If anyone is interested I posted some pics of a 503 rotax mounted upside down in a 10' x 5' boat. Just finished tacking the engine stand and threw a few pics on the MEMBERS GALLERIES section of southern airboat. Kind of a fun project. Alot easier than building a stand for a regular size boat. The pics were posted on January 3rd and under the user name CATFISH. note that the motor is mounted upside down to keep the center of gravity lower. trying to keep that small boat from being too top heavy.
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503 rotax engine stand, motor upside down
- Thread starter catfish
- Start date
Des Moines Boater
Well-known member
Hey, that's an old AirCat hull. I had a couple of those. Boy are they heavy. I think the ones I had weighed as much as my 13 x 7 foot Laser Hull and that one is only 10 x 5. I had a 1600 cc 65 horse VW motor on mine and it was fun but way under powered for such a heavy hull.
The prop is a 52" 3 blade and I believe the guy I'm building it for bought it from arrowprop. The motor is supposed to put out 54 horsepower so with the 2.26 ratio it should be throwing around 120hp to the prop. The boat is quite light so with one person and a cooler full of leinincugels it should move pretty good. I used 16guage 3/4" square steel tubing for the engine stand and it is solid as a rock. The motor with the 3/8" aluminum plate and the belt drive all weighed in at about 95lbs total. The engine stand came in at about 35lbs so the total package minus the cage is right at around 130lbs. I'll let you know how it works when it's up and running. never built a mini before so this is kind of new to me.
the hull is a 10 foot X 5 foot and about a foot or so deep on the sides. it has steel runners on the bottom so it can run on ice also. since we live in wisconsin runners are a nice option for the winter months. buoyancy with such a small boat will be an issue so I need to keep it as light as possible and keep it overpowered instead of underpowered. since rotax motors are light and snappy it seemed like a nice match for this boat size.
The motor is supposed to put out 54 horsepower so with the 2.26 ratio it should be throwing around 120hp to the prop
Are you sure thats how redrives work? Maybe I have been missing something all along. What RPM is the 54 HP rated at and what RPM do you plan on running the engine at?
Scotty :wink:
Are you sure thats how redrives work? Maybe I have been missing something all along. What RPM is the 54 HP rated at and what RPM do you plan on running the engine at?
Scotty :wink:
I am not exactly sure what the peak rpm for the motor is or where it peaks on the torque curve. The guy who I'm building it for actually bought this set up as it's used fairly often on ultralights and powered parachutes with good success, so I must admit it's use on an airboat is not known to me at this time. I was told by 3 boat builders that a good way to estimate the horsepower and the torque that will actually be transmitted to the prop is to multiply the hp or torque by the ratio of the belt or gear drive. so if the motor is running at say 35hp to maintain the boat on plane then the estimated hp to the prop would be about (35 X 2.26 =79hp to the prop). Since 2 strokes build torque right from the start it should theoretically push this boat fairly well. Well see once it's done as like I said I've never built a mini before. Believe me I've got my fingers crossed on this project! I have to admit though the prop that was recommended for this set up is fairly wide and 52" diameter so if the motor turns it like it is supposed to it should throw a nice pile of air behind that small boat.
Catfiish, first you got the right idea there. That mount lowers the center of gravity big time. Being a 2 stroke it could care less which way is up, assuming it doesn't have float type carbs. Diaphram carbs dont care.
You can not create HP with a reduction drive. HP in is equal to HP out minus any friction/gear losses. However torque on the other hand can be moved from one place to another with a reduction drive, by letting the engine run at its design speed and then letting the prop run at its design speed. Yoiu get more out of each that way.
Read the threads on reduction drives on the forum there are excellent writeups about what is multiplied and what is not and how to calculate where what will be and how much there will be. Its all right here on Southern Airboat for the reading.
I like the low engine mount it has been proved on gyroplanes and ultralights over and over and is a proven concept. Especially in gyroplanes which generally are fighting a low c/g and high center of thrust. Thats another discussion though. In an adeal situation the center of gravity and center line of thrust and the centerline of drag would all be equal but thats not attainable in a boat or ultralight, its just theoretical. But its better to approach things with that goal in mind.
In an airboat the center line of thrust is not as important as getting the weight lower , that is not to say the centerline of thrust doesn't matter. In bigger boats with big HP it can make their nose dive into the water under hard acceleration. In a mini application the low weight has more effect than the center of thrust line. You can still trim the boats by adjusting the thrust line and weight after its mounted.
I like the idea you found as a begining point for experimentation.
Scotty :wink:
You can not create HP with a reduction drive. HP in is equal to HP out minus any friction/gear losses. However torque on the other hand can be moved from one place to another with a reduction drive, by letting the engine run at its design speed and then letting the prop run at its design speed. Yoiu get more out of each that way.
Read the threads on reduction drives on the forum there are excellent writeups about what is multiplied and what is not and how to calculate where what will be and how much there will be. Its all right here on Southern Airboat for the reading.
I like the low engine mount it has been proved on gyroplanes and ultralights over and over and is a proven concept. Especially in gyroplanes which generally are fighting a low c/g and high center of thrust. Thats another discussion though. In an adeal situation the center of gravity and center line of thrust and the centerline of drag would all be equal but thats not attainable in a boat or ultralight, its just theoretical. But its better to approach things with that goal in mind.
In an airboat the center line of thrust is not as important as getting the weight lower , that is not to say the centerline of thrust doesn't matter. In bigger boats with big HP it can make their nose dive into the water under hard acceleration. In a mini application the low weight has more effect than the center of thrust line. You can still trim the boats by adjusting the thrust line and weight after its mounted.
I like the idea you found as a begining point for experimentation.
Scotty :wink:
Whitebear thanks for the info. as I am not totally clear about all this airboat stuff as of yet and I will have to do some digging around southern airboat and keep taking in info. that others have already obtained for myself and others. Southern airboat and it's members have helped me out alot on trying to figure some of the mystique of these airpowered vessels and I appreciate it immensely. thanks again.
KNUCKLEBUSTER
Well-known member
We are starting to build another Aircat "Tomcat" like the one in the picture. Our engine is a Rotex 600 cc 3 cyl. water cooled. Catfish has me thinking about hanging it upsidedown also!
Will the water-pumper aspect still work OK upside down? We want to keep the COG low, and also, mounting the prop-shaft at the bottom of the engine would be a lot easier due to lack of convenient mounting places on the back-side of the engine. We are thinking of using the snowmobile primary & secondary clutch system for a redrive.
Does anyone know where to get a tachometer that will work on a prop shaft?
He was right on about the weight of that little boat. That's why we decided to use the rotax claimed to weigh 80 lbs. and put out about 125 hp. That should handle it OK. :bounce: :bounce:
Catfish, where abouts in Wisconsin are you from? I'm about 30 miles north of St. Paul, MN.
Dave
Will the water-pumper aspect still work OK upside down? We want to keep the COG low, and also, mounting the prop-shaft at the bottom of the engine would be a lot easier due to lack of convenient mounting places on the back-side of the engine. We are thinking of using the snowmobile primary & secondary clutch system for a redrive.
Does anyone know where to get a tachometer that will work on a prop shaft?
He was right on about the weight of that little boat. That's why we decided to use the rotax claimed to weigh 80 lbs. and put out about 125 hp. That should handle it OK. :bounce: :bounce:
Catfish, where abouts in Wisconsin are you from? I'm about 30 miles north of St. Paul, MN.
Dave
SANDRAILER
Member
i bought digital tach hour meters on ebay it is accurate and will work on all engines ,hooks on plug wire and ground. thats it