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George3

Prop recommendation 2019 21 H2O 250 hp

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Hello - I have a new 2019 Chaparral 21’ H2O w/ a 250 HP Merc and Alpha Drive. It came w/ a 3 blade 14.25 x 21P and I think it is over propped. WOT only 4300 RPM and 43mph. RPM range s/b 4800-5200. Would like to go to a 4-blade for Watersports. Any recommendations on size and pitch? What has worked for you. Shocked it shipped w/ something that only can get to 4300 RPM.

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What outdrive ratio do you have?  By the sound of it you need a 17P if you want to used a 4 blade.  The Mercury Spitfire is good for aluminum and the Powertech SFS4 would be good for stainless. 
I would expect the boat to be a little faster than 43MPH. 

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Admittedly, regardless of pulling my boat out 3 times this season to have the hull cleaned growth may have a speed impact. On the Illinois River and growth accumulation is ridiculous. Will do in and out svc (valet) next year. RPM max would be the same though.

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GPS handheld ?   Accurate tachometer ?   Or the in dash units   ?

Have you been changing the parts that WEAKEN a engine with time ?  

Checked sparkplug condition AND done a compression test at that time ?

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There are super slick bottom coatings to stop the hanging ons........... But marinas do not like them. Boats slid right off on any down grade.  Special tieing to fork truck is required.  Piece of cake.

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Cyclops- thank you for the bottom coat recommendation. I likely will go with in/out service at our Marina instead of keeping it in the harbor on the Illinois River. Speed was GPS and tach was the gauge. 

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Going from aluminum to ss and 3 to 4 blades will definitely improve waterspouts handling. I wakeboard a lot on my 2017 H2O Sports 21' and by going to PowerTech SFS4 17 pitch stainless steel prop I gained a lot stability pulling and quicker hole shot - I can easily pull 2 wakeboarders out with 4 people in the boat, full tank, and about 1000lb ballast. Also, as expected, I lost some speed from the top end, but it was only 3-4 mph. I would highly recommend SFS4 - they make them in 17-23 pitch, just need to get one suited for your engine and gear ratio. I run SFS4 17 on Volvo Penta v6 240 with SX-A 1.97 gear ratio. If you cant get SFS4 work for you, I also heard good things about PTZ4 props

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