Low boat speed troubleshooting

[Wisakedjack]

My boat is 1976 Catalina 30 with 3 blade Indigo prop. At the start of the season I was able to get 1900 RPM at full throttle and slightly over 5kts boat speed. Towards the end of the season few issues have developed:

1. I could barely make more than 3 kts at full throttle.
2. There was noticeable vibration when I increased RPM, which seemed to come from under the cockpit.
3. Tachometer stopped working: its jumping between 0 and reasonable RPM, while staying mostly around zero. I don't think issue is related to the first 2, but just adding it here for completeness. I suspect this is just the case of bad connection... But because of this I don't know what was the engine exact RPM.

So, long story short the boat was hauled out this weekend and I was curious to see the state of the prop (hopefully a big garbage bag wrapped over it). There was some barnacle on the blades and some on the shaft. The later was behind the strat and kinda asymmetric. But I don't think this could explain such a loss of boat speed. The hull was mostly clean. I check the prop shaft for play and did not find any. So, obviously I will have to wait until next season to see what my boat speed will be with clean prop. But in the mean time I wanted to ask few questions:

1. Should I expect to see higher RPM with Indigo prop. I think max 2300 is advertised.
2. What else should I check in spring to explain loss of speed? Could it be ignition timing, for example, or anything else?
3. Can growth on shaft case vibration? What else should I check? Engine mounts, alignment come to mind as obvious candidates.

Any advise would be greatly appreciated.

[JOHN COOKSON]

Let's Start With The Easy Stuff

Could be a spark plug not firing or firing intermittently. Maybe the advance is sticky. Or maybe a combination of a sticky advance, a weak spark plug, and a fouled prop.
Sticky valve? Try a compression test.

Did you tune the engine when it started to lose power?

TRUE GRIT

[tenders]

If left uncleaned since launch, barnacles on the Indigo prop in the Northeast will absolutely affect boatspeed and vibration exactly as you've described by the time September (never mind October) rolls around.

Tachometer...not so much, but it seems like you're on the track with a poor connection someplace.

[Wisakedjack]

Thank you for the comments.

I did not clean the bottom/prop since launch (I don't race) and there is anecdotal evidence that things went down after I was not using boat for about 3 weeks in August due to vacation. I had similar issue last season, but it continue to question whether so little barnacle growth can have such an impact.

John, I have EI, so it can't be a sticky advance. I pulled plugs one by one and there was notable change in engine speed/sound. I did compression test in spring after I changed head gasket and the numbers were really good. I did not tune the engine before I started to loose power. But I am debating whether to look into timing. Pros: should I be getting to 2300 rpm as Indigo states? Cons: don't fix if it ain't broken. Engine started well all season and runs smooth at idle.

[ndutton]

I have EI, so it can't be a sticky advance

Oh yes it can. The advance mechanism is completely independent from electronic ignition. Per John's advice, it should be checked and lubed as long as you're there.

I don't have an Indigo prop but mine is roughly the same diameter and pitch - and with a monthly diving service, clean. Following a careful timing adjustment I can reach 2600 RPM (direct drive just like yours) although in use I'm usually around 2100~2300.

My recommendations therefore:

1. Inspect and lube the centrifugal advance system
2. Get the timing set as accurately as you possibly can
3. Clean the bottom, shaft and prop

[edwardc]

Just a few barnacles on the outer portion of the blade can completely disrupt the lamanar flow of water over the prop, killing most of the thrust from that blade.

[sastanley]

Oh yes it can. The advance mechanism is completely independent from electronic ignition. Per John's advice, it should be checked and lubed as long as you're there.

+1 - my experience is with stuck weights the engine will either want to run WOT or won't throttle up. If they are stuck advanced, the engine will stumble at low RPM, but run great at WOT..if stuck at rest, they won't swing open on throttle up and engine timing is affected.

Vibration leads me to believe primarily dirty prop. I have found the Indigo to be especially sensitive to growth..I think it is because we are getting pretty low on working blade surface, so things go south quicker with just a little growth. I try to clean mine 2 or 3 times per year.