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Old 09 November 2011, 15:06   #21
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sorry, its the sensors on the engine that are not connected, not the guages, got a used tacho to try to connect up so hopefully should take some if the guess work out if it works!
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Old 09 November 2011, 15:17   #22
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sorry, its the sensors on the engine that are not connected, not the guages, got a used tacho to try to connect up so hopefully should take some if the guess work out if it works!
Looking at the age of the engine I'd be surprised if the sensors do anything other than sound and alarm and (perhaps) limit the revs to about 2000 rpm.

At 2000 rpm, an 11.75" prop on a 2.27:1 gearbox (thats an assumption) will at best give you about 8.5 knots.
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Old 09 November 2011, 15:36   #23
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read another post on here about another DT55 that revved till 4,000rom due to an electrical gremlin, would 4,000rpm and 11.75 pitch = 20 knots?

Struggling with the tacho! see other post!
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Old 09 November 2011, 15:39   #24
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I'm not into props and the effects they have to be honest. Just guessing at its rpm based on what I see.

I've a guide which I got from somewhere on props and how to determine what you need but ive never tried its formula out to see if it works.

Here it is

PROP SPEEDS

Take max revs and devise by gear ratio, multiply by pitch of prop then devise by 1056

Deduct 10% for slip

That gives theoretical top speed (MPH) before other factors like load, conditions etc are accounted for

Simples

Peter ~ Boatsandoutboards4sale ~ askboatsandoutboards4sale@sky.com ~ 07930 421007
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Old 09 November 2011, 15:47   #25
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Looking at the age of the engine I'd be surprised if the sensors do anything other than sound and alarm and (perhaps) limit the revs to about 2000 rpm.

At 2000 rpm, an 11.75" prop on a 2.27:1 gearbox (thats an assumption) will at best give you about 8.5 knots.
Soozook guardian system limits the motor to 3000rpm

DT55 l/u ratio is 1.92:1

I reckon the data stamped on the prop suggests the pitch to be 12". This is low for this motor on a 4.8m RIB.

Replacing your data with these figures, would adjust your calculation to a theoretical speed of around 15.4 kts

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Old 09 November 2011, 18:14   #26
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Speed

Yes, you do need to connect up a tacho & run the boat on water to find out what are max the revs you can achieve with the said prop?

You also need to know what max revs should be achieved with a standard prop by searching on the web or a Suzuki manual? (I'm still guessing around the 5k mark?)

When you increase the pitch of a prop the revs drop by a few hundred!! Not too sure exactly how many tho?

Of course, if you want just top speed with a lightly laden boat, go for a big pitch! Just remember you may not achieve max revs & can cause some damage!!!

Have fun! John
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Old 09 November 2011, 18:31   #27
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Soozook guardian system limits the motor to 3000rpm
3,000 rpm sounds about right, I am now away from home till the weekend. If I can get the rev counter to work should know where I am.

On the current prop (11.75" pitch) if we assume it is easy to drive would that mean that if the engine were working as it should that should be over revving?
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Old 10 November 2011, 04:32   #28
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When you increase the pitch of a prop the revs drop by a few hundred!! Not too sure exactly how many tho?
roughly speaking, when comparing like for like props 1" of pitch = 200 -250rpm
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Old 10 November 2011, 16:27   #29
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Not sure if you are reading the size right? The 11.75 may be the diameter of the prop? The pitch will be the second figure i.e 11.75 x 12, 13, 15,17 etc?
People, please correct me if I am wrong? Ta-John
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Old 10 November 2011, 16:31   #30
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I'm not into props and the effects they have to be honest.
I quite like the effect my prop has on my boat
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