Anode position and water flow to Prop.

[Alan Saunders]

Yep, sorry guys don't know how I managed to muck up my calculation..!

 

I think subconsciously I couldn't believe such a short boat could be so heavy. But then again it is a little unusual to see such heavy plating on a boat less than 30 foot.

If your 28' x (7'?) boat really does displace 10 tonnes (very unlikely) the draught will be in the order of 44" (1.1m), the maximum depth that narrow and wide UK canals are dredged in the centre of the channel.

 

Alan

[Dalslandia]

use metric units, one liter water is one kg, one kubicmeter is one ton of water. (more or less depends on salt and temp)

we ca re calculate knowing his max if the boat is square would be 10.6 T

 

but I didn't consider the uxterplate part. how far from the transom is the swim ending? say one meter not knowing any better.

then 8,5 - 1 = 7,5 length

beem 2,05 average?

draft 24" or 0.61 m

That is 9,4 ton if square.

we do know the swim is about round and not stremlined, say the bow is the same. radius 1 meter, so area is 0.785 x 2 = 3,14 m2 (fore and after) instead of 2x2 meter say one sq meter less and draft 0.61

so 9.4 minus 0.6 = 8.8 ton or slightly less i guess

[Murflynn]

If your 28' x (7'?) boat really does displace 10 tonnes (very unlikely) the draught will be in the order of 44" (1.1m), the maximum depth that narrow and wide UK canals are dredged in the centre of the channel.

 

Alan

wrong. it would be 18 tonnes.

[Neil2]

I'm always surprised how much difference a long swim has on the draught of a narrowboat, and conversely how much ballast it takes to drag down the stern of a short swim vessel. I suspect the reason this boat has such an abrupt swim is the builder wanted to make it with 10mm plating but by the time you've added an engine the weight would have a "conventional" length swim trimming far too low at the stern.

 

With boats of 60 foot and more you can almost have the swim as long as you like but it must be quite a challenge with a short boat to achieve decent handling, if you want to use heavy plating that is.

[Eeyore]

Hmm, Are you sure it is not the bow? :-) max angle of the hull end in front of prop should be 15-30 degree each side.

 

how many inch do you have between propeller tip and skeg and uxterplate? (sp?).

19" Skeg to uxter plate, 2.5" tip clearance.

[Eeyore]

 

Well I think its bad... because she is short and she have a very round ass, and that's what make here swing the st.ass as she does.

I am checking with a prop calc program and rudder size estimate.

 

propeller seems to be correct with 7 hp at 2050 rpm and a 2:1 get 14.4x10 and 3 kts

rudder size about 41x51 cm with a K value of 4%

 

but with 2,5:1 get 16.5X13" and with 3:1 18.4x15

 

to reach 3600 rpm is a different one, it will change speed from 3 to 3,6kts, all other the same.

we get with a 3:1 12 hp at 3600, 3,6 kts, a 14.6X10"

Thank you for the rudder calculation.

The existing rudder is approx. 62.5 x 38 cm giving a K value of about 4.7%; and a 86/14 (6:1) distribution either side of the rudder stock.

I can bring the K value closer to 4% by taking 7.5 cm off the trailing edge; but will still only have a 83/17 (5:1) distribution either side of the rudder stock.

A little online research shows figures closer to 80/20 (4:1), and some images of narrowboats appear to show rudders of that configuration.

Clearance permitting I can add a little to the leading edge to improve the distribution. This should also help control the flow of water to the prop when going astern, not to steer astern, just to counteract the paddle wheel effect and help to stop in a relatively straight line.

 

The prop and gearbox may be left until the next time she's docked; although availability of a genuine Hurth HBW50 3R (2.7:1 in ahead) as a straight swop for the existing HBW50 2R would influence the decision. That would be an acceptable compromise with the existing prop.

 

Time to check availability of dry docks for next spring

 

Any comments welcome.

 

Regards

Steve (Eeyore)

 

Oh she has a theme tune - by Queen, any guesses?

Edited September 29, 2015 by Eeyore

[Eeyore]

Eventually decided on a more radical approach

Edited November 15, 2016 by Eeyore

[ditchcrawler]

Thanks again for the replies.

 

 

I'm going to have a look at alternative rudder designs, but in all honesty I'll probably just chop a bit at a time off the back edge and see how its goes. Worst case scenario is that I have to weld it back on next time she's out of the water ;-)

 

 

Thanks again

Steve (Eeyore)

I think that is the wrong way round and you need to chop a little bit off the LEADING edge of the rudder, not the trailing edge.

[Eeyore]

I think that is the wrong way round and you need to chop a little bit off the LEADING edge of the rudder, not the trailing edge.

Not really an issue now as almost everything aft of the cabin has been consigned to the scrap pile.

She's being rebuilt with longer straight sided swims and a rounded stern. The engine will be entirely under the decking, freeing up valuable cabin space. The new rudder combined with a "standard" length tiller handle should give a more balanced feel to the steering.

The original stern gland, shaft and prop has found a new home a part of Mrs Eeyores nautically themed garden feature - married to an artist; who knew!

Edited November 15, 2016 by Eeyore

[ditchcrawler]

Not really an issue now as almost everything aft of the cabin has been consigned to the scrap pile.

She's being rebuilt with longer straight sided swims and a rounded stern. The engine will be entirely under the decking, freeing up valuable cabin space. The new rudder combined with a "standard" length tiller handle should give a more balanced feel to the steering.

The original stern gland, shaft and prop has found a new home a part of Mrs Eeyores nautically themed garden feature - married to an artist; who knew!

Caught by an old thread, goes and sulks in the corner

[Eeyore]

Caught by an old thread, goes and sulks in the corner

Yes it's been a while.

I didn't want to be one of those people who ask a question but never comes back to update the members who have helped.

[Flyboy]

Hmmm, isn't a turbulator intended to cause the flow to break away cleanly from the surface (swim, in this case), rather than keeping it attached?

 

Oh no, just checked, the turbulent flow helps to keep the flow attached as you say!

You've probably got turbulator tape on your glider. Usually zig zag type just in front of the control surfaces & just in front of the hinge line. Improves response quite a bit.

[Dalslandia]

You've probably got turbulator tape on your glider. Usually zig zag type just in front of the control surfaces & just in front of the hinge line. Improves response quite a bit.

The main reason for turbulators on gliders is to eliminate laminar bubbles and make sure the turbulant layer stay attached, can be in form of zigzag tape or blow holes, on boats it can be had on the mast to reduce drag and make the wind attach to the sail earlier.

Far back on a boat the boundery layer is thick so it takes VG's to break it up in vortexes.

[David Mack]

You've probably got turbulator tape on your glider. Usually zig zag type just in front of the control surfaces & just in front of the hinge line.

 

[Eeyore]

Quick update.

Trial fit of engine showing improved swim arrangement, poor thing looks lost in there!

All new steel aft of the bulkhead as per my earlier post.

Nearly finished, just bracing myself for the bill!