Best prop for 50 hp 2 stroke blueband merc

[dynamicspot]

Hi all need some help on deciding on the pitch for a prop which one will suit my needs and a good one for fuel economy i do a lot of offshore fishing and what is the go with the pitch i dont understand it

Cheers

Greg

[bushbeachboy]

Greg,
Go have a look at the Mercury website. They have an entire section just about props. Helped me understand a lot more.

[finga64]

have a gander at this

http://www.themarinedoctor.com/Propellers.htm

There's a couple of pages.

[blaze]

the pitch of the prop is how far the prop will move the vessel through the water in one revolution. A 17" prop in theory will move the boat 17"s, but now you need to put in the equation things like slippage.
cheers
blaze

[dynamicspot]

could you manage to tell me how to work that out

Thanks or if i give you some specs on fuel for nm



Greg

[blaze]

Hi Greg
have a look at your current prop and let us know the numbers on it, some times they are stamped on where the hub nut goes or on the side between the blades.
what is wrong with your current prop?
what is your engine reving at WOT?
what type of boat?
cheers
blaze

[dynamicspot]

Hi Blaze here are the nombers of the prop propco m46-11p the current prop has rubber in the middle that has became soft after my deckie went crusing instead to be setting mud crab pots while i set up camp I think he hit a sand bar but he won't tell me He won't come again i can cruise along nice on 3500 rpm no probs att all and engine not working to hard but it does use a fair bit of fuel The motor is a 50 hp blueband merc the boat is a 4.85 m pleasurecraft seamaster fibreglass

Cheers
Greg

[finga64]

the motor maybe working too hard for a boat this size. Hence the poor economy.
I have a boat similar size and has a 90hp blue band. Economy is not too bad. But I cruise on 3100 revs (about 25 litres for 30km ocean trip)
What are other peoples thoughts???

[dynamicspot]

Hi all thats why i was inquiring about the pitch to try and drop the rpm 's for better fuel ecomany


Greg

[finga64]

to drop the rpm's you have to go up in pitch.
At the moment it looks like you have an 11" pitch ie every revolution of the prop it should propell the boat 11". To increase this you'll need a 13 or 15" inch pitch so you should go either 13 or 15" per propellor revolution.
If you went to a 9" pitch it would only go 9" per revolution. Good for take-off, no good for top end speed.
Does that make sense??
Cheers Scott

[dynamicspot]

Thanks finga got it now


Cheers greg

[blaze]

Hi Greg
I think from what you are saying that
the hp of the merc is to small to go up in pitch, say from 11p to 13p or by cupping an 11p prop you can make it about equall to a 12p. You say it cruises at 3500 rpm but it nessary to know WOT (wide open throttle, loaded as it would be for a normal trip). The reason for this is if the WOT is say 4500 and the motors recommendation is say 5800rpm if you were to run at 3500rpm the motor would be strugling and very heavy on fuel.
more info needed
cheers
blaze
IMO you are 20 to 30hp down on what is required

[Owen]

G'day Greg,
I have a Quintrex 4.55 tinnie with a 50hp merc.
My boat I'd say would be a fair bit lighter than yours.
I have just gone up from an 11 to a 12 pitch to drop the revs a bit.
Max revs on the merc (mine has a red stripe) is 5,000 - 5,500 rpm
With the 11P on I was getting 5,500.
The 12P in theory should drop that back to 5,200 - 5,300 with one person on board. Loaded I'll be back under 5,000 I reckon. As soon as the stator is repaired I'll find out

If you're not getting those sorts of revs with the throttle wide open then you are already over-pitched or more likely under powered.

cheers,
Owen

[dynamicspot]

HI all am getting 5500 rpm easy wot loaded wit the 11 p prop that why i cruse at about 3500 to save a bit of fuel and back of on the revs


Cheers
Greg

[blaze]

Hi Greg
starting to get the picture
Is your current prop in good condition, worn and thin edges, damaged?
You mention that you feel your mate may have hit something and the prop may be slipping via the rubber bush, to check this get a white permanite marker (easy to see), put a mark on the out hub near the nut and a corrisponding mark on the inner hub/spline and take it for a good work out and check to see if the marks still line up, if they dont a good place to start is to replace the rubber bush. If you can see if you can trial a prop the same (new or near new) and see if you get the same result. If you do I would purchase a new 11p prop and have it cupped.
contact solas.com.au, all details there and you find them very helpful
cheers
blaze