Mariner 25hp prop pitch on 3.8 sib

[jwatson]

Hey chaps. What pitch range can I be running on my 3.8m sib? Currently an 11". The last owner towed toys with it but I've no plans to so may up it a bit. Also is there a rev counter I can wire in to monitor rpm at wot? The boat doesn't have a battery. Cheers!!!

[bartiny]

Something like this will do for RPM.

LCD Digital Engine Tach Tachometer Hour Meter Gauge for Racing Motorcycle UK | eBay

As for prop size it all depends on what you want to do, how much weight you carry etc.

I have the same setup as you and I have 3 props 10" 11" 13", but stick with the 11" as a good all rounder, if lightly loaded (just me and light kit) I would put the 13" on, if I could be bothered that is

[jwatson]

Had two of us on yesterday and it was absolutely screaming at top end. So much so I backed it off a bit!

[bartiny]

Originally Posted by jwatson

Had two of us on yesterday and it was absolutely screaming at top end. So much so I backed it off a bit!

The only way of being sure you are within the correct RPM at WOT is to fit a tachometer like the one in the link above.

It will 'scream' each cylinder (on a 2 stroke) will be firing 92 times a second at 5,500 RPM (91.6666666666 for the pedants)

[jwatson]

Also, how do I find what rpm range the engine should be operating within?

[bartiny]

Originally Posted by jwatson

Also, how do I find what rpm range the engine should be operating within?

Probably 4,500>5,500 but more info here.

Boat Propeller Fit Guide

[dave3235]

Originally Posted by bartiny

Probably 4,500>5,500 but more info here.

Boat Propeller Fit Guide

Hi

Preety sure it is 5000-6000 for 25hp mariner but not 100% sure.Also I run a 13p on my 20hp pulls 3 adults sweet,I would like to get a higher pitch prop if it was myself only may be even a 15p but funds are the limiting factor.As it is I get 25 mph with 3 adults 50 stone on board sweet and on song.It does take may be 6-8 seconds to get on the plane but once you there much better than revving the nuts out of it.I woud have thought your pitches would be higher saying you have 25hp, would definatley want the engine to be running at around 5800 rpms.

[Downhilldai]

Firstly, it would be helpful to know whether this Mariner is a Merc, Yam or Twatsoo.

[dave3235]

Originally Posted by Downhilldai

Firstly, it would be helpful to know whether this Mariner is a Merc, Yam or Twatsoo.

Hi

My engine is a merc mariner 20 hp not to sure about the others

[jwatson]

Mariner, mercury I think?

[Downhilldai]

Yep, that's a Merc.

[Downhilldai]

I run one of these on an Avon 340 RIB. 2-up it does 26kts with an 11" pitch prop.

[dave3235]

Hi

What engine do you have.stainless or ally prop.

[jwatson]

Originally Posted by dave3235

Hi

What engine do you have.stainless or ally prop.

Me? Engine as above merc/mariner 25 2t. Alu prop 10x11 I think