Four bladed prop

[JDR]

My boat is woefully underpropped-17" on a 3-1 PRM260.Unfortunately there is not enough clearance for a bigger diameter and Crowthers dont hold patterns for high BAR props that small.Changing the box isnt an option due to expense and is a nightmare to do on my boat anyway (trad with tight access).I may have a lead on an engineering company that can make a 4 blade 17" prop.I have never seen a 4 blade prop on a narrowboat-is there a good reason for this?

[blodger]

My boat is woefully underpropped-17" on a 3-1 PRM260.Unfortunately there is not enough clearance for a bigger diameter and Crowthers dont hold patterns for high BAR props that small.Changing the box isnt an option due to expense and is a nightmare to do on my boat anyway (trad with tight access).I may have a lead on an engineering company that can make a 4 blade 17" prop.I have never seen a 4 blade prop on a narrowboat-is there a good reason for this?

I thought the AXIOM was a four bladed effort that improve propulsion and stopping but at a price that has stopped me looking into it further. you do not mention pitch which is normally increased to compensate where you cannot get a bigger diameter?

[GUMPY]

My boat is woefully underpropped-17" on a 3-1 PRM260.Unfortunately there is not enough clearance for a bigger diameter and Crowthers dont hold patterns for high BAR props that small.Changing the box isnt an option due to expense and is a nightmare to do on my boat anyway (trad with tight access).I may have a lead on an engineering company that can make a 4 blade 17" prop.I have never seen a 4 blade prop on a narrowboat-is there a good reason for this?

 

Surprised by Crowthers not being able to help, however changing the box need not be expensive

I swapped mine (PRM301) from 3:1 to 2:1 for under £250.

Take a long hard search of places like findafishingboat, ebay, and apolloduck gearboxes come up from time to time. took me 4 months and in the end I swapped mine with a guy in Devon and just paid him to service it plus carriage.

Having said that once I had done that I needed a new prop as the old one was to course.

 

There is a 260 on ebay not sure of the reduction and another PRM or two

A prm 160 would also do thats the forerunner of the 260

Edited July 27, 2011 by idleness

[JDR]

you do not mention pitch which is normally increased to compensate where you cannot get a bigger diameter?

good point on the axiom-way out of my price anyway.I dont know the existing pitch how would i work this out with boat in water?Would a coarser pitch on such a small (for the box) have a dramatic effect?

 

ps:idleness i know the correct solution is a 2-1 box but trying to work round it if possible.

Edited July 27, 2011 by JDR

[Timleech]

 

 

There is a 260 on ebay not sure of the reduction and another PRM or two

A prm 160 would also do thats the forerunner of the 260

 

The 260 on ebay is 3:1

 

Some people say that a 4-bladed prop is more likely to collect rubbish, I don;t know whether there's any real evidence for this.

If you really, really don't want to change your gearbox it's probably worth a try.

There are other makers than Crowthers, of course. Lancing offer equipoise props, which by their design have a higher blade area, as well as other shapes and 4-blades. It might be worth giving them a call.

 

Tim

Edited July 27, 2011 by Timleech

[JDR]

Thanks Tim will try them tomorrow.

[nb Innisfree]

My boat is woefully underpropped-17" on a 3-1 PRM260.Unfortunately there is not enough clearance for a bigger diameter and Crowthers dont hold patterns for high BAR props that small.Changing the box isnt an option due to expense and is a nightmare to do on my boat anyway (trad with tight access).I may have a lead on an engineering company that can make a 4 blade 17" prop.I have never seen a 4 blade prop on a narrowboat-is there a good reason for this?

 

You don't say what your max engine speed is, but on our original engine it ran at 2000rpm max with a 2:1 box and Crowther made us a superb 17" high BAR prop (17x 14 and about 75%), no hint of cavitation and excellent thrust in forward and reverse and allowed the engine to achieve slightly under full revs. When this engine let us down after a short time (long story) we changed to a 3000rpm engine which would have been ideal with a 3:1 box but we opted instead for the cheaper option of a reprofiled prop (still 17"), I have often wished we had used a 3:1 instead and left our perfect prop alone.

Edited July 27, 2011 by nb Innisfree

[blodger]

good point on the axiom-way out of my price anyway.I dont know the existing pitch how would i work this out with boat in water?Would a coarser pitch on such a small (for the box) have a dramatic effect?

 

ps:idleness i know the correct solution is a 2-1 box but trying to work round it if possible.

 

I cannot see how you are going to progress without knowing the pitch of your current prop.

 

My boat was underpropped so when in drydock I managed to swop it for one that slightly bigger 17" and greater pitch. I cannot recall the pitches now but I have it written down somewhere but I think I went up by 2. Now I am slightly overpropped and do not make full rpm 2,600 ahead. So yes changing the pitch can make a difference on smaller props and many props on the shallow draft boats produced these days are smaller

[JDR]

I cannot see how you are going to progress without knowing the pitch of your current prop.

That makes sense to me.Bear with me on this i am still learning, as i understand it a pitch x refers to the distance the prop would move through a theoretical solid medium in one 360 revolution.If you could measure the angle of the blades could that be used to work out the pitch?Or is that too simplistic?

[JDR]

You don't say what your engine speed is

bv1505 about 2600 .Revs out fully just a chronic lack of forward thrust in deep water.I would be happy to sacrife 2-300 revs off the top end.

Edited July 27, 2011 by JDR

[JDR]

Ive found some good formulas for measuring pitch but this relies on:getting a flat surface behind the prop and not dropping ruler and protractor into the river while doing it.On balance I think i'd rather wait for daylight and see if pitch is embossed on hub or better still ring the guy who owned the boat for 17 years before me!There must be a gauge that does the same job.I'm guessing 17/12 or 13 hopefully find out tomorrow.Going up to a 14/15 sounds like it may be a good place to start?

[PhilR]

Ive found some good formulas for measuring pitch but this relies on:getting a flat surface behind the prop and not dropping ruler and protractor into the river while doing it.On balance I think i'd rather wait for daylight and see if pitch is embossed on hub or better still ring the guy who owned the boat for 17 years before me!There must be a gauge that does the same job.I'm guessing 17/12 or 13 hopefully find out tomorrow.Going up to a 14/15 sounds like it may be a good place to start?

 

If you can't swing a larger diameter prop you might be better going for larger blade area. Increased pitch might increase the egg whisk characteristics?

[by'eck]

Seems that prop pitch its difficult to calculate from physical dimensions given that to maintain a constant pitch the blade angle decreases at higher radii. That's assuming a constant pitch prop. See here for more info.

 

Prop calculator

[JDR]

Thank you Richard your link is an excellent source for newbies to the dark arts.One of the suggestions I came across was the idea of measuring at the 7/10 line (of radius) which is apparently what some people in the model boating world use as a rough estimate.

 

I had a very interesting conversation with a gentleman from Lancing marine.He was brutally honest and basically told me to stop pissing about and fit a 2-1 box which I have known from the start is the correct solution and echoes a lot of replies above.I think it was wishful thinking on my part that there might be an easy/cheap fix.

 

On the upside the boat is beautiful on canals its just deep open water where its an issue.

 

Many thanks to everybody that chipped in, very much appreciated.

 

John

[JDR]

numpty double post deleted

Edited July 28, 2011 by JDR

[saltysplash]

I used to have a four bladed prop.....now wheres that pic?.....

 

 

oh yes , here it is

 

 

 

 

 

 

and this is it after a bit of scrubbing

 

Edited July 31, 2011 by saltysplash

[DickBrowne]

We had a similar under-propped issued. Our boat required 2000rpm to get anywhere near 4mph on the deep parts of the GU. It turns out that the prop (incidentally a Crowther HE) was 15x8, which is woeful. Crowthers, surprisingly, recommended a 15x12. I'd have thought a larger diameter would have been desirable as well as the increase in pitch.

 

When our boat went into a yard for blacking, I acquired a 15x12, with a much larger surface area which was then fitted. The result? I have no idea. I know this may be less than useful, but the blacking happened last week and we're not picking the boat up until tomorrow

 

I shall keep you posted, but our expectation is that we should drop our cruising revs from 2000 to a more restful 1400 or so.

 

Richard

[bizzard]

I wonder why no ones thought of making an Rose scroll type propeller for our type of boats,like a short worm drive as used on most Hydrofoil's to get them on the plane,and doesn't need a larger diameter,and has huge bite on the water,and will stop the boat almost instantly.might need a lot more power though as blade area is much greater.i'll have to work it out. bizzard.

[JDR]

I wonder why no ones thought of making an Rose scroll type propeller for our type of boats,like a short worm drive as used on most Hydrofoil's to get them on the plane,and doesn't need a larger diameter,and has huge bite on the water,and will stop the boat almost instantly.might need a lot more power though as blade area is much greater.i'll have to work it out. bizzard.

 

In your own time bizzard, no pressure. I think you're overthinking this one, I've found a 20 foot length of drainpipe a packet of beanfeast and a box of matches which should take care of the propulsion. Only problem is it's going to need someone at the pointy end to deal with reversing and don't get me started on bowthrusters.

[by'eck]

We had a similar under-propped issued. Our boat required 2000rpm to get anywhere near 4mph on the deep parts of the GU. It turns out that the prop (incidentally a Crowther HE) was 15x8, which is woeful. Crowthers, surprisingly, recommended a 15x12. I'd have thought a larger diameter would have been desirable as well as the increase in pitch.

 

When our boat went into a yard for blacking, I acquired a 15x12, with a much larger surface area which was then fitted. The result? I have no idea. I know this may be less than useful, but the blacking happened last week and we're not picking the boat up until tomorrow

 

I shall keep you posted, but our expectation is that we should drop our cruising revs from 2000 to a more restful 1400 or so.

 

Richard

 

Any news yet on new prop?

 

Calculations suggest you should get a tad more speed @ 1400rpm compared with old prop @ 2000rpm & assuming 2:1 gearbox reduction.

[JDR]

So Richard, just how many coats of blacking did you put on! You are in danger of doing a Phyllis on this one!Hows the new prop?

[DickBrowne]

Hi everyone, we're back! OK, so, first things first, I'm not sure that I'm going to suddenly start fancying a plastic boat, and defend it to the hilt, regardless of the facts. I guess I'm a Phyllistine

 

Sorry about that (to everyone, including Phyllis!)

 

The end result was three coats of blacking (after a very laborious prep routine). If anybody is on the Oxford and needs some work doing, I can really recommend Richard at the Cropredy Slip. He blacked, fitted four new anodes (supplied by me) and a prop (also supplied by me), blacked between the top strake and gunwale (not my preference, but it was already done that way so we had little choice) and still charged much less (and I'm talking a couple of hundred pounds less) than or local yards charge for a simple power wash and two coats to the top strake.

 

The new propellor was actually a used prop, my original plan was to fit a 17x10 to replace the existing 15x8, but due to my inneptitude with the size of things, it turns out that our 1.5 inch shaft was actually a 1.25 incher, well done to Rose Narrowboats for spotting that, and for having a 15x12, which as you can tell from my first post, is what I bought.

 

The end result is that I'm now having to be careful about throttle settings. I previously struggled to raise a bow wave, let alone a breaking one. It could be the lack of depth we're experiencing at the moment, but I can easily get enough speed to upset people and wildlife.

 

For purposes of perspective, our cruising speed with the old prop was 2000 rpm, with the new one, at about the same speed, I'm only at 1200 rpm. Much more relaxing, less wear on the engine and gearbox and (hopefully) less diesel consumption.

 

Hope this is useful

 

Richard

[Redeye]

My boat is woefully underpropped-17" on a 3-1 PRM260.Unfortunately there is not enough clearance for a bigger diameter and Crowthers dont hold patterns for high BAR props that small.Changing the box isnt an option due to expense and is a nightmare to do on my boat anyway (trad with tight access).I may have a lead on an engineering company that can make a 4 blade 17" prop.I have never seen a 4 blade prop on a narrowboat-is there a good reason for this?

 

Hi

 

Just seen the post about 4 blade props. I fitted one supplied by Clements Engineering in Bedford about a year ago. The prop is brilliant and I think that it is less prone to picking up weed and debris than the previous 3 blade version. This includes trips around the BCN and also the Caldon through Stoke and a short run up the weedy Chesterfield this summer.

 

Cheers

 

Redeye

[by'eck]

For purposes of perspective, our cruising speed with the old prop was 2000 rpm, with the new one, at about the same speed, I'm only at 1200 rpm. Much more relaxing, less wear on the engine and gearbox and (hopefully) less diesel consumption.

 

Hope this is useful

 

Richard

 

Those figures tie in perfectly with theoretical ones gained from the prop calculator - posts #13 & 20 - very re-assuring.