[C38] Universal M30/5424 reduction ratio -- Prop size

[Tom T.]

Max made a good point about the ration not being 2:1.

As a note on gear rations, transmissions almost always use gears with 
oddball ratios so the teeth will hit a different opposing tooth on every 
revolution which helps give even wear and reduces harmonic chatter from 
eventual uneven wear.  A true 2:1 ratio would allow the teeth to hit the 
same opposing teeth on every other rotation of the smaller gear where a 
odd tooth gear against an even tooth gear will give more rotations 
before the same teeth will mesh again.  This is why one gear most often 
has an even number of teeth and the other an odd number of teeth.

This is why differentials on cars have oddball ratio numbers like 4:11, 
3:73, etc.

Most gears today are helical cut (angled) which makes a longer contact 
between teeth to eliminate the chatter of straight teeth hitting each 
other.  Performance motorcycles have straight cut gears and they make a 
loud whine you can easily hear.

Although helical gears reduces noise, they create gear side thrust which 
robs horsepower, creates heat and shortens the life of the 
transmission.  The Navy got around this problem with their ship 
transmissions by bolting two helical gears back to back, one mirror 
image of the other, to make a single gear.  They did this with both the 
drive and driven gear in matched sets.  Since the helicals of the gears 
were mirror image of each other, any side thrust was canceled out   The 
gears were also bolted a half tooth staggered so there was even less 
tooth chatter than from a single row helical gear.  It is almost 
impossible to machine these double sided gears without resorting to back 
to back mirror image gears bolted together.

Some of these marine transmissions were built during WWII and are still 
in service today without ever having been overhauled.


Tom Troncalli
Mental depository of worthless trivia





On 6/4/2013 11:46 PM, Max soto wrote:
> Jon, the HBW 100 is suppossed to be 2:1,  and thats what the tags says, but as a fact, that right ratio is 1.8:1. Several C38 owners found that out when they ordered a new prop with the 2:1 ratio, and the prop turned out to be over pitched, so, after careful measuring, they found out the discrepancy..  This is not only C38 related. Every HBW 100 that says 2:1 is in fact 1.8:1..
> I did a rebuilt on my tranny, and just curiosity, I checked it and is 1.8:1..
>
> Hope this helps..
>
>
> Max Soto A.
> C38 #198 Estancia
> +506-8312-1367
> Alajuela, Costa Rica
>
> On Jun 4, 2013, at 8:12 PM, Jon Whitney <jonwhit23 at aol.com> wrote:
>
>> Hi All,
>> I was just researching the proper prop size and pitch to match the Universal M30/5424 on our Cat38's (as I'm sure many of you have done).  I just got off the phone with Catalina Direct and we stumbled upon an interesting question that you may know the answer to.  We were looking up the gear reduction ratio on the transmission for the original M30/5424  and found a discrepancy in the ratios reported by Catalina and Universal.
>>
>> The Universal Maintenance Manual for models 5411/5416/5424/5432 states that the transmission reduction ratio is 2.14:1  (with an HBW-10 Hurth Transmission) on the 5424 (24 hp, 3 cylinder, 2800 max rpm).  However the Catalina factory specs state that the gear reduction ratio is 2:1 (for the same engine).   Does anyone know which is accurate and why the discrepancy?
>>
>> The difference doesn't seem like much, but when they punched all the numbers to determine ideal prop size it changes the pitch ~ 1 inch.  E.g., using the 2-blade fixed prop a ratio of 2:1 gives 15" x 13" pitch (13.3" to be exact), whereas the ratio of 2.14:1 gives 15" x 14".
>>
>>  From what I could gather the older S&S C38's with the Universal M30/5424 came standard with a 2-blade 15"x13" and an optional 3-blade 15"x12". Obviously, the 15"x13" is fine as thats probably what most of us have had, but just wondering which ratio/pitch is more accurate and if anyone has used the larger (15"x14") prop size. As always, I'm grateful for any advice.
>>
>> Best,
>>
>> Jon Whitney
>> 1979 Hull #54
>> "Eye Whitness"
>> Honolulu, Hawaii
>>
>> ps. Thanks to everyone for their comments on thru-hulls sizes (especially Max, Les, and Steve).
>>
>>
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