Micro Yacht

Small offshore trailerable homebuilt yachts

About seaworthiness

The table below summarizes the estimated stability for the designs presented on this page, which I refer to as ''seaworthiness'', a very controversial concept to measure as it seems. Please note that these numbers have to be taken with caution: the estimation method is not recognized by legal authorities and does not take into account important parameters such as draft or hull design. Besides, trailer sailboats are often not really meant for cruising in blue waters, but rather for coastal cruising or even inshore racing. However, as I did not find much information on their actual seaworthiness , I took what I found to compare the different designs.

  Disp/ length Sail/ disp Hull speed Loa/ beam Capsize risk Comfort factor Moment of inertia Roll period Stability index
Roberts 25 231 16 6.1 3.2 1.85 20.5 70386 2.4 0.99
Serpentaire 7.50 scaled 208 23 6.2 3 1.95 18.5 66712 2.2 0.87
Spray 27 220 18 6.2 3.2 1.94 18.9 76415 2.2 0.85
Serpentaire 7.50 cruiser 223 20 6.2 2.8 2.05 18 75455 2.1 0.77
Etap
24i
166 21 6.3 2.9 2 16 53174 1.9 0.76
Roberts 246 190 19 6.1 3 2.04 16.2 48277 1.9 0.76
Vagabond 23 scaled 167 20 6.2 2.8 2.05 15.9 50675 1.9 0.75
Pantanal 25 146 23 6.3 3.1 2.06 14.5 40968 1.8 0.74
Vagabond 23 167 20 6.2 2.8 2.05 15.7 45687 1.8 0.72
Didi 26
97 25 6.7 3.2 2.1 12.5 38691 1.6 0.65
Etap
28s
171 17 7.1 2.6 2.12 18 186831 2.1 0.62
Poussière d'étoiles 25 107 31 6.5 2.9 2.18 12.4 38019 1.5 0.59
Etap
21i
145 22 6 2.5 2.3 12 25460 1.4 0.56
Didi mini 6.5 cruise 135 25 6.2 2.2 2.68 9.9 30145 1.1 0.37
Didi mini 6.5 109 40 6.2 2.2 2.88 8 20771 0.9 0.3

You can also download the related XL file with built-in calculations.

As you can see, I included numbers for 3 production sailboats: The Etap 21i, 24i and 28s. There are certainly other possible references, but the Etaps have the advantage of setting homogenous references across ths spectrum of dimensions. They are also very seaworthy and belong all to category "B". An Etap 21i even circumnavigated with two adults and one child on board! At the lower end of the spectrum (in terms of stability), you can look at the numbers for the Didi mini in its pure mini-transat version. The dimensions for the Etaps are the following:

 Dimensions Etap 21i   Etap 24i  Etap 28s
 Length overall (LOA)  6.26 m  7.30 m 8.60 m 
 Length waterline (LWL)  6.10 m  6.71 m  8.45 m
 Maximum beam  2.49 m  2.50 m  3.36 m
 Ballast  300 kg  500 kg 1170 kg 
 Displacement  1180 kg  1800 kg  3700 kg
 Sail area  24 m²  30 m²  40.8 m²


Roberts 25

Dimensions

Length overall (LOA) : 7.70m (25'3'')
Length waterline (LWL) : 6.34m (20'10'')
Maximum beam : 2.43m (8'0'')
Draft shoal: 1.76m (3'6'')
Draft deep: 1.07m (2'6'')
Weight : 1588 kg (3500lb)
Ballast : 898 kg (1908lb)
Displacement : 2114 kg (4960lb)
Headroom : 1.83+ m (6'+)
Category : NA
Positive floatation : No
Sail area : 26.5 m² (285 ft²)
Engine : 4 to 8 CV
Designer: Bruce Roberts
Building time : NA
Construction: Moulded Plywood/Epoxy or Fibreglass - C-Flex, Single Skin or Foam Sandwich

Web siteBruce Roberts Yacht Design

Description

Over 1000 examples of this design have already been completed. This trailerable sailboat has been very popular and performs well in either the shoal draft or regular keel versions, both of which are included in the study package. While not as sleek looking as the latest offerings from other designers this design will always remain popular because of its voluminous accommodation and good, safe handling characteristics making it a wonderful  boat for the whole family.

A nice trailer sailboat but it's a bit on the heavy side and not as fast as the most recent designs (supposed - this should be proven). There are however many construction options, which should fit most builders desires.

 

Serpentaire 7.50

Dimensions

Length Overall (LOA) : 7.65 m
Length waterline (LWL) : 6.50 m
Maximum beam : 2.73 m
Draft CB: 0.70 / 1.70 m
Draft keel: 1.50 m
Weight : 1320 kg
Ballast : 600 kg
Displacement : 2200 kg
Headroom : 1.75 m
Category : 3
Positive floatation : Yes
Sail area cruise : 33.50 m²
Sail area sport : 38 m²
Engine : 9.9 à 15 CV
Designer: Bernard Veys
Building time : 650-850 hours
Construction: Multichine plywood/epoxy

Web site: Compagnie Enthousiaste

Description

The Serpentaire 7.50 was conceived on the request of owners of Serpentaires 6.50 that crossed the Atlantic and now wanted more headroom, a
double berth under the cockpit and an enclosed head. It is based on the Serpentaire 22, winner of the first
mini-transat.

The design is offshore capable, unsinkable, fast and has an roomy interior adequate for long cruises. Construction is multichine plywood with epoxy impregnation and layer of fiberglass around the hull and deck. A really ideal micro yacht, but it is not trailerable without a special permit in Switzerland and France, unless transported it on its side, hence reducing the width to less than 2.55m.


 

 

Roberts 246

Dimensions

Length overall (LOA) : 7.40 m (24'')
Length waterline (LWL) : 6.30 m (20'8'')
Maximum beam : 2.50 m (8'2'')
Draft CB: 0.50 / 1.46m (1'7'' / 4'3'')
Weight : NA
Ballast : 524 kg (1155 lbs)
Displacement : 1703
 kg (3751 lbs)

Headroom : 1.83m (6')

Category : NA
Positive floatation : Not planned
Sail area : 26.46 m² (285 ft²)
Engine : 5 CV
Designer: Bruce Roberts

Web site: Bruce Roberts Yacht Design

Description

This sailboat has been very popular and has proved to be an exciting boat to sail. The drop keel version could quite easily be made trailerable.

Pantanal 25


Dimensions

Length overall (LOA) : 7.65 m (25'1'')
Length waterline (LWL) : 6.66 m (21'9'')
Maximum beam : 2.44 m (8')
Draft CB: 0.625 / 1.65m (5'4" / 2'2'')
Weight : 1000 kg (2200 lbs)
Ballast : 320 kg (705 lbs)
Displacement : 1550 kg (3333 lbs)
Headroom :
1.70 m (5'7'')
Category : C
Positive floatation : Not planned
Sail area : 30.38 m² (330 ft²)
Engine : 9.9 à 15 CV
Designer: Roberto Barros

Web site: Roberto Barros Yacht Designs

Description

To make the boat the most attractive possible, the designer provided an interior arrangement with a cozy and functional layout. A double berth at the forward compartment joins with two settee berths in the saloon, omitting any partition that could jeopardize the feeling of amplitude inside the cabin. Abaft the two settees a heads compartment with door and a compact galley complete the saloon arrangement. A second double berth placed under the cockpit area provide full usage of space in the whole interior Contrary to the majority of boats of this type, the designer assured a 1.70m (5'7") headroom in the main saloon, an acceptable figure regarding elbow room, if you have to stay aboard for a prolonged time. As the most requested area of the boat is the cockpit, it is the longest possible, for that matter designing a practically vertical transom. The area under the cockpit seats abaft the double berth is reserved for storage of boat gear and other equipments. The Pantanal 25 is provided with a bulbous drop-keel, together with a pivoting mast, to simplify launching without neglecting stability

The design is capable of coastal cruising, is fast, easily trailerable and has a roomy interior. Construction is foam core sandwich. A really ideal micro yacht intended as-is for racing or inshore cruising. An exchange of e-mails with the designer revealed that with simple options for reinforcing the hull, the boat fits in the CE category ''C''. There might be potential for more but not in this configuration. If you don't care abouth maximum beam as I do, look at the other designs (MC23, MC26C,...).

 

Vagabond 23

Dimensions

Lenght overall (LOA) : 7.02 m (23')
Lenght waterline (LWL) : 6.50 m (21'4'')
Maximum beam : 2.49 m (8'1'')
Draft CB: 0.63 / 1.42 m (2' / 4'8'')
Weight : 1135 kg (2500 lbs)
Ballast : 500 kg (1100 lbs)
Displacement : 1650 kg (3750 lbs)
Headroom : 1.41 m (1'43")
Category : 3
Positive floatation: Yes
Sail area : 27 m²
(295 ft²)
Engine : up to 10 CV
Designer: Jacques Mertens
Building time : 400-800 hours
Construction: Stitch and glue plywood/epoxy

Web site: Bateau.com

Description

The Vagabond 23 is the successor to the Serpentaire 22, the famous winner of the first Mini_Transat. The Vagabond 23 retain the virtues of the old design: offshore capable, well balanced fast hull that is easy on her crew, seaworthiness and strength but the designer was able to blend those qualities with better speed, more room, easier building and more strength.

The design is offshore capable, unsinkable, relatively fast but the interior is typical of smaller boats (no berth under the cockpit, no separate head). Construction is easy with the stitch and glue method. A nice micro yacht with a good compromise in all areas.

 


 


 

Poussière d'étoiles 25

Dimensions

Lenght overall (LOA) : 7.50 m
Lenght waterline (LWL) : 7.28 m
Maximum beam : 2.54 m
Draft CB: 0.625 / 1.65 m
Weight : 985 kg
Ballast : 300 kg
Displacement : ? kg
Headroom : 1.65 m
Category : B (planned)
Positive floatation : Yes
Sail area : 39 m²
Engine : 4 to 8 CV
Designer: Jean-Michel Nalpowik
Building time : 1200 hours
Construction: Diagonal round bilge wood/epoxy

Web siteLa Carène

Description

The Poussière d'étoile 25 is the bigger sister of the Poussière d'étoile 21. It was designed out of demanding specifications: maximum length of 7.50m, simple, zero maintenance, fast, roomy, easily trailerable, capable of inshore as well as offshore passages.

Construction is diagonal plywood bilge over a plywwod frame of frames and longerons, inspired by aviation technology. A really ideal micro yacht, although no prototype exist yet.

 

 

Other trailerable designs

There is a number of plans that I studied, but did not retain in the final round because they did not satisfy some of my selection criterias. This does NOT mean these are not good enough, just not adapted to my intended use. I list them here however, in no particular order other that the one in my favorites list, as readers of this site may find them of interest for their own project: Spray 27, Glen-L 25, Didi 26, Dix Cruise-Mini, TLC 27, Kavalier 800, Pizicato, Thunderbird 26 and Dolphin. There are many other ones, but I did not study them in detail as they were clearly not adapted for this project.

I actually started studying first multihulls (trimarans and catamarans) and spent first some time in this category. It turned out though - this is a subjective conclusion - that there 1) catamaran must be sensibly longer than monohulls for adequate seaworthiness and 2) trimaran could claim such a classification with a smaller hull length, but then the interior is really tiny, definitely not comfortable, and they are quite expensive due to the added complexity of folding arms to allow trailerability.

LOA/beam ratio


This ratio measures the fineness of the hull. Fine hulls, having ratios of 3.0 - 4.0 and higher, are long and slender which promotes easy motion, high speed (low drag), and good balance when heeled. Many newer designs favor wider hulls which have larger interior volume, sail flatter, and have high reaching and down wind speed potential. One note of caution when making comparisons, longer boats tend to be finer then short ones. Our template value is 3.4, which is fairly fine. Fine hulls tend to be well balanced and have low inverted stability.

Capsize risk


An empirical factor derived by the USYRU after an analysis of the 1979 FASTNET Race. The study was funded by the Society of Navel Architects and Marine Engineers (SNAME). They concluded that boats with values greater than 2 should not compete in ocean races. Values less than 2 are "good". The formula penalizes boats with a large beam for their high inverted stability, and light-weight boats because of their violent response (low roll moment of inertia) to large waves, which are both very important during violent storms. It does not indicate or calculate static stability. Some modern coastal cruisers and many racing designs have problems meeting this criteria. An interesting note, the study concluded that static stability was relatively unimportant in predicting dynamic capsize. Beam and weight were much more important factors. Wide boats give waves a longer lever arm to initiate roll and light weight boats require less energy to roll over; both undesirable attributes in a cruising boat. The template value of 1.7 is very low.

Comfort factor


An empirical term developed by yacht designer Ted Brewer. Large numbers indicate a smoother, more comfortable motion in a sea-way. The equation favors heavy boats with some overhang and a narrow beam. These are all factors that slow down the boat's response in violent waves. This design philosophy is contrary to many modern "racer / cruisers", but it is based on a great deal of real blue water data, not just what looks good in a boat show. A value of 30 - 40 would be an average cruiser. Racing designs can be less than 20, and a full keel, Colin Archer design, could be as high as 60. Our template value of 36 indicates that comfort is a high priority on cruisers.

Moment of Inertia (I)


An empirical term developed by SNAME. Large values resist rolling forces. The moment of inertia is very sensitive to the distance items are from the CG. A heavy rig can greatly increase I, with little impact on displacement.

Roll period (T) and Stability index


ROLL PERIOD (T)

Simply stated, a sailboat’s roll period, in seconds, is inversely proportional to its stability. Unstable boats have long periods, stable boats have short periods. The roll period is very easy to determine, you simply grab a shroud and push / pull until the boat is rocking over a few degrees. Then count the number of full cycles in one minute, and divide into 60. The general rule of thumb is that boats with periods less than 4 seconds are stiff and periods greater than 8 seconds are tender. The template value of 4.05 is near the stiff end of the range, indicating good static stability.

STABILITY INDEX

This is another empirical term relating period and beam to stability. Values less than 1.0 are considered stiff. Values greater than 1.5 are considered tender. I like this technique because its simple, and includes the hull form, the center of gravity, and the roll moment of inertia, all in one easy to use package. The template boat again leans towards the stiff side, with a value of 1.1.

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