need prop help and advise

[66mstgfsbk]

i dont knonw where to start but here is my set up thanks for any help
1. 1984 malibu sportster with a omc stringer and a chevy 5.0
2. 21 feet in length, 84 inches wide i have no decal for weight but maker says 2000lbs
2a. engine is rated at 220 hp
3. normally 5 people on boat total of 700-800#s and fuel is 20 gallons normally but 30 when both tanks are full to the top
4. boat is used for cruising , some tubing,
5. was told that it is a low profile hull will include picture of identical boat hull
6. 1984 chevy 305 with a omc stringer mechanical drive
7. 220 ho and was told gear ratio is 21/17
8.cannot find manufactures wot spec
9. Anti-ventilation Plate height is level with the bottom of the keel i had to redo the floor and stringers
10. not a bass boat
11. does not have a doel or trim tabs
12. currently has a 15x17p alum michigan cupped prop
13. RPM 4000, Speed 40mph, No. of people 3, Gal. Gas 20
(changed to 15x17 cupped prop since the other prop was not stamped numbered or anything, no markings at all for some reason could not get past 4000 rpms with 15X17 prop but with mystery prop got to 4800 rpms easaily)
14. at sea level
15.engine was tuned up runs good no issues at all, compression good,every thing checked ok. ran good with mystery prop but not with 15x17 cupped
16. 15x17 cupped prop installed since other was a mystery, needed a point to referance
17. no damage on 15x17 cupped prop
18. im trying to get boat to plane faster and get better top speed while getting a good hole shot( i think im asking for a lot) currently boat starts planing at 20mph and is on full plane at 25mph. does that sound right?

 

[Bondo]

Re: need prop help and advise

Ayuh,... That's a Beautiful Boat,... Too Bad it's a Stringer drive...

You should be spinnin' that 305 to 4400/ 4800rpms at WOT...
Take the Mystery prop to a Prop Shop, they can measure it,+ tell you what it is,...
'cause that appears to be Exactly what you Need for a prop...
Or possibly 1 In-between the 2...

 

[66mstgfsbk]

Re: need prop help and advise

I'd like to put a 4 blade on it. the mystery prop is damaged I have chunks the size of a quarter taken out of it can it still be measured. my boat is identical but blue not brown. thanks for the nice comment. but right on the stringer

 

[Bondo]

Re: need prop help and advise

I'd like to put a 4 blade on it.

Ayuh,... But it's a Stringer drive,...
Just like Most of the Parts for Stringers,....
Used is about the Only thing out there....

 

[66mstgfsbk]

Re: need prop help and advise

i was told that a prop from a merc will fit a omc but a omc will not fit a merc. iboats has 4 blades for the omc just dont know what pitch and diameter to go with.

 

[66mstgfsbk]

Re: need prop help and advise

can anyone tell me where to start with size and pitch thanks.

 

[hwsiii]

Re: need prop help and advise

Fsbk, I ran the numbers on your boat and I found your motor is supposed to turn between 4,000 and 4,400 RPM at WOT and your gear ratio shows a 1.24:1 gear ratio using your numbers for the gears, please verify this. At the speed you are going right now I find that your Prop slip is at 23%, and that is too high. If you buy a Solas Amita or Rubex ALUMINUM prop in a 15" pitch your RPM should go up to about 4,300 RPM and your prop slip should come down to about 12% and I believe your speed should be higher than 40 MPH, and I certainly agree that is beautiful lines on that boat. You will have a much better hole shot and acceleration at all speeds and RPM and a lower planing speed as well, if the prop you are using now is also aluminum.

Prop Slip

H

 

[66mstgfsbk]

Re: need prop help and advise

that gear ratio is what i found looking up a replacement unit from a online company. how do i verify the outdrive gear ratio? thanks for all the help

 

[hwsiii]

Re: need prop help and advise

Here is how you can know what your gear ratio is.

7. HP and gear ratio of motor IMPORTANT
If you don?t know the ratio, you need to pull the plugs out and put a piece of tape across the prop and the lower unit and then cut it between the prop and the housing then do the same thing on the flywheel and turn the motor until the tape lines up with each other on the prop, it is easier if two people do this, so one can watch the prop while the other counts the revolutions of the motor



H

 

[66mstgfsbk]

Re: need prop help and advise

i was wrong about the gear ratio it is a 1.5 not a 1.25.

 

[hwsiii]

Re: need prop help and advise

Fsbk, with that change in gear ratio my new numbers show that your prop slip is at 12% and that is good, but by changing to a 15" pitch ALUMINUM Solas Amita or Rubex prop you should be able to turn right at 4,300 RPM and your minimum planing speed and hole shot will be much improved over the 17" Pitch as well as acceleration throughout your whole RPM range will be greatly improved, and I believe you will not lose more than 1 MPH at WOT.
I do have to mention though that I show you should be getting at least another 5 MPH out of your boat from my calculations. So if I were you I would at least check the following before purchasing another prop:
15. Has your motor been tuned up lately and have you checked that the carburetor butterfly is opening all the way by just pushing the throttle at the helm and not at the carburetor, checked compression, and looked at the plugs and checked spark as well as timing and advance, all of the foregoing could be the reason your prop is not attaining full RPM.


H

 

[66mstgfsbk]

Re: need prop help and advise

thanks for the help h, one of the last few questoins that i have is if i go stainless do i need to change pitch? also does diameter matter i find the 4 blades in 14.0-14.5-14 4/5 diameters does if i go smaller do i need to change pitch accordingly? the current diamter is 15 inches. thanks for the help

 

[hwsiii]

Re: need prop help and advise

Fsbk, the most important factor is all about blade Geometry and not near as much about blade diameter. There can be as much as 400 to 600 RPM difference between two propellers of the same diameter and pitch.
If you read this you might have a better understanding of what I am talking about.
http://forums.iboats.com/showthread.php?t=303221&highlight=Black+Art


H