31' JC Casco Bay Repower & Rebuild.

Field Tester

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Well, I'm finally getting started on my repower and rebuild. I've been doing a lot of research, and speaking to a lot of different people over the past year about my ideas for this and got A LOT of different answers as to how I should go about it. The new power plant will be a Cummins 6CTA. A big issue with this repower is running a big enough wheel for the hp I will have. The second issue is even if I can run a big enough wheel, will the hull provide enough water to it? There has been many discussions on this forum and others about prop size, gear ratio, shaft speed, load, etc in any boat. I have taken a lot in, and have spoken to prop shops, mechanics, other JC owners, etc, and here is what I plan on doing. I am modifing the hull to get a bigger wheel under there. I have been told by just about everone to try to get a big enough wheel to run a 2:1 gear. The biggest wheel I can get under there with what I currently did will be a 26. And that's pushing it with having enough clearance all around. I would be running a 26X30, or there abouts, and I don't think the hull will provide enough water for that much pitch. I think I'm going to go with a 1.77:1 gear and run a 25X25 (again, or there abouts). I will have a good 3-4" above the top of the prop. So here are the pictures of where I'm at. A before shot, then the afters.









 

Field Tester

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Today, I made the mold that will get screwed to the outside of the hull. Basically a flat peice of wood with a shiny surface that I can wax.I will lay up my glass from the inside. So, here starts my questions to the builders on this site :D........I'm using 1808 (because I had some laying around). Is this sufficient? Or should I be going with a heavy cloth? I cut out some 1808, but then started rethinking my materials. How many layers will be sufficent? I was planning about 5-6 layers each overlapping a little more. In between those layers, I cut pieces with won't over lap, but basically just add thickness. The final layer will overlap about 6-8" over the existing boat. Then, after removing the mold, I was going to lay a few layers from the outside (which will suck working over my head!!) overlaping. Any advise on attacking the glass work??

 

djmarchand

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Wow! A Cummins 6CTA at 450 hp in about a 12,000 lb. hull. That thing ought to smoke.

Are you sure that hull can take that kind of hp? Also your prop shaft doesn't look big enough.

David
 

captainlarry84

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Good luck with your very aggressive repower. You wheel sizes seem to be to large. I 450 Hp Cummins will most likely swing a 24 X 24 with the 1:77 gear. That combo will give you more than enough speed to make your JC Dance real fine. I would also extend your wet new wet tube about 3" to get better water flow. What is the RPM rating of the motor?
Also you will need a ZF 280A gear to carry the power. That gear will be a tight fit into the wineglass of the hull. I would cut out the old wet tube & slide the new one in after the motor is in place. With the wet tube slide into place you will have some wiggle room to marry up the new shaft before glassing into place.
 
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Field Tester

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Larry,

WOT is 2600. We came up with a 25X25 with the 1.77, and a 26X29 with a 1.98:1. I know of a 31' BHM with the same power plant, and he is running a 26X29 with a 2:1 gear. I can squeeze the 26" wheel under there right now, but I'm leary about having that much pitch. The BHM has better water flow that the JC I think. Even if the 24 square turns out to be the right wheel, I think I'm better off being there.

It's also hard to see in the pics, and some of these pics aren't after my finish grinding, but I did grind the keel to a taper above the shaft to increase water flow to the wheel. I will build up glass on the inside to add back strength where I made the taper. I didn't do that under the shaft though because I can't get inside there to glass. Besides, with the hull design, I figured the area starving for water the most would be at the top.

For a gear I will be going with a Twin Disc 5075A. It is nearly coaxel which is what I'm looking for to keep the engine as low as possible. My 507A was about as low as you can get, so I didn't plan on touching the wet tube. I don't think I will be able to get the motor any lower than it was.
 
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captainlarry84

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If you are not planning on changing the wet tube did you check to see if the current tube will except the cutlass bearing needed for the 2" shafting?
 

tunaorlater

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Just for your info, my Volvo is 480hp 2600rpm. 1.73 to 1 26x24 3 blade wheel. Best of luck, looks like your off to a good start.
 

Ol Gilnetter

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captainlarry84

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Just for your info, my Volvo is 480hp 2600rpm. 1.73 to 1 26x24 3 blade wheel. Best of luck, looks like your off to a good start.
Based on this info with a 3 blader then you would be close to a 25 X 24 with a 4 blade propeller. Therefore my #s @ a 24 X24 with a 4 blade @ 450 HP is very close to what you will spin. I would go with the 1:77 gear for sure.
 

captainlarry84

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I would also consider this. Extend the wet tube as shown. It is a very easy upgrade. Spartan Marine will sell you a 12" section of glass wet tubing to make the extension.

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Cakosa

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Looks good, that HP is gonna blow the rudder off that thing JK.

We were going to upgrade my dads power years back but then we just stuck to the 375 hp CAT again. Do you think threat much HP will creat the keel walk? At 20 kts on our JC it seems like we're steering a lot, pretty annoying in my opinion.
 

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Cakosa,

It might keel walk at higher speeds, but just because you have the speed, doesn't mean you need to use it all the time. Capt. Larry got good speed out of his JC, and never seemed to have an issue. I know of a 31' BHM with the same motor, and it does keel walk from what I've been told, but it is nice to have the extra torque and power when you are loaded down. If I was buying a new motor, I wouldn't put this much hp into this hull. I would stick with a 370hp. It just so happened that this block came along and is the most cost effective option for me.

So, I glassed the hull last week. Haven't had any time since then to get online, but here are some pics. I screwed a mold to the outside, and laid 14 layers of 1808, each one a little bigger overlapping the existing hull. Due to colder temps, I will finish up the outside in the spring. I'm going to lay 3-4 layers overlaping from the outside also.



 

captainlarry84

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Looks like a fine lay-up with quality glass being used. Keel walking not to worry just back off the throttle on those days. Or wheel the boat so you over turn the motor. Any pictures of the outside glass work? I love my JC it always got me home and always had the look to turn a head or too.
When I fish Montuak the most popular DE I see is the JC line. People can say what they want but the hull is still in production by Judge & Eastern. My hull was # 272 and that was in 1984! They built a ton of boats and they are still in great demand.
Also consider my idea about extending the wet tube. Base on your current glass work the extension would be a walk in the park. Also with a 1:77 gear & 450 HP a 1.75 shaft is plenty strong for the HP.
 
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Field Tester

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Larry,

I did not complete the outside yet, and with the recent cold weather, I'm going to wait until spring. I have lots of other things to keep me busy through the winter with mechanical work.
 

Albert Jr.

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I've been looking for a thread of a 31-32 ft with a Cummins 6cta.
Goodluck with her, I'll be watching this thread closely.
 


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