I'm considering the purchase of a new Yamaha T9.9 high-thrust long-shaft kicker for my Mac25 to replace "Sybil", my psychotic Mariner 9.9 for purposes of lower noise and better reliability. Apparently there are three different propellers available for the T9.9, all being 11 3/4" diameter, but with pitches in 9-1/4, 11, and 12-1/4". They are given speed ratings of 7-13, 10-16, and 12-18 respectively. I would assume that, since the hull speed of the Mac25 is around 6.7k, that the proper pitch would be the 9-14" prop. Any comments, suggestions, scientific theory or general BS on the subject would be most appreciated prior to placing an order.
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Prop' Pitch
When changing propeller pitch or diameter you need to know how the propeller performs before you change it. By lowering the pitch one inch (1"), the motor will gain 150 - 200 RPM. By increasing the pitch one inch (1"), the motor will lose 150 - 200 RPM. By increasing the diameter one-quarter inch (1/4"), the motor will lose 150 - 200 RPM. By decreasing the diameter one-quarter inch (1/4"), the motor will gain 150 - 200 RPM. 1" Pitch = 150 - 200 RPM Lower pitch to gain RPM Increase pitch to lose RPM 1/4" Diameter = 150 - 200 RPM Decrease diameter - gain RPM Increase diameter - lose RPM ITG Marine-Outboards - Choosing A Propeller http://www.marine-outboards.com/choosing-propeller.htm A basic propeller tutorial from Caravelle Powerboats: http://www.caravelleboats.com/propeller%20info.htm Regards, Gord
When changing propeller pitch or diameter you need to know how the propeller performs before you change it. By lowering the pitch one inch (1"), the motor will gain 150 - 200 RPM. By increasing the pitch one inch (1"), the motor will lose 150 - 200 RPM. By increasing the diameter one-quarter inch (1/4"), the motor will lose 150 - 200 RPM. By decreasing the diameter one-quarter inch (1/4"), the motor will gain 150 - 200 RPM. 1" Pitch = 150 - 200 RPM Lower pitch to gain RPM Increase pitch to lose RPM 1/4" Diameter = 150 - 200 RPM Decrease diameter - gain RPM Increase diameter - lose RPM ITG Marine-Outboards - Choosing A Propeller http://www.marine-outboards.com/choosing-propeller.htm A basic propeller tutorial from Caravelle Powerboats: http://www.caravelleboats.com/propeller%20info.htm Regards, Gord
Prop' Pitch
When changing propeller pitch or diameter you need to know how the propeller performs before you change it. By lowering the pitch one inch (1"), the motor will gain 150 - 200 RPM. By increasing the pitch one inch (1"), the motor will lose 150 - 200 RPM. By increasing the diameter one-quarter inch (1/4"), the motor will lose 150 - 200 RPM. By decreasing the diameter one-quarter inch (1/4"), the motor will gain 150 - 200 RPM. 1" Pitch = 150 - 200 RPM Lower pitch to gain RPM Increase pitch to lose RPM 1/4" Diameter = 150 - 200 RPM Decrease diameter - gain RPM Increase diameter - lose RPM ITG Marine-Outboards - Choosing A Propeller http://www.marine-outboards.com/choosing-propeller.htm A basic propeller tutorial from Caravelle Powerboats: http://www.caravelleboats.com/propeller%20info.htm Regards, Gord
When changing propeller pitch or diameter you need to know how the propeller performs before you change it. By lowering the pitch one inch (1"), the motor will gain 150 - 200 RPM. By increasing the pitch one inch (1"), the motor will lose 150 - 200 RPM. By increasing the diameter one-quarter inch (1/4"), the motor will lose 150 - 200 RPM. By decreasing the diameter one-quarter inch (1/4"), the motor will gain 150 - 200 RPM. 1" Pitch = 150 - 200 RPM Lower pitch to gain RPM Increase pitch to lose RPM 1/4" Diameter = 150 - 200 RPM Decrease diameter - gain RPM Increase diameter - lose RPM ITG Marine-Outboards - Choosing A Propeller http://www.marine-outboards.com/choosing-propeller.htm A basic propeller tutorial from Caravelle Powerboats: http://www.caravelleboats.com/propeller%20info.htm Regards, Gord
Re-Choosing Propeller Pitch
*box The Macgregor 25 with a 9.9 engine.......... Classic Hull Speed 6.43 knots 9in prop... with 4 blades should give you full hull speed at 2837 rpm's. If you choose the 3 blade prop your rpm's to aquire hull speed will be aprox. 2005 rpm's. This is a ruff estamate figured by hp of the engine displacement of the hull and weight of the boat. Hope this helps some.
*box The Macgregor 25 with a 9.9 engine.......... Classic Hull Speed 6.43 knots 9in prop... with 4 blades should give you full hull speed at 2837 rpm's. If you choose the 3 blade prop your rpm's to aquire hull speed will be aprox. 2005 rpm's. This is a ruff estamate figured by hp of the engine displacement of the hull and weight of the boat. Hope this helps some.
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