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  Prop guess, currently 16"; merc says 20"+

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Author Topic:   Prop guess, currently 16"; merc says 20"+
peteinsf posted 01-14-2003 06:57 PM ET (US)   Profile for peteinsf  
The Challenger 25 (twin OMC '95 200hp) currently has a pair of OMC 16" SS props and has a WOT of 52-5300... My guess is that boat weighs 7000# with a full load of fuel. (she was #7000 on the CHP scale dry)

With all of that, the merc prop calculator says more like a 20"+ prop? Could the cabin wind restance make that much difference? Or do I have a motor problem?

Sal DiMercurio posted 01-14-2003 09:28 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Only a 16 pitch prop for a pair of 200 hp engines ?
My guess is, their 15 x 16 omc props, thoughts & figures say, she should twist at least 15 x 19s & possibly a 15 x 21s, or better yet, 14.25 x 19s or 21s.
Rule of thumb, if running a single engine, then going to twins, you automaticly add 4" of pitch to the props.
That boat is heavy, but I feel those engines "SHOULD" be able to twist 19s or possibly 21s.
Sal DiMercurio posted 01-14-2003 09:58 PM ET (US)     Profile for Sal DiMercurio  Send Email to Sal DiMercurio     
Sorry about having to post again, but company showed up & I only sent 1/2 the message.
I,m not familiar with that hull, but is that boat also rated for one engine, if so, those engines are out to the sides & should be up on the 2nd or 3rd holes, not all the way down.
When testing those engines for rpms, you need to rim them out as far as you can, without them blowing out.
You don't check for rpms when the engines are tucked all the way in, or down.
Where are your anti vent [ cavitation plates ] when your on plane ?
If their "not" above the water, your engines are to low.
jimh posted 01-15-2003 11:09 PM ET (US)     Profile for jimh  Send Email to jimh     
When you use the Mercury propeller calculator, you set the lower unit gear ratio they'll use in their algorithm by selecting what model of MERCURY engine is being used. Since you are using OMC engines the lower unit gear ratio may be different. This could account for differences in the prop pitch that is calculated.
Clark Roberts posted 01-16-2003 07:34 AM ET (US)     Profile for Clark Roberts  Send Email to Clark Roberts     
Good advice above! Suggest you have compression tests to see if engine (s) are to spec and if so have a "link & sync" (throttle & timing linkages) to make sure you'r getting max at WOT (wide open throttle) and max timing advance... then raise engines if needed and see what she turns! Happy Whalin'.. clark
lhg posted 01-16-2003 04:55 PM ET (US)     Profile for lhg    
Besides different gear ratios, I would imagine that '95 OMC's are no match for a pair of new 2003 Mercs. Even when new, they weren't quite as fast. And all engines do lose HP as they age.

I also have suspected that the prop selector indicates a little high, maybe by as much as 2" of pitch. Users tend to underestimate weight.

On a 25 Challenger, with new twin 200 Mercs, I would think 19" would the best all around option, Mirage series or 4 blade Offshore. These would give you about 51 mph.

I run 23" pitch props on mine when running light, and 21's when loaded with full fuel and gear for cruising and long distances.

peteinsf posted 01-17-2003 08:38 PM ET (US)     Profile for peteinsf    
Thanks guys,

Firstly the motors are at lease two holes to low in the water, spray everywhere. Secondly one of the two motor is having an ignition problem above 3500 RPM. The tach becomes jumpy and it putting out less power then the other. I tried swapping the two "power packs" and that didn't do it. The manual shows that the tach signal starts in the stater (duh) and then to the "charger/rectifier" unit. I'll try swapping that next, the stater sounds like a bigger job.

Larry after my tests I would guess that a 19" would be right also, that is what the WD/Outrage has. Your boat and that challenger have the same wetted area, the cabin and extra glass would kick me down a couple inches from your 21"...


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