WesWorld

I am trying something different - taking a drawing and trying to build a Springsharp to match it. I invite your comments to point out my errors.

The concept drawing



My current working Springsharp

Project 39R - Rev A, TBD Riverine Monitor laid down 1939

Displacement:
234 t light; 243 t standard; 250 t normal; 256 t full load

Dimensions: Length overall / water x beam x draught
157.29 ft / 155.84 ft x 21.33 ft x 4.92 ft (normal load)
47.94 m / 47.50 m x 6.50 m x 1.50 m

Armament:
2 - 3.46" / 88.0 mm guns in single mounts, 20.79lbs / 9.43kg shells, 1936 Model
Dual purpose guns in deck mounts
on centreline ends, evenly spread
2 - 1.57" / 40.0 mm guns (1x2 guns), 1.95lbs / 0.88kg shells, 1939 Model
Anti-aircraft guns in deck mount
on side amidships, all raised guns
4 - 0.79" / 20.0 mm guns (1x4 guns), 0.24lbs / 0.11kg shells, 1939 Model
Anti-aircraft guns in deck mount
on centreline aft
2 - 0.79" / 20.0 mm guns (1x2 guns), 0.24lbs / 0.11kg shells, 1928 Model
Anti-aircraft guns in deck mount
on centreline amidships, all raised guns - superfiring
Weight of broadside 47 lbs / 21 kg
Shells per gun, main battery: 150

Armour:
- Belts: Width (max) Length (avg) Height (avg)
Main: 0.79" / 20 mm 78.74 ft / 24.00 m 5.58 ft / 1.70 m
Ends: Unarmoured
Main Belt covers 78 % of normal length

- Gun armour: Face (max) Other gunhouse (avg) Barbette/hoist (max)
Main: 0.79" / 20 mm 0.39" / 10 mm -
2nd: 0.39" / 10 mm 0.39" / 10 mm -
3rd: 0.39" / 10 mm - -
4th: 0.39" / 10 mm - -

Machinery:
Diesel Internal combustion motors,
Geared drive, 3 shafts, 286 shp / 213 Kw = 12.00 kts
Range 2,000nm at 8.00 kts
Bunker at max displacement = 12 tons

Complement:
30 - 40

Cost:
£0.083 million / $0.330 million

Distribution of weights at normal displacement:
Armament: 6 tons, 2.3 %
Armour: 19 tons, 7.7 %
- Belts: 16 tons, 6.3 %
- Torpedo bulkhead: 0 tons, 0.0 %
- Armament: 4 tons, 1.4 %
- Armour Deck: 0 tons, 0.0 %
- Conning Tower: 0 tons, 0.0 %
Machinery: 8 tons, 3.1 %
Hull, fittings & equipment: 101 tons, 40.4 %
Fuel, ammunition & stores: 16 tons, 6.5 %
Miscellaneous weights: 100 tons, 40.0 %

Overall survivability and seakeeping ability:
Survivability (Non-critical penetrating hits needed to sink ship):
546 lbs / 248 Kg = 26.3 x 3.5 " / 88 mm shells or 0.6 torpedoes
Stability (Unstable if below 1.00): 1.01
Metacentric height 0.5 ft / 0.2 m
Roll period: 12.7 seconds
Steadiness - As gun platform (Average = 50 %): 90 %
- Recoil effect (Restricted arc if above 1.00): 0.28
Seaboat quality (Average = 1.00): 2.00

Hull form characteristics:
Hull has a flush deck
and transom stern
Block coefficient: 0.535
Length to Beam Ratio: 7.31 : 1
'Natural speed' for length: 14.56 kts
Power going to wave formation at top speed: 41 %
Trim (Max stability = 0, Max steadiness = 100): 45
Bow angle (Positive = bow angles forward): 10.00 degrees
Stern overhang: 0.00 ft / 0.00 m
Freeboard (% = measuring location as a percentage of overall length):
- Stem: 8.20 ft / 2.50 m
- Forecastle (20 %): 8.20 ft / 2.50 m
- Mid (50 %): 8.20 ft / 2.50 m
- Quarterdeck (15 %): 8.20 ft / 2.50 m
- Stern: 8.20 ft / 2.50 m
- Average freeboard: 8.20 ft / 2.50 m
Ship tends to be wet forward

Ship space, strength and comments:
Space - Hull below water (magazines/engines, low = better): 72.3 %
- Above water (accommodation/working, high = better): 124.0 %
Waterplane Area: 2,380 Square feet or 221 Square metres
Displacement factor (Displacement / loading): 138 %
Structure weight / hull surface area: 27 lbs/sq ft or 131 Kg/sq metre
Hull strength (Relative):
- Cross-sectional: 0.86
- Longitudinal: 2.92
- Overall: 0.98
Caution: Hull subject to strain in open-sea
Hull space for machinery, storage, compartmentation is excellent
Room for accommodation and workspaces is excellent
Ship has slow, easy roll, a good, steady gun platform
Excellent seaboat, comfortable, can fire her guns in the heaviest weather

Miscellaneous Weight - 100 tons reserved for organic troop contingent

****
You may fire when ready Gridley!

Hull strength is just a tad bit low, as is ammo stowage.

I'm pondering the block coefficient, here. Realistically, a flat-bottomed barge-sided hull should probably have a higher BC... my off-the-cuff guess is maybe .7 or .8, but I'd want someone like RLBH to confirm that and make certain. (Yes, I know the historical vessels were about this displacement, too.)

Quoted

Originally posted by Brockpaine
Hull strength is just a tad bit low, as is ammo stowage.

I'm pondering the block coefficient, here. Realistically, a flat-bottomed barge-sided hull should probably have a higher BC... my off-the-cuff guess is maybe .7 or .8, but I'd want someone like RLBH to confirm that and make certain. (Yes, I know the historical vessels were about this displacement, too.)

Oddly enough, I agree. Your 1937 Artilleriefahrprahm design, which was my starting point, had a BC of 0.447, but the 39R is a bit bigger. Does the deck layout seem to match the drawing?

Yes, a barge should have a block coefficient of about 0.8-0.9 - even 0.7 is a bit low. But there are all sorts of funny games you can play: quoted draughts for historical ships are often navigational draughts, to the lowest point of the ship, which will often be a propeller tip; this can be some way below the bottom of the hull. This is part of the reason that getting 'good' sims for historical ships is nigh-on impossible - if you use published draught figures for one RN frigate (I can't remember which) you get a block coefficient of about 0.35, which just won't work.

Alternatively, and more likely for this kind of ship, there could well be 'tunnel' propellers; there are resistance advantages to this in some cases, but it also protects the propellers if the vessel goes aground. This is a very easy thing to do with a river monitor. With cunning design, it's actually possible to have the top of the propeller above the waterline, although then you're halfway to designing a waterjet.

I'd expect that both factors are at work here: there'll be some tunnelling for the props, and quite likely some odd projections below the keel if it's been based on a historical ship.