Two-Bladed vs. Three-Bladed Props

[barefoot]

<div>I know that M-382s with the Yanmar engine had two-bladed props
and that M-383s and M-384s with the Perkins came with power-boat
(wide-bladed) three-blade props. (I'm not sure about the Perkins
M-382s.) My M-383 is great under power -- smooth, lots of
thrust. But that big, fat prop is a real sea anchor under sail,
and it causes lots of turbulance over the center of the rudder.
The former condition is a poblem in light air and the latter in a
breeze, especially hard on the wind.So.... I wanna get rid of it. The cheapest and most reliable way
to do it would be to go to a fixed two-bladed prop (with more
pitch than the original). My question to those who have two-
bladed props is how smooth the two bladers are behind that
skeg... Do you get a thump, thump, thump as the blades pass
outside the skeg shadow each half rev? What diameter and pitch
are they? Do you know what the reduction ratio is in your
transmission and what RPMs give you, say six knots in flat water?
Has anyone already changed over from a three-bladed prop to a
two-bladed one? Does this seem like a bad idea? Thanks!

</div>

 

[aurora]

Lenny: Altho I am a CM 38 I made the switch first from the three blade to a two blade and then to PYI's Max-Prop, a self feathering prop. If you really want to improve your sailing performance spend the money and do it. I have had my Max Prop over ten years and at that time it cost me $1,100.00, I do not know the current cost. But I have had absolutely flawless performance from it and it improved my sailing boat speed well over a half knot. I have changed the pitch several times in order to permit the engine to run at its optimum rpm's. You can manually change the blade angles from 10 degrees to 30 degrees giving you a wide variety of pitch angles. They make them in prop diameters from 11 inches to 21 inches in 1 inch increments. The Cm 38s have a short distance between the rear of the full keel to the front of the rudder, and I had to dish out the fiberglass at the front of the rudder to accomodate the prop but it worked well and has never given me any trouble. Again, can't say enough for the prop and its performance. JHH

 

[ashapiro]

Lenny,
I have a 382 (Perkins 4-108) that came originally with a 3 bladed prop. Hull #223! Jim is certainly correct about the sailing qualities of the Max-Prop feathering prop. There are some caveats though. Be sure that the Max-Prop is installed by a knowledgable yard person as a friend of mine had a great deal of trouble when his was incorrectly installed. (Sorry, I don't know the details.) Also, make sure that it will fit, and can be removed from the shaft without removing the rudder. I doubt that you'd want to go to the trouble of pulling the rudder if you require prop repairs, etc. One really significant benefit of the Max-Prop is that you have quite a bit of reverse power for manuvering and docking and very very little drag while sailing. My experience with other boats would indicate that you would lose the reverse power with a two blade, with the corresponding decrease in drag under sail. My 3-blade gives me tremendous manuvering ability in reverse, but of course it's like dragging a bucket aft while under sail. The Max-Prop is a great solution, although pretty pricey. I bought one wholesale a few years back for a friend with a Freedom 39 and I believe it was around $1500 without installation! Best of luck!

 

[bobclark]

Lenny,
I almost did it. Ordered a fixed 2 blade 17X13 prop. Then I found that I could not remove the current prop with out dropping the rudder. The shaft is about 1" to long. I sent the prop back.
I was racing Southerly about 4 times a season and was looking for an extra edge. I was considering doing the Bermuda Race and did not want to drag 3 blades 700 miles.
Fact is, the 3 blade is a great cruising prop, and is well balanced for the engine and transmission. During the years that I raced Southerly (w/3 blade prop) the worse we did was
fourth in division. We have taken silver for third and second. But alas, NO FIRST on corrected time, even when we were first to finish. I believe the 2 bladed would have given me the racing edge. The key to winning is also a good crew. Two of my ace crew have purchased their own boats, so we have not raced last year and this year. I am glad I kept the original 3 blade under the circumstances.
It is a tough decision, good luck.
Larry

 

[aurora]

Lenny & Mick: True enough, as Mich points out, you might have to drop the rudder when first installing the MaxProp. I even had to shorten my prop shaft, along with dishing out the leading edge of the rudder, in order to complete the installation. The rear of the unit has a bullet shaped tail extending out behind the blades about 2-3 inches.I did do the installation myself (with the help of my son who is a master mechanic). But once installed it has worked great. The ease of changing the pitch to accomodate power requirements and rpm concerns is one of the big features. This can be done on any haulout. Mick's comment about power and control in reverse is also very true.

 

[frank_remus]

Those interested in Max-Prop. I converted 5 yrs ago. There is an article in the archives. The trick is to have PYI cut the forward portion of the hub half off. It is about 2 & 1/2 inches long and they will cut 1 &1/4 off. Kevin Woody Pres. of PYI assured me it would not affect the integrity of the prop, and I have had no problems in the last five years. I have a perkins "new" 4-108 (4000 rpm )with an HBW transmission with a 1.88:1 ratio. I live aboard and have put 2000 hrs on the engine and about 10,000miles on the boat. Xanadu is a 1984 384 With a 1983 engine. With this mod there is no need to rework the rudder. although it is necessary to shorten the shaft. ( As I remember 2 & 3/4 inches) it is easier to cut the flange end and cut a new keyway.