35 Duffy 400HP C-Series Cummins Gearing

tunafishhkg

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If you already have an AQ22 1 3/4 shaft then ok but if you are going new than I would go with a 2" shaft. At 400HP 2-1 you could be in the 26X22-23 I have a 26X26 with 2-1 450 Volvo in my 35 Duffy
 

TRUE NORTH

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my friend has a 35 duffy, high shear, with a 5.9 260 cummins with a 2:1 and a 1 3/4 shaft and he swings a 26x26 and gets 16/17 knots WOT
 

captainlarry84

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The Big Choice!

We need to know a little more information. However the best diameter wheel for the 35 Duffy is a four bladed 26 propeller. Pitch to be determined based on the RPM range of the motor.

If the WOT is 2200 to 2400 a 1.77 gear
If the WOT is 2600-2800 a 2:00 gear

In either case a 2 shaft is HIGHLY recommended for a wheel that size.
 

Nighthawk

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Not to de-rail the OP's question but I would like to know if the 8.3 QSC engine fits in a 35 Duffy without a step up or engine box?

Thanks
 

lobstercatcher

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I don't think any diesel over 200 or so HP would fit under the deck of a 35' Duffy without a step or a box. Maybe one of the small Yanmars?

Mine does but I used a dry exhaust so I have a engine box just for the exhaust. But it is 200hp as you say.
 

MBILL

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Not to de-rail the OP's question but I would like to know if the 8.3 QSC engine fits in a 35 Duffy without a step up or engine box?

Thanks


I'm in the process of repowering my 35 Duffy with a 550HP QSC right now so I can tell you surely that a QSC won't fit in a Duffy without a step up or box.

Here's a photo of the QSC Cummins in the boat. After this shot was taken the water inlet on the exhaust elbow was modified to move more it to the side. The engine just fits under the hatches.

I don't have the exact measurement of my step up here but I'd guess it's about 6". Here's another photo of that...

MBILL

IMG_0677_opt.jpg

IMG_0536_opt.jpg
 

captainlarry84

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Repowers & gearing

Pushing the motor ahead of the bulkhead is to achieve more deck space for work gear. Also when the motor is put that far forward larger motors fit. The big issue on all of these repowers is getting the transmission to clear the wineglass of the keel. The problem with putting the motor that far forward is a loss of speed do to lack of bow lift. The key of these types of vessels in addition to the correct gearing is placing the motors a little more aft.

On Bills second photo it shows a step on a second Duffy repower. The original plan was a C-9 CAT However it would not fit under the step up so he drop down to a C-7 CAT.

Back to the gear issue. On Bills 550 repower he had a 3208 450 HP with a 2:0 gear it looks like he will swing the same wheel but with a 1:77! That will be a nice jump in speed. In addition even though HP is increasing because he will just be spinning the same wheel faster the 2 shafting will be more that enough. A lot of though when into doing this repower & I am sure the results will be very impressive.

More pics Please!!
 

Downeaster

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Pushing the motor ahead of the bulkhead is to achieve more deck space for work gear. Also when the motor is put that far forward larger motors fit. The big issue on all of these repowers is getting the transmission to clear the wineglass of the keel. The problem with putting the motor that far forward is a loss of speed do to lack of bow lift. The key of these types of vessels in addition to the correct gearing is placing the motors a little more aft.

A couple of points:

  1. There aren't many new builds (of current designs) where the engine is installed forward of the main bulkhead (though there are builders who can push the main bulkhead forward thus allowing for a similar outcome - Crowley Beal has done this when asked). Glenn Holland's 32 could be an exception. Another downside is fishing the rips - one can get into a real mess in the blink of an eye with this weight forward bias.
  2. Based on many of the posts that I've seen here it will come as a surprise to many that shaft diameter is more the result of wheel size than engine output.
 

Nighthawk

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MBill- thanks, that is exactly what I was looking for. With the step up do you happen to know your salon headroom? I am trying to get a feel for what is a comfortable headroom for a 6' person. A boat for example with 6'8" now and no step in my mind could be stepped a bit to dump the engine box. I need about 4" and wonder if 6'4" is too low. Thanks
 

tunafishhkg

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On my 35 with no mods to the standard molded top, I have 6 1" in the courners with a 6" total so lets say 5" in engine room.
 
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Hurdy Gurdy

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This is interesting to read.My Duffy had a 240 perkins with a borge warner 7000 series gear.She swung a 26x24 on a 1 3/4 shaft.I am repowering her with a Cummins 450 c series engine.Still need a gear though.really dont want to up the shaft size.Would a 1 3/4 to 1 gear be ok?
 

F/V First Team

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Shouldn't have to change shaft diameters at all, 1.75" shafts can take some real abuse. 1.75:1 gear reductions are fantastic, I would recommend a ZF product on the end of that Cummins personally.
 

Hurdy Gurdy

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Thats where Im headed as far as gears go ZF seems like the right move.Does anyone know what a Duffy weighs?Just the hull and top?
 
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