Contra-rotating surface drives

[Martin Tilbrook]

Hi

Does anyone have any thoughts on using a pair of contra-rotating surface propellers, in a similar way to the conventional duoprop systems? Would it have the same advantages as when used with submerged props? Or don't surface props work in the same way? Interested to hear your thoughts.

Cheers

Martin

[jehardiman]

Apples and oranges, there is no advantage for SP-CR because of the cavitation bubble. But side-by-side opposite hand rotation for SP props to balance torque effects would work.

[OutRider]

The Mercury BlackHawk drive has contra rotating props and has such a high x- dimension that it is really a surface drive.

[Verytricky]

It does not look like they are surface piercing drives.

These are the props, which are definately not surface piercing drives, and the leg looks like a standard short shaft duoprop drive.

[marshmat]

I wouldn't label the Blackhawk a surface piercer in the same sense as an Arneson is, although I think it does sometimes get a bit of air in there. Rather, it's jacked up so high to minimize the amount of drive actually in the water, for less drag. Or so Merc's literature makes it seem.

[Frosty]

There are different stages of surface props -- There are semis cavitation props and full cavitation props. In other words the more the prop becomes airiated the blades change in design.

marshmats emblem for instance is a full cavitation prop probably Rolla.

The drive shown in the picture is unusuall in so much as the cavitation plate is at an angle and comes down to the centre line of the props dive shaft This will be to assist reversing, and is therfore intended for surface use only.

Personally I have never seen a duo surface design and cant see the benefit of it.

I believe that these drives can be modified to a duo prop type. I am wondering if some one has done that to this one as an experiment.

However I do like the drive and have not seen that before, it just goes to show that the Arneson theory of having the prop on a long shaft far out from the boat is not necessary. When this drive is fitted I would guess that the prop would be not much more that a couple of feet behind the transom.

[marshmat]

Good guess Jack... my avatar's actually a Mercury full-cavitation cleaver designed for the dry-sump Six drive.
As for counterrotation on a surface piercer, I don't think there's much point. It'd be really hard to get it to work, the 2nd prop would be in a heavily aerated and very unstable stream. And you hardly ever see a single surface drive, they're usually seen in pairs or more, so counterrotation on separate shafts is a lot more practical.

[OutRider]

The BlackHawk is definitely not like an Arneson. However, it is normaly mounted so that the shaft centerline is at or above the surface. As a result, the props are at about the same height as on an Arneson and pierce the surface in a similar way. This is also true for a Six drive if the boat is fast enough. The angled plate on the BlackHawk is similar to several fixed shaft surfacing drives from Europe, except that those drives are usually mounted farther forward in a cutout in the hull. The advantage of putting both props on the same shaft is that, theoretically, the water is pushed straight back which is more efficient.

[Frosty]

Outrider I dont know what you mean by European drives being in a cut out. I think you are refering to the power tunnel,-- as it is sometimes known.

Surface drives can not be used in a tunnel . Surprising though it may be surface props need a lot of air and will cavitate if starved of it. Close swim platforms etc are a proplem to the unweary.

Some designers would say that a fixed system is all thats required. Mounting the prop at the right place, again is all thats required. making the hight adjustable is covering up a wrong prop choice.

Adjustable hight after all said and done is nice for playing with and getting a nice rooster tail up as you pass the club. But the mechanics and hydraulics used for this is not for criusers who live in the sea.

Theres horses for courses here.

[longliner45]

its my understanding that duoprops are only effective untill they reach a speed of 40 knots,,,then they fall on their face,,contact volvo for the facts though,dont even know about wave piercing props ,,,,longliner

[Verytricky]

Originally Posted by longliner45

its my understanding that duoprops are only effective untill they reach a speed of 40 knots,,,then they fall on their face,,contact volvo for the facts....

Ahem... I have just set the UIM Class V24 world record with a standard Volvo engine driving a standard Volvo DPX drive with E2 props( worked ) at 80mph, ( 70 knots ) average speed two ways between Bournemouth and Boscombe piers ( ie in the sea, not a lake! )

It is my understanding that given more power the Duoprops would give more speed. I know as fact I was hitting the engine limit, not the drive limit. Duoprop allows me to be very stable in the sea. When you get air, you back off slightly to bring the nose down, and as you hit the water again you power on to prevent a harsh bump. With any other drive I would have tourque effect on the boat, with the DPX drive I can be as silly as I want and the boat is stable.

I am investigating a 650Hp engine for a new race series in the UK, and I will definately have the DPX drive. Our initial calculations show a speed of over 100mph (87 knots ). We do not see the drives only being effective to 40 knots (46mph)!!

Pictures show the boat being 'driven silly' which I dobt you can do for any length of time on a non-duoprop setup.

[FAST FRED]

Sure does solve the problem of thin water cruising the Bahamas!



FAST FRED

[Verytricky]

It is a little tiresome, and you have to concentrate a lot, but get the balance right, and you hardly ever touch the water with the hull...

( can I call the DPX drive a surface piercing drive now? )

[longliner45]

perhaps I must stand corrected, but we had 31 ft JC hull fishing boats in the gulf of mexico,,semi displacement semi planing hull ,,we could do 20 knots with a 6000lb payload,,the manufacturer ,,volvo,,told us that they didnt do well at or about 40 knots,,they said that ( that would be that),,this was in the 80s and early 90s,perhaps it was for our application,I stand corrected,,,,,I guess its still a boat ,,as long as the prop gets wet,very nice boat ,,longliner

[Verytricky]

Regarding the Volvo information, their own information limits the speed of the E2 props to 50knots, whilst they state their E5 props can reach 80 knots. It supprises me that they can make such statements without knowing the boat attached to the drive. We are running E2's at 70 knots, and about 13 other raceboats are running E2's at 68 knots!