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View Full Version : Chrysler Nissan SD33 engines in a 32 foot RLM cruiser - outdrive ratio and prop size?



Arctic Fox
11-03-2021, 08:06 PM
Hi all,

I am based in Kent, Uk and have owned a 32 foot twin diesel cabin cruiser since 2003. The boat is fitted with two Chrysler Nissan SD33 6 cylinder diesels coupled to Chrysler badged, Volvo Penta 275 outdrives with ratio 2.15:1 and 16 inch diameter by 21 inch pitch props.
The boat has a displacement of between 5 and 6 tons, has a medium V semi-displacement hull 32 foot long and 10 foot across the beam.
I mainly use the boat on the river but occasionally take it out to sea. The performance at speed is poor and I suspect that the outdrive ratio and or the prop spec is not correctly matched to the hull. The 21 inch pitch seems excessive for such a boat. At river cruising speed of 5kts the engines rev at around 1350rpm which is ok, however at max throttle the boat reaches only 10 kts at around 3000rpm max. I tried 16 x 17 props but the performance was much worse with high 1500rpm for 5 kts. From some estimates based on the current set up and the limited data from the 16 x 17 prop runs, I guess the boat would perform much better with outdrive (sterndrive) ratios of perhaps 1.89:1 with 16 x 19 props or even 1.61:1 with 16 x 17 props. Any thoughts on this? Is there anyone out there with a similar size and spec boat also with twin SD33 diesels and sterndrives? If so what ratios and props do you run and what performance do you achieve (slow speed engine revs at say 6mph (5kts is 5.75mph) and max speed with max engine rpm?

makomark
11-08-2021, 11:49 AM
Let me start by stating I have no first hand 'tuning' experience with the drive train combination you have specified....that said, what are your expectations?

On this side of the pond, we would describe what you have as grossly under powered for anything beyond 'slow cruising'....given what you have, I wouldn't entertain the idea using a smaller reduction.

I'd suggest seeking out an experienced prop shop to get their inputs...and to bound what if feasible, performance-wise....

Arctic Fox
11-08-2021, 12:49 PM
Hi Mark, Thanks for your input. I do appreciate and agree with what you say that the power I have is no where near enough to take the boat to planning speeds. I would probably need at least another 100hp! I am just looking for feedback, expertise, recommendations etc concerning the combination I have of engine HP, outdrive ratio and prop pitch as I feel that the prop pitch is course for such a hull /engine combination and will be slipping by more than 50% most of the time. For a boat like mine, I just have a gut feel that the prop pitch should be finer so slipping less but when I put finer pitch props on the drives (16 x 17), I need to increase the prop shaft revs otherwise the engine revs too high with less performance than I get now if that makes any sense! Hence my suggestion that perhaps I should be running 1.89:1 ratios with 16 x 17 or 16 x 19 props instead of 2.15:1 with 16 x 21 inch props. I don't expect the boat to plane but I do believe the boat should do something in the region of 12-15kts in semi displacement.rather than the 9-10kts with max revs 3000rpm.
As the boat is used at low speed on the non tidal river most of its life, I am keen not to increase the engine rpm at low speed (currently 1350rpm for 5kts or 6mph). Thanks again for your input.

makomark
11-08-2021, 07:26 PM
Steve:

a few minutes on Google show you are research other sites as well.

Sorry i can't help more but that combination is rare over here.

Would still suggest visiting a good propeller shop and seeking their inputs. They typically have software that can help optimize your drive train.

Good luck!

o2batsea
11-08-2021, 07:34 PM
You aren't going to get that boat on plane with two 90 hp diesels. In addition to the drives being geared down enough so those engines can actually turn the wheels, you have a heavy boat. Because of this, you have what is really a displacement hull. That means you are making a 12,000 pound hole in the water that you are pushing around with those hard working engines. It won't go any faster even with another 100 horses. It just isn't that kind of boat.