Early 38 Convertibles with Cummins Power

[38Bill]

I'm considering repowering our '69 38C with Cummins 370/425/450. Does anyone out there have any experence with such an animal? I am curious as to fuel burn/ speed/ fit/ upgrades necessary, etc.

Thanks

[jim rosenthal]

Where are you? there is a Cummins dealer in CA, Tony Athens at Seaboard Marine in Oxnard, who would have lots of information for you about this. The 450 or 480hp C series Cummins will fit in this boat, and if you can afford them would be the ideal choice. The B series are probably too small although they would work. The C series will fit in the 36C, which is smaller, although things are a bit snug. In your boat they would be perfect. The boat would be very quick with those, lots of power and torque. Your boat would not outdrag a 427 Cobra, but it would beat anything else, probably....

[38Bill]

I'm in south florida. My concerns are performance vs. horse power. If I can avoid changing struts, shafts and logs, and the complications that they represent, I would prefer to go that route. The big step seems to be going above 370 horsepower, so I'm concerned with the marginal increase in performance when steping up from the 370s to the 450s, as the 450s weigh considerably more.

[jim rosenthal]

I am not sure that the 450, or C series Cummins, weigh a whole lot more. B series Cummins are about 1500 lbs each with gears, CAT 3126s are about 1700 lbs with gears, C series Cummins a bit more but not much. Certainly they don't weigh as much as 6-53s or 8-53s do....
The limiting factors will be 1) shaft size: above 350 hp, depending on gear ratio, you would need larger than a 1.5" shaft even with A22 shafts and 2) clearance for prop diameter in order to get all that power into the water. The largest prop that will go under a 36C is 22" and that is pushing it. If there is more room under the 38C then you have a LOT more options.
Shaft size is less of a big deal. You can buy a little shaft diameter by going to thinwall bearings without changing out the struts and sterntube. Even changing the strut and sterntube for a significantly bigger shaft is not as difficult as reworking all the angles and making more room for a larger wheel. This, I think, is why modern boats often have prop tunnels- the designer gets the added prop room by going up into the hull with the tunnel, not down below the hull into free space with a flat hull surface above the prop. Modern powerboat propulsion design often concentrates on generating thrust by having large, efficient slower turning propellers with big blades, rather than spinning a smaller wheel faster as was done in the past. The one thing most prop books seem to agree on is that diameter and blade area are the two things most associated with efficient prop function, which means pushing a lot of water backwards, and consequently the boat forwards. If I could have gotten 25" wheels under my boat, I would have done so, and used a steeper gear ratio, and driven them slower.
I think the performance would be quite a lot better with 450s, and the engine life longer, if they can be put in the boat at a reasonable cost. Bearing in mind that no costs associated with boats are reasonable.

[38Bill]

JIm,

These are all good points; and right to the center of my concerns. I turn clean 24 squares with healthy 8V53s now. I am working on a plan to boost the cruise speed from a nominal 15 knots (with 3.25" rails), to 20 or 25 knots, so that I can cover more ground when I'm running the Bahama Bank. Running her flat out at 20 kts in the bay, I can see that she is much happier at that speed.

I want to try to establish what the increase cost will be with 370 Cummins and my existing 1.5" shaft gear, (and possibly the same props) versus Cummins 450/480. Looking at Cummin's web site, the weight difference looks to be 1,280 lbs versus 1570 lbs each (almost 600 lbs difference). that could be an additional 90 gallons of fuel weight(!). Which I want to include in any repower project.

If I'll be going 20 kts, loaded, in the open sea with the 370s, and 400/ 450 gallons of fuel, instead of 300; for all the additional expense of changing struts, boring new shaft tube holes, new shafts, replacing exhaust plumbing (which is now all fiberglass green tube), and going with sexy new props, as reqiured with the 450/480s, I darn well better be smoking at 25 kts or better in calm water. Many boats have a particular engine package that that the boat is happiest with; I am trying to find out which engine our 38 prefers. Those 450s look to be a bit pricier than the 370s. If I end up going that route, I really want the beef to go with the burger.

[jim rosenthal]

I agree. However I think that a common fault of modern repowers is to put in a lightweight diesel (and most of our engines are considered lightweight diesels) which is running very high internal pressures in order to produce a very favorable power-to-weight ratio. This leads to engines that when they are seatrialed are running on the ragged edge of too much loading- then you put your gear aboard and your friends and before you know it your performance is dropping- and so is your engine life. I would rather have a larger engine which doesn't work as hard for the same performance, knowing that the extra knots were in reserve if I ever wanted to use them occasionally.
The C series engines will actually fit in a 36 boat, so they are quite likely to fit in a 38, which is significantly bigger. As you say, the limiting dimensions are the shaft diameter and prop diameter.
You might also look at the cost difference of 400hp C series engines versus B series at 370hp. Personally I think that 370 or more is too much to be squeezing out of 6 liter engines. Those are 360 cu in engines, and better than a horse per inch equates to a short life, I think. The C series engines, I think, are 514 cu in, a big difference.
Your fuel weight doesn't change, I think, since it is based on your tank size. Your 8v53s weigh more than either of the Cummins engines you are talking about. I think this comes down to a question of cost in addition to the shaft and prop factors we both noted. I suspect that if you limit hp to 400 you can use thinwall bearings and 1.75" shafts- then the question is can you get enough wheel in there?
Either way, you are going to get the performance that you want. There are also a couple of Yanmars in that range that might do the job for you. I guess my prejudice is that the weight and cost penalty is worth the security of knowing that you have installed larger diesels that are not having to work quite as hard.

[38Bill]

I agree with your points, and your plilosophy, as I am in the marine business myself. And as I always say, "it takes alot of work to live a lie". When cost is less of a concern, I wholeheartedly agree with fitting the biggest engine family in place so that they will loaf at normal cruising speed. But when we start talking about replacing all related systems; this ends up involving replacing everything relating to the engines and running gear. My problem is that I know the real cost in both time and dollars of doing these modifications: What I really want, is to catch a ride on each example- 370 vs 450, and see the performance difference, and how it relates to the different costs of the choices. If the 370 is light on performance, and merely adequate, and the bigger series is that much more robust, it is an easy choice. I just don't want to go into anything this involved based on educated guessing. I need to feel it in the seat of my pants, and see it on the GPS.