Gearbox Selection

[Rick Spleen]

I have a PRM120D gearbox on my NB with 2.5:1 ratio which is now 15 years old and in need of a major overhaul. My boat is a little too slow at a comfortable engine speed so I was considering changing to a PRM box with 2:1 ratio. The intention being a slightly higher speed at the same engine speed. I know the 2:1 box is physically the same size as the 2.5:1 box and will therefore fit the engine and shaft without the need for any mods., but are there some technical considerations that I am unaware of that may lead to problems by using the same engine and prop? The engine is a Beta 30 and the prop is a Crowther 17 x 11.4 High Efficiency style. Any advice would be greatly appreciated.

[Guest]

I have a PRM120D gearbox on my NB with 2.5:1 ratio which is now 15 years old and in need of a major overhaul. My boat is a little too slow at a comfortable engine speed so I was considering changing to a PRM box with 2:1 ratio. The intention being a slightly higher speed at the same engine speed. I know the 2:1 box is physically the same size as the 2.5:1 box and will therefore fit the engine and shaft without the need for any mods., but are there some technical considerations that I am unaware of that may lead to problems by using the same engine and prop? The engine is a Beta 30 and the prop is a Crowther 17 x 11.4 High Efficiency style. Any advice would be greatly appreciated.

Do you feel that the boat is currently "under propped?" Do you have a rev counter, and does it make the required rpm at full throttle? By changing the box in the way you suggest, and assuming you have a satisfactory prop at present (able to meet but not exceed full rpm) you will have the effect of being over propped. My boat is slightly over propped, which is not always bad, but I feel I do lose a little power at the top end.

[Dalslandia]

The boat will not, in theory go faster with a 2:1 in, instead of a 2.5:1, it will need a smaller prop, unless you are very underpropped now.

if you keep the 2.5 and have a bigger prop you will go faster at low rpm/power, but as catw. say, do you reach the rated rpm now?

with the 2:1 you will have something like a 15"X9"

 

Question is how much space you have for a bigger propeller? a 2.9:1 will take a 18.5" -19" Diameter propeller, and you boat will go faster, will be 25% more efficient then the 15" and 20% more efficient then your 17"

 

Meaning, you got 20 % more horsepower thrust, called thrust horse power, THP, with the same horse power and fuel input.

but you want 3" clearance at top and 2" at bottom from prop tip. with that big prop

Edited August 31, 2015 by Dalslandia

[Rick Spleen]

Thanks very much to both respondents for taking the trouble to respond to my query. The replies were very helpful. I checked with Crowthers (very helpful people) and they advised that to do what I proposed would most likely lead to gearbox failure in a very short time. I'll therefore play safe and stick with the same ratio gearbox and my existing prop. As a few people on the forum have said, prop selection and design is a bit of a black art, so proceed without expert advice at your peril! Thanks again to those who responded to my query.

[DHutch]

As said, it all depends if the engine has the power to turn the prop at the new ratio, or the prop at that ratio prevent the engine reaching normal/current rpm.

 

The other option is re-pitching the prop a little

 

Around 11.4 is not pitch not uncommon and the diameter good, so while I know little about prop sizing and box ratios on modern diesels