[PSUBS-MAILIST] landing craft

[Sean T. Stevenson via Personal_Submersibles]

Are you sure that it's cavitation (spontaneous vapourization and subsequent bubble collapse on the low pressure face of the propellor blades), and not ventilation (surface air being sucked into the upstream flow ahead of the propellor)?

Cavitation can be addressed by changing propellors - go to a lower pitch, or greater number of blades (increased disc area). If possible, fit a larger diameter prop and operate at lower rpm. Also check that the upstream flow path is not restricted in any way.

Ventilation can be addressed by adding an anti-ventilation plate on the skeg, increasing the immersion depth of the prop, and checking for any adverse trim issues.

Sean


On June 14, 2017 7:10:32 PM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Hi All,Got a problem to solve, my sub barge has a 30 hp outboard motor.
> When I try to cruise fast it cavitates real bad.  When I push
>something that won't move like a dock, I can push at full throttle no
>problem.  I am planning to build a landing craft 36 feet long using the
>steel hull from my paddle wheeler.  I am building it to carry and
>launch Elementary.  My concern is, because it is a displacement hull
>and it will be slow,  will I have the same problem with cavitation
> that the  barge has.  Do I need to use a jet instead?   I can't work
>on it for a year, but I want to  source all the parts, engine, drive
>etc etc.Hank
>
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