shem

I have a .21 mono that I got from Hong Kong and wondering if its better to run it with a gear drive or direct drive? I have a Novarossi 9 port buggy engine converted to marine use.

What do you think?

Ron Olson

My Feedback: (1)

I like direct drive as there is one less thing to go wrong with them. Hopefully it's not one of those converted over by just using the water-cooled head but still has the SG crankshaft. If so, I'd find a replacement crank made for boats although it may not be cheap, it will make things easier in the end.

misshydro

Well I'd talk to a guy who's uses gear drive boxes in his boats somtime ago when I was first setting up my Oberto1..He said you can get more speed out of your boat by using the gear box..It's like a real.. say prostreet car,You have a hot motor and you need to get the wright speeds out of your car..Well you have to find the wright gear combo for your motor to get the fastest speeds out of your car..without over reving your motor..Trust me if could use a gear box in my 1/12 Oberto there would be one in it. Because I have no room to put a gear box..[]

shem

Ron,

Yes its one of those that I converted by slapping on a water jacket and its with a SG shaft. I have cut off the SG shaft portion and the flywheel I've got mounted nicely on the shaft with lots of thread. I have several tanks on it to date no problems. Beside the SG shaft concern, what are the benefits of a boat crank shaft or rather a full threaded shaft?

MissHydro,

Thats what I figured too but was not sure. At present I have it direct drive and like it the clean layout. While setting it up, its either under prop or over proped. Sort of figured that these little 21 sized engine does not have the torque to spin the props directly but requires reduction gears to compensate since they usually are very rev happy.

So far I see there gear reduction units from Mocom. A little pricey though. Are there any other manufactures?

What are the typical speeds does a 21 sized mono do? This will give me an idea how far off my setup is.

regards
shem

f16man

The fastest best handeling boats I have seen use no gearbox but they run plenty of hp. and prop.

misshydro

some of the UK and euro/asia guys are running the gears boxes in there boat and there fast.I think there some in the US are running them too?I not sure?When I was doing up my Oberto.21 I had a guy who's at INTERWATERS told me he runs them.Thought it might work in my boat to get faster speeds. But I dont have the room to install one..One thing I think good about the gear boxes are you can gear then to push a bigger prop you couldnt run on a direct drive..=more power&faster speeds..If you cant aford a hight dollar motors like CMB Nova MAC. you could run a sport motor and run a gear drive to compersate for it..

f16man

The only problem is the sport engine already has issues with rpm then you gear it down and turn a big prop even slower, big push is great but you still have to move the water fast to go fast ,,,,,, I'm sure someone will chime in with better details.....

Espiga

The dimensions of the hull are important to determinate if would be better to use a gearbox or not. Assuming a standard 2:1 ratio for a .21 gearbox you will use allways a much bigger prop and the hull dimensions must be also bigger to compensate the aditional torque.
If you pay atention to the Naviga world champs results in th .21 and .45 offshore classes you will see always geared engines.

shem

I found these on a HK website. Anyone used it before? They look better then the MOCOM units. Though a little pricey.

Espiga

I never used such a gearbox. In the past I used mocom gearboxes and now when I need I make them. The main thing is the engine should be mounted in a flat surface and the the distance between gears. In almost every situation you must fine tune the fit between gears. With a little of skill it´s possible to make a gearbox assuming you can buy the gears. Anyway this gearbox you show in the photos looks fine.

shem

Espiga,

I have thought of making them myself but getting the gears with the correct ratio is the problem. Many years ago I did make one, which I used gears from RC cars. The gears have stripped and the company that make these cars is closed down.

The one in the picture looks 100X better then the one I made!! LOL


regards
shem

Espiga

These are some examples of the gearboxes a make. Now I can do the gears, but some years ago I didn´t have such a possibility and I ordered the gears, the steel gear with the flywheel and the delrin gear (you have perstwich,iansboats, and white&red RC). I think if you search you will find the gears. Always remember the MODULE 1 for the gears is the most used. I am now developing a gearbox for .21 outboards The first experience was very good

srw1960

In all my years being around boats I am yet to see a USA nats winning boat in any category running a gear box...they add weight and worse yet lots of friction in the driveline that robs torque from the engine..speed comes from turning that prop as fast as you can.and spinning as big a prop as youre engine will handle thats where torque comes into play...there several reasons why there are differant engines for the differant disciplines of RC I am yet to see a plane guy adapt a marine engine onto a plane or put a buggy engine in a heli..WHY???? the differant applications require DIFFERANT TORQUE CURVES FROM THE ENGINE...thats acomplished through variances in cylinder timing ,rod stroke length,crank throw geez the variables are stagerring..... stop trying to low buck it and get a descent .21 marine engine you can get them used in good shape pretty cheap..direct drive it..when istalling the stuffing box keep it as straight as possible to keep friction down...now then theres a thing called ,,,propping a boat...this done for rc and full scale alike and the same rules apply..you want a big enough prop so that youre engine at WOT (wide open throttle) is maybee 50 to100 rpm below redline you will have to get there by trying A BUNCH of differant size props also keep in mind that props have more to them than just 2 dimensions..theres is diameter, blade pitch and BLADE CUPPING...cupping changes how much water a blade grabs and spits out the more water it can grab and spit without overloading the engine the faster you will go it is simillar to changing the pitch angle but doesnt make as extreme a change in engine loading as pitch does...I do bellive marine engines are designed to deliver max torque mid way to the end of there usefull rpm range a car engine needs to produce more torque at a lower rpm to get the car moving off the line see what Im geting at? I applaud you for thinking outside the box if we didnt have people do that where would we be...but I truly feel that after all youre hard work you would just be disapointed..I myself have a miss lumar that has been treated to a K&B .21 with tuned pipe..BTW I bought the engine from e bay for 30 bucks..a new stuffing box the chinese one had to many tight turns in it an aluminum rudder and of course a copper berilyum prop..the radio gear was fine so I kept it....Ihope to get it on the water in the next few weeks and I bet it hauls..Ill let ya know but i allready know that Ill have to ..PROP it as well to really get it going good..I hope this has helped a bit have fun