new member So 39i

[bereboot]

Hello,
a little introduction, i am the new owner of a SO 39i, of 2008. Standard configuration, 2 cabin layout.
Before we owned a Wauquiez pretorien 35 ft (very good sailing boat, but it became too small for us.)
Hopefully we can find on this forum all the info we need about the "new" boat.
First question: propeller. This boat is equiped with a flex o fold, and won't stop. At 2,5 knts we need 2 boatlengths to get it stopped.
Has somebody any advice on this?

[alex1949]

Do not be shy, use your throttle more aggressively. In reverse ,those props need to be convinced to get fully open.  

[element]

Hi Bereboot.

Welcome on this forum.
We have a 39i (Performance) in 2 cabin version from 2007. Also first season for us.
We have the same problem. To stop we need a lot of revs and compared with our previous boat, a lot of time / distance. 
We have a 3 bladed prop. Don’t know which type, we think varifold or maybe flexifold.

I am still in the process of trying to get used to it or considering another folding prop if I would have certainty that this would solve the issue.

[rene460]

Hi Bereboot,

Welcome to the forum, you will find many friendly like minded folk here.

This problem is about folding propellors, and not anything particular to any particular model (or brand, for that matter).  If you search the archives for propellor items, plenty of threads will come up.

Our boat was purchased new in 2009, and I went through the process of selecting a propellor with less drag for sailing.  The folding ones are much favoured by the racing purists as they definitely have the lowest drag when folded, and less likely to snag weeds ropes etc, but as you have noticed, they are not so good in reverse.  In forward, the equal and opposite force exerted by the water on the blades helps them open, so they work as designed.

But in reverse, while you still have the centrifugal force trying to open the blades, that force by the water is tending to hold them closed, and they tend not to open fully, and give poor thrust level.  I have a vague memory that one forum member took some underwater shots showing this problem, Vasko perhaps?  I want to add that since those days, some manufacturers have modified blade shapes that are purported to open better in reverse, and some reports seem to indicate that they do. 

I certainly agree with you that your stopping distance is way too long for comfort, especially entering a pen with the jetty at the end or other confined locations.  I would certainly not hesitate to try more revs as already suggested, your motor is good for the full specified rpm, and a burst at full rpm is recommended by Yanmar after much running at low speed.  However, note that full rpm is not the same as wide open throttle, and your engine will not be fully loaded in reverse, so can overspeed.  So get a feel for it in open water, where you can concentrate on the tacho reading and the engine sound, and even how far to push the throttle, before you try entering the blind alley way of a pen.

There are also propellors described as feathering impellers, where instead of folding, the blades rotate to present edge on to the water flow for low drag when not driven by the engine.  These propellors have slightly more drag when feathered than a folding one, but still way better than a fixed blade.  This was what I chose, and it is certainly excellent for stopping.  There is a Yachting World report that included excellent graphs comparing different propellors, worth reading.  

I need nearly two knots just for steering when entering the pen, and I put the engine in neutral when the bow passes the stern posts, gently into reverse, then when the max beam is between the stern posts, gently increase the revs until we stop.  And stop we do.  No need for crashing from forward to reverse.   Our three bladed feathering prop also has more blade area than many of the folding ones I see in the slip yard, and no doubt this contributes the the thrust produced in both directions, but also the pitch is independently adjustable for both forward and reverse, so able to be optimised for each direction.

Changing the prop is a bit expensive, so I would first experiment with whether more rpm in reverse gives what you want before looking at a new propellor.  If you are not worried by drag when sailing, a fixed prop will perform well and probably quite a bit less expensive than a feathering one.

Lots of good stuff in the archives to fill in pleasant winter evenings.

rene460

[element]

We discussed the poor performance astern with our local boat service guy. 

He mentioned that with a shaft drive, the gearbox ratio is 2.36 ahead and 3.04 astern.
So the engine needs about 25% more revs astern to get the same number of rotations of the propellor when going ahead.

So I will try first to let the motor make more revs astern then I was used with my previous boat before I will consider a different propellor.

[zaphod]

Our 39i had a fixed 3blade prop when we bought it. After researching low drag props I ended up buying a Flexofold because it is considered one of the best folding props on the market. The performance boost while sailing was significant of course, but interestingly the boat performs better under power as well. We also found that there was much less prop walk in reverse compared to the fixed prop. It is still there, but much less pronounced.

Certainly you need to adjust your technique a bit when docking but it was an easy transition for me since my last boat had a folding prop. You simply have to give it a 2 or 3 second burst of higher rpm reverse than you would with a fixed prop. Trust me, the boat will stop! Of course the old adage "never approach the dock faster than you are willing to hit it" still applies, just as it does if you have a fixed prop.

Of course you could upgrade to a much more expensive feathering prop which might perform better in reverse, but they come with their own set of drawbacks such as higher maintenance. There is also the potential for the blades to not rotate into reverse position properly if fouled.

Here is a very good article that does extensive testing of all of the different low drag props on the market.

www.yachtingmonthly.com/gear/folding-and-feathering-propeller-test-29807

You have a very good prop on your boat right now. They are tried and true, and equipped on many thousands of yachts around the world. You just have to get used to how to use it.