Enlarging rudders to track better with one engine.

[ron6785]

On my 53MY with 8/71 naturals when I lock down one engine and run at 7-8knots my rudder indicates 15-20 degrees, which of course means I'm crawfishing and I exp.ect any fuel saving under this situation would be minimal.

Since this old gal is never going to plane again, I'm wondering if larger rudders would help reduce the rudder angle with one engine running and I could get maybe 2mpg.

I ran into a guy with a 53 late 90's model Tollycraft who has 8/92's and allison gears that claim the Allisons on his boat can be freewheeled and does so. Says he has 2:1 gears. Is this true???? Mine of course are M20's with the pump on the engine and have to be locked down.

This guy claims to get 2 + mpg at 7.5knts.

 

[REBrueckner]

It should reduce the ruddder angle but not the drag...so you'd likely never notice any change in fuel consumption....

Allison M20's are NOT supposed to be freewheeled. That has been agreed upon in these forums.

Whether freewhelling is more or less economical than locking down has been debated in these forums without, I think, a generally agreed upon conclusions...seems like it may be a boat by boat thingy.


The general consensus is that running on one engine is NOT more economical unless you also reduce speed...say you run at 5 knots on one engine versus 9knots on two....THAT may save some fuel.

"This guy claims to get 2 + mpg at 7.5knts."

When fuel is cheap, many tend to claim their boat is fast. When fuel is expensive, many claim their boat is economical. Sounds optimistic to me but I don't know Tolly's...

 

[Will]

Are you sure your rudder angle readings are accurate? That seems like a lot. Although I realize our boats are significantly different, my 43 only requires about 5 degrees rudder to offset single engine running.

If you really plan to run on a single wheel most all of the time, you should also consider having your prop pitch and diameter changed for more efficiency.

Will

 

[GJH]

I question the guy's numbers. I have 8v92TIs and MH20 gears. The manual for the gears specifically advises against free wheeling. No matter what you do it takes X horsepower to make you boat go Y speed. And generally speaking, it takes Z amount of fuel to generate that horsepower. So going a certain speed on one engine or two is not going to be dramatic mileage wise.

Rob, you were on a marathon thread on the T&T list a few years ago that came to this basic conclusion after a few folks ran some tests. I had to inadvertently do this test on a chartered Mainship 43 with Cat 3116s and FloScans. When we had to run on one engine, the fuel consumption at 8 knots (about hull speed) was virtually the same.

If you are worried about wet stacking the Detroits run them up very now and then. Personally I don't think the one engine theory is worth the hassle and downsides. I never plane my boat, and we are typically longer range cruisers, so the subject of fuel mileage is important to me.

 

[yachtsmanbill]

I think the whole debate of locking down is BS... Mine was towed "deadship" 250 miles with no evidence of the shafts being locked and there was no apparent damage at all. One trans already had a bad beaing which was documented by the service company 4 years before it sank. The tow was also run with 2 gallons of sea (lake) water in the gears.
At any point, all Allisons have an external oil pressure pump. None to my knowledge are self contained. If its a real issue, let the engine idle while your towing. JHC... mine MAKE fuel when idling LOL.
So with this being said, I got her running and ran the boat 1500+ miles from NOLA to Chicago WITH the bad bearing rumbling all the way. Those Allisons are DDs on the back end. Dinosaurs and bullet proof. PERIOD. ws

 

[BUSTER]

I believe that dragging a locked prop or a free wheeling prop through the water is going to negate any savings of dousing an engine and running on one engine. I ran my Cummins powered 42C from CT to Abaco last winter at 1400 rpms,both engines,which equated to roughly 10mph most of the trip until I got to Memory Rock and buck fever set in. My normal burn for that trip at cruise speed is about 1700 gallons. I did it that trip on a little more than a 1000 gallons but it took 3 days longer,even though I ran a couple of overnighters.

 

[REBrueckner]

The typical reasons a single engine boat typically is more efficient than a twin engine boat is because they usually swing a much larger diameter prop...via maybe a 3 or 4 to one gear reduction instead of around two to one, because they run slow...below hull speed, and of course a true displacement hull is also more efficient a slow speeds.

It's really hard to duplicate those conditions in a twin engine boat like classic Hatts by shutting down an engine.

 

[SeaEric]

I have a customer with a 52 CPMY, 8v92's not sure which trans. Prev owner did the research and decided it was OK to freewheel. Well, NOT. The current owner now has warped clutch plates (tech says caused by towing or freewheeling) and is facing trans rebuild.

As previously posted, the fuel savings was admittedly minimal.

I think for some of you guys, the "economy" of doing this crap is more about sport than the actual need for $ savings. When you add the trans rebuild $$$$ to the mix, it's got to screw up the game.

 

[MikeP]

As noted, these trannys have an external engine driven oil pump. Without the engine running, there is no oil being pumped through the transmission. So rotation of internal parts, caused by a propellor that is not locked down is not a good thing. The nice thing about ball/roller bearings is that they can run quite a long time with minimal lubricant. But at some point, if the prop is not locked down, damage to the bearings will occur because there is no lubricant being supplied. Whether the damage is sufficient to cause catastrophic failure or just result in more than normal wear requiring sooner-than-normal replacement will naturally depend on how long the shaft was rotating and the speed.

But as per the Allison service manual, the shaft should be locked down if the boat is moving and the pertinent engine is not running.

Re saving fuel - I can't imagine that running one engine while the other has a locked-down prop shaft will save fuel due to the drag imposed. If you removed the prop, THEN there should be some saving. But as noted, it takes X HP to move a boat at (say) hull speed. I'm sure that one engine can produce X HP more efficiently than two engines can each produce 1/2 X due to frictional losses. However, when you factor in the drag of that non-turning prop, I'd bet that there is no gain at all and possible a loss.

 

[sgharford]

In terms of drag, common knowledge around sail boaters is a locked prop produces less drag then a freewheeling one. I don't understand the physics behind it.

 

[ron6785]

Do to running aground south of Charleston, I bent the starboard wheel and had vibration above 1200rpm so I feathered the starboard engine at about 900rpm and ran the port at about 1400 which gave me my 8.8 knts or theoretical hull speed and I got better mileage than with one engine locked or idling. Apparantly the 900rpm on starboard reduced the drag sufficiently and the Port did not have to load as much so I reduced fuel burn.

Don't have flow meters so can't be precise but went ablut 240 miles on about 200 gallons of Deisel. Go figure.

When I discuss increasing prop pitch with prop shops they have a fit. My 53my originally came with 4 blade 28 X 26's. Some where along the line she now has 27X 26's. Because I alway run the boat at hull speed or less and never more than 1700 rpms (they're naturals) I have toyed with increasing the prop pitch 1-2 degrees. Realize that it loads the engine at a different rate and have read how dangerous it is to the life of the engine. However my props are undersized by one inch so and increase in pitch should be somewhat offset by the smaller diameter prop??? Am I crazy or just stupid ???

 

[GJH]

I too find the whole prop thing rather arcane. All I know is my boat came with a spare set of 30x32 props, and it is specced for 30x29, which were on the shafts. I had the spares hammered down to 30x31 and put them on. We go < hull speed slow, so same theory. Next thing I know my usually pristine transom is covered in soot.. fuggedaboutit.

 

[SKYCHENEY]

I have a customer with a 52 CPMY, 8v92's not sure which trans. Prev owner did the research and decided it was OK to freewheel. Well, NOT. The current owner now has warped clutch plates (tech says caused by towing or freewheeling) and is facing trans rebuild.

As previously posted, the fuel savings was admittedly minimal.

I think for some of you guys, the "economy" of doing this crap is more about sport than the actual need for $ savings. When you add the trans rebuild $$$$ to the mix, it's got to screw up the game.

I believe the 52CMY had Twin Disc or ZF 3:1 trans and big Veem 5-blades. That trans is supposed to be able to free wheel, so the rebuild may be due to other issues.

 

[Roscoe]

http://cr4.globalspec.com/thread/15712/Calculating-propeller-drag

The definitive answer from the wonks is: "maybe yes, maybe no, depending."

The fact is that according to the engineers in the above link, flow tank modelling needs to be done for the specific application. The forum consensus is that everyone is right and everyone is wrong depending on the specifics of the boat.

 

[rsmith]

On my 53MY with 8/71 naturals when I lock down one engine and run at 7-8knots my rudder indicates 15-20 degrees, which of course means I'm crawfishing and I exp.ect any fuel saving under this situation would be minimal.

Since this old gal is never going to plane again, I'm wondering if larger rudders would help reduce the rudder angle with one engine running and I could get maybe 2mpg.

I ran into a guy with a 53 late 90's model Tollycraft who has 8/92's and allison gears that claim the Allisons on his boat can be freewheeled and does so. Says he has 2:1 gears. Is this true???? Mine of course are M20's with the pump on the engine and have to be locked down.

This guy claims to get 2 + mpg at 7.5knts.

Better watch out for VMC.

 

[Trojan]

If your running a diesel engine and never plan to plane again. I think it would be in your best interest to increase the diameter of your props. Then balancing the pitch on both props and run both engines. Being careful not to over load the motors at the lower RPM's. Free wheeling or locking the prop will still induce drag. The faster the hull speed the greater the fuel loss will be. Prop design will also increase or decrease the drag along with the pitch and number of blades. I think with the proper props you will be able to tailler your boat to run very efficient. All things considered. You can still clean out your engines you just won't be going as fast. The engines don't care what speed you run only the load. So maximizing your engines at the lower RPMs should decrease your fuel use at the lower speeds and increase your handling. All boats are optimized at max rpm's. The lower RPM's are not linear so optimizing your engines at the lower RPM's should increase your fuel savings.

BILL

 

[rsmith]

Do to running aground south of Charleston, I bent the starboard wheel and had vibration above 1200rpm so I feathered the starboard engine at about 900rpm and ran the port at about 1400 which gave me my 8.8 knts or theoretical hull speed and I got better mileage than with one engine locked or idling. Apparantly the 900rpm on starboard reduced the drag sufficiently and the Port did not have to load as much so I reduced fuel burn.

Don't have flow meters so can't be precise but went ablut 240 miles on about 200 gallons of Deisel. Go figure.

When I discuss increasing prop pitch with prop shops they have a fit. My 53my originally came with 4 blade 28 X 26's. Some where along the line she now has 27X 26's. Because I alway run the boat at hull speed or less and never more than 1700 rpms (they're naturals) I have toyed with increasing the prop pitch 1-2 degrees. Realize that it loads the engine at a different rate and have read how dangerous it is to the life of the engine. However my props are undersized by one inch so and increase in pitch should be somewhat offset by the smaller diameter prop??? Am I crazy or just stupid ???

I've been told by the prop guys that if you go down 1" in dia you should go up 2" in pitch to get the same results. Could be that they reduced the dia because of repairs and balancing. I run 28x 26 with 1.9/1 gears and make 2300 rpm wot with half fuel,and I'm running 525hp 12n's. I'm supprised you can turn full rpm with the 8n's.

 

[Trojan]

Most old shops cut the dia. down when they rework them. This is how they balance them. The new prop guys build up the wheel to the proper size. You should turn the biggest dia. wheel you can throw and adjust the pitch accordingly. That way you utilize your torque to its fullest and get the maximum amount of thrust. But you also can do anything you want.

BILL

 

[REBrueckner]

I have never seen any material, including these forums, that says increasing prop pitch alone improves fuel efficiency. The standard approach to better efficiency, higher NMPG, is to get a larger diameter and slightly smaller pitch prop to maintain original WOT, or to load your engine at lower RPM, but lose some top speed, keep the pitch about the same or a bit larger with the larger diameter.

The definitive thread in these forums is here from Brian Degulis:

http://www.samsmarine.com/forums/showthread.php?t=8373&highlight=higher+efficiency

and I think is a combination of larger diameter and pitch to load his engines at lower RPM. (I don't remember if he posted his original prop size.) Note that he would have preferred to use an even larger diameter prop. It's a long read but a good one.

Another possible approach is to have your props set up via computer measurements, like Propscan, or some other program of your choice. Apparently it can improve prop efficiency by a few percent up to maybe 5%....depends on the current conditions of your props of course. At 100 hours or so of annual running, you'll probably never get your money back in any reduced fuel costs, but at 1,000 annual hours, you might make out pretty quickly.

Or you could call Hatteras or Sam's and see if they have any experience or suggestions regarding prop efficiency.

Finaally, my Allison M20 manual, 1970's, specifically said to NOT free wheel with engine off.

 

[Trojan]

My new props are 2 inches in diameter bigger and 3 degrees less in pitch and I love them . The boat just jumps out of the water. Planes at a lower speed and is much better on fuel . I don't know how much better. I don't have any figures yet. Lack of use. But all indications seam to point to a MPG improvement. Only time will tell. I'm using the new German machined nibral wheels with the larger machined blades. I will never go back to the old Michigan wheels. These are the t--ts!!!!!!!!!!!!!!!!!

BILL