Propeller

[Maudesmaster]

Hi i need recommendations for new prop size I have a 62’ narrow boat with a Gardner 3LW and PRM 500 2:1 reduction gear box and 24” between Skeg and uxter 

I have a 21” 18 pitch propeller which is very slow on the forward uptake and full revs only gives me walking speed ( little legs wife on tow path ) in locks I have to use full revs to stop boat hitting gates 

I am going to order a new prop rather than having existing prop reworked

 

[jonesthenuke]

We have a 57 foot boat with the same engine and IIRC a similar prop, but a 1.5 ratio box. If anything this makes the boat faster on tickover than I would like.

 

Before altering the prop, are you getting the correct speed range from the engine? Is the current prop ok (not damaged or fouled). 

 

Edited September 25, 2021 by jonesthenuke
typo

[Maudesmaster]

Prop is brand new boat has only been in water 3 weeks changing gearbox not an option no fouling engine revs are very responsive  

[Alan de Enfield]

19 minutes ago, Maudesmaster said:

Prop is brand new boat has only been in water 3 weeks changing gearbox not an option no fouling engine revs are very responsive  

 

 

Surely if boat is new and only been in the water for 3 weeks you have some comeback on the builder for selling something (apparently) not fit for pupose - why should you have to fork out £500-£700+ for a new prop when the fault lies with the builder ?

Edited September 25, 2021 by Alan de Enfield

[frangar]

In the past I would have said a call to Crowthers for a compensated prop would be the answer but sadly as they ceased trading that’s not an option. 
 

You could try Clements Engineering in Bedfordshire. They were recommended by Crowthers as the people to use and I’ve heard good reports. 
 

I have a JP2M with a compensated prop…basically it’s got a bigger blade area than standard and it transformed my boat. I can only swing a 22” blade and even that’s tighter to the uxter plate than desirable. We originally had a 22x22 standard blade…like you we found going and stopping were issues. I now have done many tidal rivers etc that would have been unthinkable before. 
 

https://clementsengineering.co.uk

 

My Crowthers blade was £850 about 11 years ago…not cheap but when I got them to check it over when they made me a new tailshaft last year just before they closed there wasn’t so much as Nick on a blade…and it’s done a lot of canal miles and clouted a fair few bricks. A decent prop is much stronger than the usual Chinese offering from a chandlers. 
 

edited to add. 
 

I was told by Crowthers that the compensation made my prop equivalent to a 26 x 19. I would say that 21x 18 is far too small. Don’t be tempted just to increase the pitch tho….it will be horrible to steer & stop. 

Edited September 25, 2021 by frangar

[dmr]

That doesn't sound right to me. What are full revs?

We have a Beta JD3 with a 21x20 and at 700rpm are going along at a fast walking pace and this is a heavy 70 foot boat likely about the same draft as you.

 

The 3LW has a lot more torque than the JD3 so could take a bigger prop but you are limited to about 21" by your 24" skeg depth.

I think the Gardner tickover is a little slower than mine, probably about 400rpm??, so too big a prop will give a tickover boat speed that is too fast.

So I suspect you can go just a bit bigger than 21x20, you could maybe just go to 22" but getting extra blade area might be better, this is specialist stuff.

 

21x18 is not hopelessly small so I suspect something is wrong. Are you sure of prop size and sure its a 2:1 box?????

 

 

[jonesthenuke]

1 hour ago, Maudesmaster said:

Prop is brand new boat has only been in water 3 weeks changing gearbox not an option no fouling engine revs are very responsive  

The engine may be responsive but does it run at full speed when demanded? It will be about 1500RPM but check the injection pump. Buy a cheap optical tacho and check

 

Is is possible that the gearbox is slipping? Has it got enough oil in it? As its a PRM 500 has it got a trolling valve (I believe these are an option)

 

Is everything else in the drive train properly secure and definitely not slipping? I ask this as the boat has only recently been launched and you may be looking for that kind of defect.

[MtB]

I agree with JTN and DMR above. Something is not as you think or something is not working correctly.

 

My 68ft boat has a 21 x 19 which when I had the BD3 with 2:1 reduction gear in it, was a fast and nimble boat which would stop on a sixpence and was ridiculously fast at 1300rpm. 

 

Assuming your engine is actually genuinely revving to a genuine 1500rpm (how do you know?), I'd suspect either:

 

1) Your prop is not 21x18 as you believe, or

2) You have a 3:1 reduction gear fitted, not the 2:1 you think it is. 

 

My money is on 2).

 

And swerving off at a tangent, I thought the new RCR regulations prevented vintage lumps like yours being fitted into a new boat. Have they not come into force yet? Or are you like me and entirely happy to ignore them? 

[Bee]

That does seem a bit slow, I know that on my boat that, for example, 1200rpm and a 2:1 box the shaft is not so much rotating but properly spinning at some speed, 600 rpm at the shaft is 10 times a second and a quick glance over the stern when tied up shows a lot of water being moved and the ropes are very tight. I would have a quick look at that just to see that all is in order. I don't suppose the prop is wrong handed and the gearbox running in reverse is it? Would that even work? dunno.

[frangar]

If it’s anything like my JP you are rarely running at much above 800rpm….they just don’t sound happy being thrashed. I’d check the ratio of the box as others have said but a eBay optical tachometer can be a revelation when seeing what rpm your engine “runs” at….also bear in mind that you need to be on deep water to properly assess a prop so to speak….the suckdown on a shallow narrow channel will have a massive effect….you need a decent river…preferably with not much flow….the weaver is good!…unless in flood! 

[Maudesmaster]

4 hours ago, Alan de Enfield said:

 

 

Surely if boat is new and only been in the water for 3 weeks you have some comeback on the builder for selling something (apparently) not fit for pupose - why should you have to fork out £500-£700+ for a new prop when the fault lies with the builder ?

I have been building fitting out for 5 1/2 years Prop was a guesstimate on replies to similar question 5 years ago  

3 hours ago, jonesthenuke said:

The engine may be responsive but does it run at full speed when demanded? It will be about 1500RPM but check the injection pump. Buy a cheap optical tacho and check

 

Is is possible that the gearbox is slipping? Has it got enough oil in it? As its a PRM 500 has it got a trolling valve (I believe these are an option)

 

Is everything else in the drive train properly secure and definitely not slipping? I ask this as the boat has only recently been launched and you may be looking for that kind of defect.

 

[Maudesmaster]

Thank you all I am presently awaiting a reply from Jonathan at T Norris and injector pump going to Gardner marine for overhaul 

[Alan de Enfield]

24 minutes ago, Maudesmaster said:

I have been building fitting out for 5 1/2 years Prop was a guesstimate on replies to similar question 5 years ago  

 

Ahhhh OK, you are the builder !!

 

Prop size does seem to be about right when looking at some of the replies, (similar engine, gear box and prop sizes) are you sure it is not slipping on the shaft, or any of the other suggestions ?

 

To echo on MTBs question - presumably you have not built the boat to be RCD compliant ?

 

 

I have found this prop calulator to be pretty close to reality.

 

Vicprop - Prop calculator for Displacement and semi-displacement hulls

 

For my boat it comes up with 21.5 x 17 and I actually have 22 x 18

 

 

 

 

 

Edited September 25, 2021 by Alan de Enfield

[Alan de Enfield]

It would appear that your ideal prop is 25 x 19.5 (ish) but obviously not possible with only 24" between skeg and uxter plate. It looks as if anything you use will be a compromise and give below par performance.

[frangar]

15 minutes ago, Alan de Enfield said:

It would appear that your ideal prop is 25 x 19.5 (ish) but obviously not possible with only 24" between skeg and uxter plate. It looks as if anything you use will be a compromise and give below par performance.

As I suggested!!! That’s why you need a compensated prop with a bigger blade area….not ideal I grant but the best working solution. 

[Alan de Enfield]

5 hours ago, frangar said:

 

I was told by Crowthers that the compensation made my prop equivalent to a 26 x 19. I would say that 21x 18 is far too small. Don’t be tempted just to increase the pitch tho….it will be horrible to steer & stop. 

 

12 minutes ago, frangar said:

As I suggested!!! That’s why you need a compensated prop with a bigger blade area….not ideal I grant but the best working solution. 

 

 

Indeed it looks as it the Vic-Prop calculator and Crowthers knowledge and experience are closely aligned.

 

I'd suggest, however' that the OP could end up paying a fortune and buying an assortment of random props, he'd be better off biting the bullet and speaking to the experts - buy once and get it right first time.

He may even be able to trade in his 'almost new' prop as a contribution to the cost.

[frangar]

15 minutes ago, Alan de Enfield said:

 

 

 

Indeed it looks as it the Vic-Prop calculator and Crowthers knowledge and experience are closely aligned.

 

I'd suggest, however' that the OP could end up paying a fortune and buying an assortment of random props, he'd be better off biting the bullet and speaking to the experts - buy once and get it right first time.

He may even be able to trade in his 'almost new' prop as a contribution to the cost.

Hence why I suggested Clements engineering. Not only did Crowthers recommend them as a successor but I know of several people that have had good service from them and been happy with the results. 

[blackrose]

3 hours ago, MtB said:

 

And swerving off at a tangent, I thought the new RCR regulations prevented vintage lumps like yours being fitted into a new boat. Have they not come into force yet? Or are you like me and entirely happy to ignore them? 

 

River Canal Rescue now have regulations preventing vintage engines being fitted into new boats? 

2 hours ago, Maudesmaster said:

Thank you all I am presently awaiting a reply from Jonathan at T Norris and injector pump going to Gardner marine for overhaul 

 

My prop was repitched by Norris about 8 or 9 years ago. £150. Much cheaper than a new prop if they can do it. 

[frangar]

2 minutes ago, blackrose said:

 

River Canal Rescue now have regulations preventing vintage engines being fitted into new boats? 

Steph would love that….all boats can only be fitted with their approved engines supplied and fitted at vast expense….don’t give her ideas!  

[MtB]

5 minutes ago, blackrose said:

River Canal Rescue now have regulations preventing vintage engines being fitted into new boats? 

 

Fortunately i wrote nothing of the sort, if you read my post properly. 

 

[MtB]

 

Here's a link for you, to the Recreational Craft Regulations, generally abbreviated to RCR, which came into force in 2017.

 

https://www.legislation.gov.uk/uksi/2017/737/contents/made

[frangar]

20 minutes ago, MtB said:

 

Here's a link for you, to the Recreational Craft Regulations, generally abbreviated to RCR, which came into force in 2017.

 

https://www.legislation.gov.uk/uksi/2017/737/contents/made

It’s more usually referred to as the Recreational Craft Directive or RCD……

[blackrose]

25 minutes ago, MtB said:

 

Fortunately i wrote nothing of the sort, if you read my post properly. 

 

 

Calm down dear, it was just a joke.

[frangar]

Just now, blackrose said:

 

Calm down dear, it was just a joke.

We don’t allow those on here these days! Did you not get the directive? 🤣

[Alan de Enfield]

5 minutes ago, frangar said:

It’s more usually referred to as the Recreational Craft Directive or RCD……

 

That was when the RCD (The EU name for it) was used, but since 2017 the UK has had their own version - the RCR (identical contents but the front cover is changed)