Right pitch

Johnscubanut · 15 July 2012, 20:05

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

Pikey Dave · 15 July 2012, 21:29

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.

Johnscubanut · 15 July 2012, 22:14

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

Hightower · 15 July 2012, 22:27

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?

Johnscubanut · 15 July 2012, 22:32

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?

Hightower · 15 July 2012, 22:47

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.

wellhouse · 16 July 2012, 07:26

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.

Johnscubanut · 16 July 2012, 09:27

Can't afford a bigger outfit at the moment, have to juggle the pitch size & reduce the weight somehow? Thanks for replies-John

Johnscubanut · 16 July 2012, 11:59

Anyone got a Yamaha 15inch pitch to try out?

jyasaki · 16 July 2012, 15:12

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