Prop size

[dave007]

Hi all

 

Finally took 'Happy Days' down to baiter for its maiden voyage after its rebuild, boat went really well getting up to 24 knots which was quite exciting.

Didn't manage any fishing just wanted to make sure boat was OK.

 

My question is when I bought the boat it came with two props one alloy, one plastic (ish). both have differant pitch etc but unsure which would be the best.

 

The engine is an old Susuki DT 50 any advice most welcome

 

thanks for looking

 

Dave

[Maverick Martin]

Dave

 

To make a decision as to which prop is best you will need a rev counter. You will also need to know what the manufacturer specifies as max revs. Fit a prop to the engine and load the boat with your normal traveling weight. Now try it and see what speed and revs you achieve with each prop.

 

You are looking to get as near to the manufacturers recommended rev range without going over. Which ever prop does this is the correct one...........maybe.

If it over revs you could cook the engine if you get nowhere near the rev range you will be straining the engine.

 

Hope that helps

 

Martin

[dave007]

Thanks for the info , wasn't sure if there were other things to look for, like which pitch is best and lengtht, as I understand that this can be varied for differant situations.

 

Any other tips would be grateful.

 

Thanks

 

Dave

[Rob]

 

Prop size and pitch and blade number and cupping are all critical.

 

It is a mine field and an expensive one too!

 

You are trying to select the right gear for you needs.

 

Too little pitch and you might over rev, too muh and you won't hit wot (if you want to).

 

Then you get to number of blades and size - another can of worms.

 

Did I mention materials? S\Steel or ali - thinness of the S\S can be more efficient but you coudl have more issues if you strike something!

 

Best thing is find someone who has a similar setup to you and ask them!

 

What boat is she?

 

If she works as you hope maybe best left alone!

 

If you ae thinking of experimenting the question whould be why, what is not satisfactory.

 

Remember that top end is not that useful 99% of the time!!!

 

Rob

 

 

 

 

 

 

[Wedger]

you could do the Castle Marine free prop calc just to see if you are in the right ball park.

 

EG

 

boat = fast planing

length =5.5 mtrs

weight 600 kg

engine = outboard

power = 50 hp

max rpm = 5500

redution box 2:1

 

Calc gives 33 knots

Prop = 12" x 16.5" pitch

Edited August 7, 2010 by Wedger

[dave007]

I think Rob your proably right, it does go quite well anyway.

 

it was a thought that if is was not going to be too much I might experiment.

Think I will be happy with what I have.

 

thanks for all your inputs .

 

Dave

[Rob]

 

No worries - I have recently fitted (in Jan) a different prop for JoJo.

 

I have been out a few times and I have noticed I can now hit the correct Max RPM (and hence power) at WOT. However I am yet to work out my fuel consumption - I had it plotted for the old prop (via fuel flow meter) and intend to log the consumption with this new prop.

 

Here is one on my original threads on the subject:

 

http://www.pbsbac.co.uk/forums/index.php?showtopic=8598

 

Enjoy

 

Rob