The Right Propeller Pitch

[WANNAFISH]

HI RAIDERS!!!

Im picking up tomorrow my first brand new motor for my dinghy. Its a 40 hp Mercury 2stroke long shaft and its going on a 4.3 savage gannet. I was wondering if I can choose a propeller with the pitch I want or do I have to pay extra for that and what pitch would be suitable for this boat without sacrificing fuel economy and speed?

A bit of an advise would be much appreciated. TNX

[Flightmanager]

You may be able to get some idea HERE . As for setup , your dealer should provide you with the optimum propellor for your boat. You should be able to reach your optimum RPMs at Wide Open Throttle with whatever prop you use. If your RPMs are too low , , or too high , you would need to use a prop with a different pitch.

Ross

[a boat]

Hi, the engine you are buying comes from Merc with a prop, so unlike bigger HP, which you match the prop to the engine and boat combo, you will need to pay extra if that prop does not suit your boat. That said mate, the 13.5 inch prop that is STD does a good job on boats that the 40HP suits like your Gannet. Also without a Tacho there is no way of knowing what your WOT RPM will be so the dealer should water test the boat with a portable tacho and make sure you are seeing ideally about 5300-5500 at WOT with a normal load.

Best of luck with it,

Cheers,

Huey.

[WANNAFISH]

Hi Huey,

What do u think of those PROPULSE props? I wil be installing a tacho anyway just figure out my proper rpm range.

[a boat]

Hi mate, I am sorry I have never heard of PROPULSE props so I can not comment. I will do a google and see what comes up.

Cheers,

Huey.

PS- just did a search and they are plastic variable pitch props. We have played with props like this and on the right boat they work well, but not many boats. I have not experienced this prop in particular but depending on cost it might be owrht a go. In my experience when working these props do everything claimed and it fells like a 2 speed auto when changing pitch, but the smallest engine has been a V6 so can not comment on a 40HP and whether it has the torque to turn the higher pitch in the idel RPM range. As I say to alot people propping an engine/boat combo is a "black art" and with our 45 years of experience we have a head start over most, but trust me guys I still learn some things each time I try a different prop and I would be keen to see what your results are if you gte such a prop as the PROPULSE.

Cheers,

Huey.

Edited October 1, 2008 by Huey @ Huett Marine

[WANNAFISH]

Hi Huey,

Once i save up, ill get it i think. Just thinking of it, if I hit something accidentally, thet blade will break off, at least I dont want to pay for a gearbox I rather pay for a spare blade. I need a spare prop anyway just incase.

[cruisecraft]

Hi Huey"

The term "WOT RPM" stands for "Wide Open Throttle Revolutions Per Minute", but what does it mean in practical terms.Thank you.

Regards Kit

ps excuse my ignorance in this.

[pelican]

WOT.

Basically it is the revs your motor should be achieving at full throttle when propped and set up correctly. It is important as motors have a rev range that they are tuned to run at under certain loads recommended by the manufacturer. This means you get good longevity out of your motor and good performance. If you overload the motor ( ie too taller pitch prop or too heavier boat) it luggs around and does damage to the motor. If you do the reverse and under prop a motor you risk the motor overrevving or doing high revs with low load and again this is bad for the motor and will give crap high fuel consumption but possibly a quick holeshot.

How to

Check the manufacturer specs for your engine for the WOT rev range. Different for all different models sizes and brands

Take the boat for a run with it's normal load on board (plus a bit for your fat mate that turns up occasionally) and do a full throttle run with the motor reasonably trimmed out and note revs.

If you are not getting the recomended WOT revs you need to look into the way the motor is set up and possibly props so you can get it to run in that rev range.

It is always worth comparing your numbers and prop dimensions with a sister boat owner as it can bring to your attention bad set up or a underperforming motor.

Experienced dealers who sell the same hulls all the time have a very good idea of what motor height and prop work best from doing on water tests so you get good performance , motor longevity and ecconomy.

Hope that helps

Edited October 2, 2008 by pelican

[cruisecraft]

WOT.

Basically it is the revs your motor should be achieving at full throttle when propped and set up correctly. It is important as motors have a rev range that they are tuned to run at under certain loads recommended by the manufacturer. This means you get good longevity out of your motor and good performance. If you overload the motor ( ie too taller pitch prop or too heavier boat) it luggs around and does damage to the motor. If you do the reverse and under prop a motor you risk the motor overrevving or doing high revs with low load and again this is bad for the motor and will give crap high fuel consumption but possibly a quick holeshot.

How to

Check the manufacturer specs for your engine for the WOT rev range. Different for all different models sizes and brands

Take the boat for a run with it's normal load on board (plus a bit for your fat mate that turns up occasionally) and do a full throttle run with the motor reasonably trimmed out and note revs.

If you are not getting the recomended WOT revs you need to look into the way the motor is set up and possibly props so you can get it to run in that rev range.

It is always worth comparing your numbers and prop dimensions with a sister boat owner as it can bring to your attention bad set up or a underperforming motor.

Experienced dealers who sell the same hulls all the time have a very good idea of what motor height and prop work best from doing on water tests so you get good performance , motor longevity and ecconomy.

Hope that helps

Thanks Pelican

That explained it clearly.

Regards Kit

[a boat]

+1 to what Pel says. Just like to add that it is better to be towards the upper limit of recommended RPM range-for exmaple if the recommended range is 4500-5500, it is better to be close to 5500 for best engine life and performance.

Cheers,

Huey.