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Prop help for my Southwind on steroids |
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MrMcD 
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Joined: January-28-2014 Location: Folsom, CA Status: Offline Points: 3610 |
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 Posted: May-17-2024 at 1:35pm |
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No problems with the prop logic, I struggle with the 1/4" clearance between the prop and hull, that just seems like a problem in the making? I would feel better if it was at least 1/2" clearance but I am no prop expert.
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DenDen
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Joined: July-20-2023 Location: USA Status: Offline Points: 189 |
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Posted: May-21-2024 at 11:29pm |
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It seems that I either need a 430 13 x 13 or a 474 13 x 12 5/8.
Both at .080 cup. What is the expected RPM difference between a 1 inch change in pitch? If it is as low as 200 RPMs, then these two props are basically the same. If it’s as high as 1000 RPMs, then the difference between these two props is 300 RPMs. My understanding is by changing the cup. You could turn either one of these props into the other. So my big question is why do they make so many props so close together? I am still confused, but getting close. Is it better to start with a big prop and trim it down or start with a little prop and build it up? |
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JoeinNY
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Joined: October-19-2005 Location: United States Status: Offline Points: 5697 |
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Posted: May-22-2024 at 5:26pm |
You would need to know your torque curve and where your current prop fell rpm wise to even make a guess at rpm’s per inch of pitch. Your understanding of cup is flawed at best and while it is possible to change cup or trim down the diameter of a cnc propeller it is not free and is now way to live. You want the fresh cnc goodness’s. I’ve got a boat that runs 5000 with a borrowed 431 and I bought it a fresh 475 hoping to get a few more rpm out of it - it’s 61 years old so I am hoping to give it a little easier time. - you’ve been given the best advice you are going to get knowing what you know, but a prop and go boating - or wait a few weeks and I’ll tell you the difference between the two props on a completely different boat with a completely different engine |
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DenDen
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Joined: July-20-2023 Location: USA Status: Offline Points: 189 |
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Posted: May-22-2024 at 6:08pm |
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Thanks for everyone’s help and advise it is much appreciated as usual.
I will post my findings and what Prop I decided to go with. Delta Prop is like half an hour away from me in Cleveland and will give me at least one free adjustment for what that’s worth. |
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63 Skier
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Joined: October-06-2006 Location: Concord, NH Status: Offline Points: 4248 |
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Posted: May-22-2024 at 9:48pm |
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What Joe was trying to say was, don't plan on any adjustments to the prop. Buy a CNC per the advice above and run it. By running it and then "adjusting" you'll end up with worse performance than by simply running a correctly sized prop. I found when I was first reading about modern props that the couple of rules I thought I knew about props were out the window. I think in the past people would do a bunch of trial and error, bring their prop in for more cup or to cut down the diameter as you are talking about, but they were adjusting a prop that was perhaps adequately designed but not precise, so maybe the adjustments would help performance because the base product wasn't all that great to start with.
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'63 American Skier - '98 Sport Nautique
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KENO
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Joined: June-06-2004 Location: United States Status: Offline Points: 10753 |
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Posted: June-03-2024 at 8:33am |
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I guess it's time for a little update on Den's prop story.
He wanted to drop his full throttle rpms down to around 4500, so after getting some input here and from various other places he got a new Acme 13x14 from Delta propeller on their recommendation (with one free rework if needed) and got some new baseline numbers you could say. Same tach and GPS as before, he had some new numbers 35 mph at 3000 rpm, 45 mph at 4000 rpm and 49 mph at 42 to 4300 rpm and plenty of acceleration He's sorta happy but he'd be happier at 4500 or maybe a little higher rpm. I "gently" reminded him that nobody here had recommended that prop saying it was too much prop and would do just what it did. I'm sure he'll update with his next step which will involve talking to Delta about picking up about 3 to 500 rpm. whether it's with a prop swap or a rework of what he has now Just to repeat it again, he wants to stay at or a little above 4500 rpm at full throttle. Myself, I think the answer was in the thread, but that's how you learn, get a bunch of input then take your best shot and go from there  In the meantime, I think he may be on vacation enjoying some cruisin' in his boat (or will be soon)
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fanofccfan
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Joined: December-13-2009 Location: North Bend NE Status: Offline Points: 1744 |
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Posted: June-03-2024 at 8:46am |
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Thanks for the update Ken! Seems like a speedy boat
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2004 196 LE Ski 1969 Marauder 19 1978 Ski
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KENO
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Joined: June-06-2004 Location: United States Status: Offline Points: 10753 |
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Posted: June-03-2024 at 8:51am |
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Seems it, I can't decide if it's the little brother of the BFN or the mother of the BFN.
I think it was the mother as the story goes.
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JoeinNY
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Joined: October-19-2005 Location: United States Status: Offline Points: 5697 |
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Posted: June-03-2024 at 10:39am |
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That all sounds about right - if for some reason I was dead set on overloading an engine with too much prop, or just in a position where it was the only prop I had to run and needed to get some recreation in, then I would make some adjustments to prevent knocking/destructifyering. Specifically consider dialing back a couple/few degrees of timing. Could also give it a bit more fuel jetting and or higher octane gas - particularly if you are going to pull watersports. 450 dollar experiments need not become 4500 dollar season enders.
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DenDen
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Joined: July-20-2023 Location: USA Status: Offline Points: 189 |
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Posted: June-03-2024 at 11:21am |
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Thanks for all the kind input.
We are currently trying to make sure our engine is outputting as much horsepower as possible. We have no signs of detonation and yesterday we fattened it up with some carburetor jetting and ran it again. The result was 4300 RPMs and 51 mph. I will be going to a big lake at the end of the week for a while and will experiment with the jetting some more. This is a cruising vacation with no skiers or tubers. So what I have will work just fine for now thanks again for all your help. |
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63 Skier
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Joined: October-06-2006 Location: Concord, NH Status: Offline Points: 4248 |
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Posted: June-04-2024 at 8:34pm |
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The 51 mph at 4300 surprises me. You upgraded power to a bit over 300 hp so that I guess explains it.
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'63 American Skier - '98 Sport Nautique
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DenDen wrote: