SO 32i Replacement Prop

[noiseboy]

Hi All

Just wondering if anyone has any thoughts on a replacement prop for my boat. I current have a 2 blade fixed prop, yanmar 3ym20 engine

We can achieve 3800 rpm flat out with the current prop however the recent survey highlighted that this prop was old and pinky , did not sound well with a hammer , and therefore I should replace.

It an 18” diameter prop but I don’t know the pitch, does anyone know the prop size that came from the factory?

We intend to cruise the boat, with a good amount of kit onboard, so fully loaded.

After reading online , it all seams quite daunting!

Any advice gratefully received!

Thanks

[rene460]

Hi Noiseboy, welcome to the forum.

You will find many threads on this topic simply using the search facility near the top of the page.  It is common to all sizes of boats so don’t restrict your reading to your model alone.

Certainly the pink colour and hammer test observations don’t sound good, so you are probably in the right track.

A few general principles.  We know why you are asking the question, but what else do you want to achieve with the replacement, now you are looking at the choices?

A fixed propellor is the simplest in terms of long term reliability and maintenance.  The two blades will give a bit less thrust at low speed, but less drag while sailing, so of you are happy with the performance so far, you could do worse than another two blade.  But hard to beat a three blade for low speed thrust, and stopping power.  Two blade might have a very little more vibration, as the blades alternatively line up with the keel and horizontal, but not really an issue providing the tip of the blades is not too close to the hull.  Your present propellor will give a good guide to the diameter you need for whichever type you choose.

If you are mostly a keen racer, then a folding prop is best for low drag when sailing, but a feathering type is not much worse, and is better for thrust when motoring.  Both are available in two or three blade, with the costs varying with the complexity.  The folding type is not known for really good thrust, particularly when you want to stop when entering a pen.  I have a feathering one, specifically known for good stopping power which is important with my combination of twin rudders and mooring in a marina pen, which has to be entered fast enough to maintain steering, but it was more expensive than some.

Finally the pitch has to be selected to properly absorb the engine power, and the manufacturers are best placed to recommend the appropriate pitch for their blade design.  Your current 3800 rpm at wide open throttle indicates that your present prop is well matched to your engine and hull, but a different blade shape may require a different pitch to achieve the same load on the engine.

I hope that is enough to get you started.  Have a search for threads on this topic on the forum, they are timeless and don’t really go out of date.  And please come back when you have more questions.

rene460

[noiseboy]

Just to reply to this thread for information for others

After much reading and speaking to people I went for a 16 x 10 3 blade prop. The prop came from lake engineering in poole and they were very helpful with the calculations and purchase. The 16” prop allowed for the correct hull clearance of slightly more than the recommendation of 15% of prop diameter.

I’m really pleased with my choice, the speed versus rev range has worked out well and the engine achieves the correct top end revs. The prop choice along with a new cutlass bearing has meant that the vibration I was experiencing has gone and stopping in reverse seems better. The cutlass bearing seems a very unsatisfactory design , hardly a bearing at all. I think I will be changing this every few seasons unless someone has found a proper bearing that can work in this p bracket?

Anyway just thought I would report back

Thanks

[sitara]

Hi Noiseboy,

If the cutlass bearing you have fitted is the standard fitted to new Jeanneaus, ie all rubber with a lip at one end and held in place by two grub screws, then if conditions are right it will last a long time. My current bearing is eight years old and is still fine, and the one on my previous Jeanneau is still good after 10 years (I had a look the other day when the old boat was out of the water). My only reservation is that if grit/calcium build occurs then the bearing can wear quickly.

[rene460]

Our first cutlass bearing showed excessive wear in under two years, which was a bit of a worry.  It was discovered during a haul out when we did all the normal inspections.  The only sign before the haul out was a little tingling vibration under the feet when standing at the wheel, but as it was a new boat to us, it was hard to know what that meant.

When the boat was back in the water, we got the mechanic to check the engine and prop shaft alignment.  It was way out, and required a fairly big adjustment to get the alignment back in tolerance.  We checked it again the next year and some further adjustment was needed.  However, since then, the cutlass bearing has not shown any signs of wear so is now over eight years old.  I suspect the new boat took some time to settle out after the stresses from initial construction and subsequent transport half way around the world, before being first launched.  Who knows how well it was supported through all that?  There may have been additional stresses from the initial rig tuning, as the required tensions do involve relatively high loads.

I would suggest that the cutlass bearing design is ok, but any bearing in that position is dependent on good drive train alignment, so needs to be checked as part of the engine maintenance, with the boat in the water perhaps each year until you have a history so you know whether or not it is stable.  Many people make rig tuning adjustments, and don’t seem to have excessive issues, so I suspect it is a case of when it all settles, further minor rig adjustments don’t have a big effect.  But if you have cutlass bearing wear, I would definitely recommend having your mechanic check the shaft alignment.

rene460

[noiseboy]

Hi Thanks for the reply

I have had both the engine alignment checked and adjusted as well as having the shaft checked for straightness while I had the whole thing apart

Hopefully we should be good for a few years but I will monitor every haul out and see how the bearing is getting on

It’s the standard “top hat” rubber bearing