RE: {Beneteau Owners} Motoring Speeds, vs Engine Sizes

[Kidd, James]

I have recently (sadly) sold my B 361 and will be taking a year or so off from sailing.

As I am already thinking of my next Beneteau I would like to get a little group input regarding motoring speeds, engine sizes and hull sizes.

 

Like many of us, I tend to travel long distances in a day (75 + miles) and many times find myself “not sailing”…and the 6.5 – 7.0 kts is rather tough to take.

 

Are any boats out there under 50’ approaching a consistent 9 + MPH?

 

If so, what is the Motor HP, Hull Length, and Prop type (if applicable).

 

Thanks.

James Kidd

Former B 361

[Guy]

What is your real plan...  sailing day light only ICW, coastal?;  If  24 hour days then shoot for 200 mile days.

 

If ya gotta make 75 mile days, get a fast trawler.  a 50' sailboat might get 9 kts reaching in 15+kts, but it is hard work to average that.

 

having said that, there are some mid 40's that can go fast.  A first 44.7 and 47.7 are possible.  the 461 is a good choice isf rigged for go fast... 150genoa (deck sweeper) , full battened loose footed big roach main.   the J boats (120, etc.) are easily driven as are the C+C's. 

 

Guy

[Ca...@aol.com]

Interesting question. We generally manage 7.5 kts at 2700 to 2900 RPM which works out to be about 8.5 MPH. Speeds depends a great deal on wind direction ... If the breeze is behind us we make over 8 kts consistently. Even a small breeze of 10 kts makes a difference. Yesterday we had 10 kts on the nose and had to make 2900 RPM to manage 7.4 kts, but when going with the flow we could back off to 2600 RPM.

We are using the Autoprop, and I think this helps out a bit. I'll know the next fill up what our exact fuel usage has been since entering the ICW, but historically for the last few years we have averaged 1 gph. Looks like we do about as well on mileage as some of those big motor homes. It would be even better if we didn't have this big keel to haul around.

The second day of this trip was a long one. We started from Soloman's, MD at 0700 and ended up at Salt Ponds at 1600. That's just under 100 nautical miles. We motorsailed with favorable winds for the first 1/2 day but from there on we motored. Those long spells at 9.5 kts when under sail really helped.

Cap on Zydeco
B 473 Hull #20

In a message dated 11/5/2007 10:54:12 AM Eastern Standard Time, jk...@kaydon.com writes:

I have recently (sadly) sold my B 361 and will be taking a year or so off from sailing.

As I am already thinking of my next Beneteau I would like to get a little group input regarding motoring speeds, engine sizes and hull sizes.

 

Like many of us, I tend to travel long distances in a day (75 + miles) and many times find myself “not sailing”…and the 6.5 – 7.0 kts is rather tough to take.

 

Are any boats out there under 50’ approaching a consistent 9 + MPH?

 

If so, what is the Motor HP, Hull Length, and Prop type (if applicable).

 

Thanks.

James Kidd

Former B 361

Cap & Linda Munday
Zydeco, Beneteau 473 #20
Linda - 443-994-0457
Cap - 909-969-3379
Maryland


**************************************
See what's new at http://www.aol.com

[Kidd, James]

Guy,

Thanks for the reply, but I am talking about sails down, motoring.

In my 10 years of sailing, at least 1/3 of the time when making my 50-75 mile daily treks, its either “no wind” or a condition that does not lend itself to sails being up.

I am trying not to consider a “non sail” boat…and really want to hear about motoring speeds, especially considering the new high HP boats (75 and 100 HP for a mid 40 footer).

 

James

 

---

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Guy
Sent: Monday, November 05, 2007 11:06 AM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

What is your real plan...  sailing day light only ICW, coastal?;  If  24 hour days then shoot for 200 mile days.

 

If ya gotta make 75 mile days, get a fast trawler.  a 50' sailboat might get 9 kts reaching in 15+kts, but it is hard work to average that.

 

having said that, there are some mid 40's that can go fast.  A first 44.7 and 47.7 are possible.  the 461 is a good choice isf rigged for go fast... 150genoa (deck sweeper) , full battened loose footed big roach main.   the J boats (120, etc.) are easily driven as are the C+C's. 

 

Guy

 

----- Original Message -----

From: Kidd, James

To: Benetea...@googlegroups.com

Sent: Monday, November 05, 2007 10:51 AM

Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

I have recently (sadly) sold my B 361 and will be taking a year or so off from sailing.

As I am already thinking of my next Beneteau I would like to get a little group input regarding motoring speeds, engine sizes and hull sizes.

 

Like many of us, I tend to travel long distances in a day (75 + miles) and many times find myself “not sailing”…and the 6.5 – 7.0 kts is rather tough to take.

 

Are any boats out there under 50’ approaching a consistent 9 + MPH?

 

If so, what is the Motor HP, Hull Length, and Prop type (if applicable).

 

Thanks.

James Kidd

Former B 361

<BR

[Bill Jarvis]

James,

 

Try to really define what you want to achieve. First decide on a single consistent measurement system, are you dealing with statute miles as in the AICW or with nautical miles and knots?

 

When sailing we have done 200 nautical in 24 hours, offshore on a broad reach with winds in the mid 30Kn range most of the time. In the ditch we motor at 7.25 kts (8.34 mph) with no problem. With moderate help from the wind we often exceed 8 to 9kts. When we cruise in company of some trawlers we are usually right with them.  This is with a 42 sailboat with a 50HP motor and a two blade folding prop. We average 10 mpg.

 

We can motor significantly faster but fuel consumption goes way up. I see no sense burning fuel just to make waves and thereby warm the ocean.

 

On the other hand I have delivered a 50 power boat with twin turbo Detroits, having a total of 1350 HP, up and down the east coast. Cruising at 30 kts was the norm and if pushed 38 was there. Fuel consumption 1 ½ gallons per mile. 200 miles days were the norm due to need to stop and refuel.

 

I guess the real question is do you enjoy being on the water versus what is the rush to be tied up to a dock?

 

Bill

 

 

 

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Kidd, James
Sent: Monday, November 05, 2007 10:52 AM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

I have recently (sadly) sold my B 361 and will be taking a year or so off from sailing.

[Kidd, James]

Thanks, Bill.

I am trying no to get the “Philosophical” discussion of being a destination boater…But really want to see how the larger HP boats that Beneteau now turn out, effect motoring speeds.

I had a 27 HP with a 3 bladed Gori Prop. When I sold the boat and put the standard 3 blade fixed back on, I saw a 1.5 kt difference on the hand held GPS.

When motoring from Sag Harbor to Newport, or Newport to Nantucket, that 1.5 is HUGE.

Would a 50 HP motor get me a faster speed?

What about the speeds of a 423 with a 75 HP or larger engine?

Yeah, I would have liked to be sailing, but its just not the case.

 

---

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Bill Jarvis
Sent: Monday, November 05, 2007 11:49 AM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

[Kidd, James]

To Zydeco Cap:

What is the engine on your 473?

 

---

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Ca...@aol.com
Sent: Monday, November 05, 2007 11:07 AM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

Interesting question. We generally manage 7.5 kts at 2700 to 2900 RPM which works out to be about 8.5 MPH. Speeds depends a great deal on wind direction ... If the breeze is behind us we make over 8 kts consistently. Even a small breeze of 10 kts makes a difference. Yesterday we had 10 kts on the nose and had to make 2900 RPM to manage 7.4 kts, but when going with the flow we could back off to 2600 RPM.

We are using the Autoprop, and I think this helps out a bit. I'll know the next fill up what our exact fuel usage has been since entering the ICW, but historically for the last few years we have averaged 1 gph. Looks like we do about as well on mileage as some of those big motor homes. It would be even better if we didn't have this big keel to haul around.

The second day of this trip was a long one. We started from Soloman's, MD at 0700 and ended up at Salt Ponds at 1600. That's just under 100 nautical miles. We motorsailed with favorable winds for the first 1/2 day but from there on we motored. Those long spells at 9.5 kts when under sail really helped.

Cap on Zydeco
B 473 Hull #20

In a message dated 11/5/2007 10:54:12 AM Eastern Standard Time, jk...@kaydon.com writes:

I have recently (sadly) sold my B 361 and will be taking a year or so off from sailing.

As I am already thinking of my next Beneteau I would like to get a little group input regarding motoring speeds, engine sizes and hull sizes.

 

Like many of us, I tend to travel long distances in a day (75 + miles) and many times find myself “not sailing”…and the 6.5 – 7.0 kts is rather tough to take.

 

Are any boats out there under 50’ approaching a consistent 9 + MPH?

 

If so, what is the Motor HP, Hull Length, and Prop type (if applicable).

 

Thanks.

James Kidd

Former B 361

Cap & Linda Munday
Zydeco, Beneteau 473 #20
Linda - 443-994-0457
Cap - 909-969-3379
Maryland

[Guy]

it's not clear, faster without folding? what speed ranges?

----- Original Message -----

From: Kidd, James

To: Benetea...@googlegroups.com

[Howell Cooper]

Aren't we somewhat governed by the laws of physics?  What happened to hull speed, regardless of horsepower, being limited to 1.34 X Square root of the waterline (smooth water and clean hull).  As long as the displacement hull stays upright (ie symmetrical water line) and one is not falling off a wave, how is it that one could expect greater speed with greater horsepower?  Did I miss that part of the class?

Howell Cooper

Why Knot

Port Aransas, TX

[Bill Jarvis]

James,

 

Particularly when motoring the speed is mostly dependant on waterline length.  One can get into all sorts of philosophical discussions about the hull speed and hull shape etc but it still comes down to waterline length.

 

If you want to have a speed through the water on let’s say 9kts then you’d better have a waterline of at least 44.5 feet. That almost certainly means a 50 footer. Having got the waterline then the motor need to be sufficient to push the hull at that speed. The HP needed  is going to depend on the displacement and how efficiently the power is transmitted to the water. 75HP would be more than enough with most modern sailboat designs.

 

For further study I would refer you to Dave Gerr’s “Propeller Handbook”

[Kidd, James]

Great Input.

Thanks.

[Ca...@aol.com]

I think the calculation is 1.4 X square root of water line in feet = hull speed in knots.

Cap & Linda Munday
Zydeco, Beneteau 473 #20
Linda - 443-994-0457
Cap - 909-969-3379
Maryland

[Howell Cooper]

Nope, 1.34

Cooper

[Ca...@aol.com]

In a message dated 11/5/2007 12:25:01 PM Eastern Standard Time, jk...@kaydon.com writes:

         

To Zydeco Cap:

What is the engine on your 473?

 

We have the 75 HP 4JH3-TE. The prop is probably cluttered with some growth that will be removed before we depart New Bern. I think that makes a huge difference in performance.

I noticed an outboard powered boat with nicked prop blades. That has to be a real performance killer. A power boat magazine had an article about getting the props right, and it seems like a real art/science. Fixed props are optimized for a specific speed that is determined by all sorts of things like horsepower, hull size and shape, efficiency, etc. These prop experts tweak the shape and finish and all that stuff to bring what should be a 30 knot boat from 25 up to the target speed via trial and error. Cool stuff. Think about those sports fishers that head out at 33 knots to deliver fishermen to the Gulf Stream by noon and return 'em by night. You know those props are perfect for the job they do. A small tweak resulting in an improvement would save them a bunch of fuel.

Cap & Linda Munday
Zydeco, Beneteau 473 #20
Linda - 443-994-0457
Cap - 909-969-3379
Maryland

[Kidd, James]

So in flat calm water with no apparent current, what would your motoring speed be?

 

---

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Ca...@aol.com
Sent: Monday, November 05, 2007 9:27 PM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

In a message dated 11/5/2007 12:25:01 PM Eastern Standard Time, jk...@kaydon.com writes:

[Ski...@aol.com]

In a message dated 11/5/2007 10:49:03 P.M. Eastern Standard Time, jk...@kaydon.com writes:

So in flat calm water with no apparent current, what would your motoring speed be?

I'd be interested in knowing the motoring speeds, range and fuel consumption of some of other boats..  Be conservative so we can compare and learn.

 

I have an SO 49, 100 hp Yanmar engine with Autoprop.   In flat calm I power at 8.4 - 8.5 knots at 85% power (3000 RPM) burning approximately  2.7 gallons per hour. That gives me a range of 400 miles.   At 1800 RPM I'm doing 7.1 knots and burn 1.01 gallons per hour.   At that setting it gives me a range of over 900 miles.

Dave, Nereus V

 

---

See what's new at AOL.com and Make AOL Your Homepage.

[Bob]

Fuel consumption over 8 years from 1999 to 2007 for us is:

Max 0.752 GPH

Min 0.357 GPH

Average 0.496 GPH over total of 218.87 Gals

Current Max Speed is 7.7 Knts at 3400 RPM with new Autoprop.

Prior Max was 6.8 at 3600 RPM with Std 3 bladed prop.

 

Cost per gallon has ranged from:

Min $1.15 per gal (3 fuelings in 2001)

Max $2.71 per gal (Sept. 1, 2007)

 

I average two fill ups per season June and September adding about 10 - 12 gallons each time.

I run about 20 - 25 engine hours between fill ups.  Basing my need to re fuel on .5 GPH consumption rate and Engine Hours and not the tank gage.

Tank is 24 Gallons so I estimate Max range at over 350 miles if I cruse at 2500 - 2800 RPM. 

                 Bob 
         S/V Our Dream
'97 Beneteau Oceanis 351 #195
   Slip C-17, Castle Harbor Marina
     Chester River  Kent Island
           Chester, MD
             _/)__/)__/)_

----- Original Message -----

From: Ski...@aol.com

To: Benetea...@googlegroups.com

Sent: Tuesday, November 06, 2007 6:48 AM

Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

[Bob]

I forgot to add we are running a 27 Hp Yanmar 3GM30F with the new Autoprop starting September 2007

All of the prior data was based on the Std 3 bladed prop.

[pegasu...@aol.com]

1.34 x square root of watrer line
Joe

-----Original Message-----
From: Ca...@aol.com
To: Benetea...@googlegroups.com
Sent: Mon, 5 Nov 2007 6:13 pm
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

---

Email and AIM finally together. You've gotta check out free AOL Mail!

[Ca...@aol.com]

In a message dated 11/6/2007 9:49:59 AM Eastern Standard Time, pegasu...@aol.com writes:

1.34 x square root of watrer line
Joe


-----Original Message-----
From: Ca...@aol.com
To: Benetea...@googlegroups.com
Sent: Mon, 5 Nov 2007 6:13 pm
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

I think the calculation is 1.4 X square root of water line in feet = hull speed in knots.

Thanks for this, Joe!

Hmm ... I'm still about 9 knots ... Where's the durn slide rule!

Cap & Linda Munday
Zydeco, Beneteau 473 #20
Linda - 443-994-0457
Cap - 909-969-3379
Maryland

[Kidd, James]

As an mechanical design engineer, I am very familiar with formulae to determine certain values.

I do not think “Hull Speed” is anything other than an approximation of speeds based on a number of variables.

One of those variables is the power exerted on the boat (hull) by outside forces, be it wind, motoring or both.

 

As I have only been sailing for only 10 years, and most of you “old salts” have much more experience than I, please explain to me:

Why can’t a boat that is pushed by a high horsepower engine achieve greater speeds?

I can Motor-sail my 361 at 9 kts + with 15 kts of wind……What happened to Hull Speed of 6.5 or whatever it is?

What if I took the 27 HP engine and replaced it with a 100 HP? Are you telling me I am still limited to “Hull Speed”?

 

I would think, not, but would love to hear the experts chime in!

 

James Kidd

 

 

---

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Ca...@aol.com
Sent: Tuesday, November 06, 2007 6:00 PM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

[Bill Jarvis]

James,

 

The real issue here is that we are dealing with a sailboat. That means having a great chunk of heavy metal hanging under the hull for righting moment. Hence sailboats are displacement vessels. That is they (generally) lack sufficient power to lift the hull and keel sufficiently to go into a planning mode. It is not impossible, just look at the Macgreggor 25X, with a 50HP outboard it will get up and plane. In fact it relies on that to drain the water ballast. Perhaps that’s the sort of boat you’re looking for, neither a good powerboat nor a good sailor but it can readily exceed 9 kts perhaps 15!

 

There is no magic in the 1.35 factor times the root of the waterline length, it’s an empirical number that works well for displacement hulls of traditional design. It also has a relationship the  wavelength and celerity ( the forward speed of a wave train).  Some very long, narrow easily driven hulls have much higher factors but they are not your typical sailboat hulls.

 

Another approach you might consider is a catamaran, light, potentially fast, easily driven , narrow hulls. The problem is that once you load them down with all your cruising stuff they lose those attractive features since they have a low waterline area and therefore sink quite quickly when loaded.

 

As I said before the key is waterline length to get the speed and then enough power to drive it. Sure you can put lots of HP in the boat and force it to go faster but your fuel consumption will go through the roof. You’d simply be using the energy to make waves in the ocean and unless you jettisoned the keel you won’t get up on a true plane.

 

BTW your 361’s effective sailing length is much longer than it’s static LWL. Factors such as heel, stern squat etc change the sailing LWL quite radically.

 

Perhaps the simple classic explanation is that the boat creates a bow wave. As you go faster the wave height and wavelength increase, until at “hull speed” the stern is sliding down the next crest. To go faster the whole boat has to climb the bow wave and leave the push from the stern wave.   

It can be done, for example surfing down the face of a wave but you’ll not achieve that going upwind under your motor.

 

Bill

[Bob]

James,

Here is a link to more than you would ever want to know about Hull Speed

http://www.boatdesign.net/forums/showthread.php?t=1220

                 Bob 
         S/V Our Dream
'97 Beneteau Oceanis 351 #195
   Slip C-17, Castle Harbor Marina
     Chester River  Kent Island
           Chester, MD
             _/)__/)__/)_

----- Original Message -----

From: Kidd, James

To: Benetea...@googlegroups.com

Sent: Tuesday, November 06, 2007 8:01 PM

Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

[Kidd, James]

Bill,

That was an excellent explanation of what has been baffling me,

I will use this in discussions with other boating friends regarding this topic.

 

Thanks.

 

James

 

---

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Bill Jarvis
Sent: Tuesday, November 06, 2007 9:15 PM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

James,

 

The real issue here is that we are dealing with a sailboat. That means having a great chunk of heavy metal hanging under the hull for righting moment. Hence sailboats are displacement vessels. That is they (generally) lack sufficient power to lift the hull and keel sufficiently to go into a planning mode. It is not impossible, just look at the Macgreggor 25X, with a 50HP outboard it will get up and plane. In fact it relies on that to drain the water ballast. Perhaps that’s the sort of boat you’re looking for, neither a good powerboat nor a good sailor but it can readily exceed 9 kts perhaps 15!

 

There is no magic in the 1.35 factor times the root of the waterline length, it’s an empirical number that works well for displacement hulls of traditional design. It also has a relationship the  wavelength and celerity ( the forward speed of a wave train).  Some very long, narrow easily driven hulls have much higher factors but they are not your typical sailboat hulls.

 

Another approach you might consider is a catamaran, light, potentially fast, easily driven , narrow hulls. The problem is that once you load them down with all your cruising stuff they lose those attractive features since they have a low waterline area and therefore sink quite quickly when loaded.

 

As I said before the key is waterline length to get the speed and then enough power to drive it. Sure you can put lots of HP in the boat and force it to go faster but your fuel consumption will go through the roof. You’d simply be using the energy to make waves in the ocean and unless you jettisoned the keel you won’t get up on a true plane.

 

BTW your 361’s effective sailing length is much longer than it’s static LWL. Factors such as heel, stern squat etc change the sailing LWL quite radically.

 

Perhaps the simple classic explanation is that the boat creates a bow wave. As you go faster the wave height and wavelength increase, until at “hull speed” the stern is sliding down the next crest. To go faster the whole boat has to climb the bow wave and leave the push from the stern wave.   

It can be done, for example surfing down the face of a wave but you’ll not achieve that going upwind under your motor.

 

Bill

 

 

 

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Kidd, James
Sent: Tuesday, November 06, 2007 8:01 PM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

As an mechanical design engineer, I am very familiar with formulae to determine certain values.

I do not think “Hull Speed” is anything other than an approximation of speeds based on a number of variables.

One of those variables is the power exerted on the boat (hull) by outside forces, be it wind, motoring or both.

 

As I have only been sailing for only 10 years, and most of you “old salts” have much more experience than I, please explain to me:

Why can’t a boat that is pushed by a high horsepower engine achieve greater speeds?

I can Motor-sail my 361 at 9 kts + with 15 kts of wind……What happened to Hull Speed of 6.5 or whatever it is?

What if I took the 27 HP engine and replaced it with a 100 HP? Are you telling me I am still limited to “Hull Speed”?

 

I would think, not, but would love to hear the experts chime in!

 

James Kidd

 

 

---

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Ca...@aol.com

Sent: Tuesday, November 06, 2007 6:00 PM
To: Benetea...@googlegroups.com

Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

In a message dated 11/6/2007 9:49:59 AM Eastern Standard Time, pegasu...@aol.com writes:

1.34 x square root of watrer line
Joe


-----Original Message-----
From: Ca...@aol.com
To: Benetea...@googlegroups.com
Sent: Mon, 5 Nov 2007 6:13 pm

Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

I think the calculation is 1.4 X square root of water line in feet = hull speed in knots.

[Mike Friedman]

James and Bob:

 

David Gerr, a naval architect wrote an article several years ago in one of the sailing magazines on the topic of re powering and re propping old sailboats.( If I could remember the citation I would give it, Since I can’t  I apologize.)_ In that article he pointed out that the 1.3  factor in the formula for computing   “hull speed” for displacement sail boats, was not as most of us were taught, a constant fixed number; but rather was a function of the displacement to length ratio. In the article he provided a chart showing how this “constant” varied as the D/L ratio changed.  So it seems that depending upon the design of the sailboat, boats with the same water line length could have  quite different limiting  “hull speeds”.

 

Mike Friedman

Sea Dragon

First 30

  

 

---

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[Ivars]

Hull speed is a  max speed through the water in displacement mode.   A body moving through water in displacement mode does not upset many other forces other than those which allow the water to flow past.   Once the "magic" number of hull speed is exceeded a host of new variables come into play which require exponential amounts of power.   You will need more than 100 hp to get any meaningful speed increase.  It's a case of diminishing returns.  Want more speed get a powerboat.  It ain't gonna happen in a blow boat.

 

 

----- Original Message -----

From: Kidd, James

To: Benetea...@googlegroups.com

Sent: 11/6/2007 8:01:44 PM

Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

As an mechanical design engineer, I am very familiar with formulae to determine certain values.

I do not think Hull Speed is anything other than an approximation of speeds based on a number of variables.

One of those variables is the power exerted on the boat (hull) by outside forces, be it wind, motoring or both.

 

As I have only been sailing for only 10 years, and most of you old salts have much more experience than I, please explain to me:

Why can t a boat that is pushed by a high horsepower engine achieve greater speeds?

I can Motor-sail my 361 at 9 kts + with 15 kts of wind What happened to Hull Speed of 6.5 or whatever it is?

What if I took the 27 HP engine and replaced it with a 100 HP? Are you telling me I am still limited to Hull Speed ?

 

I would think, not, but would love to hear the experts chime in!

 

James Kidd

 

 

From: Benetea...@googlegroups.com [mailto:Benetea...@googlegroups.com] On Behalf Of Ca...@aol.com
Sent: Tuesday, November 06, 2007 6:00 PM
To: Benetea...@googlegroups.com
Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

 

In a message dated 11/6/2007 9:49:59 AM Eastern Standard Time, pegasu...@aol.com writes:

1.34 x square root of watrer line
Joe


-----Original Message-----
From: Ca...@aol.com
To: Benetea...@googlegroups.com
Sent: Mon, 5 Nov 2007 6:13 pm

Subject: {Beneteau Owners} Re: Motoring Speeds, vs Engine Sizes

I think the calculation is 1.4 X square root of water line in feet = hull speed in knots.

[Guy]

Hull speed, sometimes referred to as displacement speed, is a common rule of thumb based on the speed/length ratio of a displacement hull, used to provide the approximate speed potential (i.e. maximum speed possible) of the hull. It is the speed of a deep water wave whose wavelength is equal to the waterline length of the hull. The most commonly used hull speed constant is the wave propagation speed for the hull length, and it serves well for traditional sailing hulls. In English units, it is expressed as: knots is appx 1.34 x sq of water line length.

 

The theoritical hull speed is the speed at which the BOW wave is equal to the waterline length.

 

To exceed this speed requires that power is used to lift the vessel up the bow wave.  Adding lots of power can yield speeds far greatet than the "hull speed".  Most of the power is consumed by the vertical lift component.

 

What stops a boat which is not 'planing' from going faster than its "hull speed", regardless of whether its motive power is wind or engine and regardless of 'horsepower', is the boat's resistance to ride up and over its own bow wave.

 

There are of course additional factors such as form drag, beam, flat sections of the hull, the transom offset, etc.

 

Nonetheless, it is a vertical component which consumes most of the power in excess of that reqiured for the hull speed.

 

When surfing down a big wave, the boat has a negative vertical component and can go really fast easily.

 

On the 361, the LWL is ~31.1 (from memory?) and yields a theoritical hull speed of 7.47.

 

-------------------------------------------------
Captain Guy
s/v Island Time (Beneteau 352#277)
AICW 845.5
386-689-5088

 

 

From: Kidd, James

To: Benetea...@googlegroups.com

Sent: Tuesday, November 06, 2007 8:01 PM

[Ron Witt]

To add a little confusion to the matter. Hull speed is only when you are working with an interface (air/water). If in one media, hull speed does not come into effect and drag =(coeffecient of drag)(density of the media)(velocity squared). Thats why a sub with the same power as a surface ship can go faster.

 

Ron Witt

42cc Adventure

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[Kidd, James]

More great information.

Thank you, Captain Guy.

 

James Kidd

 

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<BR

[Ca...@aol.com]

In a message dated 11/5/2007 10:49:14 PM Eastern Standard Time, jk...@kaydon.com writes:

         

So in flat calm water with no apparent current, what would your motoring speed be?

Interesting question. I did the exercise once but can't put my hands on the results. Seems we were maxed out at 9.? knots at 3,500 RPM. If I find the tabulation somewhere I'll share it with the list.

What am I thinking! We are in the Neuse River with no current so this could be an excellent location for us to head out on a calm day (with clean prop and bottom) to do the test. I'll be sure to have Bill and Dean along to verify accurate tabulation.

Linda is performing ponderations about wintering here so we'll have enough time for some thorough testing, unless we keep our southern trajectory going. This is such a great place to just hang out, and the wonderful hospitality of the natives will really make it tough to cast off the lines.

Cap in NC