Cruising Revs

blimp · 23 March 2007, 06:39

Just a quick question.

Haven't had the rib long enough yet to know what my fuel consumption is, but any suggestion what would be an efficient cruise rate in terms of rpm? Perhaps even slow cruise, and, fast cruise?

If it helps any bit:
At WOT I'm doing about 48mph for 4900-5000rpm
At 4500rpm I'm doing about 40mph
It's a Yamaha 150HPDi pushing a Cobra 6.65m

OceanEco · 23 March 2007, 07:30

3800-4000rpm

benc · 23 March 2007, 08:20

WOT on that engine should be between 5000-5500 - what propellor do you have?

blimp · 23 March 2007, 09:02

19" on at the moment - had been told that a 21" prop would give better performance at top end, but when I enquired further I was told that it would bugger acceleration which I'm going to need for wakeboarding during the Summer. I seem to remember reading on some yamaha site that WOT was 4500-5500, so I just assumed that I was fine, being slap-bang in the middle of the range!! Now I'm confused

benc · 23 March 2007, 09:20

Well, I have two of the same engine, with 23" props.

My acceleration is incredible, and top speed... about 55knots...

Is your prop stainless or Ali?

blimp · 23 March 2007, 09:49

stainless! - might well go up to the 21" then

benc · 23 March 2007, 10:16

If you weren't so far away, i'd let you try one of mine

You should be able to find one around though, let us know how you get on

blimp · 23 March 2007, 10:23

Thanks Benc - what about the other important dimensions - pitch... and the like?

benc · 23 March 2007, 10:40

19"/21" etc.. is the pitch - I have the standard yamaha 23" pitch propellors - I want to change mine for some from steel developments at some point, but can't justify it at the moment.

benc · 23 March 2007, 10:56

Just thinking about it, im talking utter nonsense.. if your propellor was too small, then the engine would rev higher... unless its hitting a rev limiter.

Maybe you have the engine too high on the boat?

I am sure with a 19", its under prop'd but I am not sure why you wouldnt be getting higher revs

The Garfish · 23 March 2007, 11:03

Quote:

Originally Posted by benc

I am sure with a 19", its under prop'd

Quote:

Originally Posted by benc

im talking utter nonsense..

i kwite aggree wiv yew onn de secund poynt

gaRf

Chris · 23 March 2007, 11:05

Blimp,

How do you know your prop is 19" are you sure you are not referring to the diameter not the pitch?

Chris

benc · 23 March 2007, 11:08

Quote:

Originally Posted by The Garfish

i kwite aggree wiv yew onn de secund poynt

gaRf

But not de fuurst?

I guess his boat is smaller, lighter too... but with the 150, I would have though 19 is too small.... but I don't really know, was just relating to what I had with the same engine - but I know its much more than that.

The Garfish · 23 March 2007, 11:14

Quote:

Originally Posted by Chris1573

How do you know your prop is 19" are you sure you are not referring to the diameter not the pitch?

beecuz ifn hee ad fittid a 19 dyameter propp itt wud av cutt arf de fkin geerbokks orf yew nobbur

gaRf

Mollers · 23 March 2007, 11:18

Quote:

Originally Posted by benc

But not de fuurst?

I guess his boat is smaller, lighter too... but with the 150, I would have though 19 is too small.... but I don't really know, was just relating to what I had with the same engine - but I know its much more than that.

My set-up is pretty similar. I run a 19" and a 21" for distance cruising. A 6m boat with 150hp is very slightly under-propped running a 19" and a 21" does numb the acceleration a tad. If I had to choose, I'd probably go for the 21". It's unfortunately not as simple as pitch. Blade profiles etc play a large part in determining the correct prop. Most of this has been covered in previous posts on this site if you do a bit of trawling.

Chris · 23 March 2007, 11:28

Quote:

Originally Posted by The Garfish

beecuz ifn hee ad fittid a 19 dyameter propp itt wud av cutt arf de fkin geerbokks orf yew nobbur

gaRf

Quite.... "note to self... think before you post"

Chris

The Garfish · 23 March 2007, 11:29

Quote:

Originally Posted by Mollers

A 6m boat with 150hp is very slightly under-propped running a 19"

fuk mee. downt yew starrt. wots appenin todaiy

evveryboddys fkin brayn seams too av gorn sofft

gaRf

benc · 23 March 2007, 11:40

Quote:

Originally Posted by Chris1573

Quite.... "note to self... think before you post"

Chris

I'll copy your note on that one!

Sixy_the_red · 23 March 2007, 11:48

Quote:

Originally Posted by The Garfish

fuk mee. downt yew starrt. wots appenin todaiy

evveryboddys fkin brayn seams too av gorn sofft

gaRf

It is friday...

jwalker · 23 March 2007, 12:05

Quote:

Originally Posted by blimp

Now I'm confused

Feck, reading this thread, it's easy to see why. GaRfie is the brainiest bloke here!

Stick with the prop you've got. Cruise at 38-40mph.

There does seem to be something wrong with your figures, though.

4500rpm = 40mph and 4900rpm = 48mph?