Propping the Monte Carlo

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Brian
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Propping the Monte Carlo

Post by Brian »

If I have, say, 400 HP available, primarily want a boat that is quick out of the hole for skiing or loads, and not that concerned about top speed (50-ish), how would I prop and gear my Monte Carlo (27')? If I drop the strut, would it make sense to use a larger diameter prop? 2/3/4 blade? Manufacturer?

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Mr Hot Rod
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Re: Propping the Monte Carlo

Post by Mr Hot Rod »

The Monte Carlo Design Page Underwater hardware Notes recommend strut # 90-122. It has a 7 1/2" Drop which will accommodate propellers to 14" diameter. A 13" diameter propeller with a 1" bore with pitch/rotation to suit motor is also recommended.

We've run our calculator with the following data :
  • LOA : 27'
  • Waterline length : 24 (estimated)
  • Displacement : 4500 lbs (Hull=1400, 8 passengers @ 175=1400, engine=1200, fuel=240, Total = 4250 lbs)
  • Engine : 400 HP
  • Max RPM : 4500
  • Transmission Ratio : 1.21:1 (ZF 63A 8°down-angle, Rated at 458 HP (gas) @ 4800 RPM)
  • Strut drop : 7.5"
  • Required speed : 50 MPH
We've included links to the PDF version of the SHP VS Speed Calculator and PDF and Excel versions of the Propeller and Gearing Calculator :
The SHP VS Speed calculator retuns the following prop sizing for various gear ratios :
  • .88:1 (12% overdrive) = 12x15
  • 1:1 = 13x17
  • 1.21:1 = 15x21
  • 1.5 = 17x26
Since the strut will accomodate propellers to 14" diameter, the recommended propellers for 1.21:1 and 1.5:1 ratios are not suitable. A strut with more drop will have to be installed to handle the extra diameter. Note that for speeds over 35 knots (40 MPH), it's desirable to reduce propeller diameter. This is because the drag force of the water rushing past the hull increases as the square of boat speed. The resistance of appendage drag of a large propeller, its strut, and its shaft impede performance and may lead to handling issues at speed.

Best fit is the 1:1 ratio with a 13x17 prop. The ZF 63C or Velvet Drive 72C inline transmissions would be a good match. You can check out the specs on our website's Transmission Page.

Increasing the propeller shafting to 1-1/8" or 1-1/4" may provide you with a wider range of steeper pitch propellers to choose from.

Performance data for the Monte Carlo is scarce. If John Amundsen doesn't chime in soon, you may PM him (Screen name jamundsen) for his data. Here's a link to his build thread : Monte Carlo update

Let me know if you wish to make any changes to your input data or have any questions about the calculator output. Accurate input data is crucial for a valid prediction !

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Last edited by Mr Hot Rod on Wed Nov 21, 2018 8:59 pm, edited 3 times in total.

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jamundsen
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Re: Propping the Monte Carlo

Post by jamundsen »

Performance data for the Monte is still scarce. I currently am running a 375 hp 454. The prop is one I acquired for $75. It's a 13 x13. I think. I have not taken the engine over 3000 rpm's yet. I plan on doing some work in the water soon so then ill have more info. I have a 1 1/4 shaft. 72c 1:1 tranny.
It comes up on plane great. No bow rise at all. It just planes.
I'll be taking Paul's advice on the prop size.
John Amundsen
Monte Carlo
Lakeland,Fl

Work tends to get in the way of boat building

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kens
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Re: Propping the Monte Carlo

Post by kens »

I found the best way to prop a new build is to get a good rapport with a reputable propshop. Here is what I did.
I paid a deposit for a used prop to my propshop with the agreement that they would 'get it right'......gauranteed!!!!
They sent me their 'best estimsted' wheel.
I ran it and reported the results. Then sent that wheel back to them for tweeking. I ran the returned prop and reported the new results. Sent that wheel back.
They sent me another and the whole process started again. and again......and again. When I was finally tired of trying all the comninations of diameter, pitch & cup, I told them to send me a 15x15 med cup and I would accept that for my original paid deposit.

I also have a couple wheels bought off Ebay that I have run & had tweeked. With all this info in hand I now have the size data I need to plunk down the hard money for a brand new wheel of cast & virgin size.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Brian
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Re: Propping the Monte Carlo

Post by Brian »

Wow, Paul! Thank you for all that great info. Since you seem to be The Wizard at this kind of thing, I have another (related) question:

Is it worth dropping the prop to go up another diameter? I am thinking of using a Hurth 8 deg drive, which would allow almost any angle from 10-15. The Monte Carlo is designed with a 15 degree shaft. High school geometry/physics is telling me a shallower shaft angle transmits more energy in the forward direction. But is a difference of 2-3 degrees enough to bother with?

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Bill Edmundson
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Re: Propping the Monte Carlo

Post by Bill Edmundson »

Brian

The shaft angle is what it is. The prop location doesn't change. If you go to a flatter angle the engine must move forward. That will change the center of gravity and adversly affect the trim of the boat. A down angle tranny allows the engine to sit flatter. This helps with clearances in the engine compartment.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

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Mr Hot Rod
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Re: Propping the Monte Carlo

Post by Mr Hot Rod »

The SHP VS Speed Calculator I posted previously determines diameter from HP and RPM using Crouch's Diameter-HP-RPM Formula. It recommends a 15x21 for a 1.21:1 gear ratio. I'd stick to that if you plan on using a down-angle transmission and aren't too concerned about speed.

At low to moderate speeds (35 knots or 40 MPH) the slower shaft RPM and the larger diameter propeller are more efficient. At speeds above 40 MPH, the resistance of appendage drag of a larger propeller, strut and shaft increases as the square of boat speed. This impedes performance and may cause hull instability issues. Smaller diameter, high pitch-ratio propellers are better suited if speed is a major goal.

We field a lot of questions about propeller sizing, so we posted an article to our website about Determining Diameter from HP and RPM : Unless you have a major hatch clearance issue, I'd suggest a 1:1 ratio transmission with the 13" propeller. Since you're also stretching the hull, you may want to produce a shaft layout diagram.

Here's a sample scaled diagram for a Cracker Box with a flywheel-forward V8 mated to a Velvet Drive 71C transmission :
For instructions on how to create a diagram, refer to link # 4 (Inboard Motor Installation Chapter 11 : Shaft Angles & Layouts by Glen L Witt) and link #7 (Riviera Shaft Hole Boring by Dave Lott) :
When creating the diagram, place the rudder, strut and strut drop as per plans. Position the engine and trans where you want them (stock location for stock hull or adjusted location for a stretched hull) and draw a line between the trans output flange and strut centerline. This will give you your shaft angle. You may have to juggle the strut drop slightly to match a stock strut's angle and drop, or shim the strut base to suit your needs.

Feel free to contact us through our website or PM me if you need some help with your diagram. Once you've got your dimensions on paper (or the shop floor !) your options will become obvious.

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Last edited by Mr Hot Rod on Wed Nov 21, 2018 9:02 pm, edited 2 times in total.

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kens
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Re: Propping the Monte Carlo

Post by kens »

There is a lot of info on this thread, but, there is also some gaps.
Most of the data does not compare the 2 most common props; 3blade vs 4blade.
If you do the math, you find that 'blade area' is a major factor in prop sizing.
In the math you will find that a 14" 4blade is approx the blade area of a 16" 3blade.
Not to mention that blade cupping = pitch changes.
All of the above formula are great if you can establish a 'standard'
So, what is the 'standard' in all the math propping calculations? Is a 3blade, standard? Is zero cupping standard math?
I have run a bakers dozen props on my boat, and the one I want to run next doesnt exist, and that is a 15" 4blade.
I am not running the HP of the V8 boats, but however, I AM running the Torque of the V8s with my 2:1 gear vs a 1:1 gear of thre V8s.
If I was to build a Monto Carlo, or similar I would install a strut that would be a standard wheel of 15" 3blade. This way you could play with wheels and run (similar blade area) 13"4blade: 14"3blade:, 15" 3blade: and start with a 1 1/8" shaft. this is assuming a hull longer than 21 foot with a BBC @ 1:1 gear no cupping at the start.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Brian
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Re: Propping the Monte Carlo

Post by Brian »

Bill, I have a 1" = 1' drawing of a BB Mercruiser with an 8 degree Hurth on it. so I can actually place it over the Monte Carlo plans to see how it fits. Because the Hurth uses a drop of about 6", the engine can remain where it is supposed to be and you can get a 10 degree shaft angle, with the prop in exactly the same place with the same clearance. The only thing that will be different is where the shaft log penetrates the keel. It looks like it might penetrate right at the frame, so if it's worth setting up a 10 degree shaft angle, I will have to reinforce a passageway through that frame. Other than that, everything fits beautifully.

This thing about a 15" prop has me confused. That would require a longer drop on the strut for clearance, and would put my 10 degree shaft log even further forward.

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kens
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Re: Propping the Monte Carlo

Post by kens »

what tip clearance do you figure with 14" dia wheel? 20%. 15%??
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Brian
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Re: Propping the Monte Carlo

Post by Brian »

Kens, the tip clearance would be the same as recommended for the 13, unless someone advises differently. I would just get a deeper strut.

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kens
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Re: Propping the Monte Carlo

Post by kens »

the strut mentioned above indicates it takes a 1" shaft. If you are planning a BBC with 1.2:1 ratio, then I suggest a 1 1/8" shaft.
That combo will put out lots of torque and you likely to run out of prop sizing for a 1" shaft. I think it best to allow for 1 1/8" shaft.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

Brian
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Re: Propping the Monte Carlo

Post by Brian »

Any reason I wouldn't want to go to a 1.25" shaft? Any of this prop/gear discussion relate to shaft angle? What if I used the 8 deg available with the ZF63A and a 13" wheel?

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Mr Hot Rod
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Re: Propping the Monte Carlo

Post by Mr Hot Rod »

Any reason I wouldn't want to go to a 1.25" shaft?
  • The diameter required for a propeller shaft depends on many variables, including SHP, propeller shaft RPM, propeller diameter, as well as the strength of the material to be used for the shaft.

    Here's a link to some guidelines : Propeller shafts (WebLetter 43)

    If you don't want to run larger than 1" diameter and have copious amounts of torque on hand, look into Aquamet 22 shafting. This is very tough shafting as we've had one mishap which twisted a strut and snapped a rudder without damaging the shaft.
Any of this prop/gear discussion relate to shaft angle?
  • Unless your main objective is speed, you shouldn't worry about it.

    Here's a spreadsheet which calculates the horizontal thrust reduction for shaft angles between 8 and 15 degrees :
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Chelsea, Quebec

Building the Glen-L Hot Rod : http://www.kanecustomboats.com

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kens
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Re: Propping the Monte Carlo

Post by kens »

the thing about the down angle trannys is that they are reduction gears. Even the shallowest ratio is about 1.2:1; some are 1.3:1. That means that they multiply engine torque and reduce shaft speed. Both these when used in shaft sizing formula are factors to increase shaft dia. Also, when using say, 400HP and run it through a reduction gear, your prop sizing goes upward. Largest prop available on 1" shaft is 14" 4blade, and you would be at the upper limit on pitch to begin with.
Go to 1 1/8" shaft and you get allowed 15' dia. and up. Though I would not go 16" on a Mahogany, but I would size it to accept 1 1/8" shaft and 15" prop.
We have a forum member with a 350chev, MPI fuel injected, 1.2:1 down angle with 1" shaft, he ran out of stock props (14" 4blade) and had to resort to custom prop to hold his engine from hitting the rev-limiter.
The tranny used is irrelevant to the boats shaft angle. It would be much easier to use a regular VelvetDrive 72c 1:1 . that would take the 400hp you mentioned, it has higher shaft speed which allow 1" shaft and the strut you mentioned for 14" prop.
Oak is over rated, everything about it takes extra time; then it warps, splits or checks !!! :roll:

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