RPMs, Bow Angle, and Fuel Economy

[Jason2tpa]

Here's one for y'all intellectuals to mull over:

RPMs are directly proportional to increasing the throttle, and fuel economy has a disproportional response to the same increase in throttle. On plane at a steady 2500 rpms, you increase throttle to hold steady at 3000 rpms. The result is you reduced fuel economy.

Here's my question:

On plane at a steady 3500 rpms, you increase your bow angle. What results is less drag and a corresponding increase in rpms. You did not increase throttle, yet you increased rpms. What fuel economy response do you expect?

Does fuel economy remain consistent with the throttle position at 3500 rpms?
Does fuel economy increase because of less load?
Does fuel economy decrease because rpms increased?

[fairdeal]

I'm no intellectual - nor have I studied / thought much about theory of naturally aspirated internal combustion engines

but we have all seen evidence that "load" on the engine factors into fuel consumption

the example that comes to mind is often seen in forums like this:

someone reports that there engine will not reach desired rpms - often "not even plane" -

but that "it will easily rev to 6,000 rpms in neutral"

and the underlying issue is a severe restriction to fuel flow such as clogged HP pump filter

[TownsendsFJR1300]

Here's the Yamaha tech bulletin for my boat / engine combo which is VERY accurate (just came in from a 35 mile run):

You can compare MPH, vs GPH, etc.




Today:

[cpostis]

Looks like that boat gets on plane between 3,000 and 3,500 rpm. Look at that difference in MPH, (almost 2X). Great difference in MPG at those RPM's as well (2 MPG).

About "bow angle", I find after the initial stern drop (minimal cause I use trim tabs) and bow rise my boat stern rises after I get on plane and I'm running on the water maybe 1/2 of the hull bottom on the surface. Bow is slightly raised. You get a feeling for where the boat cuts the water best.

Watch you tach, and speed I use GPS SOG. I recently installed a fuel management system (see the pic) and am looking forward to dialing in best performance and economy. I wonder how my "feel" will compare to the fuel management data?

[IMG]Boat Lowrance by Charles Postis, on Flickr[/IMG]

[TownsendsFJR1300]

Actually at 3,600 RPM, it's running 25 MPH (GPS) than in that spec.

About 4,100 RPM, 30 MPH.

Now mines a little lighter (than the above spec'ed boat) with the bimini top and had a half tank of fuel today. One person, almost no wind, etc, thus the slightly higher speed/RPM

As for bow up/down, etc, I don't have trim tab's and the boat is somewhat rear heavy (they didn't have 4 strokes back then).

Depending on the speed, I HAVE TO trim up or down slightly to NOT get porpoising and NEUTRAL STEERING..

It also seems, WOT, (the water needs to be smooth), I trim the motor up to 6000 RPM and the nose stays down the most.

Today was the first time I trimmed up the motor beyond 5900 (to 6K) while watching the speed and it did
pick up just a touch of speed TRIMMED UP just a touch..

Just to note, I have the T&T sensor is adjusted dead nuts on at full trim DOWN position to match the gauge.
6,000 RPM's today, the T&T gauge was right at 3 (right in the middle). I usually run one "mark" lower
once underway (normal running speeds between 3500-4000 RPM's).

[Jason2tpa]

Here's the Yamaha tech bulletin for my boat / engine combo which is VERY accurate (just came in from a 35 mile run):

You can compare MPH, vs GPH, etc.

This bulletin does not factor in bow angle. I recon because its effect on mph and gph too marginal to publish. But it's ironic you post that bulletin because it shows how little the engine industry values the importance of bow angle as it relates to fuel economy. I'm always changing the bow angle while running as sea conditions and depth change. I balance getting a comfortable ride with getting as much keel out of the water as possible. Granted I run a 17ft bay boat.

The initial question of this thread may not directly relate to your boat. The question occurred to me last time I was on the water. I normally cruise at around 3700 rpms. Maintaining the same throttle, if I increase my bow angle to just under getting a porpoise response, the rpms will increase 200-300. So now I'm running at 39-4000 rpms, but I didn't apply throttle to do so. I was wondering what y'all thought the fuel economy effect would be.

My thinking is fuel economy would increase, despite the higher revolutions, because the engine has less load to push (drag). And is the engine working harder just because it's turning more rpms? I don't think so in this scenario. It's not working harder because no additional throttle was applied, and it has less load at the same throttle position. Just my thoughts, but who knows, maybe I'm way off.

[pstephens46]

My boat has almost identical running specs to the Angler. My dry weight is 100 lbs heavier. No t top

[TownsendsFJR1300]

With that F150 being as heavy as it is, I don't have the option of playing with "bow angles". (Short of slowing for a large wake coming at me, then I'll back way down and lift the nose way up(not on plane).

What ever speed I desire, it's fairly a small trim "range" (depending on load, weight, etc) that I have to find to keep the steering neutral (cable steering) and no porpoising...

With my old V6 Evinrude, it was probably 100 lbs lighter at the rear and trimming it was no where's near as touchy as it is now..


IMO, there's alot of variables and the larger the boat, the less ability you have to play with the bow height while underway.
IE, a 30' speed boat, you don't have the option once on plane to raise or lower the bow say 1'.

For the speed I want to do, I don't really care about bow angles... What ever speed with RPM's (you need), it is what it is.

The smaller boats, (smaller lighter engine usually), you apparently have more of an option to play with the angle..

[Jason2tpa]

IMO, there's alot of variables and the larger the boat, the less ability you have to play with the bow height while underway. IE, a 30' speed boat, you don't have the option once on plane to raise or lower the bow say 1'.

The smaller boats, (smaller lighter engine usually), you apparently have more of an option to play with the angle..

You're correct. Guess I'm spoiled, sort of. I grew up on lighter, smaller boats never more than 20'.

My fathers boat is a hand laid, legendary florida flats boat called Shipoke. 18', but very heavy for its length in order to run atop of heavy inshore chop. At or near full throttle and fully trimmed to white knuckles, only about 2ft of the keel is in the water...rather dancing on the water. What's most amazing about this is even though the engine is trimmed up as far as it can be, the bow remains level. So being groomed on a boat like that you can now understand my liking of running on the edge lol. My hull is not designed for such trim angle with speed, but I do try to run it out of the water as much as a comfortable ride will tolerate.

[99yam40]

that is the way most run their boats.
Trim down a touch from being unstable.
it keeps as much of the hull out of the water

but bow angle is not what they call it

it is trim

[cpostis]

that is the way most run their boats.
Trim down a touch from being unstable.
it keeps as much of the hull out of the water

but bow angle is not what they call it

it is trim

99Yam got it down to a single sentenance.
Here's a pretty good vdeo to go with it.
https://m.youtube.com/watch?v=FxSIhhXE1Aw

[Jason2tpa]

that is the way most run their boats.
Trim down a touch from being unstable.
it keeps as much of the hull out of the water

but bow angle is not what they call it

it is trim

Trim angle? I thought bow angle refers to the angle of bow as it relates to water. As you trim up the bow angle increases. No?

[pstephens46]

Trim up till you can't stand any more....too much and you will decrease speed.
Sort of a "Oh Sht" moment.

[TownsendsFJR1300]

Back about 1980, my other neighbor had a 17' Sutphen speed boat ( http://www.sutphenboats.com/ ) with a 200 hp, Evinrude 2 stroke. (check the link, the 17' restored boat is what my friend had)

We'd take it out, it'd probably do 55, 60 MPH, guesstimating (No GPS/speedometer).

When he nailed it from idle, the nose rose I'm guessing 3', then flat out
hauled a$$..

He would trim the engine up and leave EASILY a 15' tall, 30' long rooster tail!! I don't remember if it had a tach.

From a distance, you couldn't see the boat, just the rooster tail and we didn't have half the Manatee idle zones back then either.

It beat the snot out of you but it was a blast!!!!

Talk about skipping atop the waves!!!!!

[99yam40]

Trim angle? I thought bow angle refers to the angle of bow as it relates to water. As you trim up the bow angle increases. No?

no one talks about bow angle or measures it, just trim until you get it right for the load and conditions.
If you have a trim gauge you can look at it and come back to that if you ever have the same load and conditions, but just bumping the trim until it feels right is what most do.

Scott we see plenty of the Flats boats with large outboards on them hauling a$$ throwing big rooster tails running out across the bays here also. The motors are a lot louder with the prop and exhaust on the surface like that