Getting Screwed!

[fstbttms]

Originally Posted by Renegde_Sailor

…the prop had numbers stamped on the side which I believe were 12-8

Possibly a date. It is not a pitch number.

[Searles]

As this prop runs in an appature clearances are critical,as fairing the leed in plus the exit ,must be as fine as possible ,plus shaft speed for a displacement hull is best if kept under 1200 rpm ,ventilation is possible but I will go with cavitation,a large diameter narrow blades fine pitch, if necessary a higher ratio g/ box maybe 3/1 ,but all things must have some compromise ,it’s a boat .⛵️⚓️

[Renegde_Sailor]

Originally Posted by Searles

As this prop runs in an appature clearances are critical,as fairing the leed in plus the exit ,must be as fine as possible ,plus shaft speed for a displacement hull is best if kept under 1200 rpm ,ventilation is possible but I will go with cavitation,a large diameter narrow blades fine pitch, if necessary a higher ratio g/ box maybe 3/1 ,but all things must have some compromise ,it’s a boat .⛵️⚓️

I got down and checked the tag on the back of the transmission this AM it is a KM3P4 with a 2.36: ratio, running her at 2200 rpm that gives a shaft speed of 932 RPM


Although if I wrapped it out to the 3600 rpm max it would be 1,525 rpm.



It does makes sense as when I rev the engine up to about 3000 that is where she starts to cavitate, which is 1200 rpm shaft speed (2832 rpm)


The aperture is pretty fair, far more so than most other similar style boats I have seen.

Going to be making a 31 NM trip today which we will be motoring the whole way, going to run it at about 1000 rpm shaft speed and see what it does.

[FPNC]

By the numbers, you are propped pretty well.

It may be possible that you are hearing propeller 'singing'. This can come from a poor blade shape, out of balance blades, old rounded blade edges. Someone who knows how to use a file underwater can usually remedy a 'singing' propeller.

[Renegde_Sailor]

Originally Posted by FPNC

By the numbers, you are propped pretty well.

It may be possible that you are hearing propeller 'singing'. This can come from a poor blade shape, out of balance blades, old rounded blade edges. Someone who knows how to use a file underwater can usually remedy a 'singing' propeller.

That may well be, and it is entirely possible I am overthinking this. Part of the problem is just not knowing if it is right nor not.

This morning I am running the boat at 2400 rpm, motor sounds pretty happy, and we are making better speed, but the prop sounds like it has a lot of bubbles coming off of it.

I did have the prop balanced and serviced by Port Townsend Shipwritghts CoOp, as well as the shaft and coupling when I put the new packing gland and cutlass bearing in.

What really makes me question is the boat if I want to rev it up will run at over 7.2 knots on the motor, but it makes a lot of noise from the prop, and I am asking if I am pushing it too hard doing it?

And what makes me wonder is when pushing through swells she gets slowed way down, makes me ask if the prop doesn't have enough bite and would going to a 4 blade improve matters?

Or perhaps it is as good as it is going to be and I am overthinking things?

[jamhass]

I don't know the performance curves on your engine, but at first blush, all sounds fine as-is to me.

You cruise nicely at 2200 and 6 knots, while hitting at or near hull speed at a bit less than full throttle (I assume - as you didn't note at what rpm you hit 7.2 or your redline)

As you approach hull speed your prop will cavitate or ventilate as it has to overcome the resulting bow wave and that puts great demand on the prop.

On the otherhand, if your max power and redline RPMS are way higher that 2200 + then yeah, you are overpropped.

[Renegde_Sailor]

Originally Posted by jamhass

I don't know the performance curves on your engine, but at first blush, all sounds fine as-is to me.

You cruise nicely at 2200 and 6 knots, while hitting at or near hull speed at a bit less than full throttle (I assume - as you didn't note at what rpm you hit 7.2 or your redline)

As you approach hull speed your prop will cavitate or ventilate as it has to overcome the resulting bow wave and that puts great demand on the prop.

On the otherhand, if your max power and redline RPMS are way higher that 2200 + then yeah, you are overpropped.

At this point I am thinking that I am over thinking it, going to play around with the RPM's a bit more on some longer trips, yesterday running 2400 I ran ~6.5 all day, I suspect if I wasn't towing a 10' Livingston dinghy behind me that I'd be doing a touch faster.

[Goodxcharly]

Interesting! I always associate cavitation with air and outboards popping out of the water as they crest a wave.
You could slow down and find the sweet spot where everything is working together.
I’ll fly a headsail when motoring down wind to help pull me through and over the waves.
I find bashing through the waves is reserved for powerboats. The 1 or 2 knot gain is not worth it for the wear and tear plus your still going slow, your in a sailboat.

[Kudos]

First off back to the basics. I don't care what kind of boat it is, correctly propping it will make a world of difference and to do so you want your Max RPM Red line of the engine at full throttle. If the engine over revs you need more pitch, if it does not reach the full red line RPM then you're overpitched. With that said if in fact it is cavitating, hence all the sudden you start losing speed and the RPMs go up, think of it as doing a burnout or spitting your tires on pavement. If that is happening then obviously having the correct pitch prop will help but another thing you can have the prop cupped. The trailing edge will have a little bit of a rolled edge to it. That will help a prop grip better in the water. Now I know on a couple of boats that I have which have been full keels, the prop does make noise at higher RPM and what is causing it is the turbulent water coming past the keel as the prop blade goes through it. My boat that I have now was completely messed up on the propeller size and I got a hold of Michigan wheel, filled out a rather lengthy form with all my information and they came up with the prop that I should have. They nailed it right on the nose. My engine red lines it 3000 RPM, and I get 3000 RPM at full throttle. That's what you're trying to achieve first of all.

[drdoyle]

Your prop is marked as 12-8. This means it is 12 inches in diameter and 8 inches in pitch. It appears you are under propped. Most sailing vessels use small props so as not to create sailing drag. It could be cavitation, but probably is air injestion from surface conditions. But another possibility about the noise would be a bad cutlass bearing, they get very "rumbly" as they wear out. If you are diameter limited, either 4 blades and or wider blade area will help. I would maximize diameter - check for maximum clearance. If you have some back space available then you can add a shaft spacer between the Transmission and the shaft coupler to push the prop back a inch or so. The complicated option is to add a "kort nozzle " to the vessel (square tipped prop rotating in a fixed nozzle). Or there are som exotic new propellers that have the "kort" type ring cast onto the propeller (ring rotating with the propeller). Or the newest propeller design with each blade being a complete loop back to the hub - very intriguing design.

[sailingharry]

Originally Posted by Renegde_Sailor

Yanmar 4JH3E 36.8HP @ 3650 rpm (I run at 2200)

2.36 reduction

That is a 55hp engine.

[StuartWeibel]

One of the things I have not seen mentioned in replies to your post is the importance of being able to run your engine at WOT (I think it is recommended that 5% of your engine hours should be at WOT).

My understanding of this requirement is that not doing so promotes glazing of the cylinders, which eventually leads to blow-by. Running at WOT reduces this hazard by burning off the glaze (which comes from accretion of unburned fuel over time).

It sounds like you can't do this without triggering ventilation or cavitation, or whatever is actually happening, and so you don't, which will shorten engine life.

It sounds like your propeller is under pitched to me.

A good prop-shop will have someone who can calculate an appropriate size and pitch for your boat, based on boat-length, the engine, the displacement. The answer may not be precise (as someone else said, its a bit of a black art), but it will be your best shot at making your power train work as well as possible.

best of luck.

[j.g.evans]

After getting 5 radically different propeller sizes from 5 manufacturers I started doing my own. I have done a lot of propeller sizing using The Propeller Handbook by Gerr. Recently I found a propeller calculator by vicprop.com which I think is very good. I think you can use it to get very close.

I bought a 3B autostream feathering prop which I like a lot. I did not like the reverse performance of my old prop. I also like a SS prop on a SS shaft. I can now stop in less than a boat length. I mention this because it's pitch can be adjusted under water for both forward and reverse. I fine tuned it to achieve maximuim RPM in forward and a high pitch to stop in reverse.

You want the largest diameter prop that allows maximum engine RPM. Mine is a 19" prop on a 34 HP 3GM30. If your prop is 12" it is way too small.

[merrydolphin]

Your prop is pretty deep but I suppose that extreme pitching might cause some venting. Your CCA designed boat has a lot of overhang and short waterline length. When the seas are space so as to coincide with your pitch moment, you get a lot of hobby horsing. This slows you forward progress a good bit.
I agree that if you had the space (which you don’t ). A bigger reduction of say, 3:1 and a bigger prop would make a big difference. This is a common issue with auxiliary sailboats. The designer/builders do not have proper reduction and propping for real life cruising. Eg. My Cal ll-46 had a near 3:1 reduction with a 26” prop! She could motor like a trawler! If you had to go seriously to windward you could stow sails and put the hammer down. She could make 5.5 kts even on the Baja bash into 6-10 footers and 20-30kts wind. All of the other cruising boats that I had, as with yours, you have to get well off the wind and seas ( probably 30 degrees or more and get that mainsail drawing well in order to motor sail into blustery conditions. No other way to get around it. It’s the way aux sailboats are designed. Bad way to do it. We need good reduction and larger props to punch into it. Come on designers. I could make 200 mile days under sail with that prop.

[Cheechako]

Originally Posted by Renegde_Sailor

Happens when I hit a wave, you can hear the bubbles on the prop under the boat.

Same as if you were starting from a standstill and gun it hard, you get the bubbles noise then as the boat comes up to speed it stops. Don't really see a rise in engine RPM.

nature of the beast I think. Boat hits waves, slows , prop still turns the same, boat wallows, prop cavitates until boat starts moving again. A problem with heavy cruising boats. The heavier and deeper the boat the worse it is. A lightweight racer or cat will move along on top of waves, a heavy blunt boat like my old Hans Christian was wallows terribly is a stiff chop.
Angling through the waves, like tacking ....but when you are motoring, helps a lot in a stiff chop. Motor sailing helps immensely.