Powerboat Forums at SpeedWake banner
1 - 8 of 8 Posts

·
Registered
Joined
·
195 Posts
Discussion Starter · #1 ·
I have a Scarab 7300 lbs boat 31' with two 500hp. When I take off I have to make sure it doesn't rev over 2800 rpm or it will cavitate and spin out. I use a 27" 3 blade mirage. Sould I go to a four or five blade prop to stop the cavitating or will it help the take off? Also is the three blade prop better for speed?
 

·
Premium Member
Joined
·
12,363 Posts
My 30' Sutphen was extremely difficult to get on plane with Mirage props --- it would blow out easily. With 4 blade Bravo props, it would jump right on plane with full throttle, no cavitation.

You definately need to go to 4 blade props.
 

·
Registered
Joined
·
5,508 Posts
3-blade props are usually faster but yes a four blade will help with planing, corneringing and have better cruise.

What RPM to do spin the 27 Mirages and are they standard Mirages or Mirage Plus?
 

·
Registered
Joined
·
132 Posts
The 4 blades will help with your issue and it sounds like we have a similar set-up. I have a set of 4 blades on mine right now and I have no issues getting on plane. I was considering trying a set of 3-blades for a little more top end when I wanted to go out and fly around. For all around use I think the 4 blade is still the way to go.

What WOT speeds are you seeing with the 27p 3 blades and what is your WOT rpm? Might be able to give a suggestion with that info.
 

·
Registered
Joined
·
195 Posts
Discussion Starter · #5 · (Edited)
The WOT speedo says 85 mph, not sure how accurate that is though, i need a GPS to check it. BUT the RPM reads almost 5000. If I use the 28" 4 blade will I loose 3 to 4 MPH? Your thoughts? The 3 blades are Mirage only.. whats with the Mirage Plus? What is your power plant in your Scarab
 

·
Registered
Joined
·
5,508 Posts
A Mirage will ventilate much easier than a Mirage Plus but they are faster on many hulls. The Mirage Plus has more cup and progressive pitch so they stay connected to the water better when planing and cornering.

I would either try a set of 27 Mirage Plus or Bravos. If you go with 28s you will lose a little rpm, if you go with 26s you will gain a little.
 

·
Registered
Joined
·
1,896 Posts
blackhawk said:
A Mirage will ventilate much easier than a Mirage Plus but they are faster on many hulls. The Mirage Plus has more cup and progressive pitch so they stay connected to the water better when planing and cornering.

I would either try a set of 27 Mirage Plus or Bravos. If you go with 28s you will lose a little rpm, if you go with 26s you will gain a little.
I use mirage plus 25" and don't really have a problem unless I mash the throttles (which I never do). I like my plus props. They bite pretty well.
 

·
Registered
Joined
·
132 Posts
The 27p mirage plus props aren't a bad idea. If you run a labbed set you could get your rpms up to 5200 - 5300 which would have the potential to increase your top speed a few mph and help with your ventilation issue.

If you want to try the 4 blades I would start with the 26p bravo 1s. The 4 blades usually cut a few RPMs from the numbers you see with the 3 blades so with a standard prop I would start by moving down to keep rpms up. If you are willing to spend a few more $ on labbed props you could try the 28p and hopefully you would stay near the same rpms. Assuming you can stay at 5000 with 28s you would probably gain 2 mph assuming slip remains constant.

I am running a pair of 502 HO crates with a few mild upgrades good for about about 485/550 per side. I have a pair of 24p 4 blades on their right now and running 70-72 (GPS) at 5200. The boat has great manners with these props but I am considering a test with 27p labbed 3 blades to see if I can still turn at 5000+ to increase the top speed for a few runs each year. The other option would be to try a set of labbed 26p and go to those permanently. Haven't had time in the last few weeks to think about it much so I haven't decided yet. I did test a set of 25p 3 and got them up to 5300 and had a little room to go. I didn't want to push it and over rev so I went back to the dock figuring that I could go to the 27p 3 blade if it were a labbed prop and stay down in the 5000-5200 range which should be good for 4-6 mph depending on where it lands.

When I was looking around I found a few online slip calculators through google that might be helpful to you if you want to look into it further if you know your gear ratio. If I get around to messing with it more this year I'll let you know how it turned out. Let us know what you decide to do and what rpm/speed results you see.
 
1 - 8 of 8 Posts
Top