Prop size

[t2sunquan]

Hey guys so i got my 19' 82 Mark Twain on the water yesterday and at WOT I am running 3000rpm with a full load of people (estimating 1000lbs), the boat weights 2700 as is. I bought it with a 21 pitch alum prop but i later on bought a 19 pitch SST prop which is what i used during the run. The boat has a OMC 200hp 305 in it, my question is what prop pitch do i need here to be near my operating range? I could guess getting a 15 pitch prop would solve my WOT problem which should be between 4200 and 4600 but isnt a 15 is a little small for that engine? Feed me your input on this as to what prop size i would get next, I really hate having to bounce around between pitches and spend tons of money. Thanks

 

[HT32BSX115]

Re: Prop size

Hey guys so i got my 19' 82 Mark Twain on the water yesterday and at WOT I am running 3000rpm with a full load of people (estimating 1000lbs), the boat weights 2700 as is. I bought it with a 21 pitch alum prop but i later on bought a 19 pitch SST prop which is what i used during the run. The boat has a OMC 200hp 305 in it, my question is what prop pitch do i need here to be near my operating range? I could guess getting a 15 pitch prop would solve my WOT problem which should be between 4200 and 4600 but isnt a 15 is a little small for that engine? Feed me your input on this as to what prop size i would get next, I really hate having to bounce around between pitches and spend tons of money. Thanks

Howdy,


You probably need to do a few things....and while a 21p prop might be a bit "tall" for that 305 a 19 should be closer to what you want.

Have a look at the following thread. http://forums.iboats.com/omc-i-o-in...have-technical-information-diyers-468655.html

Look particularly #14


It's not always the prop.......


Regards,


Rick

 

[t2sunquan]

Re: Prop size

I have throughly checked EVERYTHING on that list over the course of winter. Prop really could be my only guess.

 

[steelespike]

Re: Prop size

We really need the specs from a lightly loaded run. wot rpm and speed by gps.
The only way to evaluate your setup.
At 3000 your not planed so the rpm is artificially low.

 

[HT32BSX115]

Re: Prop size

At 3000 your not planed so the rpm is artificially low.

If he's not planing at 3000 that would be true. I didn't see that.....Did he say that in a different post?

Verify that your tach is correct..... They're frequently wrong, and you must use either GPS or a known speedometer.

Try it again with only you in the boat and other (heavy) gear removed (get it as light as possible)

Get it up on a NON-WOT plane and measure the speed and RPM. Try a different tach if you can get one.

I found a chart for drive ratios and it indicates that the ratio for a 5.0L engine is 1.59:1

If someone has changed that drive to a 1.41:1 model, it'll make a difference.

If you do have a 1.59:1 drive, you should be *just* starting to plane at about 2500 RPM and be doing around 30 MPH at 3000 RPM
(ref: Prop Slip Calculator using 19p prop, 1.59:1 ratio, 3000 RPM, 15% slip)

Can you get a full range of drive trim? is there an IN-limit? Do you know your drive ratio?

(is the boat water logged ? You're not dragging your anchor? and You DID remove the boat from the trailer right?)


Until you verify your tach and speedometer, you're wasting your time trying different props.....

 

[t2sunquan]

Re: Prop size

My ratio is 1.50:1. I was doing about 35mph at 3000rpm +/- 100rpm. I wonder if this sst ballistic prop isn't right for this boat. Either way I'll get a tiny tach and take an absolute reading on it this week, remove the gear and I'll let you guys know.

 

[t2sunquan]

Re: Prop size

Also, speed was read using tom tom GPS

 

[jestor68]

Re: Prop size

It's not that the Ballistic is "wrong", it's the fact that due to it's more aggressive design, it can act very similar to the 21 aluminum prop.

Regardless, you need to do testing with just one or two people and prop to get the rpm at or near the top of the range(4600). That way, when you pile the gang in there, it will be better able to handle the extra weight.

When adjusting pitch, the generally accepted rule of thumb is 150-200 rpm per inch of pitch change. But don't do anything until get a better test(without the gang in there)

 

[steelespike]

Re: Prop size

I forgot we were using a 1.50 gear ratio seems to me that most boats would struggle to be fully planed at 3000.
Especially with a big load.
Assuming the speed is accurate and if we use 10% slip rpm would be about 3300. seems reasonable.

 

[HT32BSX115]

Re: Prop size

1.5:1 is pretty "standard" for V-8 engines in stern drives that will turn around 4500-5000 RPM.

OMC spec'd 1.59:1 for certain 5.0L (305) engines.

Unless you took the top cap off and counted teeth, you might want to verify for sure that you in fact have a 1.5:1 drive.

If you don't want take the cap off, you could remove the spark plugs, put it in gear and have someone turn the engine (exactly) 3 revolutions. The prop will turn 2 if it's a 1.5:1 drive........ (or turn the engine 1 & 1/2 turns, the prop will turn exactly 1 turn etc.............)

It's sort of important to know for sure what you have, so the prop-slip calculator will give you usable numbers.

If the engine is running correctly and you prop it right, you should get 4600 (max RPM) at WOT lightly loaded. If it's REALLY only doing 3000 at WOT, you're seriously lugging the engine.

 

[t2sunquan]

Re: Prop size

1.5:1 is pretty "standard" for V-8 engines in stern drives that will turn around 4500-5000 RPM.

OMC spec'd 1.59:1 for certain 5.0L (305) engines.

Unless you took the top cap off and counted teeth, you might want to verify for sure that you in fact have a 1.5:1 drive.

If you don't want take the cap off, you could remove the spark plugs, put it in gear and have someone turn the engine (exactly) 3 revolutions. The prop will turn 2 if it's a 1.5:1 drive........ (or turn the engine 1 & 1/2 turns, the prop will turn exactly 1 turn etc.............)

It's sort of important to know for sure what you have, so the prop-slip calculator will give you usable numbers.

If the engine is running correctly and you prop it right, you should get 4600 (max RPM) at WOT lightly loaded. If it's REALLY only doing 3000 at WOT, you're seriously lugging the engine.

I know for a fact it is a 1.5:1, said it on the top cover and i know the top has not been rebuilt judged by the look of the seals. Either way i bought some cheap props off CL: a 15 and a 17. Now that i have a 15,17,19, and 21, it HAS to be one of these and the rest i will sell.

 

[steelespike]

Re: Prop size

That should solve your problem. They should all act like the numbers indicate i.e.: a 15 like a 15 a 17 liker a 17 etc.
But if they are different brands/models there may be some subtle differences. more or less cup. different rake
old technology or the latest technology. A prop with a lot of cup might act like it is an inch higher pitch.
Let us know what works for you for future reference.

 

[HT32BSX115]

Re: Prop size

I know for a fact it is a 1.5:1, said it on the top cover

THat's usually a good indication of ratio except that in 30+ years, someone could have mixed and matched parts several times.

Looks like you'll have plenty of props to try.....