conundrum ... repower engine choice for Uniflite 48 Convertible

Hey all ... I'd like to study the feasibility of using Cummins Recons to replace DD 8V-92TI's (570 Crankshaft HP / 550 Shaft HP @ 2,300 RPM [DD 'Maximum' rating], 12.07 litres, each, 3,815 pounds, each, with Allison 'MH 20' Marine Gears, 1.97:1 ratio [DD datasheet 4SA104 2-78]) in an '84 Uniflite 48' Convertible (48,000 pounds [45,000 + 3,000 owners' gear] is how Uniflite represented their weight in an August 1, 1982 specifications flyer).

FYI - I do not own this boat but it is a potential purchase candidate for personal use.

If the choice of boat model and repower engine brand / program are givens and not debated, the conundrum I face is the choice between QSC 8.3 600@3,000 'HO' rated engines, or, QSM 11 670@2,300 'HO' rated engines ... (there is a QSM 11 715@2,500 'HO' rated engine, however, the extra HP is achieved with more RPMs and would be of little benefit at cruise RPMs)

A highly respected west coast Cummins repower specialist recommends the 'wet' QSC's, however, I had / have serious reservations about using significantly smaller displacement, higher revving engines in an older boat that has likely gotten heavier over time.

After a lengthy discussion over at THT, it seems folks with personal repower experience tend to agree.

The hitch in using QSM 11's 'HO' rated at 670MHP is that the exhaust manifolds and turbos are 'dry' ... only the aftercooler is sea water cooled ... for those not familiar, this represents the potential for excessive manifold / turbo heat buildup ... and trouble ... depending on how the boat is propped ... the same repower specialist has identified fuel burn rates (19-19.5 gph) not to be exceeded in the cruise RPMs range (1,900-2,000) when using the 'dry' QSM's - and I have no reason to doubt his practical, experience-based results.

So, if someone is game, I need help in interpreting and comparing the available Cummins curve data ... and ... having recently been introduced to the concept of BSFC (Brake Specific Fuel Consumption) there is the (to me, non-intuitive) notion that fuel economy will be the same for different engine choices all depending on how the boat is propped.

I'll upload the curve data for the two Cummins ReCon engine choices noted above ... in case someone is willing to look them over for me ... even if folks here can only point me to practical articles ... I'd like to read and understand this from an engineering perspective ... but I'm no engineer.

Thank you, Kevin Mc

 

My experience with marine diesels goes back almost 60 years. I always found higher revving engines go thru rings and sleeves quicker. They use more fuel. Slow turning engines often go decades between overhaul in commercial use. Old style engines maxing under 1000 rpm usually went their entire life without an overhaul.
Some company probably makes an after market water cooled exhaust manifold. The turbo is usually wrapped with insulation to reduce engine room heat. Good ventilation and keeping the engine room cooler will help fuel efficiency. If you're planning at running near full hp you should have pyrometers installed to monitor your exhaust gas temps. EGTs that are excessive will wear away cylinder components -rings, pistons, valves and sleeves. Just because the coolant isn't overheating doesn't mean the engine isn't too hot. Turbos need a good oil supply. That's part of the cooling. If the oil is allowed to get too dirty, the soot will collect in small oil passages including the turbo line. As flow becomes restricted, the bearings and seals get too hot, and the turbo fails. Of the dozens of turbo engines I have run, I only ran at full hp when necessary. I usually ran my max at 80% of hp.
Note both engines are rated at full power only 1 hour of every 8. The other 7 hours are 200 or 300 rpm less.
Unless you have money enough to burn, I'd keep the engines in the boat now.

 

Thank you Lepke ... I understand your points re: wear w. higher revs, ensuring good e.r. ventilation, watching EGTs and keeping oil supplies clean (, plentiful & cool) ...

... my conundrum with the Cummins ReCon QSM's is that I am not comfortable investing relatively big $ in a repower product that seems to have (an) inherent weakness(es) (the 'dry' e.m.'s [& turbos]), nor would I want to modify Cummins' new 'dry' ReCon product with a/m parts (warranty invalidation issues?) ... although, the mod(s) you propose probably could be done and do(es) make(s) practical sense (but likely would require correctly engineered cooling system heat disipation capacity increases?) ... the mystery to me is why Cummins themselves have not done this 'wet' mod with the ReCon QSM 11's rated above 455MHP ... there must be solid (business) reasons (from their perspective) for not having done so by now, as this engine has been around for quite a while ... since 1998, I believe (see QSM Ratings & CPLs [Cummins Control Parts List] PDF attachment [from sbmar.com] ... interestingly, Cummins is seemingly still supplying the 8753 CPL in the ReCon line whereas the 8763 CPL has replaced it in their 'brand new' line since 2006 ... another mystery ... and ... I don't know if the 8763 CPL is still a 'dry' QSM).

I do know from my online reading that excessive heat build-up in the 'dry' QSM's can be an issue ... and I just don't think I can get behind those 'dry' variants ... on the other hand, spending $4.5-5K USD per hole to completely overhaul the DD 8V-92TI's seems like a pretty big investment in dated, dirty technology ... and that's assuming no major rebuild hurdles ... like a bad block, crankshaft, ect.

 

The issue I have with using Detroit Diesel 2 cycle is reliability. They're 100% mechanical. No circuit boards, no electronic sensors. Once started, no power is required. Because they don't have an injection pump, a failure of an injector tube (inside the valve cover) almost never happens because the fuel runs at about 35 psi, not several thousand. There are no bleeding issues. Yes it's a dated technology, that's what makes it reliable. As to overhauling, DDs are probably cheaper than other models because of their world wide use. There are aftermarket makers of engine kits that work just as well as OEMs. Other than size, the engines are only slightly harder to rebuild than gas engines.
I know this because I've been running DDs since the early 1960s and overhauling them since 1969. I also owned and operated many other diesels makes. I especially hate the new, electronic controlled engines. As they build high hours, their sensors need constant attention. I own one in my pickup (considered by many to be reliable) that I carry electronic spares. I changed 2 this month.
A case in point on the DDs: my current live aboard is powered by twin 671s naturals made in 1947. Contacting previous owners, I can document well over 20,000 hours on the engines when I bought the boat. And that's less than 1/2 of their time in service. When disassembled, by the numbers on the sleeves, the engines appeared to never have been overhauled or overhauled in the 1950s. Based on that, I assume the next overhaul will be about 2085 given the same use. The cranks are standard. The cams within spec. The heads were professionally rebuilt. I rebuilt the blowers, oil pump (with some machining), 4 water pumps. All of it relative easy on an engine designed to be rebuildable in the field. Cost was about $5000 and I did it over a winter. Had the crank been damaged, I'd would have had the journals hard chromed and then they would be even less likely to be damaged in the future. The engines are rated full hp at 1800 where I run all the time. A po ran at 2100 when making long trips. I occasionally do to. But no turbos.
The 71 series is better than the 92s and mine don't have turbos. But there is an added cost of installing a different engine to the shaft, fuel, salt water and power. If you
find a real DD mechanic, maybe semi retired, buy the parts yourself, take the heads, etc., in for rebuild, you will be way ahead of the cost of new engines by at least double.
With turbo engines of any make, the real killer of life span between overhaul is high exhaust gas temperatures. These can occur in the upper 20% of hp. Depending on the engine make. Staying at 80% of hp or below will double the engine life. I know. I've seen it and done that. Just because coolant temps don't show overheat doesn't mean high EGTs aren't damaging the cylinder parts. On a sport fisher, that maybe 25 knots instead of 30.

 

Lepke ... thanks for sharing your insights and for the DD 2-stroke education ... I've looked at several Uniflite 42' Double Cabins with 6-71N's (as well as one Uni 42 Conv with 6-71TI's [410 BHP]) so have recently bellied up to the notions of assessing and owning these engines ... I had a deal on a 42 DC w 1,600 hr 6-71N's, unfortunately, when the boat came out of the water, the bottom was a blister disaster and I had to walk away.

The simplicity of the DD 2-stroke design is appealing (I understood about the beneficial aspects of the independent mechanical injectors but did not realize they operated at such a dramatically different, lower pressure) ... what gives me pause is not really the emissions or the availability of parts these days so much as the availability of career-mechanic expertise ... I am just south of Vancouver, Canada and have previously encountered difficulty / resistance in my area finding career DD 2-stroke mechanics ... attrition has taken its' toll and although I have maybe 2 or 3 names of reputable independent mechanics to approach in my area, none of the larger diesel service providers wants anything to do with them anymore, or, would turn loose younger mechanics with insufficient DD 2-s. experience to yield highly reliable / predictable results.

I note Lepke you have indicated your location to be on the PNW coast ... if you're close to Tsawwassen or Point Roberts and are willing to offer consultational or practical mechanical services, maybe let me know via PM ... I'd be interested in discussing the DD rebuild option further ... my biggest hurdle with the object of this discussion is that the boat I'm currently considering offering on is located in San Diego and needs considerable work to her DD engines in order to make her seaworthy for a trip north to my stomping grounds ... thus ... a repower seemed like a good option ... but not as straight-forward as I had previously thought.

I have spoken with a DD mechanic local to the San Diego area, perhaps you've heard of him ... Craig Stange ... he was brought in by the current boat owner following a less than favourable outcome for the Stbd engine after an in-place (liners et al? &) top-end overhaul by a 3rd party, whose name is, as yet, unknown to me. C.S. disassembled the top-end of the Stbd engine in order to assess its' issue(s) and was then instructed to halt further work at that stage by the owner, so, he used assembly lube to preserve critical surfaces and then wrapped it all up in plastic ... some 2 years ago now. At the time, I believe, the owner had him perform certain limited work on the Port engine (turbos, intercooler, raw water pump impeller & ?) in order to be able to run it (& the boat) to satisfy a Coronado Yacht Club annual(?) mobility requirement (navigate out around some buoy and back).

If I pursue this particular boat acqusition possibility, I would hire Craig Stange to follow-up on his advice to me, which was to see if the Stbd engine can still be barred-over ... if so, an offer could potentially be formulated from that juncture. Even so, the logistics (& expense, said by C.S. to be $20-25K USD per side) of having these engines completely gone through by him while the boat is at arm's-length from my home base are worrisome ... not to mention the exchange penalty I would pay on all aspects of having a boat worked on in the San Diego area ... followed by the inherent risks of taking a vessel (with other potential unknowns) up the coast ... still ... I continue to evaluate various potential repower /rebuild scenarios for this vessel / vessel model.

 

I live aboard and winter on the Columbia River near the coast on a private dock owned by a friend. I'm 70, so while I still do my own mechanics, not interested in doing more. No problem with free advice, but my days of lifting diesel parts are about over. I cross the border a couple times a year. Maybe more if I buy something and haul it back. My experience on 92TIs and 671TIs is a good overhaul will go about 7000 hours with a good operator. 3000 and under with a bad one. The 71s usually go a little further. I made 10,000 on a pair only I operated and 8-9000 on some run by people employed by me. The problem with any turbo engine is high EGTs. So clean oil and reasonable running speed doubles the engine life. Maybe more. There are many things that extend life beyond those hours. I've been playing with continuously clean oil since the 1960s and found it greatly lengthens the life of rings and sleeves. I currently centrifuge my oil and maintain it in a almost clear form. I also run a 2 micron fuel filter and haven't changed an injector since 2011 when I had all the DD injectors rebuilt. The generators still have the injectors they had when I bought the boat.
I don't know how to PM on this site. Old dogs/new tricks.

 

Lepke ... thanks again eh? ... nobody else seems to want to weigh in on this thread ... too bad, I thought maybe someone here could offer me some advice on how to properly (from an engineering perspective) choose between the Cummins ReCon QSC 8.3 600@3,000 'HO' rated engines, or, the QSM 11 670@2,300 'HO' rated engines for the used vessel described in my first post ... if I ever went the ReCon route. I'm not getting a lot of help from Cummins either ... online and in their literature they say they will offer expert, analytical guidance in these matters, however, my experience (as a one-off owner, as opposed to a fleet manager buying engines en masse) has been that one is referred to a local sales office / salesman who seem(s) to be extremely hesistant to quote on anything, and, for actual, practical, technical guidance in turn refer(s) you to one of their dealers in the business of repowers ...

So, I'm looking further at, and thinking hard about, dealing with what's presently installed (the DD 8V-92TI's) in the target vessel and going from there.

It's interesting that you say you're on the Columbia River ... I have a friend who lives with his wife on a floating home at Jantzen Beach (pretty much under the I-5 bridge) ... I helped him build the floating home back in 2000 at a rural property on the southern shore of the river half-way out towards Warrenton ... and just recently (14OCT17) looked at a Uni 42' DC w. 6-71N's in Warrenton. Gotta' love river frontage and all the history still evident everywhere you look ... we have somewhat similar features / sights on our Fraser River up here in Canada.

I understand the desire to leave everybody else's wrenching behind ... but do appreciate the offer of advice ... I may have to take you up on that yet.

Your operational advice (reasonable vs. abusive) with respect to longevity, watching EGTs closely (especially with turbo'd DD units), clean oil (I assume your centrifuging of oil supplements regular oil / filter change-outs [i.e. you re-use engine oil (once?) after having centrifuged it] ... as opposed to lengthening the change-out intervals ... or ... have you developed a method of centrifuging engine oil on-the-fly? in your vessel? ... somewhat similar to what is done with diesel fuel by the Algae-X system [although I believe that systems relies on a magnetic field more than anything else]) and clean fuel are all well taken.

I'll send you a PM so you can have a look and decide whether you want to bother with such things. --> K_Mc edit ... you know what Lepke, this site uses something called 'Conversations' in place of Private Messages (PM's) ... a system that is seemingly not that easy to use ... (I cannot even figure out how to start a Conversation with you ... ), so, f' it for now.

Kevin Mc

 

I use the same oil as long as I get a good oil test and always do. Only new oil since 2011 has been to make up for burned/leaked oil. I have 2 mains and 2 generators. After taking samples if it's time, I pump all the oil together, centrifuge and return by hand in a large oil pitcher. Takes about a half hour. Oil was originally kept separate. I cut open the oil filters about every 500 hours and look for debris. The only way I have to see each engines individual problems. So far so good... Testing centrifuged oil is pointless. With a slow flow rate it centrifuges out like new oil. There's more to it if you go that route. I have about $2500 in the centrifuge, tanks and plumbing. It all fits in about 5x2'. The centrifuge I use is commonly used by people making biodiesel or diesel out of cooking oil. So if I ever have a fuel problem, with some plumbing I can bulk clean my fuel. I broke even on oil in about 4 years not count longer filter life and running clean oil.
I don't do on the fly centrifuging, afraid of an accident and the small individual engine centrifuges don't clean much better than a bypass filter. They use oil pressure to spin and don't spin fast enough. I've used many brands of bypass filters starting with the old toilet paper one about 1966. The best diesel bypass filters are the ones over the road truckers use. The smaller ones can't handle the soot of a diesel and are really made for cars.
When I bought my current boat it had sat for 6 years, no mothballing, etc. I used algae-x at a 3x dose in the day tank. 3 other tanks were dry - maybe an inch or two in the bottom. Who knows how much water and sludge. Tanks had no inspection ports. I put 5 gallons fresh fuel in the other tanks w/3x algae-x and pumped to the day tank. I also added the algae-x inline magnet before the primaries. I idled the 671 mains and ran up the 2 generators for a couple hours. No issues. Changed all the filters, added new fuel and did a 20 mile test in the ocean, no issues. Put plenty of filters on board and made a 600 mile trip home w/o problems. I have 1942 steel tanks and the magnet catches many little steel flakes, so the primary filters stay cleaner. I clean it out every couple years. If the magnet helps the fuel burn, I have no idea. I don't think fuel polishing is necessary. I run a 2 micron primary and have no problems going 500+ hours. I use a vacuum gauge to show when they're getting full. After a couple years, during a remodel I cut in tank inspection plates in the tank tops and went inside, walked around (about 5x5x5) and found zero sludge. Whatever may have been there was gone. I had some pits in the bottom plating I welded, but for 70 year old tanks, they were pretty good. I buy my fuel where commercial boats do. I found they have cleaner, algae free fuel than the average marina. Some also discount bigger buys.
Now I use Archoil 6200 added every fueling. I think it's a little better and gives a 6-10% better mileage. I started using it in my Ford diesel pu because the injectors are electrically controlled but actuated by hydraulic pressure using the motor oil and a separate high pressure pump. The injectors don't fire well with diesel and oil deposits. This cures the problem along with an Archoil oil additive. Probably triples injector life. And in normal driving county roads, no towing get 25 mpg in a 3/4 ton 4x4. I hate to write this because even I don't believe it. Info at archoil.com if interested. I'd use it in any common rail engine. The oil additive is also used in the boat because it's nano lube technology. I don't know if it helps. It's supposed to extend oil changes. I don't know because I also drain the pu and centrifuge about every 2500 miles. I don't drive much, that's about once a year.

 

Lepke ... I see ... thanks for elaborating on your oil centrifuging techniques ... a couple of points of clarification if you would: you say "I use the same oil as long as I get a good oil test and always do.", then "After taking samples if it's time", then later "Testing centrifuged oil is pointless." ... so, I take it you are sending out used individual oil samples for lab analysis (and cutting open your oil filters yourself for post-op inspection) but have long since ruled out the necessity of sending out the freshly-centrifuged oil for lab analysis ... having (an) in-house test(s) for the centrifuged oil that you perform yourself ... or ... am I mis-construing part of your process?

... based on your comments about bypass filters, I take it you do use over-the-road truck type diesel engine oil bypass filters in conjunction with your other oil treatment protocols (including the AR9100 Friction Modifier, aka Archoil oil additive).

Now when you refer to "algae-x at a 3x dose", are you talking about one of these products: AXI Fuel Catalyst (AFC) line of fuel additives? ... or something else?

The ALGAE-X magnetic in-line fuel conditioner devices seem to have a slightly new name, AXI Fuel Conditioners.

The Archoil 6200 Fuel Treatment webpage (and their AR9100 Friction Modifier / oil additive product webpage) has a (have) rather simplistic product description(s) and benefit claims, although, I have yet to review all of the available, detailed product literature. If you are having noticably better results using those additives, then I should probably read up on, and further research, their efficacy.

Thanks again for taking the time to offer insights into your processes ... I do appreciate it.

Kevin Mc

 

I don't use additional filters other than the engines main filter. I did in the past and the larger bypass filters work best if you don't centrifuge.
In the past to check the quality of the oil I sent centrifuged oil. I got a call back asking why, was I looking for something in particular, and saying it appeared to be almost new oil. So now I take a sample while each engine is running and a sample of the engines when together after draining. I send in the composite. If I ever get a bad test, I'll send in the individual samples. Otherwise, it's just the exam of the filters.
Algae-x was sold after I used it in the current boat. So the name is different. I still have the inline magnet and clean it about once a year. When initially dosing the tanks I used 3x the normal maintenance dose as recommended by the company. I continued to use Algae-x for about a year, adding a normal dose each time I fueled. I switched to the Archoil products because of the common injector issue in my Ford truck. Because I got a 10% boost in truck mileage, didn't need new injectors, more power, and smoother running I wanted to try it in the boat engines. What I found was better mileage of about 6-10%. I burn about 8.5 gallons an hour at my normal cruise and was saving almost a gallon an hour more than the Algae-x mix. The mains do 300+ hours a year. Some times 500. That's a decent savings. More than I pay for the additives. Algae-x is a fine fuel additive. My tanks were cleaned. I was amazed that I found no sludge at all. The 70 year old tanks had never been opened, originally contained gasoline, and the last owner was weak on maintenance. But when I switched to Archoil products, the tanks were clean and seem to be staying clean with Archoil.
My Detroit mains pump 70 gallons an hour, so I get much more circulation than smaller yacht type engines. I have talked to Archoil people a couple times.

 

I like the repower topics and will chime in with my 2 cents.

First, I would not be too concerned with Dry Turbo's /Dry Exhaust Manifolds in regards to overall engine life. CAT and others do the same for many pleasure craft engines and it is an engineered solution by the manufacturer for the right mix of combustion/heat/power for the given emission levels. Wrap the Turbo's in blankets and be done with it or even go the more expensive hardcoat ceramic route. You see it in practice all the time.

The Uniflite/Chris-Craft 48 Convertible is a good SF platform, especially out West. It is a good sea boat, we had a family friend where two brothers had them (the Chris-Craft versions) one in Dana Point and one in Newport Beach, and they thoroughly enjoyed them. They were experienced boaters and would run them hard.

The 48 is a heavy platform for its size, and the QSM 11 would be the preferred replacement. I personally would use the QSC 8.3 in SF platforms from 40 - 45', the QSM-11's from 46 - 50'. A Motoryacht repower would be a bit different.

I ran the torque numbers and normalized them to rpm and you can see where the existing 8V-92TI sits in relationship to the two Cummins options:


I think the choice is pretty easy at this point. The 3000 rpm QSC 8.3 can't match the torque of the existing 2300 rpm 8V-92TI, and will suffer in heavier seas with the boat loaded. Plus you will have to make up for the weight difference creatively, and the engine room on the 48 is a bit difficult to add systems aft of the engines to compensate for the lighter engines.

Engine weights (dry, without gear):
QSC 8.3 = 1,975 lbs.
QSM 11 = 2,620 lbs.
8V92TI = 3,335 lbs.

 

pacblue ... thank you for weighing in with your assessment of these engine choices ...

I appreciate your thoughts on the Dry Exhaust Manifolds / Dry Turbo's ... the Cummins folks have grown unresponsive (both locally, and, corporately) when it comes to direct, technical questions put to them regarding my concerns about the QSM 11 670 HO ReCon engines and excessive e.m. / turbo heat build up at fuel burn rates above 19.5 USG per hour for cruise RPMs of 1900-2000, identified as a serious issue by a respected repower specialist here. This aspect of these engines as a repower choice is unfortunate and gives me serious pause as to using them over stickng with the DD 8V-92TI's (i.e. a rebuild of same).

I tried to formulate HP & torque graphs for each of these 3 engine choices in MS Excel that could then be overlayed one upon the other as you have done, but got bogged down with the graph type selection & formatting ... plus, I was never able to find a HP / torque vs. RPM data table for the DD 8V-92TI's ... all I have to go by is the 2-page DD datasheet, 4SA104, printed 2-78 ... the scans I have of it are poor and the graphs small and blurry.

When you say you "normalized" the torque numbers to RPM, may I ask what all that (technically) entails? Are you working in MS Excel (if so, would it be possible to study your file) or something else?

I see you allowed 480 lbs., each, for the MH20 Allisons on the DD 8V-92TI's ... assuming the gears mated to QSM 11's were similar in weight, the overall weight decrease would be arond 1,430 lbs., total. I understand a dramatic change in weight (i.e. the QSC 8.3's) would have an impact on the boats' fore and aft COG, and, to a much lesser extent, the vertical COG ... would these factors still need to be addressed for the QSM 11's? If so, how might this be accomplished, given the constraints you noted? I would have thought that less weight would be mostly beneficial, however, I recognize there's more to it than that ... even if I don't understand it from a naval architect's POV.

 

I read the Cummins link and I am not as concerned as you are. There is too much of a mix of ratings (duty cycles) and rpms to draw the same final conclusion. By reading it you could also conclude that 2100 rpms would be fine as it takes you out of the 1800 - 2000 rpm zone. 1800/1800 does not equal 1800/2100 or 1800/2300 or 1800/2500. The 2 cycle Detroit's always had much better life at 1850 - 1950 rpm max cruise for a 2300 rpm engine. Not much really new here. To make the link data more applicable, the horsepower should be plotted against % throttle, since they vary by engine model/duty cycle. Normalize it by dividing rpm by the max rpm for that engine model so you get a spread up to 100% throttle, then compare. That is what I did for the torque plot for 3 different engines of 3 different max rpms.

What is the stated trade-of in the link? Some early dry exhaust manifold wear? This can be monitored and you could certainly put pyrometers in place and dial in load/fuel burn/exhaust gas temps at the same time, this would be my recommendation. How many hours do you run a year, at what percentage of hours would be at high cruise - 1800 - 2100 rpm?

He is absolutely right about propping light, this is really your ace in the hole. Get the boat at full load departure condition - full fuel, full water, no waste, all your owners gear onboard, bait tanks full, dinghy onboard, bridge enclosure complete, etc. Now prop those Cummins QSM 11's at 2350rpm. As you burn off load you gain an extra safety margin. Your Pyro's will be another level of safety monitoring on the exhaust gas temps. Remember that there are a bunch of Sea Rays running those engines, as well as other pleasurecraft. It is not such a dire proposition in my opinion and I have specified plenty of yachts with this power package.

As far as my torque calculations, I used the basic calculation, torque = (bhp x 5252)/rpm and divided each rpm point by the max rpm of that particular engine model to put it on a percent throttle basis. Attached is the 8V-92TI performance curve, a pretty rare document if I say so myself.

A Twin Disc or ZF gear will be much lighter than 450lbs.each, with the ZF being the lightest, maybe by half.

 

Thanks pacblue for expanding on the topic sub-components above and explaining your comparison methodologies ... and ... thanks for the 8V-92TI performance curve PDF ... that's a very readable set of graphs, unlike the tiny ones I had.

Sorry, but you kind of lost me with "you could also conclude that 2100 rpms would be fine as it takes you out of the 1800 - 2000 rpm zone. 1800/1800 does not equal 1800/2100 or 1800/2300 or 1800/2500." ... so ... in the last quoted portion, are you stating RPM pairs as: x divided by y (yielding products of division that do not equate), or, are you expressing different RPM ranges (that should not be directly compared)? I suspect the former but am not sure I understand your logic if that is the case.

If I understand the author of the QSM 11 propping article I referred you to, where he states, in part: "It’s the high HP and/or high fuel burn at the lower RPM’s that does the manifolds & turbos in.", I believe he is asserting that there is insufficient volumetric combustion airflow (as well as captive coolant / seawater flowrates, lubricating oil flowrate and diesel fuel flowrate) at 1800-2000 RPM to effectively purge the heat quotient released by both combustion and otherwise unburned fuel going through the e.m. / turbo and out the exhaust ... if, as he and you both point out, propping is not carefully considered and an underpropping safety margin strategically introduced ... and ... I get the impression that he developed his opinions on the QSM 11 'DRY' ReCon product from real world field results covering many examples ... as opposed to basing it only on theoretical engineering.

For myself, as someone unfamiliar with the Cummins ReCon products overall, my hope is to carefully avoid making major mistakes should I choose to move forward on that path. Your earlier suggestion to consider ceramic turbo coatings is new to me ... and bears further study ... although I don't know if any in-warranty mods to a ReCon engine would fly with Cummins. The use of pyrometers to monitor EGT's is advice well taken.

You asked what my usage would entail ...

Cruising speeds ... I have no personal familiarity with how efficient this hull was in terms of the potential 'on-step' 'cruising' speed range ... in a repowered version, a comfortable (for the engines) cruising speed of at least 22-26 knots would be nice ...

Duty cycle ... 50/50 ... 50% fastest fuel-efficient displacement mode / 50% 'on-step' cruising ... ideally, the engine choice / propping would allow for more than adequate available torque to get this vessel on-step quickly ... without the engines laboring too long to get there.

Application ... strictly recreational fishing / cruising ... likely only 200-400 hours annually, as I am in the great white north and most boating activity is typically in late spring, summer and early fall.

You commented on the potentially significantly lighter gear sets that could be mated with the QSM 11's (vs. the Allison MH20's), however, you did not comment on the COG effects and mitigation strategies / consequences, if not correctly addressed.

These COG variables lie outside of my experience, seem difficult to accurately anticipate and, if I gather correctly, could potentially be as great a pitfall (if not correctly assessed / addressed) as an incorrect engine repower choice ... may I ask for any insights you could offer in that regard?

Thanks, Kevin Mc

 

I understand the analysis that Seaboard has done for the QSM 11 Wet vs. Dry topic. I just think there are points that may not directly transfer across engine platforms that have variable max rpms, especially compared to a single rpm point (1800) or a continuous duty engine that maxes out at 1800 rpm. I can not even tell if the high heat engine pictures on the test bench are marine models or not? You have read it, I have read, you can draw your own conclusions. It is noted that a Dry QSM 11 can have premature Exhaust Manifold leaks (500 hours or so?) or not, depending on your installation (this is key) and how you use your boat (this is key as well). If you remain locked in fuel burn limiting mode, keep in mind that 20gph per side gives you a total of 40 gph, and at 0.5 nmpg, you may only be looking at a 20 knot cruise, or less if you go below that number. It does not sound as though this approach will line up with your 22 - 26 knot cruise expectations, which should be reasonable. Look at a repowered old Bertram 46 with QSM 11's - 26 knots cruise, 29 knots wot :

1979 Bertram 46 Convertible Power Boat For Sale - www.yachtworld.com http://www.yachtworld.com/boats/1979/Bertram-46-Convertible-2997840/Portsmouth/RI/United-States?refSource=browse listing#.WtYY_HmWx9A

What you have most in your favor is your boating climate. Low sea water temps, cool combustion air, a modern diesel engines dream. You may have to look into a better forced air intake/exhaust system tied to thermostats, to provide the right amount of intake air (low in the bilge if possible) and exhaust air (highest point in bilge, if possible), depending on the current set-up. Those Detroit's gave off a fair amount of heat as well. The hard ceramic comment was intended for exhaust elbow risers connected to the dry turbo's.

On your 50%/50% operating mode - a bit optimistic for planning craft/sportfishers, maybe in reality you will see 25% idle - 10 knots, 50% 10 - 20 knots, 25% greater than 20 knots?

The Allison MH20 are heavy old gears. A ZF 305 Gear is 216 lbs., a ZF 325 gear is 287 lbs. You will have about 1600 less pounds with new engines, it just becomes a balance act. This is not a big obstacle, the QSC 8.3's would be about 3200 lbs. light and could be a greater challenge.You do not want to introduce any trim by the bow (bow down moment) with this weight shift. Invertor banks are a great source of useable "ballast' as opposed to added lead ingots (the easy way out). The hydrostatics from the hull lines will dictate how your hull will be affected. Art Nordtvedt (I believe) did the original naval architecture and his son has Norstar boats, they may be able to help you out with the proper balance for the boat:

norstar-website https://www.norstaryachts.com/

You want to pick the right gear ratio for the chosen hp to keep the same shaft diameter and running gear. The Detroit's had 2300/2.0 = 1,150 shaft rpm, so you would want the same 2.0 gear ratio for the 2300 rpm Cummins QSM 11. You would need to check the shaft diameter size for the addition horsepower 670 vs 550, and make sure to keep a safety factor of at least 3.0 but preferably closer to 5.0 for pleasurecraft Sportfishers. You may need to up-grade to AQ Hi-Strength shaft material depending on the numbers.

You can also update the engine controls to modern electric and save on weight/space/cable routing mess by eliminating the most likely manual throttle/clutch cable set-up onboard.