Two shafts more efficient than one...?

[Willallison]

The most recent edition of Passagemaker magazine (Feb 05) contains an article, written by Chuck Husick, which has me a little puzzled. It describes the installation of a new transmission system called the "Geared Up System", where a single engine is connected via 3 90-degree unity-ratio geaboxes to two separate shafts, props etc.
The system is quite elegent in its simplicity and I can see some sensible arguments for its use - such as low speed manouverability and the like. But what has me bamboozled is that the system appears to be more efficient than either a single engine / single screw (from here on referred to as a 1+1), or twin engine / twin screw (2+2) installation. (The Geared up system I'll call 1+2)
The article refers to a 34 ft Mainship Pilot and gives the following performance figures:

@15mph, 1+1 gives 1.84mpg, 2+2 gives 1.49mpg, 1+2 gives 2.06mpg

@ 21mph, 1+1 gives 1.7mpg, 1+2 gives 1.4mpg, 1+2 gives 1.95mpg

@ 26mph, 1+1 not tested, 1+2 gives 1.17mpg, 1+2 gives 1.79mpg

These figures show the 1+2 system to be as much as 53% more efficient than the other more conventional drive systems!
Now all my experience and education (ok - somewhat limited I admit... ) tells me that three gearboxes and two shafts, rudders struts etc with their associated drag should be the least efficient of the three systems.
I don't doubt the validity of the numbers... so...what gives?

[woodboat]

One could only guess without more information. Looking from the outside in and "guessing" I would assume twins would be the least efficient. You are after all running two engines. One Engine two props I would guess it would be more efficient. Volvo has proven the Duo-prop afterall. My guess is you can turn two smaller diameter, higher pitched props giving more thrust and less drag offsetting any increase in drag from the extra shaft and rudder. If speed increased the appendagae drag may have increased by X.

[jehardiman]

Originally Posted by woodboat

My guess is you can turn two smaller diameter, higher pitched props giving more thrust and less drag offsetting any increase in drag from the extra shaft and rudder.

I concur. By turning two slightly smaller, higher pitched props you improve efficency by decreasing blade loading. Similar gains can be had by going to counter-rotating, where the thrust is shared between the two disks. Additionaly, by lowering the thrust on each wheel you can reduce the BAR which again increases effiency.

[yipster]

hmm... interesting! keep me posted

[Corpus Skipper]

Hold the phone!!! What happened to the whole trawler notion that a single large diameter slow turning prop is more efficient than a small high speed prop?

[woodboat]

I never heard of that notion I always thought that the less you had in the water the better, that's what makes surface piercing setups so fast.

[gonzo]

That data has no context and is missing vital information. For example, is the boat planing or at displacement speed? Also, it doesn't specify if the gearing is the same or not. Those numbers have no meaning without specifying how they were found.

[FAST FRED]

"Hold the phone!!! What happened to the whole trawler notion that a single large diameter slow turning prop is more efficient than a small high speed prop?"


Properly setup at DISPLACEMENT speeds your correct.

Of course IF you could handle the diamerter a 2 blade would be more efficent than a 3 or 4 , and a ONE blade (with suitable counterweight) would be most efficent at cruise.

The PM article is refering to a speed boat , not a cruiser.

Most of these don't have enough fuel weight carring capacity to get out of sight.

From the East Coast Bermuda is about 750 miles , so 900+ miles of range is prudent, but seldome doable in most "fast" boats .

Unless they crawl at displacement speeds where the high speed hull stinks and rolls , the engines wet stack from almost no load and the rudders are VERY small for directional controll in a following sea.

FAST FRED

[Willallison]

Gonzo - you have a point - though I think only up to a point. True, the article fails to give the gear ratio's for the boats in question, but given that the two 'conventional' set-ups are standard (production) boats, one would assume that their transmission set-ups have been optimised. So, with all 3 giving their best performance, the Geared Up system still outperforms.
The speeds quoted in the article - and which I quoted in my original post clearly indicate that the boat is planing, though at the slowest speed (15mph), you would have to say only just.
One thing I didn't mention was the power output of the various engines. The 1+1 had a single 370hp, the 2+2 had a pair of 240hp diesels, and the 1+2 was powered by a single 440hp diesel. It may indeed be this selection of powerplants that accounts for some of the efficiency claims, I don't know....

As far as two small wheels having less drag than one large one goes, I don't think that this would hold true for the speeds in question. True surface-piercing drives are more efficient at high speed: but it is generally accepted that their low-drag advantage doesn't come into play until around 40 knots. Further, you have the added drag of the shafts and their support brackets...

[gonzo]

I thought the Mainship was a trawler yacht. Maybe there are more than one company with the same name. The different engines kind of defeat the comparison. A diesel will always be more fuel efficient.

[Willallison]

same company - they also produce a couple of sort of 'picnic boats'...
Sorry - I obviously wasn't clear - all 3 ran diesel's....

I can improve on 3 -- 90's.===== 5-- 90's. ====Or, 1 two sided timing belt, it is used very eff. --------Any pay back curves?

[duluthboats]

I just found this, the efficiency increase seems high but it is interesting.
http://gearedupsystems.com/

Gary

[Willallison]

I posted this same question elsewhwere and it was suggested that some of the efficiency gain reported in the Passagemaker article might be down to an inability for the Mainship 34 to swing a single prop large enough to give its best performance.....
As an economical way of repowering a twin petrol-engined boat with a single diesel, I can see some fantastic attributes in the system. Similarly in a single engine installation it could reduce draft significantly. But as for being more efficient than a single drive system, I still have my doubts...

[kapnD]

Check out Dorado marine, they have done extensive building of single engine, multi drives. I think their original motivation was to decrease draft by using twin props mounted in pockets, but has evolved from there. I saw one boat in their shop that sported 4 outdrives driven by two engines. I know that outdrives are notoriously weak when coupled to large diesels, so possibly by splitting the power in half, it would become feasible? Also, they produce the sweetest 40' hull I've seen in a long time, that produces impressive performance/efficiency specs.