Changed Out Drive. What Prop do I need?

[John3]

I have a 1967, 165 Hp, Mercruised, straight 6 (250 cubic Inch) in a 23 Ft. Dorsett Cabin Cruiser, weighing about 5500 lbs (Empty weight 3600 lbs). bought it in 72 & use it a lot. I munched the original OD and replaced it with a used pre Alpha with different gearing. With the old OD it would get up & plane with a normal load & I could run it at about 3400 RPM & 3 or 4 inches of vacuum. I took it out the other day & with the new (used) OD and 2 adults, ? tank of gas, water & minimal equipment on board it maxed out at 2800 RPM was definitely Not on Plane & zero inches of Vacuum. The prop is a Mercruiser 48-79570-15, 15? pitch & I think 15 ? inch diameter.
I?d like to get the performance back to where it was and at this point I?d rather change props than gears.
Anyone have a good formula for calculating a different pitch prop?
John

 

[Titanium48]

Re: Changed Out Drive. What Prop do I need?

If a 15p has you so overpropped that you can't get on plane, you might need to change the gear ratio. It may be difficult to find a prop less than 13p, and large low pitch props aren't very efficient.

 

[Bondo]

Re: Changed Out Drive. What Prop do I need?

I?d like to get the performance back to where it was and at this point I?d rather change props than gears.

Ayuh,... There's No such prop,... Ya need the Right gear ratio...

 

[steelespike]

Re: Changed Out Drive. What Prop do I need?

I agree; you need 1000 rpm just to get close to the bottom of the rpm range suggested for your motor.That about a 5" pitch change,and you would still struggle because your
your motor is so far off from its power curve and the boat is underpowered to begin with.

 

[45Auto]

Re: Changed Out Drive. What Prop do I need?

Calculating prop sizes vs gear ratios is no different than calculating tire sizes vs rear end gearing on cars. To keep the same performance the ratios must remain constant. The problem you'll run into on a boat is when the prop sizes move out of the most efficient range for the diameter.

New prop size / new gear ratio = old prop size / old gear ratio

So using our basic high school algebra:

New prop size = old prop size / old gear ratio * new gear ratio

For example, if you had a 15" pitch prop and changed from a 2.0 to a 1.5 drive ratio:

new prop size = 15" / 2.0 * 1.5 = 11.25"

You would need an 11.25" pitch prop to get the same nominal performance as the 15" pitch prop on the original drive. Unfortunately, the 11.25" pitch prop would be spinning 33% faster than the original 15" pitch prop at the same speeds. That will probably put the tip velocity high enough to cause cavitation, which means that the new gear ratio will never work very well ......

Hopefully your gear ratio swap was closer together than the one I used as an example, and you can find a prop that will work.

Imagine that, there's actually a reason that the original manufacturer matches drive ratios to engine size!

 

[John3]

Re: Changed Out Drive. What Prop do I need?

Calculating prop sizes vs gear ratios is no different than calculating tire sizes vs rear end gearing on cars. To keep the same performance the ratios must remain constant.

Think you're basically right, except there's also slippage & cavitation in a prop.
There are a lot of props available & using the mechanical ratio only it looks like 13? pitch might be too little & 14? still too much. Without knowing about slippage I may spend $100 & try a 13? to see how it works. Otherwise, it looks like, new gears installed is close to the price of a new upper unit.
Wonder if it?s possible to change the pitch of a prop?

 

[steelespike]

Re: Changed Out Drive. What Prop do I need?

Yes you can chage pitch within certain limits depending on the prop and material.
Typically a 1" change nets 100 to 200 rpm. It sure woould help to know your old wot
max rpm and speed. Also sure would help to know your speed at 2800 rpm.
If it wasn't on plane then a slight pitch change might get you to the bottom of the rpm range.Have you tried lightening the boat to see if you can get it to plane?
That might at least help dtermine a prop size.Perhaps a person on the bow would get it over the hump.
You can figure your ratio by rotating the motor by hand and count the revolutions at one prop rpm.Do you know what the leg came came off of?My prop calculator indicates 22mph at 1.5 ratio(yours was likely 1.68) and 15% slip of course that would be planed with a boat of reasonable size and weight.

 

[Bondo]

Re: Changed Out Drive. What Prop do I need?

Think you're basically right, except there's also slippage & cavitation in a prop.
There are a lot of props available & using the mechanical ratio only it looks like 13? pitch might be too little & 14? still too much. Without knowing about slippage I may spend $100 & try a 13? to see how it works. Otherwise, it looks like, new gears installed is close to the price of a new upper unit.
Wonder if it?s possible to change the pitch of a prop?

Ayuh,.... Problem with yer plan is,...
The smallest prop normally available for yer drive is 15"....

Been there, done that...