Reply
 
Thread Tools Search this Thread
 
Old 07 June 2016, 12:16   #1
Member
 
Country: UK - England
Town: Kingsbridge
Make: RIB Quest
Length: 5m +
Engine: 2010 Yam 90 2 Stroke
Join Date: May 2016
Posts: 21
Choosing the right propellor

Hi,

I currently have a 5.3m RibQuest with a 2010 90hp 2 Stroke Yamaha.
Prop is 17'' aluminium standard Yamaha.

It is running at 5500rpm at 42mph top end. Acceleration is great but some cavitation when cornering hard.
Any thoughts on improving the top end? I've set the engine fairly high on the transom.
__________________
Ribquest5.3 is offline   Reply With Quote
Old 07 June 2016, 12:18   #2
Member
 
Country: UK - England
Town: Kingsbridge
Make: RIB Quest
Length: 5m +
Engine: 2010 Yam 90 2 Stroke
Join Date: May 2016
Posts: 21
Personal thoughts are trying a 19'' Stainless
__________________
Ribquest5.3 is offline   Reply With Quote
Old 07 June 2016, 14:35   #3
Member
 
MatFromPoland's Avatar
 
Country: Poland
Town: WARSAW
Boat name: T1
Make: HIGHFIELD OM540DL
Length: 5m +
Engine: EVINRUDE 115 HO
MMSI: 261026640
Join Date: May 2014
Posts: 612
But it is great speed.
What is WOT range for this engine ?
Not idea what you would like to achieve if RPM is in WOT range.
Jump from 17" to 19" can lower RPM to 5200 so speed can be the same or even lower

If cavitation is the problem you can lower engine but MAYBE top speed be a little smaller in such case.
If you lower engine MAYBE RPM be a little less than now
but it require some test.

I lifted up engine and top speed and RPM were higher but cavitation and grip forced me to put again engine to lower position.
__________________
MatFromPoland is offline   Reply With Quote
Old 07 June 2016, 15:15   #4
Member
 
MatFromPoland's Avatar
 
Country: Poland
Town: WARSAW
Boat name: T1
Make: HIGHFIELD OM540DL
Length: 5m +
Engine: EVINRUDE 115 HO
MMSI: 261026640
Join Date: May 2014
Posts: 612
Something seems to be wrong.
I do have XLS showing max theoretical speed based on achieved RPM, gear ratio, slip
If your engine is 90A gear ratio is 2.0

RPM 5500
pitch prop 17
gear r. 2
slip % 3
knots 38,39421667
km/h 69,10959

You can not go faster use 17" prop than 38 knot (69 km/h)
Even if you have 3% slip which is really incredible low ... (usually 5-10%)

If we replace 17 to 19 in this equation we can have (and lower RPM to 5200)
you can (theoretically ) have 40 knots:
RPM 5200
pitch prop 19
gear r. 2
slip % 3
knots 40,57057333
km/h 73,027032

This is all about math and physics. Not sure if spending much more money on 19" prop is really worth 3 km/h more. But again this is simple equation showing max possible.
__________________
MatFromPoland is offline   Reply With Quote
Old 07 June 2016, 15:28   #5
RIBnet admin team
 
Poly's Avatar
 
Country: UK - Scotland
Boat name: imposter
Make: FunYak
Length: 3m +
Engine: Tohatsu 30HP
MMSI: 235089819
Join Date: Sep 2005
Posts: 11,627
Quote:
Originally Posted by Ribquest5.3 View Post
Any thoughts on improving the top end?
Do you think you'll often have any need to go over 40 knots? Not many people with 5.3m boats are regularly sitting that fast for long periods. Props are all about compromise, yours sounds pretty good.

Quote:
I've set the engine fairly high on the transom.
thats probably why you are getting ventilation in tight turns. Are you trimming down/in for the turn?
__________________
Poly is offline   Reply With Quote
Old 07 June 2016, 16:28   #6
RIBnet admin team
 
willk's Avatar
 
Country: Ireland
Length: 4m +
Join Date: Feb 2008
Posts: 14,893
Quote:
Originally Posted by MatFromPoland View Post
Something seems to be wrong.
It is - you're using knots and the OP quoted his WOT speed as 42MPH = 36.5kts approx.

OP - I "think" that 5500rpm is the upper limit for this motor? You can afford to drop back a bit on the revs and not stress the engine - 5200/5300 should be fine. If you go stainless and keep the same 17" pitch you should get some improvement in overall performance as the stainless flexes less. Go up to a 19" pitch and you'll see a drop in WOT revs but you may not get much extra top end - it will depend to some extent where the motor generates it's max power? A torque curve might help you decide...
__________________
.
willk is offline   Reply With Quote
Old 07 June 2016, 16:48   #7
Member
 
MatFromPoland's Avatar
 
Country: Poland
Town: WARSAW
Boat name: T1
Make: HIGHFIELD OM540DL
Length: 5m +
Engine: EVINRUDE 115 HO
MMSI: 261026640
Join Date: May 2014
Posts: 612
Yes I was reading KNOTS not MPH In such case theory looks like this one mentioned below:

Current 17":

RPM 5500
pitch prop 17
gear r. 2
slip % 5
knots 37,60258333
km/h 67,68465
mph 42,30290625

Maybe it be like this after jump to 19" (RPM will drop about 300):
RPM 5200
pitch prop 19
gear r. 2
slip % 5
knots 39,73406667
km/h 71,52132
mph 44,700825

Still theory
__________________
MatFromPoland is offline   Reply With Quote
Old 07 June 2016, 18:20   #8
Member
 
Country: UK - England
Town: Retford
Boat name: Spy-sea-one
Make: Excel 435
Length: 4m +
Engine: Suzuki Outboard/25/4
Join Date: Oct 2011
Posts: 7,528
RIBase
Quote:
Originally Posted by Ribquest5.3 View Post
Hi,

I currently have a 5.3m RibQuest with a 2010 90hp 2 Stroke Yamaha.
Prop is 17'' aluminium standard Yamaha.

It is running at 5500rpm at 42mph top end. Acceleration is great but some cavitation when cornering hard.
Any thoughts on improving the top end? I've set the engine fairly high on the transom.
Best I got 14 x 18 Ali prop WOT 45 mph ribquest 5.85 90 hp Suzuki 4 st engine was sat on the transom no shims
No cavitation
Cheers
__________________
jeffstevens763@g is offline   Reply With Quote
Old 07 June 2016, 19:45   #9
Member
 
MatFromPoland's Avatar
 
Country: Poland
Town: WARSAW
Boat name: T1
Make: HIGHFIELD OM540DL
Length: 5m +
Engine: EVINRUDE 115 HO
MMSI: 261026640
Join Date: May 2014
Posts: 612
Quote:
Originally Posted by Ribquest5.3 View Post
Personal thoughts are trying a 19'' Stainless
Did you test having nominal load ?

If you test alone but usually will go with more people maybe 17" is optimum.
If you plan to tow a toy 17" be better also.
__________________
MatFromPoland is offline   Reply With Quote
Old 07 June 2016, 20:23   #10
RIBnet admin team
 
willk's Avatar
 
Country: Ireland
Length: 4m +
Join Date: Feb 2008
Posts: 14,893
Quote:
Originally Posted by MatFromPoland View Post
Did you test having nominal load ?

If you test alone but usually will go with more people maybe 17" is optimum.
If you plan to tow a toy 17" be better also.
My thoughts exactly
__________________
.
willk is offline   Reply With Quote
Old 18 June 2016, 20:28   #11
Member
 
Country: UK - England
Town: St Austell
Boat name: On Site
Make: Zodiac
Length: 5m +
Engine: 100hp outboard
Join Date: Oct 2015
Posts: 41
RIBase
I've been testing props.
5.5m Rib with 75hp Honda.
It's got a 17" SS prop. Went up to a SS 19" today. Revs dropped by 300rpm but so did top speed. 2-3mph less. I was hoping for 2-3 mph more. Bit slower out the water but not a lot different unfortunately. Not worth changing for me.
__________________
cornishcoote is offline   Reply With Quote
Reply


Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off




All times are GMT. The time now is 15:51.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2024, vBulletin Solutions, Inc.