2.3 ch3 or 2.52 ch4 help

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stickittoem
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2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Mon Dec 26, 2016 12:43 pm

Please only chime in if you have experience or are knowledgeable on what i am going back and forth over. Having a boat built and currently split between a ch3 2.3:1 and a ch4 2.52:1. now lets back up one step. my last boat came with a stinger 2.09:1. i rebuilt the whole rig and in the process i was able to swap gear drives for a brand new 2.3:1 so i did. I noticed very little difference if any. so back on topic. i will be running NGR blades and this will not change. small med or large hub? my motor is an all aluminum fuel injected mas 632. puts out about 850hp/850tq. the boat is a 18x8 and the front is decked and has walk boards all the way back. I am more concerned with being able to go where i want(dry running, hill running,breaking loose, etc.) versus top end speed. when i get on plane i try and run lowest rpm while staying on plane. now with all this said what do i need to know?

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Deano
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Re: 2.3 ch3 or 2.52 ch4 help

Postby Deano » Mon Dec 26, 2016 2:30 pm

For all out max push (given other things being equal), the fact is that diameter trumps pitch.
Especially, given the size of your boat and power plant, the most thrust efficient hub option will be the one that gives you the largest diameter available for the configuration you would be running (ie. three, four or five blades).

The way you describe how you want to dial in your performance, I think you would want to run a five blade. The only drawback to this approach is a potential loss of top end speed due to increased parasitic drag. Your intended use sounds very similar to mine, in which case that potential loss is inconsequential, but would aid in attaining the lower cruise rpm I believe you're after.

Whether a 2.3 or a 2.52 ratio will be preferable is largely contingent on where the sweet spot of the NGR blades is and where you spin your engine. My NGR experience is to lacking to be able to help much beyond that.
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Re: 2.3 ch3 or 2.52 ch4 help

Postby CarMotorBarge » Mon Dec 26, 2016 2:49 pm

I just checked the Century website and it said that the CH-3 is only rated for 600 HSP motors. So you might be stuck with the CH-4. My recommendation is to call Century and find out for sure.

Deano is definitely correct that diameter trumps pitch when it comes to static thrust. I would get the largest 4 blade NGR that you can fit. You might have enough HSP for a longer 5 blade. Sensenich should be able to provide advice on this.

You also might want to call GTO and ask them for some advice. They have built a number of boats with this engine package and could recommend what prop.
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stickittoem
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Re: 2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Mon Dec 26, 2016 3:09 pm

i have talked to century. they told me they under rate the ch3.

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Re: 2.3 ch3 or 2.52 ch4 help

Postby CarMotorBarge » Mon Dec 26, 2016 4:53 pm

That is good to know. Now the next question is what RPMs do you plan to run at WOT? What HSP is the motor making at this RPM? What RPM does the motor make the most HSP at? You will want to turn the NGR around 2100 to 2200 RPMs at the prop.
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Re: 2.3 ch3 or 2.52 ch4 help

Postby SWAMPHUNTER45 » Mon Dec 26, 2016 5:19 pm

A 800hp all aluminum BB is a lot of engine for a CH3.

When you factor target prop rpm and match to your engine data your probably going to want the larger ratio or want even taller yet.

I run a 2.37 ratio gear on my 542 Cad but my peak torque is at a lower rpm compared to the hp BBC platforms. If you seek a bulldozer go anywhere big boat it's hard to beat 4 or 5 big wide S blades and gear to get you there. Maybe consider speaking to Darrin at Sensenich.

In your old boat if the engine power at rpm was limited by its components abilities, swapping to another drive with a bigger ratio would yield little if any change in peak performance. The same can be true if you under gear an engine that is built to run at a higher rpm.
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Re: 2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Mon Dec 26, 2016 6:21 pm

carmotorbarge...i am used to running at 5200..talked to the guy building the motor and he said a few hundred more rpm will be ok as well if needed. peak numbers are 850 at 5800 rpm and 850 @ 4600 rpm

swamphunter45...i had just been looking at the s...max rpm is 2300 so a 2.3:1 would get you there at 5290rpm witch brings you back to the issue of the ch3. what would be the outcome with the 2.52:1? that would put the max limit on the s at about 5800rpm...that is to high in my opinion. what am i not understanding?

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Re: 2.3 ch3 or 2.52 ch4 help

Postby CarMotorBarge » Mon Dec 26, 2016 6:38 pm

The 2300 RPMs for the S blade is the max RPMs from a safety perspective and is not what you should turn that blade for optimal efficiency. I had a MAS 540 BBC and turned a 4 blade S. I didn't see much more thrust when the prop RPM got above 1900. The prop just seemed to fall on its face. I was running a 2.55 Stinger and the 540 made its max HSP at 6000 RPMs.

If you are worried about push, my recommendation is to go with the 2.52 instead of the 2.3. With the 2.52, you will be turning 5544 engine RPMs when the prop is turning 2200 RPMs. The advantage of the 2.52 is that it allows you to get to more HSP for a given prop speed. With the 2.3, you will be turning the motor only 5060 RPMs. The 2.52 allows 500 more RPMs of HSP to be used so that you get more pitch or length in the prop.

The 2.52 also gives you more flexibility than the 2.3. What I mean by this is that you can over pitch the prop with the 2.52 and improve you fuel mileage. So if you are going to be running with a heavy load, you can pitch the prop to let the engine turn 5500. You will get more static thrust, but your fuel mileage will go down. If you are going to be running light, you can pitch the prop to let the engine turn 5000. You will lose static thrust, but your fuel mileage will improve.
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stickittoem
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Re: 2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Mon Dec 26, 2016 7:09 pm

With that being said....what length would you recommend in a 4 blade s. I heard 5 is wasteful

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Re: 2.3 ch3 or 2.52 ch4 help

Postby CarMotorBarge » Mon Dec 26, 2016 7:15 pm

I would get the short 4 blade hub. I think the diameter is 83.5 inches. Also I would use the 2.67 ratio with the S blade so that you can get to more HSP. I had a 2.55 Stinger with my 540 MAS and I wished that I had went with the 2.68 ratio instead.
14x7.5 Al David hull with 14 inch transom
419 CI Horsepower Barn LS3 with 2.68 Rotator soft drive swinging 4 blade 83.5" R
GTO Rigging and B&S Tilt Trailer
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stickittoem
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Re: 2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Mon Dec 26, 2016 8:34 pm

So the 2.68 would hit that 1900 at just under 5100. Im asuming that this ratio would allow more pitch witch is more thrust. The higher the ratio the more pitch correct? The 2.68 would handle more at 5200 rpm than a 2.38...right? Wich would be more of what im going for...more bulldozer like. Im still trying to wrap my mind around this. Its something i have ever fully understood

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Re: 2.3 ch3 or 2.52 ch4 help

Postby CarMotorBarge » Tue Dec 27, 2016 7:29 am

Yes, the 2.67 will allow you to put more pitch in the prop. To help illustrate this lets talk about prop length. Most A/C will turn a 72 inch prop about 3000 RPMs. However, you will never see an 84 inch reduction prop turned 3000 RPMs. They are generally in the 1900 to 2300 RPM range. One of the primary reasons for this is the higher tip speed on the longer prop. When the tip speed gets too high (approaches the sound barrier), the efficiency of the prop drops dramatically. So you can't always just turn the engine faster to get more thrust out of a 84 inch prop. You have to add more gear to turn the engine faster and this adding gear is what enables you to add more pitch to the prop.

Another way to look at this problem is to just run thru several examples with different ratios. So lets assume your engine is being used and the prop needs to turn 2000 RPMs at WOT. Below the first column is the ratio and the second column is the engine RPM with the prop turning 2000 RPMs:

Ratio Engine RPM
1.0 2000
1.5 3000
2.0 4000
2.3 4600
2.5 5000
2.7 5400

So I did a little rounding on the gear ratios to make the math easier. As you go thru the ratios from 1.0 thru 2.7, you will notice that the engine RPMs are going up. Since your motor makes the most HSP at 5800, the HSP will also go up as you go thru the ratios from 1.0 thru 2.7. HSP is what turns the prop. HSP is also what allows you to turn a longer prop or a prop with more pitch. So in this example, the 2.7 ratio will allow your engine to get to the most HSP and turn the most prop.
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Re: 2.3 ch3 or 2.52 ch4 help

Postby SWAMPHUNTER45 » Tue Dec 27, 2016 9:48 am

S blades perform very well at RPMs below their max.

I spin mine at 1850 and Sensenich suggested I try adding more pitch slowing them and see if I gained more push. At their current setting and rpm they are like having 4x4 on the boat.

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Re: 2.3 ch3 or 2.52 ch4 help

Postby SWAMPHUNTER45 » Tue Dec 27, 2016 11:16 am

I crunched some hypothetical numbers and going off my experiences your deep water cruise with a 2.68 ratio mated to 4xS will most likely fall between 3400 and 3900rpm depending on load and headwind.
Shallow water or slick mud 2600 to 2900 rpm.
Break on dry ground 4200 to 4800rpm

Realize this is a SWAG off my data but is at nothing else something for you to forecast with.

stickittoem
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Re: 2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Tue Dec 27, 2016 3:39 pm

thanks everyone for your help. i talked to century and sensenich both and this is what i came up with. rob over at century is a veteran airboater like many of you and he said i cant go wrong with a 2.3 and the new jx prop. don over at sensenich said that the new jx blade pushes like the s series and winds up and snaps quick like the ngr, best of both worlds he says with their new design on the jx. the jx likes and performs best between 2000 and 2300. i did the math and the 2.3 will get me to 2260 at the 5200rpm i like to set at wot. the 2.52 will get me to the same number running wot at 5700rpm and i am not comfortable with that.

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Re: 2.3 ch3 or 2.52 ch4 help

Postby CarMotorBarge » Wed Dec 28, 2016 6:15 am

Sounds like you have a winner. Are you going with the 3 or 4 blade?
14x7.5 Al David hull with 14 inch transom
419 CI Horsepower Barn LS3 with 2.68 Rotator soft drive swinging 4 blade 83.5" R
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stickittoem
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Re: 2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Wed Dec 28, 2016 10:50 am

4 blade. The only length the JX comes in is 82 so i was told. on facebook it says 79-82. Hopefully the motor will turn it on or around the midway pitch mark.

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Re: 2.3 ch3 or 2.52 ch4 help

Postby SWAMPHUNTER45 » Wed Dec 28, 2016 11:49 am

Most often the hub selection adjusts length or they trim the blade for shorter lengths but this is the new design.

I would speak with Darrin in depth before I ordered anything.

Gary Barber at Outlaw Aircraft is a great resource fyi....shop him he is a Sensenich dealer and did me right

What I find interesting is this new blade and the fact it likes higher RPM which sort of mirrors the Whirlwind SW habits.

I went with the S blades because they brought push at a lower rpm range....interesting

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Re: 2.3 ch3 or 2.52 ch4 help

Postby CarMotorBarge » Wed Dec 28, 2016 12:19 pm

I have seen the JX 3 blade on 600 HSP motors. You probably have enough HSP for the 4 blade.
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Re: 2.3 ch3 or 2.52 ch4 help

Postby 21DiamondB » Thu Dec 29, 2016 9:35 pm

I am running a 4 blade 82" NGR with a 2:52 on a 700hp engine pitched at the 3 mark. I don't think that 82" with the short hub is enough prop for that engine.
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Re: 2.3 ch3 or 2.52 ch4 help

Postby stickittoem » Sat Jan 07, 2017 11:44 am

an inch wider. less gear. i guess we will find out. are you happy with how your boat runs? what wot do you set at?


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