Rubbish to stop or go into Reverse

[nicknorman]

13 minutes ago, robtheplod said:

is it an unbolt/bolt on change?  i'm fairly handy but need to know my limitations!

Well to be honest I’ve never done it, so slightly talking out of my backside but yes I think it is. A few bolts hold the gearbox to the engine bell housing, a few bolts hold prop shaft to the gearbox. Normally having disconnected the latter this allows the prop shaft to be slid back giving room for the gearbox to be extracted. Then of course there is the control cable and the hydraulic connections to the oil cooler. But that is about it I think. Someone on here who has done the job will hopefully be along shortly to give a more authoritative view.

 

Then your 1700rpm would become 1200rpm for the same prop speed. Bliss!

[David Mack]

1 hour ago, Tony Brooks said:

I suppose the alternative to another gearbox is a different prop. With a limit on diameter, it would mean more pitch or even a four bladed prop, but that probably would be less efficient. It should, however, give a better reverse and a slower cruising RPM.

A gearbox exchange can be done with the boat afloat. Not so a prop change.

[PeterF]

A bit late to this, but as a comparator for a properly set up system with a 3:1 gearbox that works well my boat details are

 

58ft narrowboat with 28" draft

Beta 43 with a PRM 150 3:1 gearbox and a 20" x 15" propeller (Michigan Marine)

Typical cruising speed 3 to 3.5mph at 1300-1400rpm

Stops quickly, the boat builder described the set up as "gripping the water well".

 

Confirms what has been said above that an 18" prop on a 3:1 gearbox is too small.

Edited October 25, 2023 by PeterF

[Tracy D'arth]

Bite on the water is better with a bigger diameter prop rather than bigger pitch. I doubt that the hull will take a larger diameter prop, you need around a 2" clearance top and bottom to cope with rubbish going through.

15" is a big pitch on a small prop.

Is the hull a Reeves? I notice that the swim does not come to a sharp edge, typical Reeves. The flat causes bad flow in reverse.

[Alan de Enfield]

54 minutes ago, Tracy D'arth said:

Is the hull a Reeves? I notice that the swim does not come to a sharp edge, typical Reeves. The flat causes bad flow in reverse.

 

My Reeves reversed really well with a lot of water moving down the side of the boat. It was a 3:1 gearbox but I don't know the size of type of prop - it may even have even been 'a square' prop.

 

 

 

[Slim]

I had my boat for 28 years and I always had the same issue as the OP. Wouldn't pull the skin off a rice pudding in reverse. Going forwards absolutely no issues. Going up a lock could be 'quite interesting' if the boat surged forwards. Bog standard Eastward Engineering budget hull with an average length swim. Any number of 'experts' looked at the hull over the years and no one could see anything out of the ordinary. Engine was a BMC 1.8 with a 2.1:1 PRM 150 box. Prop was a 17"x 12". At one point the prop was looked at by Crowthers who agreed that it and the drive train were perfectly suitable. In the end I learned to live with it.      

[blackrose]

14 hours ago, Tony Brooks said:

 

It sounds more like under-propping to me.

 

 

Sorry, yes I meant under-propping but hadn't had much sleep. My post has been edited.

 

I don't think it's under-propping either if the boat moves forwards from a standing position without a lot of engine revs.

[Bee]

The prop has to work hard to stop the boat, if you are doing, say, 3mph the prop is having to go against a flow of at least 3 mph and possibly more as the water accelerates along the swim. One of my old boats used to go and stop quite well but if I engaged reverse from approx 3mph it would smoke black until the boat lost its speed and then it would go backwards. There is a lot of variables and stuff in props / boats / hull shape. When Bee enters a lock she is already in reverse on tickover as the boat goes through the gates ( unless we have to tie on the RHS and then its all ropes flying everywhere and panic as she will not lie alongside a right hand wall - prop walk)

[Tracy D'arth]

There are so many variables about the reasons that a boat surges forwards when ascending some locks.

The length and draught of the hull, the position of the inlet culverts and the rate of filling and the precise position of the boat in the lock to name a few.

As to reversing, the shape of the rear swims is a factor.

Which is why some are different from the herd.

Also the steerer is a major factor and what a boat is expected to do in his/her experience.

[robtheplod]

40 minutes ago, Tracy D'arth said:

There are so many variables about the reasons that a boat surges forwards when ascending some locks.

The length and draught of the hull, the position of the inlet culverts and the rate of filling and the precise position of the boat in the lock to name a few.

As to reversing, the shape of the rear swims is a factor.

Which is why some are different from the herd.

Also the steerer is a major factor and what a boat is expected to do in his/her experience.

Never a truer word said.... As well as the issues im thinking this must be at least partly me - i know how she handles so need to act accordingly...  it would be nice to see if it can be improved but its such minefield and things might not change much for lots of expense....

[cuthound]

When I got my boat (Alexander 60 foot hull, Beta 43, PRM150 2:1) it went forwards well but was terrible at stopping. It was fitted with a 17" x 11" prop, which was 1" smaller than Beta recommend in both diameter and pitch.

 

When I had it docked for blackng, I changed the prop to the recommended 18" x 12". This reduced the cruising revs at 3mph 1500 rpm to 1250 rpm and considerably improved stopping power, although now I would say it is an average stopper rather than a poor stopper.

[robtheplod]

Keep thinking about this....... are there any advantages to keeping to 3:1?  i know the charging circuit will be happier but any other things to consider?

[Tony Brooks]

7 minutes ago, robtheplod said:

Keep thinking about this....... are there any advantages to keeping to 3:1?  i know the charging circuit will be happier but any other things to consider?

 

If he went to a 2:1 the boat might then be over propped, so it is perfectly possible a prop change would still be needed.

[GUMPY]

I changed Parglena from 3:1 to 2:1 for the same reason. However it was then way over propped so changed the prop as well.

Sourced the gear box from:

https://www.findafishingboat.com/list/marine-engines-gearboxes-for-sale

 

In the end it was from a chap that did overhauls and I sent him my old box and he replaced it with an overhauled 2:1 for the cost of the overhaul and carriage.

 

Prop was from Crowther. 

 

[robtheplod]

mmmm this could be a domino effect....... reluctant to go forward on this unless i know the problem for sure and the solution but it all seems a bit of a dark art....... excessive ponderings.....

[Alan de Enfield]

2 minutes ago, robtheplod said:

mmmm this could be a domino effect....... reluctant to go forward on this unless i know the problem for sure and the solution but it all seems a bit of a dark art....... excessive ponderings.....

 

The bottom line is can you live with it as it is, or, is it frustratingly annoying or dangerous ?

 

Any change will 'cost' and the knock on changes may mean its a £1000 job simply to make it reverse better.

[David Mack]

Have you tried vicprop using your boat and engine data with 2:1 and 3:1 gearbox options to see what it comes up with?

[magnetman]

On 25/10/2023 at 08:52, robtheplod said:

. I dont have much info other than the prop is 18" - 

 

 

One would need the pitch to use a prop calculator. 

[Tony Brooks]

Just now, magnetman said:

 

One would need the pitch to use a prop calculator. 

 

No, just the boat details (water line length, hull type, engine power, reduction ratio, etc.) and the maximum diameter, the calculator will then give diameter and pitch or a message about "not ideal" because of limited diameter.

[robtheplod]

Thanks all.... its not dangerous, just annoying, but liveable with..... i will do as suggested and look at vicprop though!  thanks!

[magnetman]

No I meant you wouldn't know if the existing prop was wrong unless you knew the pitch. 

 

It might not be the prop. 

 

If you assume the prop needs changing then yes the calculator gives you the data. 

 

 

 

[MtB]

56 minutes ago, robtheplod said:

Keep thinking about this....... are there any advantages to keeping to 3:1?  i know the charging circuit will be happier but any other things to consider?

 

Well yes, the main benefit of keeping 3:1 is it suits a bigger diameter prop which in turn, leads (in my experience) to (far) better brakes. If you can squeeze a 20" or better, a 21" blade in then performance in astern will be transformed. 

 

 

[Tonka]

1 hour ago, robtheplod said:

Keep thinking about this....... are there any advantages to keeping to 3:1?  i know the charging circuit will be happier but any other things to consider?

Your bank balance is healthier too

[magnetman]

 

Did it perhaps have a larger prop on there originally ? The dropped skeg seems odd. 

I wonder if someone repropped it sith smaller diameter so the travelpower worked better at cruising speed. 

 

Having said that if the prop was bigger it would be a bit close to the bottom of the weedhatch/uxter plate. 

 

 

 

I fancy it may have had a 20 inch prop on there originally. 

 

This would add up with the 3:1 box. 

 

 

[robtheplod]

it might be best if i put this on hold until she's next out the water then i can be sure of what she's got and see if a bigger prop might be viable.... what gaps are needed between the prop and sceg/weedhatch to be safe?