# dual prop shaft ?

Discussion in 'Propulsion' started by yipster, Oct 30, 2002.

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### Mike DSenior Member

Do I sense a note of frustration, Frankie? You beat me to the punch.

Yipster, it always comes down to cost versus performance. That is why you don’t hear much about contrarotating propellers because they are higher performance, higher cost, higher maintenance with reduced reliability. Below you’ll see a chart of propeller efficiency for various systems and although it is graduated at the top for ships it applies equally well for boats and small craft.

The vertical axis is efficiency and the horizontal axis is the propeller loading coefficient Bp which is Prop RPM x Power^0.5/Speed^2.5. Power is the power in horsepower delivered to the propeller and speed is a little less than boat speed, reduce it by 1 knot to keep things simple.

So if we took a boat running at 15 knots, 1800 rpm and 100 hp Bp is 24.5, or 25 knots, 1500 rpm and 200 hp is 7.5, making a twin screw instead of single screw multiply Bp by 0.7 assuming no change in speed, revs and total hp.

In this range you can see the typical recreation boat propeller – Gawn. It is less efficient than the B series but cheaper. However, if you check the luxury yachts and commercial vessels the B series is very popular or propellers having the same general characteristics.

The best are the contraprops, about 5.5% better than a Gawn but a B type is about 3.5% better. I have read many times about gains of 15 or 16% for contraprops on ships but I take a pinch of salt.

You must be careful when talking about percentage improvements. Use the cube law when comparing power and speed, in other words power is a function of speed ^3 and expressing things is percentages is the best way. It is accurate enough to use one third of the power increase for the speed provided the power increase is less than about 12% or 15%, beyond that use a calculator So Gawn to contra would give only 1.8% more speed.

If you really want improved propeller performance you should investigate the “manually operated” controlled pitch propellers. When the boat is moving slowly for prolonged periods you need something with a fine pitch whereas you need coarse pitch for speed. I have no idea about their cost but it makes as much sense as contraprops.

There is a cross-river ferry service where I live and the single screw wheel effect was awful when it was being manoeuvred. Until another skipper came on the scene, he did everything wrong and the docking was smooth! If you fight the wheel effect you’ll lose, every time, work with Mother Nature’s laws and she’ll smile at you.

Michael

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2. ### GuestGuest

Couter Rotation

Counter Rotation inboard drives would probably handle better than a single rotation propeller that is countering the torque of another single rotation propeller.

The boats that use the Volvo Duo Prop or the Bravo III Drives tend to be more docile in the slow speed manners than a non duo propeller installation, but exactly how much of your boating time are you going to be spending idling or very slow speed cruise?

There has to be more parasitic loss of power to turn an I/O with counter rotating propellers than one with non-counter rotating propellers, so I do not believe they will automatically gain a substantial amount of speed due to the power loss combined with the increased drag of the second propeller.

As for a rudder fixed shaft application, they have had very little success on any form of boat. The I/O’s are a better solution over the rudder concept, especially in slow speed maneuverability. When using the marine gear to counter rotated the propellers, you loose a tremendous amount of hp through the marine gear. It is far more efficient to have the drive units counter rotate, as they will not waste as much hp to counter rotate the propeller as the marine gear will.

A counter rotating Surface Drive was developed in the 80’s but there wasn’t sufficient market need to warrant production. With today’s market, possibly there is a need?

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### lockhughesElectricGuy

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### yipsterdesigner

electric guy lockhughes

and what a beautifull baby it is!
got specs or a URL? i noticed http://www.schottel.com also tends to go for CR instead of the nozzle, electric too!
still occasionally think of the above guest that said
how wise! is that ever done - can that work :?: :?:

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### lockhughesElectricGuy

Hi Yipster

both:
http://www.ecyclemarine.com/outboard.htm

BTW, I have Schottel at:
http://www.schottel.de/index_e.htm

Dot-com didn't work for me

`spect the CR's make more sense at higher (boat) speeds. So I was a little surprised to see eCycle go this route on their little electric outboard. There are a few e-outboards that use a kort.

Cheers

Lock
http://groups.yahoo.com/group/QCYCTender/

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### Birdman55New Member

pictures for counter rotating prop

Hay guys

I just wanted to say that I know just what your talking about. I am just 18 years old and I have seen one in person at a private marina that i used to work at. It is too late now for me to get som pictures of the props but by this weekend i will have some if yall are still interested. It is not my boat nor do I know the owner so don't ask any specifics. Just wanted to see if this is what you were talking about.

Get back with ya later.
Birdman55

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### WillallisonSenior Member

send 'em on in Birdman!....

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### yipsterdesigner

dito, show us what you mean and a pic can tell a thousend words they say! yipster

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### yipsterdesigner

going true the new honda outboards folder here i read: counter rotating engines are deliverable for the bf115 up to bf225 when using dual OB's. really!? or is that a gearbox?

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### Peter_TJunior Member

Two shafts in the same axial!! This is going to be expensive to machine.

For single screw with CP system, the shaft is hollow to allow hydraulic fluid to activate setting of the blade pitch. The makers made so many units in history, so there is no hardship, except they will bill you 20% to 25% more. The CP propellers also cost more.

Aquamaster produced a tandem counter rotating propeller style for Z-drive to compete with Z-drives with nozzles. That style did not get much market share.

The new propulsion series with POD only use pitch propellers. Since, CP system is a good combination and can be added to the system if the customer wanted. But Z-drives can rotate within a minute, so they drop the application. What matters is the cost advantage in marketing.

What tandem propellers offer can be gained by selecting better propellers with accelerated pitch like the patented work-wheel by Bird-Johnson. At least we are contended with the average propeller suppliers to offer skew propellers that can be as efficient as the tandem propeller system in a line shaft.

To save cost, perhaps look to the method used by attaching a counter-rotating hub "PBCF" www.motech.co.jp This is for application to ocean vessels, but can also applied to small craft, yachts. When fitted, the power loss in the wake stream behind the propeller can be recovered, so you gain a few percent back. This feature does not need power to drive. It will turn itself in a passive mode.

Some early invention is to reshape the rudder to gain a few percent, but this change is not as effective as the "PBCF". The powered counter rotating propellers work in the same principle.

One must be satisfied, that if you apply two or three upgrades in same area, the final gain is directly added up. You will certainly gain in combination to some extent.

It is interesting to see the torpedoes are fitted with two propellers. This may be to offer reduntancy. Note: their propellers are very simple shaped.

All the thruster companies are looking in adding new form of POD propulsion, this is the best so far. Of course this meant for the bigger vessels. I think there is a lot to improve on outboard in re-focusing the tiny propeller they supply to some reasonable sizes. Look at Volvo Penta system.

Taking two propellers, one at each end of the Z-drive shaft axle is a good idea, as adopted by "Schottel".

Peter

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### brian eilandSenior Member

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### yipsterdesigner

and that is me for one thanks Brian, and compliments on your site! fisherman did not list one but who knows, i'll reply!

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### JoelNew Member

yipster those images are going straight on my science project, which is due tomorrow....

You saved me!

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### IHTFPJunior Member

Counter Rotating Surface Drive

Mercruiser used to make a counter rotating surface drive. It was called the Blackhawk. Works very well, the only drawbacks are durability and availability of parts.

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### yipsterdesigner

yes thats rite, its a sterndrive however, not a propshaft driven duoprop

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