Another prop blowout ?

[hoytultratec]

heres the deal.

19 ft cuddy approx wt with all fishing gear and people 3000 lbs.

488 mercruiser runs awsome alpha one outdrive.

heres the problem when i put it down im getting so much prop blowout i have to back way down to keep the rpms from going to 5000 plus.

it will work if i take off real easy but i hate not being able to see over the bow so long, i know safe max rpms is about 4600 rpms being as old as it is i dont really go above 4200 rpm.

when its on plane at 3/4 throttle im at 4500 rpm at about 33 mph (gps)

the current prop is a 12"x19 pitch aluminum prop..now i would think i am seriously under propped.

the boat is only used to fish lake michigan so it runs a couple of miles out and acouple back as i troll with a kicker.

should i go to 21 pitch??? i will take any suggestions. and i do have it trimmed all the way down on take off no matter what i do with the trim i have the problem.

thanks hoyt

 

[tommays]

Re: Another prop blowout ?

In a general way on my 19' ft boat a 21" 3 BLADE will give the best top speed and a 14 X 20 4 BLADE gives better low speed and mid range performance

The 4 blade will allow me to hold plane at lower RPM in rough weather and my other normal uses


As always there is a LOT of trial and error in finding the best prop



Tommays

 

[Tail_Gunner]

Re: Another prop blowout ?

If you have a 12" hub i would assume that's the problem, i believe you should be in the 14-15" hub's

Yes that appears to be the problem

Propeller Type: Sterndrive
Manufacturer : MERCURY BLACK MAX 1983 THRU
HP / Model : 3.7 LITRE - 165-190 HP - ALPHA ONE
Year : 1986-1989
Boat Size : 17'-21' BOATS - 33-42 MPH
Material : ALUMINUM


BladesDiameter & PitchRotationPart numberStyleComments314 X 19RH031025MICHIGAN MATCHINCLUDES 470, 488, 898314-1/2 X 19RH031065MICHIGAN MATCHINCLUDES 470, 488, 898

 

[Bondo]

Re: Another prop blowout ?

the current prop is a 12"x19 pitch aluminum prop

That sounds like a Prop built for a 4" gearcase Outboard...........
I bet you've got a Serious Gap where the Prop Hub meets the Outdrive Gearcase......

Get a 19" prop for an Outdrive...........
They usually run from about 13,1/2" to 16" Diameter........

 

[hoytultratec]

Re: Another prop blowout ?

the gap is 3/8 of an inch, went to the local dealer he ran the number on the prop its as old as the boat,i did a better measuring job and its 12.5" he said if it has been rebuilt a couple of times it is possible to lose diameter????1/2 " plus????? then tried to sell me a merc prop for 180 bucks i said no.

thanks for the part numbers above.

now what i use the boat for no sking no tubing just running out on the lake should i go 19 or 21 pitch im not concerned about top end around low 30 s is fine im more concerned about keeping the rpms in a safe zone.

thanks for all help.

hoyt

 

[Bondo]

Re: Another prop blowout ?

the gap is 3/8 of an inch,

That Still sounds lke an Outboard Prop........

should i go 19 or 21 pitch

Whichever 1 that yeilds the proper RPMs at WOT...........