If you could do it over again?

[Phil Fill]

If you like the boat weather is a single or twin is secondary. The Eagle is a single with bow thruster. Weather the boat was twin or single it would not matter to me, but if a single it would have to have a bow thruster.

 

[Underway]

We've had to maneuver and dock our boat on one engine twice. I would not call it at all difficult. You have to think through what you're going to do but once you have a plan it's pretty straightforward.

Underway with one shut down is annoying because the locked- off prop generates a lot of turbulence which buffets the rudder and steering. And you lose speed because of the drag of the unpowered prop and the need to compensate for the asymetrical thrust which creates more drag.

Yes, if you have a transmission that requires the shaft to be locked, it doesn't make operational sense. As for drag, it obviously goes up....but the real question is whether net efficiency also goes up. The numbers show that it does for our boat, although I believe it has to do with operating turbocharged engines at near idle rpm, and the fact that the props on a boat with enough installed power to actually reach semi-planing speeds are fairly large. So a single prop takes a pretty healthy bite at slow speeds (yes, also comparatively more drag for the one that's freewheeling). Again, for our boat the overall thrust/drag/engine efficiency ends up as an obvious net positive.

It would be an interesting science project for members with small engine twins (that can be free wheeled), and who have a flow scan or a prop chart, to do several comparison speed/rpm runs (twin versus single). Then calculate single versus twin fuel burn for the same speed(s). I'm guessing the improvement would be smaller than for large engine, semi-planing twins. But real numbers would sure beat speculation. In that regard, next summer I'll lock down a shaft for a short test to compare free wheeling to locked prop....not that I'll ever lock one in normal operation.

Regarding turbulence and crab/rudder angle, it's a virtual non-issue for our boat in open water (free wheeling prop). And I'd always start the second engine for docking. I'm going to put this in operation for long runs next summer. Still need to mod the raw water cooling feed to cross connect the stuffing boxes to get flow to the side with engine off. I'll post long term results as I expect differences versus the prop chart.

 

[Marin]

The comparison I am most familiar with is one that was done with an Alaskan 45 with a pair of Amarine (John Deere) engines of about 120 hp and props that could be freewheeled. The owner (Bob Lowe) conducted an extensive test and charted the results. The basic conclusion was that running on one with this type of boat and engines yields little to no fuel savings and under some power conditions the efficiency actually goes down.

 

[Underway]

Do you have a link to that data?

 

[Marin]

Do you have a link to that data?

Bob, who is one of the founders of the Grand Banks owners forum, posted it there. I've sent it to other people on this forum in the past but it's no longer on my computer or this iPad. Bob's original post is in the GB forum archives.

 

[Art]

[QUOTE
Marin sounds correct about twin-engined boats being inefficient running on a single engine, and I'd add based on verbal and written accounts, very difficult to maneuver. [/QUOTE]

Au contraire, Marin and Mark.

Our twin screw Tolly cruising on a single engine at 6 to 6.5 knots (a bit below calced hull speed of 7.58 knots) improves NMPG by approx .75 NMPG over the best NMPG available when two engines run simultaneously. Our BW trany have no prob with freewheel prop at slow speed, and, just for leaning to the side of caution I rotate engine use each hour. Guarantees the 71C trany discs stay lubed and engine hours remain close to the same. Also, if water/wind conditions get too rough where single engine usage becomes hard to easily handle the boat – just start the second screw and great control is instantly in your hands again!

To OP on this thread – I recommend twin screw planing hull with hard chine... from experience and for several reasons. – IMHO!

Good luck with your boat search... May Neptune and his Mermaids’ luck stay with you!

 

[Marin]

Marin sounds correct about twin-engined boats being inefficient running on a single engine, and I'd add based on verbal and written accounts, very difficult to maneuver.

Au contraire, Marin and Mark.

The fuel savings (or not) from running a twin on one will vary with the type of hull, the speed, the weight of the boat, the engines, and a whole bunch of other variables. The tests I've seen charted used boats like ours-- relatively heavy (30,000 pound-plus), keeled, semi-planing boats with relatively low powered engines (FL120s and the like). These comparison tests showed that at best the fuel savings were minimal and under some conditions the efficiency was actually worse than running the boat on both engines.

How these comparison tests relate to a light (17,000 pounds) planing cruiser with gas engines I have no idea but the two types of boats are so different I would be hesitant to apply the results from one type to the other type.

 

[markpierce]

And here I thought you kept the bar stock to starboard.

Well, the beer is kept in the refrigerator, and it is on the starboard side. Wine (red) is kept under the saloon table along the centerline, however. Sorry, no stronger drink aboard (presently).

 

[Art]

The fuel savings (or not) from running a twin on one will vary with the type of hull, the speed, the weight of the boat, the engines, and a whole bunch of other variables. The tests I've seen charted used boats like ours-- relatively heavy (30,000 pound-plus), keeled, semi-planing boats with relatively low powered engines (FL120s and the like). These comparison tests showed that at best the fuel savings were minimal and under some conditions the efficiency was actually worse than running the boat on both engines.

How these comparison tests relate to a light (17,000 pounds) planing cruiser with gas engines I have no idea but the two types of boats are so different I would be hesitant to apply the results from one type to the other type.

Agreed!

 

[markpierce]

It's hard to imagine dragging a propeller and having the rudder creating drag to offset a working off-set-from-boat-centerline propeller could be efficient.

 

[Peter B]

If I was to do it again my preference would be(1) 48/52 ft (2)Raised Pilot House with a (3) single engine, (4) bow thruster and (5) active fin stabilisers.
Plus (6) flopper stoppers for when at anchor.
The other main items are (7) main deck toilet/shower and (8) a full walk around main bed.

1. Yep
2. Yep
3. Yep
4. Yep
5. possibly
6. Nope - too much kerfuffle deploying - too much paraphernalia all round decks, for the few times one would use - would rather just rock a bit at anchor. 'course if one could afford a gyro-stabiliser...that would obviate need for fins as well, and work at anchor. Waydago...?
7. Possibly, tho not imperative, but most have the shower anyway.
8. Definitely yes.

 

[psneeld]

It's hard to imagine dragging a propeller and having the rudder creating drag to offset a working off-set-from-boat-centerline propeller could be efficient.

Seems that way but at slower speeds...drag isn't all that much.

Turbulence enough to form cavitation I believe doesn't start till around 15 knots or so I thought I read someplace (geneneral as obviously shape matters)...

But speed is the biggest issue with drag...that's why efficiency at slow speeds between slippery hulls and drag filled hulls is almost non-existent.

When we were on Icebreakers that were painfully slow anyway...I'm pretty sure the engineer said they locked down a prop when on one shaft because at slower speeds the drag was actually less than freewheeling the prop...something to do with slipping water around the fixed prop actually took less energy than letting the water turn the prop overcoming the friction of the drive train...but only at slower speeds. When traveling thousands of miles he said the efficiency added up.

 

[Tomas]

We cruise our 40', 38k displacement, trawler single engine. The props are 2 feet off centeliner on a 14.5 beam boat. The boat tracks exceptionally well and there is no rudder feedback or buffeting. Perhaps that is because we are content cruising at sailboat speed. We just completed a cruise from Destin, Fl, to Tarpon Springs, Fl. On inland waters we ran our 120 Lehmans at 1500 rpm and made from 5.5 to 7.2 SOG, depending on tides, winds, currents, etc. On open water we cruised at 1700 to 1800 and make 5.5 to 7.5 SOG. The total engine time was 70 hours, about 35 on each engine. Based on tank soundings, estimated fuel burn was 100 gal. The velvet drive transmissions don't mind free wheeling at low speed, we did it for years on our previous sailboat. Each engine is used everyother day. I don't think we could have traveled the same distance twin engine at the same burn rate, but that aside, we only put 35 hours on each engine, there is economy in that.

 

[Anode]

Actually, I did it over again. It took four years. Got everything I wanted, done my way, new, and it all works.

 

[Ocean Breeze NL]

During the first part of Sept we were coming around the rocky shores of NL in the North Atlantic with a 30mph NE wind blowing, with 6'-10' waves plus a sea on and lost 1 engine. Single or twin? Not even a question for me! Yep had to lock down the port shaft. Yep lots of drag for the next 45nm and fuel consumption.
Docking with a twin or single? It just depends on experience and skill. I have no problem with either however love the twin.

Took the Ocean Breeze out from my wharf one day just using the motors ( normal practice ) i.e maneuvering between a rock and my wharf and didn't realize I had no steering until I got out past it only to realize I had the auto pilot engaged. Single it twin? For me I prefer twin, cause I can get to where I want just using the motors if I had no steering.

Elwin

Sent from my iPhone using Trawler

 

[Rambler]

Well, the beer is kept in the refrigerator, and it is on the starboard side. Wine (red) is kept under the saloon table along the centerline, however. Sorry, no stronger drink aboard (presently).

Maybe it's high gravity beer?

 

[Art]

During the first part of Sept we were coming around the rocky shores of NL in the North Atlantic with a 30mph NE wind blowing, with 6'-10' waves plus a sea on and lost 1 engine. Single or twin? Not even a question for me! Yep had to lock down the port shaft. Yep lots of drag for the next 45nm and fuel consumption.
Docking with a twin or single? It just depends on experience and skill. I have no problem with either however love the twin.

Took the Ocean Breeze out from my wharf one day just using the motors ( normal practice ) i.e maneuvering between a rock and my wharf and didn't realize I had no steering until I got out past it only to realize I had the auto pilot engaged. Single it twin? For me I prefer twin, cause I can get to where I want just using the motors if I had no steering.

Elwin

Elwin

I agree 100%! MOF - With a twin I leave rudders straight-ahead and don't touch the wheel when entering a dock. Also do same when leaving docks till well into a chanel, preferably with some rpm/speed a bit beyond idle has become available. Single screw is fine... Twin Screw is Great! With twins the boat can be made to dance. And, to actually walk sideways by using rudder angle with appropiate shift/throtle applications on a specific engine. Can’t do that with a single.

Art

 

[psneeld]

Elwin

I agree 100%! MOF - With a twin I leave rudders straight-ahead and don't touch the wheel when entering a dock. Also do same when leaving docks till well into a chanel, preferably with some rpm/speed a bit beyond idle has become available. Single screw is fine... Twin Screw is Great! With twins the boat can be made to dance. And, to actually walk sideways by using rudder angle with appropiate shift/throtle applications on a specific engine. Can’t do that with a single.

Art

After 10 years of operating a finicky, gas powered single engine assistance towing vessel...going out to rescue 1000's of boaters and only needing assistance myself twice...I have to say singles are just as reliable as twins in the wrong hands. Once a crab pot rebar wrapped around the prop and once I slid up aground on a high tide covered grass marsh bank. But mechanically or electrically a carbed, gas engine has never left me in a bad situation despite a lost rudder, overheating on a regular basis, etc...etc...

If you know what you are doing, a single is usually the rescue boat for the twin that has an idiot for a skipper.

If you like twins...great...but there is NO argument that a twin will get you completely and safely home.....EVERY TIME.....and that a single won't.

And we beat the living crap out of these boats...they just have knowledgeable skippers watching over them every day, 365 days of the year...meet that requirement and your commercial vessel OR recreational vessel and you'll reap the same benefits...

 

[Art]

...there is NO argument that a twin will get you completely and safely home.....EVERY TIME.....and that a single won't.

Except when they both run out of fuel... seen that! A pair of dead twins are no more use than a dead single! - LOL

 

[psneeld]

Except when they both run out of fuel... seen that! A pair of dead twins are no more use than a dead single! - LOL

I see it a hundred times a summer...fuel, fire, manueverability, etc...etc....it's all about the skipper and not about how many engine(s) and thrusters.

I love operating a twin because for me it involves a whole lot less thought most of the time....

But economically...a properly designed, slow boat with a single is much more economical and easier to maintain... You can argue it's only incrementally more expensive to own twins and you are right for a part of it...both singles and twins cost exactly the same amount on the showroom floor and are both equally as fun and safe sitting there side by side....but run them 500 hours plus a year and the economics add up.

Commercial guys aren't stupid...as long as the application allows for it...singles are almost always preferred.

 

[angus99]

Seems that way but at slower speeds...drag isn't all that much.

Turbulence enough to form cavitation I believe doesn't start till around 15 knots or so I thought I read someplace (geneneral as obviously shape matters)...

But speed is the biggest issue with drag...that's why efficiency at slow speeds between slippery hulls and drag filled hulls is almost non-existent.

When we were on Icebreakers that were painfully slow anyway...I'm pretty sure the engineer said they locked down a prop when on one shaft because at slower speeds the drag was actually less than freewheeling the prop...something to do with slipping water around the fixed prop actually took less energy than letting the water turn the prop overcoming the friction of the drive train...but only at slower speeds. When traveling thousands of miles he said the efficiency added up.

Are folding or feathering props ever an option on these higher-powered boats, as they were on my recently departed sailboat?

 

[jahillsr]

I do note that the draft of many boats is larger with a single then the same boat with twins. On a KK 52 for example (5'5" vs 4'6"). I expect this comes mostly into question based on where you expect to be cruising and the depths available along route. So would it be fair to say the trade-off includes much more than economics and maintenance considering the difficulty in keeping channels open? Or is the difference of a 11" just not a big deal on a boat that size?

 

[Pau Hana]

Another vote for twins. The only way I'd consider a single is if the vessel had a wing engine, or a way to cross-connect the generator to provide motive power.

 

[Marin]

MIT did tank tests a number of years ago to determine which generated more drag through the water, a locked down prop or a freewheeling prop. The results were posted by someone on a forum at the time. T&T or the GB forum as I recall. Anyway, logic would indicate that the locked prop would generate more drag than the freewheeling prop although there are a lot of people who believe otherwise. The MIT tank test showed that at all speeds the locked prop generated more drag than the freewheeling prop.

 

[Marin]

Elwin

I agree 100%! MOF - With a twin I leave rudders straight-ahead and don't touch the wheel when entering a dock.

I, on the other hand, use the rudders in maneuvering a lot. They can make moves much quicker and more accurate than just using differential thrust.

I started out just using thrust and leaving the rudders centered. I soon learned what a huge help the rudders can be so I added them into the maneuvering mix. More recently I have added power into the mix, finding that adding some or occasionally a fair amount of power for a moment on one engine or the other, sometimes for forward thrust sometimes reverse, adds a whole new dynamic to what I can do with the boat. I'm still getting used to this aspect-- what and when to add power is easy logic, it's learning the various situations so I don't have to take a moment to figure it out that's the learning curve. But power has already " saved" several docking situations on windy days.

This is the main reason I like multiple engines so much. It gives so many more control options to work with and it's fun to take advantage of them.

 

[RickB]

I'm pretty sure the engineer said they locked down a prop when on one shaft because at slower speeds the drag was actually less than freewheeling the prop...

Lowest drag is when the "off duty" prop is turned at a "zero thrust" speed or pitch/power.

That is how the Washington State Ferries operate. The "pulling end" prop is turned with just enough power to eliminate drag and neither contributes to thrust or creates drag.

 

[Underway]

MIT did tank tests a number of years ago to determine which generated more drag through the water, a locked down prop or a freewheeling prop. The results were posted by someone on a forum at the time. T&T or the GB forum as I recall. Anyway, logic would indicate that the locked prop would generate more drag than the freewheeling prop although there are a lot of people who believe otherwise. The MIT tank test showed that at all speeds the locked prop generated more drag than the freewheeling prop.

Consistent with your experience of turbulence and rudder vibration with a locked shaft versus other comments that a free wheeling prop was relatively benign in that regard.

 

[dvd]

Not that it makes a difference to me, since I've always been a single screw guy [although with lots of lasting power ;-) ] but I'm trying to understand the concept. As I see it, there are three options for running on one engine with a twin screw boat: 1) run one engine, turn the other one off and lock the shaft (perhaps most efficient at low speed) 2) run one engine, turn the other one off and let the prop freewheel (efficient if MIT is right) or 3) run one engine at thrust power, run the other at minimal power to eliminate drag but not produce thrust (efficient if the Washington State Ferries are right). It would seem to me that if option 3 is taken, that would underload the engine and not necessarily be a good thing?

dvd

 

[Marin]

At the risk of telling you what you probably already know, the issue with locking the shaft has nothing to do with efficiency or the lack of it. On some boats, like ours, the shaft has to be locked because the shaft logs do not get sufficient cooling/lube water when the boat is underway with the shafts turning so they depend on raw water picked off of the engine's raw water cooling system and sent to the shaft long via a hose. If an engine has to be shut down this flow of cooling water to the shaft log stops. If the shaft is allowed to freewheel the shaft log will overheat very rapidly and the heat can be enough to damage not onliy the log but even the shaft.

If the transmisison and the shaft log are such that they do not suffer excess heat and lack of lubrication from being freewheeled, then shutting down one engine and running on the other one won't hurt anything. However, on boats like ours comparison tests show that the reduction in fuel burn is insignificant at best and may actually go up with only one engine moving the boat.

Turning one engine over very slowly so as to turn the prop just fast enough to elminate significant drag is not something I think is worth doing in boats like ours. It probably is in something like a Washington State Ferry, escpecially because some (if not all of them today) have variable pitch props, and their engines are probably such that running them at idle speeds is not detrimental over long periods of time. Plus the ferry runs are pretty short so the idling engine will be moving the boat when the ferry starts back the other way. But in a cruising boat like many of us have, I don't see that this would really gain much of anything in terms of fuel savings.

 

[Marin]

Consistent with your experience of turbulence and rudder vibration with a locked shaft versus other comments that a free wheeling prop was relatively benign in that regard.

Yes. We once made a six hour run home on one engine after experiencing the start of an engine overheat at the end of the previous day's run. We had sufficient cooling flow at idle but at higher power the flow was not enough. Talked to our diesel shop on the phone and the consensus was to simply tie off the shaft and come home on the other engine.

We have cable-chain steering so anything that happens to the rudders is felt at the helm, as opposed to hydraulic steering which can act as an isolator between the rudder(s) and the helm. While the buffeting of the turbulence coming off that locked prop and hitting the rudder was not severe, it was a constant presence once we got above a few knots.

We ran the other engine at our normal cruise setting of 1650 rpm and standing at the aft rail we could see the increased turbulence in the water on the port side. In water as in air, turbulence equals drag.

In this case since the cooling water flow was sufficient at idle we untied the shaft outside the marina entrance and came in and docked using both engines. The other two times we had to dock after a precautionary engine shutdown we did not start the second engine but docked on the one.