Newbie question about RPMs
 

Hi everyone,

Thanks for this forum, have been trying to learn from it.

Been sailing for 50+ years, but never had to pay attention to engines until getting a 1978 Catalina 30 a few years ago. And I'm not much of an engine guy. So my questions are pretty basic.

Original gas Atomic 4. Engine hours when I bought in 2018 were about 850, I've been putting on 30-40/year. I suppose previous owners could have averaged 20 hours/year, but perhaps there was a rebuild along the way. I had a mechanic change the oil once (I know, I know, I should do it every 50 hours and at the end of every season -- I just bought a vacuum pump so I can try it myself soon).

Here's the question: Idling, I get a little more than 3000 RPMs. But in gear, I can't get more than about 1800 RPM, even with full throttle. Propeller is a folding 2-blade. This seems a bit low -- is it? I had a mechanic check it last year, but he didn't find anything. Prop is clean, no weeds.

Is this behavior about right for a 40+ year-old Atomic 4? Or are there things I should be looking at?

Thanks.

First, welcome to the gang!

3000? Do you mean when you "rev" the throttle?
1800 in gear is a bit low but not completely bad.

There a a few things to check and we can walk you thru learning your A4 to such a level you won't need (or trust) a mechanic to step on your boat again. :D

Do you know if your stuffing box is adjusted correctly?
Does the prop spin freely (by hand) when in Neutral?
Any idea what condition your Hot Exhaust is in?

These are some questions/procedures we can help you check.

Also, do you have the MMI Manual? A worthwhile resource that will pay you back in spades.

Finally, HERE are some really excellent videos by our host that you can learn a ton from. And... they're free ;)

Critical information is the diameter and pitch of your prop. Should be around 12 x 7 maximum for your direct drive engine.

Yes, please find out what prop you have.
Some boats have props that are overpitched and will hold the engine to 1800 RPMs and some boats have barnacles on the props.

Thanks for quick responses.

Yes, I mis-spoke -- the 3000+ is when revving the engine in neutral, not when idling.

I do have the MMI manual and have looked at the videos. But I'm at an even more basic level. To show how much of a newbie/non-engine guy I am:

Do you know if your stuffing box is adjusted correctly?
How do I check this? (I almost wrote "Stuffing box? What's that?" But I did manage to find it on a diagram.)

Does the prop spin freely (by hand) when in Neutral?
Do I check this by trying to turn the shaft by hand from inside the boat?

Any idea what condition your Hot Exhaust is in?
No idea at all. What should I be looking at/for?

Critical information is the diameter and pitch of your prop.
and
Yes, please find out what prop you have.
The diameter I can measure. How do I determine the pitch? I'm in the water now until October -- is there a way to do this without hauling? (On a calm day, I can dive to try to measure diameter.)

Given how little I understand about engines, you'll be surprised to learn that I grew up in California. :rolleyes:

Bruce

Boat Speed
 

What speed is the boat making at WOT? Typically, over-propped direct drive A4s will get a boat up to hull speed but the RPM will be in the range you are finding. Such a setup is hard on the engine as it is working quite hard at that RPM to develop the HP needed to make hull speed. It is kind of like driving uphill in 5th gear ... your are making it up the hill but you are "lugging" the engine.

A bigger problem comes into play when you hit bad weather, i.e. heavy waves and/or a lot of wind on your nose. You find that you do not have any reserve power available to such a situation.

Tom Stevens

Some boats have enough access to spin the prop shaft from in the boat and some require diving.
Props are stamped with pitch and diameter on the hub. If the prop is dirty it might be hard to find it and read it.
Speaking of that, ANY barnacles on the prop will cause a big drop in RPM. Make sure the prop is clean!

Your folding prop will have numbers stamped on the side of the hub that identify it by diameter and pitch, right hand or left hand rotation, sometimes shaft diameter too. The attached picture shows a prop that is 17" diameter, 13" pitch, left hand rotation. When you dive in take a wire brush with you.

Thanks for images of what to look for on prop. It'll be at least a week until I get a chance to dive to look -- heavy work week.

I'm pretty sure no barnacles -- I checked before splashing this year -- but I'll check again.

Will let you know what I find.

Hi all,

Sorry for long delay. Summer didn't go as expected, didn't get a chance to dive on propeller. Just hauled for the winter yesterday :(.

Prop is 2-bladed folding "Eliptec 12D x 6P 3" (see link to photo below). Is that about right for A4 for Catalina 30? Blades are fairly clean. I'm in fresh water, in Finger Lakes, upstate NY.

Trip to winter marina is 5.5 hours. When I bought boat 4 years ago, was able to get 2300 RPM under load, driving boat close to 6 kts. This year, couldn't get RPM under load above 1800, boat speed was 5.5 kts. (Traditionally-calculated theoretical hull speed is 6.7 kts.) Also, I usually figure about 1 gal/hr fuel -- this trip, was 9 gals for 5.5 hours.

I did learn how to change the oil last week, though!

Other responses to questions/comments above:
* Prop seems to turn smoothly.
* What/how should I check stuffing box?
* What/how should I check Hot Exhaust?

Any other thoughts about what I should look at over the winter?

Thanks.

https://drive.google.com/file/d/1xvn...ew?usp=sharing

Bruce, Hi...IF I had a folding prop on my boat, it would be about 12 x 6..Those numbers mean 12" diameter, 6" pitch, Mark 3 Martec Eliptec.

It sounds like at one time you were in the 2,300 RPM range which is better than most..probably due to the smaller pitch than is typical on a C-30 (I think stock was a 12 x 7, 2 blade fixed)

Anyway, welcome (back). We can help you make things run as best as possible.