Prop rake ??
#1
Prop rake ??
A greater rake angle generally improves the ability of the propeller to operate in a ventilating situation. Ventilation occurs when blades break and re-enter the water’s surface — such as occurs with 1) a Bravo sterndrive installed with a high “X” dimension, 2) a surfacing drive (NXT1, NXT6 SSM or M8) or 3) an outboard installed or jacked high on a transom. In surfacing operation, higher rake can hold the water better as it’s being thrown into the air — deflecting it aft and creating more thrust.- Fred Kiekhaefer
Seems like it would be the ticket for high x dimension cleavers and trying to lift the heavy bow of my boat.
Prop guy from Mercury thought the higher rake props have high slip numbers in the midrange but wasn`t sure..
#2
Offshoreonly Advertiser
Offshoreonly Advertiser
From a bow lifting standpoint, higher rake angles create better transom leverage and in turn better bow lift. If natural bow lift is presnt, higher rake angle can create more drag and less speed.
Brett
Brett
__________________
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
#3
Registered
Join Date: Jun 2012
Posts: 275
Likes: 0
Received 0 Likes
on
0 Posts
How do you know the correct prop rake you need? Lets say a boat,, 29Scarab, twin small blocks 25 mirage, 72 @5000....
#6
Offshoreonly Advertiser
Offshoreonly Advertiser
Brett
__________________
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
#7
Offshoreonly Advertiser
Offshoreonly Advertiser
Brett
__________________
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
#8
Brett,
First of all thanks for all your info. I've been told 5 blades over 4, still searching for the correct blades. What do you suggest???
41 Apache, 900HP, SSM3A, very low X (she sits in the basement). I have a set of 17.5x28 4 Blade, she jumps on plane quick but seems to be lacking in cruising and top speed.
First of all thanks for all your info. I've been told 5 blades over 4, still searching for the correct blades. What do you suggest???
41 Apache, 900HP, SSM3A, very low X (she sits in the basement). I have a set of 17.5x28 4 Blade, she jumps on plane quick but seems to be lacking in cruising and top speed.
#9
Registered
Join Date: Oct 2009
Location: Splendora, TX
Posts: 658
Likes: 0
Received 0 Likes
on
0 Posts
It's all a big guess. I have better cruise speed spinning out, not in. Gained 4 mph at 3500rpm. A little harder to get on plane spinning out. 4200rpm to get on plane. Nevertheless, slip numbers are high in both cases. . 1994 Cigarette Top Gun - Arnesons - 28 RAKE. 27% slip at 3500rpm. 5 BLADES. I think the only way to help my slip is to convert my props to round ear or change to 6 blades. It all cost a ton of money and you will still be guessing. It's all a gamble.
#10
Offshoreonly Advertiser
Offshoreonly Advertiser
Brett,
First of all thanks for all your info. I've been told 5 blades over 4, still searching for the correct blades. What do you suggest???
41 Apache, 900HP, SSM3A, very low X (she sits in the basement). I have a set of 17.5x28 4 Blade, she jumps on plane quick but seems to be lacking in cruising and top speed.
First of all thanks for all your info. I've been told 5 blades over 4, still searching for the correct blades. What do you suggest???
41 Apache, 900HP, SSM3A, very low X (she sits in the basement). I have a set of 17.5x28 4 Blade, she jumps on plane quick but seems to be lacking in cruising and top speed.
Plus, we weren't even making 5 blade props at this time. 5 blade props will typically give you better cruise speeds and economy since they have lower slip numbers in the middle. But, because they have more drag, they are often slower as the throttle is pushed forward as compared to their 3-4 blade counterparts.
So, if cruise is the most important goal, 5 blade props are a option.
Personally, I would be looking to dial in a set of 4 blades. Adjusting your X dimension and prop diameter will be important.
Brett
__________________
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]
Brett Anderson / BBLADES Professional Propellers
920-295-4435 http://www.bblades.com/
[email protected]