Talking of Props........Part 2

[JORIDAPILOT]

This is a question that I ask the outboard boys every year at the Southampton Boat Show but have never yet had an answer that is logical or sensible.

So who amongst our knowledgeable membership can provide an answer........

We all know that stern drives were developed from outboard motors, that is, the bottom half of an outboard was mated to an internal more powerful car/bus/truck engine mounted inside the hull and hey presto

Stern drives have gone on to be developed and become an engineering art in their own right, we now have twin contrarotating propellors in the likes of Mercruiser's Bravo 3, Volvo's Duoprop, Yanmar's ZT350 and the rest. All of the new Pod Drive systems like Volvo's IPS, Mercruiser's Zeus and other ZF Marine pods all have twin props, (except the Queen Mary 2 but thats different).

So............ why haven't any of the outboard manufactures developed a contrarotating prop in their range, it can't be that they can't handle the power, it can't be a weight issue as the extra over on a 350 Yamaha would be peanuts so what is the answer????????

A Honda man told me last year that they did develop one some years ago but it never took off.... well he should have put on a plane and not a boat..

[Capn Jack]

Now that's an iteresting question!

[JORIDAPILOT]

Now that's an iteresting question!

And the answer is

[Ed]

Perhaps...

Outboards its easy to have two engines, in a space where an inboard would have one... So you just have counter rotating pair.

Makes sense to me?

[JORIDAPILOT]

Perhaps...

Outboards its easy to have two engines, in a space where an inboard would have one... So you just have counter rotating pair.

Makes sense to me?

Good point but there are still a lot of boats with one big outboard, you still have the prop walk problem..... there must be an underlying reason why the might of Honda, Yamaha, Suzuki, Mercury, Evinrude, Tohatsu etc etc haven't done it.

[Gazjen]

Only thing I can think of is that outboards are generally on lighter boats and the duo prop was designed to get heavy boats out the water quicker and the same point as above if they are on a heavier boat there is usually several outboards strapped to the back

[Ed]

haha that pic is so mad Gazjen.

I found one recently of a similar setup with some Old 1960's mercury Inline 6 engines.

That little lot would have burnt upwards of 90gallons of fuel an hour, with a total of around 800hp..

Shows how things have moved on. Can get that with only a couple of engines these days!

[jokaboat]

OK looking at those pics why are the outer outboards mounted higher than the inboard ones you would think that once the boat has lifted onto the plane the outer ones would ventilate?

Of are they long rather than short shaft, in which case just why do it like that?

[Ed]

No, idea. And I wondered that too. I'll see if I can find out.

[markanddawn]

Not sure on the 60's picture but the other picture shows a step in the transom/engine well, which would account for them being mounted higher, perhaps.

Or maybe it is just to get grip at different depths vs. angle of plane???

[Centaur]

I think that an outboard should be mounted with the cavitation plate somewhere about inline with bottom of the hull. As the hull is a 'V' shape the outermost motors would therefore need to be mounted relatively higher to maintain the same depth below the hull. Those pictures look a bit extreme though - I should think the outermost motors would over-rev in a tight turn!

There must be a law of diminishing returns when running so many outboards. An old rule of thumb suggested that twin outboards would be 10% less efficient than a single outboard with the same nominal power. eg Twin 150hp motors would be about the equivalent of one 270hp motor. On that basis, probably two of the outboards in the picture above are effectively wasted!

[salamis]

They arn't waisted, because it is a drugs running boat, the main reason is that the Coastguard or Navy have to shoot out more engines to be able to catch and stop the boat. The British manufacture of this boat was prosocuted as development cost were paid by drugs cartels, and the moulds conferscated and destroyed.

[JORIDAPILOT]

Now that's an iteresting question!

We have drifted away from the original question, but if I know Cap'n Jack he has been reseaching the issue for a few days and he will ultimately come up with a viable reason. Am I right Cap'n..

[salamis]

Meanwhile what little I have seen is that Yamaha sold their F150 Vmax with a duo prop for a number of years but low sales numbers made them stop making it. I understand that the physical increase in gear case size combined with extra weight made no real gain from the single prop, however I read on the web that the new 350 Yamaha was going to be a duo prop outboard but the idea was dropped in preference to the single.
In this day and age you would think the Japs could get a unit made small and reliable enough to make it viable. Very good question though Jorida wonder why more people haven't asked it?

[Capn Jack]

Capn Jack he has been reseaching the issue for a few days

No!.....I've been at work looking in occasionally to delete our undesirable's...

However, I don't think sterndrives are derivatives of outboards, although I'll always listen to reason and absolutely stand corrected. I remember my Grandfather and my Dad talking about engines and drives and I don't remember there being a link. As I understand it, outboards were originally designed for craft which were powered by oars, the sterndrive evolved through a necessity to reduce space within cruisers, that is, to put the engines in the stern to give more space within the boat itself.

Outboard engines historically, have been built for fast craft revving to 5,800rpm, whereas sterndrives are more.....how can I put it, well they have more grunt/muscle pushing tonnage at speed but not at the speed of outboards.