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Old 19-04-2015, 12:48   #1
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Slow speed, Perkins aquastar....

Hi everyone, I'm new here.

I have a 33' Aquastar, with twin Perkins 6354 non turbo power units, running through aquadrives.

The boat is just a tad over 9 tonnes, 33 foot on deck, around 30' at the waterline, 3' draft, and 10' beam.

The Perkins are 120hp each, recently serviced, no excessive smoke and sound great.

My problem is that even at around 2000 rpm I am not getting much more than 8 knots, which I think is pants for the amount of hp.

The trim tabs are set at horizontal, and do not work at the moment, but I still think i should be getting more thrust to play with. The hull is fair, anti-fouled and I have recently had new replacement props put on her, though I am not sure right now what diameter they are.

I've done a prop calculation and am getting around 24-27 inches (24' ish pitch) for my hp, semi displacement hull to achieve 15 knots. I cannot believe the props are that large, but will check asap.

In the mean time, does anyone have any ideas?

Many thanks, your help is appreciated.

Bob.
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Old 19-04-2015, 17:38   #2
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Re: Slow speed, Perkins aquastar....

G'day,
More info required. 1. How old motors/boat. 2. Gearbox reduction. 3. High idle RPM. Max free running RPM. Hull & prop condition.
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Old 20-04-2015, 00:01   #3
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Re: Slow speed, Perkins aquastar....

Hi Doug,i hope this helps,built 1983,gearbox reduction,not sure,clean hull,anti fouled last june,new props,as regards rpm,throttle open full at 2000rpm,sadly i am not too mechanically minded,i hope this is off some help mate,regards bob
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Old 20-04-2015, 04:29   #4
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Re: Slow speed, Perkins aquastar....

Hi Bob,
The info is needed to give educated advice. Perkins like other makers of the era played with marine HP ratings your motor would be a Series 3 or Dot 3 Engine SHP is closer to 100HP at 2600rpm. The 2000 RPM posted is that full throttle IN GEAR?
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Old 20-04-2015, 04:42   #5
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Re: Slow speed, Perkins aquastar....

Greetings and welcome aboard the CF, Bob.
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Old 20-04-2015, 09:26   #6
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Re: Slow speed, Perkins aquastar....

Hi all,have just been onboard the engines are perkins t6.3543 150hp,and on no load port revs at 1900 and starboard at 2000,hope this helps guys
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Old 20-04-2015, 12:16   #7
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Re: Slow speed, Perkins aquastar....

Hi, I have had a number of motor boats and I think yours would compare with my Transpacific 32, in which was installed a Ford 180 hp.

I agree that the ship HP would be more in the region of 100 each so not so far different to the T/pacific.
Subject to it being a displacement vessel the theoretical hull speed would be: Square route of length water line multiplied by 1.5 which would be 8.21 knots. However I would be happier if this speed was achieved at around an engine speed of 1700 rpm. Gearboxes and prop may have a bearing on this. Please also study the fuel usage as anything above 8 knots diminish the economy of the vessel. Example: My Nordic Tug 37. Eight knots = 13 litres per hour, sixteen knots 70 litres per hour.
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Old 21-04-2015, 14:31   #8
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Re: Slow speed, Perkins aquastar....

The formula is actually the square of the waterline times 1.34.
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Old 21-04-2015, 14:36   #9
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Re: Slow speed, Perkins aquastar....

This is the square and square root police. You have been found in error.
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Old 21-04-2015, 14:54   #10
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Re: Slow speed, Perkins aquastar....

You are pretty much at hull speed. Any more power is a waste of time and money.
It's been almost a year, you don't say where you are but your bottom could be loaded with stuff by now.... as could your props...
One engine should get you to hull speed easily.
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Old 22-04-2015, 14:50   #11
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Re: Slow speed, Perkins aquastar....

You need to know that the tachs are accurate, also you WOT rpm in gear in the normal way your boat is loaded and hull condition. If you can not reach that rpm you are over propped, overloading your engine. The WOT rpm will change with how the boat is loaded, how fouled the hull and running gear are or tune of engine. I always check mine with full tanks and fresh bottom paint if it's correct then I'am sure that as I add people the total load will remain close because all the tanks will not be full at the same time. Now during the 2nd year I'll loose some rpm and speed at WOT because of growth on the hull, freshwater no diver cleaning it. Once you get propped correctly your speed at any rpm is what it is, and your engine if given proper care should last many years. The only Perkins 6.354t that I've worked on were governed at 2700 rpm, the last 4 numbers on the tag on the pump are the governor hi-idle setting, in this case I want to reach at least 2600 rpm at WOT in gear measured with a known accurate tach.


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Old 23-04-2015, 04:36   #12
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Re: Slow speed, Perkins aquastar....

Thanks for all the information guys,it really helps :-)
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Old 23-04-2015, 05:23   #13
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Re: Slow speed, Perkins aquastar....

I found a thread regarding this model where a couple of owner's report speeds in the mid teens. That means that this is not a displacement hull & you should be able to exceed hull speed.


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