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Word on the water is run a 2.68

Afternoon everyone, I'm looking to spark a debate here with factual information for new builders.

Topic on this one is: Go to gearbox combination for a SBC.

Seems everyone I have talked to on the water now days is running a Small block Chevy with a 2.68 box and a 3-4 wide blade pitched out.

Is this just the go to now? or is there actually something to this combination?

Wouldn't your cruising rpm on say a 14ftx8 be ridiculously high? Does the performance output on the hill just negate the deficiencies on the water with ( Fuel Mileage, constant higher rpm wear on engine, etc.)

Say someone put together a 383ci or 400ci 500hp-550hp with a 2.37 and a 3 blade- decent running rig-
If you took this combo and put a 2.68 with a 4 blade together- would it out perform it all around ? or just on the hill?

Lets hear it. :stirpot: :fishing
 

Swamp Thang

Well-known member
smokster said:
depends on what rpm your engine is built to run.

Exactly, that is the most important number and maximum prop rpm being second. From there you can decide the correct gear ratio for your application and preferences of performance or fuel mileage, you might sacrifice a little top end with wide blades but the new designs are so efficient all around that it probably isn't even noticeable. More gear allows you to turn more blade and more blade gives better thrust, there is a fine line between adding pitch or adding another blade... too much pitch causes torque roll and the weight of too many blades pitched light will reduce snap and push. what it boils down to is there are just too many variables involved but big horsepower, big gear and a big wide prop is a hard combination to beat on any hull big enough to carry the weight. I'm running a 400 hp 383 with a 2.88 Ox and three 82 inch race layup JX blades set at 15 on a 8x12 aluminum hull and it could use a few more degrees of pitch...spins well over 5500 with ease, just changed from three 79 inch race layup JR blades which were great on the boat but the JX blades are a big improvement. I have to say they are by far the best blades I've tried, if you want push you will probably love them. The race layup blades are quite a bit lighter and the advantages of the lighter prop are well worth the extra price IMO.
 

Slidin Gator

Well-known member
L&LLandServices said:
Wouldn't your cruising rpm on say a 14ftx8 be ridiculously high? Does the performance output on the hill just negate the deficiencies on the water with ( Fuel Mileage, constant higher rpm wear on engine, etc.)

L&L,

These are just some guidelines covering the basic physics, which favors higher gear, for most applications. Very roughly speaking, thrust ~= K * prop torque * RPM^2. This is at low speeds, say less than 40 MPH. K is a variable that varies between props, setups, top end etc. The point is, to double your thrust, you can either double your prop torque or quadruple your prop RPM, which equates to double or quadruple your Hp.

When you increase your gear, your are trading off more prop torque for less prop speed, which results in higher low speed thrust for the same Hp. This is the same tradeoff made when designing a tug boat vs. an offshore race boat. The tug boat makes wayyy more thrust for the same Hp, but that thrust falls off rapidly at speed, while the race boat prop doesn't even start pushing until the boat is up to speed.

Luckily, air is a whole lot less dense than water :cheers: Thrust falls off much more slowly vs. speed when pushing air, otherwise your basic Cessna wouldn't be able to take off with a full load and cruise well north of 100 mph.

Below say 40+ MPH (just a guess, your experience will vary) torque wins and you should net reduced engine RPM for the same thrust. Eventually the wider blade will run out of steam due to drag and you will loose top end (just think of the added load trying to tow 4 wide blades at 80 MPH down the highway). If you have the room, top end can be maintained with more diameter and less blade width.

The one key downside to all of the above is that it adds prop torque, which adds torque roll. Hull design comes into play on gear choice.
 

Waterthunder

Well-known member
Look at your gearbox as a Torque multiplier not a RPM enabler. We can tell you the exact thrust across the entire RPM range at 100RPM intervals. If you run a 80 to 84'' prop hands down the 2.68 is the way to go most of the time. I can tell you how many different tests we have done. My favorite is the guy who swears a 2.38 makes more peak thrust. I can take a prop set at 21 degrees turn it 5,300RPm's take off the prop swap to a 2.68 ratio and turn that EXACT PROP 5,600 plus and still have someone tell me the 2.38 makes more thrust.
 

Kbrewer

Member
I just got a new 2.68 rotator im paring to a 350 on a 14 foot combee I don’t have a prop yet I just hope when it’s all said and done it will run dry decent
 
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