Bought this boat with no prop. Where to start?

[Reserector_]

1993 Caravelle 1750 Classic Bowrider. 17' long. 1982 Mercruiser 140 4 cyl with OMC Cobra 3.0 outdrive.
I plan to use it for general lake cruising with occasional knee board towing, etc.
I need suggestions for what diameter, pitch and number of blades to use as a starting point.
I suppose I could call the factory and hope they could tell me what this 25 yr old boat came with.
I think I would like to end up with a stainless prop for the sake of durability, but maybe start with an economy prop to start with?
Are there certain brands to prefer or avoid?

 

[jimmbo]

Do you know what gear ratio your Cobra Drive has? Without that tidbit, it is going to be a real Crap Shoot to pick a starting point. If you don't have the ratio, do you have any idea what engine the drive came from?

If indeed you have Brand 'M' for the engine, and brand 'O' for the outdrive, the Boat Manufacturer is not going to be much help without that info either

Now for the Big Question... How long ago did you buy this boat?
DId you ask the seller, or did they mention what Prop they had been running? Or did they just cobble M and O together and then hung a For Sale sign on it?

 

[Scott Danforth]

with a 3.0, you would have about a 19p prop max

 

[Reserector_]

The PO cobbled the M and the O together over 15 years ago. He has no recollection of what prop he used.

Here are the serial plates on both units. I looked up the engine on the Mercury site which is where I learned it was an '82 model. PO says it never saw 100 hours before the floor failed and he parked it.

I'm having difficulty nailing down the particulars of the outdrive. I have been searching, but have not been able to come up with a serial number guide for the stern drive by itself.
I still have it on the hoist, and it is raining, or I would pop the cover and count the teeth. But for now I'll keep web searching.

 

[Faztbullet]

Your going to need more than a prop....From the insulation stuffed next to engine warns me it was not winterized properly. Start engine first and see if ok then get ready to spend some time and money fixing rotten transom,stringers and floor...

 

[Reserector_]

Your going to need more than a prop....From the insulation stuffed next to engine warns me it was not winterized properly. Start engine first and see if ok then get ready to spend some time and money fixing rotten transom,stringers and floor...

I'm aware. That info is on another thread in the appropriate section. https://forums.iboats.com/forum/boa...building-a-93-caravelle-1750-classic-bowrider

It would be foolish to wait until the boat is complete before searching for props. I am doing what I call "multi-tasking".

 

[jimmbo]

There is no other plate on the outdrive that has a #:# on it? You should pull the outdrive off to inspect the bellows, and the u joints. While it is off you can turn the driveshaft and determine the gear ratio watching the propshaft, in gear of course.

Does the PO know what motor was in it before he put the Cruiser in?

In hindsight, The boat likely came from the factory with a OMC engine. For whatever reason the engine needed replacing and since the OMC and Mercruiser often used the same engines...
Maybe the factory call might shed some info, if they are still in business.

 

[Reserector_]

The numbers you and I are expecting to find must have been on the engine. The PO replaced the OMC 3.0 with the Merc. He doesn't have any of that info. He's nowhere near the nerd that I am. Lol.
The drive is already removed, so I marked the shafts and turned it by hand and came up with 1.88:1 ratio.
That is to say that the input shaft had to turn 1.88 times in order to turn the prop shaft one full revolution.

Input is 17 splines. I turned it once plus another 15 of the 17.

Feel free to check my math.

 

[jimmbo]

You were close. Probably a 1.86:1.

As Scott mentioned previously, a 19 inch would be a good starting point. Your engine has a WOT range of 4200 - 4600 rpm

 

[Reserector_]

Ok. Would that be RH rotation? Three blade? What diameter?

 

[jimmbo]

The chances of a Counter Rotation outdrive in that HP range is pretty low, so yes RH. I would stick with a 3 blade, Let the Prop Engineers worry about Prop Diameter, as it can vary a bit between brands, and even models with a brand. You only need to be concerned that the hub has the proper diameter for the gear housing, and as most props these days use replaceable Hubs, the correct hub for the prop to be used on a Cobra drive.

Mercury makes some very fine Props that can be used with the proper Hub Kit. 3rd party Manufacturers like Solas, Michigan, Turning Point all market AL props that can be used, some people swear by them, I tend to swear at them. Just my preference.
A 17 ft boat with a 140 I/O is not going to be speed demon by any means, but you should be able to get between 33 and 38 mph. As I said previously the Engine WOT range is 4200 - 4600, try to prop it so it is running close, but not above 4600 rpm.

 

[Scott Danforth]

Ok. Would that be RH rotation? Three blade? What diameter?

19 pitch 3-blade by what ever diameter they sell for the cobra 1.81:a drive ratio. you will have only 1 or two diameters and that wont matter. I would look on CL and get the lowest cost prop you can (about $30)

however long prior to worrying about a $30 prop, you need to get the motor and gear box sorted.

 

[Jim Hawkins]

My experience has shown me the best place to start looking for a prop is a good prop shop. Not sure what you have available but my local shop will take your specs, find a range of props to suggest and then pull a used or cheap aluminum prop off the shelf for a test. I check results and if need be just go back to the shop and trade that prop for the next best idea. That way I'm not stuck with the wrong prop.

 

[QBhoy]

Go for a 19” to start. Unless the drive on it now was previously attached to a bigger engine.
19 and 21” would be the norm for an average 3.0 bow rider of 18ft.

 

[Reserector_]

You've all been a huge help. I've already found a good candidate on ebay, but I will hold off until they have a xx% off deal at the very least. I have a small local boat shop that I can check out, also. I've got time to search around.
One more piece of the puzzle solved.