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.?

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.

david

That's pretty much what I thought. But I like to dream; 7.5 kts. is a killer.

I suspect you have a semi disp hull. What is your boat?

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.

David

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.

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)

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.)

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.

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.

markpierce said:

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.

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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.

psneeld said:

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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Yep. GPS shows SOG. I rely on RPM for speed through water on this motorboat.

psneeld said:

Sailors are about the only group that I know that think in "Speed" (through the water)..

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That explains it. I've spent 20 years sailing on sailboats and on only 2.5 years on a motorboat.

markpierce said:

Yep. GPS shows SOG. I rely on RPM for speed through water on this motorboat.

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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.

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?

AusCan said:

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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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.

psneeld said:

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.

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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

psneeld said:

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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IS THIS JERK FOR REAL? If so, I think I may not want to join the trawler community.

motion30 said:

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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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...

capt.George said:

IS THIS JERK FOR REAL? If so, I think I may not want to join the trawler community.

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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.

As a former Professional Captain of excursion vessels, sport fishing charterboat owner/operator, delivery Captain and boat builder/restorer, I know well the frustration felt when one is forced to travel at a no-wake speed. Time is money and yet I have, for the most part, seen that the real working pros do their best to avoid damage to shoreline property whether it be seawalls, docks or boats. Not always possible given the nature,size or operating conditions of their vessel and they are a target for lawsuits.
But a 40' Albin - no excuse! Thats going to be my boat you throw up against the dock and damage, my grandchildren you swamp in their dinghy or my son's seawall you undermine. I'm all for stopping shoreside development but reluctantly respect the rights of those already there.

capt.George said:

As a former Professional Captain of excursion vessels, sport fishing charterboat owner/operator, delivery Captain and boat builder/restorer, I know well the frustration felt when one is forced to travel at a no-wake speed. Time is money and yet I have, for the most part, seen that the real working pros do their best to avoid damage to shoreline property whether it be seawalls, docks or boats. Not always possible given the nature,size or operating conditions of their vessel and they are a target for lawsuits.
But a 40' Albin - no excuse! Thats going to be my boat you throw up against the dock and damage, my grandchildren you swamp in their dinghy or my son's seawall you undermine. I'm all for stopping shoreside development but reluctantly respect the rights of those already there.

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Often I think wake discussins are about 2 different scenarios...

First of all ....after a lifetime on the water...none of my wakes have ever damaged anything or hurt anyone....even by my wake monster which was a 37' sportfish.

There are some real wakes out there doing bad things...but they are not mine.

When I refer to "whiners"...it's the guys who are complaining about a wake size that by late afternoon are dwarfed by the sea breeze against the tide waves.

What got this thread off to the wake tangent was the following picture ...which I can't be totally sure about...but I would hardly worry about that wake unless you were pretty near the docks or vessel you were passing.

My real point is that if you are on the water, sometimes wakes/waves are gonna happen (emergency vessels, nature) , some of the responsibility is yours to be shipshape and seamanlike...one hand for the ship as they say. I hate the expression "you are always responsible for your wake" ....because it suggests to some that as long as they have 1" freeboard on their 12 foot jon boat, it's within there inalienable rights that they can stand up and fish a busy inlet. While legally true...I really have an issue with that mentality whether boating or life in general.

dhiggins,

Sounds like you have a semi disp hull if it's like a GB. The formula for HS is 1.34 X the square root of the water line length. Two feet of rake on the stem and the WLL will be 2' less than the overall length.

Semi disp hulls are much less sensitive to hull speed hydrodynamics and tend to respond more like boats called planing hulls. But some semi disp hulls like many DeFevers are quite close to FD hulls and their performance resembles FD hulls more closely. Your boat is probably a tad more like a planing hull than the DeFevers but still may be fairly close to a FD hull.

What this means to you is you can pay less attention to "hull speed" as your boat is more flexible. Like Mark says ... he runs a knot slower than HS. Actually that's the only reasonable option he has. Running HS on a FD hull is making a big wake and as Mark says burning TWICE as much fuel. Sure he can but most all knowledgable FD skippers will run one knot below HS.

Semi Displacement hulls can, rather gracefully, run one knot OVER hull speed as well as exactly at hull speed. They are designed to do so. The GBs are an excellent example. A 36 GB w 250hp will run 8 or 9 knots all the time. That's the glory of the SD hull. It goes faster and does it gracefully. Actually I'm quite sure a FD hull will burn more fuel at hull speed that a SD. Burning considerably more fuel and going more than HS is so desirable (since fuel is cheap) most all trawlers are SD as I think your's is.

It sounds like you've got a GB or a boat like a GB w a single engine. If you want (looks like it) to go over 7 knots there are lots of twin engined boat owners that are fixated on low fuel consumption that will probably trade for your more economical boat. Or do a good job of selling yours and buy a twin that will probably be cheaper. Most trawler owners and shoppers think single engined boats are better. You can use this to your advantage.

Edit:
Just read your OP. We need to know what kind of hull you have but if it's like a old GB yes you can put a bigger engine and go 8 knots or even 9 or 10 if you are willing to pay the fuel burn. If you have an Island Gypsy it may go faster than a GB. What have you got?

Don't want to hear any complaints about this 5-plus-knot wake!:



Further up the river/slough it is narrower; we got "dirty" looks from a couple of on-shore fishermen. They didn't seem concerned with our itty-bitty wake, but they had their lines out in the middle of the channel, and we were apparently disturbing them.

psneeld said:

...
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.

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If one is wealthy enough, you can afford both a boat lift and a personal "no wake" sign (as well as a very large house with comparable swimming pool and acreage), leastwise along the Petaluma River:

In California: "The maximum speed for motorboats within 100 feet of a bather (but not a water skier) and within 200 feet of a bathing beach, swimming float, diving plaform, or life line, passenger landing being used, or landing where boats are tied up is five miles and hour." (emphasis added) Who said anything about a no-wake zone?

markpierce said:

In California: "The maximum speed for motorboats within 100 feet of a bather (but not a water skier) and within 200 feet of a bathing beach, swimming float, diving plaform, or life line, passenger landing being used, or landing where boats are tied up is five miles and hour." (emphasis added) Who said anything about a no-wake zone?

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Which makes sense...but only to my point...5 mph is which?

5 mph through the water or over ground? If 5 through the water...then if a 4-5 knot current...commerce all but stops or backs up...5 mph over ground is 10 mph through the water and results in a huge wake...

Wake laws are written poorly, adhered to poorly and supported without the big picture in mind much of the time.

Is appears some of you guys are on the wrong post. This was a question/info post on hull speed/engine HP, not wakes and law enforcement. Thanks.

I'm sad to say my boat has a full displ. hull. So I guess I'm stuck with the 8 kt. hull speed.

Psneedl. Some people like to run off half cocked before knowing the whole story.

Everyone needs an education now and then.

In this regard you are spot on.

SD