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Old 04 July 2005, 05:35   #1
Country: Australia
Town: Brisbane
Make: Gemini
Length: 5m +
Engine: Evirude 135
Join Date: Feb 2005
Posts: 14
Prop Selection Woes

Hi all

After some guidance on prop selection fine points

Currently have a Gemini 550 with a 135 Hp evinrude fitted.

Prop is a cupped 14 3/4 x 17. (Evinrude SST)

Performance - WOT is 5000 rpm giving 38 knots with 3 people and 6 tanks, or

35/36 knots with 5 people and 12 tanks. (4900 rpm in this case)

Running this through a prop calculator give a very low slip of between 5 and 11 percent depending on load.

My only problem with this is WOT is given as 5250 to 5750 rpm. Since I cannot get the motor to rev to this range I am wondering if the boat is fundermentally propped wrong or should I be satisfed with the choice of prop I have

If I was to change props should I be looking to decrease dia to allow it to spin to full revs. looking at buying a SS 14 1/2 x 19 non cupped prop as an alternate. Do people think this would be a good choice, or more likely to be very similar overall.

Boat is used for diving. Usual load is 4 divers and 14 tanks, or 5 divers and about 10 tanks.

Boat currently criuses at 28 to 30 knots (3900 to 4200 rpm) with nominally 11% slip range and I am happy with the hole shot and fuel economy. Am uncertain about the motor not reaching WOT range in practice for the most part.

All thoughts appreciated


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Old 04 July 2005, 05:54   #2
Pete7's Avatar
Country: UK - England
Town: Gosport
Boat name: April Lass
Make: Moody 31
Length: 9m +
Join Date: Aug 2001
Posts: 4,865
Matt, the 14 1/2" is the diameter and the 19" pitch. 19" will be a courser pitch which is going the wrong way, you need 15" - 16". However try a borrow a prop first just to see what its like.

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