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Old 07-31-2013, 12:32 PM   #1
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Cool Hull Speed

From all the info I've read a boats hull speed "is what it is"; give or take a kt. or two. But, my question is, if I replace my FL120 with an engine with more HP can I get a reasonable increase in speed? Keep in mind this is all number crunching. So save all suggestions on why not just buy a faster boat. Let's deal with any given 40ft, full displ., single eng. trawler. Can replacing a 120hp eng. with a 240hp. eng. increase the speed from 8kts. to 12kts. or 14kts., etc.?
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Old 07-31-2013, 01:24 PM   #2
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You say your boat is a full displacement hull. So the answer is no.

Full displacement hulls just dig in the stern and try to climb up over the bow wave if you try to go faster than hull speed with more power. If you double the horsepower you just plow along a bit faster but not much.

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Old 07-31-2013, 02:33 PM   #3
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That's pretty much what I thought. But I like to dream; 7.5 kts. is a killer.
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Old 07-31-2013, 03:26 PM   #4
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I suspect you have a semi disp hull. What is your boat?
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Old 07-31-2013, 04:30 PM   #5
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I plugged in Litton 41 into Yachtworld and saw one with a forward slanting pilot house window that looked very much like a semi-displacement hull. Couldn't be sure as there were no pics out of the water.

So if she is a semi-displacement and I agree with Eric that it might be, then you could go faster with more hp, but you won't like the fuel bill.

Plugging some assumed numbers into boatdiesel's calculator gives 150 hp to cruise at 10 kts, about as much as you want to pull out of a 240 hp engine. You will burn about 8 gph or 1.25 NM/gal at that speed. 12-13 kts is about what you would get at wot and 240 hp.

Your FL120 probably burns 2-3 gph at 7 kts.

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Old 07-31-2013, 04:44 PM   #6
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Mine is a full displacement hull. It has a FL120, 26x22 4 blade prop. Speeds are 1600rpm - 7kts, 1800rpms - 8kts, WOT 2100rpm - 8.5kts. At 7kts I burn 3 gph. The boat's looks is on the style of a GB Europa.
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Old 07-31-2013, 07:00 PM   #7
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My boat's engine is very "comfortable" at one knot under hull-speed: 6.3 knots. It's about twice as efficient as going at hull-speed of 7.3 knots. Six-something knots:



(Lower San Joaquin River)
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Old 07-31-2013, 08:01 PM   #8
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Although six knots is too fast for a "no wake zone" (approaching), the resulting wake is still gentle.



(Lakeville, CA on the Petaluma River. ... Last two photos thanks to Ray/Giggitoni.)
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Old 07-31-2013, 08:17 PM   #9
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The trouble with the whining "no wake" crowd is that the wake produced by 6 knots SOG with the current is markedly different than 6 knots SOG into a 4 knot current.

I see/hear the complaints every day.

I'm sorry for the folks that live on the ICW...but if the moon tide is ripping at 4 knots and I need to get someplace at 5 knots SOG (the minimum I will do when people need service)...that puts my speed through the water at 9 knots and the wake can be larger than people in small boats, tubes, swimming, etc like.

To them I say buy a house someplace else, tube and swim in the lagoons etc...etc...going any slower is not an option unless there is LE vessels, guys working on docks/bulkheads or buoys, or someone obviously in distress.
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Old 07-31-2013, 09:03 PM   #10
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I've been referring to speed through the water. Speed over ground is something else here; frequently plus or minus 2.5 knots (more and less). Always assumed the speeds, wake-wise, were based on speed-through-water, not SOG.
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Old 07-31-2013, 09:12 PM   #11
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I've been referring to speed through the water. Speed over ground is something else here; frequently plus or minus 2.5 knots (more and less). Always assumed the speeds were based on speed-through-water, not SOG.
Sailors are about the only group that I know that think in "Speed" (through the water)...most people going places (not racing around in circles) care about getting someplace so SOG becomes more important and is what the more accurate instrument (GPS) shows anyway.
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Old 07-31-2013, 09:17 PM   #12
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Sailors are about the only group that I know that think in "Speed" (through the water)...most people going places (not racing around in circles) care about getting someplace so SOG becomes more important and is what the more accurate instrument (GPS) shows anyway.
Yep. GPS shows SOG. I rely on RPM for speed through water on this motorboat.
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Old 07-31-2013, 09:28 PM   #13
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Sailors are about the only group that I know that think in "Speed" (through the water)..
That explains it. I've spent 20 years sailing on sailboats and on only 2.5 years on a motorboat.
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Old 07-31-2013, 09:37 PM   #14
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Yep. GPS shows SOG. I rely on RPM for speed through water on this motorboat.

I too just set RPM and forget...what I get ...I get...

....but ultimately it's SOG that let's me know when I will arrive at the next bridge or destination...not SPEED (thru water)...so that's what I and most capts I boat/deliver with think in.
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Old 07-31-2013, 10:12 PM   #15
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I have a question about calculating theoretical hull speed.
As it's based on hull length at the water line, how do you measure LWL? I assume it is from the bow to the corner of the transom along the WL.
What about on a canoe stern hull? Is it from the bow to the mid point of the stern?
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Old 07-31-2013, 10:24 PM   #16
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I have a question about calculating theoretical hull speed.
As it's based on hull length at the water line, how do you measure LWL? I assume it is from the bow to the corner of the transom along the WL.
What about on a canoe stern hull? Is it from the bow to the mid point of the stern?
Unless you have a tiny boat...the square root of that difference isn't gonna matter much...the more important number is the constant (1.34) times the square root of the WLL...it really isn't a constant and varies between 1 and 1.6 or so...I've seen it all over the place and it has to do with the hull shape. The more'"efficient", "slippery" the hull (as in sailboat design and some power boats) the higher that number resulting in a higher theoretical hull speed.
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Old 08-03-2013, 10:56 PM   #17
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The trouble with the whining "no wake" crowd is that the wake produced by 6 knots SOG with the current is markedly different than 6 knots SOG into a 4 knot current.

I see/hear the complaints every day.

I'm sorry for the folks that live on the ICW...but if the moon tide is ripping at 4 knots and I need to get someplace at 5 knots SOG (the minimum I will do when people need service)...that puts my speed thruough the water at 9 knots and the wake can be larger than people in small boats, tubes, swimming, etc like.

To them I say buy a houste someplace else, tube and swim in the lagoons etc...etc...going any slower is not an option unless there is LE vessels, guys working on docks/bulkheads or buoys, or someone obviously in distress.
nice to know you don't consider the. value of other people's property ,or the rule of law as long as you can run 10 knots nice. and you're supposedly a professional ? I think IT WOULD be better if people like you stayed in your truck and drove on the interstate
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Old 08-04-2013, 06:53 AM   #18
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the trouble with the whining "no wake" crowd is that the wake produced by 6 knots sog with the current is markedly different than 6 knots sog into a 4 knot current.

I see/hear the complaints every day.

I'm sorry for the folks that live on the icw...but if the moon tide is ripping at 4 knots and i need to get someplace at 5 knots sog (the minimum i will do when people need service)...that puts my speed through the water at 9 knots and the wake can be larger than people in small boats, tubes, swimming, etc like.

To them i say buy a house someplace else, tube and swim in the lagoons etc...etc...going any slower is not an option unless there is le vessels, guys working on docks/bulkheads or buoys, or someone obviously in distress.
IS THIS JERK FOR REAL? If so, I think I may not want to join the trawler community.
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Old 08-04-2013, 06:54 AM   #19
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nice to know you don't consider the. value of other people's property ,or the rule of law as long as you can run 10 knots nice. and you're supposedly a professional ? I think IT WOULD be better if people like you stayed in your truck and drove on the interstate
As a professional...I see the "whiners" every day...there are WAYYYYY more of the regular joes that "get" the big picture and think like me.

To prove my point...if you run the ICW...you'll see almost all new properties along the way have boat lifts and there aren't ANY state placed no wake buoys.

The states see the point that the ICW is a roadway on the water and if you chose to buy/build along it, tough, they are not going to endorse the whole thing becoming a "no wake zone". In fact many of the states have now enacted laws AGAINST putting up "private" no wake zone signs that have not been state approved. See the pattern here?

So whether it me being respectful or not.... or the people who "don't get it" being respectful or not...I'll let the people that see me operate ever day on the water be the judge of me...
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Old 08-04-2013, 07:06 AM   #20
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IS THIS JERK FOR REAL? If so, I think I may not want to join the trawler community.
Do you have something constructive or factual to add to the "wake debate"?

My points are valid, shared by many "seasoned" recreational boaters as well as many pro including Law Enforcement types. There are also being considered by government entities that regulate the waterways as "they" become educated to the problems that arise from overcrowded waterways.

What some boaters forget....the ACIW maintenance is funded heavily based on the amount of commercial traffic that uses it.

Commercial traffic by it's nature is going to produce wakes larger than much of our smaller craft...not in all cases, but in a lot of it. Especially using the example of a fast running tide. If you are paying a $1000/hr to have a barge moved even a 1/4 knot increase against the current is saving large sums of money. So strictly adhering to "no wake" is never going to happen. In fact, no wake is impossible...so it's really just a matter of "how much wake".

Choke off the reason for commercial traffic to use the ACIW...and all the complaints about shallow spots, bridge restrictions, lack of services, etc...etc will definitely be falling on deaf ears.
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