TAPruitt

Does anybody know if there is a overdrive type gear box for large engines. I'm looking for one for my next build. About a 2 to 1 gear ratio, or 50 % overdrive. Thanks Tim

Justaddwata

My Feedback: (3)

Something like this. Not sure how strong it would be or what ratios they have as an option.

http://hyperprod.bizhosting.com/v_drive.html

TAPruitt

I did check on that one. Also another one that I found on dragboat.com. I emailed them and they sent me info on gear ratio's and power handling. They use it with a 122cc engine so I believe it will work. I would need another mhz gear box behind that one for twin props. I don't believe the mhz box would take the power. Here's a link to the engine that I want to use. I know its crazy, but I like crazy stuff like that. The hull is going to be a mhz 96inch v-hull. If you will give me your opinion on what I should use. Or is it just to much. The engine weight is 34lb's and 48 hp. If there is some way, then its a go ahead. There sending me a brochure on the engine. So I can get a good look at it.
http://www.cometkartsales.com/store/engine/aixro.htm

glennb2006

[X(]

I want to see the video of that running!!

Glenn

Justaddwata

My Feedback: (3)

Sounding a little too much for an RC boat (and believe me - excess is something I strive for). But a 34 pound 50hp rotery motor may not give you 1/2 the performance you think it will (I could be wrong though). Things you really need to consider are total weight. Motor + Pipe + Cooling + Rails and additional support + Fuel load + gear boxes + other hardware. Your talking about a boat that 2 people wont be able to lift and no amount of floatation will keep from the bottom if there is a problem.
True MH$ did sell this boat with the Rotax Max which was almost 30#. It ran a 2:1 Overdrive to the counter rotating 1.3:1 gearbox. This was a bit of an industrial setup and you may ask yourself why it is no longer offered in the US or Germany.

Also worth noting - The 96" MH$ hull has some pretty ugly handling properties from what I have seen. To the extent that a guy in germany cut the bottom vee off his boat from the last step to the transom (quite a decision on a $1700 hull). I have pics I will gladly share.

The box from RC Drag boats is likely inadequate. While they may have run it behind a 122cc motor it was also a motor that puts out less than 10hp (No offense RCDB but they dont even run tuned pipes on those boats).

I personally think you have a lot more things to consider than just getting all this stuff into a boat but rather - what is a good power / weight / performance setup.

Sharing my plans for the same boat
I am still waiting on prices for a counter rotating pair of Mathe 70's. These motors will put out around 18hp each and rev to around 17000rpm. They will weigh around 18 pounds each with clutches, pipes and mounts and cost less than $2k each.
No gearboxes are needed - just direct drive.
While I dont know the RPM capabilities of that rotery 300cc motor (pretty high if other roterys are anything to go by but lets say 12000). Running through a 2:1 gearbox your at 24000rpm and 25hp. A twin gearbox with 33% o/d your at less than 10hp per prop while nearing 30 000rpm. I am not saying it wont work but think you will have a lot more weight and a lot more $$ in a boat that may have questionable performance.

If your serious about this motor and think it will work then I would definately spend the extra $$ and have a gearbox made to fit. It is more power than anything RC related is designed to handle and you will ultimately save money by getting something made for it in the first place. Also keep in mind that no 1/4" flex cables will handle these power numbers. Even the Mathe 70 breaks them quick. You will need 5/16 or 3/8" cables and will need custom made hardware to suit.

Certainly let me know if I can help with any ideas you have kicking around.

TAPruitt

I did some racing with rotary engines Years ago. They have a very flat torque curve. That means it would be real easy on parts. Also there very easy to hop up. They already have them up to 65 hp. @ 14000 rpm's. Stock is 10.500. With very little modification. So I probably could get away with a MHZ gear box be its self. As long as it didn't come out of the water much. Ive seen there gear box inside and out. Its really tough little box [the billet one]. If I could get away with it then it would be real easy to do. If I could find props big enough I could just use one v-drive and a single out the back. I haven't looked for any yet but I will. One thing I will say about a rotary engines is they have a very smooth power band. If you didn't get crazy with it, it would probably last a long time. The top end would be just be unreal. I'll have to give this some thought first. About the engine and hull. Its allot of $$$$ Any time you do something like this your gambling. But I gotta build something crazy. The funny part is when I met with the fabricator that working on the pocket boat. He mentioned that hes been doing some work on a race car with a rotary and that's what got this started. I was hoping to find a little smaller one. Thank you sir VERY MUCH for your input. Tim

TAPruitt

You gotta check out this video. Its the second one down. You'll see what I mean by flat torque curve.
http://www.youtube.com/watch?v=MXW3oTWZ5YQ

Paul M

Matt, Twin Mathe 70's? Sounds like you and Googly have got a nasty little contest brewing

TAPruitt

Does the Mathe engine have two carbs or one. The reason I ask is twins with two carbs could be hard to tune. With a single carb it would be easy. Two strokes will fry a piston quick if they lean out. If the Mathe engines are single, they would be a killer choice.

Paul M

I'm pretty sure the Mathe 70 has a single carb. They make a great engine. I had a Casa 35 and it was ballistic until I let it eat a carelessly placed hand towel and bent the crank[:@]

Justaddwata

My Feedback: (3)

The Mathe 70 have a single carb.



Nah - no contest. He wouldnt have a chance and he knows it.

Paul M

Matt, I thought we were going to see that 70 in a GTX soon?

TAPruitt

I really like the single carb idea. That's the best way to go. Is it true that you need to have prop speed in the hydro-dynamically range. It sounds like BS to me. I'm getting to where I don't believe anything they on MHZ site. Does that apply to all props. Or just surfice props. I was concerting using a submerged prop.

Justaddwata

My Feedback: (3)

Paul,

that 70 is in a GTX that I am almost finished. I am shopping for a Counter rotating pair (mentioned above) for another future project.

Tim,

I am at the point where I am confident that the site you referred to knows as much about their products as my cat does.

It is certainly easier to find smaller props (and more affordable) but theres nothing wrong with big props (within moderation)

You were talking about sourcing bigger prop for a direct single drive. Theres plenty of options in the racing outboard market. Also Propshop do make props up to 115mm in diameter. (heres a pic of my 110mm prop - be a bit small for 50hp though!!)[link=http://groups.msn.com/_Secure/0UQDWDscYHvi!hA4FjiZ8395zGS*3CCNH2JsnNs7BhUxQQbTgL sId8SwMO0wkZTI7I2nIuTQwl6Sp4RnkZKUaqZuqgnikc6U6!Zm OePOgbNccExTxKuVDAiGpCPGG6i8j/Props%20031.jpg?dc=4675531577090853365]Pic 1[/link]
[link=http://groups.msn.com/_Secure/0UQDWDskYp*i!hA4FjiZ83w0FCMhVRPSfKv9OxY16TNN0XVI9T bdHST421kBE*eIni6S8OnzFcKprDzGn8DcWE1UbLFgO*7wPa9f uayYlabohiEs5JfdiqV8pU2TNvumY/Props%20024.jpg?dc=4675531577086378645]Pic 2[/link]

TAPruitt

WOW that's a mans prop there. I need one of them just to play with. Not sure on the rotary. They have a new, well new to me billet case for pocket bike engines. So I may just do two of them. They say with the new case there getting 20 hp. That means it probably has 15. Ive learned don't believe any HP figures unless there showing you it on a dyno.

Paul M

I've gotten to the point where I feel like advertized HP numbers are about meaningless as well.

Hey Matt, that's some cat you have there. Next time I have an engine problem can you send her over??