You can provide more thrust by providing more engines though. In files, see a "Pulse Projection" article by Len Stevens from the April 1996 issue of Model Boats magazine.
--- In email@example.com, "papypp44" <papypp44@...> wrote:
> Thrust augmenters are well known but...you cannot win everywhere. The thrust evolves with the mass flow and with the jet velocity. If you increase the mass flow (for instance with a ventury) the jet velocity will be less. It means that at low speed or at bollard pull you could increase the thrust, but the maximum attainable speed for the boat will be less. And the pop-pop jet velocity is already so slow...
> --- In firstname.lastname@example.org, "x022435b" <x022435b@> wrote:
> > hi this is my first message.i made a popop boat years ago from memory of one i got in a lucky dip when i was a kid .i was on you tube and sawn experiment to inccease the speed of popops by using multi tubes and engines.i then had a flashback to my young experiences with jetex modfl planes,there waswhat they called an augmenter tube to increase the thrust.it was a tube with a bell like end and the jetex motor out let was mounted with the otputend in thre center of the bellmouth .what happened was the thrust from the jetex drew air into the tube and added to the thrust.the point is i think you can do the same wiyh the popop egine and it would act like a ventury pipe i do not have the equipment to make this to try it but some of you may.let me know what you think of the idea