Propeller rake

[dognduck]

Lets talk about propeller rake. Can some one walk me though what it does? I see on outboards, alot of props have positive rake. With an inboard, in particular the B31, what is a good rake angle? pos or negative? and how does it affect the boat?

[Carl]

I'm going with the layman's terms...rake condenses the water pattern being thrust astern.

This is done by kind of pushing the blades back, think of tulip petals compared to daisy petals...daisys are flat 0 rake. Tulips have positive rake.

What rake does for performance...that I'll leave to someone else

[pschauss]

Carl,

That’s a great explanation.

I read your post to Francey and her response was “Wow, fabulous!”

[dognduck]

Does positive rake, (angle aft) provide lift of the stern?

[Tony Meola]

If you do a search under Bob Lilco you will actually find a number of discussions on this.

Bob customized his props more than most of us would even attempt to do. I know of one other person who followed the same path and actually saw the same results Bob did, but his was on a 38 Henriques. They both saw an increase in efficiency and how the boat cruises. All positive.

The issue you may have with playing too much with the rake is idling down low enough to troll slow for stripers if that is your thing.

I would post the link up to the discussion, but it will not open for me. It was a discussion from 2016. Just search using "rake"

[bob lico]

i like that explanation carl and when i spent that midnight oil in back of a prop shop raking inboard props (you can't do that) that's exactly why i did it! one other simple question how do you slow down or stop a canoe or kayak??????? please tell us what you do with the paddle

[Carl]

But Bob we have…


On my Penn Yann I gained almost 13 mph adding rake on a tunnel drive with an OJ Johnson wheel. On my Bertram…I could not get close to spinning my gas prop fast enough to get on plane. Rake eats power…


Dad with diesel was able to add rake and his boat picked up speed, about 2mph


As to adding lift or...I really have no idea.

[Ironworker]

But Bob we have…


On my Penn Yann I gained almost 13 mph adding rake on a tunnel drive with an OJ Johnson wheel. On my Bertram…I could not get close to spinning my gas prop fast enough to get on plane. Rake eats power…


Dad with diesel was able to add rake and his boat picked up speed, about 2mph


As to adding lift or...I really have no idea.

Penn Yann!!!!

I haven't heard those words in a long time. Back in 1982 a friend bought a 24' PY with a tunnel drive in Crystal River Florida for $1500. He found a trailer and I pulled the boat with my 4 WD Luv truck. That was the slowest friggin boat I've ever been on. I think we topped out about 8 MPH and were getting about 0.5 mpg or possibly less fuel mileage. Hell everybody in Crystal River would pass us.

[bob lico]

carl there is no one change "fix all " with prop. design. you need to have massive engine torque to customize the props. to max you fuel economy and your planing angle at cruise. i was trying to get at that with the paddle statement in my above post ,trim tabs would kill my boat. totally destroy all the custom prop work.tabs are pushing bow down and rake pushing it up.remember the paddle theory to slow or stop a kayak you put the flat side of the paddle down in the water (like trim tabs) . the object is to keep the boat level but lift the entire boat out of the water from bow to mid ships to eliminate the drag but you must coincide this with your engine torque curve for maximum fuel economy or speed. look at those single outboard tunnel race boats only the prop is in the water! and a hair more they roll flip backwards. you get the idea. extreme cup and rake only works with massive prop torque 4 cylinder need not apply.31 Bertram fact number 1 the shaft angle from 14 degrees stock to highly custom 10 degrees and everything in between will push the bow down and the faster you go the more the bow will be push down by this factor until lt gets to the the point when it starts "ruddening",very bad situation!!!!!

[trace elements]

Bob,

Do you have any input on props for gas-powered boats? I am going to have my 3-blade 18x19 reworked over the winter.

[Carl]

Bob, what you just said makes a ton of sense to me, thanks, I never quite understood all the prop improvements, especially rake. I knew what it is...some idea of what it did for me. We really only dealt with workboats, tugs, commercial fishing and a slew of recreational boats. Race boats, we did nothing, high performance boats, we did nothing. Lots of sportfish boats...

[Carl]

Penn Yann!!!!

That was the slowest friggin boat I've ever been on. I think we topped out about 8 MPH and were getting about 0.5 mpg or possibly less fuel mileage. Hell everybody in Crystal River would pass us.

Yup...mine was a 23' and I had a few acquaintances with the same boat. Boat ran 13mph, just barely on plane even when pushing with a regular wheel. It would top out at 16pmg. A friend with a prop shop said I should be going MUCH faster and loaded me up with a bunch of props to try.

The OJ Johnson, a ski boat wheel had me slow cruising at 21mph and could push to 25-26. She topped out a tad over 30. Fuel burn was better with the OJ at 21mph than the regular wheels at 13.

I kept no secret from the other guys, told them exactly what wheel to use...even offered to lend them my spare to try. The price of the wheel kept them away...dumb. They wasted a ton of fuel and time trying to save a couple hundred on a wheel.

[Tony Meola]

Bob,

Do you have any input-on props for gas-powered boats? I am going to have my 3-blade 18x19 reworked over the winter.

Not sure if I understand everything Bob is referring too, but it sounds like you may not have enough torque for it to work with a gas engine. You would really need to talk with a good, I mean a really good prop shop.

[Carl]

Bob,

Do you have any input on props for gas-powered boats? I am going to have my 3-blade 18x19 reworked over the winter.

Before you do anything chart your performance. Speed at each rpm, fuel burn and your WOT RPM.
Without that info, no good prop shop is going to offer any suggestions.

Any that do...walk away.

[bob lico]

correct tony you need at least 1000 pounds of torque output at cruise speed to utilize any effects of raking the propellers. number one you have to lift the boat up to ride on inners strakes and yes a pair of 454" mercruisers can attain 26 knots but cannot sustain that speed with any fuel economy so if you were to rake the props on a gas boat it would not be beneficial .

[trace elements]

correct tony you need at least 1000 pounds of torque output at cruise speed to utilize any effects of raking the propellers. number one you have to lift the boat up to ride on inners strakes and yes a pair of 454" mercruisers can attain 26 knots but cannot sustain that speed with any fuel economy so if you were to rake the props on a gas boat it would not be beneficial .

Understood, thanks Bob.

I am going to try Atlantis Propeller in Jersey. They have propscan. I'm getting 24 knots at 1.0MPG, which is good enough for me. I just want to see if I am leaving any performance on the table. I think some high blade-area props from ACME might be the ticket, but at $1300 a side, its a tough sell.

[bob lico]

that's about right for 454" mercruisers. you can go to fuel injection it really helps on a gas boat.

[Tony Meola]

Understood, thanks Bob.

I am going to try Atlantis Propeller in Jersey. They have propscan. I'm getting 24 knots at 1.0MPG, which is good enough for me. I just want to see if I am leaving any performance on the table. I think some high blade-area props from ACME might be the ticket, but at $1300 a side, its a tough sell.

1200 a side should be NIbrals. You can save a few dollars with Bronze, but you lose some efficiency due to flexing of the blade which will give you more prop slip.

Where are you in NJ?

Atlantis does a good job. They pretty much hit it on the nose first time out after we repowered. I had the cup tweaked after the first year and that actually helped.

[trace elements]

1200 a side should be NIbrals. You can save a few dollars with Bronze, but you lose some efficiency due to flexing of the blade which will give you more prop slip.

Where are you in NJ?

Atlantis does a good job. They pretty much hit it on the nose first time out after we repowered. I had the cup tweaked after the first year and that actually helped.

Tony, I am in Lacey. I worked with Ocean Propeller in Manchester last year, they weren't bad, but couldn't provide me with an actual sizing of my existing props. That's why I was looking forward to using Atlantis with their prop scan.

[Rawleigh]

A little bit of cup helped with my 440 Chryslers, but it has been so long ago I cannot remember the exact specs.

[Tony Meola]

Funny, I have a house in Lacey also. My boat is kept for the winter at Rick's Marina. Been at Ricks since 1987. Used to Winter at South Winds before that.

Small world.

Due to some unforeseen surgery I did not go in the water this year. I was using Atlantis also for prop work. I used Ocean for the first time last year, but have not had a chance to try the props out. We will see this summer how they work. Problem I have, is I asked the yard if Ocean said how bad they were, they said they did not say. Atlantis gives you a full break down and they use computer analysis to make the adjustments.

I hope Ocean did a good job. If I lose speed, I will not be happy.

[trace elements]

Tony,

Hope you're doing better. I wasn't disappointed in Ocean, just looking for a little higher degree of accuracy since it's the first time I'll have these props done.

[Tony Meola]

You will be happy with them. If you do not want to drive across the State, take them over to Forked River Diesel. He ships all his props over to Atlantis.