Prop Rake angle

[kfxguy]

WHY do you think it's "holding the bow higher in the air"?? Think about it.

Because the prop has more bow lift. I won't argue the fact that it may be pushing the stern down because there has to be a pivot point but there is bow lift. Call it whatever you like. Here's a couple videos to back what I'm saying up.


This is 442 props with hardly any rake.

[kfxguy]

And no changes at all except to 17 degree rake abc props. You can clearly see here the nose (bow) of the boat is clearly being lifted higher. This is the same battery packs too. Brought boat in and swapped props is all I did.

[Darin Jordan]

Because the prop has more bow lift.

The prop is on the back of the boat. It's not capable of providing lift to the bow. Changing thrust angle I suppose can. Increasing the AoA of the hull bottom can. More speed, which puts more air under the bow can. The prop doesn't.

And it doesn't "push the stern down". It either lifts, or doesn't lift. If it doesn't lift, then the stern settles deeper, which in turn raises the bow.

You can believe what you will, but a correct understanding of the dynamics of exactly what is happening will one day help you advance your RC programs...

Nice videos.

[Bduncan]

TO raise the bow, the thrust line of the propeller must project forward and be below the boat's center of gravity. Also, the deeper the propeller runs in the water, the farther below the boat's center of gravity it will be. Therefore, its thrust will have more leverage to raise the bow.

It may be that these "bow lifting" propellers actually accomplish this because they tend to remain deeper in the water when running at high speed. That is, they do not tend to lift themselves up toward the water surface until they break out or ventilate. Because they run slightly deeper, the thrust they produce has a longer arm on the boat's center of gravity, and therefore it can raise the bow more.

Hence Travis is correct

[kfxguy]

The prop is on the back of the boat. It's not capable of providing lift to the bow. Changing thrust angle I suppose can. Increasing the AoA of the hull bottom can. More speed, which puts more air under the bow can. The prop doesn't.

And it doesn't "push the stern down". It either lifts, or doesn't lift. If it doesn't lift, then the stern settles deeper, which in turn raises the bow.

You can believe what you will, but a correct understanding of the dynamics of exactly what is happening will one day help you advance your RC programs...

Nice videos.

If I was good at drawing I'd draw it out for you so you can see what I mean. I'll try to give you an example and put it in words as best I can so you can see where I'm coming from. I'll also reiterate that I'm calling it bow lift (because really it is anyway) just to make things less complicated for the newbs reading this thread. So..imagine this in your head...grab a garden hose with an adjustable nozzle. Set it to fan where it spreads the water in a great area. There's not much push, right? Well look at the rooster tail on a low rake angle prop. It's a wide and tall roost. The let's tighten the nozzle up to where the water spray is tighter. Same thing a higher rake does. You can feel it pushing more straight now. Well a higher rake concentrates that power cone and makes the spray angle tighter in turn pushing in a straighter angle on the boat causing the nose to rise more while essentially pivoting the rearmost bottom of the boat on the water. I don't know how else to explain it. You (whomever it may concern) can pick apart my example and my explanation all you like, but I just posted two back to back video within five minutes of each other illustrating what I mean by the bow being lifted. Not quite sure why some people find it imperative to correct someone when they aren't wrong.

[Bduncan]

and to further support Travis,
Propellers which are not "bow lifting" thus tend to be "stern lifting." What is meant by this is the propeller tends to walk itself upward in the water until it reaches an equilibrium, and it runs near the surface of the water. As a result, its thrust line is nearer to the boat's center of gravity, and therefore the lever arm is shorter. So the tendency to raise the bow is not as great. In fact, the tendency is to raise the stern slightly, so these propellers are known as "stern lifting" propellers.

An extreme example of a stern lifting propeller is the cleaver type, which tends to push itself up and out of the water until it runs with only that portion of the blades below the propeller shaft actually in the water.

[dasboata]

another thing that needs to be considered in Bow lift,, is prop depth in the water, ( i know about strut angle I am not talking about that now ) a lower prop in the water will push your bow down, and a higher prop in the water will not,, think of it as a seesaw BETWEEN THE BOW AND TRANSOM

[Bduncan]

well said dasboata. I find the 1815 and 1915 on my mono and cat can be run lower with a neutral strut angle. I get the best of both worlds. my bow is where it needs to be and performance is exceptiona not to mention the load. Probably why an 1815 (17degree) rake is about 10mph faster than a M645. Some of the debate above can probably be deduced to semantics amd or chicken or the egg. (do trim tabs on a fishing boat lower the bow when they are trimmed down? or is it the transom lift resulting from the trim tabs lowering the bow).

[kfxguy]

another thing that needs to be considered in Bow lift,, is prop depth in the water, ( i know about strut angle I am not talking about that now ) a lower prop in the water will push your bow down, and a higher prop in the water will not,, think of it as a seesaw BETWEEN THE BOW AND TRANSOM

^^^^^ this guy knows props more than all of us combined so I'd take his word before I'd take anyone else's.

[Bduncan]

If I run say a 1815 in same posistion as a M645 my boats will blowover at a lower speed. In conclusion, if a 17 degree rake prop has to be set at more of a neutral posistion and a lower strut posistion to avoid excessive bow angle and blow overs. it would be fair to say a 17 degree rake ABC in the same posiston as the M645 the bow is lifted more regardless. Dasboata made seesaw anology. Is the lighter kid on the seesaw in the air because he is lighter or is the heavier kid on the ground because he is heavier?

[keithbradley]

Whatever you say.....

I'm not quite sure how this is a response to what I posted...
I won't argue it. Keep boating, you WILL understand exactly what I'm talking about at some point.

[keithbradley]

TO raise the bow, the thrust line of the propeller must project forward and be below the boat's center of gravity. Also, the deeper the propeller runs in the water, the farther below the boat's center of gravity it will be. Therefore, its thrust will have more leverage to raise the bow.

It may be that these "bow lifting" propellers actually accomplish this because they tend to remain deeper in the water when running at high speed. That is, they do not tend to lift themselves up toward the water surface until they break out or ventilate. Because they run slightly deeper, the thrust they produce has a longer arm on the boat's center of gravity, and therefore it can raise the bow more.

Hence Travis is correct

This is so wrong...

[Darin Jordan]

Whatever...

[keithbradley]

.............

Unfortunate that the most useful post in this thread was deleted.
OP, it would be worth sending a PM to Brian if you didn't catch his post.

[Darin Jordan]

Unfortunate that the most useful post in this thread was deleted.
OP, it would be worth sending a PM to Brian if you didn't catch his post.

Keith, no, they shouldn't. Everything I know about props I learned from Brian, so he's completely wrong as well...

Guess all those 18-hour drives back and forth to the LA SAWs were all a waste. I could have gotten it all resolved right here.