Selden Hydraulic Systems 1, Practical Guide — Owner's Manual
Selden's practical guide to its earlier hydraulic sail-handling systems: basic hydraulic theory, motor sizes and speeds for hydraulic Furlex and in-mast furling, thread standards and JIC adapters, and how the boom outhaul valves behave – useful for troubleshooting.
Basic Hydraulic Theory
- Hydraulic power is pressure × flow. P = F / A; Q = V / t = A × S / t; for a rotating pump or motor Q = D × n and torque M = ΔP × D / 2π.
- Units: 10N/cm2 = 100.000Pa = 0.1 MPa = 1 bar ~1kg/cm2; 1l = 1000cm3; flow in l/min; displacement D in cm3; torque in Nm.
- When the load on a cylinder or motor rises, system pressure and pump-motor current rise automatically and flow drops – the pump curve.
- Pressure loss ("friction") depends on hose diameter and length, bend radii and geometry, and rises with flow.
- Worked example (piston pump – cylinder): pump piston A1 = 10cm2, cylinder piston A2 = 100cm2. 100N on the pump piston over 100cm in 1 min gives P = 10 N/cm2 (1 bar), Q = 1000cm3/min (1 l/min), so the cylinder delivers F2 = 1000N = 1kN over S2 = 10cm. This is the principle of a hand-pumped backstay adjuster: the bigger the area ratio, the bigger the load.
Selden Hydraulic Motors
| Application | Motor | Displacement | Recommended flow | Speed |
|---|---|---|---|---|
| Furlex 300H, 400H and in-mast furling | OML12.5 | 12.5 cm3 | 10 l/min at 40 bar | 10000/12.5 = 800 n/min |
| Furlex 500 | OMM20 | 20 cm3 | 20 l/min at 40 bar | 20000/20 = 1000 n/min |
| Cutter Furlex (400H) and in-mast furling on the 20 l/min powerpac | OML20 (instead of OML12.5) | 20 cm3 | 20 l/min | 1000 n/min |
On a 20 l/min powerpac an OML12.5 would turn 20000/12.5 = 1600 n/min – too fast – so it is changed for an OML20. An OMM20 has different dimensions and would not fit.
Threads & JIC Adapters
Selden hydraulic fittings are BSPP with 60° cone, British Standard BS 5200 "K4" (also written G1/4", G3/8", G1/2"). Where a system uses JIC fittings, hoses with JIC 37° female ends can be fitted with an adapter in place of the original male BSPP coupling:
| BSPP | To JIC 37° male | Parker Triple-Lok SS adapter |
|---|---|---|
| 1/4 BSPP | JIC 7/16-20 | 4-4F4OMXSS |
| 3/8 BSPP | JIC 9/16-18 | 6-6F4OMXSS |
| 1 /2 BSPP | JIC 3/4-16 | 8-8F4OMXSS |
The adapters include a plastic sealing washer on the BSPP side.
| G BSPP | Thread | Dash size | ØM (in) | ØM (mm) | A (mm) |
|---|---|---|---|---|---|
| 1/4" | 1/4-19 | 4 | 0.51 | 13 | ~11 |
| 3/8" | 3/8-19 | 6 | 0.65 | 16.5 | ~12 |
| 1/2" | 1/2-14 | 8 | 0.82 | 20.83 | ~14 |
The guide also has approximately 1:1 sketches of the 1/4" and 3/8" couplings for identification, and hose / coupling layouts for Furlex 300, 400 and 500 (glands 540-895 and 540-889), mast motors OML12,5 and OML20, and booms 200, 250 and 300 – see the PDF.
Hydraulic Furlex & In-Mast Furling Circuits
- Selden does not supply the powerpacs and valve blocks for hydraulic Furlex and in-mast furling; the guide shows a common solution.
- If one powerpac also drives winches and two functions run at once, oil takes the path of least resistance – the lightly loaded motor runs faster.
- Fit a pressure-reducing valve in the in-mast furling circuit: working pressure is generally 140 bar and must be reduced, at least for RB systems (120 bar). It acts only in the MAIN IN direction, bypassing oil at the set pressure to limit torque on the tack assembly and profile if the sail or outhaul line jams while furling.
- In each direction the flow-control valve acting is the one on the return from the motor.
- Control box 532-426 is the same for 12 and 24 V; only the relays K1-K8 differ.
How the Boom Outhaul Valves Behave
| Situation | What happens | Manometer 3 / 38 |
|---|---|---|
| BOOM OUT pressed | Overcentre valve 33.2 closed, oil passes its non-return valve; 33.1 opened by servo. | 3 = working pressure; 38 slightly less |
| BOOM OUT, line jammed or too much sail / wind load | Relief valve 2.4 opens; no flow except through 2.4. | Both show the max working pressure set on 2.4 |
| Sailing, normal wind | Wind load pressurizes the cylinder B-chamber; 33.2 stays closed below its setting. | 3 = zero; 38 = pressure from wind |
| Wind load exceeds the 33.2 setting | 33.2 opens and the piston moves; rod-volume difference is made up from the reservoir. | 3 = zero; 38 = 33.2 setting |
| BOOM IN pressed | 33.1 closed, oil passes its non-return valve; 33.2 opened by servo. | 3 = working pressure; 38 lower |
| Outhaul gives way in moderate wind | A and B hoses mixed up between valve block and cylinder: 33.1 (much lower setting) takes the load. Refit the hoses correctly; temporarily swap A and B at the quick couplings. | Both zero |
Hose B is on the starboard side. Pos 35 is the emergency valve.
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