Is it Possible to Put a Bigger Propshaft on My Bertram 42?

[pascal bertram]

Hello everybody

I live in thailand and i recently bought an old bertram 42 (1984) i totaly rebuild the boat and my last probleme is that i didnot get decent propeller there are 22 inches bad local one dont i didnot know the pitch..
I get in topuch with propeller maker or dealer and it appeared that with the engine decribe below and a 2-1 transmission it is allready too much for my 1 3/4 propeller shaft .
if i want get a little speed i got to have 28 or 29 inches propellers and for that i need at least 2 inches shaft the minimum is 1.82 mm for 27 inches propeller.
Any body able to said me how to do here in thailand no professional and nobody you can belive is it possible to change shaft size without touch the fiber or ........................................
Thanks for your help in the states 10 000 peoples could for sure make it but here wooden fishing boat and boudha for the maintenance
With my best regards

Pascal

- Twin engine detroit diesel 671TI in line injecteur of 90 so
328 hp each one i suppose
model 6-71(1062-4310)serial 6A287260 PORT et 6A287354 STED
- boite transmission Allisson manufactured by detroit diesel allisson
Ratio 2.1
serial 42708number 6770016 PORT M20Ra on the right and on the left
serial 42416 STBD
number 6770015 M20L
- propeller shaft 1.3/4 inches
- Maximum RPM transmission in neutral 3000 RPM
- Cruising RPM 2100
- Maximum RPM 2300 RPM .ExternalClass .EC_hmmessage P {padding:0px;} .ExternalClass body.EC_hmmessage {font-size:10pt;font-family:Verdana;} - IDLE RPM 500/900

[GordMay]

Originally Posted by pascal bertram

... if i want get a little speed i got to have 28 or 29 inches propellers and for that i need at least 2 inches shaft the minimum is 1.82 mm for 27 inches propeller...

It's not usually practical to increase shaft diameter.

The size or diameter required for a propeller shaft depends on many variables, including the horsepower delivered through the shaft (called the "shaft horsepower", or SHP), the RPM of the propeller shaft, the propeller diameter, as well as the strength of the material to be used for the shaft.
In the absence of any information other than propeller diameter, the propeller size as a general rule should not exceed about 14 times the diameter of the propeller shaft. This means that a 24.5" diameter propeller, for example, would use a 1-3/4" propeller shaft.

Approximate Max Prop ∅ = 14 x Shaft ∅

1.75" Shaft x 14 = 24.5" Prop ∅

[James S]

Your cutlass bearing outside diameter will determine whether you can physically do the change with out any glass work.
Same size cutlass O.D. will have normally two sizes of I.D available. measure your O.D. (and length) and see what the maximum I.D. you can replace it with....that will be the Maximum shaft size.

[Fishspearit]

I'm not sure why Bertram uses such small shafts, but I doubt you'll be able to go much larger without glassing in a new shaft tube. We recently straigtened a bent shaft on a smaller Bertram and I was surprised to see only a 1-1/4" shaft for the 250 hp motor. I have the same (30 mm) for my 37 hp engine.

[pascal bertram]

Hello

And for you glass in a new tube is a big difficult and technical work or an eassy one ??????????????

kind regards

pascal

[GordMay]

To glass in a new tube and a new shaft strut is a big, difficult ,and technical job.

For more information on Propeller & Shaft calculations,
see page 87 onwards in “The Propeller Handbook” ~ by Dave Gerr
➥ http://books.google.com/books?id=8w09O4hooWUC&printsec=frontcover&dq=%E2%8 0%9CThe+Propeller+Handbook%E2%80%9D+~+by+Dave+Gerr #v=onepage&q=&f=false