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Key West 239 FS - low rpm

Old 12-10-2018, 05:37 AM
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Default Key West 239 FS - low rpm

Hi All,

I’ve been getting to know my new KW 239 FS for the last year ... 250 Yamaha with 19” pitch SWS II prop, Fiberglas t top, no additional bait or convenience center add ons.

Best I’ve gotten thus far is right at 5400 rpm at about 50 mph (GPS) ... have had power posts for 30 years and can trim effectively.

Have collected enough info from others with exactly the same boat and motor to note that no one has ever seen 6000 rpm and most are 5200-5500 rpm, with a very few at 5800 rpm - with the 19” SWS.

I’ve played with prop calculators (1.75 reduction gear) and looks like 10-12% is about the norm ... to get close to 6000, using the prop slip calculator, would seem to indicate 17” pitch with about the same top end.

So, thought I would ask a bit more broadly about the same size boat and motor combos relative to the prop being used and results ... would appreciate thoughts and experiences if you have either a KW, or SH, or SF, etc in this size/weight with the Yamaha 250 (non SHO).

Thanks.



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Old 12-10-2018, 04:00 PM
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Seriously - THT and no responses?
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Old 12-10-2018, 04:38 PM
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Originally Posted by stephenson View Post
Seriously - THT and no responses?
You have to put Freeman in the thread title or you will be subject to 8 hours of soak time

If you're confident of engine height don't pass go, don't collect $200, proceed directly to a 17P prop...is enough Yam wheels out there you should be able trade for one without much hassle. You might even keep your 50MPH but with a boat that runs better in every other regard and the motor not being lugged.

Some majority of new boats are delivered with a low motor and too much prop.
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Old 12-10-2018, 05:48 PM
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What HTJ said. There's been a lot of discussion on this on the KW forum, and yes it seems few 239's run optimally with 19p. For me, raising the engine one hole and running 17p is best for mine. I played with a 19 for a while and never got the RPM I wanted. YMMV
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Old 12-10-2018, 05:52 PM
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HTJ,

Thanks ... motor is in correct hole/location.

Agree, re 2” reduction netting around 400 rpm or so ...

My dealer has offered to replace the 19 with a 17 SWS ... my biggest concern is that my mpg will decrease quite a bit ... but, I usually cruise around 3200-3500 rpm so may not be a big deal?

I also keep wondering if my 80 hour motor is gonna loosen up ... I was hoping to get massive quantities of info from other owners of the same combo, but just haven’t been able to get more than about six other owners to come on line at keywestboatsforum.com.
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Old 12-10-2018, 06:07 PM
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What does it matter what others with similar boats and motors are seeing with respect to performance? Are you interested in their boats are you interested in your boat?

To quote a wise man "If you're confident of engine height don't pass go, don't collect $200, proceed directly to a 17P prop". "You might even keep your 50MPH but with a boat that runs better in every other regard and the motor not being lugged."

Of the same brand and model that you have installed now.

This assumes that you are seeing 5400 RPM at WOT with the boat loaded, engine trimmed up only to the point of most efficiency, on a hot and humid summer no wind day.
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Old 12-10-2018, 06:36 PM
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5400rpm is within spec. But, if it really bothers you try a 17P prop. I'd be happy with it if it were me, hitting 50 and I imagine you getting around 3-4mpg? maybe just a touch less?
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Old 12-10-2018, 07:09 PM
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alloyboy - I started to try and determine WHY the 19 pitch was in the motor in th first place since almost no one was reporting reasonable rpm ... btw, evan, 5400 is not in spec (although apparently this is an arguable point).

So, I’m looking for some “ground truth” that will help all those of us not getting suitable rpm from exactly the same boat and motor combo make decision for the right prop.
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Old 12-10-2018, 07:20 PM
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well here ya go https://yamahaoutboards.com/en-us/ho...150-hp/v6-4-2l it is in spec........ i'm not a complete idiot

copied from Yamaha's website
Engine TypeV6Displacement4.2LBore x Stroke96 x 96mm (3.78 x 3.78in)
Prop Shaft Horsepower250hp at 5500 rpm
Full Throttle RPM Range5000 ~ 6000 rpm
Alternator Output at W.O.T.50 - 70 Amp
Compression Ratio10.3:1
Fuel Induction SystemEFI
Weight*250 kg (551 lbs) - 254 kg (562 lbs)
Recommended FuelOctane 87
Recommended Engine OilYamalube® 4M (See owner's manual)
Recommended Fuel FiltrationYamaha 10-Micron Fuel / Water Separating Filter (external).
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Old 12-10-2018, 07:30 PM
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Evan ... you do sound a bit touchy, though.

Seriously, you think 5000 rpm is acceptable?
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Old 12-10-2018, 08:46 PM
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Originally Posted by stephenson View Post
HTJ,

Thanks ... motor is in correct hole/location.

Agree, re 2” reduction netting around 400 rpm or so ...

... my biggest concern is that my mpg will decrease quite a bit ... but, I usually cruise around 3200-3500 rpm so may not be a big deal?

I also keep wondering if my 80 hour motor is gonna loosen up ...
You wont loose anything other than premature wear and tear on a fine motor.

Loosen up? Drive it like you stole it and stay away from synthetic oil for at least 200 hours. I don't think there is an outboard on the market that doesn't run better at 4000 - 4400 than it does at lower RPM. Peak torque, peak efficiency and peak warm fuzzy happy sound all usually coincide at the same RPM and that same RPM is usually within an RCH of 4000 RPM or 75% loading.

Aside from my anti synthetic sentiment above - synthetic in the lower from the git go
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Old 12-11-2018, 10:44 AM
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HTJ,

Your anti syn logic is due to rings needing to seat more effectively than they do with syn?
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Old 12-11-2018, 01:31 PM
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Originally Posted by stephenson View Post
HTJ,

Your anti syn logic is due to rings needing to seat more effectively than they do with syn?
Yes and raw water cooled engines with 143F thermostats are extra needy...things just don't expand like they do in a car motor running at 180 - 210.

Now were it me...Im an old hotrodder and every motor I own gets the oil changed out to synthetic after the first heat cycle or at most 8 hours of use.

Disqualifier: I am not afraid of the throttle, YMMV.
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Old 12-11-2018, 01:56 PM
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HTJ,

Yep - concur ... been synthetic in everything for me since around 1976.

I do wait on some motors like lawnmowers and outboards ... will probably just keep non-syn in the Yamaha since the oil changes are so close together ...

Want to make sure I don't get sidetracked, though ... you do believe I should not accept 5400 RPM as the normal range on my 250, right? That is the RPM when it is light ... with three guys and fishing gear with ice and bait, it is around 5150 RPM.
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Old 12-11-2018, 02:07 PM
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IMO U gotta get top RPM between 5800-6k for best results and to prevent the dreaded "making oil."
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Old 12-11-2018, 02:08 PM
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I’d go with a 17 , not an expert. But my thought is that you will still nearly hit the top speed with a 17 @ 6000 rpms and handle a loaded boat much better than a 19 @ 5400. It is gonna be laboring with more weight
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Old 12-11-2018, 02:10 PM
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OK - I've spent 100 hours sorting through this topic - most with background in the topic seem to believe that, as well. The "making oil" thing has really gotten everyone's attention.

I look at the Yamaha manual number 5000-6000 as pretty darn fuzzy ... it allows probably waaay too much latitude for dealers to just toss whatever prop they have on a boat.

I started this quest to try and get enough data to feed back to KW and other manufacturers that they are putting "generic" props on boats without suitable rationale of follow-up.
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Old 12-11-2018, 05:20 PM
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Lightly loaded , I like it to hit the top of the rpm range or above and back off.. Then when loaded up your not bogging it down.
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Old 12-11-2018, 05:43 PM
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Although the spec per the manual is 5000 to 6000 rpm, it doesn't make 250 HP until 5500 rpm. So at 5400 rpm you're not getting the full advertised HP from the motor, I don't call that "in spec" but that's just my opinion.
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Old 12-11-2018, 05:53 PM
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17 pitch and your right where you should be. Easy as that.
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