Propeller

Propeller

Propellers shall have taper 1:12 and boring consequently 31.7 - 25.8 - 70.0 mm / Groove 8mm.

Moonraker is as standard from the Yard supplied with 1"1/4 stainless steel shafts, with 1:12 taper in both ends.

1. Fundamentally:

There is a tendency sailing with propellers with a too coarse pitch. The layman may be of the opinion that supplying the boat with propellers with coarser pitch, gives a bit extra! But it isn't that simple! It’s a connected whole, a correlation between the installed engine power, water line length (hull shape) and the propellers.

Moonraker exists in to versions, when it comes to the part of the hull, which has an influence on choice of correct propellers, that is with and without keel. The version with keel is far the most common.

Moonrakers most are powered with two Perkins 175Hp (135kW), which the engines do at 2600 revolutions. If your engines don’t reach that at full load (throttle), you presumably – other things being equal – not have the correct propellers fitted!

Powered by 2 x 175Hp are the correct propellers for vessels with keel 17”x13” and for vessels without keel 17”x14” (3-bladed Aquapoise 55% DAR). Calculations made by a Propeller factory shows, that the above sizes even are in the upper limit and may be should be 17”x12” respectively 17”x13” instead.

If you power really long distances with engines, which don’t reach nearby the correct revolution, the load on the engines is higher than necessary just like increased smoke development occur.

A rough rule of thumb tells, that with every inch the pitch is increased the engine revolutions decrease by 150-200, so does your meter show 2200 your pitch should probably be reduced with 1-2 inches.

2. ...However:

With eternally increasing prices on diesel, it isn't quite uninteresting to make sure, that one is sailing as economic as possible. This obviously can be done by putting the brake on yourself and "not give her the gun", but also by making sure that the propellers are optimized to the engine revolutions you normally do.

Now, - if you always are doing 1500 rev/min. and never or seldom are "giving her the gun", you can choose to fit propellers, which gives you a better fuel economy isolated seen by the low engine revolutions, and that of course will be with coarser pitch.

3. A bit more about propellers:

Moonraker is fitted with three-bladed "Aquapoise 55" type, with 55% surface. According to propeller producers, a lot has happened within "propeller research" since Moonraker came about. They are telling that fitted with a 1:1 gear, which Moonraker is, the propellers should be four-bladed and have 90% surface. This would result in smother running and greater efficiency.

A company making profit by selling propellers might naturally be interested in stating just that? Never the less I am considering changing my propellers, and if I do, I'll return with my experiences, and if some of you are able to tribute with something about this subject please contact us.

09/2003.

4. And more about propellers.

When I am measuring out the consumption of diesel, it has always been by registering from full tank to full tank = tanked amount of litres. Here I have very precise calculations, which tells that with 3-bladed 17x13 propellers, the consumption at 1500 revs (8-9 knots) is 2,65 L / nm.

After switching to 4-bladed 17x13 has my pattern of refuelling been different, and I have therefore not many precise refuels to make calculations from. However I have over 762 nm at 13-1500 revs (8-9 knots) registered a consumption at 1466 L, which means approximate 2,0 L / nm.

My engines doesn't reach the recommended 2600 revs at full throttle, but only 2200 revs, so as the rule of thumb says that each inch change in pitch gives a change in revs on 100-200, the propellers rather ought to be 17x12, if it wasn't because, I mainly on the whole sails just at 1500 revs, and that I want to optimize the fuel economy, by having propellers fitted, which gives the best economy at exactly these revs. The top speed hasn't changed, as it still is 19 knots.

Incidentally I through the years have had propellers with different pitch mounted, and this scheme ”Propeller_305”, is showing the connection between size, pitch, revolutions and speed.

Barry Greenwood, who owns Golden Sands nr.259, wrote in Moonrakers International some years ago:

“I fitted new four-blade propellers, size 16x13 with a blade area ratio of 0.87. Earlier this year and during trials with clean antifouling the boat achieved 20.3kts at 2350rpm with full fuel tank, half-full water tank and two crew. Later in the year when slime had built up we could only achieve 18.8kts. then made these changes:

The props were made 16x12x, and on speed trials I got 20.3kts. At the end of the following year the speed was down to just under 19kts. with clean antifouling etc.

I had the props repitched to 16x11 and although I could get 2500 rpm the boat speed was down to 17.3kts.

I had the pitch changed back to 12 but the boat speed never went above 19kts. apart from once, when I had tide and wind with me-it did 20.2kts! - again with clean antifouling.

On recent trips the max I can get is about 18.8 at 2350-2400. I tend to cruise at 15-16 at about 2250rpm.

Propeller/speed relationship is not an exact science, and the conditions of the boat, the sea, the atmosphere, can all have a bearing on the outcome. Although I had the pitch changed back to 12, each time it is altered they balance and fettle the blade edges and you lose a bit off the diameter. I also have Strippers fitted and carry a RIB, which I believe knock off about 1kt.”

11/2006