Loss of foreward thrust from BMC engine

[Amh300]

Hi,

This may not be the correct forum to discuss but as my nb has a BMC engine I thought I'd post it here. I have a recurring issue with foreword thrust (recurring because it also happened last year). Boat is fine when I start out but after about 30 mins, speed drops until barely any forward movement. Reverse thrust is good. When I stick it in reverse and crank up the revs, when I put it back in foreword it regains thrust. But it drops down again after 15-20 mins. If I switch off the engine and leave it for a while, when I start up again, foreword is fine. But then drops off again. Things to note:

1. weed hatch is clear

2. RCR engineer has been out to check it. As it was over night, engine had totally cooled down etc and engineer couldn't find anything wrong - foreword and reverse thrust strong.

3. RCR engineer checked and slightly adjusted the cable;

4. RCR engineer checked gearbox oil - right amount in there just dirty. Gearbox was replaced in May this year by qualified engineer.   

So we carried on but same problem has recurred.

 

Just wondering if anyone has had a similar experience?

Thanks

Andrew

 

 

[haggis]

Sounds like the symptoms of leaves on the prop. A common occurrence at this time of year 🙂 

[bizzard]

2 minutes ago, Amh300 said:

Hi,

This may not be the correct forum to discuss but as my nb has a BMC engine I thought I'd post it here. I have a recurring issue with foreword thrust (recurring because it also happened last year). Boat is fine when I start out but after about 30 mins, speed drops until barely any forward movement. Reverse thrust is good. When I stick it in reverse and crank up the revs, when I put it back in foreword it regains thrust. But it drops down again after 15-20 mins. If I switch off the engine and leave it for a while, when I start up again, foreword is fine. But then drops off again. Things to note:

1. weed hatch is clear

2. RCR engineer has been out to check it. As it was over night, engine had totally cooled down etc and engineer couldn't find anything wrong - foreword and reverse thrust strong.

3. RCR engineer checked and slightly adjusted the cable;

4. RCR engineer checked gearbox oil - right amount in there just dirty. Gearbox was replaced in May this year by qualified engineer.   

So we carried on but same problem has recurred.

 

Just wondering if anyone has had a similar experience?

Thanks

Andrew

 

 

Do the engine reva remain the same when it slows down.  Which gearbox and is it's oil level ok

[Rob-M]

I agree with Haggis, this time of year lots of leaves in the water that collect around the prop but as soon as you give a blast of reverse they all drop off and forward picks up again.

[Amh300]

5 minutes ago, bizzard said:

Do the engine reva remain the same when it slows down.  Which gearbox and is it's oil level ok

No, revs stay the same. Don't know what kind of gearbox it is but. here's a photo. Oil level is ok.

3 minutes ago, Rob-M said:

I agree with Haggis, this time of year lots of leaves in the water that collect around the prop but as soon as you give a blast of reverse they all drop off and forward picks up again.

That would make sense in as much as I've been out twice in the summer and had no issue. Is it really a common problem then? My boat pretty much stopped moving forward at one point. Can't believe it could be something as simple as that - not a wind up (leaves on the line etc

[Tony Brooks]

It is a Hurth mechanical box, well known for not being the most reliable of gearboxes. When it next happens put it out of gear and look over the sides and stern of the boat, if you see clouds of leaves in the water then it is normal. If you don't then it might be a worn ahead clutch.

[Amh300]

Ok, sounds like I having to work with for next time. Thanks everyone (this forum is awesome) 👍

[bizzard]

5 minutes ago, Amh300 said:

No, revs stay the same. Don't know what kind of gearbox it is but. here's a photo. Oil level is ok.

Seems like clutch slip. Looks like a mechanical box so metal cone slip.  When  sthe boatlows down and the engine revs remain the same does the prop shaft stop revolving or almost stop revolving.

[Amh300]

Just now, bizzard said:

Seems like clutch slip. Looks like a mechanical box so metal cone slip.  When  sthe boatlows down and the engine revs remain the same does the prop shaft stop revolving or almost stop revolving.

No difference to speed of the prop shaft - it carries on at normal speed

[bizzard]

1 minute ago, Amh300 said:

No difference to speed of the prop shaft - it carries on at normal speed

If your cerain of that than then go for a fouled prop.

[Tony Brooks]

2 minutes ago, bizzard said:

If your cerain of that than then go for a fouled prop.

 

But leaves will just float away when you stop the prop, so you never find them

[Amh300]

So the leaves get stuck on the prop you can see in this pic? Hard to believe it can make such a difference. Not doubting, just surprised! 

[Tony Brooks]

2 minutes ago, Amh300 said:

So the leaves get stuck on the prop you can see in this pic? Hard to believe it can make such a difference. Not doubting, just surprised!

 

It worries almost all boaters new to inland waterways, especially canals.

[Rob-M]

Going through a tree lined section in the autumn can be a real slog sometimes, spending what seems like more time in reverse than forward.

[beerbeerbeerbeerbeer]

with a fouled prop and giving the boat some welly I usually get a lot of vibrations and a juddering below my feet,

 

would I be wrong thinking a slipping clutch will just spin and not cause such vibrations on the prop?

 

OP, I struggled to get in and out of Wheaton Aston lock the other day it was so full of leaves,

once clear of the lock mouth it was all fine,

so yes, OP, leaves will cause a problem,

one trick as said above is put the engine in a gentle reverse for a moment or two, pause and let the boat drift a little and give the leaves time to clear, no real need for any hard blast, look down and watch them come up you’ll be surprised at the amount, then gently carry on.

Another I do is put the engine in neutral and drift for a little while. Sometimes I’ll just drift through a large gathering of leaves anyway.

 

[Amh300]

As I now appreciate - still mind blown that this looks like the explanation! Leaves on the prop - who would have thought it.

1 minute ago, Amh300 said:

As I now appreciate - still mind blown that this looks like the explanation! Leaves on the prop - who would have thought it.

At least I didn't fork out £235 RCR sub + £150 call out fee or anything expensive like that to find out  via this forum about this common autumn phenomenon 🤦‍♂️

[beerbeerbeerbeerbeer]

4 minutes ago, Amh300 said:

As I now appreciate - still mind blown that this looks like the explanation! Leaves on the prop - who would have thought it.

Ha, let’s hope that’s the problem 😃

 

surprised your RCR man didn’t come up with the idea,

....or just swap and charge you for a new gear box 😂

Edited November 26, 2023 by beerbeerbeerbeerbeer

[Amh300]

The RCR guy was diamond - no complaints there.

[Tony Brooks]

14 minutes ago, beerbeerbeerbeerbeer said:

would I be wrong thinking a slipping clutch will just spin and not cause such vibrations on the prop?

 

No, by and large I would expect it to be smooth, even if it was s ill driving, but the difference between static and dynamic friction suggest that as soon as it started slipping t would slip totally on a mechanical box, so the prop would not turn.

[bizzard]

Folk need to be careful stopping the boat too. leaves can destroy prop thrust when in reverse gear as well. Go dead slow when approaching lock gates for example.

[Tony Brooks]

9 minutes ago, bizzard said:

Folk need to be careful stopping the boat too. leaves can destroy prop thrust when in reverse gear as well. Go dead slow when approaching lock gates for example.

 

That reminds me of LH150 boxes that are low on oil, especially in the hands of inexperienced hirers.

[bizzard]

6 minutes ago, Tony Brooks said:

 

That reminds me of LH150 boxes that are low on oil, especially in the hands of inexperienced hirers.

Yes, My ST2 has the LH150 box. Lovely smooth operating box.

[Big Bob W]

Almost certainly leaves on the prop.

Last autumn I was convinced I had a problem mid way through Blisworth Tunnel. Hardly any forward motion despite increasing revs. Then I noticed through the gloom, the water surface was covered in leaves. Burst of reverse and all sorted. The surface was clear entering and leaving the tunnel, but the leaves all accumulated half way through.

[Bee]

If you are absolutely certain that the shaft  is spinning at the same speed as normal and not just slowly rotating then the problem is not the gearbox (or the coupling becoming loose) which only really leaves the prop. The prop being loose on the shaft is extremely unlikely so something like weed or maybe leaves is pretty much all that is left.  It only takes a bit of weed or something to mess up the science of propellers, I spend a lot of time on weedy canals pulling strands and lumps off the prop in summer.

[beerbeerbeerbeerbeer]

11 minutes ago, Bee said:

If you are absolutely certain that the shaft  is spinning at the same speed as normal and not just slowly rotating then the problem is not the gearbox (or the coupling becoming loose) which only really leaves the prop. The prop being loose on the shaft is extremely unlikely so something like weed or maybe leaves is pretty much all that is left.  It only takes a bit of weed or something to mess up the science of propellers, I spend a lot of time on weedy canals pulling strands and lumps off the prop in summer.

 Ain’t it weird how sometimes little things will upset the prop, not just the heavy stuff. 

I follow Dmr’s rule: don’t go down til you definitely have to 😂

 

anyway it don’t matter what time of year it is or where you are the water rarely seems to be in your favour. 
Always a matter of finding the sweet spot with speed to make the most of what you got when in a ditch.