Dear Balladeers,
We are looking at replacing our fixed 3 bladed 13x9 prop with a two blade folding prop, firstly to go faster under sail, and secondly as the current selection seems underpropped.
We have a Betamarine 20hp with 2:1 reduction on the gearbox. Max engine revs are 3600 in theory, but we can easily reach that with the current prop, and to get to hull speed needs almost full revs.
I have looked at lots of info about folding props and feathering props, and was hoping that someone had some experience to share. I notice that looking through the old MSN system, Aemilia, hull 978, had the same choice but there was no follow up about whether the choice was successful.
We are considering Flexofold (expensive), Gori (early design), Radice (cheap but look a bit primitive) and Varifold (made in UK so less penalty from weak sterling).
There have been a number of articles and test of varying rigorousness, and a two bladed flexofold comes out well.
I have done some preliminary contacts, and most suppliers seem to suggest a 14" diameter. Gori suggested a 14x11.5, but suggested a 14x9.5 for Aemilia with an identical engine and gearbox, and recommended a 16x10 to Josin (although with a higher gearbox reduction ratio of 2.3:1). This concerns me a lot, as they are very expensive for a bit of cast bronze, and I dont want to have to buy two if I make a mistake.
A quick spreadsheet calculation, using the 50% slippage quoted by Josin, suggests that you need at least 11" pitch to get 6.5knots at less than 3000 engine revs/ 1500 shaft revs.
Any comments would be much appreciated.
Cameron, Cracklin Rosie #73