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47' - 9" Cutter 'DINAH'
A Fast Blue Water Sailing Yacht in Aluminum
Copyright 2014 Michael Kasten
Aft Perspective | Forward PerspectiveThe Concept
The 'Dinah 47' has been developed as a variation of our Sarah 50 design. In keeping with the names given to our designs 'Zebulun', 'Leah', 'Rachel', and 'Sarah' this design has been named after another family member, Dinah, daughter of Jacob and Leah, sister of Zebulun, and wife of Job.
Planned for fast ocean voyaging, this concept design is configured with a single skeg hung rudder and a single fin keel. The keel is relatively shoal draft and fitted with a bulb. The keel in combination with the skeg hung rudder will provide 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 materials.
I say 'semi-radius' because it is not a simple 'radius chine' hull shape. With this hull form, both topsides and bottom, though curved, can be plated with flat plates and do not require any special forming. 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, this effect is achieved without any penalty in terms of added construction labor, and therefore also involves no increase in the cost of fabrication.
The Hull Shape
Many fast cruising designs offered these days tend to mimic all-0ut racing sleds rather than wholesome cruising yachts. A few key features are outlined below which differentiate the the Dinah 47 from those thoroughbreds...
STERN WIDTH: Contrary to those so-called "cruising" yachts, a significant feature of the Dinah 47 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.
That is the very reason why most modern racing sleds must make use of twin rudders…! 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 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.
BALANCE: 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 hull from being lifted and keep the WL from being shifted so far to leeward when heeled. Thus, the CL will not become out of parallel by much when heeled, resulting in much greater directional stability, whether heeled or just simply rolling…
COUNTER STERN: In addition to having a more balanced WL outline, the Dinah 47 also has a modest counter stern, i.e. a slight overhang. In other words, the hull bottom extends beyond the end of the waterline. A modest counter stern allows the effective WL to lengthen at speed and when heeled, providing greater speed potential, as well as added buoyancy.
TRANSOM RAKE: The forward-raking transom originated among racing boats, the result of racing rules that penalize length on deck. The forward-raking transom has become popular, mainly to emulate those thoroughbred, but also because cruising sailors have realized it is a practical transom shape for life on the water. With the actual transom recessed somewhat forward of the end of the hull bottom and sides, we have the now-common sugar-scoop transom. Unfortunately, combined with the trend toward having a very wide stern, this could not be a worse shape for sailing in following seas...! Call it a 'water scoop..!'
SWIM PLATFORM: On the Dinah 47 in addition to limiting the 'width' of the transom for the reasons given above, we have kept the 'recess' to a minimum for the sake of achieving maximum buoyancy. On center, a modest swim platform allows easy access to and from the water or shore boat and a protected recess for a boarding ladder to the aft deck.
THE RESULT: Combined, these features provide convenient access to the water, the greatest speed under sail, the best balance when heeled or when rolling, less fatigue at the helm, less tendency to broach, and far greater peace of mind for safe blue water sailing.
PARTICULARS: Preliminary design particulars are as follows:
- Length overall: 47' - 9" (14.55 m)
- Length on Deck: 44' - 8" (13.61 m)
- WL Length: 43' - 3" (13.18 m)
- Maximum beam: 14' - 6" (4.42 m)
- Fixed Keel Draft: 5' - 10" (1.77 m)
- Rudder Draft: 5' - 5" (1.66 m)
- Displacement at DWL: 35,400 lb. (16 mt)
- Displacement at Half Load: 40,000 lb. (18.1 mt)
- Displacement to Length: 207
- Hull Body Prismatic: .57
- LCB: 52 % aft
- Sail Area Target: Approx. 1,300 sq. ft. (121 sq m)
- Sail Area to Displacement: 18
- Comfort Factor: 38
The Interior
You can visualize the interior layout by reviewing the Sarah 50 Layout to which it is similar, as well as by studying the images in the following links:
Aft Port Sketch | Aft Starb'd Sketch | Forward Sketch | Cockpit Closeup Sketch | Aft Cabin Closeup Sketch
When creating a new boat design, it is usually best to imagine the interior first, and then wrap an envelope around it and see what eventuates in terms of overall size and the arrangement of exterior features such as cabins, cockpit, hard dodger, etc… In general I prefer to use a modular approach to the interior. The layout described here is based on a regular frame spacing of 70 cm (@ 27.6 inches). This works nicely for most interior spaces, 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, and so forth.
Using that approach, and thinking in terms of ‘modules’ for the interior layout, in no particular order, we usually have an owner’s sleeping cabin, a head / wash room, a saloon / settee / dining area, a galley, navigation area, an outside steering station on sailboats, and maybe some place for guests if at all possible.
In the Dinah 47 model shown here, some of those ‘modules’ are placed inside an enclosed pilot house. I’ve envisioned the interior more or less as follows…
- Below Forward:
- Owner’s cabin with 'island berth' on center and wardrobe port & starb’d aft
- House batteries below 'island berth'
- Aft of owner's cabin a WC & lavatory to starb’d and shower to port
- Aft of WC & shower, the Saloon with settees or dinette, possibly with pilot berths outboard
- Pilot House:
- Galley to port inside pilot house
- Chart / Nav Desk to starb’d inside pilot house
- Helm station to starb’d inside
- Engine space below Pilot House
- Aft:
- Small exterior cockpit aft of the Pilot House with an auxiliary steering station
- Private Guest cabin aft of and below the cockpit, with an entry from the cockpit
- Right aft, a small swim platform with boarding ladder & steps to the aft deck
The Rig
For this size vessel the cutter rig is ideal. We find it best to place the mast according to what works best for the interior rather than according to some pre-conceived notion as to the 'perfect spot'. For this interior layout, the mast will ideally be in the forward hallway, where a widened bulge in the hallway can allow easy passage to either side as well as an aesthetic focal point.
The mast and sail are intended to have details similar to the Sonja 50 Proposed Sail Plan, except that in this case it will be arranged as a cutter.
Summary
In all, I think this is an ideal platform for a couple with two kids or two guests, and will still allow sleeping for others 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 Interior 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...!
Click for Larger ImageOther Designs in This Series:
45' - 60' Zeb | 46' Dinah | 50' Sarah | 65' Rachel | 96' Zebulun | 118' Leah | 120' Leah - II
Please see the AVAILABLE BOAT PLANS web page.
Home | Intro | Our Design Process | Stock Design Info | Motor Yacht Designs | Sailing Yacht Designs | Prototype Designs
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