Prop Design Calculations
Paul Winter raised a question about a 3-bladed prop. in the last magazine and here is the reply from Rowland Shannon (Dia):
Benefit of a 3-bladed prop
In short, yes, a 3-bladed prop. will significantly improve your performance under power. As a 2-bladed prop rotates, when it reaches the 12 / 6 o/clock position there is effectively no drive (fairly substantial dead wood in the water flow). However, with a 3-bladed prop. this ‘dead area’ is massively reduced giving increased efficiency.
Calculating size
As to calculating size, it’s not really a science as the manufacturers would have us believe, with their computer modelling and hi-tech formulae. It’s more of an art. There is, however, a fairly simple method of calculation.
Pitch
Initially you need to know the theoretical max. hull speed, which for a Twister is approx 6.25 knots. (square root of waterline (in feet) x 1.35)
Then you need the max. engine r.p.m. You should aim to achieve max. hull speed at 80% of this figure.
Now for the sums!
6.25 knots x 2000 yards = 12500 yards = 37500 feet = 450000 inches
Thus in 1 hour at hull speed the boat moves 450000 inches. Engine speed is expressed in minutes hence
450000 inches / 60 minutes = 7500 inches per minute.
Now say the max. engine speed is 3000 r.p.m. Take 80% of this which is 2400 r.p.m.
With a gearbox ratio of 2.21:1 the shaft speed is 2400 / 2.21 = 1086 r.p.m.
For pitch you then divide inches per minute by shaft r.p.m.
i.e. 7500 / 1086 = 6.9 inches
The fudge factor
Now for the fudge factor. Obviously a prop. isn’t 100% efficient. There are plenty of factors to be taken into consideration. Unfortunately the maths is well beyond my scope (despite studying engineering at University!) , but for a long keeled hull form losses will typically amount to 30%. To gain this you simply add to the theoretical pitch figure thus:
(6.9 /(100 – 30)) x 100
Which give a pitch of 9.8 inches.
The nearest round figure to this is 10” but you’ll find that by over propping the boat the engine will smoke more readily when under load, so I would go with a 9” pitch. (This holds for max. engine revs. of 3000 r.p.m. – yours may be higher in which case the final figure will be lower – and vice versa)
Diameter
I don’t know of any ‘simple sums’ for calculating diameter but there’s a reasonable graphical method I’ve seen in some publications. Anyway, it comes out at 12 or 13 inches. I’d be inclined to go for the 13” and if the engine struggles a bit you can always have it reduced fairly cheaply!
Rowland Shannon, Kia