1972 Cathedral Trihull Prop Size

[Trihulled]

Hello! I'm looking for help/information on getting the right sized prop for my boat. It is currently not running at the factory recommended RPM range for WOT.

Here is a link to a forum post about the boat itself: LINK

Boat Specs:

• Year/Make/Model: 1972 Thunderbird Cathedral Trihull w/Cuddy Cabin, Comanche(I think... not sure)
• Length: 21'
• Weight: 3,360lbs
• Engine: Mercruiser L6 250 165HP
• Drive: MR
• WOT RPM's: 3350 @ 30.5MPH (two people in the boat)
• Prop PN: 48-78120-19
  • MFG: Mercury Marine
  • Material: Aluminum
  • Diameter: 14''
  • Pitch: 19

ISSUE: The WOT specs for this motor is 3800-4200 RPMs and it is not getting there. The motor is a fresh rebuild with an electronic ignition and rebuilt carb. I'm very confident in the performance of the motor, but am looking for any information if the prop is not sized properly.

Ideally, I would love to get 5-10MPH more speed, but physics might be a problem.

The fuel tank max capacity is 37gal and is located in the rear of the boat. Not sure if that could be a limiting factor.

Any info would be much appreciated!

 

[jimmbo]

You say the engine is rebuilt. How was it rebuilt, like an Auto Engine or as a Marine Engine, as in it has Marine grind Camshaft. Same goes for the Carb and Ignition, are they the Originals or some Auto Parts replacements? If those parts are in fact the correct Marine parts, is it tuned properly, and is the Timing advance is correct? If so, and the engine is making its full hp, and the Tachometer is accurate, then you are seriously overpropped. to gain 800 rpm you will need to drop, about 3 1/2 - 4 inches in pitch.
Another thing to check, is the Throttle opening the Carb fully?

 

[Scott Danforth]

Compression numbers? Should be about 140 psi

 

[Trihulled]

Carb is original marine unit and motor was rebuilt using marine-grade parts. Ignition is a Sierra electronic distributor and plugs are the marine spec'ed plugs. Timing is verified correct at 6-degrees and the tach is reading accurately (verified with timing light). Engine compression is in the 140-150psi range. Throttle valve is 100% open at WOT.

Only thing that I can think of electro-mechanically is that the ignition runs from the battery to the switch and then back down to the coil and NOT thru a relay-switch at the engine, but there isn't a voltage drop at the coil. I don't know if the coil demands more current from v_batt as RPMs increase...

 

[jimmbo]

What is the Final Timing at WOT, and what is the Advance Curve?
This might start an argument, but what cam was used, the Marine Grind?
Electronic ignitions usually don't restrict the Voltage while running as there are no points to Burn, unless the EI uses points, and some upgrade systems did.
One other question. The prop, 3 blade, or one of those god awful 4 bladers?

 

[Bondo]

Ayuh,..... It sounds like yer gonna need a 17, or probably a 15" pitch prop,.....

 

[Trihulled]

What is the Final Timing at WOT, and what is the Advance Curve?
This might start an argument, but what cam was used, the Marine Grind?
Electronic ignitions usually don't restrict the Voltage while running as there are no points to Burn, unless the EI uses points, and some upgrade systems did.
One other question. The prop, 3 blade, or one of those god awful 4 bladers?

I haven't checked the final timing. I do know it is advancing as you throttle up, to what end I have never officially measured. The CAM was the original from the Mercruiser and the shop that did the machine work specializes in boat motors.

Prop is a 3-blade.

Guess a general question I have is, for this sized boat and engine, should it be able to go faster and hit spec'ed WOT RPM with the given prop?

 

[Trihulled]

Ayuh,..... It sounds like yer gonna need a 17, or probably a 15" pitch prop,.....

Does changing the diameter from 14'' make any sense? Going to a 14.5'' or 15' diameter or should I stay with the 14'' and try to get a 14x17 or 14x15?

 

[Bondo]

Does changing the diameter from 14'' make any sense? Going to a 14.5'' or 15' diameter or should I stay with the 14'' and try to get a 14x17 or 14x15?

Ayuh,..... As far as yer concerned, diameter is off the table,.....
As prop pitch goes down, diameter goes up,.... It's the designer's call,....
Props are bought by Pitch,.....

What's the gear ratio of yer drive,..?? It should be 1.84:1, or more likely 1.65:1,.....
If somebody put a 1.50:1 on it, that would explain the need for the tiny pitched prop,.....

 

[Trihulled]

Ayuh,..... As far as yer concerned, diameter is off the table,.....
As prop pitch goes down, diameter goes up,.... It's the designer's call,....
Props are bought by Pitch,.....

What's the gear ratio of yer drive,..?? It should be 1.84:1, or more likely 1.65:1,.....
If somebody put a 1.50:1 on it, that would explain the need for the tiny pitched prop,.....

I don't know what the ratio is, only that is is a MR outdrive. Is there an easy way to determine this?

 

[jimmbo]

On Mercrusier Drives, the Ratio is usually on the Branding on or around the upper Gearcase

 

[jlh3rd]

Does changing the diameter from 14'' make any sense? Going to a 14.5'' or 15' diameter or should I stay with the 14'' and try to get a 14x17 or 14x15?

according to what I've learned, mostly on here, if you go to a 15x19 from your 14x19, your rpm is gonna drop even further at wot...like another 200 rpm....
every change on a prop affects the final max rpm.... number of blades, pitch, diameter, and material.....
I don't quite understand the statement "props are bought by pitch". Not only did I choose mine by looking at diameter, pitch, blades and material, I also had to consider my gear case diameter.

 

[airshot]

Had a similiar issue some years back, bought an I/O, 3.0 engine with an extra outdrive. Owner couldn't get the rpm up high enough. Examination found he had a 1.65 ratio outdrive, the "spare" outdrive was the 1.98 ratio needed for the 3.0 engine!! I replaced the seals and installed the 1.98 drive and everything was back in sync!

 

[jimmbo]

Some people just think cause it 'looks' the same, that it is the same. The 1.65 would have been fine on a 5 liter.

 

[Trihulled]

I'll see if I can determine the gear ratio and post it.

 

[Texasmark]

I'll see if I can determine the gear ratio and post it.

No biggie. Mark the crankshaft and one of the blades on the prop. Put it in gear and rotate the engine until the prop has made 360*. Note the position of the mark on the crankshaft pulley. 1.65 means that the engine had to turn 1.65 turns for the prop to turn once. .65 x 360 = 234* For reference, North would be 0, South 180, West 270. 270-180 = 90, over 2 + 180 = 225 (half way between S and W).
So, if you started with your mark at the top of the crankshaft pulley and had a 1.65 your mark would be slightly above South West (WSW). If higher it would be SSW or less.