Does my prop look fine in this?

[Rivelin]

Hi all,

 

I'm keen on having my engine run at lower revs so it's quieter so if possible would like to increase the propeller pitch.

 

Are there any prop experts that might hazard a guess to the pitch in the attached photo?

 

fyi the engine is a 33bhp Vetus M4.15, which is based on a 1.5L Mitsubishi engine, driving a 38ft narrowboat.

 

Thanks in advance.

 

Nick

 

 

[Neil2]

You need to be careful mucking about with prop sizes, the chances are unless your boat has been put together by an idiot, the prop size will be about right, or possibly slightly overpropped already.  Increasing the pitch might just push you into real overpropping which you do not want.  

 

The Vetus/Mitsubishi is a naturally high revving engine, you can't turn it into a low revving slogger just by altering the prop size.

 

If you want it quieter a bigger silencer or a hospital grade silencer will make a world of difference - if you have the space. 

[tehmarks]

It might make it quieter, but it won't do any good at all for the long-term health of your engine.

[Mike Tee]

Looks like you can take clear underwater pictures, so have a go at the central boss on the prop with some wet and dry - very often the diameter and pitch will be stamped there.

[Bod]

 

This is what you are looking for.

There may be also a serial/part number.

 

Bod

[booke23]

I am having similar thoughts about my prop. It is a 14x10 (Beta 30, 40ft boat) and is quite badly bashed up and one blade is bent a bit. (Didn't happen in my ownership) So it needs replaced anyway.

 

When I first got the boat my initial thought was it was very underpropped. But now I have a bit of experience with it, I realise it is actually there or thereabouts......if anything maybe just very slightly underpropped. 

 

1650 rpm gives me 2.7 mph in still waters and if I open the throttle wide the engine won't go past it's max torque revs of 2600 rpm (max power is 3600 rpm on this engine) so it's certainly not grossly under propped. When I replace it I will probably put a 15x9 on it. 

 

What RPM do you cruise at and what gearbox do you have?

 

Don't forget that sound deadening foam under the deck boards can make a difference along with a hospital silencer. If you can't see the stamped specs, you could easily measure one blade on your prop and multiply by 2 to get the diameter. The pitch is trickier to measure and can't really be done through the weed hatch. 

 

 

 

 

Edited September 9, 2021 by booke23

[David Mack]

13 minutes ago, booke23 said:

and is quite badly bashed up and one blade is bent a bit. (Didn't happen in my ownership) So it needs replaced anyway.

It may well be possible to get it repaired at lower cost. You could also have it repitched at the same time.

[PD1964]

6 minutes ago, David Mack said:

It may well be possible to get it repaired at lower cost. You could also have it repitched at the same time.

These people do repairs/re-furbs. With a fast turnaround. Lots of other places around to post prop to.

https://www.aewoodward.co.uk/

[Neil2]

I'm curious - is there anyone who is 100% happy with their propeller?   Don't we all cruise around thinking, maybe a bit bigger, maybe a bit smaller...  

 

I remember meeting a guy who had just had his BMC 1.5 reconditioned, and he was moaning because it was smoking when it didn't before.  Then it turns out he had also changed the prop from a 17 to a 19 because "I wanted it to sound like a trad".  He just assumed the "new" engine would have more power.   

[booke23]

1 hour ago, David Mack said:

It may well be possible to get it repaired at lower cost. You could also have it repitched at the same time.

 

1 hour ago, PD1964 said:

These people do repairs/re-furbs. With a fast turnaround. Lots of other places around to post prop to.

https://www.aewoodward.co.uk/

 

Thank you both for that. I did briefly consider getting it repaired but discounted it as I wanted to try a slightly bigger prop. I didn't consider getting it repitched but I will look into that. I was concerned about turn around times on a repair/repitch as it would have to be done when the boat was out for blacking, but on looking at a few repairers it seems they all can do fast turnarounds for that very reason. 

[booke23]

1 hour ago, Neil2 said:

I'm curious - is there anyone who is 100% happy with their propeller?   Don't we all cruise around thinking, maybe a bit bigger, maybe a bit smaller...    

 

It certainly seems to be a common gripe. So much so it's surprising you don't see more controllable pitch propellers on canal boats!!

[PD1964]

8 minutes ago, booke23 said:

 

 

Thank you both for that. I did briefly consider getting it repaired but discounted it as I wanted to try a slightly bigger prop. I didn't consider getting it repitched but I will look into that. I was concerned about turn around times on a repair/repitch as it would have to be done when the boat was out for blacking, but on looking at a few repairers it seems they all can do fast turnarounds for that very reason. 

They will refurb and re-pitch to suit your boat, turn around is days, if your close you can drop off and collect same day with some.

[Neil2]

22 minutes ago, booke23 said:

 

It certainly seems to be a common gripe. So much so it's surprising you don't see more controllable pitch propellers on canal boats!!

 

Funny you should say that, I have a good friend who designed and built a heavy duty variable pitch prop for narrowboats, I have the prototype in my shed somewhere (it's probably in the box with the glass hammer and the wooden blowlamp).  

[booke23]

29 minutes ago, Neil2 said:

(it's probably in the box with the glass hammer and the wooden blowlamp).  

 

It would certainly be an exuberance on a narrowboat!

[Bee]

Have you a rev. counter? I guess the tick over is maybe 7-800 rpm and I would guess you are cruising at maybe 1400 - 1500rpm. If that is the case then that is probably about right.  There is a lot of guesswork in that though. If you are having to rev it over about 1600 to get reasonable progress then maybe the prop is a bit small. The prop looks a bit battered and could do with a bit of fettling and the distance between the prop and the back of the boat could usefully be shortened a bit, maybe there is space to push it into a coupling a bit. That's a bit picky though.  To be honest without knowing the prop size and the gearbox reduction its all guesswork really. Just saying what my thoughts could be if I steered it for an hour.

[beerbeerbeerbeerbeer]

26 minutes ago, Bee said:

Have you a rev. counter? I guess the tick over is maybe 7-800 rpm and I would guess you are cruising at maybe 1400 - 1500rpm. If that is the case then that is probably about right.  There is a lot of guesswork in that though. If you are having to rev it over about 1600 to get reasonable progress then maybe the prop is a bit small. The prop looks a bit battered and could do with a bit of fettling and the distance between the prop and the back of the boat could usefully be shortened a bit, maybe there is space to push it into a coupling a bit. That's a bit picky though.  To be honest without knowing the prop size and the gearbox reduction its all guesswork really. Just saying what my thoughts could be if I steered it for an hour.

Without blinding me with maths/science how do you work out best/optimal distance between propeller and arse end? What difference will it make?
It’s something I’ve never given thought to, till now. The OP’s prop appears closer than mine. 
(I’ll have to check)

I guess in extremes when too close or too far away it simply can’t pull/draw water effectively?

 

 

[Rivelin]

1 hour ago, Bee said:

Have you a rev. counter? I guess the tick over is maybe 7-800 rpm and I would guess you are cruising at maybe 1400 - 1500rpm. If that is the case then that is probably about right.  There is a lot of guesswork in that though. If you are having to rev it over about 1600 to get reasonable progress then maybe the prop is a bit small. The prop looks a bit battered and could do with a bit of fettling and the distance between the prop and the back of the boat could usefully be shortened a bit, maybe there is space to push it into a coupling a bit. That's a bit picky though.  To be honest without knowing the prop size and the gearbox reduction its all guesswork really. Just saying what my thoughts could be if I steered it for an hour.

Spot on with your rpm estimates! Probably nearer too 1400 for cruising, so maybe I just need to get better soundproofing / silencer.

The prop is 440mm according to the pre-purchase survey.

 

[blackrose]

4 hours ago, Neil2 said:

I'm curious - is there anyone who is 100% happy with their propeller?   Don't we all cruise around thinking, maybe a bit bigger, maybe a bit smaller...  

 

I am now, but only because I had my prop repitched.

 

I had the pitch reduced because it was well overpropped and I needed to release more engine revs to get some more power out of the engine. It's all worked very well. However, as others have said, you don't repitch a prop because you think your engine is too noisy!

Edited September 9, 2021 by blackrose

[DaveR]

Want a quieter life onboard, get a high attenuation silencer fitted.

 

Had Legacy done at the  TW Marine Engineering place on the Peak Forrest.

 

So many comments now such as "is that an electric boat".

[Rivelin]

3 hours ago, booke23 said:

 

It certainly seems to be a common gripe. So much so it's surprising you don't see more controllable pitch propellers on canal boats!!

When I was a kid, my Dad always said "pitch fully fine, 49/50" before we set off in the car (he was in the RAF) - maybe this is where it all came from? 😋

[PD1964]

14 minutes ago, Rivelin said:

When I was a kid, my Dad always said "pitch fully fine, 49/50" before we set off in the car (he was in the RAF) - maybe this is where it all came from? 😋

Maybe if he was in the Navy, but I think the RAF had slightly bigger propellers and at the other end😀

[Bee]

There is a formula for the max distance between the shaft bearing (probably Cutless type on most of our boats) and the back of the propeller hub, I think its equal to the dia. of the shaft (probably 1.5"), The usual explanation for this is if your prop is unbalanced it will try to run in an oval and wear the bearing.  I guess there is a risk of this but its not something to lose sleep over. I guess that if the prop is too close to the rudder there is some sort of theoretical loss of efficiency steering or something too. This might be an issue in an offshore power boat at 40 knots but its probably equal to two barnacles clinging on like grim death to the bottom.

[IanD]

13 hours ago, Bee said:

There is a formula for the max distance between the shaft bearing (probably Cutless type on most of our boats) and the back of the propeller hub, I think its equal to the dia. of the shaft (probably 1.5"), The usual explanation for this is if your prop is unbalanced it will try to run in an oval and wear the bearing.  I guess there is a risk of this but its not something to lose sleep over. I guess that if the prop is too close to the rudder there is some sort of theoretical loss of efficiency steering or something too. This might be an issue in an offshore power boat at 40 knots but its probably equal to two barnacles clinging on like grim death to the bottom.

If you look at the recommendations for marine propellers you'll find the following (propeller diameter=D):

 

http://www.ricepropulsion.com/TNLS/Clearances.htm

 

Minimum space to deadwood (end of swim) = 0.27D -- about 4" for a 16" prop. A bigger gap than this is even better to get good water flow into the prop

Minimum space to rudder = 0.1D = 1.6" -- but should be at least 2" to reduce risk of jamming with debris. Too big a gap can reduce rudder effectiveness

 

These recommendations are for slow boats like trawlers not power boats, so narrowboats should be similar.

 

If the shaft bearing plus gap to propeller puts the prop closer to the hull than the 0.27D recommendation then you get more prop noise and lower thrust. Note that this assumes the end of the swim is pointed not blunt, if it's blunt then the gap needs to be bigger, add approximately the width of the blunt end to the minimum gap (e.g. 1" width of blunt end, minimum gap = 5" for a 16" prop) -- if you want to be exact, extend the sloping sides of the hull backwards until they meet, and take the 0.27D from here.

 

Looking at the OPs photo, the gap to the (very blunt-ended!) hull is much smaller than recommended, and the gap to the prop (can't see this) is much bigger -- just like a lot of narrowboats because it's easier, you don't need a protruding stern tube to support the shaft.

 

Probably yet another reason they don't slip through the water as well as they could and use more fuel, just like all the other terrible design practices that some boatbuilders use to churn out hulls which swim like a brick...

Edited September 10, 2021 by IanD

[Tony Brooks]

If it's any use. On the hire fleet where jambing the prop with rocks, logs or stones was more likely that getting something wrapped around the shaft badly enough to stop the prop dead we did not like a large distance between aft bearing and the prop so tended to have about half an inch on flexibly mounted engines and less on solidly mounted ones. Some had only a 1" shaft diameter.  We had to allow for mounting flex when going into ahead and the engine moving forward a little. However, this was with proper boat shaped hulls and a number only had  a P bracket in front of the prop. In all cases I am sure the water flow into the prop was far better than you get with a NB swim.  How much difference to water flow moving the prop back an inch makes I have no idea, but suspect It's not much.

[OldGoat]

19 hours ago, Neil2 said:

I'm curious - is there anyone who is 100% happy with their propeller?   Don't we all cruise around thinking, maybe a bit bigger, maybe a bit smaller...  

 

I am ! - I fitted a Crowther!

In those days there weren't any forums (? fora) where you could ask the assembled multititudes - but there were a few hireboat companies and boat builders who knew 'what was what' - thus I took theer advice and forked out the extra cash.....