optimum speed for fuel economy m26

[yukonbob]

Thanks for the tips - I want to do some night runs to Avalon too so I figured on a lot of WOT on those legs during the times of night that the wind might die.

I don't know about where you are, but up here that's asking for trouble. Nothing like a 4 ton water logged semi buoyant log through your hull to ruin your evening.

[BOAT]

Captain Seahouse, I have a 2013 ETEC-60 motor. If I understand your post the WOT is not really such a neccesity for clearing fouls and carbon? (Or whatever?)

In the same grain I would hope that WOT would also not be a bad thing over a 20 mile leg? On most vehicles i have ever operated you would never run at WOT for extended periods of time and my history is as a sailor, not a stinkboat operator - so I'm not well experienced at long distance travel under small motors between 30 and 70 HP

All my stinkboat experience is on large cruisers (deisel or bunker c) and I am by no means an engineer for such a thing. I can only go by what I observed the skipper do on those boats.

[iredrider1177]

Lots of variables to consider, my M seems to do best at 6-7 mph with a yamaha high thrust 60

[yukonbob]

20 miles is fine, as is operating at wot for both 2s and 4s. just uses more gas.

[Tomfoolery]

O/B manuals usually suggest getting up to speed at WOT, then backing it down a little once on plane. Saves gas, and the engine doesn't have to work quite as hard. Still planing and therefore moving fast, though.

[seahouse]

If I understand your post the WOT is not really such a neccesity for clearing fouls and carbon?

Correct. There might be other benefits to varying your rpms, but keeping the plugs clear, on E-tecs, is not one of them.

-Brian.

[seahouse]

O/B manuals usually suggest getting up to speed at WOT, then backing it down a little once on plane. Saves gas, and the engine doesn't have to work quite as hard. Still planing and therefore moving fast, though.

Yes, the "optimimum" speed that gives a good balance between fuel burn and speed for power boats is usually around 2/3 to 3/4 of WOT, as suggested by manufacturers.

In power boats a higher speed is required to get up on plane, than is required to maintain it. So the procedure is to speed up to over planing speed, and then throttle back to a lower speed at which you can maintain plane. Running along at just below plane and you're plowing, wasting energy, and not making good use of your fuel. In that case, you could very well be using the same rate of fuel burn as you would if you were at a higher speed and up on plane.

It is this point that I was referring to in my post above where I referred to the "high-drag plane" of our boats. Because the hull design in our boats is a compromise for sailing and motoring, the plane that our hulls attain (at the factory-recommended HP ratings) is less efficient than that of a dedicated powerboat hull (a more "true" plane). So I the gain in fuel economy from being on plane is very much less pronounced on our boats is than it would be in a similar-sized powerboat with a dedicated planing hull, which would "suck" for sailing, BTW.

-Brian.

[dlandersson]

let's be clear

A has a 5 degree V-slope - more of a powerboat - planes faster and better.

A has a 15 degree V-slope - better sailing - except for blue hull drag factors.

O/B manuals usually suggest getting up to speed at WOT, then backing it down a little once on plane. Saves gas, and the engine doesn't have to work quite as hard. Still planing and therefore moving fast, though.

Yes, the "optimimum" speed that gives a good balance between fuel burn and speed for power boats is usually around 2/3 to 3/4 of WOT, as suggested by manufacturers.

In power boats a higher speed is required to get up on plane, than is required to maintain it. So the procedure is to speed up to over planing speed, and then throttle back to a lower speed at which you can maintain plane. Running along at just below plane and you're plowing, wasting energy, and not making good use of your fuel. In that case, you could very well be using the same rate of fuel burn as you would if you were at a higher speed and up on plane.

It is this point that I was referring to in my post above where I referred to the "high-drag plane" of our boats. Because the hull design in our boats is a compromise for sailing and motoring, the plane that our hulls attain (at the factory-recommended HP ratings) is less efficient than that of a dedicated powerboat hull (a more "true" plane). So I the gain in fuel economy from being on plane is very much less pronounced on our boats is than it would be in a similar-sized powerboat with a dedicated planing hull, which would "suck" for sailing, BTW.

-Brian.

[seahouse]

dl -

A has a 5 degree V-slope - more of a powerboat - planes faster and better.

A has a 15 degree V-slope - better sailing - except for blue hull drag factors.

Good to point out that distinction.

-B.

[Tomfoolery]

Same here. I was aware that there was a difference, but I didn't know what it was, nor did I know that the has a more powerboat-like hull shape. Unfortunately, that also translates to weaker sailing, along with the smaller rig.

Can't have everything in one package, I guess.

[seahouse]

Can't have everything in one package,

Very true, and regardless of price paid.

[dlandersson]

I seem to recall my significant other saying something like that once.

Can't have everything in one package, I guess.

[Boblee]

Think you are right Seahouse except that getting over the hull speed generally 1200-1500 rpm of at least an and onto plane will never get you anywhere near the fuel economy of hull speed (flat surface no current 8-10kph? etec) or the 2/3 rpm speed of a dedicated powerboat.
As an anecdotal eg when we put the new powerhead on in Darwin we were told to open it up so did a run across Darwin harbour at various speeds but rarely below 2000 rpm and mainly on plane, when it came time to head back next day the above half tank of fuel was a mere half inch on the bottom,
We curtailed our plans to another destination and headed the thirty odd k's back to Darwin at 1200rpm and on arrival found the level had barely changed.
On a recent trip down the Murray here, we would have averaged between 1200-1500 rpm for the trip but at the low side and we covered above 280k''s for about 40l of fuel but with a light river current at some stages and higher at narrower shallower parts but the speed ranged between 6-12kph.
If we had used the old technology of the Mariner 2 stroke even on the efficient hull we wouldn't have covered a third of that distance for the reasons seahouse mentioned above but at wot they would be much closer together.
Mind you we did open it up when we found a bit of deep water to clean the chambers and saw plenty of smoke but we were pulling a heavily laden 3.6m aluminium dinghy and the was fully laden for the trip.
My advice is to do a run under controlled conditions at a set high speed 30k's or more and then do it as near as possible under the same conditions at a lower rpm and then take fuel readings, alternatively do a run of x hours at one speed at y revs and then do another run of x hours at z revs.
Distance covered should not be a consideration as on the water there are too many variables from even one hour to the next, the challenge is finding the optimal rpm range for your boat with fixed conditions (load etc) and translating that to the distance you need to cover taking into account wind,tides, swell etc but if you know your boat does x gps speed against certain conditions at a certain rpm range that is as good as it gets, flow meters etc only tell a small part of the story IMHO .

[bartmac]

Just an indication....we run a 15HP Suzuki 4 stroke and it will run a 26X up to hull speed at less than 1/3 rpm...1-1.5 litres per hour...tried at full rpm and no real increase in speed ie its not going to plane...so my thoughts are that no matter the motor anything above hull speed is going to consume more fuel but then the extended low speed rpm running becomes a worry??

[seahouse]

At any particular boat configuration (weight, balance, trim etc), if you arrive at a throttle setting where adding more power does not produce more speed or rpm, (and you can't make it up on plane) then you would normally not want to exceed that throttle setting (it would labour the engine), except maybe to accelerate quickly up to that speed.

Assuming your prop isn't cavitating or having other “bite” problems, you might want to consider a different prop (finer pitch) in that situation. But if your top speed now equals hull speed, you cannot exceed that without going up onto plane. A different prop will not get you to planing speed, but it will let you run at more efficient rpm/load settings at all speeds below hull speed.

Also note that when I was using the term “powerboat” above, I was not referring to our (Mac) boats. While I can't speak for an “X”, and suspect it's not much different, I seriously doubt that the “M” hull has a throttle setting on plane where the fuel consumption rate (that would therefore give higher mpg) is better than that of any displacement speed. The “pseudo-plane” of our boats still has too much drag to arrive at that point.

In making a fuel burn test run, travel in a straight line for half the distance, turn 180 degrees, and follow the reciprocal track to eliminate or minimize a number of variables.

Accounting for adjustments for external factors like current, wind, sea state, the lower the rpm on a Mac with and E-tec or similar engine, the better the range will be. To compensate for headwind going upstream against the current etc, you would need to raise the rpm up above idle somewhat to get maximum range.

-B.