New prop - what a difference

[Portholme]

I had my first outing with my new prop this weekend and the difference was great.

I have a 2400 SCR, with 2000 V8, 220 hp with Alpha 1 and was always concerned about the cruising RPM which was 4,000 rpm to get 23 knots. WOT was 4,800 for 31 knots. It seemed to be working too hard. the prop was a 15 x 17 three blade, which I know was as per delivery spec because I have the Delivery Advice Note from when she was brand new. What i don't know was if this was a standard or owner specified prop.

I have replaced it with an ally 4 blade 14.5 x 17 FOUR blade prop and the difference is remarkable. Notable improvements are acceleration, hole shot and cruising rpm. It's now at 3,500 rpm for a comfortable 21 knot cruise speed and it doesn't feel like it's labouring. This was upwind, downwind, cross wind averages with a full camper canopy up. I'm excited to check it in the warmer weather with no canopy.

My old boat was a 27 foot RIB with a 225 opti outboard and i expected the V8 inboard to be lazier but was disappointed until now.

The prop cost me £150 but i guess it will save me that much in fuel in around three months (rough maths).

[shrew]

What is your WOT RPM's now?? WOT @ 4,800 would have been perfect.

[Portholme]

don't know yet. ill let you know when conditions allow

[Water Babe]

Hi! I am wondering about this...thinking to change my prop from 3 blade to 4 as well. How did it work out?

[Portholme]

It has worked out really well for cruising speeds. Top speed seems compromised because WOT is now around 4500 rpm. I've got to 32 knots but prefer crushing at 3400 rpm at around 22 to 27 knots depending on conditions. My prop is an alloy one and was pretty cheap to try. I'm sticking with it too!

[shrew]

Well, you're still within the max rpm range (4400 - 4800). Top speed and commonly hole shot (time to plane) get decreased, while engine load increases. However, theoretically fuel consumption also gets decreased as the RPM also gets lowered for a given speed.