Propeller pitch advice pls

[chuckaduck]

As I am not clued up on this so I thought of asking you lot !!

the boat
22 foot Binliner deep v hull 130 horses cobra out drive

the Prop currently 19 inch pitch

we use the boat for cruising not 4 skiing etc would like to have the best option for cruising speed and economy

so its up to you guys now !!

 

[spannerman]

That engine should pull between 4400-4800rpm so if your current prop gives you that at full throttle then you have the optimum prop, if its lower then you need a 17".

 

[omega2]

www.clementsmarine.com

 

[Nauti Fox]

Have a looky here and click on calculator.
Oops, sorry it wont do it if your not a member.

 

[duncan]

he's got a starting point anyway, and isn't going to want to move away from the standard 3 bladed for his stated requirements so, as spannerman says, simply increase or decrease pitch until you are getting the correct revs with normal loading. relatively light loaded you should aim to get within 200rpm of max revs - fully loaded it can be further away. Over proping in an attempt to gain economy simply doesn't work across a range of loadings and useage - if you are always running light and on long runs (ie not constantly on and of the plane) you might gain a small mpg advantage at certain speeds but thats all - and you risk other damage etc.

 

[Jinks]

Would you have an idea of the sort of revs at WOT I should be getting from a Yanmar 240hp on a 25ft. planing hull?

 

[chuckaduck]

[ QUOTE ]
That engine should pull between 4400-4800rpm so if your current prop gives you that at full throttle then you have the optimum prop, if its lower then you need a 17".

[/ QUOTE ]

It is flat out at 28knts with engine revs at 4500 rpm is this right for the boat ??

 

[PCUK]

One inch of pitch equals 300 rpm roughly either way. Add an inch and you will lose 300 rpm from maximum but gain more thrust at lower revs. If you don't need maximum speed, over prop to gain thrust at mid range revs. It won't hurt the engine, mild over propping is no different to running with a dirty bottom.

 

[chuckaduck]

I have a 21 pitch prop to try what do you think ??

 

[oldharry]

You'll do 23 kts at 4000 rpm. And no more. /forums/images/graemlins/smirk.gif

 

[duncan]

engine is governed at 3650rpm

max power generated at 3300rpm

continuous rating 3100rpm

if you can't get 3300 you have a problem prop wise but if you are on an outdrive, esp BIII, you may not be able to go to a bigger pitch without other problems - ie relationship between 1700rpm (turbo starts) and hull speed is limiting.

What drive have you got? Shaft or OD? BII or BIII?

 

[duncan]

don't bother to change it - looks pretty much spot on!

 

[spannerman]

The 21" is too big, you will struggle onto the plane and if you don't achieve the minimum WOT rpms you can damage the engine (burn holes in the pistons)
You are OK with the prop you have, if you can borrow a 17" you will find it will plane quicker but I doubt you will get anymore speed as you are in the area where you can swap rpm for knots but the gains will be insignificant. If you are going to just cruise stay with the 19", if you want to waterski or always have a full boat then try a 17", if it over revs you can back off the throttle but I am pretty sure you will lose on top speed.

 

[Jinks]

Thanks Duncan.

It was more to check how accurate the new(ish) replacement rev counter is. It shows 3300 at WOT and I try to cruise at about 2900 which gives around 20kts and suits me fine. It's a B2 outdrive.

Of course, I still don't know if it's actually doing 3300 or 3600 at WOT!!