1.7 Litre Rooster Tail

[martynwhiteley]

30 hours on the clock now and there's no stopping me.

Did the 30 mile or so trip down the Humber to Spurn on Saturday, and gave it a bit of a blast on full throttle once back in calm waters neer Reeds Island.

It now revs to 4300 rpm, only 100 short of the limiter, so has loosend up a bit as one would expect. At full revs, with the leg fully down, getting about 22 knots, I looked back to admire the usual 'nice' wake, to find the addition of a small 'Rooster Tail'.

Now I am aware that I might need to alter some spacers on the Alpha drive when the boats next out, because if I trim out at all (from fully in), the boat loses speed and soon starts to cavitate.

But I did set my X dimension 20mm short (too high) than recommended, so the prop will site a little higher in the water.

Obviously I've got no complaints about the performance, but would one expect a little rooster from the Alpha on normal running, or could it mean the prop is a little high?

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[duncan]

it's surely the angle of the prop shaft (thrust) rather than it's height that's the issue

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[martynwhiteley]

I think your right Duncan, although since my leg was fully 'down', I couldn't lower the angle of the prop any more. Perhaps my transom angle is less than 13 degrees which I think is the datum that Mercruiser use for designing their drives. I never bothered to measure it, since I couldn't have changed it.

However, as I mentioned, there is apparently a way of allowing to leg to be lowered further, by removing (or rotating) some spacers on the end of the rams. Since the boat needs to be out of the water to do this, I'll wait until then.

If other Alpha drive users also see a small rooster tail on full throttle, then perhaps the depth/angle of my prop is just fine.

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[duncan]

if it's the same as the BIII there is a spacer that can be placed infront or behind the pin that the rams are located to. You want the spacer on the transom side of the pin.
No real reason why you need the boat out of the water to swap it over - but if you don't have nice warm clear water 3ft deep over golden sand (without waves) it' sprobably worth waiting! However if you put the drive fully up you should be able to tell which way round the spacer is currently

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[Russell]

There was a Mercruiser service bulletin about the spacers on Bravo drives:-
<A target="_blank" HREF=http://www.lowerunits.com/bulletin/oemcruiser/bullet/94/om941r.pdf>http://www.lowerunits.com/bulletin/oemcruiser/bullet/94/om941r.pdf</A>
It sounds most entertaining !

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[jon_bailey]

Not quite sure what you mean by a rooster tail, but if you are comparing your Alpha to one of those PWCs with water jets, then I would not describe my Alpha drive as producing a rooster tail at any throttle setting.

Also suggest that if rooster tail is being produced then this is inefficient as all of the thrust should be under water, not above it producing 'go faster' affects. Appropriately adjusted you may even be able to get a bit more top end speed from your 1.7.


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[duncan]

yep - if you open the throttle wide on mine but don't trim out a bit as the speed goes through 25 knots it doesn't like it!
However it does like to be fully trimed in to lift the stern a bit to start off so fitting a spacer to limit the trim in would be rather defeating the object.
I presume it is a function of the transom angles on some boats - the B drives being designed for one and the being used for another!

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