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Old 04-27-2020, 07:56 PM   #1
Lt. JG
 
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Default Standard prop for force 90 outboard 1991 Maxum 1880 XR

Hi,

Just wondering; when I purchased my 1992 1800 XR with a force 90 outboard, it came with a spare prop. The spare is a 13x19P, I don't know what the fitted one is but it looks like it certainly has a different pitch. Does anyone happen to know what the standard recommended prop is for this boat/motor combo? On the water it accelerates really well but I suspect that top speed is somewhat below par. I could take the prop nut off the fitted one and have a look at the spec but I just wondered if anyone could suggest what it actually should be before I do.
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Old 04-28-2020, 12:47 PM   #2
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Max RPM for the Force 90 OB should be 5000 - 5500. What is your current RPM at Wide Open Throttle (WOT)? You should be as close to 5500 at WOT (properly trimmed) as possible.
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Old 04-28-2020, 02:05 PM   #3
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Thank you for the info, that's really helpful. I've been looking into it a bit more actually and watched a really good youtube video that explained things nicely. I honestly can't remember noticing the what the revs were at WOT but having done some more research I suspect that what's on there now is probably OK but I'll take the prop nut off and have a look anyway. Next time I take it out I'll know to watch the rev counter at WOT and go from there.
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Old 04-28-2020, 02:13 PM   #4
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I've just had a closer look and this time put my glasses on! The one on there is a 15" which is probably about right. I would think there's no way the spare 19" would be suitable!
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Old 04-30-2020, 01:32 PM   #5
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Is the 15" and 19" the diameter, or the pitch? Diameter wouldn't change, only the pitch. 1" of pitch change should translate to roughly 200 RPM. Increasing pitch will decrease max rpm at WOT. Decreasing pitch will increase max rpm at WOT. They might have been using the 19" pitch for cruising and the 15" for pulling skiers.
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Old 04-30-2020, 02:06 PM   #6
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It's the pitch and you could be right about the skiing/cruising. Most of my time on the boat is on a lake with strict speed restrictions (Windermere) and being an old 2 stroke outboard it's extremely thirsty on fuel! I do intend to some skiing/kneeboarding on there, albeit at a sedate pace but most of the time I'm just cruising around slowly. I wonder what sort of pitch prop would give the best fuel economy? Would it even have that much effect on fuel economy?
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Old 05-01-2020, 02:46 PM   #7
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Don't mess with introducing overloading or over-revving to chase ounces of fuel burn. Get the WOT RPM to as close to max as possible.
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