How to tell if an engine is lugging

[tie_one_on]

Boat:
1988 28' Baha Cruisers Weekender
Twin 302 Ford 220 hp (engines are mid ship) PMC's
Velvet Drive 10-17-105 straight drives
3 blade 18 x 18 props
Max RPM's 4400 (per manual)
I've owned the boat for 3 seasons now

Situation:
The maximum RPM I see know is 2800-2900 (both tachs show almost identical numbers according to throttle location) speed about 30 mph. Engines seem to run effortlessly at all ranges. Because the engine weight is mid ship the boat has no problem getting up on plane. Usually on plane about 1500 - 1800 rpm running about 15-18 mph. Speed is per GPS.

Questions:

1. How would someone know an engine is lugging?
2. Is there someone or some place that would have data on boat configurations and estimated props?
3. Does anyone know of a place that lends props to test?


My rough calcs are I need 18 x 14 props. Before I spend $1200-1400 for a set I'd like to test them.
Thanks for your help.

 

[tommays]

Re: How to tell if an engine is lugging

If your current 2900 RPM is the max you can reach than you are luging it should be able to reach the rated 4400 RPm with a NORMAL load



Tommays

 

[Don S]

Re: How to tell if an engine is lugging

Moving to Prop Questions and Topics.

 

[Texasmark]

Re: How to tell if an engine is lugging

Other than the tach readout, my opinion of lugging is the inability to respond (noticeably) to increases in throttle.....like when you give it the gas you feel an immediate lurch as the boat picks up speed, or if your jacked up high enough with a light enough load, the prop spins up immediately and then the boat catches up to the new rpm's.

Mark

 

[QC]

Re: How to tell if an engine is lugging

Mark's definition is very close to the technical definition which is(was) increasing the throttle does not raise RPM and/or RPM drops at max throttle. However, that is for diesels and gas engines don't really like it period. Are you sure the tachs are right? I'd check with a photo tach before I did anything.

BTW, lug (or full load) is also the only place that your engine makes full advertised power. It is also the only time an engine is "on the torque curve" as many incorrectly describe many other conditions.

 

[MikDee]

Re: How to tell if an engine is lugging

Those 18x18 props seem way too big in diameter for a direct drive inboard with twin motors? not so much the pitch, but the diameter, are you sure you measured them right? I'm no expert on this, but I'd guess the maximum diameter you'd need considering twin engine power to weight ratio would be 16". I would think that props that diameter would be more suited for a displacement hull, rather then a planing hull. There are plenty of big heavy boats running around with single, or twin engines, using no bigger then 15 1/2-16" diam. props. That alone would free up a lot of load on your motors, the original pitch seems about right to start with though, In My Opinion. If your engines are loosing power do to age, then yes, less pitch will help them unload, but make sure it's not due to other issues, before you make the investment. I know there must be a chart, or formula for figuring out prop size on your boat, but I don't know it, or where to find it.