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Philip

Propeller size for Beta 20 engine

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Can I please ask about the best dimension and type of propeller for my boat? I have a Beta 20 engine (110 saildrive) which can pull a maximum of 3600 revs. Currently I have a two blade prop which I think is 11x8 (not certain about this), but I'm not getting good fuel economy from it and the performance is fairly poor (as an example even at 2000 rpm, the boat is only just touching 3mph and if I turn the wick up to about 2500 rpm, the engine water temperature creeps up towards boiling point, 80 degrees is the temperature it should be at).

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More details of the boat please.  Type, weight, length, draft at least.

More details of the gearbox too- what is its reduction ratio?

N

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If it really is a 'saildrive' then they are not ideal for use on shallow water - hanging out 18"-24"  UNDER the boat means that it is probably spending its life dragging thu' the mud and is 'screwing the boat' along the bottom rather that screwing itself thru' the water.

They are designed as 2 bladed units for sail boats to use to 'get them into a marina' rather than as a main form of propulsion.

I posted this pic t'other day showing how far they hang below the bottom of the boat

 

What sort of boat do you have, and does the drive  go out thru' the bottom of the boat, or thru' the transom ?

 

 

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We have been through all this for you before and the consensus is that a saildrive is not really suitable for canals and yours does no seem to be installed properly.

 

Lets have some photos showing the drive, any keel, rudder etc. and we may be able to suggest how to do something about the consumption.

 

If the prop really is dragging through the mud then I doubt altering the size will make much difference to the consumption.

 

Normally saildrives tend to be direct cooled and raw water up through the leg but there are a variations. If yours is like this you may be filling the cooling system up with silt, hence the overheating.

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The boat is a Norman 24 fibreglass, draft of 1'11, 6'10 beam. I will try and upload a couple of pictures to give a better idea and find out the reduction ratio, but I can assure you that neither the prop nor the leg are dragging on the mud, since the keel of the boat is lower in the water than both of them! It is a 110s penta saildrive to be precise.

 

IMG_20170331_160050.jpg.b94f8a7543b8807d9429fffebfff6f4d.jpg

IMG_20170331_152700.jpg

Edited by Philip

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I can assure you it is a saildrive, a 110s model, it's not an outdrive. Reduction ratio is 1.66:1. 

 

The saildrive is on the 'bow' side of the engine on this boat, the engine being closest to the stern, which might be why it looks more like a stern drive from underneath.

Edited by Philip

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Those pics were taken March '17, when it still had the Volvo Penta 23hp engine in it. We changed the prop to the current 2 blade one when the Beta was installed in July 2017.

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I think that much of your problem is that the water flow over the leg / prop is actually masked by the stern &  keel.

What a weird set-up I've never seen anything like it.

 

The whole of the prop should ideally be clear of the keel to get water flow over it - having the prop too high (which on your pic looks as if it is) means that you are not getting water flow over the prop and you'll be reving the engine and getting nowhere.

 

This pic (although an OB) shows the position the prop should be in.

 

 

Transom Leg length.jpg

Edited by Alan de Enfield

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I understand what you're saying, but I don't think there's much way round it. These boats were built to house the saildrive and inboard engine arrangement. Beta do their own saildrive, but I believe the leg is the same depth as it is on the old Volvo saildrives. 

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I have been pondering this because it seems something is "wrong".

 

My first thought is that although a diesel engine has one "most efficient" speed the difference in fuel consumption at other speeds tend to be similar except at very high speeds on indirect injected diesels where it tends to increase rather more.

 

On any given boat the power required to drive it through the water at a given speed will be much the same whatever sized prop is fitted, it is just with undersized props the engine will have to rev faster but the power delivered will be the same and it may not reach its top speed, likewise an oversized prop may also prevent the engine reaching its maximum speed.

 

This problem is twofold

 

1. The engine has to rev higher than the OP would like to reach an acceptable speed.

2. When the revs are raised the engine starts to overheat.

 

Number 2 suggest the engine is delivering more power but the cooling system can't cope with the extra power so fitting a larger prop is unlikely to cure it and may make it worse.

 

The typical max hull speed calculation (sq rt waterline x 1.3 (don't know the constant for a Norman) suggest that at around 7m the maximum hull speed will be about 3.5 knots which is not far off what the OP is getting. Trying to drive the hull any faster unless you have the power to make it plane (if a Normal can) will just make the stern dig down more and try to push the bow out of the water - that will take ever increasing amounts of power and the prop will make little or no difference.

 

The hull shape in my (and others') view will tend to starve the prop of water so making the stern sit down even more, especially on narrow canals.

 

My conclusion, along with Alan, is that the prop is too high.

 

The OP says the engine is fitted onto the INNER hull moulding so unless the hull and inner moulding have no gap between them at that point the  height of any gap raise the prop by the same amount. As the OP said in another post the space between the hull and inner lining filled with water I suspect there is a gap.

 

So, it looks to me as if Norman did not design the boat properly (or whoever fitted the sail drive). I also suspect that for some reason the cooling system is somewhat compromised but sorting that out is unlikely to do much apart from reducing the overheating.

  • Greenie 1

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25 minutes ago, Tony Brooks said:

I have been pondering this because it seems something is "wrong".

Saved me typing it all out - pretty much in full agreement.

 

I'd always viewed Norman as 'knowing what they were doing', I just cannot imagine that this is a Norman original installation.

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I know that when new or newish the boat was part of a fleet of Broads based hire cruisers. All of the fleet were Norman 24s with the Volvo inboard and saildrive arrangement (from an old online brochure I found), similar to the short Owl class narrowboats that were built/based at Barbridge Marina. It's possible that originally it was a conventional Norman moulding and the stern layout was later changed by the hire company to accommodate the inboard.

I know that when new or newish the boat was part of a fleet of Broads based hire cruisers. All of the fleet were Norman 24s with the Volvo inboard and saildrive arrangement (from an old online brochure I found), similar to the Owl class short narrowboats that were based at Barbridge Marina. It's possible that originally it was a conventional Norman moulding and the stern layout was later changed by the hire company to accommodate the inboard.

Edited by Philip

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To update, we've gone for a 2 blade 14x8 Volvo propeller, which is an inch higher in pitch than the current 14x7 one. Any higher than 8" and it'll probably overload the engine a bit (the 14x10 3 blade one which we took off was far too over-propped for the 20hp engine). Even allowing for the unconventional back-end arrangement, an extra inch in pitch should still improve performance and efficiency.

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To update, the above-mentioned 14x8 propeller was fitted a couple of months ago and the result is it just isn't the same boat! I'm getting the performance I should be getting from the engine with 2000rpm now allowing approx 4mph running and thus it's not guzzling the fuel anything like it was doing. Seems to be handling better too, particularly in reverse. Surprising how an inch in the propeller pitch can make such a difference!

 

The engine also runs a hot water calorifier and an extra car heater-style blown-air heater, using the engine cooling water - rather pleased with this. No need for the 'must-have' solid-fuel stove! Thank you for the help from this forum btw when I asked about the installation of this system a year or so ago.

Edited by Philip

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3 minutes ago, Philip said:

To update, the above-mentioned 14x8 propeller was fitted a couple of months ago and the result is it just isn't the same boat! I'm getting the performance I should be getting from the engine with 2000rpm now allowing approx 4mph running and thus it's not guzzling the fuel anything like it was doing. Seems to be handling better too, particularly in reverse. Surprising how an inch in the propeller pitch can make such a difference!

 

The engine also runs a hot water calorifier and an extra car heater-style blown-air heater, using the engine cooling water - rather pleased with this. No need for the 'must-have' solid-fuel stove!

Glad its worked out for you but I'd still suggest with that type of drive you avoid the shallow canals.

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20 minutes ago, Alan de Enfield said:

Glad its worked out for you but I'd still suggest with that type of drive you avoid the shallow canals.

For most of its life it was based on the Norfolk Broads, so I guess this arrangement was done so with this in mind. I guess it isn't the best arrangement for shallow water use but, the whole structure of the stern would have to be changed to incorporate something different, so probably more cost-effective to buy a new boat! I don't often get things round the prop and the drive and prop are both in good nick. If I'm honest I quite like the slightly quirky (if that's the right word?) side to it!

Edited by Philip

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