Prop size and pitch BD722 Beta Marine on Mac 27

[Colin Jones]

Macwester 27 with Beta BD722 / Beta 20, rated 20 hp at 3600 rpm. Looking for real-world propeller matches. What gearbox model and ratio are you running, shaft diameter, prop diameter/pitch/blade count, and what max rpm do you achieve under load with a clean hull? I need to source a new prop for our new Mac 27 but suspect the current 2 blade is not right.

[Stuart Marklew]

Hi Mate, my Stag 28 with the bd722 was sized for 13″x 9″ p , however a 12″ x 10″ pitch should be equally as efficient if you need a bigger clearance, it must be a Right hand prop preferably with a 25mm dia shaft as opposed to the old 1″ dia shaft

Regards Stuart Marklew.

[Colin Jones]

Hiya Stewart. Many thanks for the info one second question, 2 or 3 blade?

[Colin Jones]

Sorry Stuart for the mis-spelling

[Bob & Freddy Tuffnell]

Hi Colin. I have a MW 27 (1976 : not that that matters, for all years of production, the hull and keels were not changed). I also have the Beta 20, with 2:1 reduction gearbox.

Unfortunately, I can’t find where I noted the inscriptions on the prop, and as the boat is afloat for the season, I can’t go and have a look. From memory, it’s 15” diameter, and 12” pitch, 3 blades, R.H. rotation.

That seems to be very well matched, I can reach around 6 knots in calm water at 2800rpm, without any fuss. More throttle will increase the size of the bow wave, and perhaps add half a knot, but no black smoke from the exhaust like my previous MW27 would do with its Volvo Penta MD2B 25 hp!

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[Bob & Freddy Tuffnell]

Sorry, I forgot some of your questions: It’s a 1” shaft, and a PRM150 gearbox. The entire drive train, prop and prop shaft, etc. was replaced in 2014 by a previous owner in 2014/5, when the new engine was fitted. Personally, I would have preferred a classic stern gland with a stuffing box , but there is a dripless  “Volvo” style one fitted.

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[Stuart Marklew]

Hi Colin, sorry for the delay in getting back to you, yes it was a 3 blade prop.

I am sure the tech team at beta would advise you.

[Colin Jones]

Thank you Bob and Freddy. Your information is very helpful. Although conflicting with other research. Beta Marine think, based just on the engine and gearbox ratio, that a 13 x 9 RH 3 bladed prop is the correct size. They don’t give opinion regarding the boat. The 15 x 12 is interesting with an rpm of 2800. How do you find it when manoeuvring at slow speed in the marina? Thanks. Colin

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