37 Bertram 6v92 550HP shaft size

A swap to rebuilt 6V-92's would not be advised. Too much weight and will negate the extra 100hp. I would never consider it.

A Cummins ReCon engine is the real way to go. Either an 8.3 or a 6C. The weight savings alone will give you a bump that is not available with the Detroit's.

After looking at their ReCon site, I would probably lean towards a ReCon QSB5.9 at 480hp.

https://cumminsengines.com/showcase-item.aspx?id=330&title=QSB5.9+ReCon+for+Marine&#overview

The 92's in the 37 were way too much iron and killed the engine room space. It was a bad engine match.

We had a 38' Blackfin SF with 6V92-TA's (550hp) and it was a terrible match as well - loud, sucked too much fuel, overwhelmed the engine room , and had trim issues as the aft fuel tank was consumed, etc. It is just not the right engine for a 38' SF, no one today would select it if it were still available. There are blogs about the Blackfin's having success with Cummins repowers and note that the Blackfin President at that time came from Bertram.

A 6V-92TA weighs at least 3,000 lbs. without gear. A Cummins QSC8.3 500hp weights 1.975 lbs. without a gear. A Cummins 6B 430hp weighs 1,570 lbs., a Cummins QSB5.9 480hp weighs 1.470 lbs.

Weight is your 38 Bertram's enemy, not its friend. If you want to rejuvenate the boat and have more than what you started with, the Cummins is the way to go. If you have a budget that allows for only a rebuild, keep the costs down and go with the existing 6V-71 rebuild and save all the "minor" issues you will encounter with changing to a 6V-92 that will eventually run into much more money than you anticipated.

As far as the shaft question goes, what gear ratio do you have?

The shaft calculation is easy and can be found at Western Branch Metals website: http://www.wbmetals.com/shafting.asp

The calculation is:



You want to have a Safety Factor of at least 5.0 for Diesel Recreational boats. the other properties are in their catalog, with the yield in torsion for AQ 22 Shafts at 70,000 psi for 2" diameter.

Your Safety Factor for the 6V-71TA rated 435bhp @ 2300rpm (1.50/1 gear ratio) works out to 6.34 for 2" diameter AQ22.

Your Safety Factor for the 6V-92TA rated 550bhp @ 2300rpm (1.50/1 gear ratio) works out to 5.01 for 2" diameter AQ22.

Good news for you since you would not need to change shaft diameter (as long as it is AQ 22).

$30K/12 cylinders is $2.5K per cylinder for rebuilt 6V-92TA's. Quire possible but I would only use a DD Distributor, with DD parts, maybe W.W. Williams in your area? There will be some unseen costs, but that will be discovered during the swap.

For $30K you most likely will not see a re-sale bump down the road. For the $100K investment you will have a rejuvenated boat, quieter, more fuel efficient, smoother, faster and a better overall "feel". This will translate to some kind of re-sale bump down the road, call it $30K, so the difference is coming down a bit.

I have to admit though, for 1350 total hours since 1986, it would be hard to justify the extra $50K - $70K. Assuming that you are the original owner, glad to see you keeping up on her. Walker Airseps will go a long way towards keeping the bilge clean from blow-by oil mist.

As far as speed goes, here is the 1986 Boating magazine published test with 6V-71TA's:


You should be able to compare your boat numbers to the chart above, but I would say a 27 knot high cruise at 2100 rpm is possible with the 6V-92TA's, although I like to cruise these Detroit's no higher than 1950 rpm myself for better longevity. Keep in mind that fresh water (Great Lakes) speeds are less as you displace more than boats in saltwater.

That's a really nice looking 1986 Bertram, your pride of ownership really shows well. Freshwater and Winter storage are easy on FRP boats when compared to all that saltwater and UV robbing Florida sun!

Here is the torque comparison for conversation sake: