Good prop for 150 pound boat?

[sloper steve]

About how wide should a prop be for a boat of this weight going up to 10mph or so using about 600w on brushless motor?
Direct drive is preferred if possible.

Any links to basics for props in the water would be very helpful.

[CG Bob]

What is the diameter of motor can? A basic rule of thumb for electric boat models is that the prop diameter should not exceed 125% of the motor diameter. A prop over 125% of the motor diameter generally requires gear reduction.

[martin richards]

Strange as it may seem, prop diameter does not come into calculating boat speed. Basically the pitch of the prop is the distance travelled per revolution (in the case of direct drive, the same as the motor speed). Your application sounds as if a fairly fine pitch + the same as the diameter would be called for. Probably it's a fuly submerged prop, so true speed will be about 20% less than theoretical to allow for prop slip.
Just curious. Why brushless?

[sloper steve]

Brushless because the outrunners have so much torque and don't need the gearbox- at least with airplanes. If there are motors for boats that are brushed and have this kind of power just let me know.

Here is the type of motor I am looking at:
http://hobby-lobby.com/actro-24.htm
or the model under it in watts

The boat is super draggy and heavy so it needs power not speed more than a few mph. I'm thinking because a motor like that can drive an 18" prop in the air it may be able to handle a 2 or 3 inch prop in the water and produce some nice thrust.

How would the width rule apply to an outrunner?
What brand prop range could you recommend for giving this a try?

Thanks for the answers!

[martin richards]

The only modestly priced brushed model motor that come to mind is the Graupner 820. Any of the other large brushed motors, such as the Graupner Ultra or Plettenburg are for higher speed applications. However, the battlebot crowd seem to have the same motor parameters in mind as you and there are probably many recommendations on their sites for economical motors.
Considering the diameter of most outrunners, I'd hazard a guess that the prop/can diameter rule should be at least reversed. Also, if your hull is very draggy, I'd increase the prop slip factor to 30/35% to calculate speed.

[Greenflight]

A 150# boat? Is that weight a typo.
Sounds like it needs a 15 horse Johnson outboard.

[Keith43221]

Yea might I ask what you are doing with a 150 pound boat?

[Umi_Ryuzuki]

150 # boats are not hard to find.

The cruisers weigh at least 100#, and i have no Idea how much Brads Battle ship weighs to the water line.

The Light colored Destroyers are 56" long and weigh 15lbs.
The cruisers are 96" in length,
The Bismarck is pushing the 12 foot pp. ;)

The propellers I want to say 2 inches...

[jmolwitz]

Dave at harbor models told me some time ago those ship drives (pittman for example) only use 30 whatts under go (talk about effecency). The biggest scale prop he has is 4"x4" . A theoretical 10 mph is at about 2500 rpm . The actro on a low cell count will work . Also after 5mph the drag from pushing a displacement hull beyond its design speed will push ,or load the motor quite a bit . Do watch motor temp at 10 mph .

[sloper steve]

150 pounds is right
It has many amps of lipos and 650w plus of power

Any info at all on props please?

[sloper steve]

Also looking for links on coupling motor to prop while keeping motor dry.
What is the seal called?

[mike_victoriaBC]

Try the "Prop Shop"
Simon@prop-shop.co.uk
He has a variety on models all over.

[Umi_Ryuzuki]

We use Pittman motors almost exclusivily. Although "Maxxon" motors are highly treasured and horded. These motors will run half the day on a 2000mah battery. Since the boats need to weigh so much, everyone typically will throw a huge 7-24ah gel cell battery into the boats for ballast. ;)

Steve,
I believe we all thought you knew where to get the propeller, and that you just wanted to know what size? :p

For motor shaft connection, you want to pick up a u-joint.
Dumas, Traxxas, Graupner all make good u-joints.

For passing the shaft through the hull in a water tight manner, you want to install a stuffing tube. This can be built up from K&S Brass tubing from the local hardware store, starting with your shaft size. I usually use three sizes of tubing. Or you can buy a premade stuffing tube for/or with a shaft.

http://www.harbormodels.com/Harbormain/Templates/m14-propulsion.htm

http://www.loyalhannadockyard.com/RUNNINGHARDWARE.htm

Below is a typical stuffing tube build up for a 1/8"(3mm) brass propeller shaft built fro K&S brass tubing. Just cut some short lengths for either end, about a 1/4"(6mm) or less in length and solder them into the ends of the stuffing tube.