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Old 12 February 2008, 12:55   #1
K&S
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Suzuki 175 - need new prop

Hi we have a dilema.
our engine is proped with a 3 blade S/s 15 1/4 x 19 and anything above 4000 rpm does not give any more speed than approx 22knt

the acceleration between 1000 and 4000 is impressive but it's the top end i need to sort out.
my dilema is should i go for

3 blade S/s 14 3/4 21
or
3 blade S/s 14 3/4 23

The supplier is willing to exchange my prop foc this time. but at the price ogf new props id rather try and get it right first time.
So any help, experience would be greatly recieved.
many thanks
Kim
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Old 12 February 2008, 13:01   #2
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Should probably also mention
It is the four stoke engine 2007
and boat will be used light (2 adults 3 kids)
Boat is an xs 650 (6.5m)

I know the what prop question gets asked alot but i really would be gratefull for your help.
Thanks in advance
Kim
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Old 12 February 2008, 19:03   #3
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What revs are you reaching at WOT?
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Old 13 February 2008, 05:28   #4
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Kim, 22 knots is dreadful, as Matt says what are the max revs you can achieve at the moment.

However there could be something else wrong with this, like massive ventilation causing the prop to slip. Before you start swopping props can you post a couple of photos of the transom so we can see the height of the cavitation plate in relation to the botom of the hull. I have a feeling the height of the engine may be at fault. Alternatively the rubber bush in the prop is shot and the prop is just spinning at high revs, bit like a clutch slipping in a car. Is the prop in good condition?

Pete
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Old 13 February 2008, 09:17   #5
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Quote:
Originally Posted by K&S View Post
Hi we have a dilema.
our engine is proped with a 3 blade S/s 15 1/4 x 19 and anything above 4000 rpm does not give any more speed than approx 22knt

the acceleration between 1000 and 4000 is impressive but it's the top end i need to sort out.
my dilema is should i go for

3 blade S/s 14 3/4 21
or
3 blade S/s 14 3/4 23

The supplier is willing to exchange my prop foc this time. but at the price ogf new props id rather try and get it right first time.
So any help, experience would be greatly recieved.
many thanks
Kim
I don't quite understand the 22knot top end - surely 4000rpm will give you faster than 22 knots ?
Back to your question, I have Rib X 6.5 metre with DF150. The Rib X is quite a light boat I estimate about 1000kg with engine and couple of passengers. After trying a few combinations I ended up with the 14.75 X23 which allows the rig to rev to 5900rpm and attain ~50mph

With the DF175 achieving its extra 25hp after 5500rpm via the variable cam mechanism you will maybe want a "longer geared" prop than the 14.75 x23 ?

The only way to be sure is to try it since it depends so much on your hull and the way the engine is hung onto it -the 14.74X23 may work perfectly if you hull/engine combo "drags" slightly more then mine OR it may rev over the 6100rpm limit if your set up has less drag !

I would firstly establish accurately what revs the current prop will hit flat out then ring Steel Developments who will give you the best advice......
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Old 13 February 2008, 13:34   #6
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Many thanks for the replys so far.

We had the boat out again today and tried it up at WOT it hits 6100 whith travel left in the throtle speed 34knts

6100rpm is Wot. The rev limiter kicked in if i tried pushing the throtle any harder ( good to know this feature actually works )

As for ventilation I know what that feels / sounds like and can safely rule that out, the engine set up looks right.

I assume that that only leaves pitch.
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Old 13 February 2008, 14:00   #7
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Hello K&S.
Am I missing something here?
This is a new boat and if 22kts was your top speed from 175 hp I would be going back to where you bought it in AUG 07 and get them to sort it out.
Suzuki will tell you what the Top speed at whatever revs one can expect from that hull and weight.
Go back to where yo bought it and ask???

Good luck with it, would love to know fuel consumption when you sort it out.
Aidan
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Old 13 February 2008, 14:07   #8
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Hi, Aidan.
Supplier is recomending 21 pitch but my reason for thinking it might be 23 inch is because they initially recomended 17 or 19. Not sure if they are forgetting the 1:2.5 ration on the gear box.
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Old 13 February 2008, 14:11   #9
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Will Def post fuel figures etc when we finally get it all set up, as when looking into this motor we found it difficult to find an info.

What i can tell you so far is that it is incredibly quiet, if there is any wind or conversation while at idle i have to look at the rev counter to see if the engine is still running - it seems even quieter than our previous boats engine (Mercury 60hp fs)
Will get back with figures as soon as i can
thanks,
Kim
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Old 13 February 2008, 18:08   #10
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Quote:
Originally Posted by K&S View Post
We had the boat out again today and tried it up at WOT it hits 6100 whith travel left in the throtle speed 34knts
that sounds a bit healthier. and is about 11% slip.

Quote:
6100rpm is Wot. The rev limiter kicked in if i tried pushing the throtle any harder ( good to know this feature actually works )
so you know the engine can "give" a bit more "oomph"

A quick back of the envelope calculation suggested that with the same slip and still getting 6100 rpm your best possible speed for a 21" prop would be 37.5 knots (is that enough of an increase for you?) and with a 23" would be 41 knots. Those assume the engine has enough power to deliver full revs.

Now I think even if the 23" prop is too big, and the engine manages only 5600 rpm you should still get similar speed to the 21". All very back of the envelope calculations which completely ignore the boat and engine in the calculation - and assume "anything is possible".

Of course what you gain in top end speed you may lose in acceleration/time to plane.

Not sure why someone would suggest a 17" prop for this - unless you were driving a very heavy boat (lots of divers, fuel etc); you would be doing well to get much over 30 knots with that.
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