Help with prop

sean35

Recruit
Joined
Oct 18, 2009
Messages
3
Hi everybody...I'm a newbie and been bitten by the boat bug!! Question.. I have a 1991 21' open bow with a mercruiser 350 magnum & a alpha one drive engine serial # OD 425381 Drive # OD 567128 and the serial # listed on the alpha 1 is OD 465908 on the motor sticker it sez WOT max 4200-4600 rpms.
The boat does 57mph at 4000 rpm's if i trim the boat to higher rpm's it just doesn't sound right..the above #'s the boat seems to run best. I can't find a pitch # on the prop but its a 3 blade s.s. and it sez hill propellers it measures 14" in dia. HELP..what should the prop be... 3 blade....4 blade...s.s....what pitch for top end....or hole shot for skiing..... Thanks in advance for all the help you can give me.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Help with prop

Welcome,I'm a little outside of the prop experts but I'll try to get you started.
What brand 21 footer?
Really can't tell much without your prop size.Usually on the barrel or under the prop nut.Need more detail When you trim to higher rpm what happens.to rpm and speed?You may be getting ventilation(cavitation) rpm goes up speed may go down.Is the speed by gps? speedo is usually not too accurate up to 5 mph off+ or -.57 seems pretty good at 4,000 is that wot(wide open throttle)? See if you can find the prop size.
 

sean35

Recruit
Joined
Oct 18, 2009
Messages
3
Re: Help with prop

Steelspike....boat is a caliber 1....when i trim up rpm's climb to about 4200-4300 and speed goes down...so i imediatly trim down...speed by speedo...and all numbers are by WOT....can't get prop size till off work....i'm guessing take nut off and remove prop....
 

propper_advice

Seaman Apprentice
Joined
Dec 21, 2005
Messages
31
Re: Help with prop

Here are a couple common guidelines for props, hopefully they will help get you started.
3 blades tend to be a tad faster and the typical general prop.
4 blades offer better hole shot and usually better handling at speed. These are popular for watersports and heavy boats.
Of course everything can be adjusted by change pitch :)
There should definately be a number on your prop, so you will have your pitch. Best test is that RPM's at WOT should be as close to top of rev range as you can get without going over.
Good luck.
 

steelespike

Supreme Mariner
Joined
Apr 26, 2002
Messages
19,069
Re: Help with prop

Assuming the motor and boat are in good health; if the max rpm at reasonable trim is 4,000 it appears you need to drop in pitch to a 19. this should put you at about 4,300 to 4,400.Will improve your hole shot.
You should try to stick with the same brand and model.At least stick with the same material. a 4 blade will further improve hole shot and low end performance and handling;it will also introduce some unknowns as far as
performance changes. a 18" 4 blade should help maintain your rpm.
You may be able to find props right here that iboats will let you try till you find the best one.
 
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