Recommended prop RPM

[RGinCanada]

What is the ideal prop speed? I know this varies widely depending on the boat, the prop, and what they are supposed to do, but what are the general guidelines here?

Does the speed vary with prop size? Number of blades? Pitch? At what speed will a given prop begin cavitating? My interest is primarily displacement hull workboats (tugs, trawlers, coasters, etc), but any information on this topic would be greatly appreciated.

I'm looking at picking up a couple of Pittman motors that have an unloaded RPM of 1700@5.0V and 2700@7.2V. I'm trying to decide whether they should be geared down or not.

Regards,

Ray

[martin richards]

How many volts are you intending to use. If no more than 12 V, direct drive should be reasonable for hulls up to about 302 - 36".

[Knotaddicted]

From what I have read it more depends on motor efficiency than prop speed. I beleive you want the motor running at close to full speed as possible, as this is where it is most efficient, for longer run times. Of course it also depends on the size and weight of the vessel, and if you plan on pulling/pushing something.

[CG Bob]

What is the ideal prop speed? I know this varies widely depending on the boat, the prop, and what they are supposed to do, but what are the general guidelines here? There are lots of variables, but you can use information based on the full size boat types. A lot of tug/trawler hulls typically turn props at 200-500 Shaft RPM. On two of the CGC cutters I served on (210' WMEC), 300 SRPM was about 18 knots.

Does the speed vary with prop size? Number of blades? Pitch? Yes, sometimes the speed does vary with the prop size. If the motor RPM remains the same, sometimes a higher speed is generated by using a smaller prop, or one with two blades instead of three. Prop pitch also affects the boats speed. A speed prop has a higher pitch than a power (tug) prop.

A "rule of thumb" for scale boat model and props: Prop diameter should not exceed the motor can diameter. If the prop diameter is bigger than the motor can, you need a gear reduction unit. The approximate gear reduction is equal to the proportion between the prop and motor diameter. 3' prop, 1.5 inch motor = 2:1 gear reduction (prop connects to larger gear)

You might want to check out the"Propeller Handbook" by Dave Gerr; published by International Marine; ISBN 0-07-138176-7; price $19.95 USA. The book has a lot more prop theory than we'll ever need for our models, but there is lots of useful info for model building.

[patmat2350]

My rule of thumb:
Go to http://www.raboesch.com/ , see their prop tables, which have max recommended speeds for many size "scale" props, which varies with prop diameter... probably based more on cavitation than mechanical stress.
Then check out your motor... yes, you should have the motor loaded down to no more than 75% of it's free speed when running full throttle, otherwise you're pulling more amps than it's designed for, and it will get hot (water cooling is an UNNECESSARY CRUTCH in a scale boat if you've matched the motor to the prop).
How fast does the motor go? You can get a good idea of the free speed by chucking the shaft in a drill press with a known speed (say, 3100 rpm). The motor will generate a voltage which you can measure with a voltmeter... say it generates 2.3v. When it runs as a motor, it will do 3100rpm/2.3v = 1348 rpm/volt. At 12v, it will do 12x1348= 16,170 rpm... typical for a 400 or 500 size motor.
If the speeds are too far apart, you may need gearing.
And note that the motor size means little... it's all in the WINDINGS... more winds and/or smaller gage wire cuts current draw and the speed.

Now, is the motor too small, regardless of whether the speeds are matched? A small motor won't turn the big prop, it will slow down. Best way to test is with a tach... but failing that, listen... does the motor slow way down when in the water vs. in the air? And feel the can- if it's pretty warm, you can be sure you're pulling too much current (lugging the motor).

Of course, none of tells you what's the right power for your boat. My "steam" tug has a scale size prop, running below it's limits with a 3:1 geared-down Speed 600 at only a lowly 6v, and at full stick it still looks like it could pull a water skier!

Pat M
http://geocities.com/y2patmat/

[jimjel]

I would like to preface this post with the caveat " I know nothing about electric boats".

Beyond that, I would like to build a non-scale push tug to use as a retreival boat for my gasoline powered raceboats. The boat would be approx 32" long, flat bottomed w/skeg, 12" beam and weigh about 25 lbs all up. The bow would be squared like a river tow boat to provide a pushing capability.

I have a Robbe Power Series 1000 motor and would like to use it if possible. It is a 12vdc motor, rated at 2500 rpm no load, has a 6 mm output shaft, and is intended for direct drive applications. It is rated to operated at 2 amps for max effiency, and 10 amps max power draw. The diameter of the motor case is 2.5" and the motor is 4" long.

I was thinking of using the Rokraft 100 controller with this motor.

I contacted Robbe via e-mail concerning use of this motor and they suggested using a swiveling Kort nozzle and a 4" diameter, 4 bladed prop with this motor.

Seems like a prop that large would really load the motor heavily, don't know, just guessing. The thing that bothers me is that I read somewhere that if you are running a lot at lower rpm such as manuvering speeds, the batttery power is dissipated into the speed controller, causing it to overheat.

I guess my question is would this be a good setup or should I go to a smaller prop allowing the motor to spin up more freely. Robbe also makes a Kort nozzle that takes a 4 bladed 2.3" There are also many props in the 3" range on the market.

One other thing I need some expertise on is that I would like to use a driveline setup like I have in my raceboats which basically is a wire wound 1/4" flex shaft contained in a brass stuffing tube, rather than solid shafts with a u-joint. A solid shaft system with a u-joints would require the u-joint to handle about a 22 degree shaft angle delta. Don't know if that much angle would cause problems. The flex cables I use are left hand lay and they turn in the 17-18k rpm range connected to a 5 1/2 hp gas motor spinning a highly pitched 2 3/4" prop. The question is, will the the flex shaft take running the boat in reverse as there would be a normal tendancy to unwind the flexcable. I do know that if you use a right hand lay cable and try to spin it at 17k rpm with a high power gas motor it will unwind the cable causing a great deal of unpleasantries. On the other hand, the 1000 electric motor doesn't put out nearly the power of the gas motor, nor spin anywhere the speed of the gas motor, and the effiency of a prop running in reverse is greatly diminshed. Seems to me the flex shaft would work fine, but again just guessing.

Any input on the above would be appreciated.

Thanks,

Jim

[CG Bob]

My first recommendation is for you to contact the Bay Area Electric Boaters (http://www.bayrcboaters.com/default1.htm) up in St. Petersburg and talk to Marcus Krest, Duane Curtis, or Larry Green - all three have been running tugboat models for a few years. Secondly, I would not use a flex cable in the tug because of the unwinding cable in reverse problem you mentioned.

For your motor/prop set up, I would use a 2:1 gear reduction.

For a comparion, I have a 51" long, 9"wide, 30 pound fleet tug boat. I runs a 3 inch 4 blade prop off a solid 3/16: shaft. The motor is a MACK (http://www.mackproductsrc.com/Power%20p2.htm) Twin Motor Gear Box Unit, geared at 3:1. 12V power is fed thru a Vantec 411P ESC.

[Knotaddicted]

I rememeber reading somewhere that a direct drive prop shouldn't be any larger than the motor housing. I would post the link if I could remember where I saw it. For a 4 inch prop in a Kort I would agree with CG Bob that a 2:1 reduction would be good. For direct drive the 2.3" prop in a Kort would work, but you may not get the thrust needed to push the sailboat back to shore easily. The other question about 22 degrees on a u-joint, that is too much angle and would cause vibration or binding.

[jimjel]

Would the Robbe recommended setup work ok? i.e the Robbe 1000 motor connected direct drive to a 4" prop with a swiveling Kort nozzle, or would that be a speed controller killer?

JIm

[Knotaddicted]

If they are recommending it I'm sure it would work. Just make sure the speed control has the amp rating for the motor, for example, if the motor pulls 10 amps full load, a speed control rated for at least 12, but 15 amps would be safer for controller. Note these amp figures are random as I do not know your motors amp rating. Also be sure to check that the motor and the speed control isn't getting to hot under the use you plan for it.

[OhioMike]

I've always read and have been told by more than a few, to keep your rpms to around 3000 max for scale projects. For what thats worth.

[jimjel]

The specs on the Robbe 1000 Power Series motor are as follows:

Description: (from Robbe)

"Extremely high torque (65Ncm) Low rpm, Low Noise motor is ideal for direct drives of large and heavy work ships with prop diameters up to 100 mm (4").

Specs:
Diameter: 63.5mm
Length: 96mm
Weight: 1000g
Voltage: 6-12 volts
Rotations: 2200 rpm
Current amps at max efficiency: 2.5A
Max power:10A
max efficiency 70%

The Rokraft 100 Speed Controller Robbe recommended is rated at 30A momentary load. They also have another model called the 120 Navy which is water cooled for a few bucks more. Same specs, just water cooled. Both controllers have something called a "braking" feature which apparantly prevents you from slamming it into reverse when going forward at higer rpms.
Both have BEC circuitry which would not be used with a 12v power source.

Sound viable?

Thanks,

Jim

[jimjel]

Does anyone besides Dumas make a mechanical speed controller with both forward and reverse that will work on 12 volts and handle a 15 to 20 amp load plus surge capability?

Jim

[CG Bob]

Forget the Dumas mechanical speed control - it's a large variable resistor. It can generate a lot of excess heat and burn out rather quickly. I used a Dumas mechanical speed control in my first boat over 30 years ago, and it got to hot to touch. I changed to electronic speed controls within a few months of launching that first boat. You're better off getting a Vantec (http://www.vantec.com/) 411 series electronic speed control.