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Propeller advise for a Kelvin J2


gbclive

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The water pump (both sorts) and governor (both sorts) are fairly simple items. The thing to watch on the water pump is that the ram on the single acting one is not worn hourglass shaped and that the valve springs are made of non-rusting material ( stainless or Monel wire are best).

 

 

On the governors the wear area is where the packing runs, especially if it is, or has been, overtight. It is possible to get this welded up and re-ground.

 

 

N

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Is bottom-end rpm set under load or in neutral? As the engine obviously slows under load!

 

Hi Clive, just put two and two together, I'm Orca (Stuart) from around the corner, we spoke last year when I was painting our boat in the end shed!

Any chance I might borrow your tachometer next week? Just started our engine in the boat for the very first time this morning! :)

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  • 2 months later...

An update on my prop saga?

 

The original prop was 20 x 16 and and was definitely too much for the J2 in a 64 footer.

Long story short, ended up with a new 19 x 14, equivalent to 3 inches off the pitch.

Now feels a tad under propped.

This is the result in deep water - gut feeling is to get 2 inches pitch taken off the original and try that next time the boat is in dry dock.

Any thoughts?

 

imagejpg1_zps795aa53d.jpg

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You are getting ~900 rpm at 6mph. There are two outstanding questions: 1. What power is being developed at that point and 2. How fast can the hull be reasonably made to go for that power?

 

To answer 1 is relatively easy- Where is the rack at this point? If it's fully forward then you will be getting no more than 90% of 22 HP (less any alternator losses and losses in the shafting etc) and are very slightly over propped still.. If it's not fully forward then more power can be had, but the prop can't handle it- underpropped still.

 

To answer 2 is much more difficult. 'Reasonable' hull speed is the speed at which wavemaking begins to take off and that is proportional to the waterline length. IMHO 6mph is about all you can expect from 22 hp in your boat, so I'd be pretty happy with that at 900 rpm, providing there's no black smoke.

 

Looking at it another way, 900 rpm on a 14 in pitch in a perfect fluid is 1050 ft/minute. This is equivalent to about 12mph. You are actually getting ~6 mph so you have about 50% slip. That is a little high for a well-shaped displacement boat, but altogether typical for a narrowboat.

 

By all means have your other prop adjusted but I bet it isn't any better in the speed stakes, although the extra inch of diameter may produce slightly better stopping.

 

N

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...........Where is the rack at this point? If it's fully forward then you will be getting no more than 90% of 22 HP (less any alternator losses and losses in the shafting etc) and are very slightly over propped still.. If it's not fully forward then more power can be had, but the prop can't handle it- underpropped still.

 

N

Thanks (again!) N

Once more I get to realise how little I know?

I chickened out when the throttle lever on the governor was about 2/3 to 3/4 open due some grey smoke and an overwhelming feeling of trepidation!

This gave about 860 RPM = 6 MPH in deep water. Until your reply I would not have considered the rack position.

 

I will pluck up the courage to firewall it and note the rack position, RPM and resulting speed.

That said, what I would like to achieve is not an increase in speed but a slightly more relaxing RPM at normal speeds, but without causing the baby J to labour as it did with the original prop.

 

Re your stopping comment, the current set-up does seems worse than with the original 20 inch prop, so perhaps adjusting the original 20 x 16 prop by 2 inches of pitch would improve things - would be about 2/3 less reduction that the new 19 x 14 and hopefully result in a tad less RPM for a given speed and better stopping.

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The propeller pitch is measured at 70% of the radii, on the Flat face, the chord is not straight but circular in shape. (from the centre of hub)

 

so diameter * Pi() * 0,70 * Tan Beta (angle degree) = pitch

If not stamped on the hub. or re-pitched

 

there is protractors for phones or pads ...

 

Diameter and pitch is not all that counts, blade area also

Edited by Dalslandia
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The propeller pitch is measured at 70% of the radii, on the Flat face, the chord is not straight but circular in shape. (from the centre of hub)

 

so diameter * Pi() * 0,70 * Tan Beta (angle degree) = pitch

If not stamped on the hub. or re-pitched

 

there is protractors for phones or pads ...

 

Diameter and pitch is not all that counts, blade area also

Thanks for that info Dalslandia.

The original prop has these numbers stamped on it:

20 x 16

7 95

20603

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Can it have been made July -95? and a serie Nr.

or 20" 60% DAR 3 blade ???

Thanks - the boat was built 1993 and it is a 3 bladed prop.

I'll try to find out the %DAR for the new 19 x 14 prop and factor that in.

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Thanks - the boat was built 1993 and it is a 3 bladed prop.

I'll try to find out the %DAR for the new 19 x 14 prop and factor that in.

I though blade area ration (BAR or EAR) was more important. Disc area ratio (DAR) is that of the blade area when projected directly onto the full area described by the props diameter, which is distorted by the angle of the blades. BAR uses the actual area of the blades (as if flat) compared to the full area.

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I though blade area ration (BAR or EAR) was more important. Disc area ratio (DAR) is that of the blade area when projected directly onto the full area described by the props diameter, which is distorted by the angle of the blades. BAR uses the actual area of the blades (as if flat) compared to the full area.

This pdf say it all http://hydrocompinc.com/wp-content/uploads/documents/HC135-BladeAreaRatio.pdf

BAR is the common name, but there is 3 versions of BAR, PAR, DAR & EAR

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This pdf say it all http://hydrocompinc.com/wp-content/uploads/documents/HC135-BladeAreaRatio.pdf

BAR is the common name, but there is 3 versions of BAR, PAR, DAR & EAR

Thanks for that, so as its says BAR (generic) as expressed as expanded area ratio (EAR) is the most important since it takes into account the total area of the blade as if completely flat in all planes.

  • Greenie 1
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I'd be careful fitting one of those Crowther-specced propellers with the thin blades. I took the one off my boat with a K1 as the performance, especially in reverse, was as dire as you'd imagine from looking at the blades.

 

Over-propping is good with a small Kelvin unless you want the engine to be screaming at 750rpm to achieve any sensible sort of cruising speed.

 

MtB

You are on to something important here Mike, that hardly ever is mentioned when talking about boat propellers, the pitch and camber relation, a thicker blade have a higher real pitch then a thinner blade, even if those have the same pitch angle at the (often) flat face.

 

the lift is created of the angle from the zero lift line and not the bottom surface of the airfoil.

 

the zero lift line is found if the half thickness at half chord point is connected to the trailing edge, meaning a thicker airfoil will have a greater angle from the flat bottom to the zero lift line then a thin airfoil, so a propeller brand A with a thick airfoil don't have the same effective pitch as a thin airfoil propeller brand C, even if the angle at 70% radii is the same.

 

then also they can have different airfoils, but that mostly effect the blade drag and cavitating, and not the pitch, if measured from zero lift line!!

 

http://traktoria.org/files/personal_submarine/propulsion/propeller/lecture_notes/propeller_geometry.pdf

Edited by Dalslandia
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Thanks for that, so as its says BAR (generic) as expressed as expanded area ratio (EAR) is the most important since it takes into account the total area of the blade as if completely flat in all planes.

Yes, confusing it is.

 

When in a BAR talking about airplane propellers, looking, at the woman, listen with the EAR, PAR is never on the topic, but more the aspect ratio, AR, that is based on the total blade area, (EAR in boat propellers) in relationship to the radii or blade area X Nr of blades and diameter.

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  • 4 years later...
On 25/08/2014 at 14:42, BEngo said:

My J2 has a 21 in dia 13 in pitch blade in a 60ft boat. This gets to about 650rpm in open water (Thames or Severn), so is a bit too big, but the boat goes well enough and stops beautifully. I would replace it with a 20x13 if I ever needed a new blade.

Thought I should report back on my quest for a better propeller for our J2 - can’t believe this was nearly 5 years ago!

T. Norris in Middlesex recently did the honours with our original heavy 20” x 16” propeller, reducing the pitch by 3” so now 20” x  13” in accordance with Nigel’s suggestion. (£235 inc VAT and including balancing and polishing).

They completed the work in just a few days.

 

So far just 6 days on the T & M, but it feels good, so thanks Nigel (and everyone else)?

Seems to go well in a slightly more relaxed way and importantly, stops better compared with the modern 19” x 14” that was my first attempt at solving the over propped original. 

When we next get onto a suitable river I’ll have a go at re-doing this graph which was for the for the 19” x 14”.

 

 

imagejpg1_zps795aa53d.jpg

 

 

 

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