Operating Cost of Single versus Twins

[Hydrospud]

We are new to this type of boating. Stepping up from a 19.5' open bow to hopefully a 36'-44' trawler. I would prefer a single engine but there are several nice vessels for sale with twins.
What would the operating cost difference be between the single verses twins.
Standard Preventative Maintenance Costs?
Fuel Consumption?
What motors to stay away from?
Deb and I have been happily married for 28 years. I don't think the docking learning curve with either (single or twin) is going to have a negative effect on our marital bliss.
Are there any other +/- of single engines verses twins?
Thanks for your input and this forum is great!
Peter and Deb

 

[Ken E.]

Search the archives here, much has been written on this in the past and it will always be an unending debate on which is better, a single or twins. A few highpoints....a single has better access for maintenance, the cost of maintenance is one engine rather than two, probably better fuel economy, protection for the single shaft and prop by the keel. Twins engines have the benefit of redundancy and are easier to handle in close quarters. Many more issues are part of the equation which I'm sure others will mention.

 

[rslifkin]

If the boats in question are powered equally (bigger single vs smaller twins), the twins will typically burn only slightly more fuel (10% at most). And the twins will have a little higher maintenance costs, but far from double, as they'll each hold less oil, have smaller parts to replace, etc. Depending on the depth of the keel with twins, the twin may have a little less draft (as the props will be off the centerline and typically a bit smaller diameter, so they can be tucked up tighter).

 

[Comodave]

I like twins for handling but they are more difficult to access. Guess I can live the the access issues since that is what I have been buying for the last 5 or 6 boats. The fuel burn difference isn’t twice, more like the 10% mentioned above. But it is nice when one engine quits to be able to get home on the other engine. Also the docking is easier with twins. Yes, the maintenance is double but typically with diesels the maintenance isn’t terrible. Oil changes, impellers, heat exchanger maintenance, etc. in the scheme of life buy the boat that fits your needs and desires and live with the engines that it comes with, except Volvos. I had them once and never again.

Check for insurance before you buy. You are making a big jump in size and you may have hoops to jump through in order to get coverage. Better to find out before you buy what you need to do, if anything.

 

[rslifkin]

Access with twins varies a lot depending on the boat. Unless it's a really big boat, twins with fuel tanks outboard of the engines will generally have poor access. But if there's no equipment mounted outboard of the engines themselves, access is often fine on a 40-ish foot boat.



On my boat (38 feet, 14 foot beam) I've got about 2.5 feet between the engines and about that much again on the outboard side of each. Only thing out there is some plumbing to the above waterline thru hulls, as my fuel tanks are located elsewhere. There's a slightly awkward crawl to get outboard of the engines, but once you're out there, access is good.

 

[ofer]

I don't think the docking learning curve with either (single or twin) is going to have a negative effect on our marital bliss.

even twin engines and thrusters can Challege marital status while docking.

I prefer single and am divorced

 

[Sealife]

Very few boats come with both options. Find the boat you want, and the decision will be made for you.

 

[O C Diver]

If you're OCD about preventitive maintenance and boat near coastal or inland, a single will be fine, more fuel efficient and roughly half the maintenance.

If your idea of preventive maintenance is fixing what breaks, by a boat with twins so you will always have one working engine.

If your plans include traveling to extreme remote locations or crossing seas and oceans, buy twins for the above reason.

With the inclusion of bow and stern thrusters, a single engine boat equipped with them will be easier to maneuver in tight quarters than twins without thrusters.

If you boat in the continental USA and its near coastal waters, over 99% of the time you will be easily reachable by Seatow, Towboat US, or the Coastguard should your boat break down.

Ted

 

[BruceK]

Even boats of apparently similar dimensions can have different access to twins. My Integrity 386 has excellent access whereas with a Clipper 40(think NP 40) you can`t pass between engines.
The need for redundancy for safety is less of a factor with increased reliability.

Biggest plus of twins for me is close quarters handling, but thrusters one or both ends can help on a single.
Go carefully, with open mind. A boat is a total package, be ready to compromise within reason. I like twins, but would have bought a Mainship 395 with single if not for tiny water tankage with nowhere to add more.

 

[koliver]

Is an actual data point allowed here?
When doing oil changes, the same week, I went to the local auto parts store for oil and filters. I bought their own brands, Wix, is what they had for my Volvo TAMD41s of 200 hp on my boat, Baldwin is what they had for my Cummins ISL400 on my Motorhome.
My invoice shows that I paid the same for the pair of filters for the Volvos as for a single filter for the MH. I also needed exactly twice as much oil for the single 400 in the MH than for each of the engines in the boat.
The MH doesn't have a sea water pump, but has fuel filters, again, twice as much for that size.
Overall I likely took more time to pump out two engines on the boat than to drain one on land, but really, are we dancing angels on a pin here?
The reality is that maintenance depends more on how much hp your boat uses than on how many engines you use.

 

[Flatswing]

An interminable thread potential! Bottom line: single is cheaper to maintain, but minimally cheaper to run at < displacement hull speeds. Many get hung up on the $$. But the harsh reality on a daily basis for most of us has more to do with access to the outboard side of each engine on the boat you are considering. Do not underestimate this importance if you intend to do any significant maintenance/repairs yourself.

 

[Art]

I didn't notice mention of cruising speed or top speed or D, SD or P design hull shape.

There are lots of reasons to have either twins or a single.

Go into the search feature here and read a lot about engines and hull designs. In other words - get educated. If you are used to 19' open bow runabouts... 40' boat of any type is a whole different world with different requirements/opportunities.

Happy Boat-Search Daze! Art

 

[socalrider]

Very few boats come with both options. Find the boat you want, and the decision will be made for you.

This. It's a purely theoretical discussion unless you're buying a new boat and get to choose. These days inventory is so tight you need to grab the first good boat that meets your major criteria at a reasonable price point. I don't think this should be one of your major criteria. Generally 90-95% of the market in your size range is twins so you're really limiting yourself if you specify a single.

 

[Gdavid]

I think you would find a difference in maintenance costs among brands and models of engines comparable to single or twin engines cost differences.

Doing your own maintenance will always provide a massive savings but check around your area to see what brands/models are well supported in your area as well as checking parts availability. A lack of service or parts availability can lead to long periods of laid up boat.

 

[mvweebles]

A Grand Banks 36 is one of the few trawlers that was offered in both single and twin (very early model years of the GB42 also offered a single, but those are rare). I have operated both and in my opinion the single is very mannerly for docking even without a thruster. The twins have a higher top speed and the props are a more exposed. The engine room with twins is super tight. With a single, space is comfortable.

If it were me looking at a GB36, I would seek-out the single for engine room access and maintainability; and prop protection. However, most buyers like the idea of additional speed and redundancy.

Not sure I agree with the blanket statement that fuel economy is the same between a single and a twin. Under very narrow circumstances it's true - the narrow circumstances are if the cumulative power is the same. The GB36 example, the twin configuration likely has a pair of Cummins with a total of close to 425-450 hp and can do 15-17 kts. The single configuration might have a single 210hp Cummins and tops out at maybe 10-kts. At 10-kts, they would both burn around the same, but no one actually operate a single at 10-kts. She'd be comfortable at around 7.0-7.5 kts burning 1.5 gph. At that speed, the twin would be closer to 2.25 gph total. While less than a gallon per hour difference, a 50% premium. Given the price of diesel these days, it may make a difference in your calculus of usage. Another very popular thread topic right now is effect of diesel costs on cruising plans. The folks who say it has no effect are the slow, single engine crowd such as myself. The twin crowd are scaling back and throttling back, some dramatically.

The Defever 44 is one of the few twins I can think of in this size range with decent engine room access. It is powered with smaller engines as it's a displacement boat, and was well designed from the outset.

The Nordhavn 40 is the penultimate small-boat engine room with near standing room and plenty of space for spares and tools. It's arguably a twin as it has a wing auxillary engine.

Engine room on my Willard 36 is tight (but serviceable) due to carrying close to 500 gals in saddle tanks. Engine room on the Willard 40 is excellent because they went with a single large tank placed side-to-side in front of the engine room that separates the engine room from staterooms.

If you're doing much of your own work and plan to use the boat, accessibility of the engine room is important. If you're writing a check and hiring a mechanic, makes less difference (though stay away from over-sized mechanics).

The big tradeoff is redundancy of twins vs maintainability of a single. Twins also typically offer faster speeds. Singles usually have more protected running gear. OC Diver above defined use case for twins pretty well.

Good luck. Please update with your thinking and decision.

Peter

 

[rslifkin]

The GB36 twin vs single isn't a great comparison, as you pointed out. Typically the twin just had 2 of the same thing the single did (so 2 FL120s instead of 1, or 2 Cummins Bs instead of 1, etc.). For a true equivalence, the twins would have been lower powered engines (such as a single 250hp Cummins vs a pair of small 130hp Perkins or similar). The GB36 is also a fairly narrow boat (particularly before they enlarged them slightly), so that further hurts engine access with twins. I'd expect the same pair of Cummins in a GB42 should have much better access.

My boat isn't really a trawler, but I'll be there later today, so if I remember I can grab some pictures of engine room layout for access.

 

[Art]

For comparison regarding relatively small boats' [say, 34' to 50'ers] engine compartment:

Some call them engine rooms - but they usually are not as such until you get into boats in the 60' and longer range - because... in a room you can usually stand erect with ceiling over head and an entry/exit door.

That said - Before you purchase a boat [with single or twins] make sure you would feel comfortable spending an hour or more working diligently on the power train in the engine compartment of that boat.

Personally... Being a large fellow. I won't own a boat wherein its engine compartment requires me to crouch/kneel down continually. Thus, I always purchase boats that have open-floor-hatch access to engines so I can standup; and, that has ample room to work on the engine.

This can be easily found in sedan style boats where its engine hatches are in the cockpit. Can also be found in tri cabin boats that are laid out well. Following photos are easily, quickly opened hinged/stand-alone hatches to the engine compartment in our Tollycraft salon floor. Makes life easy to work on engines, gen set and batts. Because our Tolly has 100 gallon side tanks it is a squeeze to work on outside portions of engines... but doable! There's a tradeoff for everything on boats!!

 

[rslifkin]

I've got full floor hatches like Art, but with the exception of a few tasks on the generator, I've never found it worth the effort to open them. Opening them lets me stand, but then I'm bending over the thing I'm working on. It's usually easier to just crawl in and either kneel or sit next to it instead of being bent over.

Here are a few engine room shots on my boat for comparison. The only spot that's really a bit rough to access is around the generator. You can get to most basic stuff pretty easily by either crawling around on top of the holding tank or sitting between the stbd engine and generator. But for anything on the forward end or outboard forward side of the unit it's easier to just pull the floor over it (conveniently the easiest of those panels to remove), as crawling past the generator exhaust system, etc. is pretty tight. Sitting on the angled hull bottom outboard of the engines can take some getting used to (and there's a little less headroom out there), but there's plenty of space.

 

[Seevee]

One could make an argument either way, but:
Comparing the SAME boat with twins or single, the Mainship 390 and 400 are good examples.

The twin usually has two smaller engines.
The fuel economy at hull speed is virtually identical. We ran 2 Mainship 400s side by side for 500 miles and the twin actually burned less fuel... and with the twin, you have the option of speed if you want it.
Yes, the maintenance is almost double, but that's usually a rather small part of the bill.
The twin is much more maneuverable, especially in tight area, and there's a lot of twins that come with a thruster, and have a very easy time maneuvering. My Mainship 430 has twins and bow and stern thrusters, which are rarely needed.
And YES engine room access IS a big deal... if you can't get to the outside, you can't maintain them. When I looked at the MS400, the twin was tight, but doable. Some are worse and others are great.

 

[Hippocampus]

Had several long conversations with the gentleman who consults at present for Lugger/NL. He said singles fail less often and are rarely towed. He thought with a single any new noise, vibration or hiccup is identified and addressed ASAP. He thought this was both due to a higher level of concern if failure occurred as well as it being easier to be aware of new deviations from normal.
I used to do virtually all my maintenance. Now with a engine 5x as large and complicated I’ll need to watch and train before resuming that program. Labor is billed by time. If it’s difficult for you to get to a spot or difficult to move once in that spot it will be for a professional wrench as well. That means more hours and expense.
Twins move the bow around. Yes much better than a bow thruster. Not so good moving the stern as the wheels are smaller so less prop walk than the bigger wheel of a single. Still it’s more intuitive to use twins than walk, wash and thrusters. Have several pro captain friends. They’re amazing. It’s beautiful to watch their close quarters handling on really large boats. Chatted them up when starting to look for a trawler. They said you will need to learn with either. Twins aren’t meaningful insurance against failure. Good maintenance is. 17kts v 20kts doesn’t matter in the real world. Prop protection is not complete with a single. You still need to be careful. Pick the boat on condition and price.

 

[rslifkin]

Twins move the bow around. Yes much better than a bow thruster. Not so good moving the stern as the wheels are smaller so less prop walk than the bigger wheel of a single. Still it’s more intuitive to use twins than walk, wash and thrusters. Have several pro captain friends. They’re amazing. It’s beautiful to watch their close quarters handling on really large boats. Chatted them up when starting to look for a trawler. They said you will need to learn with either. Twins aren’t meaningful insurance against failure. Good maintenance is. 17kts v 20kts doesn’t matter in the real world. Prop protection is not complete with a single. You still need to be careful. Pick the boat on condition and price.

How the bow vs stern moves with twins varies a lot depending on the boat. Some twins pivot on the center or aft of it and move the bow over easily. Others (like mine) pivot further forward and move the stern sideways more than the bow (moving the bow sideways with no thruster is a challenge on my boat). Generally a deeper forefoot and more pronounced keel will move the pivot point forward.

Twins with small, fast turning props or prop pockets will tend to have limited prop walk, but with no pockets and slower turning (bigger) props there will often be plenty of prop walk to move the stern around. And like a single, you can always use some rudder to help move the stern.

 

[Hippocampus]

Excellent post. Thanks for the education.

 

[Art]

Find a "seasoned" boat pilot [who has access to using a boat for instructions = maybe their own boat??!!] who has spent years doing transport and many other items of handling as well caring for boats; [you know... the real deal, with experience... not a marine school graduate that has limited "personal experience"].

Then, cough up a grand or three $$$. Spend several days aboard a boat with the captain letting him/her Teach you the ropes [pun intended] of owning, caring for and handling a 40' +/- boat.

If you are really going to purchase a 40' +/- pleasure boat... Those couple of grand will be the best way for you to catapult forward into knowing what a substantial sized cruiser really is all about.

Happy Boat-Learning Daze - Art

 

[krouth]

Not trying to get too far off topic - but one question that came to mind: is it practical to run a twin-equipped boat with just one of the engines, especially if all you want to do is trawler speeds. You could just alternate engines to equalize run-times. I have heard of catamaran owners doing this.

It seems you could then have the best of both worlds: dual engine redundancy/maneuverability when needed and also reduced single engine maintenance costs since you are effectively only using one engine at a time.

Or is this a big no-no?

 

[rslifkin]

Not trying to get too far off topic - but one question that came to mind: is it practical to run a twin-equipped boat with just one of the engines, especially if all you want to do is trawler speeds. You could just alternate engines to equalize run-times. I have heard of catamaran owners doing this.

It seems you could then have the best of both worlds: dual engine redundancy/maneuverability when needed and also reduced single engine maintenance costs since you are effectively only using one engine at a time.

Or is this a big no-no?

It can be done, but it will noticeably hurt maneuverability. And in many cases, assuming you're going the same speed on one engine vs 2, it saves very little fuel (if any). The only time I'd do it is if forced to run at no wake speed for an extended period where I'd consider shutting one engine down to increase load on the other (vs having both just barely off idle for a long period).

 

[Art]

Not trying to get too far off topic - but one question that came to mind: is it practical to run a twin-equipped boat with just one of the engines, especially if all you want to do is trawler speeds. You could just alternate engines to equalize run-times. I have heard of catamaran owners doing this.

It seems you could then have the best of both worlds: dual engine redundancy/maneuverability when needed and also reduced single engine maintenance costs since you are effectively only using one engine at a time.

Or is this a big no-no?

Very doable and does save fuel... albeit... eats up time if you want to go faster than about 4.5 to 5.5 knots. Also, at that speed, plan for the direction of tidal currents... which is always smart to do no matter the speed traveled. But, if a 3 knot current is going against you and you are only doing 4.5 knots tdw = 1.5 kol.

There is a thing about not hurting your trany of the shut down engine. Check with trany manufacturer to learn if you need to lock your shaft.

 

[rslifkin]

Very doable and does save fuel... albeit... eats up time if you want to go faster than about 4.5 to 5.5 knots. Also, at that speed, plan for the direction of tidal currents... which is always smart to do no matter the speed traveled. But, if a 3 knot current is going against you and you are only doing 4.5 knots tdw = 1.5 kol.

There is a thing about not hurting your trany of the shut down engine. Check with trany manufacturer to learn if you need to lock your shaft.

It saves fuel with gas engines mostly because they're so horribly inefficient under light loads. But with diesels, the fuel savings from better engine loading is usually lost to the drag from the dead prop plus having the rudders angled the whole time (which also adds drag).

 

[Ken E.]

Not trying to get too far off topic - but one question that came to mind: is it practical to run a twin-equipped boat with just one of the engines, especially if all you want to do is trawler speeds. You could just alternate engines to equalize run-times. I have heard of catamaran owners doing this.

It seems you could then have the best of both worlds: dual engine redundancy/maneuverability when needed and also reduced single engine maintenance costs since you are effectively only using one engine at a time.

Or is this a big no-no?

I have experimented with running on one engine and freewheeling the other quite a bit. As far as fuel savings, I have found that the biggest effect comes from going slower rather than running on one vs two. Also, your transmissions must be rated for freewheeling to prevent damage, or the off- engine's shaft has to be locked so it doesn't spin. Shaft seal cooling can also be an issue with one engine off. No doubt though that you'll run up fewer engine hours doing this but whether that equates to lower maintenance costs is debatable. Long and short for me, I just run them both.

 

[High Wire]

You're going to spend more on maintenance for twins vs single for obvious reasons. Fuel 10-20% more for twins at the same speed. Docking is pure preference.

 

[Mac2]

Not trying to get too far off topic - but one question that came to mind: is it practical to run a twin-equipped boat with just one of the engines, especially if all you want to do is trawler speeds. You could just alternate engines to equalize run-times. I have heard of catamaran owners doing this.

It seems you could then have the best of both worlds: dual engine redundancy/maneuverability when needed and also reduced single engine maintenance costs since you are effectively only using one engine at a time.

Or is this a big no-no?

Just experimenting with this. I'm planning on putting ShaftLoc's on my boat, but wanted to test the steering and speed differences. I lost 1.5 knots at cruising speed (freewheeling) and didn't notice any difference in steering (Puget sound 1-2 foot seas). I have Cat 3208 NA and I'm only doing this for plans of running 24/7-each engine doing 12 hour shifts.

I just got back from the San Juans. I've never seen as many tree sized logs floating in the water. A key issue when talking about single versus twin would be the protected running gear. To the OP I would say it would be a consideration to consider the cost of bent shafts and props in your consideration (not to mention safety). The slower trawler gives you a lot more time for dodging, but my nine hour run to the San Juans last week was done mostly in fog. It was clear and sunny on the mainland. I've had both single and twins. Both have protected running gear. I love my twins for the maneuverability, but more importantly for the redundancy. I do all my own routine maintenance, so the cost difference between single versus twins is really minimal in boat dollars.
The only thing I would add that others havn't, is to add every warning sensor possible on the single (exhaust, intake water flow, oil, etc) along with a fuel polisher and oil bypass filter. Make that single as bullet proof as possible.