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MobjackB32 04-06-2010 02:06 PM

Cruising RPM
 
I just bought a 1981 Bristol 32 with a Universal 5424 that has 1150 hours. Bringing her home (30 hours) I noticed she hit cruising speed 5-6 mph at 1400-1500 rpm. I had heard that crusing rpm should be over 2000 on this engine. Is there any harm in having the crusing rpm between 1400 and 1500? Will it lug the engine or cause undo wear, overheating, etc? She has run this way for almost 30 years it would appear.

Thanks,

Jeff

sailingdog 04-06-2010 02:20 PM

Sounds like she's propped wrong. What is the WOT RPM on your boat and what speed do you get???

k1vsk 04-06-2010 02:35 PM

Jeff
Before getting overly concerned with the prop and engine, check the tack to make sure it is reading properly. If it's the same engine and prop combination for 30 years, someone would have noticed this a long time ago.
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christyleigh 04-06-2010 02:40 PM

Since your hull speed only about 6.8 - 7 I'd agree that's a low RPM to hit 6. As SD suggested what RPM and speed do you get at WOT ? Black smoke ? Soot on the transom ? I'd guess your WOT should be about 2800 unlike a high revving Yanmar at around 3200.
So if you get a good WOT and your speed says anything over 7 your knot meter is wrong/high. If you can't get to 2800ish you are probably over-proped as SD suggested. Of course there is always the possibility that your tach is wrong too. You have to verify what you can - Knotmeter with GPS at slack tide. Stock prop ? Engine manual for WOT RPM ? Heavy soot on transom ? ........

MobjackB32 04-06-2010 02:48 PM

She will hit 2500+ rpm in neutral which is the correcWOT range for the engine. I fully suspect the prop pitch is too aggressive. After 30 hours of running there was light black residue on the stern but not bad. With the number of hours on the boat (1150) and the age (29) it averages out to be about 38 hours per year usage. We put almost that many on it in two days. PO's may not have noticed since it wasn't taken very far. My concern is if the low RPM will hurt the engine long term. Ideally I would like tosee her around 2000 rpm at 6 mph. Speed during th cruise was verified with knotmeter and GPS.

sailingdog 04-06-2010 02:49 PM

Just curious, how do you know she's run this way for 30 years.... The engine has 1150 hours on it, and may not be the original engine or prop.... since many boats are typically run for 75-100 hours a year, 1150 hours would be pretty low for a 30 year old boat. Was it a one-owner boat? That's pretty unusual... so I kind of doubt it.

Quote:

Originally Posted by MobjackB32 (Post 589690)
I just bought a 1981 Bristol 32 with a Universal 5424 that has 1150 hours. Bringing her home (30 hours) I noticed she hit cruising speed 5-6 mph at 1400-1500 rpm. I had heard that crusing rpm should be over 2000 on this engine. Is there any harm in having the crusing rpm between 1400 and 1500? Will it lug the engine or cause undo wear, overheating, etc? She has run this way for almost 30 years it would appear.

Thanks,

Jeff

That's a pretty big if, and damned unlikely given only 1150 hours on the engine.


Quote:

Originally Posted by k1vsk (Post 589705)
Jeff
Before getting overly concerned with the prop and engine, check the tack to make sure it is reading properly. If it's the same engine and prop combination for 30 years, someone would have noticed this a long time ago.


MobjackB32 04-06-2010 02:56 PM

I'm the third owner. Original engine. New Hurst Trans in 1999 with less than 600 hrs on it at the time. All documented. The lady i bought it from had the boat for 12 years. PO to that had it since purchase. Can't say for sure if it is the original prop but know it has been on there for 12 years and half the hours on the engine.

klem 04-06-2010 03:18 PM

It does sound possible that the gearing and propeller pitch are not set up properly.

To the original question, there isn't a magic rpm, there are a few things that you want to avoid but a relatively wide range of workable rpms. If you are in fact overpropped, then you are lugging the engine. This means that you are trying to get too much power out of the engine per combustion cycle. For the same power output, lower rpm means less combustion events which means that each event must supply more power. This puts greater stress on the mechanical parts of your engine (a common problem from lugging is blown bottom end bearings) and raises EGT.

Making the assumption that the prop is correct, then the dangers of low rpm are related to engine temperatures. If the engine is not doing enough work to bring all of the components up to the working temperatures, you could have problems with wet stacking or stuck rings. Wet stacking is when not all of the fuel is burned so it starts to wash your cylinder walls, coat valves, coat the exhaust, etc. Since I doubt that you have an EGT gauge, the only gauge that you have to help you with this is your temp gauge. If it ever reads low, give the engine more throttle. Also, if you see signs of unburned fuel, then you need to throttle up. Taking a guess since I don't know the exact setup, I would think your boat with the correct prop would be safe over 1500rpm but you could always rev up a bit more to be safe.

The issues with really high rpm are related to longevity. Fatigue is due to two factors, stress and the number of cycles. More rpm means more cycles for a given period of time. Also, high rpm tends to also mean high stress. These factors combine to mean that parts fatigue more quickly.

The acceptable rpm range is relatively wide. You may find that the sweet spot is something like 1800-2400 on your engine but it really comes down to how you value your time (boat speed), engine life and fuel consumption. It does sound like you may have a more fundamental setup issue to solve first though.

k1vsk 04-06-2010 03:53 PM

Before making any rash judgments about the prop pitch, start with the obvious - is the prop clean and not bent? These kinds of maladies are difficult to diagnose without having all the info and jumping to a conclusion about prop pitch, particularly given the history you provided, isn't the first option.
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MobjackB32 04-06-2010 03:55 PM

So far there havebeen no underheating issues. She runs about 185 -190 at 1500 rpms and the water is cold. If anything, I would say she may be on the warm side when the water gets up to 80 in the summer. I haven't noticed and fuel being blown out either. Maybe a slight adjustment in the pitch would help bring her into the 1800 rpm range. She only used about .75 gals per hour at 6-7 mph. Max speed was 7.2 and she never dropped below 6 all day for 18 hours.


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