3 or 4 blade prop ?

[Jacq]

By having a 4 blade prop ,how much roughly does it allow you to reduce the diameter compared to a 3 blade ,if any ?

I have been quoted as needing a 28 x 25 , 3 blade. Would a 4 blade 25 x 33 be similar ?

[PhilR]

By having a 4 blade prop ,how much roughly does it allow you to reduce the diameter compared to a 3 blade ,if any ?

I have been quoted as needing a 28 x 25 , 3 blade. Would a 4 blade 25 x 33 be similar ?

 

I doubt it. 33 ins pitch sounds far too much, but I would be interested to hear expert opinion.

[mykaskin]

As far as I understand it adding another blade increases the blade area, but possibly at the expense of reducing efficiency. I'm not sure if a 4 blade prop is better than a "high effeciency" (read large BAR) 3 blade prop.

 

Mike

[nb Innisfree]

As far as I understand it adding another blade increases the blade area, but possibly at the expense of reducing efficiency. I'm not sure if a 4 blade prop is better than a "high effeciency" (read large BAR) 3 blade prop.

 

Mike

 

I think it's a balance between increased blade area and trying to reduce the tendency for blade turbulence to get in each others way?

[mykaskin]

I think it's a balance between increased blade area and trying to reduce the tendency for blade turbulence to get in each others way?

Thats my understanding.

[Albion]

For my recently built barge I bought the prop from Jooren in Holland. They told me that they would specify a four blade prop if they felt it necessary and they didn't in my case. Now the builder generally fits an UK sourced 4 blade prop but I chose my own 3 blade one.

During the trial cruise the builder remarked how much more efficient my 3 blade prop seemed than their normal 4 blade choice, despite the fact that my prop wasn't such a big pitch as theirs. The way the stern sat down as the prop bit the water as I dropped it into gear for example and similarly when stopping.

I have also been told, by quite an experienced boat builder/repairer in France that, depending on the design of the swim etc, you can get some slight vibration/noise as the prop rotates because, with a 4 blade, you have 2 blades pssing the dead wood (end of the swim) at the same time whereas on a 3 blade you only have 1.

Roger

[Jacq]

For my recently built barge I bought the prop from Jooren in Holland. They told me that they would specify a four blade prop if they felt it necessary and they didn't in my case. Now the builder generally fits an UK sourced 4 blade prop but I chose my own 3 blade one.

During the trial cruise the builder remarked how much more efficient my 3 blade prop seemed than their normal 4 blade choice, despite the fact that my prop wasn't such a big pitch as theirs. The way the stern sat down as the prop bit the water as I dropped it into gear for example and similarly when stopping.

I have also been told, by quite an experienced boat builder/repairer in France that, depending on the design of the swim etc, you can get some slight vibration/noise as the prop rotates because, with a 4 blade, you have 2 blades pssing the dead wood (end of the swim) at the same time whereas on a 3 blade you only have 1.

Roger

 

Thanks,I have had a quote from Jooren, Are you happy with the prop and there service ?

[RobinJ]

As far as I understand it adding another blade increases the blade area, but possibly at the expense of reducing efficiency. I'm not sure if a 4 blade prop is better than a "high effeciency" (read large BAR) 3 blade prop.

If you look at the actually propellor circle, I'm sure there is not much difference in blade area between 3 and 4, since they occupy a similar amount of space around the circle. I suspect there is a benefit at higher pitches in preventing cavitation and vibration, but there is also probably more drag?

Can't say I've seen any 4 blades on the canal, are they any more common on rivers and wavy stuff?

[Timleech]

For my recently built barge I bought the prop from Jooren in Holland. They told me that they would specify a four blade prop if they felt it necessary and they didn't in my case. Now the builder generally fits an UK sourced 4 blade prop but I chose my own 3 blade one.

During the trial cruise the builder remarked how much more efficient my 3 blade prop seemed than their normal 4 blade choice, despite the fact that my prop wasn't such a big pitch as theirs. The way the stern sat down as the prop bit the water as I dropped it into gear for example and similarly when stopping.

I have also been told, by quite an experienced boat builder/repairer in France that, depending on the design of the swim etc, you can get some slight vibration/noise as the prop rotates because, with a 4 blade, you have 2 blades pssing the dead wood (end of the swim) at the same time whereas on a 3 blade you only have 1.

Roger

 

Maybe there's the clue, they used an increased pitch to compensate for limited diameter?

Big bats with finer pitch will generally give better starting & stopping, whether it's by having 4 blades, big blade area or bigger diameter, but may give a reduced theoretical top speed through the water.

Yes the vibration thing with an even number of blades is often quoted, but I don't know how much of an issue it can be in normal inland waterway circumstances.

 

I have a suspicion that some makers don't offer 4 blades because 3 bladed props are their standard product & they're not kitted up for anything else.

May be totally wrong there, of course.

 

Tim

[Naughty Cal]

We couldnt choose so took one of each

[Albion]

Thanks,I have had a quote from Jooren, Are you happy with the prop and there service ?

 

Very satisfied. It wasn't cheap as there was a significant transport cost and, at the time, Dutch VAT was more than ours (slightly less now after our rise to 20%, of course). The quality of the prop is superb, it comes ready ground to prevent prop singing etc. You do have to specify the taper though as there are two tapers in use 1:10 and 1:12 IIRC. 1:10 is normal for Europe and 1:12 for the UK IIRC. IIRC there would have been a slight extra charge for a 1:12 taper from Jooren. I had the misfortune to damage the prop slightly en-route through the UK canals to Nottingham (and eventually France). Jooren repaired and returned the prop in 2.5 days (from Holland to France).

 

Maybe there's the clue, they used an increased pitch to compensate for limited diameter?

Big bats with finer pitch will generally give better starting & stopping, whether it's by having 4 blades, big blade area or bigger diameter, but may give a reduced theoretical top speed through the water.

Yes the vibration thing with an even number of blades is often quoted, but I don't know how much of an issue it can be in normal inland waterway circumstances.

 

I have a suspicion that some makers don't offer 4 blades because 3 bladed props are their standard product & they're not kitted up for anything else.

May be totally wrong there, of course.

 

Tim

Identical diameter. The builder normally uses a 24 x 19 x 4 blade. Mine was a 24 x 17 x 3 blade.

 

For Jooren, being based mainly in the barge (up to full size continental commercials) and Dutch cruiser market they produce props of all blade configurations (see their web-site: http://www.joorenpropellers.nl/uk_bedrijf.html). On Page 1 of their Inland Shipping tab there is a picture of a 4 blade prop of bigger diameter than the height of the man working on it.

Roger

 

We couldnt choose so took one of each

 

Now there is an interesting thought. With your twin contra-rotating props on a common axis they use a 3 blade and a 4 blade. That surely has to be something to do with having odd numbers of blades passing each other at the same time to reduce the noise/vibration I would guess. Also, although not a nautical theme, I know that automotive radiator fans often used to have unevenly spaced blades to reduce the harmonic noise.

Roger

Edited March 24, 2011 by Albion

[WotEver]

I know that automotive radiator fans often used to have unevenly spaced blades to reduce the harmonic noise.

Roger

Another new thing I've learned today

 

Tony

[nb Innisfree]

 

I know that automotive radiator fans often used to have unevenly spaced blades to reduce the harmonic noise.

Roger

 

A similar trick I used to do with the front brake pads on my old Yamaha XS750 to stop them squealing was to put a few irregular spaced hacksaw cuts across the face of the pads.

[ditchcrawler]

I have also been told, by quite an experienced boat builder/repairer in France that, depending on the design of the swim etc, you can get some slight vibration/noise as the prop rotates because, with a 4 blade, you have 2 blades pssing the dead wood (end of the swim) at the same time whereas on a 3 blade you only have 1.

Roger

I have heard that before

[Proper Job]

........I have also been told, by quite an experienced boat builder/repairer in France that, depending on the design of the swim etc, you can get some slight vibration/noise as the prop rotates because, with a 4 blade, you have 2 blades pssing the dead wood (end of the swim) at the same time whereas on a 3 blade you only have 1.

Roger

Simple physics says that there must be some out of balanced forces caused by this, but would it be a real problem with the relatively low speeds that NB props turn at?

 

I suppose that it could become more apparent with worn stern gear?

[Albion]

Simple physics says that there must be some out of balanced forces caused by this, but would it be a real problem with the relatively low speeds that NB props turn at?

 

I suppose that it could become more apparent with worn stern gear?

 

I doubt it would be a problem with a slow revving prop such as yours but, with some egg-whisk props it might have some effect. My guess is that there will be many factors that add or subtract from the effect to a greater or lesser extent; swim shape, proximity of the prop to the swim, water depth, adequate flow to the prop, possibly prop diameter etc. The guy that told me about it isn't doing very many nbs in his yard as it is in France but I thought I'd throw it into the equation as I don't think I've seen a nb with a four blade prop ever and so we don't know whether this might, or might not, be problematic.

Personally I'd always take the advice of the prop specialist. In the case of my barge I specifically asked Jooren about why they choose 3 as opposed to 4 blades because I was going against the builders norm of a 4 blade. Jooren told me that they would recommend a 4 blade if they felt it necessary and they didn't on mine. The results of the Jooren 3 blade have been excellent so, I guess, they know what they are doing.

Roger

 

Edited to add: The guy in France running his boatyard used to be an engineer on a large UK nb hire fleet many years ago before he moved to France so he is quite au fait with nbs etc.

Edited March 25, 2011 by Albion

[Timleech]

I doubt it would be a problem with a slow revving prop such as yours but, with some egg-whisk props it might have some effect. My guess is that there will be many factors that add or subtract from the effect to a greater or lesser extent; swim shape, proximity of the prop to the swim, water depth, adequate flow to the prop, possibly prop diameter etc. The guy that told me about it isn't doing very many nbs in his yard as it is in France but I thought I'd throw it into the equation as I don't think I've seen a nb with a four blade prop ever and so we don't know whether this might, or might not, be problematic.

Personally I'd always take the advice of the prop specialist. In the case of my barge I specifically asked Jooren about why they choose 3 as opposed to 4 blades because I was going against the builders norm of a 4 blade. Jooren told me that they would recommend a 4 blade if they felt it necessary and they didn't on mine. The results of the Jooren 3 blade have been excellent so, I guess, they know what they are doing.

Roger

 

Edited to add: The guy in France running his boatyard used to be an engineer on a large UK nb hire fleet many years ago before he moved to France so he is quite au fait with nbs etc.

 

I know of one narrow boat which had a 4-bladed prop from new (on a 2LW), it propelled the boat perfectly well & I'm not aware that there were any vibration issues initially. It got changed for a 3-blade mainly because the 4-blade was too lightweight for canal use, and quickly became rather dog-eared.

 

My little tug has a 4 bladed prop about 36" dia., and vibration is not an issue with that.

 

Tim

Edited March 25, 2011 by Timleech