[PSUBS-MAILIST] landing craft

[Sean T. Stevenson via Personal_Submersibles]

The water flow isn't the same though. In the bollard pull scenario (zero speed of advance), the upstream flow has zero speed and maximum pressure. Slip, and hence thrust, is maximized, and the flow through the propellor is parallel to its axis.

Once the boat gets moving, its own wake has an effect on the angle of flow through the propulsor. With a heavy displacement craft with a pronounced bow wave, as you approach hull speed you'll be sitting in a hole, and both the flow angle and prop immersion depth can be affected. You can also start ventilating the prop as the effective water level changes. Also, as you gain speed, the propellor now has some advance, and therefore less slip and less thrust. The prop is unloaded somewhat, and may rev at a higher rpm as a result, which can lead to cavitation if you have too little blade area to absorb the pressure difference.

Sounds like it might just be ventilating though.

Sean


On June 14, 2017 7:53:55 PM MDT, hank pronk via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>Sean,I only use the word cavitation loosly, since I have no clue.  The
>water flow is very unrestricted as it is a pontoon vessel.  It could be
>ventilation.  But why does it work perfect when it is pushing against
>an immovable object, same trim, same water flow.Hank 
>
>On Wednesday, June 14, 2017 7:47 PM, Sean T. Stevenson via
>Personal_Submersibles <personal_submersibles at psubs.org> wrote:
> 
>
>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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