Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

[jmw129]

What is the best "Happy Medium" prop size for a 305 V8 230hp application on a 19ft open bow weighs roughly 2100 lb's with merc alpha 1 outdrive?

Ideally what I want is "Hole Shot & Top End" all in one.

Recommendations?

 

[Bondo]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

Ayuh,... Standard easy to find props run from 15" to 23", maybe 25" on a good day...

that's why there's different drive ratios for different motors...
'bout the best you can do with such limited Info, is start in the middle of the available range,
'n Hope yer close....

What was or is on it now,..??
That's 'bout the Best place to start...
After this of course....

 

[rte592]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

May not be much help

The boat (21 foot open bow 1990 Ultra)
I just picked up has a Solas 14.25 x 21 pitch (3 blade aluminum prop with a few minor dings)
On the test drive with 2 people in it........it came out of the water right now with not a whole lot of throttle and has a good top end......I'm not too concerned with WOT as this is a family boat.

 

[Tail_Gunner]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

May not be much help

The boat (21 foot open bow 1990 Ultra)
I just picked up has a Solas 14.25 x 21 pitch (3 blade aluminum prop with a few minor dings)
On the test drive with 2 people in it........it came out of the water right now with not a whole lot of throttle and has a good top end......I'm not too concerned with WOT as this is a family boat.

....Ok prop selection is taken a bit serious around here...

There are many reasons for your given speed at Wide Open Throttle...wot...and the engines rpm at that point. With that a good selection can be made and one that will insure longevity for your engine...Off the cuff 21 sounds right

 

[jmw129]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

Ayuh,... Standard easy to find props run from 15" to 23", maybe 25" on a good day...

that's why there's different drive ratios for different motors...
'bout the best you can do with such limited Info, is start in the middle of the available range,
'n Hope yer close....

What was or is on it now,..??
That's 'bout the Best place to start...
After this of course....

This boat I bought a few months back, the pitch on the prop was 15P. Seemed very much under propped. Ideally I would like to get 47mph to 50mph top while having a good cruising speed in the 30mph to 35mph with a load of people.

 

[jmw129]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

....Ok prop selection is taken a bit serious around here...

There are many reasons for your given speed at Wide Open Throttle...wot...and the engines rpm at that point. With that a good selection can be made and one that will insure longevity for your engine...Off the cuff 21 sounds right

This boat I bought a few months back, the pitch on the prop was 15P. Seemed very much under propped. Ideally I would like to get 47mph to 50mph top while having a good cruising speed in the 30mph to 35mph with a load of people.

Will I get these results with a 21P?

 

[Bondo]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

Ayuh,.... "Seems under propped" ain't much to go on...
'n what you Want, really don't matter...
Ya get what Works...

Ya can do this the Right way, as in the link I posted,.....
Or, we can end up with a thread 20 pages long, 'n you Still won't know what the boat needs...

Pickin' a prop is a mathamagical equation,.....
Supply the base numbers, 'n you'll get the prop you need....

otherwise,... We're all wasting keystrokes...

 

[Silvertip]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

How to achieve a base-line:
Note: Wantng killer hole shot and high top end is not a reasonable request. You cannot have drag car performance (hole shot) and top end speed (NASCAR/F1) at the same time. You compromise or choose one or the other.
1) Install a prop - any prop designed for that particular outdrive.
2) Ensure the engine is properly tuned and making full power and that you have a clean outer hull.
3) Load the boat the way it will be "most" of the time. If that means towing water toys or hauling people, make that the load.
4) Make several wide open throttle runs noting the "exact" engine rpm and speed.
5) Those numbers are the baseline.
6) If the engine cannot reach the manufacturers recommended WOT rpm the rig is "over-propped". If it tends to rev higher than the manufacturers WOT rpm the rig is "under-propped".
7) You can now select a prop that adjusts the WOT operating rpm,. The engine rpm drops or increases about 200 RPM/inch of pitch change up or down respectively.
8) Armed with the base-line, you can use any of the on-line prop calcualtors to determine slip % by plugging in the number.
9) As Bond-O pointed out -- we are already into the 8th post and are no closer to a recommendation. Details, details, details.

 

[steelespike]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

As suggested a 21 is a reasonable prop to start from.
Assuming a healthy motor a 15 is really low pitch.
Hopefully the 15 isn't on there because it is all the motor could spin.
15 could be on there because the po didn't want to give up the right one.
You could start with the 15 just don't overev it too high or too long.

 

[fkboatman]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

boat should have come from factory with a 14.5x19 pitch in which should put motor wide open and trimmed out about 4600 rpms. going to a 21 pitch will drop rpms about 2-3 hundred but will be a little slower coming out of the hole. do you use boat to to pull kids on tubes and ski. if so i would drop down to a 17 pitch to keep hole shot good with a medium to heavy load of people and gear. look into a 4 blade of a 19 pitch.

 

[jmw129]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

How to achieve a base-line:
Note: Wantng killer hole shot and high top end is not a reasonable request. You cannot have drag car performance (hole shot) and top end speed (NASCAR/F1) at the same time. You compromise or choose one or the other.
1) Install a prop - any prop designed for that particular outdrive.
2) Ensure the engine is properly tuned and making full power and that you have a clean outer hull.
3) Load the boat the way it will be "most" of the time. If that means towing water toys or hauling people, make that the load.
4) Make several wide open throttle runs noting the "exact" engine rpm and speed.
5) Those numbers are the baseline.
6) If the engine cannot reach the manufacturers recommended WOT rpm the rig is "over-propped". If it tends to rev higher than the manufacturers WOT rpm the rig is "under-propped".
7) You can now select a prop that adjusts the WOT operating rpm,. The engine rpm drops or increases about 200 RPM/inch of pitch change up or down respectively.
8) Armed with the base-line, you can use any of the on-line prop calcualtors to determine slip % by plugging in the number.
9) As Bond-O pointed out -- we are already into the 8th post and are no closer to a recommendation. Details, details, details.

The WOT rage is 4200 to 4600 based on what the valve cover says. What you suggested makes sense. I'm thinking that a 19P or 21P is going to be the right size.

I would rather have top end speed vs hole shot since I can't have both. I prefer more speed and the GF does too.

 

[jmw129]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

boat should have come from factory with a 14.5x19 pitch in which should put motor wide open and trimmed out about 4600 rpms. going to a 21 pitch will drop rpms about 2-3 hundred but will be a little slower coming out of the hole. do you use boat to to pull kids on tubes and ski. if so i would drop down to a 17 pitch to keep hole shot good with a medium to heavy load of people and gear. look into a 4 blade of a 19 pitch.

No plans for pulling skiers or tubing at the moment. Thank you for the helpful info. 19p seem's the most reasonable. I just can't believe the previous owner would put a 15P on unless he planned to pull the whole neighborhood.

 

[Silvertip]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

You are missing the point. What the valve cover says is the range Merc wants the engine to run at WOT. What you ACTUALLY see on your tach at WOT is what we need. Just because the valve cover says 4200 - 4800 does not mean that's what is actually happening and if it isn't. that tachometer reading tells you which way you need to go with pitch. You also want to be at the very top of that range, not the bottom.

 

[jmw129]

Re: Merc Outdrive Gear Ratio 1.50 Designed for V8 350 / 305 - Proper Prop Size?

You are missing the point. What the valve cover says is the range Merc wants the engine to run at WOT. What you ACTUALLY see on your tach at WOT is what we need. Just because the valve cover says 4200 - 4800 does not mean that's what is actually happening and if it isn't. that tachometer reading tells you which way you need to go with pitch. You also want to be at the very top of that range, not the bottom.

I certainly see your point & will follow your advice in making sure the RPM is close or almost at 4800 to get the max WOT without running the engine too hard or cause pre-mature engine wear.

Thank you & everyone else with helpful advice!