Prop size

[Andynoz]

Hi I'm new to this forum just wondering if anyone has any info on prop sizing 

I'm currently rebuilding a 48ft trad stern which is basically a bare shell the original engine was I believe a bmc 1.5 but the engine I have is a sabb 2jhr which has a hurth 2:1 box 

The existing prop is 18 x 12 and I'm thinking this will be undersized for the new engine given that I want a low engine speed for normal cruising 

Any help appreciated 

[Alan de Enfield]

5 minutes ago, Andynoz said:

Any help appreciated 

https://www.vicprop.com/displacement_size.php?action=calculate

[mrsmelly]

7 minutes ago, Andynoz said:

Hi I'm new to this forum just wondering if anyone has any info on prop sizing 

I'm currently rebuilding a 48ft trad stern which is basically a bare shell the original engine was I believe a bmc 1.5 but the engine I have is a sabb 2jhr which has a hurth 2:1 box 

The existing prop is 18 x 12 and I'm thinking this will be undersized for the new engine given that I want a low engine speed for normal cruising 

Any help appreciated 

Much will depend on the boats draught anyway? If you want to swing a larger diameter prop you need the space to do it?  Pitch of course can be altered.

[bizzard]

Those engines are about 30hp so I'd try the prop you have. Its nice to be able to go dead slow in confined places. Also I like to be a little under propped, it helps to keep the engine from overheating if the cooling system happens to be a little inadequete, common on narrowboats. And who knows you cooling system might be inadequete too.

Edited April 10, 2020 by bizzard

[Onewheeler]

7 minutes ago, Alan de Enfield said:

https://www.vicprop.com/displacement_size.php?action=calculate

I'd not used that before, having used an Excel spreadsheet "propcalc" a few years ago when replacing our prop. It seems to give very similar results to propcalc, allowing for doing rough mental conversions to stupid measurement units. Guessing the parameters of the OP's boat, 18" x 12" sounds rather oversized.

[mrsmelly]

5 minutes ago, Onewheeler said:

I'd not used that before, having used an Excel spreadsheet "propcalc" a few years ago when replacing our prop. It seems to give very similar results to propcalc, allowing for doing rough mental conversions to stupid measurement units. Guessing the parameters of the OP's boat, 18" x 12" sounds rather oversized.

Ive just tried it and it came out with there or there abouts the 18 x 12  I have fitted. 18 x 12 works on most modern narrowboats between 50 and 70 feet with the standard modern engine and box I find.

[Canal321]

24 minutes ago, Andynoz said:

Hi I'm new to this forum just wondering if anyone has any info on prop sizing 

I'm currently rebuilding a 48ft trad stern which is basically a bare shell the original engine was I believe a bmc 1.5 but the engine I have is a sabb 2jhr which has a hurth 2:1 box 

The existing prop is 18 x 12 and I'm thinking this will be undersized for the new engine given that I want a low engine speed for normal cruising 

Any help appreciated 

Hi,

There are a couple of sabb 2jhr owners on here.

 Looking in the manual it suggests a 20x16 RH 

You can find the manual free online if you have not already.

[Onewheeler]

3 minutes ago, mrsmelly said:

Ive just tried it and it came out with there or there abouts the 18 x 12  I have fitted. 18 x 12 works on most modern narrowboats between 50 and 70 feet with the standard modern engine and box I find.

You might be right. Our water draft is a bit deeper than many modern boats. 16.5" x 11" is near perfect.

[PaulJ]

18 x 12 will be a bit small-you will lose all the advantage of having a slow revving engine.

I have same engine/box on a 50fter and have a 20 x16 which is spot on. Chop! has one too but used a four bladed to get round the diameter issue.

Edited April 10, 2020 by PaulJ
Spelling

[Alan de Enfield]

32 minutes ago, Andynoz said:

I'm currently rebuilding a 48ft trad stern which is basically a bare shell the original engine was I believe a bmc 1.5 but the engine I have is a sabb 2jhr which has a hurth 2:1 box 

I suppose its best not to mention that re-engining requires a Post Construction Assessment to the RCD

 

The new Directive has also clarified the definition of Major Craft Conversion, which is now also included in the new Directive. Major Craft Conversion means ” … a conversion of a watercraft which changes the means of propulsion of the watercraft, involves a major engine modification, or alters the watercraft to such an extent that it may not meet the applicable essential safety and environmental requirements laid down in this Directive”.

[Andynoz]

Is it not the case that I dont need to comply with the rcd if I mean to keep the boat and not put it up for sale within 5 yrs ? 

[Alan de Enfield]

3 minutes ago, Andynoz said:

Is it not the case that I dont need to comply with the rcd if I mean to keep the boat and not put it up for sale within 5 yrs ? 

 

Not when it comes to using 'old' engines that are non-emission compliant.

 

Lets not talk about it and then no one will know you fitted it.

 

Just as a matter of interest what age is the boat ?

[Bee]

Like many others I have a Beta 43, this has an 18 x 12 as a rule of thumb Beta recommendation, with a 2 ; 1 reduction.  The engine is quite happy at 1000 rpm or 500 rpm at the prop at the slowest boat speeds and gets along at a good speed at about 1.500 rpm or 750 at the prop. Obviously this is a modern 4 cyl relatively high revving engine that will handle 2000 rpm for those 'Damn, we're going over the weir ' moments. I don't know the Sabb engine but I would think that is running it rather too fast for its own good to give that sort of performance. I guess that the Sabb is stronger at lower speeds so a bigger prop would be more sensible. E Bay sometimes has props.

[Andynoz]

Hi thanks for the advice 

Seems to be the same idea I've got

The Sabb is at it's best at low speed they sound really strident when revving hard its rated at 30hp but at canal speeds that probably equates to 10 hp at best which is why I was thinking a bigger prop would be better 

[Tracy D'arth]

18 X 10.5 Crowther high efficiency with a larger blade area, cruises at 700 rpm aprox 3 mph with a 30" draught on 50 ft hull.  

If it helps.

[wergie]

If you download the manual for your engine you'll find that the maximum prop size is 20 x 16,so if you use your current prop you'll be a little underpropped.

I have the same engine with a PRM 2-1 box but I need to fit a smaller prop as my engine is overpropped at the moment and is a little smoky . I was planning to fit a smaller prop the week before last but I'm  under house arrest at the moment so it will have to wait.  If you want a copy of the manual PM me and  I'll send it to you.

 

Steve

[Andynoz]

3 hours ago, Tracy D'arth said:

18 X 10.5 Crowther high efficiency with a larger blade area, cruises at 700 rpm aprox 3 mph with a 30" draught on 50 ft hull.  

If it helps.

I'm still a bit confused is that 700rpm engine speed or prop speed ? 

[Andynoz]

2 hours ago, wergie said:

If you download the manual for your engine you'll find that the maximum prop size is 20 x 16,so if you use your current prop you'll be a little underpropped.

I have the same engine with a PRM 2-1 box but I need to fit a smaller prop as my engine is overpropped at the moment and is a little smoky . I was planning to fit a smaller prop the week before last but I'm  under house arrest at the moment so it will have to wait.  If you want a copy of the manual PM me and  I'll send it to you.

 

Steve

Hi Steve 

Yes I'm in the same boat ( if you'll pardon the pun ) I have a copy of the manual downloaded thanks 

Just out of interest what size prop are you running at the moment ? And what sort of displacement do you reckon you have ? 

I love the sound of the Sabb on low revs and just over tick over but as it's in an open engine room it's far too noisy when its revving hard which is ok for maneuvering but not for cruising all day 

[Tracy D'arth]

16 minutes ago, Andynoz said:

I'm still a bit confused is that 700rpm engine speed or prop speed ? 

Apologies, engine revs into 2 to 1 gearbox.

So 350 prop rpm.

[Andynoz]

6 minutes ago, Tracy D'arth said:

Apologies, engine revs into 2 to 1 gearbox.

So 350 prop rpm.

Ah yes obvious really 

So is that with a Sabb engine ? 

 

[Tracy D'arth]

15 minutes ago, Andynoz said:

Ah yes obvious really 

So is that with a Sabb engine ? 

 

No, 37 HP BMC.

[wergie]

Hi Andy,

I'm not sure what size my prop is but I've deduced through the final law of engineering i.e 'when all else fails,read the manufacturers instructions', that having eliminated everything else, all that's left is that my prop is too large.. The old engine, a 1937, 16hp Lister CE had a maahoosive flywheel and generated a huge amount of torque,so  was able to turn a big prop. The Sabb, although twice as powerful doesn't have such a large flywheel so isn't as torquey, and as a consequence is a bit smokey...its a bit like trying to drive off in third gear... all good fun though... It does make a lovely noise though...

also, it's fitted in a 57' narrowboat...

[wergie]

Do you have a silencer fitted into your exhaust?