Props for the 1.8

[Neil2]

I've recently been in the company of a guy with a bmc 1.5 who has experimented with a number of different prop sizes in order to get the most out of his engine. I'd like to try something different on my 1.8 but it can be an expensive business. I have the ubiquitous 17 x 12 (well, I know it's 17" in diameter anyway) but Cleo is relatively small (47') and light (12 tons) and I wonder if she would stand something bigger.

 

I wonder if any 1.8 owners have tried different props and what the effects were? Eg my new friend finds an 18 x 13 too big for his 1.5 but had good results with a smaller "high efficiency" rig.

 

Any thoughts?

[cereal tiller]

the standard and well proven prop. for the 1.5 is 14x14(assuming 2-1 reduction).

a i.8should be comfortable with a 16 x 15 due to the increased mid -range torque.

assuming the aplication is for a narrow boat.

[Tony Brooks]

the standard and well proven prop. for the 1.5 is 14x14(assuming 2-1 reduction).

a i.8should be comfortable with a 16 x 15 due to the increased mid -range torque.

assuming the aplication is for a narrow boat.

 

 

Just to confirm the above, Crowthers and Castle Marine Propcalc both came up with 16 x 14 for my Bukh which has a very similar quoted BHP to the 1.5 but slightly higher torque (giving the BHP at lower revs).

[Neil Corbett]

Interesting. So in practice, what difference do you think a 16x15 would make over a 17x12 in terms of performance ad engine revs?

 

Just to confirm the above, Crowthers and Castle Marine Propcalc both came up with 16 x 14 for my Bukh which has a very similar quoted BHP to the 1.5 but slightly higher torque (giving the BHP at lower revs).

[Tony Brooks]

Interesting. So in practice, what difference do you think a 16x15 would make over a 17x12 in terms of performance ad engine revs?

 

My old boss used to say that 2 inches on the diameter is worth about 1" on the pitch so I would expect the boat to be slightly over propped (assuming the 17 x 12 was correct) with possibly very slightly more slip. However as I keep saying I consider prop sizing and getting it correct first time on akin to black magic so would ask a proper expert.