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Ampen Spekersohn

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The boat I am planning is to be mid-engined, probably a JP3 with a PRM 260 gearbox.

The thing is, I want to make proper use of what would normally be the boatman's cabin and use the space as a galley. I need the space to have similar headroom to the rest of the boat, but of course there is the matter of the prop shaft. I have briefly considered and have pretty well rejected, hydraulic drive on the grounds of cost, complexity and increased fuel consumption.

The next option I'm considering is to build an offset prop shaft, i.e. an independent lay shaft, supported by plummer blocks and driven either by triplex chain and sprocket or HDT belt and pulleys at either end. This would allow the shaft to be dropped down to the level of the bearers. This arrangement would also give the opprtunity to finely tune the engine:prop ratio. The prop shaft itself would be quite short and I assume, would need a thrust bearing to resist the force of the prop. The short shaft from the gearbox would be supported by a plummer block.

I'm sure I remember seeing photos or a video of a similar arrangement some time ago, but I think the objective in that instance was to allow the engine to be offset one side.

My questions are: has anyone else done this? What issues were encountered? Any thoughts re chain/sprocket vs. belt/pulley?

Help/input appreciated.

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Yes!! In essence, that's very much what I mean! Thank you for the photo. It looks like they have used pretty standard V-belts, by the look of it.

In terms of torque/power loading, I have dialled the numbers into Fenner's very good design tool. They do a range of toothed belts called HDT, which is Heavy Duty Transmission. The result was for various toothed pulleys with a massive 85mm wide belt (!)

The result also came in at over 95% efficiency, which I'd be happy with.

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I have no idea of your swim length/shape but the available base plate is fairly restricted to achieve max amount of headroom, if your boat is or resembles a replica working boat your cabinwill be more or less all swim reducing the available floor space, a friend fitted his motor in the engine "ole more or less in the centre but wit GB coupling aimed at the lleft hand rear corner, a shaft with UJ's, a length of propshaft in 2 plumber blocks & a UJ'ed rear shaft to the tailshaft I don't know what the actual problems he had but I remember him telling me he swapped the UJ's on the shafts for CV' joints, as his job was makng /refurbishing prop shafts he had all the info to work out what was required I got the general impression the gain was not worth the hassle.

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Thank you for your input guys. As I'm at the design stage, I do have control over the shape of the swim, (my preference is for an S-shape) but can indeed gauge whether the gain in useful floor space is worth the hassle. I have seen another boat recently, where they had managed to locate the galley where the BMC would have been, but they had not tackled the prop shaft problem. Consequently, anyone over about 5' 2" would have to stoop. But it was enough to highlight the value of the approach, as - purely in my view, a BMC is a waste of space as I really wouldn't use it. Plus the kettle would be in easy reach of the helm!

The design would allow the galley and engine room to be fitted into the rearmost 18' of the cabin, allowing the remaining 28' to be allocated to saloon, cabin and bathroom, whilst still retaining a 'proper' 10' bow. So I feel it's worth trying to crack this.

I've sent an enquiry to ARS Anglian Diesels to get an idea of costs for a hydraulic solution, so am not eliminating anything at the moment. My gut feeling is that the belt drive option would be quiet and elegant, but have a nagging doubt that I'm over-simplifying it!

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1 hour ago, Tony Brooks said:

I have a feeling that modern piston pumps and motors can get close to 95% efficiency so may be worth a quick look at a hydraulic drive. You could almost put the floor on the baseplate then.

True but even if not the engine can be run in its most efficient zone so OK overall.  Also think of the fun to be had annoying the Mere cats.  On the head out of burrow and  scream of 'Slow Down!!' to be able to reverse, stop and ask 'What say?' with no change in engine pitch. :rolleyes:. More seriously, X Alan W has a good point about swims & bace plate area.  OK for worktop size but cupboard space cramped.

Edd.  Bog.  To slow a response.

Edited by Taslim
OP is on the case.
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Melaleuca achieves the same effect by combination of the dropped output of the Blackstone gearbox and a lowish mounting position for the engine. That puts the shaft at just above (normal) floor level through  the back cabin. There are a couple of steps which raise the floor up to the rear door at the back of the cabin, and under those there are in order a bearing, UJ, section of shaft sloping upwards, second UJ, plummer block and normal prop shaft at normal height. The shafting and UJs look like (and probably are) the sort of think you see under 7.5 ton trucks. This all works well, and has never given any trouble.

 

MP.

 

 

 

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Depicted here is another method. I apologize for the complicated blueprint of the arrangement, and to those who can't read blueprints. When at rest and the cabin is needed for walkin about in, halt the cranked propshaft at bottom dead centre and replace floorboards. To go sailing along, remove the floorboards to allow the crank to whirl around. Skipping can be practiced, just like in the school playground. Just imagine that the propellor and gearbox are the two children whirling the skipping crank round and round for you to skip over. I recommend a slow running engine and 6.1 ratio gearbox at first for skipping. When you've become adept at leaping over the crank at this low speed without cracking your shins. The ratio can then be upped until your on 1.1 ratio  at which speed your leggy pegs will be a twinkling blurr.  :closedeyes:     Important= The crank counter balance weights are very important. :)

 ETA. In the event of engine failure the cranked propshaft can be turned by hand to propel the boat along.

 

002.JPG

Edited by bizzard
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Wow, that's a novel bit of thinking! I'm not entirely clear if you are suggesting this as a serious option though! All of the obvious 'danger' issues aside, if properly counter-balanced, it would probably work.

Curious to know what a BSS examiner would make of it!

I'm convinced there has to be a way of overcoming this problem. Do they make flexible shafts heavy enough to transmit 20-30HP?

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Hang around for a bit and you will realise that Biz is very good at coming up with such bright ideas. Its his sense of humour although just occasionally it makes it harder to take his very good and well founded technical advice seriously.

I don't think the BSS man is mandated to inspect such things so I doubt they would make anything of it as part of their inspection.

If by flexible shafts you mean things that flex all the way along their length such as you may find in a socket set then my experience of them would be that for it to be reliable it would have to be so large as to render it impractical. If you mean a complex line shaft with plumber blocks, thrust block, and sundry universal or constant velocity joints then yes, they can be fabricated.

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2 hours ago, MoominPapa said:

Melaleuca achieves the same effect by combination of the dropped output of the Blackstone gearbox and a lowish mounting position for the engine. That puts the shaft at just above (normal) floor level through  the back cabin. There are a couple of steps which raise the floor up to the rear door at the back of the cabin, and under those there are in order a bearing, UJ, section of shaft sloping upwards, second UJ, plummer block and normal prop shaft at normal height. The shafting and UJs look like (and probably are) the sort of think you see under 7.5 ton trucks. This all works well, and has never given any trouble.

 

MP.

 

 

 

OK, right, I see exactly what you mean. This looks like it would be a very elegant and relatively simple system. I will research the dimensions relating to: the engine bearers to bottom of the sump; the relative height of the gearbox output shaft. This will tell me how low the main prop shaft can go.

You referred to 'couple of steps which raise the floor up to the rear door at the back of the cabin' - these are needed anyway as a matter of course, so no disadvantage there. My only concern is running UJ's constantly at a fairly acute angle. I think I read somewhere that the maximum is about 20 degrees.

I'm wondering if I could take part of your idea - to arrange a low propshaft as you suggest, but then have a single belt drive from the low propshaft to the higher one.

I appreciate your suggestion and will follow up these ideas. The thing is, it's terribly easy to over-think, over-engineer and over-complicate things like this, which inevitably means more things to go wrong.

I assume that the boat you mentioned must have had a thrust bearing on the shaft supporting the propeller, in order to resist the forward/backward force of the prop?

Cheers

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1 hour ago, bizzard said:

Depicted here is another method. I apologize for the complicated blueprint of the arrangement, and to those who can't read blueprints. When at rest and the cabin is needed for walkin about in, halt the cranked propshaft at bottom dead centre and replace floorboards. To go sailing along, remove the floorboards to allow the crank to whirl around. Skipping can be practiced, just like in the school playground. Just imagine that the propellor and gearbox are the two children whirling the skipping crank round and round for you to skip over. I recommend a slow running engine and 6.1 ratio gearbox at first for skipping. When you've become adept at leaping over the crank at this low speed without cracking your shins. The ratio can then be upped until your on 1.1 ratio  at which speed your leggy pegs will be a twinkling blurr.  :closedeyes:     Important= The crank counter balance weights are very important. :)

 ETA. In the event of engine failure the cranked propshaft can be turned by hand to propel the boat along.

 

002.JPG

Amongst many other useful uses that I can think of for the cranked propshaft, this particular one will save you a lot of labour and you'll keep lovely and clean too. An elongated slot will need sawing out in your cabin roof. A flexible rubber seal around the slot and connecting rod will stop the rain gettin in.

003.JPG

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If you are fitting a JP2 in an engine room then I assume its going to be a traditionally styled boat, so why not just fit a back cabin?. Although some feel they are a waste of space the opposite is true, they are a very efficient use of space, in fact an entire family can live in one full time :D. They make lovely places to sleep and it only takes seconds to make up and put away the bed. You could cheat and make the bed just a little wider than is conventional if you wished.

If you go with your lowered floor plan then its going to be quite a big step up onto the back counter (or strictly several steps).

..............Dave 

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21 minutes ago, dmr said:

If you are fitting a JP2 in an engine room then I assume its going to be a traditionally styled boat, so why not just fit a back cabin?. Although some feel they are a waste of space the opposite is true, they are a very efficient use of space, in fact an entire family can live in one full time :D. They make lovely places to sleep and it only takes seconds to make up and put away the bed. You could cheat and make the bed just a little wider than is conventional if you wished.

If you go with your lowered floor plan then its going to be quite a big step up onto the back counter (or strictly several steps).

..............Dave 

My main reason for adopting a reverse layout was to have a lengthwise 4' 6" double bed, permanently made up. I just find that after a hard days boating, it's nice not to also have to build your bed! A permanent bed also gives the opportunity to have a decent quality domestic mattress and to use an ottoman mechanism to raise the base to expose under-bed storage, as per photo. The hull will be Birmingham Square, which will gain that bit of extra width at floor level to allow passage past the bed.

A BMC bed really requires a sectional mattress, not so comfortable IMO and effectively blocks the route through the boat when made up. It also means that the person sleeping nearest the stern would struggle with night-time visits to the loo. That might need another small design feature adding. ;)

However... I take your point. I have previously refitted an extended back cabin in a butty and agree it is possible to make use of every cubic inch. If I adopted the BMC as the main sleeping area, it would free up more 'square space' (as opposed to space constrained by swims, tumblehome, etc) further down the boat.

I will re-visit my drawings and look seriously at changing it all round. At the moment, it's only lines on paper, so easily altered!

0943883f76cafe04a9e97d64ddc373b9.jpg

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Sectional cushions plus a thin memory foam topper make a lovely bed, but then I have always preferred a harder bed.

We do also have a fixed double but that has degenerated into a sort of storage area with a bit of space for the dog to sleep in. I can see the lack of fixed bed is an issue but I just so much prefer the back cabin option. Its actually quite nice for one of us to sleep in whilst the boat goes along though the engine is a bit loud.

As for the night time wee, this is a challenge, there are many solutions but we are told that this is a family forum so best not discussed.

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

Its

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There is another small side benefit to this approach. By having to have a raised floor in the BMC, I would continue this level through the engine room, which I think should provide room to fit the battery bank under the walkway floor. This would help resolve an issue I currently have with weight distribution.

Thanks for the input everyone.

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16 minutes ago, Ampen Spekersohn said:

There is another small side benefit to this approach. By having to have a raised floor in the BMC, I would continue this level through the engine room, which I think should provide room to fit the battery bank under the walkway floor. This would help resolve an issue I currently have with weight distribution.

Thanks for the input everyone.

You might want to leave enough height under the floor to fit Trojans wot are a bit taller than standard batteries, but with typical back cabin floor heights this should be fine.

But, a back cabin floor is quite high and this might be just a bit too high if extended into the engine room. I do rather like to stand in the engine and look out of the side hatch and a lower floor level makes this so much nicer, I think its about been closer to the water and feeling "down inside the boat". Could you have a lowish floor in the engine room walkway and then locate the battery bank behind the engine below a higher floor?

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

 

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More good points... but I really want to install a 120L vertical calorifier in the engine room, which has got to go on the non-walkway side. I was thinking that 160Kg of batteries would balance that out if they were placed on the opposite side. I'm trying to achieve a reasonably even weight distribution down the boat, to avoid the usual ballasting 'issues'! Will have a play around with the layout. I may set the engine a little further forward and build in a hot press at the end of the side bed.

Given that the doorway from the back cabin needs to be central, what is a sensible distance to allow between that bulkhead and the engine, do you think?

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7 hours ago, Ampen Spekersohn said:

OK, right, I see exactly what you mean. This looks like it would be a very elegant and relatively simple system. I will research the dimensions relating to: the engine bearers to bottom of the sump; the relative height of the gearbox output shaft. This will tell me how low the main prop shaft can go.

You referred to 'couple of steps which raise the floor up to the rear door at the back of the cabin' - these are needed anyway as a matter of course, so no disadvantage there. My only concern is running UJ's constantly at a fairly acute angle. I think I read somewhere that the maximum is about 20 degrees.

I'm wondering if I could take part of your idea - to arrange a low propshaft as you suggest, but then have a single belt drive from the low propshaft to the higher one.

I appreciate your suggestion and will follow up these ideas. The thing is, it's terribly easy to over-think, over-engineer and over-complicate things like this, which inevitably means more things to go wrong.

I assume that the boat you mentioned must have had a thrust bearing on the shaft supporting the propeller, in order to resist the forward/backward force of the prop?

Cheers

Do they geometry before assuming that the propshaft will be at an acute angle. Clearly it depends on the dimensions of the steps and the length of the counter, but on Melaleuca, the angle is probably less than 10 degrees. Indeed it was only when I had to remove the bottom step for another reason several years into my ownership that I realised there was a UJ and a bearing under there, and that I was the proud owner of a kinked propshaft.

 

There is indeed a thrust bearing on the shaft with the propeller on it. This picture was posted long ago for a different reason and showns the UJ disconnected, but it give the general idea.

 

Cheers,

 

MP.

 

 

IMG_20150114_132612.jpg

 

 

 

 

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Reading your later post, I see that you're planing exactly what we have - a lengthwise permanent double bed rather than a BMC. The photo below shows the general arrangement. The end of the bed is at the bottom of the frame. The floor to the left of the bed and at the end of the bed on the left is at the same height as the floor elsewhere in the boat. The small step in front the drawers on the right is precisely high enough to allow the propshaft to sit underneath,  running parallel to the floor. The next step (with rectangular mat) has the bearing and UJ underneath and is high enough to allow the shaft to rise to the level of  the final propshaft. Under the bed the shaft runs in box about 10cm wide by 10cm high.P1040512.JPG.13dff16ea1f4d07ad1e539c0b34cdd4e.JPG

 

 

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4 hours ago, Ampen Spekersohn said:

More good points... but I really want to install a 120L vertical calorifier in the engine room, which has got to go on the non-walkway side. I was thinking that 160Kg of batteries would balance that out if they were placed on the opposite side. I'm trying to achieve a reasonably even weight distribution down the boat, to avoid the usual ballasting 'issues'! Will have a play around with the layout. I may set the engine a little further forward and build in a hot press at the end of the side bed.

Given that the doorway from the back cabin needs to be central, what is a sensible distance to allow between that bulkhead and the engine, do you think?

Hot Press ???? Have only ever heard that term from an Irish lady, usually called airing cupboards. 

We have a similar arrangement but the calorifier is on the walkway side on the rear wall of the engine room and there is still room to easily step between the engine (or gearbox) and calorifier to the central door to the back cabin. We have the JD3 which is a smaller engine, and its offset slightly away from the walkway side (another option to consider) but then we also have a smallish engine room. JP3 is a big lump but if you can add a few extra inches to the engine room it might all work. Backcabin floor height extends into the engine room right up to the gearbox, then steps down to a lower floor level.

...............Dave 

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