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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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Old 10 November 2011, 16:39   #31
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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
Might be wrong but close enough. am i right in think ing that a 12" prop should rev the nuts of the engine?
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Old 10 November 2011, 16:45   #32
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Might be wrong but close enough. am i right in think ing that a 12" prop should rev the nuts of the engine?
I would have thought so.
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Old 10 November 2011, 16:49   #33
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Cool that's what i had thought but as its all new to me i needed to get a few more opinions! iF i get the rev counter to work at the weekend all should become clear!
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Old 19 December 2011, 03:30   #34
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well took a few weeks to get a working rev counter fitted and decent weather to try it out. Took it out yeaterday and max revs are just under 5,000rpm and max speed on phone GPS was 23.5knots so a bit better than before but not as much as I had hoped for.
The water was lumpy and it was an entertaining ride! Played with the trim to get max speed but did not see much differrence in the lumpy water. Would the next step be change prop to get more rpm?

current prop is 11.75 x 12.

Also annoyingly the engine started to miss fire so I came back in and it conked out in the Harbour. Managed to start it at home but only by changing the plugs, the old middle plug was slightly wet. Now the tick over is about 3,000 rpm! Any ideas what would cause this? Stuck carb float?

Cheers TomKat
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Old 19 December 2011, 03:41   #35
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have you put a compression gauge on the engine before?

what results did you see?

a 4.8m boat with a 55hp you are ball park at 24 knots i'd suggest. maybe could see 26/27 with a prop change and if the compression test is ok.

i would also inspect the floats since a plug is wet and balance the carbs.

cheers
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Old 19 December 2011, 04:55   #36
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was sold to me as having been tested but will check it out myself, been a while since I tested compression do you just test it on the started with the kill switch out or do you run on 2 cylinders with the compression guage in the other cylinder?
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Old 19 December 2011, 08:12   #37
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ideally you want the engine warm so run it up on muffs if you can before doing this.

if however you can't do that then fair enough as the readings will be lower-but it is more important they match-ish.

just leave the killcord off will be enough as your only cranking it for 2/3 seconds.

what i do is take all the plugs out and leave the killcord off and note the PSI reading. if there is a large discrepency then you have found your problem, if readings are the same and a decent reading PSI wise then you probably could try a diff prop.

as a word of warning, NEVER buy an engine without doing a compression test and witness it being done-even better if it is a gauge you know/trust.

i don't know what PSI your motor should have, if you have a manual then it probably is in there but you don't want more than 5psi or so a difference between your cylinders.

cheers
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Old 19 December 2011, 08:35   #38
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cool, will try tonight if dry outside!
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Old 19 December 2011, 08:44   #39
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pic of the boat being 'tested'!
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Old 19 December 2011, 08:46   #40
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