Worth changing prop size?

Jack h · 04 October 2006, 19:28

Hi I have a 5m humber assault with a mariner 60 bigfoot 4/. The rib came with a 13" pitch prop which was over revving so before we bought it had it movd up to 17" pitch, this is way to big and under revs, will do 4250rpm but engine will go up to 5750rpm. Is it worth changing down to 15" pitch prop? will i notice a big difference in excelleration,top speed, and power?
because at the monet the rib takes about 30 seconds to get on the plane with 6 people in and seems quick but not that quick, thanks.

ashbypower · 04 October 2006, 19:35

As a general rule 1" change in pitch results in 200 RPM change.
i.e up 1" Revs down 200, and vice versa.

Regards

Brian

Forgot to say Hi and Welcome.

Lets go boating · 04 October 2006, 20:33

I would say it is worth you while changing props.

sparkey · 04 October 2006, 22:50

you should change prop.
if you do not rev to correct WOT then you will damage your engine .

Lets go boating · 04 October 2006, 22:56

If you don't watch out you could blow the con-rod out of the engine. This happened to mine, and it has just come back from repair after 2 months.

The Garfish · 04 October 2006, 23:50

lots ov gud addvise ther butt unfawtewnatly itts orl bollux.

heers a likkul cloo...............

Quote:

Originally Posted by Jack h

at the monet the rib takes about 30 seconds to get on the plane with 6 people

chukk forr ov de bastuds owt an yorr revvs wil mysteriously rize.

fuk mee dis prowpeller stuf iz a blakk arrt.

ohh an beesydes orl dat de bigfut iz 120 pir sent de rong enjin forr dat bote

gaRf

BogMonster · 05 October 2006, 03:12

Quote:

Originally Posted by Martha Focker

If you don't watch out you could blow the con-rod out of the engine. This happened to mine, and it has just come back from repair after 2 months.

How does that happen then?

My engine indicates a WOT range of 5000 to 6000 in the book, it was doing 5500 with the old 17" prop now 5200 with the 19" prop. Is that ok?

Lets go boating · 05 October 2006, 18:10

Quote:

Originally Posted by BogMonster

How does that happen then?

My engine indicates a WOT range of 5000 to 6000 in the book, it was doing 5500 with the old 17" prop now 5200 with the 19" prop. Is that ok?

This happens when the engine is over reved, or is over worked.

My engine has full revs at 5500 revs, and i had it prop tested, they said that it was perfect. i have got a 19p on mine so yh should be fine.

hope this helps.

Jack h · 05 October 2006, 19:30

Quote:

Originally Posted by The Garfish

ohh an beesydes orl dat de bigfut iz 120 pir sent de rong enjin forr dat bote

gaRf

thanks for the help everyone but, why is a bigfoot not the right engine for the job?

samt · 05 October 2006, 21:32

They are designed for pushing heavier boats, i.e a fishing boat that needs the power but not speed. Not a light rib! The gearbox ratios are different to the standard gearbox. The prop rpm will be less. Replacing your o/b with the standard one may solve many of your problems.

Pete7 · 06 October 2006, 12:56

Quote:

Originally Posted by Jack h

thanks for the help everyone but, why is a bigfoot not the right engine for the job?

Thats a big prop turning slowly which generates lots of thrust. Its actually a 75 - 90 gearbox with a 2.33:1 ratio box rather than the smaller 60 gearbox with 1.64:1 ratio. Mariner even say,

"for added durability and push, the Bigfoot models feature a taller gearcase and 33% larger components, just the job for bigger boats"

Good for a rib full of divers and kit that needs lots of thrust and will only ever travel at 20 - 24 knots to the dive site, but not the right engine for you if your after speed and acceleration. That bigger gearcase means greater drag too. If its new take it back and get the dealer to put the right engine on it, they should have known better.

Pete