Dodgy propellor or something else?

[adamv]

Hi, I've always felt my boat is under powered. The hull is clean and it will happily reach hull speed when under sail with 12-14kts wind but under power it struggles even at high revs. It also feels like it vibrates a lot. So im keen to hear if i should replace the prop and if so, which one?

The engine is a universal m18 (14hp). The prop is a 2 blade (dont have the measurements) and on a 7/8ths prop i believe.

Sometimes at slack and no wind and at nearly max revs (the boat doesnt have a tach but from sound, its really revving at this speed) i can get over 4knts, but if i cant plan to cruise with the current, i can only expect 3kts. Yesterday i was headed into a nominal 3kts wind and very very slight current (basically slack) and it was hard to get it above 3kts SOG- spent most the time at 2.8kts. When cruising over a longer distance, I have to use 3.8kts as my average speed and this is at almost max engine power. I should also note i regularly see similarly sized boats blow right by while i have the engine running at high revs

Its probably worth stressing that although i cant provide an exact RPM, it does sound very high and i dont believe the engine is unable to turn the prop so much as the prop isnt efficient. I do have a manual optical RPM guage (so i can install a tach once i get time!) so perhaps can confirm the max RPM

I dont know what the prop is. Ive included a picture taken last year. You'll spot one blade in a photo was omnce bent - (i did straighten this with pliers after spotting this)

Is an upgraded prop worthwhile here? If so, which one
Thanks in advance

 

[Foswick]

Based on your description - and especially the fact that your Universal M18 sounds like it's revving high but you’re only getting 2.8–3.8 knots SOG under power - I’d agree that the current prop is likely mismatched or underperforming. The excessive vibration, poor thrust, and the fact that you can sail to hull speed with ease all support that.

A few thoughts:

• Bent blade: Even though you straightened it, a previously bent blade can throw the prop off balance and reduce efficiency (not to mention increase vibration). That alone could be a major factor.
• Two-blade prop: A 2-blade is often a compromise for less drag under sail, but it may not give you enough bite for close-quarters maneuvering or head-current situations. Upgrading to a well-pitched 3-blade (or a good feathering/folding prop if you want to preserve sailing performance) might be worthwhile.
• Engine power vs. prop load: The M18 should give you better than 3 knots under power in calm conditions. It sounds like the engine is under-loaded, meaning the prop isn’t offering enough resistance, allowing the engine to rev but not deliver much thrust. That usually points to too little pitch or too small a diameter.

If you can get a measurement of your current prop (diameter and pitch stamped on the hub, or measured), plus confirm your shaft diameter and reduction gear ratio (usually on the transmission), folks here can help suggest ideal specs. Also, using that optical tach to confirm max RPM at WOT would be very helpful. Ideally, I think you want to hit around 3000 RPM for that engine under load - if you’re blowing past that, it’s a clear sign the prop is too small or flat.

 

[Tally Ho]

It does seem like your prop may be too small or not pitched enough.

My O’Day 322 has a Yanmar 2GM20F and I was running a 14x14RH 2-blade prop. I didn’t like it either…dragged under sail, not great reverse thrust, and a bit of vibration while motoring. I could not get hull speed (6.9 knots) while motoring, but I could get 5-6 knots depending on wind and waves.

I put a 3-blade Maxprop feathering prop on her last season. Expensive prop, but I really like it. I gain a half knot of speed under sail, I have great reverse thrust, less vibration, and importantly, I can adjust both the Forward and reverse pitch. I tweaked both this spring before I launched, giving me stronger reverse and a bit more forward bite too. It allows me to really dial in the pitch I want and adjust if the engine gets weaker with age.

MaxProp can take your boat, engine and transmission info and provide you with the correct size and starting pitch, but you can dial it in after you run it for a while.

They are a thing of beauty!


Greg

 

[Davidasailor26]

I agree with the others that the prop must be too small, assuming your tach is accurate. We had an O’Day 28, about 10% heavier than the Newport 28, with a Universal M12. We had no trouble cruising over 5 knots with that. I don’t know what size the prop was, but it was 2 blades.

I don’t know if it would be worth going to 3 blades for a boat that size. I’d talk to some prop manufacturers about what they would recommend for your engine and transmission specs. I’m a fan of Flexofold but my experience with them is with 3 blades and I’ve heard the 2 blades have more vibration issues.

 

[dlochner]

There may not be much room to enlarge the prop as tip clearance is a concern. There needs to be sufficient clearance between the prop tip and the hull to avoid turbulence that will negatively affect prop efficiency.

Even though the blade has been bent back to shape, having a good prop shop take a look at it to ensure the prop is well balanced and the shape is correct would be beneficial.

The prop's pitch may be incorrect. Give Campbell Props a call, ask them what they would recommend. You'll need the engine, transmission, and gear ratio. Compare that to the prop you now have. If you can't measure the props pitch, check on the hub it is often stamped in to the hub. There should be a label on the transmission with the gear ratio.

 

[jssailem]

I had a mismatched prop. I could make the stern sit 8-12 inches down at a high RPM.

I changed to a CampbellSailor engineered by WestbyNorth. Now I get thrust not underperformance.

before

After

Improvement is unbelievable.

 

[higgs]

I would guess the once bent prop is the culprit.

 

[Richard19068]

Did you straighten the prop after the pictures were taken?

Did you have the problem before the prop was bent?

Before replacing the prop I'd have a prop guy look at it for damage and/or improper repair.

FWIW I get a good 5.5+ KTS from my M18 in an O'Day 31 with 2 blade prop. Similar hull speed and about 3,000 pounds heavier.