Help prop an old boat?

[e.Rock]

Long time lurker, finally registered!
I'm nearing completion of my restored 1958 Fleetform Commodore 16' runabout. The boat is mostly complete, and I'm working on refurbishing the motor - 1972(ish) Mercury 1150 Tower of Power. I've just finished salvage and installation of the power trim, so I can finally get it propped correctly and make it cosmetically look more like the 1950's Mercury's...

Anyway, the motor runs amazingly great and I don't have any problems, but I'm just running the beat up prop that came with the motor. It holeshots fine, doesn't seem to cavitate at all, but its top speed in the upper 30's seems low for an admittedly over-powered boat. Not that I really want to go that fast on this old of a boat, it just seems to be an indicator that I'm not propped right.

Currently, of the fixed positions offered on the transom mount, one is too high and the boat porpoises badly, and the other is too low and I can feel the nose of the boat pushing down and plowing at WOT. I also have a whale tail on it, which doesn't help drag, as I think it sits too low in the water for this boat (we tried it to see if it could be run in the upper position and I never took it off). Thus, I'm hoping the trim will let me get me to a better position and get rid of the whale tail.

I'm looking for help to just find a good all-around prop. Here's the important tech info:
1. It's a light boat (I don't know the weight)
2. relatively narrow beam (~68")
3. don't do any water sports with it - she's just a cruiser
4. the max load is 4 adults and a dog
5. my starting point prop is beat up, so WOT/RPM tests won't tell us anything
6. I don't have a tach (although I could put on one of those cheap conductive tachs if it's really necessary).

Yep, exactly what you guys want to hear. I know.

None of the online "prop wizards" can handle this old of a motor, let alone the boat. Like I said, I'm just looking for a good compromise that's "close" that I'll probably never take off. I feel like this is just going to be a gut call from someone with lots and lots of experience.

What's on there currently (assuming I'm reading the prop correctly):
RH 12.75 x 21P - 15 spline

I've thought about picking some sort of an arbitrary used prop in good condition as a starting point, but I can't even figure that out - nobody says what newer props fit on this old Merc, and I'm still not sure about the old props I think will fit. Is there some sort of a compatibility guide out there somewhere? Can you tell from a glance what props fit what generations?

Attached pic of the boat for hull shape reference.

Thanks in advance,
-e

 

[Scott Danforth]

Get a tach. That is important

 

[e.Rock]

Get a tach. That is important

Ok, I understand why this is important when fine tuning your prop selection, but can you tell me how it will be used to pick my FIRST prop?

-fellow cheesehead

 

[Willyclay]

Welcome to the forum. The tach will help you avoid overrevving that great old ToP. Good luck!

 

[Bondo]

Ayuh,..... Welcome Aboard,...... Considerin' the standard range of prop pitches runs from 13" / 15" to 21" / 23", 'n you think the 21" on it is to small,.....
I'd look for another 21" or a 23" to test it out,....

 

[jimmbo]

A 17" or 19" would be a good place to start. WOT range for that Engine is 4800-5300
BTW your engine, well the Cowl Wrap, is a 74. A 72 would have Kiekhaefer Mercury on the Faceplate. Yours has Mercury Marine

 

[Scott Danforth]

Ok, I understand why this is important when fine tuning your prop selection, but can you tell me how it will be used to pick my FIRST prop?

-fellow cheesehead

Spin that ToP at 6000 RPM for a bit and you will blow the motor as cylinder #3 welds itself

You want a tach to tell you what RPM you are spinning at as you test any prop.

 

[JimS123]

Ok, I understand why this is important when fine tuning your prop selection, but can you tell me how it will be used to pick my FIRST prop?

-fellow cheesehead

Look up your engine in a Michigan Wheel catalog and select the prop that is suggested. Tach it out and probably you will be OK. If not, you have the data to select a better one.

 

[e.Rock]

Look up your engine in a Michigan Wheel catalog and select the prop that is suggested. Tach it out and probably you will be OK. If not, you have the data to select a better one.

Good tip - I had no idea this site existed. According to their online outboard sizer, it's calling for a 13 diameter, 21 pitch (for a 15-18' boat).

It's a bit confusing though, as they have three different options that would match my boat length. If I go with the smallest boat (up to 17') it tells me to go with a 23 pitch. If I go with the largest option 16-19', it's a 19 pitch.

Either way it sounds like my existing prop might be close, even though it's dinged up. Guess I'll order a tach and see where it stands.

Thanks all for the responses!
-e

 

[jimmbo]

Those are only Charts. And based on my 50+ yrs of Propping, I feel a lot of them are a little Happy/Optomistic Pitchwise. 19" would be a far better choice than a 23, unless the boat, motor, gear, fuel, and driver add up to less than 700lbs

 

[Texasmark]

The tach is a must to start. You didn't say you had power trim so you select the thrust angle with a pin location on the transom bracket.

After you get the tach, load up with your 4 people and the dog and adjust that pin for the tilt angle of the engine with respect to the transom position that gives you the best performance. Take GPS speed and RPM numbers and post back on here.

 

[nola mike]

Also, getting the trim sorted first is important. Your wot rpms will be low if you're plowing