More thoughts on repower

[bestfish]

After the great discussion about repower options, hp vs torque. Here's my situation. 36' Harris cuttyhunk, royal Lowell design (think cross between 32 holland and 35 Bruno), I was running a 420 cummins with a 2:1, 29x29 wheel. I had a cruise of about 16.5 and 19.5 wot. For a couple reasons, (the main being $) I put in a 6v92 550hp with a 1.5:1. I'm at least a year away from launching but here's my train of thought because all i can do right now is think about it. With keeping the same wheel I previously had a wot shaft speed of 1200 rpm+/- that equaled 19.5 knots. Now with the Detroit and all it's noise I'll have a cruise shaft speed of 1300 @1900 rpm. Maybe somebody can give exact specs but I'm pretty confident the Detroit puts out alot more torque for any given rpm. I understand this is a heavier combo for sure, but I look at it as 3 decent size guys onboard. I had a brief communication with Lowell boats about it, they thought I 'might' get 20kt cruise. I've used the boatdiesel calculator and even adding 1000lbs to my displacement it gave me a cruise well above 20 more like 24. My goal is a 20kt cruise. If I get that I'll be happy. What do the pros think without shattering my dreams!?

 

[FPTMarineDiesel]

After the great discussion about repower options, hp vs torque. Here's my situation. 36' Harris cuttyhunk, royal Lowell design (think cross between 32 holland and 35 Bruno), I was running a 420 cummins with a 2:1, 29x29 wheel. I had a cruise of about 16.5 and 19.5 wot. For a couple reasons, (the main being $) I put in a 6v92 550hp with a 1.5:1. I'm at least a year away from launching but here's my train of thought because all i can do right now is think about it. With keeping the same wheel I previously had a wot shaft speed of 1200 rpm+/- that equaled 19.5 knots. Now with the Detroit and all it's noise I'll have a cruise shaft speed of 1300 @1900 rpm. Maybe somebody can give exact specs but I'm pretty confident the Detroit puts out alot more torque for any given rpm. I understand this is a heavier combo for sure, but I look at it as 3 decent size guys onboard. I had a brief communication with Lowell boats about it, they thought I 'might' get 20kt cruise. I've used the boatdiesel calculator and even adding 1000lbs to my displacement it gave me a cruise well above 20 more like 24. My goal is a 20kt cruise. If I get that I'll be happy. What do the pros think without shattering my dreams!?

If you are focused on an efficient cruise speed, and worried about weight, increasing the displacement of the engine and torque is smart, and be sure to keep as much diameter under the keel as you possibly can for efficient cruise speeds. From my experience it is the best option for a focus on cruise speeds. I am sure there are others with more experience that may or may not agree.

 

[captainlarry84]

You are removing great power & 16.5 is not a bad speed, but like all of us we love to squeeze out another knot or two. With my back yard math can you give me the following info? Then I will give you my thoughts:

420 Cummins HP & what RPM?
550 6V92 HP @ what RPM?
420 Cummins rated continuous running RPMs
550 6V92 rated continuous running RPMs
Shaft size
Weights of both motors.

Also you posted your current wheel is a full 29 X 29 correct, seems like a lot of wheel for a 420HP Is it Four bladed?

 

[djmarchand]

bestfish:

I certainly am no pro, but I can read datasheets, so I found a data sheet on a similar DD engine on boatdiesel, but the file size is much too big for this bbs system. So bear with me while I try to explain.

We know that your boat will go 19.5 kts and requires 420 hp from the Cummins at 1,200 prop rpm and 2,400 engine rpm to do so. The DD will require the same hp to do the same thing, right? But since you changed the gearing, to get 1,200 prop rpm will now require 1,800 engine rpm on the DD and it will need to pull 420 hp from the engine to go 19.5 kts.

The data sheet I got from boatdiesel is for a 450 hp 6-92TA and a 550 hp 6-92 TA. I suspect the injectors or something is different. They both produce their maximum hp at 2,300 rpm.

So when I look at the 550 hp engine's max hp curve, it shows about 450 hp at 1,800 rpm. The 450 hp engine shows 400 hp at that rpm. Don't know if your engine is in between or maybe the hp spec is wrong, or I just couldn't find a 500 hp datasheet. But what it says is that the 550 hp engine can push the boat to 19.5 kts but the 450 hp engine cannot.

And both will be operating near the maximum hp curve to do so which is not a good thing.

I would prop down at least 2" or maybe even get a new prop specific to the engine/transmission combo. You can probably cruise that engine safely at 2,000 rpm but if for example the 550 hp engine is propped right then it will produce 375 hp at 2,000 rpm (using the prop curve) which won't get you to 19.5 kts. More like 18 kts I would guess.

And there is a lesson in this exercise. You don't have to look at torque curves. HP curves will tell the whole story.

David

 

[captainlarry84]

A review of specs:

Cummins 420 HP @ 2400 RPMs Shaft speed 1200 RPMs

6V92 550 HP @ 2300 RPMs Shaft speed 1533 RPMS

Therefore you have increased your HP by 30% & you have increased your shaft speed by 28%. Based on those increases you should spin the same size wheel. If your WOT with the 420 Cummins was 19.5 knots then @ 1800 RPMs with the 6V92 you should be at 91.5 Knots or close to it.

 

[petrel]

That's another way to look at the math. Cool, but is the Cummins not 2600 rpms?

 

[bestfish]

Sounds like I was thinking correctly, the prop might change someday but as a starting point my reasoning seems pretty close. And yes I agree the 29x29 seemed a bit much or the cummins, but it did make wot and that engine was tired. The DD version I have is the twin turbo aftercooled (tta). Just for thought, what if I used a 2:1 gear instead of 1:5? My fear is that I would have to put so much pitch that the boat would idle too fast for trolling.

 

[captainlarry84]

Sounds like I was thinking correctly, the prop might change someday but as a starting point my reasoning seems pretty close. And yes I agree the 29x29 seemed a bit much or the cummins, but it did make wot and that engine was tired. The DD version I have is the twin turbo aftercooled (tta). Just for thought, what if I used a 2:1 gear instead of 1:5? My fear is that I would have to put so much pitch that the boat would idle too fast for trolling.

If your cummins is 2600 RFPMs then I must redo the math. A 2:1 gear would require a wheel with more pitch & shaft load.
If the Cummins was spinning 2600 RPMs then shaft speed only increases by 23% therefore it should make the current wheel spin even easier. I would stick with a 1:5 gear. Maybe a 1:77 but hard to call.

 

[bestfish]

Thanks capt Larry, I'm glad you agree I should keep the 1.5 because that's what's bolted up and sitting in the hull! I finally got the engine sitting in place and didn't want to doubt myself at this point, not that it's too late but I really didn't want to pull it out again. With my work schedule and the boat a couple hundred miles away from home I don't get alot of time. On my next visit I will be removing the fly bridge in preparation to get hauled to my house in Maine, hopefully then I can make some decent progress.

 

[captainlarry84]

Your current wheel is a good start it is all about shaft speed & the RPMs that the motors develop HP. So you are there.