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50' Cutter 'SARAH'

A Fast Blue Water Sailing Yacht in Aluminum

Copyright 2013 - 2016 Michael Kasten

50' Aluminum Cutter 'Sarah' - Kasten Marine Design, Inc.
Aft Perspective  |  Farther Aft Perspective  |  Forward Perspective  |  Cutaway Perspective

The Concept

The 'Sarah 50' has been developed as a variation of our 96' Zebulun design. In keeping with the names given to our designs 'Zebulun', 'Leah', and 'Rachel', this design has been named after another family member, Sarah, wife of Abraham and mother to Isaac.

Planned for fast ocean voyaging, the Sarah 50 design is configured with a skeg hung rudder and a fin keel.  Draft has been kept relatively shallow for the sake of expanding the cruising area to all waters outside the one fathom line.  The bulb creates an 'end plate' for the keel, effectively increasing the aspect ratio of the keel.  Combined with the skeg hung rudder there will be excellent windward ability and accurate tracking at sea.

The hull shape is planned for easy construction in Aluminum, including deck and house structures. The "semi-radius" chine shape is easy to plate, with the majority of the surfaces using flat panel material. 

I say 'semi-radius' because it is not a simple 'radius chine' hull shape.  With a slight transverse curvature given to the topsides and bottom plating, combined with a transition that makes use of a free form curve to blend the topsides into the bottom gracefully, the visual effect is that of a fully rounded hull form.  As a bonus, since both topsides and bottom can be plated with flat plates and do not require any special forming, this effect is achieved without any increase in the cost of fabrication.

Preliminary particulars are as follows:

  • Length overall: 50'
  • WL length:  45.2'
  • Maximum beam: 14.5'
  • Keel Draft: 5' – 10"
  • Rudder Draft: 5' – 5"
  • Displacement Light: 42,000 lb.
  • Displacement Loaded: 51,000 lb.
  • Displacement / Length Ratio: 184
  • Hull Body Prismatic: .57
  • LCB: 52.1% aft
  • PPI Immersion: 2,345 lb / inch
  • Sail Area Target: Approx. 1,500 sq. ft.
  • SA / Displacement Ratio: 219.8
  • SA / Wetted Surface Ratio: 3.1
  • Typical Hull Speed: 9.2 knots
  • Maximum Hull Speed: 11.2 knots
  • Sailing Speed Potential: 10.0 knots
  • Relative Motion Comfort Index: 39

Although planned for construction in Aluminum, it would be a simple matter to convert the Sarah 50 to fiberglass construction, which would weigh the same and would not require changing the hull form.  It would also be possible to build the Sarah 50 in steel.  For this, it would be necessary to increase the displacement by deepening the hull and possibly also adding some beam.  At this point, the design is a concept only, so any option is possible...

The Hull Shape

Many fast cruising designs tend to mimic all-0ut racing sleds rather than wholesome cruising yachts.   For example, the older IOR racing types are well known for poor handling, in particular when off the wind.   And as the Fastnet tragedy has amply shown us, neither are they safe in heavy weather.  As a contrast to the older IOR racing types and their 'cruising boat' counterparts, the Sarah 50 is not overly fat in the middle... 

These days, the trend is toward extreme wedge shapes in plan view, resulting in a sharp entry and a very wide stern.  Contrary to those so-called "cruising" yachts, a significant feature of the Sarah 50 is that the stern is not too wide.   Allow me to explain...

On sailboats with an overly wide stern, the water-plane outline (looking down at the heeled water plane) will move off-center to leeward in the aft part of the boat, whilst remaining more or less on center forward. This effectively moves the heeled centerline to leeward in the aft portion, and therefore out of parallel with the upright centerline.  The heeled CL is thereby canted to windward causing excessive weather helm.

This can be tolerated to a small extent, however if the CL moves out of parallel by very much there will be excessive yaw when rolling under power, and excessive weather helm under sail.  As insult to injury, the stern at the centerline will be lifted and the bow will be depressed.

With a very wide stern, a centerline rudder would be lifted too far out of the water to be effective, exactly at the moment when it is desperately needed to maintain control due to the excessive yaw induced by the wide stern…  This is the very reason why modern racing sleds and the so-called "cruising boats" that emulate them must make use of twin rudders…!  

This is not at all conducive to steady course keeping, nor is it safe... The person at the helm must remain extremely vigilant and fatigue becomes a very real issue.  In heavy conditions these factors can quickly get out of control and readily induce broaching.  A broach can even result in less severe weather conditions when carrying a spinnaker.

Far better is to have a balanced WL shape, both upright and when heeled.  In other words, the WL outline should not be excessively wedge shaped.  In general, the transom width should be no more than half the overall beam.  Narrower yet is even better.  This will keep the aft portion of the WL shape from being lifted, and from being shifted so far to leeward when heeled.  Here, the CL will not become out of parallel by much when heeled.  The result is that the vessel will have much greater directional stability, whether heeled or just simply rolling… 

In addition to having a more balanced WL outline, the Sarah 50 also has a modest counter stern, i.e. a slight overhang.  In other words, the hull bottom extends beyond the end of the upright WL.  A modest counter stern allows the heeled the effective WL to lengthen, providing greater speed potential.

Combined, these features provide the greatest speed under sail, the best balance when heeled or when rolling, less fatigue at the helm, far less tendency to broach, and much greater peace of mind for safe blue water sailing. 

The Interior

In general I use a modular approach to the interior. The layout described here is based on a regular frame spacing of 68 cm (26.8 inches). This is works well since three stations make a full length berth space or saloon settee space; two stations are just right for a head compartment or galley; four stations make a generous double berth suite, etc.

Sarah 50 Interior Profile - Kasten Marine Design, Inc.
Preliminary Interior Profile - Click for Larger Image 

Sarah 50 Interior Plan - Kasten Marine Design, Inc.
Preliminary Interior Plan - Click for Larger Image

Sarah 50 Pilot House - Kasten Marine Design, Inc.
Alternate Pilot House Layout - Click for Larger Image 

  Above Aft Sketch |  Far Aft Sketch |  Forward Sketch  |  Cockpit Sketch |  Cockpit Closeup Sketch

Right forward will be a generous forepeak for sail storage.  Aft of that is a large Owner's Cabin having an "Island" type of berth on center.  The Owner's Cabin is 9' – 4" in length, so there is plenty of room for a large hanging locker to port and starb'd aft.  To help balance the weight of the engine which is located well aft of center, the house bank of batteries will be located below the Island Berth.

Just aft of the Owner's Cabin is a Head compartment to starb'd and a Shower compartment opposite to port.  A pair of doors forward allow direct access to either compartment from the Owner's Cabin.  Inboard, each compartment also has a door to the hallway on center.

Aft of the head / shower / hallway is a 7' long Saloon with Settees arranged parallel to center, each having a pilot berth above and outboard of the settee.  A table is located on center having leaves that can be lowered for easy passage through on either side.  The starb'd settee can be arranged to slide out, to create a double berth for additional sleeping.

Aft of the Saloon is the Galley to port, and a Navigation Station to starb'd.  Aft of the Galley / Navigation Station on center below the interior cockpit is the Engine Space, accessed via a door forward and via WT hatches in the interior cockpit sole.

Outboard of the Engine Space and aft of the Galley / Navigation Station are a pair of quarter berths which extend inboard to the Engine Space below the interior cockpit.  If single berths are used, there is sufficient room to make these spaces into Mini-Staterooms. The Galley counter and the Nav Desk are sufficiently far outboard to allow a doorway into each of these aft cabins.

Sarah 50 Cutaway View - Kasten Marine Design, Inc.
Cutaway View - Click for Larger Image

On deck aft, located above the engine room and aft cabins there is a generous Interior Cockpit with a hard dodger and windows to each side and forward.  A bulkhead with sliding doors will separate the Interior Cockpit from the Exterior Cockpit.  The Interior Cockpit length is 6' - 8" so the well protected settees can be used for sleeping.  The aft Exterior Cockpit is of a good size as well with a wrap-around horseshoe shape.  A single helm is on center. 

Aft and above the Exterior Cockpit is an arch which can be used as a foundation for instruments and as a convenient plenum for the engine room ventilation ducts.  The arch allows a Bimini cover to be extended from the Pilot House to the Arch.  Side curtains or screens can be fitted as needed.  The Arch is also intended to house an articulated stern davit, which when retracted will hold the shore boat above the transom, and when deployed will allow the shore boat to be retrieved and stowed easily.

Right aft is a raked transom having a swim deck on center, and ample reserve buoyancy to each side.  The intent with the swim deck is that it not be so big as to be a hazard in following seas…  The vertical transom forward of the swim deck will have steps let into it creating a stairway to the aft deck.  Aft of the cockpit is a generous expanse of deck allowing access to the transom ladder

Below the cockpit there is quite a large lazarette space for storage, which will be nicely accessed via a WT door in the vertical transom formed by lifting the integrally built stairway / hatch and via a door / hatch at its forward end from the interior cockpit.

The Rig

For this size vessel the cutter rig is ideal.  Ordinarily it is most convenient to place the mast according to what works best for the interior, rather than according to some fixed notion as to the 'perfect spot'.  In this case the mast will be in the forward hallway where a widened bulge in the hallway will allow easy passage to either side as well as providing an aesthetic focal point.

The mast and sail will have details similar to the Sonja 50 Proposed Sail Plan, except that it will be arranged as a cutter.


This is an ideal platform for a couple with two children, and will still allow sleeping for two additional guests in the pilot berths outboard of the Saloon settees.  For long range voyaging, the owners can occupy the pilot berths as needed and still have accommodations for six onboard. 

Naturally more people can be accommodated by making use of the Inboard Cockpit and the Exterior Cockpit, both of which have ample length for sleeping.  Alternately, these locations are available even with only two onboard, ideal for sleeping where the view is at its best...!

50' Fast Aluminum Cutter SARAH - Kasten Marine Design, Inc.
Click for Larger Image

Other Designs in This Series:

45' - 60' Zeb | 46' Dinah | 50' Sarah | 65' Rachel | 96' Zebulun | 118' Leah | 120' Leah - II