Offshoreonly.com - Prop rake ??

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Quote:

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

Anyone ever try the higher rake angle props on a heavy flat bottom?
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..

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

How do you know the correct prop rake you need? Lets say a boat,, 29Scarab, twin small blocks 25 mirage, 72 @5000....

Quote:

Originally Posted by bbladesprops (Post 4549125)

I have the same question. 30' old heavy V. Arneson conversions, 700HP. 18" dia.? 18 degree rake?

Twins of course. Trying 5 blades, also turning in.

Quote:

Originally Posted by 01scarab29 (Post 4549471)

How do you know the correct prop rake you need? Lets say a boat,, 29Scarab, twin small blocks 25 mirage, 72 @5000....

Use the least rake you can that provides adequate boat attitude. 18 degree should be most common.

Brett

Quote:

Originally Posted by Cary 29 (Post 4549482)

I have the same question. 30' old heavy V. Arneson conversions, 700HP. 18" dia.? 18 degree rake?

Turning in is what I would do. Also with arnesons, I typically am going to 20-22 degree. Not always needed but rarely a step backwards.

Brett

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.

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.

Quote:

Originally Posted by dmaxximus (Post 4555109)

You boat was actually a 3-4 blade boat. Reason being, at this time straight v bottom boats were the norm. These boats tended to have lower X dimensions and required more transom leverage to create bow lift for better speeds and handling characteristics.
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