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Prop size


Laurencr

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8E188C5C-12B5-4E36-A497-0FB2DC799F57.jpeg.5aafcca4a92e4b5fbf33ac1ce476d4a8.jpegWhat is the correct calculation to work out the size of the prop required my boat is running at 24 horse buhk engine through hydraulic pump with a 16 inch propeller I feel it is under powered the engine has to work very fast to get any momentum 28 foot Icebreaker hull 

Edited by Laurencr
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Bukh engines are high quality units but they are high revving  - not like a vintage engine at all, even if they weigh as much.!  

 

You shouldn't worry about high revs though, unlike most diesel engines they are rated at peak power because they can work at maximum revs indefinitely.  Also at low revs they run too cool which is why you typically see them emitting smoke when fitted in a narrowboat - unburnt fuel.

 

You have to bear in mind that their natural environment is fishing boats, so they are ideally meant to be mated with a relatively small prop. 

 

So in some respects they are not ideal for a narrowboat and I find it odd that an icebreaker should have one - a boat like that surely needs an engine that can swing a big prop.

 

I had a boat with a DV24 which had a 17 inch screw and I always though it was overpropped.  IIRC you could get two different gearboxes for that engine with different reduction ratios, the one I had was the lower ratio so it was a case of either change the gearbox or the prop.  I was going to change the prop and when I gave all the details to Crowthers they concluded that 16 dia inch was about right, can't remember what the pitch was.  

 

I wonder if you have the higher ratio box which is why it feels underpowered.  Why not contact Crowthers and see what they think?

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36 minutes ago, Neil2 said:

I wonder if you have the higher ratio box which is why it feels underpowered.  Why not contact Crowthers and see what they think?

 

How does the hydraulic drive affect relative speed between engine and prop? I'm not familiar with these at all.

 

8 hours ago, Laurencr said:

I feel it is under powered the engine has to work very fast to get any momentum 28 foot Icebreaker hull 

 

Well no wonder. Looking at the photo, I think I see the problem. You need to add more ballast to the stern so the prop is actually in the water. 😀

If that isn't an option, then perhaps you need a prop more like this:

Air_boat.jpg.8b48cb2fc66b44dfd182033d4c77b2b5.jpg

By USGS South Florida Information Access, Public Domain, https://commons.wikimedia.org/w/index.php?curid=1140200

Edited by Jen-in-Wellies
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17 minutes ago, Jen-in-Wellies said:

 

How does the hydraulic drive affect relative speed between engine and prop?

 

 

 

 

Doh I completely missed that - too early in the morning.

 

It's generally accepted that hydraulic drive involves greater power losses than shaft drive isn't it.  I've seen/heard estimates up to 30% but I guess it depends on the installation.

 

I did have a boat with hydraulic drive once and it certainly felt like the engine had to work noticeably harder.

 

But again it depends how the reduction ratio is set up, between the pump and the drive motor so the same logic applies - find out what reduction ratio is and present the details to a prop specialist who will take into account the likely power losses involved in hydraulic drive.

Edited by Neil2
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Sounds like an undersized prop problem to me, but in this case I suspect the prop may be correct for the hull, but the hydraulic reduction is wrong. Get the prop size of your prop and then play with the pro calculator above until you find the recommended  reduction ratio, then check that against the pump to motor displacement ratio. I suppose that if money is no object, you could fit a variable displacement hydraulic pump.

 

The other think it might be is PRV related. An oversized prop may well cause the PRV to lift while accelerating and at high speed/power, and that would give high revs and little thrust. The PRV may be maladjusted or the spring has gone weak, but to check that you need a hydraulic pressure gauge, fitting point and site of the PRV dump output.

Edited by Tony Brooks
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1 hour ago, David Mack said:

I'm not too sure how well the online prop calculators model the situation of a long square box-shaped narrowboat in a confined channel, compared with a boat-shaped boat in open water, but you could have a play around with this:

https://www.vicprop.com/free-propeller-sizing-calculators

The Vicprop calculator comes up with sizes which are pretty close to those recommended by engine suppliers (like Beta Marine) and suggested by people on this forum (and Crowthers in the past...), assuming you want to match the prop to the engine for maximum power.

 

Some people like overpropping above this size to cruise at lower revs and get a better "grip" on the water (bigger prop), but the consequence is that the engine won't reach full power.

 

If you want to fit a bigger prop (and have space to do it) the right way to do it is to use a bigger ratio gearbox (e.g. 3:1 instead of 2:1) to drop the propeller rpm. With a hydraulic drive you can do this by swapping either the pump or the motor for one with different displacement, at which point you'll probably be horrified by how much they cost... 😞

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8 minutes ago, IanD said:

The Vicprop calculator comes up with sizes which are pretty close to those recommended by engine suppliers (like Beta Marine) and suggested by people on this forum (and Crowthers in the past...), assuming you want to match the prop to the engine for maximum power.

 

Some people like overpropping above this size to cruise at lower revs and get a better "grip" on the water (bigger prop), but the consequence is that the engine won't reach full power.

 

If you want to fit a bigger prop (and have space to do it) the right way to do it is to use a bigger ratio gearbox (e.g. 3:1 instead of 2:1) to drop the propeller rpm. With a hydraulic drive you can do this by swapping either the pump or the motor for one with different displacement, at which point you'll probably be horrified by how much they cost... 😞

 

I remember having this conversation with Nick at TW Marine, the Bukh specialists, he said he frequently has to caution people who want a bigger prop.  I reckon most boats are overpropped but it's definitely a bad thing with Bukhs as they are designed to work hard at high revs.  I didn't really get this until we did the Severn against the current and the engine felt right at home.  It was also about the only time that engine didn't give off smoke.

 

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32 minutes ago, Neil2 said:

 

I remember having this conversation with Nick at TW Marine, the Bukh specialists, he said he frequently has to caution people who want a bigger prop.  I reckon most boats are overpropped but it's definitely a bad thing with Bukhs as they are designed to work hard at high revs.  I didn't really get this until we did the Severn against the current and the engine felt right at home.  It was also about the only time that engine didn't give off smoke.

 

 

I can confirm that. Much happier being thrashed up the Trent or down the Thames in near flood.

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1 hour ago, Tracy D'arth said:

Sadly, Crowthers are no longer trading.  Greatly missed.

Very very true. I had two props from them on different boats and plenty of advice. The quality of the props for the additional price was a bargain.

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