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Old 11 June 2010, 05:42   #1
Member
 
Country: Italy
Town: genova
Boat name: vfi 555
Make: hydrosport
Length: 5m +
Engine: outboard 4 st 60 hp
Join Date: Jun 2010
Posts: 2
Need advice

Hi everybody,
I' m new in the forum and looking for some help.
I have a 5,5 m rib with a VFI hull (ventilated flow interruption, so i was told) with a mercury 4 stroke 60 hp big foot, 17 PT propeller.
The thing is, the motor never run above 4900 rpm max throttle, and the motor can go on 5500 rpm to 6000 rpm.
Why does the motor donŽt go on top rpm? Should i change for a 14PT or 19PT propeller?
By the way the boat GPS top speed with one person in a lake is 28 Knots? I think it should go at least 30 Knots?

I would realy apreciate some help because me and my friends are sailing boat people, without experience in ribs, motors or propellers

THANK S
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Old 11 June 2010, 08:20   #2
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Country: UK - Scotland
Boat name: Wildheart
Make: Humber/Delta Seasafe
Length: 5m +
Engine: Merc 60 Clamshell
MMSI: 235068449
Join Date: Apr 2007
Posts: 4,611
Firstly, welcome to ribnet.

Sounds like you could go down in pitch. the usual rule of thumb is 1" of pitch = 200 rpm. So, based on you aiming at 5500, I'd see if you could borrow a 14" and see what happens.

For what it's worth, my rib is a similar size & HP and I'm running a 14".


I'm now going to go & Google VFI.......
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Old 14 June 2010, 06:41   #3
Member
 
Country: Italy
Town: genova
Boat name: vfi 555
Make: hydrosport
Length: 5m +
Engine: outboard 4 st 60 hp
Join Date: Jun 2010
Posts: 2
Thank s

Quote:
Originally Posted by 9D280 View Post
Firstly, welcome to ribnet.

Sounds like you could go down in pitch. the usual rule of thumb is 1" of pitch = 200 rpm. So, based on you aiming at 5500, I'd see if you could borrow a 14" and see what happens.

For what it's worth, my rib is a similar size & HP and I'm running a 14".


I'm now going to go & Google VFI.......
Thank s for the tip iŽll try to borrow a 14".
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