Reply
 
Thread Tools Search this Thread
 
Old 15 July 2012, 16:05   #1
Member
 
Johnscubanut's Avatar
 
Country: UK - England
Town: Beds/South coast
Make: Ribcraft 585
Length: 6m +
Engine: Yam 115
Join Date: Jul 2010
Posts: 294
Right pitch

Hi guys, just need some confirmation on our set-up? 5,8m ribcraft Yam 115hp four stroke.
Reduced the prop pitch from 19inch down to 17inch to increase revs due to using our rib for dive purposes.
Fully loaded yesterday with 7 people, 6 sets of dive gear & full fuel tank of 80L

Struggled to plane & would only get to 4-4500 revs! 18-20kts eventually!

Reduce pitch even further you say? Would going down to a 15inch pitch cure? Or even further????
I believe the max revs for this engine are around 6k?
We don't need top speed, just able to get on the plane & back off for fuel economy.
Your thoughts would be appreciated before we purchase another prop?

p.s. please don't say we need a bigger engine? LOL
__________________

__________________
Johnscubanut is offline   Reply With Quote
Old 15 July 2012, 17:29   #2
RIBnet supporter
 
Pikey Dave's Avatar
 
Country: UK - England
Town: South Yorks
Boat name: Black Pig
Make: Ribcraft
Length: 5m +
Engine: DF140a
MMSI: 235111389
Join Date: Feb 2008
Posts: 7,590
RIBase
Roughly, 1" of pitch = 200rpm, I don't think you're going to find another 1500rpm with that combination of boat/load/engine. It's a big ask of that combo to carry all that weight & achieve WOT. If you continue to run it as is, you are lugging the engine which will shorten its life. Dropping down to a 13" might be as good as you can get, also a stainless prop will get you a few more RPM over the same sized ally.
__________________
Rule#2: Never argue with an idiot. He'll drag you down to his level & then beat you with experience.
Rule#3: Tha' can't educate pork.
Rule#4: See rule#5
Pikey Dave is offline   Reply With Quote
Old 15 July 2012, 18:14   #3
Member
 
Johnscubanut's Avatar
 
Country: UK - England
Town: Beds/South coast
Make: Ribcraft 585
Length: 6m +
Engine: Yam 115
Join Date: Jul 2010
Posts: 294
Wow! as low as a 13inch you reckon? On the return journey with 1/2 tank of fuel & empty bottles (air does wheigh), we got 5k on the tacho & speed of around 24kts but I know we will stuff the engine if we can't achieve higher revs. So we will have to drop down to that low a pitch I guess? have to be careful not to over-rev when lightly loaded tho? Thanks for your reply-John
__________________
Johnscubanut is offline   Reply With Quote
Old 15 July 2012, 18:27   #4
Member
 
Hightower's Avatar
 
Country: UK - England
Town: Fareham
Length: 6m +
Join Date: Sep 2003
Posts: 7,855
Really laboring the engine at that RPM. A 15" will help, but as Dave has suggested a 13" is the way to go.

BTW I thought the Alloy/Stainless prop worked the other way around in that a stainless would lower RPM for a given size?
__________________
Andy

Looks Slow but is Fast
Member of the ebay Blue RIB cover club.
Hightower is offline   Reply With Quote
Old 15 July 2012, 18:32   #5
Member
 
Johnscubanut's Avatar
 
Country: UK - England
Town: Beds/South coast
Make: Ribcraft 585
Length: 6m +
Engine: Yam 115
Join Date: Jul 2010
Posts: 294
Thanks Hightower, I believe he's right re the stainless prop, thinner metal, spins faster!
I'd like to try a 15 pitch at first tho?
__________________
Johnscubanut is offline   Reply With Quote
Old 15 July 2012, 18:47   #6
Member
 
Hightower's Avatar
 
Country: UK - England
Town: Fareham
Length: 6m +
Join Date: Sep 2003
Posts: 7,855
Well I'm not sure? Someone told me that Alloy props lose efficiency when they get loaded which in turn distorts the blades, wouldn't this make the prop run at a slightly higher RPM?

Anyway, whoever is right I doubt it would make any changes to you set-up, I would go for the Alloy prop purely on the ground of cost.
__________________
Andy

Looks Slow but is Fast
Member of the ebay Blue RIB cover club.
Hightower is offline   Reply With Quote
Old 16 July 2012, 03:26   #7
Member
 
Country: UK - England
Town: surrey
Length: 7m +
Join Date: Apr 2012
Posts: 322
with that amount of gear and people i think it's time to invest in a bigger rib and engine if it's going to be a regular event. I'm getting about 40mph 3 or 4 up full fuel on a 6.2m with the same engine and 13 x 19. I think you're expecting to much of your rib to be honest.
__________________
wellhouse is offline   Reply With Quote
Old 16 July 2012, 05:27   #8
Member
 
Johnscubanut's Avatar
 
Country: UK - England
Town: Beds/South coast
Make: Ribcraft 585
Length: 6m +
Engine: Yam 115
Join Date: Jul 2010
Posts: 294
Can't afford a bigger outfit at the moment, have to juggle the pitch size & reduce the weight somehow? Thanks for replies-John
__________________
Johnscubanut is offline   Reply With Quote
Old 16 July 2012, 07:59   #9
Member
 
Johnscubanut's Avatar
 
Country: UK - England
Town: Beds/South coast
Make: Ribcraft 585
Length: 6m +
Engine: Yam 115
Join Date: Jul 2010
Posts: 294
Anyone got a Yamaha 15inch pitch to try out?
__________________
Johnscubanut is offline   Reply With Quote
Old 16 July 2012, 11:12   #10
Member
 
Country: USA
Town: Oakland CA
Length: 3m +
Join Date: Feb 2005
Posts: 6,653
I'm running a roughly similarly sized/engined boat (18' aluminum RIB, F115), and run a 15" 4-blade SS prop for general use. I have a 13 around in case I have more than 4 divers and gear (and enough advanced planning to actually swap it out.)

jky
__________________

__________________
jyasaki is offline   Reply With Quote
Reply

Thread Tools Search this Thread
Search this Thread:

Advanced Search

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





Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -5. The time now is 23:38.


Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.