Suzuki 175 - need new prop

[K&S]

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

[K&S]

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

[Nos4r2]

What revs are you reaching at WOT?

[Pete7]

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

[TNDB]

Originally Posted by K&S

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......

[K&S]

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.

[Aidan]

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

[K&S]

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.

[K&S]

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

[Poly]

Originally Posted by K&S

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.

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.

[TNDB]

You have written that you hit 6100rpm "whith travel left in the throtle speed 34knts"
Then you said:"6100rpm is Wot"

So a bit contradictory but presuming that the current prop 15.25 X19 will over rev the engine I would try that 14.75 X 23.

Because your engine specifically produces its extra 25hp (over the DF150) from 5500 to 6100rpm IMO you should really endeavour to get the WOT to max out right around the 6000 to 6100rpm - If you only hit 5500rpm then you might as well have bought the DF150 !!

[Pete7]

Originally Posted by K&S

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.

That will make a difference, my opti is 1:1.87. So you will have a big slow turning prop. Go for the 23, you can always have it adjusted by an inch to fine tune it.

Pete

[K&S]

Many thanks for taking the time to post your reply Polwart.

The reason the engine was supplied with the 17 inch pitch i was told was because that was the only one available at the time of collecting the boat and to be fair to the supplier it arrived at the factory an hour after we did.

I think your calculation matchs my gut feeling, so thanks

Tndb - It does sond like a contradiction, i will try and explain a bit better :
WOT is 6100 (as prescribed in owners manual). When the boat reachs 6100 the throtle is at approx 3/4 of its travel. The engines 'brain' prevents any more rpm being produced, although the engine doesnt sond / feel like it is working hard to get to these revs. That and the rather pathetic top speed lead me to think i need the higher pitch. Does that help or did i confuse the matter further lol

I dug out a copy of Rib international and looked at a boat test they did back in Dec / Jan issue.
The boats were solent 6.5m ribs one had a verado 175 and the other had a suzuki 175

the suzuki recorded 47.95 knot

I know its not the same boat as mine but does suggest that there is a lot of speed lacing in mine.




So with all this in mind and all your help i am off to wrap the old prop for sending back to exchange for the 23. will hold off until tomorrow just incase someone comes along with more info.


Many thanks to you all for your help on a question that must have been asked a thousand times.
Kim

[K&S]

Thanks Pete 7 - good idea Anyone you would recomend to make the alteration if needed?

[TNDB]

Thanks for you explanation re the 6100 and half throttle -I understand now . Basically looks like you are severely under propped - , my Df150 will push my 6.5m to 5900rpm with the 14.75X23 so your extra 25hp should certainly manage an extra 200rpm (to your max revs) one would think ?

You may even end up with a 14.5 X25, but the 14.75 X 23 is certainly worth trying since as I said before you really want to use the full 6100rpm rev band as that is what you have paid for (DF175 vs DF150)

[K&S]

Yep all makes sense TNDB and as Pete 7 suggested I could always have it altered to 24 pitch.

[Pete7]

Originally Posted by K&S

Thanks Pete 7 - good idea Anyone you would recomend to make the alteration if needed?

Yep, Geoff at Streamlined, over the years he has helped quite a few folk out on Ribnet so worth pointing business back in his direction.

http://www.streamlinedpropellers.co.uk/

Pete

[K&S]

Hi, was out this weekend with new prop (23 pitch)
managed to get 6000 / 6100 rpm and speed of approx 41knts
a much better set up with this propfuel figs later in the season
many thanks for everyone's help
K&S

[TNDB]

Originally Posted by K&S

Hi, was out this weekend with new prop (23 pitch)
managed to get 6000 / 6100 rpm and speed of approx 41knts
a much better set up with this propfuel figs later in the season
many thanks for everyone's help
K&S

Excellent - sounds spot on