The original scheme was to bring the apex of the windscreen frame forward to the level of the back of the hatch, and to swing its side frames back to the level of the dash bulkhead, leaving a dashboard behind the screen sitting on top of the bulkhead. That is not only ugly, but it leaves no room to attach the spotlight, which traditionally went between the windscreen and the hatch.

To overcome this difficulty I propose to bring the apex of the screen back behind the dash, as seen above, and to flare the screen frame even further back again. Therefore, in order to support the screen frame, the deck stringers were extended back from the dash bulkhead into the cockpit, and they were cut at appropriate angles on their ends to accommodate both the rake of the frame, and its athwartships angle. Those compound angles were easiest to cut off the boat, so the angles had to be decided  before any further deck work was commenced.

From looking at photographs of traditional slipper launches it appears that the rake of the windscreen is about 25°, and the bottom frames appear to flare back about 300 mm. from the central post by the time they reach the coaming. With that in mind I was able to trim the foredeck stringers to their approximate length and angle, but they now need to be cut precisely.

To allow for the spotlight in front of the screen the forward apex of the windscreen has been set back 100 mm. from the dash bulkhead. The design called for a single central stringer between the back of the hatch and the bulkhead, but in order to accommodate the spotlight I have replaced that with two stringers spaced slightly apart. They join the deck half beam at the rear of the hatch, and provide a more bulky fixation point for the central post of the windscreen.

If the junction of the bottom frame and the central post of the screen is to be a right angle in the plane of the screen, which looks right, then the two members can be represented by an "L" where the upright is the central post, and the horizontal is the bottom frame. The meeting point of the L is the point where the front of the central post emerges through the deck, and the end of the horizontal bar is the point where the bottom frame meets the carling, viewed from the side.

In order for it to actually meet the carling, which is at a lower horizontal level than the midline of the dash bulkhead, the entire L structure has to tilt backwards (ie. rake). The rake angle depends on the point on the carling at which you want the frame to meet it. The further sternwards you want it to meet, the greater the drop from the midline, and the greater the rake necessary.

Arbitrarily choosing a point on the carling 190 mm. to the stern of the apex of the windscreen results in a vertical drop of 75 mm. from the midline. The rake angle for that works out to sin ¯¹ (75÷190) , or 23.25º. If you want a steeper rake you will have to bring the bottom frame more sternwards to a lower point on the carling, or, if you want a less flared screen (which has a greater angle between its two halves), you will have to decrease the rake.

At the 190 mm. point the inner sides of the carlings are located 1340 mm. apart from each other. That means that the angle between the two halves of the frame is 148º in the bird's eye view, which equates to a real angle of 146º. Therefore, the central post can begin to be shaped by having cuts at 17º made on its front surface, to give an apex in the midline, 17 being the reciprocal of half of 146 (ie. 90 - 73). Secondary cuts up the side of the post, at right angles to the front surfaces give the mating edges for the bottom frames, allowing the bottom frames to be cut square across their width and thickness for the joint.

Of course, all these angles are fine in theory, but there is no point in being proven wrong with a piece of valuable mahogany, so I intend to make a mock up of the windscreen frame before committing to the hardwood.

With both bottom frames glued on to the central post it became apparent that the trigonometry was perfect. The structure sat exactly where it was predicted to...a rare victory for theory in woodwork!

Next, the foredeck stringers whose ends protrude into the cockpit were to be cut to the appropriate angles to allow for attachment to the bottom frames. These angles, and those to be cut in the actual windscreen timbers will be taken off the mock-up. However, the task of lining up those cuts on timbers which are not in the same plane (because of the camber of the deck) proved to be very troublesome. The central two were done to house the central pillar of the screen frame, but the other stringers were cut off short of the bottom frame, and two precut stubs were slid along the cut ends to butt up against the frame.

The batteries should be housed as near to the boat's centre of buoyancy as possible to minimise rocking horse movements and increase comfort, especially for the rear seat passengers. The original plan for the 20' version of this boat has a centre of buoyancy right in the middle of the cockpit, which is ideal. That is before the engine is added, and before the alterations to the windscreen and passenger compartment with the increase to the 21'9" version. Nevertheless, the situation has not changed dramatically, and it seems likely that the ride would still be comfortable.


However, with the replacement of the engine by the lighter electric motor (106 Kg. for a Yanmar 2GM20 which was specified, becomes a 12 Kg. electric), and the dispensation of necessity for a fuel tank, the centre of buoyancy moves back quite a way. Batteries, weighing in at around 30 Kg. per 100 AH can be used to move it forwards again, but there are six of them, resulting in the equivalent of a large extra body sitting forward in the boat. Whether this has a detrimental effect on handling I will not know until the boat is launched, but in case it does I am going to allow for moving the batteries around to achieve the best trim.


Currently, I am planning to house them in the motor compartment on specially constructed racks, just forward of the dash bulkhead; but, if necessary, I want to be able to bring some back into the cockpit area. They will then need to be housed under the driver's seat, which requires the passage of cables back into the motor compartment under the sole, and, therefore, under the waterline. That, in turn, requires some plumbing of waterproof duct from the cockpit to the motor compartment.

The best route for that duct will be to pass lateral to the central bilge compartment to stay out of the way of the shaft. It can be attached to the inside of the outer the longitudinal floor, and come up through the sole directly underneath the seat. Forward of that it will pass through the dash bulkhead underneath the sole, and emerge into the motor compartment between the batteries. The batteries themselves have to be as low down in the boat as possible because of their weight, so their bodies are actually below the waterline, although their terminals are above it. The duct will pass up above the level of the terminals, and both ends will, therefore, be clear of the line.

The batteries I have in mind are AGM deep cycle marine12 volt batteries, series connected in pairs to 24 volts, three pairs in all. That will give me the option of running the motor at 24 volts or 36 with reconfiguration , should the 24 prove to be underpowered. I need about 100 AH for an hour's cruising, and six small 100 AH batteries is preferable to three 200AH ones because of their weight. A 100 AH battery weighs about 30 Kg. and is manageable for getting into and out of the motor compartment, whereas the 200 AH ones are too heavy.

The dimensions of a 100 AH battery are about 330 mm. x 170 mm., so only four will be able to fit side by side across the width of the dash bulkhead, but there will be room for the cable conduit to pass up between them. AGM batteries do not need a battery box, because they cannot spill acid, but  racks onto which they can be strapped for stability is a good idea. The obvious place to put the racks is between the motor mounts and the hull on either side.


The remaining two batteries will then go either in the cockpit or forward of the first four in the motor compartment.

Slightly oversize battery trays are built, to accommodate larger batteries in the future, and they are screwed down onto the racks, leaving just enough clearance for the conduit to pass between the batteries into the driver's compartment.



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Every compartment of the boat should be either available at least to air flow, if not to direct manual access, or be sealed off entirely.  It is not practical to make the forward and aftmost compartments directly accessible. The forward hatch opens into the motor compartment (between bulkheads B and C), and will make the fuel compartment (between bulkheads A and B fairly easy to reach, but the space forward of bulkhead A, which is divided into two halves by the stem girder will not be. One option is to fill any totally enclosed areas with foam, acting as buoyancy compartments, but the foam would have to be one which would not suck in water like a sponge. Another is to vent these areas, in which case holes in the bulkhead will have to be made.

In the aft end of the boat, the space between bulkhead F and the inner transom frame will be unavailable, unless there is to be a rear hatch opening into the rudder compartment (between bulkheads E and F). Furthermore, the arc between the inner transom frame and the transom proper in totally inaccessible. Foam is probably the only solution here.

One means of aiding airflow is to place cowl vents on the deck, and this is certainly a solution used on traditional slipper launches, especially over the engine compartment. Many also feature a rear facing vent over the aft deck, although this may not appeal to any boat owners who anticipate the occasional breach of water over the transom from passing water skiers or the like.





Another help is a blower located in the compartments to hasten air exchange between inside and out. In the S-F plans a single blower is specified to be attached to bulkhead B, to ventilate the engine compartment. That requires that air be sucked in to the fuel compartment from somewhere, and blown back out through the engine compartment, a mechanism which would be greatly assisted by an outside vent opening into either the fuel compartment or the area forward of bulkhead A, which would be in communication with the fuel compartment. However, I have not been able to find any evidence of cowls in these locations on pictures of traditional slipper launches, so I presume that air circulation is sufficient. It is not so much of a problem with the electric motor anyway, as there are not fumes to disperse.

The area of bulkhead B which is away from the load bearing parts near the engine bearers and girder fillets is dispensable, and can be cut away to revel the fuel compartment.


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The motor has been bolted to its baffle in sliding grooves, so that the drive belt can be removed easily without having to loosen the sprockets. The thrust bearing is also bolted to the baffle, using hex head bolts with as shallow a head as possible, so as not to interfere with the belt or sprocket on the shaft. Even so, the sprockets both have to be fitted a little proud of their shafts to give adequate clearance.


The baffle is dropped into grooves cut into the inner surfaces of the motor bearers, and then sits on top of the bearers. It is reinforced in a forward/aft direction by blocks and hinges screwed and glued onto both the bearers and the baffle.

The wooden block behind the baffle also acts as a shim to raise the baffle from the level of the motor bearers by 9 mm. This was necessary to realign the propeller shaft with the centre of its hole in the dash bulkhead. (The thrust bearing is not fitted with slots for its bolts, so cannot be adjusted).