MYStargazer
Well-Known Member
Cheers, Paul.
Propeller aperture is 11½"
Propeller aperture is 11½"
Propeller replacement calculations – please help me!
- Thread starter MYStargazer
- Start date
Latestarter1
Well-Known Member
Back of a fag packet Crouch's method.
Crucial specs:
Hull type: semi-displacement
LWL: 30’ / 9.15m
Displacement = 8000 Kg
BHP = 2 x 180 @ 2,600 RPM
Gearbox = 1.52:1
Propeller aperture is 11½"
You have the POTENTIAL to swing a wheel between 19 ½ to 18 ½ inches. Largest diameter is more efficient however will likely rumble, 18½ diameter will be sweet but I will live a little on the edge, so let’s plump for 19 inches.
Wind the handle the slide rule and out pops a 'square' 19X19 propeller giving 18.5 knots @ 2,600 rpm WOT and 16.5 knot cruise @ 2,400 rpm.
A slightly 'safer' 19X18½ will likely bring WOT speed down to 18 knots.
Give prop manufacturers the raw data and see what sexy programs come up with.
Good hunting.
Crucial specs:
Hull type: semi-displacement
LWL: 30’ / 9.15m
Displacement = 8000 Kg
BHP = 2 x 180 @ 2,600 RPM
Gearbox = 1.52:1
Propeller aperture is 11½"
You have the POTENTIAL to swing a wheel between 19 ½ to 18 ½ inches. Largest diameter is more efficient however will likely rumble, 18½ diameter will be sweet but I will live a little on the edge, so let’s plump for 19 inches.
Wind the handle the slide rule and out pops a 'square' 19X19 propeller giving 18.5 knots @ 2,600 rpm WOT and 16.5 knot cruise @ 2,400 rpm.
A slightly 'safer' 19X18½ will likely bring WOT speed down to 18 knots.
Give prop manufacturers the raw data and see what sexy programs come up with.
Good hunting.
MYStargazer
Well-Known Member
Thanks, Paul - very kind of you and very useful.
Episode
New Member
Fitzroy
Managed to get this sorted yet? If it helps, I have a Wey 32 running the original perkins HT6.354M engines (145 shp at 2400 rpm). She had new props a few years ago and is currently fitted with 4 blade 22" X 24". According to data that came from the previous owner she previously had 3 blade 22" x 23". Current props look enormous but engines nave no problem reaching max rpm so they can't be too big.
As to weight, not put mine on a load cell but suspect 8,000+ kg is going to be closer to the truth. Boats only get heavier with age!
Managed to get this sorted yet? If it helps, I have a Wey 32 running the original perkins HT6.354M engines (145 shp at 2400 rpm). She had new props a few years ago and is currently fitted with 4 blade 22" X 24". According to data that came from the previous owner she previously had 3 blade 22" x 23". Current props look enormous but engines nave no problem reaching max rpm so they can't be too big.
As to weight, not put mine on a load cell but suspect 8,000+ kg is going to be closer to the truth. Boats only get heavier with age!
fergie_mac66
Well-Known Member
Latestarter1
Well-Known Member
To spin a pair of four blade 22X24 wheels at 1,579 rpm with 8 tonne displacement will absorb around 582 hp.........Impressive
MYStargazer
Well-Known Member
Not sorted yet, but getting there, thanks.
The largest recommended to me by suppliers so far has been 20", with pitches of 17" to 19". And that's with only three blades. In line with Paul's (Latestarter1) calculations above.
Not sure how engines react to props which are too large - what speeds are you making?
MYStargazer
Well-Known Member
brunel made props not that different !
That was exactly my first thought! But they haven't got back to me with a quote so I think all-in-all this is an unconventional alley I won't be venturing down...