24' Water Bee - A camper power boat


24' Water Bee - A camper power boat


Design #301.

 "Water Bee".  A camper power boat.

 LOA 24' X Beam8'-0" X Draft 24".

DRG.   301-01   Profile.                                            Scale: 1: 12     1" = 1'-0"

DRG.   301-02   Lines Plan.                                                1: 12  1" = 1'-0"

DRG.   301-03   GeneralArrangement.                            1: 12  1" = 1'-0"

DRG.   301-04   Construction Plan.                                    1:12  1"=  1'-0"

Table of Offsets.

US$400.00. Buy.

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Design #301 WaterBee. 24

An efficient Inboard Cruiser designed for Family use.For affordable launching the boat is trailerable, this gave me the idea to keep the project small, at the smallest end of the request. Being small is advantageous when the beam is limited to 8' and can find place in limited storage space such as in the driveway. The trailer should have a load capacity of 4000 pounds and a front dolly wheel for its handling away from the vehicle.Therefore, here are the Principal Characteristics for this model. (See full Hydrostatics for more details).

L.O.A24'-0"  7.315M. xL.W.L 24'-0"  7.315M. x BEAM. 8'-0"  2.438M. x DRAFT.  2'-0"  610MM. Dspl. 3,465Lbs. 1.571T. 

Being limited in length gives an added incentive to be built by Amateurs and or by a small shop. This is important in regard of the methods of construction, not limited to only wood. This will be addressed later. So is the question of the need for a decent Accommodation Plan, even if simple. 

LINES. A hard chine boat was for the asking. In this small size, a round bottom boat would not get to speed. The result is a double chine boat with the panels having their own individual function. The bottom panel for speed, the mid panel to prevent water from clinging to the hull in the bow and to slim the shape aft. The top panel, to deflect spray forward and streamline the air flows aft. The high freeboard was a necessity to implement the use of the boat. A keel thus becomes a must, to control a craft with a whole lot of windage.  

POWER. The problem of a small boat is that given the short length, in parallel with a modest power plant for fuel efficiency requires a Speed/ length ratio of well over 2, to achieve the 10 knots requirement, with power in reserve. That is the question.To solve the equation, we will have to minimize the weight with the available power burning less than 2 gallons per hour. We know that the total weight with engine shall not exceed 3,500lbs as per specification. We are just under that number, which represents the weight of the vessel, fully laden, but without the burden of 800 lbs. representing a family of 4.The weight estimate, is careful enough to be in agreement with the total sum. But, for our true goal of performance and speed, the displacement of the craft will be increased to 4,265Lbs, to include the crew weight.

Now, for this boat, a Speed/Length ratio of over 2.0 is necessary. This requires a planning hull. A hard chine-planning hull with a prismatic coefficient of .70 delivers the best result in the speed region necessary to get over the line. The bow remains sharp as you might expect, but the widest part of the waterline is at midway, which can be a surprise. This has the advantage of creating a pre-planning loaded surface ahead of its time, to promote lift. At lower range speed, the angle of incidence can be a little higher because of it. The maximum waterline beam will also give more stability to the boat. 

The sections are kept at a 12-degree angle from the mid-planning area all the way aft. Hull depth is also kept at as a constant, therefore the chines lines keep on getting narrower, for a slimmer exit. Attention is paid in keeping the center of buoyancy well aft. This shape treatment will give better sea keeping ability at low and high speed.

Now, for calculation of the power required with speed and fuel efficiency in mind.

The Hydrostatics gives a High Hull Speed figure of 7.323 knots with only 4.208 for Brake Hose Power. B-H-P is the true power delivered to the water with no losses that might occur in the engine transmission, or due to propeller efficiency. You can expect to double the H-P figure to get to the true number with this formula. Furthermore, in the planning mode that we need to get to the speed of ten knots minimum, and the added crew weight, the boat will even require considerably more power. 

To transpose all this to the choice of a power plant, I chose a Diesel engine. I this case, and for a first approach, a lower range engine, new model Yanmar of 29H-P. Rated at 3600rpm. We only want to use it at, maybe 65% of its rate on a continuous basis. Or 2340 RPM and 18.85 HP; the available shaft horsepower.  

Based on the weight of 4,265Lbs, and HP of 18.85, the pound per horsepower is 226.lb/hp. This number will give a Speed/ Length ratio of approximate 1.65. Short of the 2.05 needed to get to the 10 knots mandated. Therefore, the engine is too small for our purpose.

The next engine, the 39Hp from the same manufacturer, has a maximum output of 3400rpm. Reduced to 65%, the output is now 25.35H-P. The pound per horsepower is now down to 168. Approaching the 2.0 S/L ratio, but still not enough.

Of course, by pushing the pedal and looking closely at the Performance curves given with the engines, it actually looks like the 39HP engine can do the work. Why, because the efficient running of that engine is around 2700 rpm, at 79.4% of the horsepower, or 31HP. The LBS/HP(137.5) chart, indicates a Speed length ratio of 2.1;  just over the number we are looking for. I have to add, that the curve at the rate of 2700rpm indicates a gallon/hp.hour of 1.98, just short of 2 gallons. The torque is also at its maximum. So, we have obtained the goal of maintening a 10 knots cruising speed while carrying the family of four. Albeit, with the little reserve of a 1/4 of a knot.

To see what happen next, lets try the big 53Hp 4JH5. With the correction of adding 172lbs to the displacement, weight of the boat.

With a reduced output of 34.45HP at 65% of rpm. the Pound per Horsepower is 129, to deliver a speed/length ration over2.1, to 2.15.  The latter represents the cost of adding power without being able to lower displacement or increase the length of the vessel. 

Again, looking closely at the manufacturer curves, the seemingly ideal output, torque and specific fuel consumption measured at the propeller shaft is as follows: 2800rpm (42.4HP), for a maximum torque and a consumption of 1.75 G/hp.h. With this input, the speed length ratio is now2.2. Or, 10.3/4 knots. The later based  

The above represents the difficulty associated with finding the properly sized power plant. Without mentioning the right size propeller and transmission. 

CONSTRUCTION PLAN. Given the possibility of designing in any material, I choose to explore several options. The hull lines are developable and therefore offer the advantage of sheeting large areas with panels made of different nature. The first one is for an all-wooden construction. 

Water Bee has frames, floor timbers, keel, stem and stringers planked with plywood. Using this medium, bottom and topsides are 5/8" thick. Deck is 1/4'. Fiberglass covered with a 6 and 4 oz/yd finishing cloth. The drawing shows scantlings of the main members. The weight study is partially based on this model.

The second method is to use the above system, as an integral mold for a sturdy skin of fiberglass.  Applied to hull and deck, the plywood thickness can be reduced to 3/8" for the bottom, 1/4" for topsides and deck. Fiberglass construction calls for a mold to be built in which to shape the boat. Considering the need for the hull to be made stiff enough to take the installation of motor, propeller struts, etc.This might not be a bad method of building.

The third: all fiberglass. I have had no time to develop the construction aspect for this medium. But, I have no doubt that a lighter boat can be built. Especially if using a foam core building method.

The fourth: aluminum. Well she will be strong! But again, I have had no time to seriously calculate it all. But it is possible of course. Most of my work is in aluminum these days, and I like to work with metal.

ACCOMMODATION PLAN. This an efficient inboard cruiser for family use despite her rather short size. Even if the definition is from Sparta, we have to pay attention to give decent accommodation. We can make it more comfortable by extending the floor plan, making the cockpit literally part of the interior. Sleeps two in wide, comfortable berths in main cabin; sleeps two in seat and bridge deck berths - in the cockpit cabin. Has full headroom in cabin and full headroom in cockpit with the top up. Has, down below a galley with ice box and stove, sink and lockers. An enclosed Head.

A Bimini with zippered sides, which folds up from their permanent mounting, covers the cockpit area. It takes, maybe ten minutes to convert daytime into a nighttime boat. The folding table, 36" X 36" is surrounded by comfortable seats with mattress cushion, for four to six for dining. 

COST. The crux of the matter. This is a catch 22 question.To get estimated cost, from a Boat Builder, you have to have on hand some drawings, even if preliminaries. But, at least,  a General Arrangement Plan and a Midship Section Plan showing the Construction method, Material and Scantlings. Without the designs, there is no point. Further more, trying to get answers, even in these trouble times is not easy. Instead, I chose to base the rough estimate on previous similar work, with an inflation figure plugged in. 

Hull. Deck and cabin top: $4,743.40. No labor is included, for I expect this boat to be attractive to the Amateur building His or Her own boat.

Inboard Motor. Plus Battery propeller shaft and accessories.  Based on a quick call to my favorite engine representative, the approximate retail quote for the 29HP Yanmar is $10,500; 12,500 for the 39hp; and $14,500.00 for the 53hp. Installation: $4,000.00 to $6,000.00 from one engine to the next.

Equipment. Fittings. Head. Sink. Pumps etc. $3,025.03. Again, no labor is accounted for.

But, if I were to give a total wild guess, I think that $67.500 will get you on this boat.

Now, I remember a few years ago when I submitted drawings for a cost estimate on a similar size boat. The estimate went from a high of $85,000.00 down to an actual expenditure of $35,000.00. It is hard to get the right answer.

CONCLUSION. WaterBee 24 is an unusual boat, if not totally unique in all of her aspects. She is light enough to be carried on a trailer and can be launched at most vacation spot. The diesel engine, safe and efficient will motor at reasonable speedwith reserve, for a low consumption below 2 gallons of fuel per hour. The construction is stout and a choice can be made between all wood; wood and fiberglass, fiberglass; even aluminum.