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Oct 06, 2020, 09:46 PM
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Oct 08, 2020, 06:33 PM
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MKII sea trials vid:

RC Rescue Boat MKII (1 min 0 sec)
Oct 17, 2020, 03:34 PM
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Rescue Boat – MKIII


Winter is fast approaching and I need to find a project to keep me busy throughout the next few months.

My little green roadster needs some head work, which I plan to tackle soon…but that’s only 40-hours of work.

So, with my sanity in mind, I’ve been considering a new rescue boat with the following missions tasks:

1- Must be sized to fit in one of those folding wagons with an IOM, toolbox and radio gear.

2- The craft shall be able to set course marks / anchors.

3- The craft shall be able to rescue up to an IOM sized sailboat which has:
a. Lost radio control
b. Lost its rig
c. Stuck on shoreline
d. Entangled with another sailboat

4- The craft must be capable of towing a rescue line to man in the water.

5- The craft must have a shallow draft.

6- The craft must be able to operate within weeds.

7- The craft must be able to support FPV and GoPro cameras.

8- The craft must be able to operate at least 30-minutes on a battery.

9- The craft must be able to be launched by 1-man in under 1-minute.

With the above in mind, I’m leaning towards a tug boat design. The course marks / anchors will be dropped from the bow end, using a mechanism similar to the MKI and MKII crafts.

Sailboat rescue will be accomplished using the tugs front bumper to free sailboats stuck on the shoreline, and a tow line (lasso) which will circle the distressed sailboat(s) with a line, and pulled back to the dock.

Propulsion will be a conventional propeller / rudder (or azimuthing prop), with brushless motor and ESC w/reversing capability.

Big torque will be required to ensure adequate performance in all conditions. And although I have lots of expertise sizing motors/props for RC aircraft, I’ll be relying on the good folks in this forum for assistance.

Here’s a sketch of a Springer “Rescue” Tugboat which looks like a good candidate for this project:
Oct 18, 2020, 11:32 AM
The wind is free, go sailing!
Scratchy101's Avatar
A number of years ago I considered building a Springer tug for this purpose.
The existing springer tugs are only 18" long, so based on the following diagram. I scaled it up to 24" in Excel.
I just added a 3rd size of 27.5" long (see exlel in zip file).

Scaling this may give an exaggerated draft, so I would use this as a starting point only.
Oct 18, 2020, 08:51 PM
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Quote:
Originally Posted by Scratchy101
A number of years ago I considered building a Springer tug for this purpose.
The existing springer tugs are only 18" long, so based on the following diagram. I scaled it up to 24" in Excel.
I just added a 3rd size of 27.5" long (see exlel in zip file).

Scaling this may give an exaggerated draft, so I would use this as a starting point only.
I was going for 30-inches (largest size that fits most tote wagons), however your 27.5" calcs will do perfectly!

Have you given any thoughts to propulsion? I want to use a brushless motor, which will necessitate some type of reduction gear box. Something I have no experience with.
Oct 19, 2020, 10:58 AM
The wind is free, go sailing!
Scratchy101's Avatar
Quote:
Originally Posted by Trogster
I was going for 30-inches (largest size that fits most tote wagons), however your 27.5" calcs will do perfectly!

Have you given any thoughts to propulsion? I want to use a brushless motor, which will necessitate some type of reduction gear box. Something I have no experience with.
I think you can get away without using a reduction gear box.
Have a look at the BDC motors hear:
https://www.robotshop.com/ca/en/bldc-motors.html

First thing to do is to get the prop you want to use, then match the RPMs to the motor.
Example: 3600 RPM prop = 630Kv motor at 6V (2S LiFe battery).
Oct 19, 2020, 07:58 PM
Registered User
Quote:
Originally Posted by Scratchy101
I think you can get away without using a reduction gear box.
Have a look at the BDC motors hear:
https://www.robotshop.com/ca/en/bldc-motors.html

First thing to do is to get the prop you want to use, then match the RPMs to the motor.
Example: 3600 RPM prop = 630Kv motor at 6V (2S LiFe battery).

Thanks for the above...in addition, I'll need to cacl the desired power needed for a given boats displacement and mass. Plus I haven't been able to source data on propeller thrust vs diameter/RPM characteristics.

I would estimate that I would need to generate at least 400Watts to motivate a 5kg tug with a 28-inch NWL and have sufficient power to launch marks and rescue stuck sailboats.

To generate 400 watts at 6V (2s) I'll need to pull 66Amps.... don't know of too many 2S out runners that could generate 400Watts.

Too keep the amps and heat down, I would prefer to run 4S-6S and gear it way down...




I'm leaning towards a z-drive setup.
Last edited by Trogster; Oct 20, 2020 at 05:25 PM.
Oct 28, 2020, 05:43 PM
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MK III Progress report:


Over the last three days, I've hobbled together a test hull using the scaled-up plans provided by S101 above. I elected to go with the 27.5-inch hull as it fits the transport wagon perfectly.

The test hull is made from 1/8-inch MDF and will receive a couple of coats of varnish which should get it through it through its buoyancy tests before melting.

I've also ordered the Oceanworks 60-mm Z-drive. The lack of specs are of concern, with hope it will provide sufficient power for the tug.
(I suppose I could always twin the drive if necessary)
Last edited by Trogster; Oct 28, 2020 at 11:08 PM.
Oct 28, 2020, 08:39 PM
Registered User
Hi, what is the primary motivation for the new tug design ? I was under the opinion that the previous air boat design was a success with perhaps a bit more refinement and tweaking ? The lack of any propellers to snag weeds and rocks seemed like a win.
Oct 28, 2020, 10:26 PM
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Quote:
Originally Posted by marylandsailor
Hi, what is the primary motivation for the new tug design ? I was under the opinion that the previous air boat design was a success with perhaps a bit more refinement and tweaking ? The lack of any propellers to snag weeds and rocks seemed like a win.
With an abundance of time at my disposal, I needed a new distraction. Figured a new project would do the trick
Last edited by Trogster; Oct 28, 2020 at 10:50 PM.
Oct 29, 2020, 08:39 AM
The wind is free, go sailing!
Scratchy101's Avatar
Quote:
Originally Posted by Trogster
To generate 400 watts at 6V (2s) I'll need to pull 66Amps.... don't know of too many 2S out runners that could generate 400Watts.
Too keep the amps and heat down, I would prefer to run 4S-6S and gear it way down...

I'm leaning towards a z-drive setup.
Of course that was just an example calculation.
You will have to match the motor power and RPM to the z-drive specs (unless you ordered one with the BDC motor).
Oct 30, 2020, 01:22 PM
**NOT GUILTY**
Capt.Crash's Avatar

Here's my rescue boat.


Oct 30, 2020, 03:54 PM
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Quote:
Originally Posted by Capt.Crash
That look's like it's ready to take-off!!
Oct 30, 2020, 04:09 PM
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MK III Progress report:


Pardon the delay...I literally had to fix the kitchen sink. BTW, if you're looking for a quality faucet may I recommend Kohler. My faucet is 15-years old and has started to have have issues. A quick call to Kohler and they sent out a bunch of rebuild-parts all free of charge. It pays to buy quality!

OK, I ran some buoyancy tests today. Findings:

1- The hull displaces 35Lbs (15.87kg) of fresh water
2- With 3lbs-2.7oz (1438 grams) of weight on the front end, the bow sinks 3/4-inch (20mm)

I'm not crazy about the amount of ballast this hull requires, but I am pleased that I will not have to come up with some complex ballast shifting device to correct the hull attitude once the mark-anchors are dropped. To address the 3/4" drop in the bow, I'll rear bias some of the ballast, and I'll add a buoyancy bow-bumper just above the bow NWL.

I've been requested to consider fabricating the hull out of fiberglass. A great idea!!!

As my test-hull isn't sturdy enough (or near perfect) to act as a mold, if I do proceed with a FG hull, a better mold will be needed.

If I do proceed in that direction, I'll make a few hulls and offer them to anyone interested (one has been spoken for).

One final concern is how I'm going to launch this boat. With a final ready to sail weight approaching 40-lbs (18kg), I have no idea how I'm going to launch or retrieve this tug from dockside?
Last edited by Trogster; Oct 30, 2020 at 04:58 PM.
Oct 30, 2020, 06:39 PM
Registered User
Update:

I've received feedback that the bow of the Springer needs to be much higher than my test illustrates.

So I de-ballasted the tug to 20-lbs to bring the bow up. As an added bonus, loading the mark-anchor test weight on the bow has a less dramatic effect.

Launching a 20-lb tug off the side of a dock is a much more manageable task.

These results are encouraging.


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