Thinner blades on sail drives?

[cmedsailor]

Looking around in the marina at the various boats (out of the water!), I have noticed that 2-blade fixed propellers on sailing boats with a sail drive have thinner blades than those with shaft. Is this a coincidence? Are sail drives more efficient and can keep blade area reduced? Just wondering...

 

[oldsaltoz]

Most cats with twin engines or outboards have high thrust props, lower speed and more push. Catamarans tend to sit on the water rather than in the water, with no heavy cast iron or lead keels to add to the drag or pull them down deeper in the water.

We run twin 40 Hp VP's with fixed 3 blade stainless steel props, 1,800 rpm around 7 knots for just under 1.5 litres an hour for each engine.

But much better under sail.

Avagoodweekend......

 

[johnalison]

A saildrive prop sits deeper in the water and ahead of where a shaft prop would be. My saildrive 2-blade certainly bites more effectively in choppy water than I would expect from a shaft, but I don't know any comparative figures. Some craft have been built with both types, and experience of these would be interesting.

 

[Will_M]

[ QUOTE ]
I have noticed that 2-blade fixed propellers on sailing boats with a sail drive have thinner blades than those with shaft....

[/ QUOTE ]

Fixed props on SDs will typically be of an alu. cast construction… Fixed props on shaft will typically be cast bronze, requiring a ‘chunkier’ profile.

Other than the fact an SD is more likely sat in clear water, think of hull profile say long keel v's shallow easy driven fin keel hull. Efficiency should be much of a muchness other than an SD's shaft line being horizontal compared to shaft drive at a given down angle.

The SD installation affords said designer more accommodation space inboard of cause.