Gear Ratio vs Performance

Harry Babb · Post by **Harry Babb** » Sep 10th, '07, 21:19

Hey guys
I will soon repower DeNada with a pair of 300 HP 6BTA's. Today I realized that the gears I have are 1.23:1 ratio. The gears are ZF IRM 220 A............the engines are CPL 970

Anybody have any idea how a 1966 31 Express will perform with these gears when propped correctly???

Harry Babb

CaptPatrick · Post by **CaptPatrick** » Sep 10th, '07, 21:28

Harry,

1.23:1 gears aren't going to allow you to utilize the power & torque potential of the 300 6BTAs...

1.5:1 would be better and allow you to turn a bigger wheel.
Br,

Patrick

Capt. Mike Holmes · Post by **Capt. Mike Holmes** » Sep 10th, '07, 21:30

Harry, with the gas engines, my 1962 express had 1:1 gears. Bad, very bad. When I first repowered with the diesels, I used 1.45:1 gears, pulled a 20 x 20 prop. Wanted better performance, went to 1.91:1 gears. Wanted a little better performance, had the injector pumps recalibrated and changed injectors. Now running at least 250 hp, probably closer to 270 or above. Turning 22 x 24 three blade Nibrals with a slight cup. My boat is pretty heavy, bulky half tower, teak deck, AC, genset, frig, heavy helm chairs (came off a 72' Halter), etc. I hit 25 knots wide open at 2700 - 2800 rpm. I'm happy with this, at least for now. My goal when I started this project was a comfortable and economical 20 knot cruise, without working the engines too hard. Got it.

mike ohlstein · Post by **mike ohlstein** » Sep 10th, '07, 21:32

If you want to take full advantage of those engines, I think that you should be changing gears as well. With your current gears, you won't be able to swing a 20" or 21" prop.

Consider going to 1.5:1.

scot · Post by **scot** » Sep 10th, '07, 22:07

Not to get into too much discussion surrounding dia vs pitch etc....but on the 25 I'm building the max dia I can handle is 19". The engine is 275hp @ 2300 rpm. All the calculators (boatdiesel & the local prop shop) show a 1.12 or 1.23 gear and somewhere around 22-25 pitch.

I understand that I can't take FULL advantage of the diesel at this diameter, but if you can't fit a larger diameter what's a fellow to do? A 1:5 gear results in a 35 pitch with an 18-19" wheel!

Which is better: 1:12 w/ 19x22....or 1:5 w/ 19x35????

Another factor of confusion for me (there are many)....lots of the original Detroit's came with 1:1 Allision gears. I'm assuming this was because of the low rpms these engines produce and the fact that the "power band" occurs between 1.000-2,000 rpm.

Should I give up drugs?

Second question: Tony Athens does not feel the ZF220A would be up to the task of 275hp @ 2300 rpm....but Harry "may" have (2) RTO ZF220A's coming on the market (with the correct ratio for me)....do you guys think the ZF 220A could handle a 275hp Detroit?....aren't they rated for 375hp?

mike ohlstein · Post by **mike ohlstein** » Sep 10th, '07, 22:10

Are you doing a single engine installation?

scot · Post by **scot** » Sep 10th, '07, 22:39

Yes it's a single in-line. Shallow prop pocket is required to bring everything into alignment with the engine at the correct location, based on the 25's COB. It's the pocket, shaft angle, draft, etc. that limits the wheel to around 19" max.

mike ohlstein · Post by **mike ohlstein** » Sep 11th, '07, 06:10

OK.....I'm lost.......

Rawleigh · Post by **Rawleigh** » Sep 11th, '07, 08:14

Mike: Check out his website. The address is in his profile.

Bruce · Post by **Bruce** » Sep 11th, '07, 08:43

but 2:1 is the best option

Only when dealing with engines in the higher rpm ranges.

You couldn't swing a prop under a 31 with a Cummins with a 2:1 gear without going to an absurd pitch.

1.5:1 would be better than the 1.23:1. But they will work.

CaptPatrick · Post by **CaptPatrick** » Sep 11th, '07, 10:14

Bruce,

Thanks for setting me straight...

Br,

Patrick

Harry Babb · Post by **Harry Babb** » Oct 7th, '07, 08:01

How does the gear ratio affect fuel burn............

Will the 300 HP Cummins with a 1.23 Reduction burn more or less fuel (Per Hour) than the same engine with the 1.5:1 gear......... both setups proped correctly to turn 3000 RPM

Or does it make any difference at all

Captian Mike.............did you notice any fuel consumption difference as you experiemented with different gears (per hour)

Harry Babb

Capt. Mike Holmes · Post by **Capt. Mike Holmes** » Oct 7th, '07, 08:38

Harry, I don't have enough running time yet to make a definite ruling, but I think in my case I will be doing better on fuel burn. Because I was having to work the engines harder with the 1.45 gears I started with, and I am turning quite a bit bigger props, I get better performance at lower rpm. Of course, I never intended this to be a high speed 31, just wanted a consistant cruise somewhere over 20 knots with any sort of load - when the Gulf will allow me to run even that fast.

Capt. DQ · Post by **Capt. DQ** » Oct 7th, '07, 09:19

Harry,

I have zf220a's 1:53.1 gears on mine with 250hp Cummins, swinging a Michigan DJX Nibral 3-blade 21 x 20 with no cup and crusing @ 25kts @ 2200rpm. The DJX'S kinda give you both worlds of a 4-blade and a 3-blade speed. But the gears need to be just like Capt. Pat & Bruce are saying. Now I also have a full marlin tower, genset, AC and all the extra's to that.

DQ

Harry Babb · Post by **Harry Babb** » Oct 7th, '07, 10:48

Doug

As I understand it although my engines are rated at 300 HP since they are 970CPL I should prop them and treat them as though they are 250 HP.

Have you figured a GPH at cruise since your repower????

Harry

Capt. DQ · Post by **Capt. DQ** » Oct 7th, '07, 16:11

Harry,

If you are going to get the 1:53.1 ratio zf gears....yes with reguards to going with 21" wheel. First check to make sure your shaft angle will let you go to a 21" wheel, then I would go with a 21 x 21 wheel to start out w/no-cup to see if you make your rak, but that should put you close to where you need to be with a 3-blade wheel now.

If its a 4-blade wheel is what you want....then 20" is probaly where you want to be. I have not personally put enough time on mine yet to give actual gph yet....but should be somewhere around 12-14 gals @ 2200 rpm cruise @ 25kts with these new props. But that is not hard data yet, but should be close. There is no need for me to run @ 2400 rpm which puts me @ 28kts, thats just more fuel.... then if I really needed to push it up to the pins @ 2600rpm for a short run @ about 30kts.

Go see Tyler Dalhgren over in Orange Beach @ Intercoastal Prop to check your pitch & balance of your props when you get ready.

DQ

Harry Babb · Post by **Harry Babb** » Oct 7th, '07, 20:38

Doug
I was not aware of Intercostal Props over here in Orange Beach............

That's funny.............I guess the grass is always greener on the other side of the fence.......I've been using the prop guy in Pensacola, I think he's located somewhere close to where you were on drydock......never been there myself........my wife usually runs those errands for me.

When the time comes I will go visit Tyler

To put my project in prospective...........Yesterday I disassembled the Starboard engine for overhaul and today I finished the port Shortblock assembly.

Thanks
Harry

Capt. DQ · Post by **Capt. DQ** » Oct 7th, '07, 21:24

Harry,

David or Dennis @ Accu-prop in Pensacola in good too, thats who sold me mine and who did my set up...just thought Orange Beach was closer for you. Either one can explain the difference in props. But I,m gonna recommend the Michigan DJX's/Nibral 3-blade, if you are gonna buy new or if you can find a good used set thats great also. Its like having the benefit of a 4-blade....but the speed of a 3-blade, really sweet set of props. I highly recommend those set of props.

DQ

Capt. DQ · Post by **Capt. DQ** » Oct 7th, '07, 21:47

Harry,

We are always talking about the right diesel engine & gears being matched-up, but you have to have the right set of props to make everything come together, like putting all that power to the water. Don't put a cheap ass pair of props on, if you are going to invest money in the engines & gears put the right set of props on to make whole system perform right.

DQ

J Graham · Post by **J Graham** » Oct 8th, '07, 07:33

I've been following these comments and wondering how they might relate to my boat. B-25, single 330B, 1.52 ZF V drive. Currently running 20 X 23 4 blade dqx. Performance is pretty good so far, still getting the kinks out.
2200-22.4 kts
2300-23.5 kts
2400-25.2 kts
2800-30.2 kts WOT
Boat is pretty light, Tony Athens is recommending 19 X 24 #4 cup to bring revs up a bit and up the top end. Hull seems very happy at 2400 not sure of fuel consumption. His advise has been impeccable to date and will go in that direction this Christmas.
This is my first Cummins and I don't want to beat it, but I love that 25 kts.
Jim

Harry Babb · Post by **Harry Babb** » Oct 8th, '07, 10:06

Hi Jim
I would love to help you but in this area I am not qualified to give any advise........thats why I am leaning on the experience of our friends who have already slain this dragon.

But believe me.......I will be taking it all in........

Harry Babb

JohnCranston · Post by **JohnCranston** » Oct 8th, '07, 12:57

Harry,

For what it's worth...Bob and Minerva of "Minerva's sport fishing" out of Cabo prefer the 1.23 gears behind their 250 bta's. Bob claims that top speed was 32 knts. Said had over 12,000 hours on them. He didn't know why everyone else were running 1.5 or 2.0's. I heard that he re-powered with 270 hp's and is still running the 1.23's. I ask alot of questions and that was his spin on it.

JohnCranston · Post by **JohnCranston** » Oct 8th, '07, 13:04

By the way..their boat is a fbc and has been around a long time, being fished ober 300 times per year. If you want further info from them on this subject, send my wife, Pam, an e mail and she'll give you their contact info.

In Memory of Vicroy · Post by **In Memory of Vicroy** » Oct 8th, '07, 15:44

Harry, its really an issue of engine loading. The 6BT 210s without a turbo can be pretty much loaded up with 1.2 gears, while the turbo versions need the 1.5 to lessen the load. You CPL 0970s need to be run like the CPL 1247 250s like Doug & I have, max cruise at 2400, and propped to top out at 2850 or 2900 rpm (the 250s need to be propped to reach 2650) I run 21x22 three blade Michigan Dyna-Jets with no cup thru ZF IRM 220As at 1.53. I'm probably underpropped as load means nothing. At 2400 I cruise 24 kts at about 16 GPH total. I can go from 1000 RPM to WOT of 2650 in about 7 seconds with one big puff of black smoke. At 2650 the GPS shows about 27 kts, but 2400 is my "happy cruise" where the wake flattens out and everything just hums.

Paul (Capy), a noted engineer, did a study on our diesel boats years ago and concluded that the bigger the diameter prop we can run, the more effecient the system is - to a point. 21" seems to be the max we can run without chopping a hole in the bottom, or using prop pockets. I can tell you that 24 kts is damn fast and the boat handles super good at that speed, and just feels light in the water as she's waaaaay up on the strakes.

UV

J Graham · Post by **J Graham** » Oct 8th, '07, 18:03

UV, do you and most of the other 31 guys use trim tabs to get level at speed. Even at 25 kts I seem to be a bit high in the bow. Didn't want tabs but they may be necessary to get level. May even pick up a little speed and be able to drop RPM. Man this is complicated.
JG

In Memory of Vicroy · Post by **In Memory of Vicroy** » Oct 8th, '07, 19:02

JG, most of us with higher hp diesels don't use tabs....the B31 runs just fine without them...the boat runs bow proud naturally and was designed by Ray Hunt to do that so she will get up on the strakes but keep the 24 degree deadrise to the transom in the water. Bow proud is how she runs, and part of the mystery of The Best Boat Ever Built.....

UV

J Graham · Post by **J Graham** » Oct 8th, '07, 19:19

Thanks, I will ask Scot to try to post a couple of recent shots for your opinion.

Dave Kosh R.I.P. · Post by **Dave Kosh R.I.P.** » Oct 8th, '07, 20:24

UV,
Simply put your right on the money. My bow runs almost level and with my 2-1 gears, military thicker hull and 18,000 lbs wet and full I turn 22 X 22 (close but works well, no noise) 3 blade Bronze . She runs wot @2950 RPM with 288 Gallons of Diesel and 24 Kts (not MPH). A few others are running the same as me here. Now I can get away with a slightly larger diameter because of absolutely no flex to the hull with the increased thickness. I had the Volvo dealer check and test run and my engine surveyor(who use to own the Volvo dealer here) . The 250 60C series are very similar to the Cummings in the torque curve.
I am just a happy camper at my 20 knot cruise out at sea with 1.15 KMPG too.
Now if I can only shake this cancer thing and get to use it again I'll be even happier.
Go get the Pelagics and run AJ. She is a beauty! Dave K

Capt. DQ · Post by **Capt. DQ** » Oct 8th, '07, 21:59

Harry,
Click on the link and go down to compare the difference between the E.A.R rating on the DJX verses the Dyna-Jet 3-blade propellers and then compare the Dnya-Quad verses the DQX 4-blade. The E.A.R. on the DJX is very close to the Dyna-Quad, the DQX is much more than the Dyna-Quad.
That percentage of blade surface in water made me have to go back 1 degree of pitch. I had Dyna-jet nibrals 21 x 21 with no cup and then went to the DJX's @ 21 x 20 no cup. added 1-2 kts of speed compared to the Dyna-jets. The DJX's were a very noticable difference veres the Dyna-Jets in smoothness of ride & speed. Now taken the fact that both sets of prop were computer balanced & matched on pitch there was a big difference in performance. The engines and gears are also matched up, so all three components are fine tuned together.

http://www.miwheel.com/MIWheel/pdf/Michigan03.pdf

DQ

scot · Post by **scot** » Oct 8th, '07, 22:30

Here a shot James sent me of the boat on plane, nice....hope mine comes out half as good as yours did.

Harry Babb · Post by **Harry Babb** » Oct 8th, '07, 22:35

Thanks Doug
I really appreciate your help and interest.

Harry Babb

bertram31.com

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