Help--Need quick answer about cruising speed

David Morgan said:
Not in my experience with planing vessels. After on a plane, the Speed vs MPG curve increases at an increasing rate. It does not decrease after a vessel is on a plane. In fact it is true that in order to double the speed of a vessel, you must anywhere from a squared to a cubed relationship increase the horsepower. Off a plane, the MPG is much better, but this is not what I am talking about.
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I agree with what Gary is saying here. I always word it as follows: Your most efficient cruise is the slowest speed at which you are comfortably on plane. This means that the bow is down, the boat isn't schlogging... The MPG curve is relatively flat in this region. There is a sweet spot, but it varies with current, wind, weight...

The bottom line is that once your boat gets up on plane and accelerates out a bit, you are going to be fairly close to the max range cruising speed. Push the throttles up a bit and your fuel economy begins to drop off at an increasing rate. Allow your boat to fall back and allow the nose to come up, and your economy begins to drop off as well.

Max range is just above idle in calm currents, in case you need that.
 
David Morgan said:
In fact it is true that in order to double the speed of a vessel, you must anywhere from a squared to a cubed relationship increase the horsepower. Off a plane, the MPG is much better, but this is not what I am talking about.
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That fact is only true for a displacement hull boat. It is not a "fact" nor true for a planing hull boat. The background assumption you are making is the coefficient of drag is constant and it is not for a planing hull. It changes with the "angle of attack" the hull has with the water. This cuases the CD to change as the boat increases speed. The faster you go, the angle of attack of the hull with the water decreases, the wave drag decreases and the skin friction drag increases. There is a sweet spot where the drag is minimized... manufacturers try to match the most efficient load of an engine with the lowest drag for a planing hull. That number is not "barely on plane" and it has nothing to do with a displacement hull behavior like you stated.

A practical way to see this is water skiing. If you have ever been on a water ski, you know the faster you go, the less tension there is on the rope pulling you (to a point). If you are "barely on plane" on water skis, it wears your arms out. The tension on the rope is the same as the drag.

My boat is "barely on plane" at about 12-13 knots. That is also my worst fuel efficiency other than WOT. At 25-26 knots I get the best MPG and below 19 knots, even though I am on plane, the MPG goes into the toilet.

The statement of the most efficient cruise is barely on plane would be like telling a pilot that an airplane's most efficient cruise is "right before stall"... It doesn't work that way...
 
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Thanks for the info everyone! I guess the true question should have been "Will this boat cruise at 28 mph?":)

On the original sea trial the boat had not really gotten to the speed that he likes to run. It had really started worrying him (especially that it had v-drives) and he was thinking of passing on this boat. There were the issues with "old gas", "a slight problem with one of the props", but he was not fully satisfied that this was the real problem. He did not want to buy a boat that took him "forever" to get where he wants to go...which is generally Chattanooga or Joe Wheeler from our dock.

He wanted to take it back out and run it (we had paid earnest money and were carrying around a certified check for the agreed upon purchase price so the broker knew we were fairly serious about buying the boat) and notified the broker. Shortly afterwards that same day, the broker contacted him and said he could meet him so he called me from the office and said "quick...find out what is supposed to be the cruising speed for this boat if everything is working correctly". I did a quick search on Google and then thought I would ask the question on this forum. All he was really interested in knowing was if the boat would cruise at 28mph with everything in running order.

I guess you can tell that he is not interested in "tinkering". He knows a lot of basic things because he has had boats all of his life. He also knows good mechanics because all he really wants to do on a boat is drive it which he does everyday weather permitting. He is a mechanic's dream client. His philosophy is "I trust you. If you say it needs to be done, do it." We have been lucky so far in finding good, honest mechanics. We live in a small town so word gets around fast on who to use.

All of that being said, I have gotten some wonderful information on this forum that I have passed on to him. I believe it has been very helpful in buying this boat. The boat is still at our local marina awaiting repairs (2 pages long) that runs from the very basic (replace cracked SeaRay logo on port side...I told you he is not a "do-it-yourselfer") to truly mechanical issues.

If anyone is interested in the list, I would be glad to post it:)

We will be of no help concerning mechanical issues (except how much we paid to fix it) or any other repair, mainenance, or upgrade issues (except how much we paid to have it done)....but we will be available for any advice on how to "boat" in the Goosepond/Roseberry Creek area of Lake Guntersville on the Tennesse River. Our areas of expertise include but are not limited to: the best places to eat and buy adult beverages (except on Sundays); a good place to anchor; great places to water-ski; a great dock to stop by if you are travelling the Tennessee River (ours); the name of a good mechanic.

Again, thanks for all of the info.
 
I was told years ago, as a rule of thumb on a gas engine a good cruise is 1000 rpm's less the WOT. I have proven that out a few times by comparing my boat and friends boats to articles and performance stats.

Let me know what you think!
Rich
 
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Rich said:
I was told years ago, as a rule of thumb on a gas engine a good cruise is 1000 rpm's less the WOT. I have proven that out a few time by comparing my boat a friends boats to articles and performance stats.

Let me know what you think!
Rich
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That is the BEST advice ever given to anyone when given by any engine rebuilder or salesman.

Deisels yes. Gas no.

Optimum cruise is between 70 and 80 percent of the manufacturers recommended WOT. That trick is getting the engines propped correctly and having the proper tabs on the boat. I speak from experience on this one on both I/O and inboard boats.
 
OK the newbie has a question :smt100
please be gentle :smt089

What is planing??????? :huh:
 
David...no four sons is correct. at minimum plane we are doing nothing but "plowing" tons of water. Optimum mpg is quite a bit more than "minimum plane" Ric
 
Since David has a bit of a nasty EDGE to him, I will explain further. Once on plane and cruising, make certain trims are properly set and you can see fuel burn on the smartcraft along with the sound of the engine.

Hey guys only looking to help but when people feel the need to be rude,well then they must be right!!

Go for it David.
 
Rich said:
I was told years ago, as a rule of thumb on a gas engine a good cruise is 1000 rpm's less the WOT. I have proven that out a few times by comparing my boat and friends boats to articles and performance stats.

Let me know what you think!
Rich
Click to expand...

I like it. It's a wag, but it's probably not too far off.
 
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