Prop question
 

Hi,

i'm wondering if someone could give some advice on what prop i should use

i have a 15ft fibreglass runabout with a 70 hp 3 cyl evinrude motor with
trim tilt

i have a propco-19p prop that i would like to use on it , i'm guessing its
a 19 in pitch prop

would this be sufficient for decent holeshot and reasonable top end ?

i cant really find alot of info in goog le.com

its not a heavy boat


thanks

Prop question
 

mememememe! wrote:
Hi,

i'm wondering if someone could give some advice on what prop i should use

i have a 15ft fibreglass runabout with a 70 hp 3 cyl evinrude motor with
trim tilt

i have a propco-19p prop that i would like to use on it , i'm guessing
its a 19 in pitch prop

would this be sufficient for decent holeshot and reasonable top end ?

i cant really find alot of info in goog le.com

its not a heavy boat


thanks


Like airplane props, boat props specify diameter, # of blades and pitch.
Diameter and blade count are a function of shaft HP X prop shaft speed.
Pitch varies with desired boat speed of operation and prop shaft speed
Boat speed depends on hull shape - principally if it can hop onto the
plane.

It's sometimes easier to refer to a guide (of similar types) and try two
or three props to bracket the desired result.....

Brian W

Prop question
 

On Thu, 23 Jul 2009 23:03:02 +1000, mememememe! wrote:

Hi,

i'm wondering if someone could give some advice on what prop i should
use

i have a 15ft fibreglass runabout with a 70 hp 3 cyl evinrude motor with
trim tilt

i have a propco-19p prop that i would like to use on it , i'm guessing
its
a 19 in pitch prop

would this be sufficient for decent holeshot and reasonable top end ?

If you have this prop in hand, install it and give it a go.

Prop question
 

brian whatcott wrote:
mememememe! wrote:
Hi,

i'm wondering if someone could give some advice on what prop i should use

i have a 15ft fibreglass runabout with a 70 hp 3 cyl evinrude motor
with trim tilt

i have a propco-19p prop that i would like to use on it , i'm guessing
its a 19 in pitch prop

would this be sufficient for decent holeshot and reasonable top end ?

i cant really find alot of info in goog le.com

its not a heavy boat


thanks


Like airplane props, boat props specify diameter, # of blades and pitch.
Diameter and blade count are a function of shaft HP X prop shaft speed.
Pitch varies with desired boat speed of operation and prop shaft speed
Boat speed depends on hull shape - principally if it can hop onto the
plane.

It's sometimes easier to refer to a guide (of similar types) and try two
or three props to bracket the desired result.....

Brian W

Here's one rough datapoint: on a 16 ft trihull which planes easily,
(but bangs in chop....) the 85HP Johnson (which is really too heavy for
the boat), has a three blade 14 diam 12 in pitch (guessed not read off
the prop...) does 33 mph....

Brian W

Prop question
 

np's thanks for the replies









"mememememe!" wrote in message
...
Hi,

i'm wondering if someone could give some advice on what prop i should use

i have a 15ft fibreglass runabout with a 70 hp 3 cyl evinrude motor with
trim tilt

i have a propco-19p prop that i would like to use on it , i'm guessing its
a 19 in pitch prop

would this be sufficient for decent holeshot and reasonable top end ?

i cant really find alot of info in goog le.com

its not a heavy boat


thanks

Prop question
 

memememememe!!

You might want to start by calculating you boats resistance or the
estimated power required for a particular speed first.
Use can this calculator www.psychosnail.com/boatspeedcalculator.aspx

This will give you the SHP, shaft horsepower for a desired speed.
Since you have an outboard? it's likely that the manufacturer will be
able to tell you what diameter is best for the engine. So what's left
is calculating the pitch which isn't all that difficult, once you have
the results from the speed calculator.
Because your boat would be relatively fast the Crouch slip method
should give reasonably accurate results.
Slip is calculated as follows:
Use about 90% SHP/RPM and the speed from the BoatSpeedCalculator
P = (kts*1215.6) / (RPM* (1- (1.4* (kts^0.57) ) )

RPM is at the shaft so you need to find your reduction ratio

The inputs you'll need for the calculator are
Displacement, Length at waterline, desired speed
and the to calculate pitch RPM (with reduction)

Hope this helps