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Old 21 October 2007, 17:33   #11
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Quote:
Originally Posted by Andy Moore View Post
Two engines look more sexual.
A pair of Mariner 40's?! Does the thought of Ann Widdecombe get you into a bit of a lather also?
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Old 21 October 2007, 17:34   #12
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Originally Posted by Downhilldai View Post
Yes, you're quite right there. If one engine breaks down, you'll have an available source of spare parts on the other one
Or use it as an anchor!
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Old 21 October 2007, 17:35   #13
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one engine up, and throttled from standstill, with 2 adults and 2 small children.
25mph sounds really high. How were you measuring your speed.

Lots of low power twin set ups would struggle to get on the plane on just one engine.
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Old 21 October 2007, 17:38   #14
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25mph sounds really high. How were you measuring your speed.

Lots of low power twin set ups would struggle to get on the plane on just one engine.
Are you sure you don't have one 40 and a 4hp?
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Old 21 October 2007, 17:39   #15
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With a GPS, it was calm though and most of the fuel was proberley gone.
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Old 22 October 2007, 06:49   #16
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Sounds like you have relatively fine pitched props on there.

One vs two if the revs went up whilst running both, you could probably load the engines a bot more, up your props' pitch & get some extra speed, however you might be gubbed should one break down, as it would be way over propped for one engine alone.

I reckon if you go twin for safety, there's no point in doing so if you can't get on the plane. Like all things, you pays your money, you makes your choice.
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Old 24 October 2007, 18:21   #17
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If you would like to obtain the best acceleration & final speed in your rib, you must perform the WOT (Wide Open Throttle) Test. you must install one tachometer for each engine. There are some induction hour meter/ tachometers available in the market for free battery operated engines ( Rope Start) and electrical tachometers for battery operated engines.

With one person and 2 gallons of gasoline per engine, make a full throttle acceleration run , see how many revs per engine is shown in each tachometer. According to the Mariner Manual your 40 HP long tail assuming they are 2 strokes, must work proper between 5,200 and 5,800 rpm when in full throttle.

The ideal is to have both engine working at 5,800 rpm at Wot. Get same diameter prop delivered with the engine, but with different prop pitch size and perform WOT rpm tests. runs. For any pitch size there are 200 more revs, for any + pitch size there are 200 less revs. Have not specified if its a 2 or 4 stroke engine.

You can ask your engine dealer what pitch alternatives are there to your present diameter 40 HP prop. Try to match the picked prop to match 5,800 rpm or as close to this lecture in full Wot per engine. Never pass the maximum rpm specified or you will be over reving your engine.

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Old 24 October 2007, 20:30   #18
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Are you sure you don't have one 40 and a 4hp?
My friend has a 5.5 rib with 2 mariner 40's top speed 29 knts and can't get on the plain with one engine.
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Old 25 October 2007, 06:27   #19
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But I've driven a SR5.4 with 5 bods aboard on a single 30 - got on the plane no problem.... (eventually - but most of the wait was down to loosing the water ballast!).

Like I say, you need to decide why you want twins, and if you want to get back on the plane with one dead or not. Bottom line is that to get home on one, you'll need to drop your prop pitch, killing your top speed....
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Old 25 October 2007, 15:05   #20
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My friend has a 5.5 rib with 2 mariner 40's top speed 29 knts and can't get on the plain with one engine.
You are supposed to use them in the water that's why!!!
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