Propped right?

[RANDY HASSELL]

I am running 370 Yanmars (6LY-STP)

With the boat about 1/2 loaded at the end of the season my WOT rpms were 3375 and 3390 with Aetna digital tachs.

I have the props off now and getting them tuned up.  Should I be looking to get those numbers about 100 or so higher if possible or should I be happy with where I’m at?  There’s no smoke from the exhaust, but I do get a little soot on the transom after a day offshore.  I have a spec sheet from Yanmar that shows 3425-3525 as the WOT rpm goal.  Valves were adjusted just before the haul out at 1700 hours this year.  Not sure if that changes anything.

Any help is appreciated.

Topic

[Rob Schepis]

A #1 cup is the lightest measurable cup.  I doubt removing that would get you any real rpms back.  In my experience some cup is important for prop efficiency and this depends on the prop specs and the boat/hull characteristics.  I don’t think you are going to be happy with the change but a small cup change like that should have been inexpensive.

[RANDY HASSELL]

So the prop shop reconditioned my propellers.

My desire was to get them to turn about another 100 rpm.

The props had a #1 cup which they removed in order to accomplish that goal.

My question is:  Is there any other benefit or detriment to removing the cup and was that the right move?

 

[RANDY HASSELL]

Thanks for the info.

[Rob Schepis]

Randy, your questions/concerns were exactly why Tony put this article together quite some time ago: https://www.sbmar.com/articles/engine-life-vs-engine-loading/

[RANDY HASSELL]

more questions…

I have an info spec sheet for my engines from Yanmar that shows max output of 3300 rpm and suggested wot rpm for “prop matching” from between 3425-3525.  What’s the reason for getting them to turn over 3300 if that is where the max hp ends? 

Should the correct cruise rpm be about 90% of 3300 or the highest achievable numbers?   

and should the cruise rpm change as the max numbers change later in the season due to water temp, fouled bottom, loaded weight etc?

[RANDY HASSELL]

Those numbers were 3/4 fuel, and just the normal stuff we carry on board but not fully loaded for fishing.

It’s an older heavy boat (1989 Sport-Craft 360 Express, 25k loaded).   I’m getting about a 21-22 knot cruise at 2900 rpm and can hit about 26-27 knots wide open at 3375.

[Rob Schepis]

If she’s an offshore runner subjected to various loading and sea conditions I’d want to get the WOT rpm’s up a bit, especially if you already have props in hand.  Your end of season numbers were with what kind of fuel, persons and misc. gear loads?

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