<html><head></head><body><p dir="ltr">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.</p>
<p dir="ltr">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.</p>
<p dir="ltr">Sounds like it might just be ventilating though.</p>
<p dir="ltr">Sean<br>
</p>
<br><br><div class="gmail_quote">On June 14, 2017 7:53:55 PM MDT, hank pronk via Personal_Submersibles <personal_submersibles@psubs.org> wrote:<blockquote class="gmail_quote" style="margin: 0pt 0pt 0pt 0.8ex; border-left: 1px solid rgb(204, 204, 204); padding-left: 1ex;">
<div style="color:#000; background-color:#fff; font-family:Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:13px"><div id="yui_3_16_0_1_1497490952929_10957"><span>Sean,</span></div><div dir="ltr" id="yui_3_16_0_1_1497490952929_10956"><span id="yui_3_16_0_1_1497490952929_11021">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.</span></div><div dir="ltr" id="yui_3_16_0_1_1497490952929_10956"><span>Hank</span></div> <div class="qtdSeparateBR"><br /><br /></div><div class="yahoo_quoted" style="display: block;"> <div style="font-family: Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 13px;"> <div style="font-family: HelveticaNeue, Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif; font-size: 16px;"> <div
dir="ltr"><font size="2" face="Arial"> On Wednesday, June 14, 2017 7:47 PM, Sean T. Stevenson via Personal_Submersibles <personal_submersibles@psubs.org> wrote:<br /></font></div> <br /><br /> <div class="y_msg_container"><div id="yiv6907338076"><div><div dir="ltr">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)?</div>
<div dir="ltr">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.</div>
<div dir="ltr">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.</div>
<div dir="ltr">Sean<br clear="none" />
</div>
<br clear="none" /><br clear="none" /><div class="yiv6907338076gmail_quote">On June 14, 2017 7:10:32 PM MDT, hank pronk via Personal_Submersibles <personal_submersibles@psubs.org> wrote:<blockquote class="yiv6907338076gmail_quote" style="margin:0pt 0pt 0pt 0.8ex;border-left:1px solid rgb(204, 204, 204);padding-left:1ex;">
<div class="yiv6907338076yqt1049892225" id="yiv6907338076yqt93352"><div style="color:#000;background-color:#fff;font-family:Helvetica Neue, Helvetica, Arial, Lucida Grande, sans-serif;font-size:13px;"><div id="yiv6907338076yui_3_16_0_1_1497488281118_2683">Hi All,</div><div id="yiv6907338076yui_3_16_0_1_1497488281118_2683">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.</div><div id="yiv6907338076yui_3_16_0_1_1497488281118_2683">Hank</div></div></div><div style="margin-bottom:1em;border-bottom:1px solid #000;"></div><pre class="yiv6907338076k9mail"></pre><hr /><br clear="none" />Personal_Submersibles mailing list<br clear="none" />Personal_Submersibles@psubs.org<br clear="none" /><a rel="nofollow" shape="rect" target="_blank" href="http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles">http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles</a><br clear="none" /></blockquote></div></div></div><div class="yqt1049892225" id="yqt63622">_______________________________________________<br clear="none" />Personal_Submersibles mailing list<br clear="none" /><a shape="rect" ymailto="mailto:Personal_Submersibles@psubs.org" href="mailto:Personal_Submersibles@psubs.org">Personal_Submersibles@psubs.org</a><br clear="none" /><a shape="rect" href="http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles"
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