Repower 6bta to 6cta

[Rubbah Sleepah]

Hello 

We have a 32 Radon currently with a 6bta 315 and ZF85 IV 1.64:1 turning a 20×21 blade prop on 1.5″ shaft.  

Boat is quite heavy for it’s size and loaded with full fuel, no fishing gear, ice, etc weighs 12500 lb.  Needless to say, it is a 9 knot boat as it is not practical to push the 6Bta to get on plane. 

I am considering repower with a 6cta 430/450 to make long runs more tolerable.  Don’t want to be cheap yet don’t want to break the bank in this repower.  Can the 85IV and 1.5″ shaft handle the additional torque of the 6cta or will those components need to be upgraded as well?  The fuel lines are 1/2 ” and exhaust is 6″ so shouldn’t be a problem.  Based on measurements from engine specs, engine box should have more than adequate room for the larger engine.

 

Appreciate your advice. 

 

 

 

 

 

 

Topic

[Tony Athens]

I was introduced to Radons and Wilsons in the early 80’s. The Radons were 1st (early 70’s) and Wilson came along after about 10 yrs of Radon’s introduction.. Both builders were about 30 miles from my shop and both builders built their reputation and business based on the successful commercial sea urchin industry in S. Calif. The hulls were perfect for what these 1,2,3 or 4 person dive boats needed.  Since the early 80, I have repowered not less than 30 of them from 24 ft to 36Ft. All of them went from outdrives to V-drives and all have been the building blocks for what my company got on the map for.  There are two distinct caveats to help this immensely– One is the major increasing prop diameter, typical might be in a 27ft from a 19-20 inch to 24″ prop) and or lengthening of the LWL of the hull –Like from a 27 to 31. Engine stays the same but now the boat will plane 4000 lbs vs. barely 1000 pounds on a good day. Another example-30 x 10 ft Wilson -Extended 6 ft in the center- No longer a 2-step hull- 26″ 4 blade and travels at 8-22 kts cruise at 20000-24000 lbs – 1.4 to 1.5 MPG above 13Kts. I have most on this on this website. Learn it.

[Rubbah Sleepah]

Appreciate your reply.  Agree that all of the above you mentioned would require replacement if 85IV and 1.5″ shaft are found inadequate for the 450c, which based on what you wrote, assume that it isn’t.  Would like to stay with same shaft and drive to minimize cost but not opposed to it if the optimal engine solution requires it.  Before going that far though, I’ll try and explore other options that might allow me to retain them.

These Radon’s, especially heavier ones tend to squat if not on plane.  Anything above 9-10K (~2000RPM) and its pushing a lot of water, advancing throttle just loads it up, raising EGT’s with little difference in speed.   At 2600 RPM, still not on plane and only 14Kts, at ~2850 or so, its hitting a blistering 15.2Kts (almost over the hump).  Totally not worth it Lol.   It’s been suggested to me that trimtabs might help but I have absolutely no experience with them nor have any idea if it’s something worth trying.  I’ll be glad to give it a try but wondering if they’d make any difference considering how severely underpowered the boat is already.

Would the QSB 5.9 380 allow reuse of 85IV and shaft?  It has a bit more torque than the 330B but is the additional power enough?  The QSB 6.7  480 looks like a good match, too but can the drive/shaft tolerate it?

My end goal is to ultimately have a cruise speed of around 17-18Kts, which brings the boat on plane.   If anyone has any suggestions, I’d be more than grateful.  

Thank you.

 

 

 

 

[MATTHEW]

you will likely need new:

• shaft
• seal
• strut
• cutlass bearing
• engine mounts/brackets
• transmission
• prop

I’m probably missing some things.  a re-man QSB might be a better option but either way it’s a major refit.

[Author]

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