Recommended prop for volvo 250 outdrive

[KDG]

My Volvo 250 drive has a 1:61 ratio. We're running an aluminum 14 x 19 prop on our 66 18' with a 351 Windsor. W.O.T is 4800 rpm and in the low 50's. Any assistance regarding the correct prop size would be great. Thank you in advance.

[mattyboy]

running those numbers you are at around 3% slip which means the prop you are running is almost perfect

[KDG]

Thanks Matty. Could a change in prop size get her in the upper 50 MPH or possibly higher? I will say that with the current prop, she jumps on plane and also accelerates quickly.

[mattyboy]

the prop you have must be cupped and acts larger than a 19 with your numbers if I add an inch of pitch your slip numbers are still low. most of the alum props had high slip numbers. the solas titan has 7 % slip which is the lowest slip number I have seen.

say i suggest a solas 19 your rpm are gonna drop 200-300 rpm and with 7% slip your speed will drop. the other option would be that a healthy 351 should be able to turn a 21-22 pitch to 4900-5000 rpm so I would guess your prop is more like a 21 22 pitch if not more. low 50s are the wheelhouse for a volvo classic so you are not off by much .

if you want high 50s you need to add power and spin a solas 21 ultra 24 to 5000

[KDG]

Thanks

[mattyboy]

KDG

it is always good to have a spare prop . I re ran your numbers using 52 mph and 4800 rpm on 1.61 gears using 10% slip which is a good slip number,
means the prop is around 21 pitch. So if you are looking to try props a 19 solas or a 22 ultra would be props to look at.

the ultra 22 should keep your rpms or may gain some rpm the prop will give you some bow lift and reduce the wetted surface so it may net you a mph or 2 it has around 12-14% slip

The ultra will drop your rpm but will probably keep the same speed it will handle well and keep the boat at a steady angle of attack and carries the entire boat it has a 7-8% slip