referring to the drawings , in fig1 is seen a typical open cockpit water craft of the kind commonly seen in recreational use , both in inland waterways and along the coasts , in the united states and elsewhere . although the drawings herein represent the invention to be utilized on an open cockpit type of craft , the principle is equally applicable to large and small craft alike , and whether or not such craft are equipped with closed or open cockpits ; the principle being that any boat ( with the possible exception of some very large or very specialized craft equipped with highly sophisticated piloting and / or navigational gear ) presents a hazardous potential when the pilot is not properly situated at the station designed for him . the safe operation of small craft normally calls for the pilot to be seated , except at very low vessel speeds . in some larger vessels piloting may be proper from a standing position , but nonetheless the pilot should not leave the piloting station except under very limited circumstances , such as slow vessel speed in calm open water , or the like . although i describe my invention in the setting of small craft where safety would dictate seated operation , the invention can obviously be extended for application to other types of craft where standing vessel operation would be permissable . in such application , instead of the body pressure sensing apparatus being situated in the seat portion of the pilot &# 39 ; s station , it could be situated for example in the floor portion so that it would detect the presence or absence of a standing boat operator . thus it would not mandate seated operation , but would sense a standing pilot &# 39 ; s body weight and function accordingly , to prevent the pilot from carelessly walking about the boat while it is under way . referring again to fig1 small craft 11 is shown driven by motor 12 contained in engine compartment 13 , and controlled from within cockpit area 14 in which can be seen steering wheel 15 and throttle lever 17 . the proper position for the operator of the boat is seated in the driver &# 39 ; s or pilot &# 39 ; s seat 19 , upon which is disposed a seat cushion 21 . contained within seat cushion 21 is seat pressure responsive mechanism , more particularly described hereinafter , from which electrical wires 23 lead to the ignition circuit of the boat engine . it should be appreciated that although an inboard situated engine is shown , the principles of the present invention are equally applicable to water craft with outboard and other types of power plants . in fig2 can be appreciated the seat support structure or surface 19 , underneath the broken away seat cushion 21 , upon which is supported an air bladder structure 27 , sometimes hereinafter referred to as the seat pressure bladder . seat pressure bladder 27 is formed of normal air bladder material , such as rubber or other durable , flexible , airtight composition . the particular bladder 27 shown in fig2 can be not unlike the structure of an ordinary hot water bottle , forming a substantially rectangular platform with relatively thin cross - section ( so as to be conveniently encased within a boat seat cushion ), and having an outlet 33 at one end from which an air exit pipe 34 leads to an air switch 35 via coupler 37 . the air switch 35 is better appreciated by reference to fig9 which shows it to be comprised of a pair of normally opened electrical contacts 36 , situated between switch bladder 41 and switch compartment 38 . the switch contacts are held normally open by any convenient means , such as spring - loading or by the inherent resilience of the contact arms 39 . the air switch bladder 41 is smaller in size than the larger seat pressure bladder 27 , and it is normally formed of a thinner membrane , so that it is sensitive to small pressure changes and inflates or deflates , accordingly . the structure of the air switch bladder relating to its size , shape , and membrane structure , can be appreciated by likening it unto the structure of an ordinary balloon , in a substantially deflated configuration . the switch contact arms 39 and their electrical contact elements 36 being situated between the bladder 41 and the air switch compartment 38 , a small inflation of the bladder 41 to alternate position 40 compresses the contact arms 39 together to alternate position 37 , whereat the contacts 36 close , thereby completing the circuit in which they are contained , via connecting wires 23a and 23b . this circuit is interposed in series with the engine ignition circuit so that engine ignition is enabled , until the boat driver , voluntarily or otherwise , is displaced from his normal seated orientation , whereupon his body pressure on the seat bladder 27 is relieved , thereby decompressing seat bladder 27 and diminishing the pressure in switch bladder 41 . the switch arms 39 , then separate to kill the engine ignition and stop the vessel &# 39 ; s forward motion . the contacts 36 must be designed so that they close against normal bladder system pressure , i . e ., when the seat bladder is unloaded . the &# 34 ; unloaded &# 34 ; or &# 34 ; no - load &# 34 ; pressure may be determined by whatever seat weight is considered sufficient to indicate the presence of a competent pilot for the water craft . some weight in the neighborhood of 50 to 75 pounds would be preferred . it is important not to design the contact arms 39 so as to be separated only by an absolute no - load condition , for in such a design minimal seat pressures such as those exerted by a package or other object randomly placed on the driver &# 39 ; s seat would prevent the contacts from opening in the event of operator seat evacuation ( by the driver going overboard , for example ), and would thereby defeat the primary function of the safety system . also , such a no - load design would facilitate undesirable overriding of the safety system by irresponsible boat operators , since they could accomplish this by simply placing a relatively small , light object on the driver &# 39 ; s seat while they continued to operate the boat in careless fashion from an improperly seated position . on the other hand , the load point for actuation of the safety system should be designed so as not to require an excessive weight to close the contacts , otherwise only persons possessed of a very substantial body mass would be able to exert the quantum of seat force prerequisite to operation of the water craft . an additional factor to be considered , of course , is that the seat force exerted by a human body is not exactly the same as its full body weight , inasmuch as some of the body weight is supported at the feet of the person operating the boat , and another portion of his weight may be absorbed by some boat structure other than that of the seat ( depending upon how much the operator may lean on the cockpit side , the seat back , the steering wheel , or other portions of the boat structure ). in operation , therefore , the system is actuated by the presence or absence of a substantial downward force on the driver &# 39 ; s seat cushion 21 to compress the seat bladder 27 . decompression of bladder 27 is caused by operator seat evacuation e . g ., the driver is thrown from the seat through an encounter of the craft with irregular water surface or with other objects in the water , or even by running aground , or the driver suffers a disabling seizure , or falls overboard , or perhaps he voluntarily evacuates his station in a negligent manner . in any such event , the seat evacuation will remove the seat force pressure on the cushion 21 and also the underlying seat bladder 27 , which is pneumatically connected to the air switch 35 , and break the marine ignition circuit through connections 23a and 23b . it should be appreciated that the components of a marine safety system such as the one described herein , will be continuously exposed to the deteriorating effects of water , either salt or fresh . thus the electrical portions should be properly conduited or otherwise protected by such means as are well known to the electrical marine art . as for the mechanical and electromechanical elements , i have shown particular structures which achieve this protection : the seat bladder is made of a water bottle , or water bottle - like construction which by its very nature is impermeable to aqueous deterioration . moreover , the kill switch is protected from water incursion by means such as are seen in fig3 ( discussed in more detail hereinafter ) where the air switch 35 &# 39 ; is enclosed in a plastic waterproof compartment 38 , into which waterproof wires 23a and 23b , and waterproof air hose 45 , are heat sealed or otherwise water - tight imbedded so as to preserve the waterproof integrity of the switch compartment . it should be noted that as a convenient structural unity and as augmentation of the waterproofing effect i have heretofore described , the air switch 35 may be encased within the water protective seat cushion 21 as is seen in fig2 . notwithstanding the desirability of the operator remaining properly seated while the boat is in motion , it is essential in certain situations at low speeds , that a boat be navigable from an unseated position . this is normally required when docking or the like , where it is necessary for the pilot momentarily to stand or otherwise displace himself from the pilot &# 39 ; s seat in order to better view the dock structure , or surrounding docked or moored boats , and other such navigational obstacles and hazards . however , since docking takes place at very slow speeds it is not hazardous for the pilot so to operate . in order to enable such non - seated operation it is necessary that some override to the system be provided . such an override is easily introduced by electrical circuitry bypassing the engine interrupt system , and such an electrical bypass means 42 is shown in fig2 connected in parallel to the engine ignition interrupt circuit , across wires 23a and 23b . when the bypass element 42 is actuated , the engine ignition circuit is completed through it so that the engine continues to function independently of the status of the interrupt system , and irrespective of whether the boat operator is seated or not . the structure and function of the override system will be described in more detail hereinafter . it should be apparent that a broad range of substantially equivalent alternative structures can be utilized to result in the same system , only one embodiment of which i have disclosed here . for example , instead of interrupting engine ignition by completing the safety circuit , the same result can be accomplished by the opposite process of opening the safety circuit . the circuit opening concept is expedient for application in connection with engine ignition systems which are interrupted by breaking the engine circuit by an interrupting switch connected in series with it , such as was explained in the description of fig2 . however , some engine ignition circuits are interrupted not breaking the ignition circuit but rather by completion of a separate &# 34 ; grounding &# 34 ; circuit to kill the engine . in such a situation a different type of air switch might be utilized , in which deflation of the air switch bladder would cause closing rather than opening of the air switch , for grounding of the engine ignition . such an air switch 35 &# 39 ; can be appreciated by reference to fig3 in which the air switch bladder 41 &# 39 ;, contained in air switch casing or compartment 38 &# 39 ; is pneumatically connected to the air seat bladder 27 by air hose 45 , through which air pressure is conducted between the seat and switch bladders . thus in a system utilizing this type of kill switch 35 &# 39 ;, evacuation of the driver &# 39 ; s seat would cause decompression of bladder 37 &# 39 ; and concomitant closing of the switch arms 39 &# 39 ; until the circuit of wires 23a and 23b is closed by contacts 36 &# 39 ;. other alternative configurations may be used instead of the rectangular shape seat pressure bladder 27 , as may be appreciated by reference to fig4 a and 4b which respectively show an elongate serpentine tube configuration of seat pressure bladder 47 , and a manifold branched configuration of bladder 49 , leading to the integrated pneumatic kill switch 35 &# 39 ;. moreover , the pneumatic kill switch 35 may be alternatively , and in some cases preferably , enclosed within the waterproof seat cushion , as is shown in fig2 with the kill switch 35 imbedded in the seat foam 19 , and its lead wires 23a and 23b exiting therefrom at 40 . it should further be appreciated that the kill switch 35 or 35 &# 39 ; may be constituted not necessarily of the specific structure heretofore descirbed , as containing a separate air bladder , etc ., but rather any known pneumatic switch can be substituted componentially for the specific switch structures shown , it being merely necessary to pneumatically connect such a component to the basic seat bladder 27 . it is sometimes found undesirable if the motor boat engine is interrupted merely on account of the boat encountering ripply water , or merely due to momentary seat liftoff of the operator for the purpose of changing his position . such temporary , nondangerous situations cause annoyance if they cause shortduraton seat lift - offs which in turn cause frequent engine interrupt . therefore , to avoid engine interrupt or momentary seat lift - offs , i propose the addition of a device to introduce a delay between the time of seat lift - off and engine interrupt , so that short duration lift - offs cause no interrupt at all . one method to introduce time delay is to introduce a constriction 50 ( see fig1 ) in the air passage 34 between seat bladder and air switch . the cross - sectional dimension at the constriction would control the duration of the delay , and a maximum time delay of two seconds is recommended . the constriction is not essential if instead a pneumatic kill switch is utilized of the type ( which are available ) having a built - in ( selectable ) time delay . returning now to the override system , reference is directed to fig5 a and 5b , in which is seen a mercury switch 67 mounted on a portion of the throttle linkage , which moves mechanically in conjunction with motion of the throttle lever 17 . the mercury switch is of the type that includes a floating globule of mercury ( not shown ) which moves to alternate closed or open switch positions , in response to tilting of the switch structure . thus in the figure of 5a with the throttle 17 in a forward or high engine speed position , the mercury switch 67 is tilted to a more nearly horizontal position where it closes and consequently has no effect on the cutout circuit through lead wire 53 by which it is placed in parallel with the engine cutout circuitry . fig5 b , however , shows the throttle lever in an alternative position 17 ° corresponding to low engine speed , where mercury switch 67 &# 39 ; is shown in a corresponding position tilted substantially away from the horizontal , completing circuit 53 and bypassing the engine interrupt circuit , thereby overriding the marine safety system . as previously explained , this permits of boat piloting without necessarily being seated on the operator &# 39 ; s seat cushion , for low - speed maneuvering , docking , and the like . another override switch means can be appreciated by reference to fig6 a and 6b in which , instead of a mercury switch , the safety system is overridden by means of a microswitch 68 operated by direct mechanical contact of the throttle lever 17 , which when displaced to a low - speed position 17 &# 39 ; depresses switch plunger 69 to switch closing position 69 &# 39 ; for electrical bypass of the safety circuit . still another override apparatus can be provided in the form of a tachometer circuit , which senses the rotational speed of the marine engine , say in revolutions per minute , and is provided with limit switch means so as to complete a bypassing circuit at any engine speed from 0 r . p . m . to a predetermined , relatively low number of r . p . m . designated &# 34 ; b &# 34 ; in the schematic diagram of fig7 . but at a higher engine r . p . m ., as for example at all speeds between those indicated by the alternate indicator arrow positions &# 34 ; a &# 34 ; and &# 34 ; b &# 34 ; in the diagram , the override circuit would be in the closed range so as to keep the safety circuit enabled . the schematic diagram includes a source 77 of electromotive force ( voltage ), a fuse , and a neon indicator light to show the status of the safety circuitry , as &# 34 ; on &# 34 ; or &# 34 ; off &# 34 ;. a dangerous situation already referred to above in the summary of this invention , is the hazardous operation of boats which do not , by their design , impose seated operation upon the pilot . some boats , particularly larger pleasure craft and commercial vessels , may accommodate standing piloting , such as shown in fig8 . my system can be easily adapted to function in this setting , as by substituting a foot pad or cushion 71 for the seat cushion 21 of fig1 to enclose the body pressure responsive bladder for engine interrupt should the standing operator 74 ( shown in phantom line ) be thrown from his piloting post on the deck 73 of the flying bridge 75 , or should he voluntarily evacuate that position while the boat is under way at more than minimal speed . another aspect of my invention is that it is not necessarily limited to application with electrical ignition systems ; it could be applied , for example , to a diesel engine craft , where the air switch of my invention could be employed to kill the diesel engine by solenoid or relay interconnect to the diesel engine kill switch , if it be mechanical ; or by direct connection where the engine interrupt is electrically actuated . the alternative structures above have been described for the purpose merely of showing some of many possible modifications , adaptations , substitutions , and equivalent embodiments which would be obvious to anyone skilled in the related arts , and it is my intention that all of such modifications , adaptions , substitutions , and equivalents are to be included within the spirit of the invention as claimed hereinbelow .