200 RPM Rule?

[Inter20sailor]

Hello Salts.

1991 Cat 30 MkII. Looking for optimum 'good for engine' power setting.

The Universal 25, red line about 3400 RPM. I was told by a few sailors that you should run a diesel engine at full (to get a red-line reading) then back off 200 RPM and use that for cruise power setting.

I would like to sample what many here do as well. Does that seem like an appropriate 'rule-of-thumb'?

Thank you....

 

[Maine Sail]

For cruise RPM 80% of wide open throttle, or max rated throttle, is the general rule of thumb.

So if your engine is rated at 3400 max then you should be able to get it to 3400 with a clean prop, clean bottom and no head wind or current. You would then run at 80% of that number for cruise speed. I however did not recall that any of the M-25's were marine power rated at 3400 RPM?

The Universal M-25XP is rated at 3200 RPM @80% =2560 RPM

The Universal M-25XPA is rated at 3200 RPM @80% =2560 RPM

The Universal M-25XPB is rated at 3000 RPM @80% =2400 RPM


From the M-25XPB tech sheet:
"Universal recommends a propeller that will allow the
M-25XPB engine to turn 3000 RPM underway at full throttle"

 

[Inter20sailor]

Thank you for reply.

I do indeed have the XP (so, 3200) would be stated red-line. This is a new to me boat.... I am sure the Tach should be/ needs to be calibrated.

Thank you for the 80% rule. That sounds more practical than 200 rpm off max.

Thanks again.

 

[fox islander]

Mine is the M-25XP also...my experience is that 2400 rpm just feels right from a vibration and noise standpoint.

 

[Anonymous 121765]

Inter20,

All the guys gave good info. Also depending on the age & previous speed usage the engine is used to, you will find a "sweet spot" in your RPM's in which the engine vibration smoothens & harmonics feel "sweet". I have a 3M20A and run cruising between 2200 & 2500 RPM. That's my "sweet spot" & gives me close to hull speed @ 2500. If motoring for distance, i ride between 2200 & 2350 RPM.
c'ron

 

[Inter20sailor]

If I may ask a question:

What is hull speed for the Cat 30? I know there is a formula... I just don't know the formula.

Thank you.

 

[Anonymous 121765]

Inter20,

Here's the site for quick hull speed formula. http://www.sailingusa.info/cal__hull_speed.htm

Weight & conditions will have an effect, but my 1980 cat hull speed is about 6.3 knots. What with changes in hull dimensions & keel configurations over the years, speed may vary. You are in a liquid/weather environment. nothing is exact or equal.
c'ron

 

[FOB]

Congrats Inter20. I also have a 1991 Catalina 30 MII with an M25xp. I find 2800- 2900 rpm feels good on her. ive owned the boat now for a couple years, and the PO used to run the engine at 2900 also. Kind of like C'Ron says, the engine has always run at that speed when going the distance, so shes happy there.

 

[Stu Jackson]

You may be interested in this link

Please see: http://www.c34.org/wiki/index.php?title=Diesel_Engine

 

[Bad Obsession]

I thought that the hull speed for the Catalina 30 is 6.8 knots. I remember seeing that somewhere.

 

[Anonymous 121765]

Bad,
I have the 1980 cat 30. Your hull/keel dimensions are somewhat different I believe, & 10 yrs. different in age. You may also have the wing keel? Catalina told me years ago that on my boat's theoretical hull speed is 6.3kts.
c'ron

 

[Bad Obsession]

According to Catalina owners web site, the LWL is 25 feet. Putting 25 feet into the calculator you get 6.7 knots. A boat that is fully loaded and under sail the water line will drop a little so it would give you about 6.8 knots, theoretical, that is.

 

[Anonymous 121765]

Bad,
I hear ya man. However, theories are written on flat sheets of paper and in a perfect world. For my boat going to windward (gulf sailing), I usually need about 13-15 kts of wind to go over 6KTs. (150 gen & roached mainsail) with that, now I have anywhere from 3-4ft seas. For me, that kind of kills that 3/10 of a knot.

I do know however that the boat likes weight at the bow. Put 1 or 2 people even out on the bowsprit, what with that fat beam, it seems to be running down hill, and you can feel the acceleration right away.

Been toying for some years now of adding weight under the v-berth locker. Have not decided what to use for ballast. I know some put bags of sand, lead etc. It is somewhere one of my "to do" lists. Do you experience the same pickup in speed with 1 or 2 people at the nose?

A friend of mine had a Sparkman-Stevens around 1984-86 vintage with that fat guppy like beam. I got the idea of weight up front from sailing his boat. 4kts & up you could see the bow dip and standing forward looking aft you could see you were going downhill. It was one quick Catalina. They did not make many, but it was sweet!

I have a little issue with the weight arrangement on the boat though. Starting just aft the motor there is heavier weighting aft. You have the fuel tank, hot water heater, gear, stern locker lunch hook, spare lines pedestal and passengers. I have always noticed that the cockpit always hangs a little lower in the water. Also at the dock it seems to roll tender towards port if I am anywhere in the galley. Do you notice the same?
It was this issue & sailing the 38 footer that gave me the idea of bow weight.

c'ron

 

[Bad Obsession]

I have a 33lb bruce and 30 feet of chain in my anchor locker so I don't worry about weight up front. I have been in some pretty big waves (classic steep Chesapeake Bay waves) and not noticed an issue when under sail. When I am under power she will hobby horse unless I am drawing some sail.

Where you will see hull speed in the Catalina 30 is when you are reaching. The Catalina 30 LOVES to reach. Mine is a wing keel so she doesn't like going to wind very well.

The transom of the boat will go down when you are underway. It is designed to do this and it is a non issue. It actually adds to the waterline so that is why I am saying that you get about 6.8 theoretical hull speed.

As far as the boat sitting in the dock, I have noticed that a lot of Catalinas, 30s and others, tend to have a VERY slight port lisp. This means that she sits a little off of straight up and down and favors port. My Uncle's Catalina 34 (1988) does this and I have another friend with a 1990 Catalina 30 MKII that does the same.

Also remember, you do have your large water tank under the front V-berth so that adds a lot of weight forward when you are cruising.

 

[LDK]

First I support the 80% rpm rule, I have adhered to it except durring extreme need for speed/thrust or stopping responce. Second I would like to comment on bow weighting: there too I am in support. I hang a 30 lb bruce and 380 ft of 3/8 chain in the bow under the V-birth. Neptune asked for 20ft of it and I could'nt refuse. Also in the bow behind the chain locker and also under the v-berth is a 35 gal watertank. So I have lots of weight forward. When heading uphill in 3-4ft chop that weight keeps the bow down and we cut right thru the stuff. As stated by others, sand bags or lead can be used but neither offer the multi-use aspect that potable water and chain anchor rode provide.

Fairwell

LDK
Razmataz
C-30