Vacuum formed hull and homemade drive - RC Groups
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May 02, 2017, 06:28 PM
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Vacuum formed hull and homemade drive


I'm a long time lurker and first time poster, so bear with me. I'm a long time rc addict and if it flies, floats, or rolls, I'm interested. A quick explanation of my "posting" name - I got involved in rc helis some years ago and posted on RunRyder and scalerchelis using this posting name, and just continued using it here. The name derived from the fact that I joined the Marine Corps in 1961 and was assigned to fly around the H-34 Sikorsky known as the Seahorse. It's better known as the HUS in the Corps. It's always been my favorite heli and I made some vacuum formed fuselages because none were available commercially.
After learning a lot about vacuum forming techniques, I've become a fan of it. If you want to design and build your own models, it's a good way of prototyping and easily making more of them economically.
I began running rc crawlers last year and was influenced by some of the guys wanting to pull a jet boat with crawlers and then launch them and run them in creeks - which we have plenty of here in Colorado. I decided to see if I could design and build my own version of a jetboat for that purpose.
I'd had good luck making the forms or "bucks" for vacuum forming from balsa with my helis and decided to continue using that method. Balsa is easily cut and sanded to get a shape you want and is easy to change if you want to. It's strong enough to withstand the forming pressure and heat.
I was influenced by the Thrasher design, which was just getting ready for production. So, I carved and sanded a form that was 16 " by 6" and started making some hulls. I've always had an affinity for military vehicles and decided to make a model "inspired" by the Light Seal Support Craft also known as the Seal Team Assault Boat or STAB.
Here are some pics of my efforts.
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May 02, 2017, 06:49 PM
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This first boat utilized a cheap drive we found on E-Bay. While it worked, it's performance was very poor, in my opinion, so I thought I'd try to build my own drive. In this, I was influenced by an article in Model Boats magazine called the "Stinger". This author used plumbing parts (p trap) to make his own drive and I did the same. I used 1.25" parts and the shaft and prop from the Aquacraft Atlantic Tug. I had to trim down the metal prop to fit in the plastic tube. The coupler is also from the same source. I used a 22-26 outrunner with 2200 kv and 2s lipo. The performance was much improved - the speed was about twice as fast with the same motor and battery and ran for the same amount of time.
Here are some pics of it.
May 02, 2017, 06:55 PM
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In an effort to improve the scale looks aspect, I used some parts from a kid's toy I found on Amazon. I also pull a "Zodiac" behind the STAB and it's fun to see how long it remains upright!
May 02, 2017, 07:09 PM
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Some of the guys wanted a boat that would look more like it would actually be pulled by a crawler truck and I did too, so I carved a new top for the boat.
I also started using a plastic prop from the Aquacraft "Miss Seattle" which was also cut down. It improved the speed a little bit.
May 02, 2017, 07:30 PM
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I felt that I had taken the drive about as far as I wanted to. It was tedious to cut and fit the p trap so I decided to try using the very good KMB drive. It really transformed the boat! I began using a bigger motor suited to the drive - a Turnigy 35 -36 with 1400 kv and 3s lipo. It's almost scary fast. I have a Proboat River Jet 23 and we think both boats are about the same speed on 3s. I haven't run the Proboat on 4s yet, but I assume it would be faster. My vacuum formed boat is smaller and I don't think I want to run it on 4s! It was so much faster, that I made a fully enclosed version to hopefully protect the electronics.
I'm enjoying running my vacuum formed boat and am waiting for some bicycle spokes to make a protective grill and hopefully less worry when running in our rocky creeks and streams.
May 02, 2017, 07:40 PM
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I'd always liked the scale looks of the Tear Into boat, so I bought one. I thought the drive was a bad joke so I replaced it with one of my homemade drives. It performs about the same as my own hull design.
My latest effort is to lower the KMB drive about 5/16" to help it stay primed more easily. I'll post some pics of it soon.
Let me know if there are any questions and I'll try to answer them.
Bob
May 02, 2017, 08:38 PM
Registered User
Love that simple hull design Seahorse Bob...loads of access and room and I can see the temptation for fitting the KMB drive, for the money and along with the availability of parts and the fact that they don't claim it to be a high revving powerhouse.
I will be adding a few bike spokes in mine as a grill/grate as there are more rocks than weed where I run.
Does the hull turn well in tight turns and track well at speed?
May 03, 2017, 10:27 AM
Registered User
Hi, Oyley. I should have mentioned that I found it necessary to add the spray rails along the chine. They help get the boat on immediate plane and enhance the cornering ability and speed a great deal. I suppose that they also act like what sundogz refers to as sponsons on the Thrasher. I hope to get some video this weekend when I get together with my boating friends and will post it when I can.
Bob
Last edited by seahorse bob; May 16, 2017 at 12:13 PM.
May 04, 2017, 04:27 AM
Jet Drive Design & Development
sundogz's Avatar
Oooh Raah! I joined the Corps in '72 and was fortunate to stay stateside during the conflict. Nice boats there to be sure. That nacelles on the bottom makes me wonder if it wants to hop at high speeds. I had a similar arrangement on a custom vacuum formed BBY Oval Master and couldn't tame it down.

The NQD drive will work well if you give it enough power, but that drive you made outclasses it for sure. Thanks for your post on the mods thread. I thought I had closed it, but was pleasantly surprised.
May 04, 2017, 02:10 PM
Closed Account
seahorsebob- post #5 is good and room to install everything. My hat goes off to you!
May 05, 2017, 11:20 AM
Registered User
Hi and thanks for the compliments and observations, you guys. And a "Semper Fi" to you, sundogz.
The first version of the STAB would raise it's bow and sort of "plow" ahead as it got up to speed but didn't hop - probably because it didn't have a lot of power to go fast enough to do so. I changed battery placement and cg and that kept the bow down. Adding the spray rails at the chine also improved this trait.
My newest version with the KMB drive has really surprised me with it's performance. It goes about twice as fast and stays level - even in turns. So far, it hasn't hopped except when hitting a wave of sufficient amplitude. Even then, it's only once or twice. Please note that I'm not claiming any special knowledge or technique of designing hulls. There's a lot of TLAR (that looks about right) and serendipity in this hull! I'm posting some pics of my effort to lower the KMB drive to improve self priming. I've been wondering why the KMB drive is so high at the transom. The only thing I can think of is that they want to have enough room for their reversing mechanism. I've got the new lowered version ready and hope to get some video of it tomorrow while running with some friends.
I glued the drive to a piece of wood and then cut it against the rip fence of my table saw. You can see how much it's lowered in the boat.
Bob
May 05, 2017, 11:48 AM
Jet Drive Design & Development
sundogz's Avatar
That oughta do it. One cause for it to not self-prime is insufficient grease - air will get sucked into the intake. There is no seal on the shaft, so it gets sucked out with use. Use a good sticky marine grease such as Proboat as it will stay with you longer.
May 05, 2017, 12:20 PM
Registered User
I looked at cutting my KMB down to see if may help with priming but decided against it for one reason only...
It really limits the diameter size of the motor I could fit into the hull..and following measuring and before the cut...it did.
This is even more so as the motor fits directly onto the pump...the only option then for me would have been for a different and separate motor mount remote and further away from the pump with a support tube and a flexi drive...so I binned the idea on this hull, just takes away the simplicity of the pump mounted motor.
May 05, 2017, 03:26 PM
Registered User
Hi, guys and thanks for the tips and suggestions. I'll keep them in mind. The diameter of the adapter plate is 40 mm. and I've got at least another 2 mm. under that, so, I guess I could put a 42 mm. dia. motor in there. Right now, I'm using a 35 by 36 motor at 1400 kv that I love. I've also got a 35 by 36 1800 kv motor I want to try. It's hard to imagine needing a bigger motor, but, you never know.
Here are some pics of my two KMB drive boats for comparison. As you can see, my lowering cutting has also reduced the intake area and moved it rearwards a little. It'll be interesting to compare the performance of each. I reluctantly reduced that nice NACA duct inlet of the stock KMB. It'll probably reduce the speed a little. For me, this trade off is ok for my intended use, but I can see that other people might not accept this compromise for their use.
I'll keep you posted and hopefully post some video tomorrow.
Bob
Last edited by seahorse bob; May 16, 2017 at 12:12 PM.
May 15, 2017, 04:08 PM
Registered User
Hi, guys. I finally got some video edited, so I'm posting it here. Thanks to Tim for taking the video footage. The location is a local lake where my boating friends and I meet on Sat. mornings. I feel that the cut down version of the KMB drive will be my preferred setup because it really does help with self priming when running. The 35mm dia. motor fits fine and allows enough clearance with the hull and is definitely powerful enough for this boat. My running was stopped when the boat "swallowed" a twig. It made a lot of noise but thankfully didn't damage the impeller. I just got some bike spokes and am making a grill to hopefully prevent that in the future . Our creeks and streams are beginning to run high and swift with our snowpack melting here in Colorado and I hope to get some video running in our local creek soon.
Bob
(I don't know where the extra "e" in the title came from, but I can't get rid of it!)
Vacuume Formed Jet Boat (0 min 59 sec)
Last edited by seahorse bob; May 15, 2017 at 05:54 PM.


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