Correct Propeller RPM

[Senza Timone]

I am building the Ulises Tugboat by OcCre. It is designed to be an RC ship.  I do not plan to "play" with it and will not install the Radio Controls but before I finish the top I am going to install an electric motor because is going to be much more difficult once the hull is closed up.  Somebody in the future may wish to add RC ability.

Which brings up the question: What kind of RPM is just about right.  Is easier to find PM DC motor spinning at 20,000 RPM but without any scientific support I think such RPM is way too high. It would seem like the propeller would act more like a blender than do real propulsion.

The propeller has 45mm (1.75") diameter.

Any idea of what a correct RPM range would be?

[lmagna]

A small prop like that would tend to need to spin faster than you may think to get realistic or enjoyable operation. But you are probably right in that you will need nothing like 20,000 RPM to get the job done. More importantly, motors that run at those speeds are normally meant for RC cars and such and draw many more Amps than you would need for a tug of that size. 

 

I think you would be much happier with a motor equivalent to the Dumas line of scale type motors,  https://www.micromark.com/Dumas-6V-10-000-RPM-Boat-Motor  I prefer a 12V motor set-up as they tend to develop more torque and draw less current, but the motor listed should get the job done and includes the direct drive components you will also want without putting too much strain on the bank account. It should give you reasonable power, speed, and handling, while not sacrificing running time by being over powered and drawing unnecessary current.

 

If you are going to run RC on this tug then you may want to enlarge the rudder or make a clip-on rudder extension for when you run it. Single screw/rudder models tend to have very poor rudder response at slow speeds, and even worse in reverse and the smaller/lighter the model the worse it is. If the model was a larger/heavier tug then this would not be as big of a problem. If you are just going to run around the pond/lake back and forth then you may not be bothered too much by this but if you do a lot of close in "tugboat work" and maneuvering then you will appreciate the larger rudder.

 

Good luck on your build. Do you have any pictures or build log information?

[Senza Timone]

Thanks for the response.  I appreciate the pointer on the rudder size since I had no notion.   A clip on is a great idea since it achieve the purpose leaving the right appearance on the stand.

This is how fare I got so far.  There is almost as much work on the two lifeboats then on the entire ship hull.

After seeing the amazing work documented here I am not going to make a build log for a modern ship from a kit. All pieces are precut this is little more than a glue job.

 

I found a 12 V motor in the salvaged box that should be fine when run by 6 Sub C NiCd cells.  Found a piece of 3/4" Schedule 20 PVC pipe that will make a perfect battery holder.

 

[kurtvd19]

I second the idea of using a Dumas type motor but with a 2:1 drive - not direct to the shaft.  A tug needs more low end power than RPMs and 2:1 with a Dumas type motor works great fro a tug such as you show.  CAUTION - Do not under any circumstances use a Dumas geared drive for this - pure garbage and the noise is very loud - a belt drive is great.

[lmagna]

4 hours ago, Senza Timone said:

After seeing the amazing work documented here I am not going to make a build log for a modern ship from a kit.

No reason to not show your build of this tug. There are plenty of builds in the kit section. If you share your experience in a build log then others who may be interested, (You might be surprised how many people here would be interested) can follow along and get answers to possible builds of their own.

 

This type of tug is as much of a static display model with wide appeal as well as a comfortable size for RC. Over the years I have built, run, and competed with about twelve RC tugs ranging from small harbor tugs to 65 pound ocean going tugs with dual 4" props that could tow real boats with people in them at surprising speeds. It was a fun hobby.

 

You may just want to buy one of the 7.4V battery packs that are commonly available for RC cars and such. They are easy to find, normally not all that expensive, and there are a number of chargers that will both quick charge and slow charge them.

https://www.amazon.com/Zeee-Batteries-Dean-Style-Connector-Vehicles/dp/B076Z778MJ/ref=asc_df_B076Z778MJ/?tag=hyprod-20&linkCode=df0&hvadid=312111912863&hvpos=&hvnetw=g&hvrand=612196785035678968&hvpone=&hvptwo=&hvqmt=&hvdev=c&hvdvcmdl=&hvlocint=&hvlocphy=9033347&hvtargid=pla-571251577875&psc=1 And that price is a bit on the high side. A little looking around will probably get you a much better price.

 

Unlike Kurt I am a fan of direct drive, especially for smaller props like yours. Not only is simpler and quieter, but smaller props will need to spin faster  just as you will benefit from a larger rudder. I DO AGREE with his comment on the Dumas Adapt-A-Drive! Pure junk is the only fitting description.

 

For larger props in the 3 to 4" range I found that finding a large motor suited to driving that size of prop was the best answer, although this may be hard for some people to acquire. In years past I was able to locate 18V motors from scrapped out copiers and printers that were real powerhouses and very high quality. I used a number of them in my bigger boats, all direct drive with excellent results, AFTER learning that I needed to make my own universal joints from metal to handle the torque! I won several Tug-A-Wars against other model tugs over the years with this arrangement. The biggest prop I ever installed on an RC tug was a 4" four blade prop from a US WWII torpedo. It was used on the nose of the torpedo to arm it as it traveled through the water. I can't remember where I found it but it was a fantastic tug prop, but again the loads imposed on the running gear were impressive and all had to be made extra heavy. No off-the-shelf stuff in that boat. I started with 2:1 belt reduction but eventually ended up with a much larger motor and 1:1 direct drive. 

 

If you end up running with other similar sized tugs, sooner or later you will either get into a race, or Tug-A-War, probably both. It is in the blood. Even real tugs do it. I have been on board real tugs a couple of times in the middle of tug races and believe me it is a lot of fun. If in the future you end up building one of these monster RC tugs. They are not fast as boats go like plaining hulls but they are impressive when pushing half of the lake in front and dragging the other half behind!

Edited June 17, 2021 by lmagna

[Senza Timone]

I have  Johnson Control motor C4557-6003   12V 3 pole salvaged from an HP Printer

I characterized  V; I; RPM alone, connected to the propeller shaft spinning on air and the immersed in water

Here is the set up,   I give you a minute to finish laughing...

Direct coupling via neoprene tube

All is pretty predictable until the prop hit the water, then my power supply current limit and all I get is 2.9V 4.8A and 200 RPM

You can see it churns up a fair rooster tail of water, I do not think I need more than 400 RPM at the propeller to push the tug just to see it cruising.  Even at this level of drive the motor is cool.

 

I though a direct drive would be acceptable but is clear the motor is bogged down and is operating far from optimal.
So I am going for reduction 2:1 running the motor with 6 NiCd cell (7 to 9V)

The question is belt or gears?  I can buy nylon gear for next to nothing and built the transmission case as I have a fully equippet metal shop.  Belts is more of of unknown what type of belt to handle the torque? How about a 1/8 diameter O  ring on V grove pulley?

Is the small pulley 1/2" diameter adequate?

 

[lmagna]

Just a few comments that you may be interested in or not.

 

In this case you are tying to run a 12V motor @ 2.9 volts and your tube alignment is off, causing extra load on the motor as well. This motor is designed to provide it's optimal torque at at least 12V or even possibly more. Many printers/copiers operate at 24V. The high current draw you are experiencing could be from alignment + low voltage + motor design. You should see your currant draw go down a little by making a little better alignment with the prop shaft. Also, even though it is a little harder to measure, you will find that your current draw goes down and your RPM up as the boat gains speed in the water.

 

If you have machining skills and equipment and want to go to reduction drive then go the gear route if you want. Belt drive has noise advantages in many cases and is certainly easier to design and build, but a well designed and built gear drive can work just as well. Just try and choose quality parts and try to build it as sturdy and precisely as possible. Your motor and battery will like it.   

[Patrick Matthews]

There's a whole book to write regarding preparations for RC use, all best done before getting too far along... not only motor and prop, but prop tube and bearings... ballast, how much and where to put it... battery choice and how to mount it... wiring... steering linkage and servo... access to EVERYTHING, as everything needs to be serviced... and super important, waterproofing. I would never consider floating a wood planked hull that doesn't have a fiberglass outer skin (and not just resin- actual glass in there).