C-30 motoring speed

kloudie1 · Mar 30, 2021

The first two clarifying questions are: Is that GPS speed, or speed through the water ? and what is thje condition of the bottom and the prop?
Based on this attachment, 2400-2500 RPM is fine for that engine.
M25XPB.pdf (torresen.com)

stephen_i_w · Mar 30, 2021

I always heard that you can comfortably cruise at 80% of the maximum RPM of a diesel. According to the information sheet, I found on the Internet, the maximum is 3200 RPM. So, 80% of 3200 is 2560 RPM. Also, how fast is the river current? That will slow you down for sure.

kloudie1 · Mar 30, 2021

GPS speed can be misleading because it does not take currents into account, or show boat speed through the water. I am not familiar with that area, but any tidal current may be subtracting from GPS speed .. or adding to it.. Sounds like the underwater stuff is clean, so the next questions would be: has the engine speed been verified compared to the tachometer reading? and what is the prop pitch and diameter compared to another Cat 30 with same engine and transmission gear ratio?

Stu Jackson · Mar 30, 2021

"At Ease" said:
Two questions. The original owner told me not to run over 2000 RPM but everyone I talk to say 2400-2500 RPM is not a problem with

the M25XP your thoughts?

Nonsense.

Engines 101 - The BIGGEST & BEST collection of M25 Series Universal Engine Information on the Internet, plus some M35, too

Diesel Engine - c34.org
**********

I have an M25, 2 hp less, essentially the same engine, I run at 2600-2800 (3000 wot) all the time. I have owned this boat for 24 years.

Hayden Watson · Mar 30, 2021

I have a 1988 C30 with the M25XP and get 5.4 - 5.6 knots at 1900 RPM with clean prop and bottom. I get 6.4 at 2400 but do not like the sound. Might be doing more of that 2400 stuff this year after I finish replacing the motor mounts and fully sound insulating the engine compartment.

sailme88 · Mar 30, 2021

There are many rivers that cannot be navigated against the current with a sailboat. The fact that you were making 5 kts is positive. Even with the tide behind you, rivers can be tricky. Next opportunity, check you speed up river and then about. You should acquire a good understanding of how the river affects your boat.

jssailem · Mar 30, 2021

"At Ease" said:
Two questions. The original owner told me not to run over 2000 RPM but everyone I talk to say 2400-2500 RPM is not a problem with

the M25XP your thoughts?

Also would a 3 bladed prop make a difference?

Let examine the data sheet provided by @kloudie1 (always a helper).

WOT - Wide open throttle is identified as 3000. That is what the wizard @Stu Jackson stated and he has years of experience.

Jumping back to the questions - Prop - 3 blade?
That is a personal choice. I would base the selection on the type of cruising you are considering. If you are constantly in conditions that the wind is fickle then you might want to emphasize the auxiliary power and maximize your prop performance. The 3 blade prop is identified as a smoother prop in action.

If you are focusing your sailing on racing or maximizing your sail performance perhaps you want a 2 blade either fixed or folder. The fixed can be set to hide behind the keel reducing the prop drag while sailing.

Your boat your choice. Ain't America Great....

jssailem · Mar 30, 2021

"At Ease" said:
The original owner told me not to run over 2000 RPM

If this is still a concern you might want to go back to the owner and ask why he feels that is important information.

Hayden Watson · Mar 30, 2021

jssailem said:
Let examine the data sheet provided by @kloudie1 (always a helper).
View attachment 192220

WOT - Wide open throttle is identified as 3000. That is what the wizard @Stu Jackson stated and he has years of experience.

Jumping back to the questions - Prop - 3 blade?
That is a personal choice. I would base the selection on the type of cruising you are considering. If you are constantly in conditions that the wind is fickle then you might want to emphasize the auxiliary power and maximize your prop performance. The 3 blade prop is identified as a smoother prop in action.

If you are focusing your sailing on racing or maximizing your sail performance perhaps you want a 2 blade either fixed or folder. The fixed can be set to hide behind the keel reducing the prop drag while sailing.

Your boat your choice. Ain't America Great....

FYI, the OP's engine is an M25XP not M25XPB and they have a different block and more horsepower. They both do have the same WOT so other than a bit less power, the advise for one applies to the other. Mostly this becomes an issue when you get parts because they ain't the same thing. FYI to get parts that are not a part of the marine conversion, the M25XP is a Kubota D950 engine and you can get part cheaper at the Kubota dealer ship and much cheaper on line. A full overhaul kit is only $450 or so.

jssailem · Mar 30, 2021

Thanks for the clarity Hayden. I was a little confused as to why the Previous Owner said “Don’t go over 2000 RPM”. Perhaps it is the 0.5 per gallon fuel consumption. Perhaps the motor really rumbles. Or there is a problem with fuel flow. Very confused.

Kingjim91 · Mar 31, 2021

sailme88 said:
There are many rivers that cannot be navigated against the current with a sailboat. The fact that you were making 5 kts is positive. Even with the tide behind you, rivers can be tricky. Next opportunity, check you speed up river and then about. You should acquire a good understanding of how the river affects your boat.

I agree with this comment. I have a C-30 with a different engine, but 5 knots GPS against current and/or tide might be good. Regardless of the engine and prop combo, you’re not going to push a 5 ton sailboat much more than 6-6.5 knots speed through water. Running the boat up and about would give you a good indication of how much you’re losing to the current.
After you understand that factor, there are lots of good tips in this thread to optimize your setup.