Propeller Speed Theory

This is for the racers and physics majors. For a given boat, what makes for the highest top end speed, turning a higher pitch prop a little slower, or turning a lesser pitch prop very fast? And how does size matter?

It seems clear that craft designed to go fast use smaller props. So let's say for a 17 foot runabout, v-hull, v-drive, small block with about 380hp, prop size would be 13 or 14 inches. Or would a 12 better? Just for the sake of this discussion, assume there is no need for acceleration performance whatsoever, only maximum speed for an upper-mid cruising rpm, (like 3000), and WOT top speed. Hope for 40 mph and 65 mph respectively.

Consider that if one were designing the drive train for a boat, there are options on the final drive ratio. In the above scenario, a stock transmission might be used that would provide a 1.5:1 ratio. A good start for a stock propeller would be about a 14x17, (three blade). At 5000 rpm engine speed, the prop would be turning 3300 rpm.

But if top speed were the only concern, would it make sense to go to say, 1:1, or even 1:1.5, and turn a smaller or lesser pitch prop faster? Such as a 13x13 at 5000 rpm? Or a 12x18 at 5000 rpm? Or an 12x15 at 7500 rpm? Or 11x20 at 7500?

Thanks in advance for your thoughts on this topic.

 

Starting from scratch, prop dia. is kind of a function of displacement.
Boat total weight w. fuel and driver say between 1800 - 2600# would probably use a 12" dia 3 blade prop. 2200 - 3400# a 13"dia etc. If additional loads are imposed such as skiers etc. additional blades and/or dia might be required.
Pitch is than determined mathamatically by taking anticipated speed, slip and desired rpm.
Too small a Dia will increase slip (decrease effciency), Too large, decrease rpm's and speed.

Assuming your 17' boat is a fairly lite racer, lets say 2000lb with fuel and driver and your 380HP is brake HP. Your Max. top speed calculates to about 80 MPH. w. 323 Prop HP.
At 80 MPH, with about 6% prop slip you would be turning a 12x15, 3 blade @ 5991 RPM. w. 1:1 gears
Unless your setup is less than 2000 lb with more HP, I would stay with 1:1 gearing, and a 3 blade prop.
If optimim RPM is higher, decrease pitch or increase pitch to lower RPM.

Hope this helps, jango

 

It's not that easy. It seems empirically that diameter/pitch matters, and for the pitch that works you have to find the minimum diameter (stop before speed is lost) experimentally. See Dave Gerr's Propeller Handbook rfor what's more or less known.

Fro my experience once diameter is found, and pitch is determined (I always label 'pitch' from the leading edge for reasons I discovered accidentally) there's mainly the problem (experimentally) of more or less matching the leading edge to the induced inflow to minimize prop drag. With racing outboards the motor must wind 1000-3000 RPM over the 'rated' RPM (about 6000) for top speed, so you never want a too large diameter. Nor do you want a too small diameter, you have to experiment to find out what works.

Physics Prof who raced outboards for years and won

 

You can get some result for proppeler theory from: http://www.partenovcfd.com

 

Where the lines between art and science cross.

"Physics Prof who raced outboards for years and won"

PERFECT!

 

If we call:
Vblade - water speed "seen" by the blades
Vboat - tboat's speed
Vrpm - tangential speed at the blade due to the rotation
AoA - blade's angle of attack

then you have that:
Vblade^2 = Vboat^2 + Vrpm^2 (approximately - I'm neglecting the induced velocity)
AoA is a function of the blade pitch and of the Vboat/Vrpm ratio, increasing as the later increases.

So it follows that:
- the lower pitch / higher rev prop' blades works at lower angle of attack but at higher Vblade. That, on one side means less blade load due to the smaller AoA but more blade loading due to the higher Vblade.
- vice versa for the other solution.
The solution for the optimum prop lies somewhere between and finding it is the task for the obscure science called "prop sizing".

 

Since we're talking Inboard power, for Max Top Speed, Prop should ALWAYS be sized at MAX H.P rpm. Also, for any given boat displacement and speed, Prop Dia requirements DECREASES as RPM increases. Therefore, a 1:1 gearbox will turn a Smaller dia prop with Equal efficiency as the required Larger Dia Prop with a 1.5 :1 gearbox.

Ex Inboard Racer & current owner of 87 mph, 20' Chris Craft Clone

 

Since a prop is selected based upon:
1) prop diameter
2) speed of water in way of the prop
3) RPM

what influences the above?
well, looking at what are the important characteristics of a prop:
1) prop diameter
2) Pitch
3) No. of Blades
4) Surface area of blades
5) Shape of blades
6) Shape of blade sections
7) Blade root thickness

In 4, it is influenced by the diameter and no of blades and in 5 one can also add skew, rake and camber too.

Pitch ratio is simple pitch/diameter of prop.

Changing the pitch affects the diameter, changing the diameter changes the BAR and this affects the allowable thrust on the blades, changing the rpm changes the speed of advance which affects, well.... you get the idea and so on and on....

I think you summed that up pretty well Daiquiri "...The solution for the optimum prop lies somewhere between and finding it is the task for the obscure science called "prop sizing". ...

 

I expect batboat was happy with Jango's advice when he asked the question 4 years ago. I hope he has not been waiting around all this time.

Rick W

 

All this said actual design, shape, profile, mounting location and even surface of prop has something to do with all this.

Science, art, trial and error.