Engine position - distance from rear of swim?

[Pen n Ink]

Just sitting here sifting through the murk that is my design process a question occurred to me.

 

In a trad design with engine room and given a completely free hand (blank sheet) is there an optimum distance from the rear of the swim to the engine (or gearbox) position? I guess I'm thinking about prop shaft lengths, bearings etc.

 

I also know that some people have gone for offset shafts to increase boatmans headroom. Is this something which people would generally consider a good idea if the option was there?

 

The engine in question will be a Gardner 3LW with PRM500. Not that that makes much difference I guess!

 

ED: Because I'm trying to write numbers before I'm awake enough to be licenced to do so.

Edited December 6, 2014 by Pen n Ink

[Tony Brooks]

As long as you provide sufficient support for the shaft and if offset provide a thrust bearing you could put the engine right at the front. Before hydraulic drives many Broads boats did and so do the Thames slipper sterns. You may have to use several plumber blocks to support the shaft depending upon length.

 

If you are going to offset the engine then make sure that you still have fair access to anything on the side nearest the hull for maintenance. Just watch that you do not exceed the maximum drive angle for any flexible joints you use (CV/Universal).

[dmr]

wouldn't a 260 be the better box for a 3LW?

 

..........Dave

[twbm]

I used to steer a 48ft ex-Amsterdam water bus with the engine (a Thorneycroft as it happens, see Irish thread) under the front deck.

[The Pipe]

dmr, on 06 Dec 2014 - 12:30 PM, said:

wouldn't a 260 be the better box for a 3LW?

 

..........Dave

No a PRM500 as per normal the low speed torque is what you need to be looking at. Gardner’s fitted Twin Disk boxes on LW's when they stopped making their own and Twin Disk changed the hydraulic pump because of the torque issue.

 

[Pen n Ink]

wouldn't a 260 be the better box for a 3LW?

 

..........Dave

 

Err... yes. But because I'm a complete numpty I was trying to type 500 anyway!

 

The 500 has been specified because of the ratio available which has been suggested by Crowthers.

[dmr]

No a PRM500 as per normal the low speed torque is what you need to be looking at. Gardner’s fitted Twin Disk boxes on LW's when they stopped making their own and Twin Disk changed the hydraulic pump because of the torque issue.

 

I am not an expert on this, but surely the full torque will only happen at top speed as at lower speeds the prop just won't apply much torque.

Anyway I have just looked at the 260 specs (we only have a Betas JD3 but you have me worried), got an estimate of the 3LW power and it is indeed marginal. I reckon the 3LW is ok at the 260 light commercial rating but would not meet the heavy commercial rating. So I reckon you are right, I would go for the bigger box. I like the way PRM specify torque as power per per 100rpm!!!.

 

........Dave

[dmr]

Just sitting here sifting through the murk that is my design process a question occurred to me.

 

In a trad design with engine room and given a completely free hand (blank sheet) is there an optimum distance from the rear of the swim to the engine (or gearbox) position? I guess I'm thinking about prop shaft lengths, bearings etc.

 

I also know that some people have gone for offset shafts to increase boatmans headroom. Is this something which people would generally consider a good idea if the option was there?

 

The engine in question will be a Gardner 3LW with PRM500. Not that that makes much difference I guess!

 

ED: Because I'm trying to write numbers before I'm awake enough to be licenced to do so.

 

Not sure what you mean by offset shafts. The boat dictates the position of the prop and hence height of the shaft at the prop end and hence the height of the back cabin floor. I would not fancy a toothed belt drive etc to lower the prop shaft, not with such a high torque engine. The engine is usually offset sideways a bit to give a better gangway to get past the engine to the back cabin. An offset engine with universal joints in the propshaft works fine. Do note that if using conventional UJ's (Hookes joints) that a bigger offset can be safer than a very small offset!

 

............Dave

[hughc]

On Wyrd from the face of the coupling by the thrust block to the face of the gearbox coupling is 93.25" There is an aquadrive shaft and thrust block. the shaft is slightly oversize and is foam filled to reduce 'singing'. The BSD3 produces 42 BHP at 2200 and this drives a PRM 260. when we were constantly towing I did have to rebuild this box a couple of times but as a single motor it seems to cope well. The engine is mounted as low as possible in the engine room and the shaft runs uphill at IIRC 7.5 degrees which , apart from a step at the rear of the cabin, gives reasonable- 5' 11"- headroom. Regards, HughC.

[Kieron G]

I'm sure someone will post with greater detail but If you look at the power / torque curve for a diesel engine torque peaks at much lower rpm than max power. Although whether you can load the engine at max fuel to pull the rpm down on a boat like you can on a truck going up hill i'm not sure.

[Loafer]

On Wyrd from the face of the coupling by the thrust block to the face of the gearbox coupling is 93.25" There is an aquadrive shaft and thrust block. the shaft is slightly oversize and is foam filled to reduce 'singing'. The BSD3 produces 42 BHP at 2200 and this drives a PRM 260. when we were constantly towing I did have to rebuild this box a couple of times but as a single motor it seems to cope well. The engine is mounted as low as possible in the engine room and the shaft runs uphill at IIRC 7.5 degrees which , apart from a step at the rear of the cabin, gives reasonable- 5' 11"- headroom. Regards, HughC.

 

What's a BSD3? You didn't mean BD3 did you?

[magnetman]

 

I am not an expert on this, but surely the full torque will only happen at top speed as at lower speeds the prop just won't apply much torque.

Anyway I have just looked at the 260 specs (we only have a Betas JD3 but you have me worried), got an estimate of the 3LW power and it is indeed marginal. I reckon the 3LW is ok at the 260 light commercial rating but would not meet the heavy commercial rating. So I reckon you are right, I would go for the bigger box. I like the way PRM specify torque as power per per 100rpm!!!.

 

........Dave

I've got a prm 260 on my Beta 90 (90hp@2,600rpm) . Bloke at Beta marine told me that they had done some testing with a 260 and it would handle over 100hp (@2,600rpm) without issues so I expect a 3LW while being a fairly large unit with high torque would be fine with the 260. Don't see that the 500 would be any harm though.

 

Not sure what you mean by offset shafts. The boat dictates the position of the prop and hence height of the shaft at the prop end and hence the height of the back cabin floor. I would not fancy a toothed belt drive etc to lower the prop shaft, not with such a high torque engine. The engine is usually offset sideways a bit to give a better gangway to get past the engine to the back cabin. An offset engine with universal joints in the propshaft works fine. Do note that if using conventional UJ's (Hookes joints) that a bigger offset can be safer than a very small offset!

 

............Dave

 

 

I know someone who's got a JP3 in his engine room with an offset cardan shaft drive, simply so he can get past the engine easier as it is the entrance to the boat. seems to work ok. looks a bit odd though

 

What's a BSD3? You didn't mean BD3 did you?

BSD3 is a different product, same base engine I think but its not a Beta Marine engine

[magnetman]

BSD3 topic previously on CWF http://www.canalworld.net/forums/index.php?showtopic=17469

Its worth a read for Beta BD3 owners as well that thread

[Split Pin]

3Lw torque 170 Lbs ft

 

PRM 260 is rated at 160 Lbs ft so nearly but not quite big enough

 

Pen-n-Ink out of interest what size prop and reduction ratio have Crowthers recommended?

 

Bright drawn steel bar is usually sold in 3mt lengths if you are going with solid shafting upto 6mt lengths can be purchased if needed.

 

Steve

[The Pipe]

3Lw torque 170 Lbs ft

 

PRM 260 is rated at 160 Lbs ft so nearly but not quite big enough

 

Pen-n-Ink out of interest what size prop and reduction ratio have Crowthers recommended?

 

Bright drawn steel bar is usually sold in 3mt lengths if you are going with solid shafting upto 6mt lengths can be purchased if needed.

 

Steve

Beat me too it.

As for the Beta engine 100 BHP @ 2600 rpm is out of the PRM figures for the 260 gearbox. PRM state 3.36 BHP per 100 rpm ie. 3.36 x 26 = 87.36 BHP no idea about the Lbf load allowable.

[magnetman]

edoted

Edited December 6, 2014 by magnetman

[by'eck]

Although even the heavy commercial rating of a PRM260 is 3.08 bhp per 100 rpm. If your 3LW could reach it the 53.5 bhp @ 1700 rpm is only just on the limit.

[Pen n Ink]

what size prop and reduction ratio have Crowthers recommended?

 

 

 

23 x 17 - 1.5:1 - bearing in mind that this will be paired with a butty full time.

 

edoted

 

Quite like edoted!

[Split Pin]

 

23 x 17 - 1.5:1 - bearing in mind that this will be paired with a butty full time.

 

 

Quite like edoted!

I run a 23" X 23" 65% B.A.R (similar to Crowthers compensated design) with a 2:1 reduction which does limit the max revs to about 1100, (by then either the cut is empty behind me or I am up on the plane).

 

Normal running speed on the cut is 500rpm at which the engine runs sweetly

 

Having bashed away at the calculator for a few minutes the figures come out about the same for either configuration so will probably work well

 

I do know of two other boats with a 1.5:1 reduction one is fitted with a 24"X 24" but consider it to be well over proped

 

The other has a 24" X 18" which works well, It does however have a very fine shape more like a Thames slipper

 

The torque figures that I mentioned in an earlier post may well be incorrect

 

I was told many years ago when the PRM range of gearboxes were rebranded that the designation indicated their torque rating so a 160 was 160 Lbs ft which became the 250 Newton Meters which I never actually checked.

 

I have an old R&D drive plate catalouge which lists the 160 as 160lbs ft, looking on the website it has now been uprated to 214 lbs ft

 

The 250 is rated as 290 newton meters so on torque handling the 250 is now suitable.

 

I ran a PRM 160 on my 3LW for about 5 years having previously having had it fitted to different engine for about 4 years, Before it was fitted to another smaller engine it was stripped and checked for wear

The forward clutch pack actually measured thicker than the reverse pack so hadn't worn much, I did suffer a failure of the clutch pack bolts which take the full drive load from the clutch plates,hence my desire to replace it with a larger version

 

Steve

Edited December 6, 2014 by Split Pin

[Bee]

I think this means that you can put the engine wherever you like and there will be some way to connect it all up. I used a Python drive on the inboard end of the 'tailshaft', useful as it has a thrust bearing and will take a fair degree of angular offset (can't remember the amount) then a solid bright drawn bar, about 3m, to a fenner coupling just behind the gearbox, that takes care of engine wobble, it is important to put in enough plummer blocks, I saw an installation once with none (!) over about 10' length and the thing revolved in a blur of whip and oscillation, terrifying.

[Bod]

To state the obvious.

Where ever the engine is, it has to be serviced.

Also other items, placed around the engine must be accessible. Not only for service, but replacement.

 

Bod

[Peter-Bullfinch]

I'm six foot and the normal back cabin arrangement works fine. Yes, I stoop going through the back cabin door and when we first got the boat I had a couple of bumped heads. Having the engine centrally means good access to the engine room side hatches whichever side we tie up on. Getting folding bikes out of the engine room is easier too. I have seen a few engines offset and the negatives seem to outweigh the advantages for me.

[magnetman]

One advantage of an offset engine is that you can have a 'vintage power unit' (a JP a Gardner Russell Newbery etc) in a stern engine room while still being able to have a full height walkway around the side of the engine with plenty of space. this is good if you want a vintage engine, you want it at the back of the boat, you are tall and your main entrance to the boat;s interior is via the back doors.

 

'vintage power units' are noisy if in stern engine rooms. I had a Russell Newbery DM2 about 6ft forward of the steerer in an open engine room on a narrow boat and I went all over the canal system with it but it was very LOUD. I would look at the other people with their boatmans cabins and 'quiet' engines and think about how I would change it but never did.

[by'eck]

Check out this thread (although it drifts off topic) on a Pelapone industrial twin that was fitted to trad boat with engine room and back cabin. The drive was taken from front of engine via toothed belt down to propshaft running down low near base plate. It was stepped up at the stern to PRM gearbox in line with propshaft via second toothed belt. This allowed much lower floor in back cabin.

 

Pelapone BTW was an acronym for Prudence's Engines Lighting And Power Oil and Nautical Engines quite a mouthful and not as quoted in the linked thread.

Edited December 8, 2014 by by'eck

[Neil2]

As long as you provide sufficient support for the shaft and if offset provide a thrust bearing you could put the engine right at the front. Before hydraulic drives many Broads boats did and so do the Thames slipper sterns. You may have to use several plumber blocks to support the shaft depending upon length.

 

If you are going to offset the engine then make sure that you still have fair access to anything on the side nearest the hull for maintenance. Just watch that you do not exceed the maximum drive angle for any flexible joints you use (CV/Universal).

Years ago we hired an old Broads cruiser that had such an arrangement (BMC 1.5) and it was an absolute delight, incredibly quiet and the best handling single prop boat I've ever steered. I've often fantasised about a similar drive system on a narrowboat but one hurdle would be the loss of ballast room in the cabin bilge - or would it? Starting from scratch I suppose with a thick enough baseplate you could overcome this. Also, thinking about it, the engine access in that cruiser wasn't great...