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Best Cruising RPM for twin 270 crusaders

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Dan Eagle

New member
I can plane at 2000RPM in my new to me 1987 36' Mainship 20K semi-displacement hull which will produce about 16mph. The boat seems to run well there but I've had advice that the engine is working too hard cruising at such a low RPM. The prop is OEM. Original mainship manual suggests a cruising speed of +/- 20 mph with top end at 30mph. I'm interesting in hearing opinions. Thanks
 
Jeff, WOT is 4200RPM so my preferred speed is less than 50% WOT. If I rarely cruise over 2000RPM do I need to exercise the engine at a higher load and if so how much?
 
I find it hard to believe that a 20K (I assume you mean 20,000 pounds) could get on plane, or even sustain a plane, at 2000rpm with 270 Crusaders. Are you sure your tachs are accurate? I would expect a much higher rpm would be needed.
 
Dan, to ask what is the best RPM involves many other relative factors. Your particular hull, and the way in which you load the boat, may cause this to vary some.

Is this semi-displacement hull a true planing hull? IOW, is it actually intended to plane out?

The least water resistance will be at either hull speed, or at planing speed and then some! The "then some" gets the hull at the correct planing attitude with the least resistance for this hull attitude, IMO.
This may possibly be your RPM sweet spot!

I'd double check your WOT RPM. Make sure that it meets your OEM specs.
This means engines in full tune, Tachometers accurate, etc, etc.



Here's the other side of this.

Marine gassers are prone to heavy load detonation.
Just to be clear.... Detonation and Pre-Ignition are not the same phenomena.

IMO, detonation with the SBC Marine Engine is at a greater potential at/near 2,000 rpm, compared to lets say 3,200+ rpm.

Now we need to add the engine build into the equation.

Many of the SBC Marine engines are fitted with the GM style full dished pistons. These pistons are terrible at combating detonation. In fact, they can actually contribute to and promote detonation.
Ignition advance is held back on this engine build to help in this area, but it does not necessarily eliminate all potential.

Detonation can also be self perpetuating....... meaning that once it begins, cylinder temperatures increase. With the increase in cylinder temperatures, detonation potential also increases.
You can see where we're going with this!

Point being..... 2,000 RPM is perhaps a lousy RPM to be putting the SBC under heavy load (such as planing)......... unless the build accommodates for this..... and typically it does not!

Add to this the long duration at which you may attempt to remain on plane at this RPM.


For the record.... GM, and many of the Marine Engine builders and re-builders, have done us a terrible disservice by continuing the use of these full dished pistons that were originally used during the 70's automobile emissions control erra.
They have no place being in the SBC Marine build. :mad:

But they continue using them because they are inexpensive, and one piston p/n fits all 8 bores, etc.
These can be upgraded in material from cast aluminum to a Hypereutectic Aluminum alloy, or even forged aluminum.... but the profile is what's wrong with these when used underneath a SBC cylinder head "wedge" area.

As said, ignition TA is held back as their cure for this.


OK... I'm off my horse now! :D



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Rick, my hull is a semi displacement. The original mainship literature states it will cruise comfortably at 17knots and reach a top speed about 27knots. This is consistent with my experience wit the boat. The 2000-2200rpm at 17knots does feel like the sweet spot so far. Push it past that and the secondaries open and then it just feels like I'm pushing it too hard.
 
Perfectly understandable--that is a SEMI dislacement hull (very different). As you stated, your engines are NOT overworked, so why not run them that way?

I run my 32 Marinette at 2,500 rpms or so and it too will effortlessly hold plane at 2,000 rpms. WOT rpms are about 3,900 since i've deliberately over-propped it.

Jeff
 
Depends upon how you define "best"... if you are focused on fuel efficiency, I'd suggest investing in a FloScan setup and finding out empirically.

Also, when it comes to factory literature, see if there are any specific details before you "accept" the data they offer...many represent a best case scenario that rarely reflects real world (typical) use of the hull...
 
Forgot to add this: I marked my throttles at cruise speed, then checked to see how much the primary butterflies were open--I could not slip a pencil into the opening! I suggest you try this test yourself and, if yours is similar to mine, put your mind at ease.

Jeff
 
Dan Eagle said:
Rick, my hull is a semi displacement. The original mainship literature states it will cruise comfortably at 17knots and reach a top speed about 27knots. This is consistent with my experience wit the boat. The 2000-2200rpm at 17knots does feel like the sweet spot so far. Push it past that and the secondaries open and then it just feels like I'm pushing it too hard.
Click to expand...
Dan, let's look at the relationship between the OEM specified WOT RPM, prop pitch, and cruising RPM.
Any one of us could make a minor change to your prop pitch, and could either bring the RPM UP, or could bring the RPM DOWN, while still maintaining the same hull speed. (within limited perameters)

The entire idea behind the WOT RPM test, is to keep from over-burdening the engine at ANY RPM.

I've roughly explained detonation, and have roughly explained how this relates to a certain load and RPM range.
I'll suggest that @ 2k rpm, your engines are at greater risk for detonation potential than they'd be @ 3.5k rpm.
I'd carefully watch what your progressive ignition advance is doing at this RPM.

Basically in lay terms (baring no engine component issues, high heat, lean F/A, etc.), our progressive ignition advance (per engine load) is given the "last word" as to whether or not we'll have detonation potential..... (that, and perhaps a poor combustion chamber design).

However, I suppose that if at 2k rpm, you are still into the primary throttle bores ONLY (I.E., not yet into the secondaries), you may perhaps be OK.
If this works for you..... I can't disagree with what you've been doing!



makomark said:
Also, when it comes to factory literature, see if there are any specific details before you "accept" the data they offer... many represent a best case scenario that rarely reflects real world (typical) use of the hull...
Click to expand...
Amen! Ditto that one!

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Rick:

That is the reason for my "mark the throttles at cruising speed" test. If his carbs are open as little as mine are at 2,500 rpms, then his engines are in no danger whatsoever. But, however, if his primaries are WAY open, then his motors are straining to maintain that 2,000 rpms cruise speed.

Jeff
 
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