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Old 10-26-2015, 10:17 AM   #21
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[QUOTE=Art;382559]
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Originally Posted by semi-planing View Post

"Then pushed remaining engine up to achieve 8.4 knots."

Therein lies the difference between running on one engine to conserve fuel and running on twins. The $$ fuel savings (actual per nautical mile fuel use reduction) that can be accomplished by using single instead of twins only comes into play of a notable amount when the speed is considerably reduced. If instead of pushing the single to reach the twins' speed of 8.4 knots you had rather reduced speed to say 6 knots then the nmpg fuel usage will notably diminish... albeit your miles per day accomplished surely diminish too!

Happy Fuel-Use Daze! - Art
+1
a twin cruises at 8kts/1500revs with a nice load on both engines, so should produce no slow running probs....that's exactly why you're doing 8kts not 6-7kts!

In other words you can't go 6kts in a twin because your engines will be idling.

Well surely you should then run on a single engine at equally economical 1500 revs, which will probably get you 6-7kts and save 50% on fuel.
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Old 10-26-2015, 10:18 AM   #22
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The best way to run on a single engine is buy a single engine boat. ;-) Best tool for the job.
I ran my planning T/480hp boat 40 miles on one motor after cracking a manifold. What a freekin mess trying to maneuver around currents, traffic... ugly
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Old 10-26-2015, 10:50 AM   #23
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Originally Posted by markpierce View Post
I suggest one operate the boat as it was designed. If that doesn't conform to your needs, get a proper boat.
Absolutely the correct approach.

But most of these boats were built when fuel was cheap (cheaper) and there's sooo many people that have the philosophy that you can't have too much power. Think I remember Walt saying that several times. But now to most of us most boats are overpowered.

Like Mark says there's only one solution and that is to purchase a boat that is configured to meet your needs. If a SD craft is too inefficient you need a FD boat but few are availible. And those few are not in a market position of extremely high demand ... few seem to want them. Most stick to the notion that you can't have too much power or one needs to outrun storms or other excuses to overpower.

Something that hasn't been mentioned thus far I belive is that a boat w engines close to the keel or center line of the boat will suffer far less than those w far apart engines .. and props. But there aren't many configured like that.

So the solution to the problems resulting in all this "running one engine" talk is as Mark says ....... get a propper boat. Even repowering w small engines you still have the wrong boat because of it's hull shape.
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Old 10-26-2015, 12:21 PM   #24
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Another point I like to make in these sorts of discussions...fuel cost is one of the smallest you will have in keeping your boat in tip top condition. Slip fees, insurance, maintenance, etc will shadow your fuel cost. Unless you're on the move everyday throughout the year, fuel cost will be less than 10% of the cost of ownership. Don't sweat it!
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Old 10-26-2015, 12:37 PM   #25
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[QUOTE=Rustybarge;382570]
Quote:
Originally Posted by Art View Post

+1
a twin cruises at 8kts/1500revs with a nice load on both engines, so should produce no slow running probs....that's exactly why you're doing 8kts not 6-7kts!

In other words you can't go 6kts in a twin because your engines will be idling.

Well surely you should then run on a single engine at equally economical 1500 revs, which will probably get you 6-7kts and save 50% on fuel.
Don't jump to conclusions. I was simply collecting data for comparative purposes, not trying to establish how to best operate the boat on one engine. I already had the twin speed/fuel burn number for 7.9 MPH, which is about 1300 rpm @ 3.0 gph. This is a pretty slick hull. It's 2 gph on one engine at that speed (see post #20). And no, if I elected to run the boat on one, I would NOT run it that slow. First of all I can't stand going that slowly, but more importantly, as I mentioned in post #5, the engine coolant temp is down around 165. One of the main benefits of pushing it up when on one engine would be that the temperature gets up to 185 where it should be. Anyway, the rule of thumb you presented is not universal.


I elect to operate on both even though the engines temps are running cool. I do the occasional high power push, and while it seems to clear out what is obviously slight overfueling...probably by heating up the combustion chambers, I doubt I has any benefit beyond that. I change the oil every 100 hours max. Anyone with a big iron turbo engine with lowered compression to accommodate turbo boost has the same problem.
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Old 10-26-2015, 12:49 PM   #26
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[QUOTE=semi-planing;382611]
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Originally Posted by Rustybarge View Post

Don't jump to conclusions. I was simply collecting data for comparative purposes, not trying to establish how to best operate the boat on one engine. I already had the twin speed/fuel burn number for 7.9 MPH, which is about 1300 rpm @ 3.0 gph. This is a pretty slick hull. It's 2 gph on one engine at that speed (see previous post). And no, if I elected to run the boat on one, I would NOT run it that slow, since as I mentioned in my first post, the engine coolant temp is down around 165. One of the main benefits of pushing it up when on one engine is that the temperature gets up to 185 where it should be. So, the rule of thumb you presented is not universal. In any case, I elect to operate on both even though the engines temps are running cool. I do the occasional high power push, and while it seems to clear out what is obviously slight overfueling...probably by heating up the combustion chambers, I doubt I has any benefit beyond that. I change the oil every 100 hours max.
If you're using fresh water cooling with a heat exchanger , why not change the thermostat for a higher temp one?
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Old 10-26-2015, 01:47 PM   #27
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Running a twin engine boat on one engine means having the rudders over 40 degrees to compensate. That is a head scratcher.
You may run your engines at light load and not have issues like some cigarette smokers live to 80. Reality is we can compression test, oil sample the 75% load engine against the run cold engine and the loaded motor will score much higher.
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I don't think any of the dozens maybe hundreds of different boats I have run as a delivery/corporate captain EVER required 40 degrees of rudder for cruising reasonably on one versus two.

Obviously there are differences or exceptions...but I believe the standard rudder stop is somewhere between 35 and 40 degrees.....usually just at or past 35.
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Old 10-26-2015, 02:32 PM   #28
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I suggest one operate the boat as it was designed. If that doesn't conform to your needs, get a proper boat.
Yep... This.

IMHO, you don't gain much of anything and stand to lose a lot if you add excessive wear to a trans or add unnecessary usage to each engine during it's turn being the single. Just use the boat and the engines the way they are designed and try not to worry about trying to save a relatively small amount of fuel.
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Old 10-26-2015, 10:12 PM   #29
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My friend and I have been running our boats from New Orleans to the Bahamas for years. He has a 61ft Hataras with 12-71s. This last trip he put a feathering prop on one side, giving him a maneuvering/ get home engine on that side. The boat cruises at 8kts about 20-30% more efficiently running on a single engine with the feathering prop. Honestly, if you don't run long distances, it does not matter, but for our trips where we'll burn several thousand gallons over a cruise, it does add up. One of the biggest benefits on a long cruise is not needing to change oil on the feathering side.

On my Defever 48 I have 3208s. I will install a feathering prop on the port side next month when I haul out.
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Old 10-26-2015, 10:15 PM   #30
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Old 10-26-2015, 11:11 PM   #31
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Quote:
Originally Posted by semi-planing View Post
[QUOTE
Quote:
Originally Posted by Art

+1
a twin cruises at 8kts/1500revs with a nice load on both engines, so should produce no slow running probs....that's exactly why you're doing 8kts not 6-7kts!

In other words you can't go 6kts in a twin because your engines will be idling.

Well surely you should then run on a single engine at equally economical 1500 revs, which will probably get you 6-7kts and save 50% on fuel.
Sorry to tell ya semi - Somehow post quotes got mixed up - that one's not mine! - Art
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Old 10-27-2015, 05:00 PM   #32
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My friend and I have been running our boats from New Orleans to the Bahamas for years. He has a 61ft Hataras with 12-71s. This last trip he put a feathering prop on one side, giving him a maneuvering/ get home engine on that side. The boat cruises at 8kts about 20-30% more efficiently running on a single engine with the feathering prop. Honestly, if you don't run long distances, it does not matter, but for our trips where we'll burn several thousand gallons over a cruise, it does add up. One of the biggest benefits on a long cruise is not needing to change oil on the feathering side.

On my Defever 48 I have 3208s. I will install a feathering prop on the port side next month when I haul out.
Thanks for that info; those numbers sound right on the mark.

What sort of ball park figure is it for a feathering/folding prop for your size of boat?
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Old 10-27-2015, 05:03 PM   #33
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Sorry to tell ya semi - Somehow post quotes got mixed up - that one's not mine! - Art
That's my quote, (putting on tin foil hat!)

OK, so I exaggerated a bit to get my point across, it should be 20-30% fuel saving according to post#29
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Old 10-28-2015, 07:16 AM   #34
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"On my Defever 48 I have 3208s. I will install a feathering prop on the port side next month when I haul out."

Be sure the blade area matches the existing prop.

A difference can mean different engine RPM are required to motor ahead , which gets hard on your ears in time.

Are the shaft rotations such that you can switch sides every few transits?
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Old 10-30-2015, 08:20 AM   #35
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This has also been discussed in several threads over the years. I believe Timjet gathered some real world numbers and I've played around with it myself.
Yes I did post questions/comments on this several years ago. A member at the time came back with some very precise engineering data most of which was above my head.

To summarize in my case cruising at just below displacements speeds and for me that was 1300 rpm and 8 kts when I shut one down my speed dropped by only 1 kt to 7kts. So I thought that it was definitenly a fuel savings. In fact I thought that if you note your speed and rpm with both running and shut one down as long as your resultant speed is greater than half the speed with both running then there is a fuel savings running on one. Not so.

As I understand and perhaps some experts will chime in, a diesel engine will try and maintain a set rpm at a set throttle position. So when one is shut down, the other engine will increase fuel flow to maintain it's rpm, in my case 1300 rpm. How much more fuel is used can only be really determined with fuel flow meters.

Also hull design, prop pitch, rudder size and the amount of rudder deflection required to counter act the assymetrical prop thrust all come into play and are all different for each boat.

However on the other side of the equation any engine will require a certain amount of power that does not go to the prop, coolant pump, raw water pump, transmission, turbo, that all take power away from doing any work, - power and fuel that is required just to operate the engine but not making the boat go.

So bottom line you won't know for sure unless you have accurate fuel flow.

Do a search with author timjet around the 2011 time frame and you will see some excellant info on this subject.

I decided to operate on 2 engines, it's safer especially when running in narrow channels and fuel is cheap.
Don't worry about ruining a diesel engine by not running it hard enough. That doesn't happen to much, more likely is early failures due to over propping. With those 500 hp engines you mentioned you can get up on plane when needed, but save fuel and cruise at displacement speeds the rest of the time, that's what I do.
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Old 10-30-2015, 09:48 AM   #36
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"As I understand and perhaps some experts will chime in, a diesel engine will try and maintain a set rpm at a set throttle position. So when one is shut down, the other engine will increase fuel flow to maintain it's rpm, in my case 1300 rpm. How much more fuel is used can only be really determined with fuel flow meters."


Absolutely correct - exactly what a diesel genset is doing talking it from no load to 100% of load. This is also the reason why changing props on diesels to run at lower rpms will most often yield little or no results.
Not so with gas powered boats. The rest of your points also make sense but this is the large contributor.
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Old 10-30-2015, 10:49 AM   #37
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"as i understand and perhaps some experts will chime in, a diesel engine will try and maintain a set rpm at a set throttle position. So when one is shut down, the other engine will increase fuel flow to maintain it's rpm, in my case 1300 rpm. How much more fuel is used can only be really determined with fuel flow meters."


absolutely correct - exactly what a diesel genset is doing talking it from no load to 100% of load. This is also the reason why changing props on diesels to run at lower rpms will most often yield little or no results.
not so with gas powered boats. The rest of your points also make sense but this is the large contributor.
bingo!!
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Old 10-30-2015, 12:10 PM   #38
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As I had one engine down for an extended period last year, I learned this:
At the same rpm as before, my remaining engine burned the same fuel as before. My boat speed dropped 2 knots, from just over 8 to just over 6. I had no trouble docking on the side away from the operating engine. I had a lot of trouble docking on the side nearest to the operating engine.
Conclusions:
Fuel is cheap. I would rather burn more to get the extra 2 knots.
I prefer twins.
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Old 10-30-2015, 08:14 PM   #39
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We recently had a bit of a scare on our boat when we had a runaway starter motor on the stbd engine, at a time when we were transiting a lock and had just gotten secured inside the lock.


Long story short, we ended up having to cruise home on one engine, a distance of about 20 miles. Here's what I discovered as far as speed/rpm is concerned:


Two engines--
to cruise at ~10.5 kts both engines are set to about 1050-1100 rpm's (depends on load(


Single engine--
to cruise at ~10.5kts, the rpm's on the port engine needed to be boosted up to about 1250 rpm's.


That got me to wondering if cruising on one engine, on purpose, would do any harm to the boat. I sent an email off to Sea Ray to pose that question to them.


When I get a reply I'll post their answer on here.
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Old 10-31-2015, 06:36 AM   #40
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That got me to wondering if cruising on one engine, on purpose, would do any harm to the boat. I sent an email off to Sea Ray to pose that question to them.


When I get a reply I'll post their answer on here.
The transmissions may need lubrication due to the free wheeling prop. Check your manual and call the tranny manufacturer.
Also your shaft logs, depending on what type you have.
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