Prop advice for stepped 50kn 8.2m diesel rib

Scooby Doo · 14 June 2015, 20:28

Hi everyone,

I'm new here but have been lurking for a while. I run an Arctic Blue 27 with a double stepped hull, 350hp diesel and bravo 1 x diesel drive. She's 3 tonnes on the water and does 50knts.

I'm currently running a 28P Bravo 1 prop and was wondering if switching to a 3 bladed Mirage Pro would increase the top end a little. I've also been led to believe its normal to up the pitch by 1 when changing from Bravo 1 to Mirage Pro?

With the 28P Bravo 1 she runs well at max RPM at WOT at normal load.

Any advice would be gratefully received.

Scooby Doo · 15 June 2015, 18:10

I need to buy a new prop as a spare and was wondering if its worth taking a punt on a Mirage Pro to free up a few more knots top end.

Worry is it'll get les grip in the corners and cavitate more when pulling off. I've read the US threads which have lots of info for Outboards but nothing for inboard boats.

Has anyone swopped from Mirage Pro to Bravo 1 or visa versa?

Searider · 15 June 2015, 19:50

Have you looked at the mercury propeller selector on their website?

kubcat · 16 June 2015, 21:54

50 knots is moving for such a big boat.

There is a guy on the offshore boat forums that has a prop shop that is considered the guru for fast boats your size and above. Stern drives are his specialty.

He is in the U.S but we have racers in Australia sending him props for tweaking.

Google BBlades.

Mollers · 16 June 2015, 22:00

That must be a very smart hull if that rig will do 50knts.

Scooby Doo · 17 June 2015, 04:08

Thanks for the replies I've sent an email to a couple of prop guru's in the states and got some really helpfull answers. Looks like the

The hull is fairly nippy, double stepped with tricky chimes. The cavitation plate is pretty close to the bottom of the V which helps top end with a good prop. Below is a link to a really bad picture of the hull. I will post some better pics once I get home (early July).

The hull is the 2nd generation; the 1st was based on a Norwegian racing rib hull. The crazy Norwegians have one of these boats with a 700hp V10 that pushes 70-80knots. So I'm not even stretching its legs at 50.

http://www.rib.net/forum/f64/hello-f...tml#post682453

Mollers · 17 June 2015, 19:22

Quote:

Originally Posted by Scooby Doo

The crazy Norwegians have one of these boats with a 700hp V10 that pushes 70-80knots. So I'm not even stretching its legs at 50.

As you have half of the horsepower and more weight, that's impressive.

Scooby Doo · 18 June 2015, 01:06

The boat weighs 2.7t light and 3t heavy. One of the reasons for the good speed is the engines huge torque, 704nm (519ft-lbs)! Which means it can turn a high pitch prop, pretty much the highest pitch mercury make.

B58 · 18 June 2015, 11:04

I wouldn't recommend Mirage Plus, its not designed for diesel, Bravo 1 is the better choice, if you want that extra mph, you could try a lab finished Bravo 1 prop will give you the edge but I think your doing well as it is.

Scooby Doo · 18 June 2015, 16:33

Thanks for the replies, consensus from the states seems to be stick with the Bravo 1. A Mirage Pro 29P may give 2 knots more top end but will suffer everywhere else.
As B58 said, perhaps during the winter I'll get the prop lab finished and see what difference it makes.

So it’s another Bravo 1 as a spare then..................better start saving!