Prop change

[hypno-toad]

hi all, I have lurked here on and off for years, a great forum. here is my situation i hope someone can help me out - i have a 16' 1987 4 winns with a merc 3.0 4 banger, this spring, I tore up the out drive (upper gears). so, i ran across a complete drive on craigslist that was supposed to be off of a 4 cyl boat, and in fact it had the 1.98 stamped near the ujoints, so I bought it. long story short - it turns out that it has 1.50 gears in it. the boat can get on plane, but can't get over 3000 rpm but travels at 32 mph (boat speedo) before "the incident" it ran 4400 @ 34mph it presently has a 19 degree prop. my question is, can i change the prop to make up for this gear ratio problem and how can i determine what size to go to?
Thanks,
Rob

 

[180shabah]

Re: Prop change

You can't make up that difference with a prop. You need to regear/replace the drive, or at least the upper.

 

[hypno-toad]

Re: Prop change

Thanks for the reply, i have run across some online prop calculators that suggest a 15 p prop may be of some use. is there something in my case that the calculator does not take into account?

 

[Titanium48]

Re: Prop change

You can't make up that difference with a prop. You need to regear/replace the drive, or at least the upper.

Why not? If the motor could spin a 19 p prop at 4400 / 1.98 = 2222 rpm, why shouldn't it be able to spin a 13 p or 15 p prop somewhere near 4400 / 1.5 = ~2900 rpm?

 

[180shabah]

Re: Prop change

I have found most prop calculators to be very optimistic, they usually suggest that I use a 23" for watersports and 25 for general use. Both about 4" too much pitch, campared to what really works.

 

[hypno-toad]

Re: Prop change

i know a correct upper gear is the right way to solve this but, the calculator i used was pretty close with my pre-disaster setup which was 19p 4400rpm 35 mph give or take due to slip.with a 15p same slip i got 4200 @ 34mph. i guess it comes down to do i want to risk the cost of a prop? i'll hunt for a upper case see what i can find. i really thought about getting the SEI here (before i bought the used one, which is a good lesson, had i bought SEI i wouldn't be here!!) @ $600 which is a great deal but this is an older boat that only sees occasional use. thanks again.

 

[180shabah]

Re: Prop change

Why not? If the motor could spin a 19 p prop at 4400 / 1.98 = 2222 rpm, why shouldn't it be able to spin a 13 p or 15 p prop somewhere near 4400 / 1.5 = ~2900 rpm?

Less effective power TO the prop.

To make this work he would need to find a prop somewhere in the range of 13" pitch, but WITHOUT increasing the diameter.

Most 19" props have a diameter of about 14 - 14.5". When you start looking at lower pitches like 15" you diameters of 15-15.5 and 13" props will increase diameter to the 15.5-16" range. He does NOT have enough power to swing a prop that large.

Online prop calculators are a great feel good tool, you get to click, and type in some numbers, and it tells you exactly wht you need..... Except when they don't - which is frequently.

All things being equal, yes you could just change pitch to make up the difference. The problem is, things are not equal. Diameter, rake, cupping etc ALL change a little as you increase/decrease pitch on a propeller. Up or down one size = no problem. 4-5-6 inches, doesn't work too well.

 

[steelespike]

Re: Prop change

If we're going to try to force it into submision lets try a 15 if its not enough
have it pitched down to a 13.Thus keeping the diameter.You might consider removing some cup as well. Just boondoggling.??

 

[thompy]

Re: Prop change

it would be worth a try

 

[hwsiii]

Re: Prop change

Hypno, Shabah is right about

Less effective power TO the prop.

To make this work he would need to find a prop somewhere in the range of 13" pitch

When you changed from a 1.98:1 to a 1.5:1 gear ratio you lost right at 25% of your actual motor Torque at the prop itself. Also when you change from a 19" pitch prop, in the same model, you also lose the higher prop efficiency that higher pitch props always produce. And since you are having to go to a 13" pitch prop, to get your RPM back up to a reasonable level, that is a fair amount of efficiency lost.

Prop Torque Loss

1.50 Gear Ratio

1.98 Gear Ratio

Shabah

All things being equal, yes you could just change pitch to make up the difference. The problem is, things are not equal. Diameter, rake, cupping etc ALL change a little as you increase/decrease pitch on a propeller. Up or down one size = no problem. 4-5-6 inches, doesn't work too well.

I also agree with this statement by Shabah, but for all the money that it might cost to change this I think if you will buy a 13" pitch, in the EXACT same Manufacturer and MODEL of prop you have now you will lose about 3 MPH top speed, but it will work.



H

 

[hypno-toad]

Re: Prop change

well, i picked up a SS 15" prop cheap it did help, it takes a little bit to get on plane, but I can get 36mph@36-3700 rpm, the worst part is when trimmed for top speed, the boat "porpoises" so i have to trim the bow down. no big deal i usually just cruse @ 20-25mph tubing etc. but, i think i will keep an eye out for a 1.98 or 1.84 (use the 15" prop) upper. thanks for everyone's advice it was helpful.