Suddenly "revving up" when in water -Does your Amphi do this?

Jon March

When driving in water alone: should you be able to go above 65% throttle?.... without getting sudden "over revving"/slippage from prop-cavitation? I cant use more than 60% speed or so, unless i have someone in the passenger seat.

How about you - can you do this in yours, without your motor revving up suddenly?....and how might i fix this - add prop shield/ water flow-director on passenger side??
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I can only use about half throttle unless I have four passengers. The rear seat passengers make the difference. So throwing ballast on or under the back seat would probably help, maybe 300-400 lbs. Then no mods have to be made to the car. I think the flow of water to the props is the problem, and that the car sits a little high in the back when not fully loaded, so the props aren't deep enough. I've also wondered if a different prop pitch or design would help? All part of the compromises of a boat car.


Active Member
It's an Amphicar, not a speed boat. Do not trust the first 7 in 770.
My Amphicar with prop shields does not go over 4.8 knots / 5.5 mph. More gas only increases fuel consumption.

Excursus on hydrodynamics: With displacement hull boats, there is a hull speed that can not be exceeded by more power. The shorter the boat, the lower the hull speed.

Jon March

Peter - i know, and im fine with that. But when i was with other Amphis and could not keep up and the motor kept randomly speeding up without me doing it, it concerned me why mine was dong that

Does yours cavitate when you are driving alone?


Active Member
Ok Jon, i forgot about the difference between european and american Amphicar captains. Herb wrote about his irritation in the story of his trip to europe in "Wheels and Waves". We in europe try to prolong our trips on the water by driving slowly. Only our kiddies, when at the rudder, speed up.

I don't think that my propellers are drawing air, but there is a point where more gas does not increase rpms and way before not increases water speed. Cavitation is a problem caused by highly destroying vacuum bubbles.


Jon March

Ja, Peter.
I am not looking for speed - Im just wondering if everyone has this "driver only" cavitation.
(PS - im not a heavy person, so i am not "tilting" the car excessively to pull in air to the prop)

I may have to get a fellow to put a GoPro camera underwater to record what is happening.

Jon March

What a great "Cavitation / ventilation / prop slip" comment by Ed Cajon from 2003:

I do think that the flow near the props is way too turbulent, and I
suspect that the cause is the rear wheel and wheel well. I think that the wheel
and wheel well allow the prop to "suck air", causing the prop to be working in a
spongy mix of air and water. Since air is compressible, some power is wasted in
simply compressing air and blowing it aft into the turbulent wake.

I would love to see some underwater photos of an Amphi cruising by the
photographer. (Hint to you Florida amphibians; that would make a nice summer

>>> Maybe a shot of an Amphi going directly overhead. <<<

That would easily show what's going on with the flow (plus it would make for some really
nice pictures).

About a year ago, I had speculated that perhaps the propeller "shields" were
originally intended to improve the water flow past the props, by blocking any
wheel well induced turbulence. Comments from the group left me unconvinced about
the utility of the shields, although most felt that the shields were more of a
debris trap and increased the likelihood of propeller damage. Maybe we still
don't understand the real purpose of those barrier vanes.


Gold Subscriber
yes, cavitation does occasionally occur when alone, sitting on the seat back toward the door. it especially happens while rolling in waves from the side. just one more reason to always be giving someone a ride!


Amphicar Expert
When alone I sit in middle or on pas. side with foot on gas pedal.
When I had my LIGHT fiberglass car I would stand on rear(Where top would
be ,and steer by leaning from side to side. )


Jon March

you just hit an aha! Lightbulb for me, boss! Altho it happens in calm, it was indeed especially pronounced in chop- i thought a key had partially stripped in one of the large water trans output gears; grabbing ok at low rpms, but slipping at high rpms.

Which raises a question- thers no “independent”-slip in the water trans, correct?
In other words, when one prop loses water friction—-im not just hearing that one prop slip..,actuallty both props overspeed, right?
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Amphicar Forum Admin
Staff member
I have also found that my Amphi does not always keep up with some of the others. In my case I find that it tends to starts acting like a porpoise once it builds up speed and the nose wants to take a slight dive. Billy Sxy had a theory that the trailing edge of my Amphi was bent (perhaps from a rear end bump at some point in its life) and that the back was acting like a horizontal rudder causing her to nose down as after rushed past. Do not know if that is true or not. I stopped worrying about it and just cruise a bit slower in the water.


Active Member
When I was racing outboards many, many years ago, I would run into cavitation issues. We would cup the rear edge of the propeller blades to reduce the problem. It worked quite well but that was with bronze propellers. I doubt you could do anything like that with the nylon props that Amphi uses.

John Friese
67 White
67 Red
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