31 JC Repower

tailhook

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Well its time, eight to nine knots was fun, I decided to repower.
out with the Perkins 135 HP and in with a 315 yanmar, I know its a little underpowered, hopefully 15 to 16 knots

135 p.jpg
 

captainlarry84

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Nice power you will get your speed @ 14-15 knots. It the new motor is 2:0 gearing you can use 1.5 shafting. If the new gear 2.5 you should use 1.75 shafting.
 

Toolate

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tailhook

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2.5 gearing and will be going with 1-3/4" shaft with new wet tube,
couple of questions Larry was thinking about going dripless on the packing do you think I will need a cutlass at each end or the tube? and my feed water outlet for the old tube came from my exhaust will that be sufficient it has 4" exhaust and plenty of water to keep the old one cool
 

tailhook

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200 more hp (and a shit load of torque) should get you something!

Please post pics of each step along the way. Are you not sure its going to fit?

the yanmar will fit nicely in the compartment, I just started stripping the motor for removal on Saturday and doing the usual garbage can full of wires and junk from a repower on sunday
 

petrel

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Good choice for the reduction, get some wheel under there.
 

captainlarry84

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2.5 gearing and will be going with 1-3/4" shaft with new wet tube,
couple of questions Larry was thinking about going dripless on the packing do you think I will need a cutlass at each end or the tube? and my feed water outlet for the old tube came from my exhaust will that be sufficient it has 4" exhaust and plenty of water to keep the old one cool

NO! only one cutlass bearing at the tail end or wheel end of the shaft. Cutlass bearing that close to the transmission is not a good idea. The shaft will be held in a mush to tight of tolerance. Ailment will be very hard. In addition with the bearing that close to the gear the shaft starts to work. The process is call kneeing the metal. Over time with the shaft being kneed by the front bear it will break at the coupler.1.75 shafting & one rear bearing is all than is needed. More is not better. As for the dripless the PSI is the best choice. Just make sure you get a good water feed to the bearing. Do not remove a zinc for a water feed. High a plug high in the system to feed the bearing.
 

tailhook

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The wheel on there now is rather large not exactly sure. but I will find out
I am trying to apply what I have learned on this site about the JC. So far the info about making it handle properly was spot on, big wheel, weight down low, to a point the auto pilot could run the boat all the time until I messed with the settings (granted the biggest I ran in was around 4')
 

petrel

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Just sayin' compared to 2 to 1, you'll have a bigger wheel.
 

captainlarry84

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With a 2.5 gear I EST your wheel at 23 X 19 or a 22 X 20. Speed about 16 knots @ 3500 RPMs. With a 2:0 gear your wheel would drop to a 22 X 17 EST. You need the largest wheel possible on a JC to keep the large keel in line. My JC had a 370 Yanmar with a 2:0 gear and it handled a 23 X 21 no problem, plus I had a F/B Casco Bay. 2.5 is the winner for that motor.
 

tailhook

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NO! only one cutlass bearing at the tail end or wheel end of the shaft. Cutlass bearing that close to the transmission is not a good idea. The shaft will be held in a mush to tight of tolerance. Ailment will be very hard. In addition with the bearing that close to the gear the shaft starts to work. The process is call kneeing the metal. Over time with the shaft being kneed by the front bear it will break at the coupler.1.75 shafting & one rear bearing is all than is needed. More is not better. As for the dripless the PSI is the best choice. Just make sure you get a good water feed to the bearing. Do not remove a zinc for a water feed. High a plug high in the system to feed the bearing.

As always thanks Larry that's what I thought. I should have said the water for the shaft came from the exhaust at the first bulkhead where the exhaust hose from the motor met the tubing that goes aft
 

machaca

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I repowered 31 jc w a 315 cummins 2:1 zf gear 23 by 24 inch 4 blade wheel drip less water out of wet exhaust 16-17 knot cruise 20.5 at2800 rpm that yanmar should run great in there good luck!
 

tailhook

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I repowered 31 jc w a 315 cummins 2:1 zf gear 23 by 24 inch 4 blade wheel drip less water out of wet exhaust 16-17 knot cruise 20.5 at2800 rpm that yanmar should run great in there good luck!

Thanks, I like your jc, it looks like the motor is set back more than most and I know that boat has been just everywhere on her bottom
 

machaca

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When I did the repower I talked w Jackie jr and he said to put the weight as far back as possible which I found odd because the Jc s tend to ride a little bow high but it rides good the boat is in Falmouth if you want to check it out see how we did the dripless or what ever
 

captainlarry84

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I repowered 31 jc w a 315 cummins 2:1 zf gear 23 by 24 inch 4 blade wheel drip less water out of wet exhaust 16-17 knot cruise 20.5 at2800 rpm that yanmar should run great in there good luck!
Correct gear for you & nice size wheel. The 315 Cummins turns much slower that the 315 Yanmar so even with the small est wheel you will be turning it faster so your speed should be the same. Keep in mind that the Cummins is rated at 2800 & the Yanmar 3800 RPMs. 1000 RPMs is a big difference. YOU vessel only has a shaft speed of 1400 RPMs which explains the larger wheel. The Yanmar with the 2.5 has a shaft speed of 1520 RPMs. That 120 additional RPMs explains the smaller wheel by about 3" of pitch. Which puts my EST of a 23 X 19 pretty close.

As far as the water feed goes you really do not what to feed the dripless with a water feed off the hot exhaust water. The bearing should be fed with cool water. Search the motor a raw water pump you will find a cool water feed off the motor.
 
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Keelboater

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the yanmar will fit nicely in the compartment, I just started stripping the motor for removal on Saturday and doing the usual garbage can full of wires and junk from a repower on sunday

Sounds like a good combination. You should love it. Good luck with your project and keep the pix coming!
 
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