props?

[yeungy]

hey guys i have an 75hp etec and i am looking to buy a new prop 4 it purely 4 extra speed. im justing wondering what props you hav chosen 4 your motors and what type of speed and price differences.
cheers yeungy

[Brad Y]

Need to know your current

rpm at WOT
max speed
prop size

Cheers

Brad

[yeungy]

well im not sure about rpm etc but i assume its the same as every other 75etec. but the top speed is around bout 65kms with 1 person 60 with 2.

[Brad Y]

Need to know exact figures mate, otherwise it wont be accurate. Eg if your getting about 5300 rpm at the moment you probably wont get that much more out of it.

If you were getting 5800 rpm then you could increase prop size 2" and that will bring you into the recommended rev range and get extra speed. Vice versa too, if you are revving about 4600rpm you can decrease prop size and spin faster to get speed.

The prop size will be on the prop itself and all you need to do to get WOT rpm is trim it out far as it will go before it ventilates at full throttle. This will also help in determining if you should get a 3 or 4 blade prop and having cupped blades too.

[Rossco32]

Originally Posted by yeungy

hey guys i have an 75hp etec and i am looking to buy a new prop 4 it purely 4 extra speed. im justing wondering what props you hav chosen 4 your motors and what type of speed and price differences.
cheers yeungy

First off: I see with two POB you lose 5 kph....... Your tabs is a plate hull (heavier than a pressed hull ie Quintrex) isn't it?
Do you trim it correctly?

Prop pitch is like the gearing in a car:

Low pitch = high engine RPM & lower speed (but more punch off the mark)

High pitch = lower Rpm but higher speed (Slower off the mark)

If you're under pitched (like a ski boat) : the boat will accelerate fast but to a lower top speed

If you're over pitched : the engine will not produce it's full potential (rpm below peak power) will drink like a fish and feel sluggish off the plane.

Boat props are a compromise.

I have two props (both 4 bladers) a 19" pitch for tournament loads (ie full live wells, tourny gear etc) and a 21" for when I go out by myself.

If I use the 21" during a tourny I actually halve my fuel range, the boat is a pig in 8 knot zones with the 21 due to too much prop slip (2800 rpm = 8 knots but 3000 rpm = 22 knots, the boat falls onto the plane and unloads the engine a LOT!).
The top speed is lower too because the added tourny weight creates the situation where the engine can't reach the top of it's powerband (5600 rpm on my 90). Did I mention drinking fuel like a fish?

The 19" is perfect for my rig in tourny's, speed it a little lower (38 knots with the wells full) but economy is nice (averaging 35 litres for a perth event).

All hulls are different, some hulls will only go so fast due to the design regardless of what prop (or Horse power) you throw at them.
The amount of cup & rake is important too as Brad said, Rake adds lift to the stern and cupping reduces prop slip and cavitation.

you can work out the basics for yourself using this online prop calculator:

http://www.go-fast.com/Prop_Slip_Calculator.htm

The formulas are pretty simple, remember that fast planing hulls have an average of 10% prop slip.

Engine height (prop depth) is another thing to look into, running a 4 blade prop so it pierces the surface as the boat nears top speed gives you extra rpm (you can more pitch) to play with too. but at the cost of 'grip' in turns.

Add the pros and cons.

Mike @ just propellers is the man you need to speak to, he will probably know exactly what combination of blade number, cup & rake and pitch is best to suit your rig off the top of his head (seriously!!).