Repower Considerations

[Steven Borcherding]

We have a 37 foot, 16,000 pound 1987 Roughwater 37′ that is currently powered by twin Detroit 8.2 350 hp engines with TD 506 gears. We love the boat and want to repower. It has newer fuel tanks, 1.5 shafts, new logs, cutlasses and struts. The steering system is also updated. The bottom was just compleatly stripped and re done.
Currently with the 8.2s, we cruise at 8-9 knots at 1750 RPM 14-16 at 2,200 and exceeed 20 knots WOT. We are not rich people and want to repower but I think that the current HP is excessive. We run the boat at 14 knots out and backwhen we go ocean fishing but most of the time cruise at 8-10 knots. I do not ever cruise in excess of 16 knots. I have pretty much decided on recon Cummins for the new engines but am unsure what HP I should use or the gear. As we are on a budget I would like to re-use as much of the existing gear as possible. What would you recommend? We are looking at next fall to do the repower. I will remove the old engines and have the new installed at a good local shop.
I posted this question before but it seems to have disappeared? Something I did I am sure.

Topic

[pwrobert]

Bye the Way

Are you sure they are 350 HP engines.  These look just like mine – standard Detroit/Allison Turbocharged which were rated at 205 HP..Only J&T and a couple of others made Aftercooled or twin Turbo and Aftercooled engines with the 350 HP.  I don’t see any of that hardware in your pictures.

What is your fuel burn at 16 kts and at 20 kts?

My 38 ft Chris Craft with the DD 8.2L weighs more and at 18 kts it burned 18-19 GPH.  I repowered for reliability as well but with the now 350 HP Cummins 6BTA’s they cruise at 19 kts and burn 17-18 GPH.

I was very pleased with my result.  Other than fishing and doing nightime crossings, I seldom run my boat at 8-9 kts.  Mostly run at 2200-2400 depending on load and at 18-21 kts.

Everybody has a different way of looking at things but my feeling has always been to decide on the engine package that fits as far as size and weight and then choose the Highest HP engine version in the series.  You can always throttle back and reduce fuel consumption but when the need arises the extra HP at no weight penalty is a plus.

 

 

[pwrobert]

Let the 8.2’s Go

A couple of years ago I replaced my 8.2L Detroits (205HP model Turbo no aftercoolers) and replaced with Cummins 5.9L 370 mated to the 220A transmissions.

For your same reasons – reliability.  It was a good move and all specs were improved.  Most of all it got rid of the Mickey Mouse Raw water setup that was always a source of trouble for me.

Top end improved by 6 MPH, Cruise speed went from 20 mph at 2550 RPM to 21-22 at 2350-2400 and from 0.9 miles per gallon to 1.2 M/G at the  increased cruise speed.  

Giving up the GM’s and MG502 tranny and going with the Cummins and ZF 220A saved  somewhere over 1500 pounds and made it easier to get to the outboard sides of the narrower inline 6’s

Tony made all the engine and trans mounts and designed the new exhaust system since the Cummins do not have Left and Right turbo mounts like the GM’s  the design is a little more involved in that each engine has it’s own configuration.  Tony and his guys made a few other suggestions to my project and all have incorporated relatively easily.  List of accessories I upgraded for reliability were – SMX Raw Water Pump, SMX 160 degree thermostats, a lot of the fuel filter setup parts and filters, engine Flush Setup, Environment Kit to reduce engine room soot.  All have made the repower give me what I was looking for – added reliability with the extra bonus of fuel savings and performance as a bonus.

 

 

 

 

[Steven Borcherding]

Why Repower

Hi Tony,
We are repowering for reliability. We make the trip from the Columbia River to Puget sound yearly. The 8.2s are starting to have a fair number of minor failures, all non critical but for the type of boating we do simple, robust systems give a lot of peace of mind and ad to the enjoyment of being on the water. This is our forever boat and I have a good ten years of boating left in me and my son also uses the boat and may keep it after I am gone.
I was surprised/not surprised that you would recommend the 315s. I know that most people are looking for more speed and HP when they repower, but do I need that much HP if I do not operate the boat at the speeds that it currently is capable of? As I said we can easily achieve 20+ knots but never cruise above 16. I think I may have the R & L engines. I will have to try to get the info. I do know that my twin disk plaques both say the gears are MG 502 RH with a 2:00- 1 ratio. I miss spoke when I said they are 506. I should have know that as we replaced a tranny cooler last year…..

[Tony Athens]

Exactly why are you repowering–That’s a major question that you did not dwell into..

But on the surface you should be looking at the Cummins 6BTA 5.9 315’s as the perfact match to what you do now. Unless you have R&L 8.2’s, the current gears can be reused

[Author]

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