Using single engine of dual engine boat?

[mcarthur]

What I've done in the background to the blog post is collect as much data on consumption for powercats that I can. I have pretty good data for:
- PDQ 34
- Highland 35
- MY37
- Aspen 120
- Leopard 43
- Summerland 40
- MY44
- Leopard 53
- Aquila 32
- Aquila 44
- Aquila 45
- Aquila 48
- Powerplay 56
- Sunreef 70
- Horizon 52
- Africat 42
The rest is only barely enough quality and quantity to use for comparisons.

With enough data, I've fitted a consumption polynomial so can work out for most common speeds what the consumption will be (ignoring wind, wave, over-and-under weight, etc.).

For the Horizon 52 - if that's the one - 9.5kn is reported to use 2.6lpnm (9.7gph), so your friend's 8gph is 20% under or about 2.1lpnm.
To compare similar sized boats:
- Powerplay 56 is 3.2lpnm
- Aquila 48 with 370's is 3.2lpnm
- Tennant cat Catbyrd is 1.2lpnm
- Leopard 53 is 1.7lpnm
- Cumberland 47 with 300's is 1.6lpnm
- Tennant Domino is 1.9lpnm
So the Horizon 52 is in the middle at that speed in my list. Not too bad given her displacement is about 27 tonne.

 

[FF]

IF the boat is a new build the stock tranny (for landing craft) has the most advantages .

One larger on center prop , very heavy duty shaft , and either or both engines can operate.

Could make sense with 2 different sized engines if long range cruising was required with fast speeds at defueling time.


Two identical engines would make repair parts available ,mid ocean..

 

[rmbaker47]

Running twin engine boat on one engines

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?

I have run 2 twin screw boats for 100s of NM on one engine for fuel efficiency. Both boats had Crusader 270s one was straight shaft the current boat is V drive. Puget Sound has a 3 knot current with the tide so I would run up and down the Sound at 1800 rpm on one engine showing 7 knots but making 10 knots over ground. Keep dead engine transmission in neutral for prop free wheeling. From what I read in you replies, better check with your transmission mfg on free wheeling the prop

 

[markpierce]

On my dad's auxiliary sloop, when transiting from power to sail, we'd look in the engine compartment and check the propeller shaft and align it so the two-bladed propeller was vertical, hidden by the keel, than placed the transmission in gear. Thus, no/minimal propeller drag.

 

[rgano]

Keep dead engine transmission in neutral for prop free wheeling. From what I read in you replies, better check with your transmission mfg on free wheeling the prop

Here's a query. Does a hydraulic tranny care what the position the shifter is in when the engine is not running to power its pump?

 

[IfItsWorthDoing]

What about engine running hours though?

Surprised nobody has factored this into the discussion - clearly running only one of two engines (at hull speed or less) creates ‘savings’ in terms of engine maintenance intervals?

 

[catalinajack]

Perhaps, but not a savings worth even talking about.

Surprised nobody has factored this into the discussion - clearly running only one of two engines (at hull speed or less) creates ‘savings’ in terms of engine maintenance intervals?

 

[koliver]

My "real world" experience is from a summer on one engine while I waited to get the other repaired. I put the usual ~100 hrs on the operating engine, at the same rpm and operating temp as when both were operating. My fuel consumption averaged exactly the same as when that engine had an operating mate, so my total fuel cost was 1/2 of the usual for all of that time. The contribution of the second engine was the difference in speed that I was able to maintain, ~8.2 on both, ~6.5 on one.
No, I have not reduced speed to 6.5 on 2 engines for any measurable length of time. At 6.5 I make hardly any wave. At 8 or higher, wave making is a significant use of energy.
On 1 engine the boat is driven crabwise, with the engines ~2.5' off center, so a few degrees of rudder are needed to go in a strait line, cutting into efficiency.
As others have noted, one side heats water, one side drives the big alternator, each side draws from its own fuel tank, so trim is affected, so there are many reasons to always run both. Fuel efficiency will never be a reason, on my boat, to run on only 1.

 

[mcarthur]

For my boat, extra cost of running 2 engine vs 1 engine (alternating so both get used), at 6 knots (only!), is 100% for maintenance and rebuild and 70% for fuel. So if spending $1000 per year on engine maintenance and $1000 on fuel for using one engine @6kn, if both engines were on instead and still going 6kn, I would spend $2000 on maint and $1,700 on fuel (or $1,700 extra). Of course this isn't real as you wouldn't go 6kn all the time, but gives a flavor that maintenance savings aren't minimal if you can work one engine use into your allowable revs and acceptable speed.

 

[koliver]

For my boat, extra cost of running 2 engine vs 1 engine (alternating so both get used), at 6 knots (only!), is 100% for maintenance and rebuild and 70% for fuel. So if spending $1000 per year on engine maintenance and $1000 on fuel for using one engine @6kn, if both engines were on instead and still going 6kn, I would spend $2000 on maint and $1,700 on fuel (or $1,700 extra). Of course this isn't real as you wouldn't go 6kn all the time, but gives a flavor that maintenance savings aren't minimal if you can work one engine use into your allowable revs and acceptable speed.

Well, why would you think the use of a second engine and no increase in speed would still cost you for maint? At 6 knots for one, you may be needing some maint after a yr=ear, but add the second engine, keep your speed at 6 knots, you will never get much above idle, you won't use much fuel in either, likely less total between the two than your previous example of using only one, you won't likely need an oil change, filter change, or any other usual maint, as you haven't really used your engines at all.


I recently bought a pair of oil filters for my 2 200 hp boat engine and a single oil filter for my single 400 hp engine in my motorhome. Also oil for all three. The boat oil and filters cost exactly the same as the MH oil and filter, as the sump size is double and the filter cost is double for the bigger engine.
Still can't see how you can be spending so much for maint.

 

[rgano]

When I had two engines in a trawler, I ran them both at best cruise speed virtually all the time. I owned it for over a quarter of a century and never had to overhaul except in the first year when one went "tango uniform" due to a faulty repair well before I bought the boat. So on the whole I think factoring in a rebuild is not a real world concern for most of us running diesels. Run em because that's what they like. Running one engine all the time at 6 knots? Go get a snailbote.

Now I have a single and I run it on plane virtually all the time because fuel is the least of my expenses, and even if it was not, why putz around going nowhere fast in a boat which can? I am not that poor. And at my age the darned engine will likely outlive me.

 

[Andosan]

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.

Lots of sportfish boats with high power diesels, in the order of 100hp/metric tonne, troll all day at barely above idle with no ill effects.