CMB 90 - Geared or Direct Drive

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Jake Ellwood

Well-Known Member
Joined
Aug 11, 2019
Messages
122
Hi all,

Currently eyeing up a Dave Marles Orion 48 cat (racing kevlar edition) and thinking a 90 sized nitro engine would go like **** off a shovel in one of those. So, looking at CMB engines, I notice in their new M-Line range they have both geared and direct drive options available.

Now, I struggle to get my head around how a the geared version works but I know the basics is that it allows it to throw a bigger prop, therefore shifting more water, equalling in more speed at the same RPM. Thats how it was basically broken down to me anyway.

So, is there someone who has much experience in this field and first of all can they tell me if it is a significant difference in terms of potential performance by going for the geared version? I am not looking at competing, just casual (extreme, lol) hobbying.

Is it worth the extra €? or are we talking about a minimal difference in performance here?

Any thoughts and discussion is welcome :)

Note: Before anyone suggests it, I am just not interested in Gas engines! Thanks!
 
The theory is a larger prop at a lower rpm is more efficient than a smaller prop at higher rpm, and it is that efficiency that is supposed to allow for additional speed. The caveat is you introduce additional parasitic drag into the driveline from the gears and hence you have less horsepower to put to the prop, and you have additional mass in the drive train that slows the acceleration of the hull. Worth noting is straitaway record holding boats, to my knowledge, do not use gear reduction systems.
 
The theory is a larger prop at a lower rpm is more efficient than a smaller prop at higher rpm, and it is that efficiency that is supposed to allow for additional speed. The caveat is you introduce additional parasitic drag into the driveline from the gears and hence you have less horsepower to put to the prop, and you have additional mass in the drive train that slows the acceleration of the hull. Worth noting is straitaway record holding boats, to my knowledge, do not use gear reduction systems.
Thanks for the reply Paul.

So to throw a larger prop at a lower RPM, does the geared version cause the prop to turn at a lower than 1:1 ratio than the engine?
 
Jake,

You might want to consider two engines geared to one shaft in a ratio of 1:1. A setup like this using spur gears would double the HP & the torque minus gear losses.

Jim Allen
 

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The theory is a larger prop at a lower rpm is more efficient than a smaller prop at higher rpm, and it is that efficiency that is supposed to allow for additional speed. The caveat is you introduce additional parasitic drag into the driveline from the gears and hence you have less horsepower to put to the prop, and you have additional mass in the drive train that slows the acceleration of the hull. Worth noting is straitaway record holding boats, to my knowledge, do not use gear reduction systems.

Let me tell you a story.....In 2006 David Hall and myself decided to assault the .12 tunnel SAW record....
In 2006 there were no reverse rotation outboard cranks made for .12 Novarossi motors ....also there were very few props available for small displacement motors.....
We decided to build a gearbox to not only to reverse motor rotation but to match it to an Octura backcut 1440 which was a proven to be a great 3.5 tunnel prop.....
After many discussions we used the SWAG estimate system to determine the gear ratio of the gearbox....we decided on a 20% underdrive and David built the gearbox .....
Long story short..... We ran the boat at the November IMPBA record trials and in a couple of passes upped the existing record from 38 to 49+ mph......That record still stands today,13 years later.....
Obviously there has to be some sort of parasitic drag in the gearbox but I don't know how you can easily measure or quantify that but by crunching the numbers we do know the efficiency of the prop raised considerably by reducing the props rpm and setup depth....
One thing we also know.....there is a ton left in that .12 tunnel record.....The Novarossi .12 we used was the very first generation Nova 12 with a 10.5MM crank and there were virtually no props available like there are today....
Another thought , virtually all FSRV boats run in Europe today all use a gearbox and they run very well on 15-20% fuel....
You make the call....but I for one am not discounting the gearbox idea....
Rod Geraghty
PS: I have a video of that boat running some where .....I will try to find it and post it....
I also have some gearbox photos somewhere ....I will also try to find them and post them.
 
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Been running a gear drive in my 20 mono since 2003, I do pretty good with it.

full


Ran the same class direct drive for a few years before and found you had to run very small props to get the little motors to rev into their HP range, that meant lots of slip if the boat was tightened down for heat racing. With the gears the motor revs to where it's happy and the prop is more efficient. I can run as fast as the fast guys but my boat runs through the chop better from my experience.

My ratio is 1.5:1 so if the motor turns 30K the prop is at 20. My props are an X448/2 or a X447/3 whereas typical props are X437/3 or X440/3.

I doubt the advantage would be worth the extra effort with a 90 motor tho as the "band" is prolly in the mid 20K range. :)
 
my old fsro 21, narval f3 hull French made, gear box 20_30 prop octura x447/3 cup, fuel 25% nitromethane
 

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FSR hulls are a lot different than the monos we run and would not be legal here, but drag robs power to no end.

Brad
What in the hell does mono bottom design have to do with gearboxes????
I am not arguing that there isn't any drag in a gearbox , but by definition there has to be drag....I said that in my post...
All I asked was how do you measure gearbox drag and how much parasitic drag is there in a gearbox....
All I said was 99% of FSRV boats run a gearbox and they run very well....
All I did was relate my personal experience with a gearbox.....

Question ; How much parasitic drag is in a gearbox?
Obviously you have a lot of facts and experience with RC boat gearboxes....so educate me .....

The following link is one of the best I have found on spur gear drag and power losses....
https://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19800010132.pdf
 
Been running a gear drive in my 20 mono since 2003, I do pretty good with it.

Terry,
I have seen your 20 mono run with the gearbox and it runs very well....
You experienced the same thing we did....When you reduce the prop rpm , prop efficiency goes up...
Our experience was the increased prop efficiency far surpassed the drag loss in the gearbox...
I don't believe anyone in North America RC boat racing has fully tested this approach of power transmission...
I do not know for sure if there is only an advantage with a 20 motor....Who knows??????????



full


Ran the same class direct drive for a few years before and found you had to run very small props to get the little motors to rev into their HP range, that meant lots of slip if the boat was tightened down for heat racing. With the gears the motor revs to where it's happy and the prop is more efficient. I can run as fast as the fast guys but my boat runs through the chop better from my experience.

My ratio is 1.5:1 so if the motor turns 30K the prop is at 20. My props are an X448/2 or a X447/3 whereas typical props are X437/3 or X440/3.

I doubt the advantage would be worth the extra effort with a 90 motor tho as the "band" is prolly in the mid 20K range. :)
 
the top speed with or without gearbox is almost identical but the engine with gearbox is used with maximum rpm, in turn the boat loses less fat speed to a larger propeller that brings more thrust, my narval F3 was first with the direct drive engine but it was more efficient with the gearbox
 
I think that I saw a formula similar to the one used here in my bowl one day when I was eating some "Alphabet Soup":):):)


If the drag was less than 2% and everything else being equal, then that means that it would only account for about 1 MPH difference at 50 MPH.o_O
Now we need to know how much of an increase in efficiency the larger prop brings.

I Love Science.

Good job Rod....
 
Quite the report and calculation. Going back to Jake's original question, the more moving parts you have the more problems you can have. If you have lots of extra money then go ahead with all the toys, gear drive and so on, but if it is just for fun a 90 powered cat direct drive is the simple way way to go HAVE FUN !!

Brad
 
Fellas,

Not sure about propeller selection for a gear drive 21-motor, but I have friends in Japan
Running pitched up 1455/3’s and they are starting to try 1457/3 next. It must be
their hull designs that allow that much propeller diameter. They are running pretty
fast with their gear drive 21-Monos. With all the new designs from Columbus, Ohio
I am pretty sure we can make them go even faster. Still a lot more testing to come
on this subject for sure. We are just starting to scratch the surface.

Thanks For Reading,

Mark Sholund
 
Fellas,

Not sure about propeller selection for a gear drive 21-motor, but I have friends in Japan
Running pitched up 1455/3’s and they are starting to try 1457/3 next. It must be
their hull designs that allow that much propeller diameter. They are running pretty
fast with their gear drive 21-Monos. With all the new designs from Columbus, Ohio
I am pretty sure we can make them go even faster. Still a lot more testing to come
on this subject for sure. We are just starting to scratch the surface.

Thanks For Reading,

Mark Sholund

The gear ratio of the gearbox would be the major determining factor as to prop selection....
 
In Europe virtually all racing surface drive monos 3.5cc and 7.5cc use gearboxes for the simple reason that they are faster than direct drive in the oval races here. The bigger props are more efficient and give better bite in the rougher conditions that we often race in here with up to 10 boats in a race. The races are longer than in USA and so the boats are a little bigger to carry the fuel required. Most races also use the transponder system and contrary to what I read on IW :) we do use nitro. We call our racing Offshore but actually it is oval racing, same as USA just a bit less of a sprint. The advantage of taking the start is much less in our racing.

https://docs.wixstatic.com/ugd/36beee_69a40eccb1c74a7c9ad042b8f2d42132.pdf

To answer Jake Ellwood's question, in my experience the difference on gears with a 91 in a mono or cat would be marginal but there is still a small advantage although the extra complexity and weight is probably not worth it.
 
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