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Prop size for 2DM?


billh

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Now then

I would hazard a guess that a 27x25 would be fine on an empty boat, and be able to cruise at tic over, possibly a bit fast into locks and prop walk for England though.

Big props are great for getting a grip on the water (including stopping), but when loaded/towing/dragging through the mud, the engine needs to get enough revs on to create the power otherwise it will bog down smoke for England and knock. I wouldn't buy a car that was so over geared it couldn't hold top into a head wind, but at least in a car you can then change down.  

A  24x17 will have the same theoretical speed as a 27x17 but less grip on the water (not good for stopping) which could cause you to go slower, or achieve higher revs and possibly go faster , however Battersea will pull max revs in decent water. Using a turn to much throttle in a restricted channel sucks you onto the bottom and looses about 1mph per turn as measured with a navigation app

If you permanently run empty, don't tow or go onto narrow/shallow canals then a 27x25 might be fine.

 

On 16-8-18 Battersea should be leaving Great Haywood going to Gayton Junction she is trimmed to approximately a 10 ton load,

unless they have dredged the bit after Fradley  since I last passed, it provides a good demo of why I don't run a bigger pitch.

We aim to leave the war weekend at Stoke Bruerne Sunday 9th Sept in the evening heading back to the Caulden you are welcome to have a go on either trip.

I believe the only practical answer is to try as many working boats with a National or RN and go with whatever you feel is right for you.

Happy hunting

Bob Tidy

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  • 2 weeks later...

Now then

Interesting document,

The prop calculator https://www.vicprop.com/ document earlier in the thread says

"Failing this, you can reduce the diameter and increase the pitch at the expense of your propeller efficiency. The rule of thumb is 1 inch of diameter is equal to 1 1/2 to 2 inches of pitch".

therefore the opposite must apply ie increase the dia and reduce the pitch to achieve the correct loading. This leaves me with approximately the correct prop once you allow for the large alternator we are driving. If we hit a tough patch or have a bit on the prop whilst the Alternator is bulk charging, we get black smoke and a slow increase in revs when opening her up, but it,s fine with a clear prop and a bit of water.

Pity we can't have access to a dock and a selection of props when theses decisions are being made.

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  • 2 weeks later...
On 14/08/2018 at 23:08, Bob Tidy said:

Now then

I would hazard a guess that a 27x25 would be fine on an empty boat, and be able to cruise at tic over, possibly a bit fast into locks and prop walk for England though.

Big props are great for getting a grip on the water (including stopping), but when loaded/towing/dragging through the mud, the engine needs to get enough revs on to create the power otherwise it will bog down smoke for England and knock. I wouldn't buy a car that was so over geared it couldn't hold top into a head wind, but at least in a car you can then change down.  

A  24x17 will have the same theoretical speed as a 27x17 but less grip on the water (not good for stopping) which could cause you to go slower, or achieve higher revs and possibly go faster , however Battersea will pull max revs in decent water. Using a turn to much throttle in a restricted channel sucks you onto the bottom and looses about 1mph per turn as measured with a navigation app

If you permanently run empty, don't tow or go onto narrow/shallow canals then a 27x25 might be fine.

 

On 16-8-18 Battersea should be leaving Great Haywood going to Gayton Junction she is trimmed to approximately a 10 ton load,

unless they have dredged the bit after Fradley  since I last passed, it provides a good demo of why I don't run a bigger pitch.

We aim to leave the war weekend at Stoke Bruerne Sunday 9th Sept in the evening heading back to the Caulden you are welcome to have a go on either trip.

I believe the only practical answer is to try as many working boats with a National or RN and go with whatever you feel is right for you.

Happy hunting

Bob Tidy

I'll agree with you on deep water towy flew down the new and old main line, could really wind her up, but as soon as hit narrows or a shallow I just stopped.

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Thanks to  everyone for the helpful comments. Sarson's original RN listing defines the prop size for the 2DM with 2:1 reduction as 24x21, which is exactly what we have now. A full circle then. I guess we can live with the lacklustre performance  for a while yet and put the money saved into the oak tree fund. Cheers.

Bill

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  • 4 months later...

Apologies for this late response.  Swallow has a National 2DM with Brunton gearbox and 2:1 reduction.  The propellor is a 'weed slipping' (Roger Lorenz design he said) 24 inch x 17 inch.  It pushes Swallow along just fine with plenty of power in reserve, 3 mph on narrow canals and 6 mph or a bit more in deep water.  She also stops well.  I gather the slightly  'swept back' design of the blades may mean a slight loss of efficiency in deep water but it's a small price to pay as we rarely pick up anything on the blades (famous last words!).

 

regards

 

David L

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21 hours ago, fanshaft said:

Apologies for this late response.  Swallow has a National 2DM with Brunton gearbox and 2:1 reduction.  The propellor is a 'weed slipping' (Roger Lorenz design he said) 24 inch x 17 inch.  It pushes Swallow along just fine with plenty of power in reserve, 3 mph on narrow canals and 6 mph or a bit more in deep water.  She also stops well.  I gather the slightly  'swept back' design of the blades may mean a slight loss of efficiency in deep water but it's a small price to pay as we rarely pick up anything on the blades (famous last words!).

 

regards

 

David L

nope it's your water system that does that for you!

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  • 2 weeks later...
  • 2 years later...

Reviving an old thread!

 

In summary the previous discussion established that the GU boats with Nationals were originally fitted with 24" x 17" props. @fanshaft was happy with his 24x17 on Swallow, but others mostly suggested larger props.

@billh thought his 24x21 was underpropped.

@Movin' on on Persia had a 24x20.

@alan_fincher repitched Flamingo's prop to 25 1/4 x 21 (for a Lister HA2 with 3:1 box), and could go a little bigger

@Bob Tidy on Battersea has 27x17 (which can't give much tip clearance on a Big Woolwich).

@billybobbooth has a 27 (pitch not stated) on Towy.

RN recommended 24x21 for 17hp engines (and 80+ year old Nationals which were originally built as 18.5hp, would probably struggle to deliver that full power now).

But Vicprop suggests something rather larger - around 27x25.

I've been told about another boat with 26x18 that could do with a little more pitch.

 

Clements Engineering have recommended I go for a 26" diameter, but have yet to recommend a pitch.

 

So what should I go for?

 

 

 

 

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Most of the nationals have probably been rebuild a good few times by now be it some very worn parts in places,

I can't comment on my pitch as I've never checked but did check size.

The blade I have be it not in the best condition or sharpest blade, gives very good stopping power (when the gearbox didn't slip) will be interesting to see what the new engine will be like as its near all new and what hasn't been replaced is very very near new or been refurbished.

Top end I do struggle to get any extra power down but i also have a very deep draft, on a shot engine on river I was able to very well keep up infact slightly quicker than a 59 foot boat with near new fr2 engine and box.

If was buying a new one I would go 24inch with 21-24 pitch just as I find my tickover quite fast (can't/ couldn't reduce the rpm as the starting rachet would start to click)

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  • 3 weeks later...

Turns out the clutch had been slipping  for years at higher revs. Much better now with rebuilt gearbox. I am surprised that there was no sign of overheating, burning oil etc , but there wasn't. Thanks to everyone for all your help.

Bill

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Hope it was rebuild correctly,  if was slipping did they recut the casing and cone to eachother, mine had been slipping and put a odd lip/ wear marks on the casing. Had to have it all ground ro rematch,

Throwing a new cone in dosnt always work as the cones can be diffrent.

But least you found problem

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