Rescue boat build questions

[eehess81]

I am looking into building a rescue boat out of PVC, foam, plywood and an airplane propeller. I have extra outrunner motors in the 4000 kv range. Would use a 2s lipo and a 45 amp rc car esc I have. I know nothing About rc plane propellers. Was wondering if anyone can recommend a size that would be appropriate. Was thinking a 5" with a pitch of 3. The 2s with 4000 kv would be around 28000 rpm. I think you divide this by the diameter to get about 6k rpm..is this a good range? This is the link to the build I wanna do.

https://m.youtube.com/watch?v=ajW0ciXoGYU

Any input would be appreciated as the video doesn't specify motor or prop sizes.
Was thinking I could but skis on this and drive in the snow in the winter too!

[704Authentic]

https://forums.offshoreelectrics.com...70-Rescue-Boat

[eehess81]

Yeah, I saw that, I don't have any low kv motors on hand. Was wondering if 5" prop will work with such high rpm. Don't wanna buy anything besides prop and PVC. Thanks though!

[jkflow]

Try this:
https://oscarliang.com/table-prop-motor-lipo-weight/

[eehess81]

That's for drones but could apply. I'm gonna get the 5" prop and test it out. Will post when I get some results. Thanks

[jkflow]

No, it was a reference to prop size and motor kV. Your prop will disintegrate at those speeds. If not initially, it will happen at the worst possible moment.
By all means, don't go full trigger and should be ok. At the end of the day, you want to push a big load. You have to go with a larger prop and less kV to be successful.
Give it a try, it's a free motor, you can also use fewer cells.

No idea of how your math works with starting out at 28000rpm, and ending up with 6000rpm. The motor will spin at 28k and that will be the rpm at the tip of the propeller as well as any other point in your setup.

You want to be in the 10-14k range for any prop, again, half throttle will work and keep the prop intact.

[eehess81]

I don't know..didn't sound right to me either..but I keep seeing a formula for airplane propellers, rpm/diameter of prop. I must be missing something. I figured I wouldn't be able to go full throttle.

[eehess81]

I guess the formula is app. 120000/ prop diameter to get max rpm. I ordered a 6" an 8" and 10" props. Will have to take it easy on throttle.

[Dr. Jet]

I've built several rescue boats over the years. I'm currently using a John Deere farm implement called the "Rescue Rigger".

IMG_0798.jpg

I designed it using a geared Speed 400 motor spinning a large and super-flat pitch 3-plade prop through an old Graupner steerable outdrive. It was designed to be easily disassembled and stored in one of my cases.

DSCN1733.JPG

DSCN1728.JPG

If you watch this video, you can see some of the issues with "pusher" type rescue boats.

https://www.youtube.com/watch?v=_dso00zhhAM

You have to be perfectly aligned with the victim, otherwise steering is an issue. It's also hard to see at a distance if you are properly aligned.

Were I to build another rescue boat, I wouldn't build a boat at all. Instead, I'd build a floating "Trailer" in the basic shape of the letter "T" with serifs. The upright of the "T" would be a long and elevated (to be above the victim) tongue that would attach to any old boat you have lying around. The crossbar of the "T" would be really wide, probably made of PVC pipe/pool noodles, with the underwater catchers similar to those on the John Deere. To rescue a victim, pull the rescue boat alongside it, then pull past the victim and turn into it. The victim is snagged in the crossbar of the "T" and can easily PULLED back to shore.

[jkflow]

Added FPV to my Tug, no more alignment issue.
$50 investment for camera and monitor. Works like a charm.
Camera plugs into receiver, 5V compatible (5-36V) and monitor has power for ~ 2.5h.

[eehess81]

That's cool. Thanks. I might proceed with the pusher rig just because it will be a fun project for the winter. Thanks for the info