boat shutting off one engine will cause that shaft to overheat.

The transmission has to be able to cope of course so check it can freewheel, but many seals (like our PSS) are good for upto 12 knots without cooling water according to the manufacturer. So no need to run cross-lines from the other engine for that brand unless you're going very fast on one engine .

Definitely consumption benefits. Depends on your boat, but I'd guess anything from 4.5kn or so upward you'd see better consumption on one vs two. Obviously your top speed may be less though, so you may be better on one engine from 4.5-7.5kn, then about equal to 8, then better on 2 from 8 upward. You will lose a little efficiency due to rudder compensation, but that entirely depends on your hull shape and loading - a high prismatic round-bilge sailing cat may not track as well and need more offset than a deeper 'v' shaped boat. But there'll still be efficiency gains compared to 2 engines.

 

[jbinbi]

I fully understand the power/speed/fuel usage curve. But I remain unconvinced that traveling at six knots with one engine running on your boat uses less fuel than going the same speed with both running. Maybe it does, maybe it doesn't. The big gain in FE that you cite comes from slowing down, not from shutting one engine off.

No, it comes from what part of the power curve you are using. to move the boat at 6kts doesn't need much from a 30hp engine. So when you have 60 hp available, and only need a fraction of it, you can use 1 engine.

Maybe it doesn't fully translate as a catamaran is not a true displacement hull. Under sail it can go much faster than power. That is not true with a sailing displacement hull where you really can't go faster than 1.37 * sqrt(LWL) whether under sail or power.

 

[tiltrider1]

With a sailing cat you probably had feathering props. With out feathering props you won’t save any fuel running just one engine. Do a search and you will find this has all been discussed before.

 

[mcarthur]

I fully understand the power/speed/fuel usage curve. But I remain unconvinced that traveling at six knots with one engine running on your boat uses less fuel than going the same speed with both running. Maybe it does, maybe it doesn't. The big gain in FE that you cite comes from slowing down, not from shutting one engine off.

Nope. But then I don't feel the need to convince anybody - it works in reality and that's the only facts I need. I'm happy just doing it and getting the benefit of running on one upto changeover speeds. For the OP, as another example, the power cat Domino does it too (<=8 kn it's one engine only). Our particular break-even is about the same about 8.2kn. One engine is more fuel efficient than two by an even 60% from 4kn through to 6kn (1lpnm vs 1.7lpnm), then progressively less to the turning point at 8.2kn where we are at a total of 2.1lpnm either way.

 

[darkside]

On our 46 sail cat using one engine used far less fuel than two engines at 6kn.
We probably motored 8 to 10,000 miles so not just a casual observation.
We did have feathering props however with far less drag than fixed.

If you want to do it on a trawler controlled pitch props like Hundested would be a big help.

 

[mvweebles]

No, it comes from what part of the power curve you are using. to move the boat at 6kts doesn't need much from a 30hp engine. So when you have 60 hp available, and only need a fraction of it, you can use 1 engine.

Maybe it doesn't fully translate as a catamaran is not a true displacement hull. Under sail it can go much faster than power. That is not true with a sailing displacement hull where you really can't go faster than 1.37 * sqrt(LWL) whether under sail or power.

Huh. I would have thought that a single engine 8-feet off centerline would cause the boat to crab. Surprised that any efficiency gained by running one versus two engines overcomes this obviously inefficient dynamic.

Back to the question, I recall a PMM article 15+ years ago about a GB42 that went from California to Hawaii. The owner, who was single handing, removed one prop and ran on the other engine. Halfway there, he shut down and removed the prop and installed the other one. I don't know whether that's brave or stupid, but choices like that may explain why he was alone.

The closer together the engines are the less the crabbing and the less hurtful the effect. To suggest a catamaran (power or sail) is more efficient under one engine than two is counterintuitive. At least in a properly designed and equipped boat. Sailboats sometimes sacrifice efficiency under power for reduced drag under sail. Perhaps any perceived efficiency effect is related to that versus single vs twin

Peter

 

[Ken E.]

FYI, Arleigh Burke class destroyers, as a fuel saving measure, routinely shut down one shaft and run the ship at close to 15 knots for long transits, but they have controllable pitch propellers too.

As I read in the magazines, superyachts typically shut down one engine in Atlantic crossings or similar long hauls to save fuel. There have been several threads on this in the past - check the archives. I have run on one side but as mentioned above, most of the fuel savings seems to be from just slowing down, with either one or both engines on.

 

[Peter B]

......Maybe it doesn't fully translate as a catamaran is not a true displacement hull. Under sail it can go much faster than power. That is not true with a sailing displacement hull where you really can't go faster than 1.37 * sqrt(LWL) whether under sail or power.

Here's a thought. I've often wondered why it is that a multihull, whether sail or powered, does appear to not be limited to that usual formula for calculating the hull speed as quoted above, although my memory of it was 1.34 x sq root waterline length is feet.

My query is, is this because the hull length, even though side by side, rather than all in one length, mean that the calculation then becomes 1.34 x sq root of WLL in feet of both hulls, added together, or is the fomula more complicated than that.

However, we are shortly going to see all our preconceived ideas re the physics of hulls, mono or multi, called into question come the next Americas Cup races, using foiling monohulls, not catamarans, over in Auckland in March 2021. Please covid don't ruin that..!

https://www.americascup.com/en/home

https://www.americascup.com/en/video/694_TE-KAHU-RETURNS

https://americanmagic.americascup.com

 

[Insequent]

Bob Lowe (Oak Harbour, WA) is well known particularly in GB circles, but he may not have much www presence these days. On his website he used to have a comparison of one engine versus two, prop locked or free wheeling and maybe some other tests. He found that at any given speed running on one engine used slightly more fuel than running on two engines. A fixed prop (locked shaft) used a bit more fuel than a freewheeling prop.

If you think about it, it makes sense. It takes a certain amount of HP to move a boat at a particular speed. It does not matter where the HP comes from. Sure, parasitic loads on a second diesel will consume a few additional HP, but its a very small impact. Bigger effects are the crabbing, and energy consumed by the freewheeling prop.

After my refit I did some sea trials and obtained identical results to Bob. In my case I needed 7° of rudder to hold a straight course running on just one engine, and that was probably where much of the additional fuel use came from. The attached graph shows both our results. Remember to look at fuel consumption at the same speed when comparing. In order to go the same speed, a single engine has to run at higher rpm than the rpm's of each of the two engines will be at that speed.

My conclusion was that I would only ever run on one engine if for some reason I needed to go so slow that the engines were at too low an operating temp for extended running. In that case running just one, at higher rpm, is better for the engine even though fuel use is slightly elevated.

Now I do think that you need to test for yourself as results could vary from one hull shape to the next. Bob's Alaskan and my OA are quite similar hulls. But anyone who sees big differences for running one versus two, on a boat with several hundred HP installed, is very likely not comparing apples with apples (eg same speed). Now for sailing cats, with tiny engines parasitic loads from the second diesel could be a larger % of total power (fuel being used).

Oh, and another thing that is obvious from the graphs I attached is that slowing down has a huge impact on fuel consumption.....

 

[Flatswing]

Not a good idea unless you have what is called a trolling transmission, it allows the transmission to freewheel. If you have a dripless seal on your boat shutting off one engine will cause that shaft to overheat.

Really nothing to be gained at trawler speeds anyway.

While I agree not much benefit except reduced maintenance at trawler speeds, I beg to differ. My ZF transmissions manuals specifically ok to freewheel (Surprising how fast they turn) but PSS seals stay cool @8 knots without crossover.
The reality however is one motor drives the Naiad hydraulics & hot H2O while the other drives the high output alternator to charge the house bank so few opportunities to run on just 1.&

 

[darkside]

Many underestimate the power developed by a decent sailing rig.
We had two 40hp engines on the sail cat, flat out on both, around 8kn.
Our highest speed in flat water under sail around 15kn, best surf was 22kn.

There certainly are displacement powercats but most are powered not to achieve maximum hull speed.
There really is no point in adding too much power as just like an over powered monohull displacement trawler the fuel burn soars.

As the hulls become long and skinny (over 12:1) the factor of 1.34 (x sq root of WLL) no longer applies and much higher displacement speeds are possible with enough power.

Can't wait for the next cup either, I was out watching team NZ practice a couple of weeks ago. They were doing better than 30kn (my boat can do 36 WOT) in around 8kn - 10kn of wind.

 

[Peter B]

Hi Darkside from an ex-Kiwi. We used to live in HB, but frequently visited Tauranga because my parents lived there and loved it, for the last decade or so of their lives.

What do you think about my query re the conventional calculation of hull speed..? Can cats go faster without planing because the length of the hulls is additive, or is it a different formula entirely..?

I've looked on here, https://www.brighthubengineering.co... the limiting factor for catamaran hull speed.

but still somewhat fuzzy as to how it is calculated. Maybe someone better at maths than me can explain..?
They say..."The formula for catamaran hull speed is 1.34*(wetted length)1/2; however, this drag formula is generally not the limiting factor for catamaran hull speed."

Does that mean that the theoretical hull speed of say a 30' WLL cat is 1.34 X 30, divided by 2 = 20.1..? Or am I missing something here..? Anyone..?

 

[darkside]

Hi Peter
Sorry I don't know if the formula becomes additive for the two hulls.
Your link describes the effect of drag on cats which is very real. A catamaran for a given displacement has far more wetted surface than a monohull. That's why you see large rigs in performance sailing catamarans to get them going in light winds.

I really like this hull design which is a 40 ft full displacement powercat but can achieve 23kn.

https://www.alloycats.co.nz/our-boats/kingfisher-cats/kingfishercat-12m-displacement/

We bought our sailing cat (a Grainger) just up the road from you in Airlie Beach.

 

[Peter B]

Well, I wouldn't call Airlie Beach quite just up the road from us, but ok, in Aussie distances compared to NZ distances, I guess...

Love your boat. I've always like cats. Yours would be good for a quick trip up to the Poor Knights for some scuba diving, for sure.

However, I bet you still miss the sailing one at times, as I miss my Gazelle maxi trailer yacht I brought over to Oz with us, event though I grew to love the diesel cruiser, and the turn on the key, drop the lines and go, and much more roomy convenience of the Clipper 34.

Sorry, thread hijack over...

 

[psneeld]

My experience...... it depends.

Depends on the boat, the speeds used, the size of the engines, etc.

All these posted half theories are just that.

I have seen somewhat scientific studies and the results show sometimes this and sometimes that. So what I have gathered is till you have your boat, set up the way you want it, and are cruising it the way you want it.....it will be difficult to even guess. Much of the time, my guess is the numbers will be so close as to not matter. In extremes though., like where you are willing to crawl to make it across an ocean on limited fuel....then all bets are off.

The people who say it takes the same energy to drive a boat at a given speed. ...often leave out so many details I have to choke reading them. The most basic idea that conflicts is a hull that is driven at a certain speed at one engine idling, keep the same speed (if you even can) and start the other engine. What makes sense now if these are 2 fairly large engines both burning X fuel at idle? The minute you add power to the single to get to the idle speed of 2 (often the case)...then all sorts of other things come into play that are mostly left out of these one or two paragraph theories. This is assuming that the energy available at idle is enough to overcome all the variables in drive train and drive the boat at the same governed rate across the board. So the measurement really needed is the difference of unloaded idle burn versus, in gear at minimum throttle setting. This combination of factors is probably what throws most studies off. If the engines are small and the speed for a single at idle is ridiculously slow or still adds fuel at the minimum throttle setting....then yes the numbers for 1 or 2 engines will probably be close.

So, some rough rules of thumb come to mind, but they really only apply if they fit your boat, engines and very specific cruising speed......

And for the locked shaft theory.....it to seems to be all over the map. The USCG engineer on an icebreaker I was on..... told me locking the shaft of the bad motor would save a ton of fuel over the next several months. That was what his info was, I believe from both engineering school and the US Navy. But I have also seen research on small vessels where that isnt absolutely true.

 

[FF]

Hull speed is the vessel being trapped in its own wave system.

For most 3-1 L/B the usual 1.34 formula is close the hull has a hard rime climbing its own bow wave , until the stern falls into the midship hollow and drag really soars.

At 6-1 L/B ratio the bow wave begins to be smaller and some skinny boat reductions in power required are felt. But only if the boat is light , shoving tons of water aside still requires energy.

The problem with high L/B ratios over about 1-3 is the drag from increased wetted surface does increase the power required .

So if a higher speed at all times is contemplated 8-1 or 10-1 can make sense , but will cost more energy at more common lower speeds.

As folks worry about fuel costs few multihull folks run at high speeds very often.

For cruisers that tend to get heavier after being outfitted only a large skinny boat can handle the excess weight added and still perform .

Folks desiring a less expensive easy cruise ,( about at the sq rt of the LWL) would gain from a pair of CPP, controllable pitch props , on either a monohull or a multihull.

 

[Mako]

The advantage of locking one engine and running the other on a long haul would be just being able to better load the engine.

Most of the recreational boats we see are semi displacement. If your 40ft powerboat is equipped with twin 375hp fire breathing Cats and you run 7knots at 1,000 rpm, wouldn’t it be better to shut one down and then run at 1200 or 1300 rpm?

I would think so, but my associate doesn’t. He just runs both engines at barely over idle and then after a long day he throttles them up high and brags about the billowing black smoke “cleaning out” the engines. In the long run that isn’t healthy, not in my book.

 

[psneeld]

Hi Darkside from an ex-Kiwi. We used to live in HB, but frequently visited Tauranga because my parents lived there and loved it, for the last decade or so of their lives.

What do you think about my query re the conventional calculation of hull speed..? Can cats go faster without planing because the length of the hulls is additive, or is it a different formula entirely..?

I've looked on here, https://www.brighthubengineering.co... the limiting factor for catamaran hull speed.

but still somewhat fuzzy as to how it is calculated. Maybe someone better at maths than me can explain..?
They say..."The formula for catamaran hull speed is 1.34*(wetted length)1/2; however, this drag formula is generally not the limiting factor for catamaran hull speed."

Does that mean that the theoretical hull speed of say a 30' WLL cat is 1.34 X 30, divided by 2 = 20.1..? Or am I missing something here..? Anyone..?

I have read the same over the years in many forms.

One rule was that vessels with a length to beam ratio greater than 5 : 1 start to break away from the tired 1.34 rule. I have also read 7 : 1 and the linked article I thought said 8 : 1.

That's the reason why old timers also say the long, skinny cruisers of old were "slippery" hulls.

I am no engineer or naval architect but I have long discussed here that the 1.34 hull speed rule wasn't as hard and fast as it often is made out to be.

I am not sure about the calculation as I am sure there are variations to that rule, but 20.1 know does not seem out of the question for some 30 foot cats. Those record breaking transatlantic cats I think averaged over 25 knots ...but they were much longer. Usually just to keep from pitch poling carrying so much sail. Like all boats, cats have to compromise between ultimate speed and load carrying vs hull shape.

 

[psneeld]

The advantage of locking one engine and running the other on a long haul would be just being able to better load the engine.

Most of the recreational boats we see are semi displacement. If your 40ft powerboat is equipped with twin 375hp fire breathing Cats and you run 7knots at 1,000 rpm, wouldn’t it be better to shut one down and then run at 1200 or 1300 rpm?

I would think so, but my associate doesn’t. He just runs both engines at barely over idle and then after a long day he throttles them up high and brags about the billowing black smoke “cleaning out” the engines. In the long run that isn’t healthy, not in my book.

My 37 sportfishing with 320 CATs definitely fit my other post and yes, there was significant savings in shutting one down and running the other at or slightly above idle.

 

[sunchaser]

Aspen power cats with asymmetric hulls have one inboard engine. Or two different size outboards which have some further advantages. Given the ever increasing popularity of today's outboards and the overall cost advantages over diesel the two different outboard sizing is intriguing.

A 10-12 knot cruise with good efficiency is enticing. The Aspen design and engine options wade right into the shutting an engine down discussions of this thread.

 

[mcarthur]

Bob Lowe (Oak Harbour, WA) is well known particularly in GB circles, but he may not have much www presence these days. On his website he used to have a comparison of one engine versus two, prop locked or free wheeling and maybe some other tests. He found that at any given speed running on one engine used slightly more fuel than running on two engines. A fixed prop (locked shaft) used a bit more fuel than a freewheeling prop.
…

Brian, I think the difference is in hull shape and cat vs mono. The cat owners here (and elsewhere) have as much or even more evidence for their boats as Bob may have for his. As often the case, there isn't a single (and simple!) answer - it depends on various factors, for this it means boat type and hull type is a critical element.

 

[mcarthur]

Aspen power cats with asymmetric hulls have one inboard engine. Or two different size outboards which have some further advantages. Given the ever increasing popularity of today's outboards and the overall cost advantages over diesel the two different outboard sizing is intriguing.

A 10-12 knot cruise with good efficiency is enticing. The Aspen design and engine options wade right into the shutting an engine down discussions of this thread.

I'm interested what people think is a "good efficiency" at say 10kn?

My review of power cats consumption (which I must update some time!) and the data behind it indicates that for powercats between about 34' and 60', average consumption at 10kn is 2.2lpnm (1.7nmpg) but with std deviation of 0.96. Separating out into those below and above 40', those below have a 1.6lpnm/stddev 0.39 (2.4nmpg), while those above 40' are 2.5lpnm / stddev 0.99 (1.5nmpg). Going back to the OP, these are all on two engines at that speed (except for the Aspen which only has one :smile.

What is a "good efficiency" at 10kn? Is 2.2lpnm/1.7nmpg?

 

[Insequent]

I'm interested what people think is a "good efficiency" at say 10kn?

My review of power cats consumption (which I must update some time!) and the data behind it indicates that for powercats between about 34' and 60', average consumption at 10kn is 2.2lpnm (1.7nmpg) but with std deviation of 0.96. Separating out into those below and above 40', those below have a 1.6lpnm/stddev 0.39 (2.4nmpg), while those above 40' are 2.5lpnm / stddev 0.99 (1.5nmpg). Going back to the OP, these are all on two engines at that speed (except for the Aspen which only has one :smile.

What is a "good efficiency" at 10kn? Is 2.2lpnm/1.7nmpg?

Firstly, yes update your graph! But I would suggest using nmpg for the vertical axix as the majority of people will be more familiar with that metric.

As to 1.7 nmpg (10kn) - very good for mono! Reasonable for a cat. Personally I'd want over 12kn at that economy to go for a cat.

 

[mvweebles]

A friend's 2014 Horizon Power Cat with Cummins burns around 8 gph at 9.5 - 10.0 kts (1.2 nmpg on a good day which may not sound fantastic, but is about 15%-20% better than a 65-foot mono which has commensurate space). Nowhere near 1.7 nmpg even with very modern engines with electronic controls and readout (including consumption) and a well designed hull from a respected builder. I've been aboard this boat for a couple hundred running hours.

My buddy would be delighted to get 1.7 nmpg. Can someone please tell his boat that she's a laggard compared to reported data from other Cat owners? Of course, assumes the reported data is accurate. As a past delivery skipper, had I relied on reported fuel economy from owners, I would have routinely run out of fuel well short of my destination.

Peter