I Picked Up a Used Gas RC Boat – What am I doing?




OK, so most of you know me as an aircraft guy. While I do love just about anything that flies through the air, I also enjoy ALL forms of RC! Recently, I was perusing a Facebook page, and  guy was looking to trade an airplane for a gas RC boat.

My in-laws live on a wide calm spot of the Mississippi river, and I’ve been looking for a boat to run when I go up to their place. As I’m looking at the page, this bright yellow catamaran catches my eye. This thing looked quite large, so I jumped at the deal and asked him what he was looking for in a trade! Long story short, we made a deal, and one of my 3D planes was crated and left on the FedEx truck. A few days later, another large crate arrived at my front door! Without hesitation, I opened the crate, and I had my very first water-cooled, gasoline engine powered RC boat.

I set the boat and its stand on my dining room table – it was actually larger than I had originally though it would be! These are the first photos of this boat. It is an AC Models Blast Cat 40 racing boat. The 40 means it’s 40″ in length. In addition to that, there’s about 6-8″ of propeller and rudder sticking out behind that!

I took several more photos that first night. Now the guy I got it from said it was ready to run – I would have to disagree. So, what follows is my story. This is how I received an RC boat, and the repairs I made to get it REALLY ready to run! Interested? Read on and see what I had to do…

Like anyone that’s excited about a new ‘toy’ (Yes, I realize this is NOT a toy), I started digging in to see what it was all about. I had a small electric boat about 15 years ago, but this was WAY cooler! The first thing I see when I pulled the cover off is this – a very ugly yellowish-brown spot on the floor. This is where the propeller shaft goes through the hull (bottom) of the boat. A closer inspection reveals that the fiberglass and epoxy used to seal and strengthen this spot is thin, weak, and leaking. I can see through the fiberglass in spots! Many years ago, I had a job at a marine repair shop, so I’m no stranger to fiberglass repair. This was going to make a for an easy repair – or was it???

Thankfully, I have a good friend that races gas RC boats. In fact, he’s really good at it! Matt placed 2nd in the IMPBA Gas Nationals Open Offshore class back in June! His boat of choice is very similar to mine – an AC Models 42″ Catamaran! While I had some knowledge of real boats, and have some RC experience, I knew not so much when it came to large gas RC boats. Thankfully, Matt was kind enough to answer my questions and get me going in the right direction!

OK – back to the fiberglassing issue – so as it turned out, I needed a little more than to just reseal the hull. The first thing I had to do was remove the old fiberglass, and get the hole cleaned up correctly. The hole was not straight with the hull, so I cut it a little larger. This gave me a nice opening with which to work! In the first image, you can see the rear engine mounts – I removed the engine and Radio box (a water-tight box that houses the radio system) to make working inside the boat easier. with everything out of the way, and the hole opened correctly, I roughed up the inside and outside of the hull with sandpaper. This will allow the new epoxy a good surface with which to bond. Some denatured alcohol and a shop rag were used to remove the sanding dust and oily residue inside the boat.

This is the stuffing tube – it is the brass tube that supports the flexible propeller shaft. These photos are of the old tube that I removed. It is bent poorly and in bad shape! A quick visit to Zippkits.com and advice from Matt had me placing an order to get the new parts I would need.

As you can see in the first photo I ordered two brass tubes. One is just large enough for the other to  fit inside. the inner tube is the stuffing tube. the outer tube is the stuffing box. “Why two?” I asked my friend. He said this makes it easier to replace the inner tube if ever needed – it will simply slide out of the larger one, and the hull won’t need to be repaired a second time!  Also in the picture are a couple of new drive dogs – these are what lock the propeller to the propeller shaft. They yellow nozzle and the purple Y- Fitting were purchased to make an apter that will allow me to hook a water hose to the boat. With this adapter, I can run the boat out of the water, and not risk overheating the engine.


With the engine and radio box out of the way, I started looking over the rest of the boat. I noticed right away that the fuel tank was set up with silicone fuel line designed for glow fuel. This is a big problem, as gasoline will cause this type of fuel line to swell and start deteriorating very quickly. I removed all of the silicone tubing and replaced it with DuBro Large Tygon Fuel tubing.

While the engine was removed from the boat, I decided to disassemble it and give it a good cleaning. The first step was disassembly – though there are a lot of different parts to a marine engine, it was pretty easy to see how it all worked. the mounting system, as well as the pull start and collet (this connects the propeller shaft to the engine) appear to be all from Zippkits.com, making iteasy to get replacement parts when needed.


With everything disassembled, I started cleaning and reassembly. I began with the pull starter – as well as cleaning, I pulled out the rope and checked it for fraying and made sure it was free from damage. I also cleaned and checked the starter dog (the part that attaches to the engine’s crankshaft). All parts looked to be in good shape, so the starter and forward engine mount (The pull-starter sits to the front end of the boat) were reassembled. I also removed the covers from the carburetor to check for damaged diaphragms and debris – thankfully, All was in good order, so the carb and isolation mout were reinstalled.

The rear engine mount, water-cooled exhaust manifold, and header pipe were all cleaned and reinstalled after a quick inspection. Other than being dirty, the engine was in great shape! Speaking of the engine, it is an RCMK 24.5cc engine. Although this engine appears to be available at several places on the web, I have not needed to buy anything for it yet… I hope it stays that way!

I reinstalled the engine and gathered all the parts I would need to install the new stuffing box and tube, as well as the propeller shaft. Matt had told me to apply a a little heat from a propane torch to the stuffing tube prior to bending it. This would prove to be helpful when I made the bend. Prior to bending, though, I had to figure out how much of the stuffing box I would need. A few measurements and ‘eyeballing’ the location a couple of times, and I had my stuffing box length. Out to the garage I went, and I cut the stuffing box on my band saw. A sharp Exacto blade made quick work of the internal burrs left by the saw, while a piece of sand paper took care of the outside of the tube. With all the parts laid out in front of me, I started assembling and installing them. After making two marks on the tube, I heated it and gently bent it WITH THE PROPELLER SHAFT INSIDE THE STUFFING TUBE. Having the propeller shaft in the tube allows an easy bend without the worry of kinking the stuffing tube.

With the tubes and propeller shaft in place, it was time to start repairing the hole in the hull!

I taped the stuffing tube and box in place on the outside of the hull – not only did this keep the tubes in place, but it made a ‘mold’ for the fiberglass matting and epoxy I was about to lay in place. With the tubes taped, I mixed up a large patch of 1.5 hour epoxy. This stuff was awesome, because it gave me plenty of time to lay all the fiberglass in place, and get the epoxy worked into the matting very well without the fear of the epoxy curing. At this point, I was done for the night, and let the epoxy cure for a couple of days.

Once the epoxy had cured fully, I removed the tape from the bottom of the hull – the fiberglass had definitely adhered to the hull nicely, but the resulting transition was less than pretty. Thankfully, I had remembered an old trick I had learned to make ‘Tiger Hair’ epoxy resin. I took a small piece of the left over fiberglass matting, and cut it into small ‘strings’ about 1/2′ long. I added these strings to a small batch of epoxy as I mixed it, and it made a strong epoxy/fiberglass filler. this filler was laid over the existing, ugly resin, and worked into a respectable looking shape. This stuffing box isn’t going to go anywhere!

It was time to reinstall the radio box! I really love Hitec servos, and I happened to have some extras in my ‘spare parts’ box. I figured it couldn’t hurt to put a digital metal geared servo on the rudder, and a standard servo would more than take care of the throttle. The servo mounts, I found out, were also from Zippkits.com. These mounts are really cool, and made side mounting the servos a breeze!

I replaced the rudder pushrod with a few parts from DuBro – they’re all 4-40 parts, so they should stand up to stress well. A 4-40 ball joint on the servo end and a solder connector and locking pin clevis on the other end made for a slop-free setup.

Before I forget to mention it, I decided to go the economical route for a transmitter. This will be the first time I have ever used a FlySky radio setup, but I have heard good things about them. This 3-channel pistol grip transmitter came with a 3-channel receiver, and cost a whopping $37.00!!!

The throttle servo was installed like the rudder servo, but the pushrod is connected to a 90° bellcrank – this allows for an easy throttle connection!

Even though the radio box is supposed to be water-tight, I wrapped the receiver in a balloon. This should make for a nearly waterproof receiver. With everything in place, the clear cover was set in place on the radio box and taped in place with 1/2″ wide clear tape.

When it came time to install the propeller shaft permanently, I greased the shaft liberally – this is a job much easier (and cleaner) done with a rubber glove on. with the prop shaft ready, I added some extra grease to the inside of the strut and slid the shaft in place. As the shaft exited the stuffing tube inside the boat, I slid the shaft saver in place, then secured the collet and prop shaft to the engine. The collet MUST be tight!


The tuned pipe was slid onto the header and adjusted so that the widest part of the tuned pipe was 12″ from the exhaust port. If I plan to race the Blast Cat in the future, The pipe will be further adjusted for top speed, but 12″ is the best starting point. the rear pipe holder was adjusted so the pipe cleared the canopy when it is in place, and the muffler was added last.

Although I didn’t HAVE to, I replaced all the silicone water lines – they felt sticky and looked bad. A 4′ piece of DuBro 5/32″ water line runs about a dollar a foot – for that price, I thought it was worth replacing. The new lines were routed in the same manner as the original lines.

The last thing to do was to set the cover in place and secure it at the four corners with the O-rings. With this, she’s ready to go!


I have put a video together showing my first startup and run since getting it ready. When you watch the video you’ll see two nozzles squirting water out of the side of the boat – one is from the engine’s cylinder head, and the other is from the exhaust manifold. I had the water pressure opened very little to get this amount of water running through the lines, which proved to be more than adequate for cooling purposes. If you’re used to an airplane engine like I am, the high idle really sounds weird, and that’s why I grabbed the transmitter so quickly to slow it down. Matt tells me to keep the idle up – since there’s no clutch in this boat, the propeller is a direct drive from the engine. Low idle speed will cause the engine to die in the water. The key is to keep the idle set high enough to keep the engine running when the throttle is at idle – this will most likely be a trial and error system, one that I don’t want to do without a rescue boat available! Check out the video below!

That’s about it – I’ve learned a lot from this boat, and I’m already thinking about entering a race next year! Perhaps next summer I’ll give you all a ‘Part 2’ on running and racing!





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