Diesel Gear Ratio Question

[STraenkle]

Just wonder about people with 250+ HP Diesel engines, basically 500+ Ft lbs of torque (assuming 2500 RPM engines) at crank, are you guys running 1.5:1 or 1:1 gears and 1 1/2 inch shafts?

[Tony Meola]

Cummins 270 which is basically the same as the 250. 1 1/2 shaft and 1:51 ZF

[STraenkle]

Thanks Tony, I have gas 496 with 450 ft lbs of torque, but running 2:1 ZF, Knew I was pushing too much. but snapped shaft after 7 years. Trying to find out how much a conversion to 1:1.5 costs, but dam that thing jumped out of the water. Would send anyone not hanging on over the stern.

[Bruce]

Tony is correct.

With 2:1 on those 496, that was a lot on 1 3/8. But you got 7 years, not bad. 2:1 is correct for current setup due to rpm

I saw the same setup on many new boats all the time when 7.4l went away and big boy gas came along, then a few years down the road 1 1/2 shafts became more standard.

[STraenkle]

Trying to decide whether going to 1:1.5 ratio and keep the 1 3/8 shafts aquamet 22, or go to 1 1/2 inch and keep the 1:2 gears and get bigger wheels. I could only go to 66% of throttle according to the engine's computer at 4,900 RPM. not sure of the cost of changing the ratio.

[John F.]

I may be wrong, but smaller wheels at 1.5 slip more ar trolling and allow you to go slower at troll without trolling valves.

Crows Nest has 8.2 Detroits at 300 hp, and 1 3/8" shafts. Shaft guy told Doug that he was right at safety edge. I run her up to cruise slowly, and generally run at 2200-2300 rpms at 21-22knts. Shafts are still there. I'm gentle with her be cause of shafts, and because parts for the 8.2s are pretty nonexistent.

[Bruce]

Scott,
If I remember correctly that engine is rev limited at 5150. So 4900 to 5000 would be WOT. At 4900 it would not be 60% throttle unless you were way under propped and just ran out of cable throw on the controls only opening 60%.

Have you checked at wot at the cable/engine throttle connection to see if the arm was hitting the wot stop?

Correct me if I am wrong on your rev limiter.

[Carl]

Not sure what hardware you have, but sometimes you can swap out to a thin wall Cutlass Bearing or have strut bored out to accept, that takes care of strut.
Log is usually fine, may be able to open packing gland to accept 1.5 shafts...if not, packing glands are not costly.

Coupling...same story, if enough meat, they can be machined for 1.5 shafts.

Wheels...some can be bored open, but cost of that plus having to tweak prop after can get close to used wheels. So new to yu wheels may be in order.

As to shaft, you have to buy one anyway, so its the cost of 1 more. price difference between 1-3/8 and 1.5 is not a huge factor.

Now you have a boat you don't have to worry about snapping a shaft, losing a wheel and getting back in on one motor...plus spinning faster leads to more vibration and they are less efficient.

upgrade rather then degrade is my .02

[Carl]

Not sure what hardware you have, but sometimes you can swap out to a thin wall Cutlass Bearing or have strut bored out to accept, that takes care of strut.
Log is usually fine, may be able to open packing gland to accept 1.5 shafts...if not, packing glands are not costly.

Coupling...same story, if enough meat, they can be machined for 1.5 shafts.

Wheels...some can be bored open, but cost of that plus having to tweak prop after can get close to used wheels. So new to yu wheels may be in order.

As to shaft, you have to buy one anyway, so its the cost of 1 more. price difference between 1-3/8 and 1.5 is not a huge factor.

Now you have a boat you don't have to worry about snapping a shaft, losing a wheel and getting back in on one motor...plus spinning faster leads to more vibration and they are less efficient.

upgrade rather then degrade is my .02

[STraenkle]

Bruce, you are correct about Rev limit is 5,000 according to sticker on the engine, the rev alarm buzzer goes off and backed it a touch to get the 66% throttle, I have a software program that plugs into the engine and records everything. I always knew the 1 3/8 40 year old shafts were an issue. I think I will keep the gears but go 1 1/2 shafts. That way when I do go diesel, that part will already be done.

As far as going slow enough, the boat at idle on 2 engines is around 3 knots. at 4900-5000 RPM about 34 knots. Light boat, no tower, just Bimini top. cruise at 25 knots ~3,300 and according to computer 33 to 35% throttle, 1.1 Nautical miles to the gal. I figure the engines aren't working that hard to push this boat.

[Tony Meola]

Scott

Those numbers are good for gas engines.

[farmer]

I have a dumb question I keep seeing 1.5 to 1 , 2.0 to 1 , and 1 to 1.5 . Which number is crankshaft speed, and which is output shaft?

[Bruce]

Engine will always be the higher rpm vs shaft using a reduction gear.

[Marlin]

Consider asking a professional prop/shaft/strut guy, especially if your thinking of diesels in the future. All these items were engineered to function within the overall wall thicknesses to allow the bores that were put into these running gear items. Cracking a hub and loosing a blade, hitting a submerged something and trashing thin wall strut, bending/warping a Shaft coupling while backing down on fish are all things to consider . I'm cautious and got the recommendation when I put in 330 hp Cummings,new stainless struts with the barrels to accommodate 1 3/4" shafts, blank couplings that were of the maximum outside diameter to allow for the bolt pattern to work, bored then faced off in a lathe once the shafts were inserted and match marked to avoid mis matching. Grasping a Shaft with a 3 jaw chuck could give u a different centerline than if a collect were used . I made shafts for many years and learned this lesson the hard way . The 1 3/4" size was recommended as the cost difference was just the wt difference, machining time would have been negligible . Cummings calculated the transmission shaft angle and ratio for this hull, they had done it many times before

[Carl]

Grasping a Shaft with a 3 jaw chuck could give u a different centerline than if a collect were used . I made shafts for many years and learned this lesson the hard way .

Marlin, please clarify why a 3 jaw chuck can give a different centerline. Unless shaft was not properly indicated prior to starting. But that is no different then if collets are used...still have to check to verify collet chuck and collet were properly installed and running true before starting.

[Marlin]

The 3jaw chuck has 3 kinda rack and pinion mechanisms all theoretically working in unison. If there was no wear /tolerances dirt/chips any where in the system and the same operator set up the turning with a dial indicator,it could work. I found it not the best system , as once the collet was confirmed /dialed in, it was a reproducible system approach . CNC turning equipment that I have been exposed too uses the collet concept, no matter who the operator is, take out the variable potential. I still have a 3jaw chuck lathe that I used to make prototypes on, prove market acceptance/feasibility. Once those concepts were proven, on to a statistically proven manufacturing method.

[scot]

Scott,
What prop do you run behind the 496 with the 2:1 gear? I gave up my diesel dreams awhile back and now have a 454 with a ZF 2:1 going in my single 25B inboard conversion. Trying to get a feel for the prop requirements, what size and pitch to start with. The prop online calculators and my local prop guy are disagreeing LOL.

[farmer]

Scot, I really wish I knew how to post pictures of my 25 inboard conversion, or take u for a ride. Think u could get some ideas on yours. Been a mechanic all my life , but still new to this boat thing . Not very good at explaining it. My boat had a 454 prior to the 383 mercruiser . I feel prop , tranny, and engine are balanced perfect for speed, performance , trolling, and fuel usage , but don't have the experience of these guys.

[Carl]

Scott,
What prop do you run behind the 496 with the 2:1 gear? I gave up my diesel dreams awhile back and now have a 454 with a ZF 2:1 going in my single 25B inboard conversion. Trying to get a feel for the prop requirements, what size and pitch to start with. The prop online calculators and my local prop guy are disagreeing LOL.

I'll take a "good" prop guy over an online calculator anyday...

That said...never hurts to be able to borrow a prop or two and verify before tossing down coin.

[Carl]

The 3jaw chuck has 3 kinda rack and pinion mechanisms all theoretically working in unison. If there was no wear /tolerances dirt/chips any where in the system and the same operator set up the turning with a dial indicator,it could work. I found it not the best system , as once the collet was confirmed /dialed in, it was a reproducible system approach . CNC turning equipment that I have been exposed too uses the collet concept, no matter who the operator is, take out the variable potential. I still have a 3jaw chuck lathe that I used to make prototypes on, prove market acceptance/feasibility. Once those concepts were proven, on to a statistically proven manufacturing method.

I kinda thought you were referring to a 3 jaw universal chuck. I agree they "can" run a little wonky, especially if its a cheap or clapped out chuck.

But "some" 3 jaw chucks can also run quite true. I have a 3 jaw that can be dialed in, granted not alot of adjustment like an independent chuck, nor is it quick and easy. The adjustment allows for different jaws and/or uneven wear, once set it is very repeatable. In the CNC's we often run 3 jaw chucks using soft jaws; they are machined in place, to size and run truer then most collet systems I am aware of.

Yes, collets are the go to for production, especially with less then operators and feeders.

Universal chucks also come in other jaw configurations; we regularly use 2,3,4 and 6 jaw universals (maybe not so much the 2 jaw)...in addition to the independent chucks.

Sorry, just a sore point. Once in awhile I have customers come in and question why I run their product the way I do...and in not so much of an inquisitive manner. From where I sit, provided the end product meets or exceeds specifications, expectations, if we hit their price point and timeline...how we go about doing it should not be of concern. More often then not, its out of curiosity or something they heard. I have no issue with explaining and going over, especially because they have a better way we haven't thought of.


Back to the Gear Ratio question

[scot]

Carl..

I'll take a "good" prop guy over an online calculator any day...

I always tend to defer to my local (really good) prop guy, but in this case I think his software program is lost. The results just doesn't fit within any parameters I know to be accurate. So, looking for some first hand, user info on props for a 454 with a 2:1 gear. I have a pretty fair gut feel on the prop based on my past single inboard, deep V hull. I believe the boat, engine and gear "should" turn 19" diameter, with 20"-23" pitch. Yes, loaner props for trials are nice

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