Prop size question

[GILow]

Originally Posted by Panope

I just switched from a 3 blade fixed prop to a 3 blade feathering (Maxprop).

Huge difference. It's like an entirely different boat.

-Faster. Not quite 1 knot faster, but close.

-Boat is now able to come about under the (very small) headsail alone.

-Boat LOST a significant portion of her ability to self steer under sail (I guess the old prop was acting like a drogue).

-Only one tenth of a knot boat speed was lost from my normal cruise RPM power setting. A small price to pay for the much improved sailing performance.

Steve

Steve,

Have you had a chance to do a bollard-pull test like you did with the old prop? I know the test is not a perfect simulation of real world motoring, but I’d be curious to know what effect it had.

I’m replacing my prop next month, going from a 3 blade fixed to a four blade Kiwiprop, and I am wishing I’d done the pull test on the current prop (no point doing it now, it is pretty badly fouled)

Matt.

[boatpoker]

Originally Posted by Vancefish

My 1977 Peterson 34, came with a 14" Martec two blade folding prop. It's in terrible shape, pits, floppy pin, a hole in one blade.

Don't install a new prop til' you find out where the stray current that caused the pits and hole is coming from.

[Vancefish]

LOL...

This boat sat on the hard for 15 years in Napa.

The second owner threw it back together with a reliable rebuilt engine.

The prop that's on the boat, may have only been used to get the boat out of Napa.

Although I dove the boat last. I had a diver clean and examine the prop. He showed the hole, and loose pin.

I know my reverse is weak, due to this prop.

I know the flat surface of a feathering prop looses efficiency (Plus they are quite expensive).

I'd somewhat prefer fewer moving parts, over 1/6th of a knot.

I'm thinking that this thin three blade, may be the optimum choice.

A good "Compromise" between performance under motor, versus drag.

[MicHughV]

.... trying to do a " bollard pull" with your own sailboat and a spring loaded scale will not give you an accurate determination of your prop's capabilities.
...the engine is unlikely to reach maximum operating rpm speed while tied to a dock.

...the simplest method to determine if your prop is " right" for your engine and boat is to run the boat under power away from dock lines in smooth water....assuming you have a clean prop and clean hull.

...if your engine can reach it's maximum rated rpm.....your prop is " right"
...if your engine cannot reach it's maximum rated rpm....you are " over propped", whether it is diameter or pitch..even a 100 rpm difference means you are either under or over propped.
...if your engine spins to its maximum rpm with room to spare, you are " under propped"

...some other points to ponder....speed in calm water will not be the same as speed in rough water...

...some years ago, " Cruising World" magazine ran an experiment with props.
They arranged to have a surfboard fitted with a prop shaft and shaft log.
They fitted a wide variety of propellors to the shaft in turn...2 blade....3 blade...a variety of feathering and folding props..etc...maybe 20 or more props...I can't recall.

The surfboard was pulled behind a boat fitted with a " bollard pull" scale.

The results were tabulated and provided an interesting insight to prop drag.
If you search through the " cruising world" library you can still find this report.

If memory serves me a correct, it was a folding prop that provided the least drag...with maxprop a close second...
On the other side of the scale, fixed 2 and 3 blades provided substantial drag...not unlike trying to drag a bucket through water behind your boat..

if you want the best of both worlds.....a feathering or max prop is your answer.

Max prop will sell you " reconditioned" props at substantial discount.

I have fitted two " reconditioned" max props to my boats ( 2 blade) and both have worked flawlessly.

Having sailed with both fixed and feathering props, I can say without hesitation, a feathering prop makes a quantum difference in boat performance and can recommend them highly.

One must remember that a maxprop has basically flat blades.....ie, it has no blade twist...this is both good and bad if you are "fine tuning" a boat but for the average sailer does not amount to much.

[Pete7]

Originally Posted by Vancefish

I fully agree with everyone who has contributed to this so far. However I need to get sailing as cheap as possible right now. This prop has definitely been one of my hold ups to actually getting on the sail.

For $300 I would buy it and try it, assuming it has the right taper. Longer term find a decent folder or feathering prop and keep this one as a spare. At least it gets you on the water and moving.

Pete

[MicHughV]

...one last thing to consider....
...your maximum hull speed is large dependent on LWL....
..in your case....28'
...the standard definition of theoretical hull speed is
1.34 x square root of LWL....about 1.34 x 5.3 = 7.1 knots...

....8 knots might be attainable sliding off the face of a large wave...

...secondly...a speed of 7.1 knots means you are running wide open....which invites an engine problem...

...a 12 hp engine is very tiny....I don't know what the top rpm of that engine is...but I'm guessing 3,000 rpm or thereabouts...given that most small diesel engines have an " operating" rpm of 75% or thereabouts...your engine is likely turning 2,300 rpm at cruise...which means your " cruising" speed is likely going to be around 5 knots....

cruising your boat at 8 knots is simply not going to happen with a 12 hp engine!!

[Panope]

Originally Posted by GILow

Steve,

Have you had a chance to do a bollard-pull test like you did with the old prop? I know the test is not a perfect simulation of real world motoring, but I’d be curious to know what effect it had.

I’m replacing my prop next month, going from a 3 blade fixed to a four blade Kiwiprop, and I am wishing I’d done the pull test on the current prop (no point doing it now, it is pretty badly fouled)

Matt.

Not yet, Matt.

I would like very much to re-test my bollard pull, and will do so as soon as I find a suitable load cell to rent or borrow.

I'll add (to the performance data that I submitted previously) that the engine with new prop (maxprop) achieves the same rpm at full power (as the old fixed prop) while boat speed is about 2/10ths of a knot slower (flat water).

Also, as expected, reverse performance is noticeably better.

Steve

[a64pilot]

Originally Posted by Panope

Not yet, Matt.



I would like very much to re-test my bollard pull, and will do so as soon as I find a suitable load cell to rent or borrow.



I'll add (to the performance data that I submitted previously) that the engine with new prop (maxprop) achieves the same rpm at full power (as the old fixed prop) while boat speed is about 2/10ths of a knot slower (flat water).



Also, as expected, reverse performance is noticeably better.



Steve

Unless your a tug a bollard pull won’t tell you much.
It’s often done trying to pick a seaplane prop, and the Guys at Hartzell propeller will tell you it’s a waste of time, cause the results are only valid with near zero induced flow, soon as your moving, your in a different realm.
Your speed at WOT is a much better indicator, and if your only .2 kt down from a fixed prop, that’s outstanding, so much so I’d suspect the fixed prop was a little off, which is real life, no one is propped perfectly.

A fixed prop will have a poor reverse as of course it’s very inefficient spinning backwards with the leading and trailing edges swapped, where the efficiency will be less in forward for a Maxprop than a fixed, but it’s just as efficient in reverse, and therefore more efficient in reverse than a fixed of course.

[Panope]

Originally Posted by a64pilot

Unless your a tug a bollard pull won’t tell you much.
It’s often done trying to pick a seaplane prop, and the Guys at Hartzell propeller will tell you it’s a waste of time, cause the results are only valid with near zero induced flow, soon as your moving, your in a different realm.
Your speed at WOT is a much better indicator, and if your only .2 kt down from a fixed prop, that’s outstanding, so much so I’d suspect the fixed prop was a little off, which is real life, no one is propped perfectly.

A fixed prop will have a poor reverse as of course it’s very inefficient spinning backwards with the leading and trailing edges swapped, where the efficiency will be less in forward for a Maxprop than a fixed, but it’s just as efficient in reverse, and therefore more efficient in reverse than a fixed of course.

I need the bollard pull data so that I can match/repeat my previous thrust for future anchor testing.

Steve

[GILow]

As I said, I know the bollard test is not a perfect simulation.

(And Steve, of all people, probably knows that fact better than most people. )

I’m just curious to see how that particular metric compares.

Strangely, the way I use the engine, a bollard pull is more relevant than dynamic data. Practically all I do with my engine is manoeuvring in and out of the pen.

Reverse thrust is the one I am most interested in.

[Compass790]

You are never going to get 8knts with a 12hp engine. First work out your hull speed then feed the figures into the prop calc based on hullspeed as your desired speed.
As for fixed versus folding depends on wether you like to sail or motor. If you like sailing go for folding/feathering. If you mostly motor fixed is better.
Personally i'd never have a fixed on a yacht ( sailboat ) but YMMV

[MicHughV]

....I have to echo Compass 790 thoughts here....
....if you think you are going to make 8 knots with only a 12 hp engine on your size boat, it is time to put the crack pipe down !!!

[Vancefish]

LOL!!

The internet sure love to point out the obvious!

The Panama canal charges less if you can pull 8 knots. Not knowing what I can do now, I entered 8 knots! Which was fun and funny (the rea

[a64pilot]

Originally Posted by GILow

Reverse thrust is the one I am most interested in.

Then you want one of the feathering props that are flat plates as they work the same in reverse as in forward.

Or an Autoprop, because an Autoprop blades actually reverse direction so the leading edge is still the leading edge in reverse.
It’s hard to find, but if you look you’ll find that the Autoprop was designed way back in WWII, specifically to get a landing craft off of the beach, they were after max reverse power.
We want good reverse power, but if your stuck on the beach and being shot at, you need good reverse power.

[Opie91]

Originally Posted by Vancefish

Just found this. Thin blades, very close to the right pitch. Might be an inch long, but would definitely fit.Attachment 200699

That prop is way too big for 12 hp per Campbell Sailor. They recommend a 12x6 for a 16hp and a 12x7 for a 20hp.

I started with a 12x7 on my Beta 20 and ended up with a 12x6 and it is still a little too big. It fells smooth as can be, but it feels more like there is a (smooth) blender down there than a propeller.

If you haven't figured it out I'm not a big fan.

I had planned on replacing it this year if you want a 12x6 1" taper shaft I'll sell it shipped for $200 to the continental US.



No Refunds.....