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Old 02-02-2016, 19:00   #1
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Question about speed...

Hey Guys and Gals,
I've been thinking about putting a couple outboard 200-300 hp engines on the back of a Beneteau 33-47 footer. The reason for the engines is to get to shelter in case a bad storm pops up quickly or there is no wind I could use a single engine to go. My question is,is this an absurd idea? If so,why? Do these boats have a speed limit or capability? I've watched several videos of sailboats but everyone seems to get nervous above 15 knots except for the racing boats or cats. Would it be wiser to just go with a powered boat. My dreams are to do the Greater Loop and travel as far as Puerto Rico. I know if I go with a sailboat I will be taking lessons in Florida somewhere as I have never sailed before. Thankz for any and all input.
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Old 02-02-2016, 19:24   #2
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Re: Question about speed...

Can't tell if this is a serious question but in case it is I'll try to answer. NO. The speed of a displacement boat is limited by its waterline length. Without going into the physics and design formulas suffice it to say bigger engines and more horsepower will only create a bigger bow wave and bigger wake but not much more speed. If you want to get up on a plane for speed get a boat designed to plane.


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Old 02-02-2016, 19:32   #3
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Re: Question about speed...

Troll anyone? Any person who has stepped foot on a sail boat knows how and why this is ridiculous. Now if you wanted to put a turbine engine on the back that would be killer!
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Old 02-02-2016, 19:34   #4
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Re: Question about speed...

My main concern is if the weather should get bad when traveling the Gulf between islands. I've watch several sailboats out in gale force winds and high seas and that spooks me a bit. But info about not being able to convert one is greatly appreciated. Thank You,
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Old 02-02-2016, 19:37   #5
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Re: Question about speed...

Never been on a sailboat yet. Biggest boat I've been on was a 50 Viking out in California over 20 years ago. Offshore fishing. It was awesome. Thanks.
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Old 02-02-2016, 19:38   #6
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Re: Question about speed...

Look into hydrofoils.😄
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Old 02-02-2016, 19:39   #7
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Re: Question about speed...

You posted this in another thread you opened:

Quote:
Originally Posted by Johnyo View Post
Hey guys and gals,
I am in the about to stage of looking for a 26-47 ft boat to live aboard and hopefully travel the Greater Loop and the Caribbean on. I really like the diesel engine boats,(being an ex truck driver for the past 30 years) but at the same time I like outboard engines. I've thought about getting a shallow draft twin keel sailboat and putting one or two electric engines on the back and also a 150 hp outboard as backup plus the sails. I've never sailed or even been on the ocean for more than a few hours in a decent sized boat. I don't mind learning to sail but I want to be able to get to safety should the weather go to getting a bit nasty. I've also thought of a sailboat and putting outboards on the back maybe 200-300hp but most sailors get nervous when the boat gets upwards of 15 knots. I've also been looking at single engine boats but if something happens to the engine out to sea I'm stuck. I know a lot of the Greater Loopers are on single engine boats but like I said I'd like to travel as far as Puerto Rico. Is it safe to do so on a single engine boat say like a trawler. I know some or most say they are for long distance cruising but the single engine and speed thing bothers me. I'm not wealthy or have a bit of money as I'm on disability.
Any insight you have to share would be greatly appreciated, Thankz.
You should probably ask this on SA
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Old 02-02-2016, 19:40   #8
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Re: Question about speed...

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Originally Posted by Tayana42 View Post
Can't tell if this is a serious question but in case it is I'll try to answer. NO. The speed of a displacement boat is limited by its waterline length.
===

This smells like a troll but almost any displacement boat can be made to plane if you put enough power to it. How much is enough? It's not uncommon for 20,000 pound planing power boats to have 700 or more total horsepower. With gasoline engines you will burn about 70 gallons per hour at full throttle, for a speed of maybe 25 to 30 knots.

How much fuel do you want to carry?
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Old 02-02-2016, 19:54   #9
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Re: Question about speed...

Thanks Everyone, I will look into Hydrofoils as I've only heard of them a couple times but have no idea what they are. I have found several boats that are long distance cruiser and can run just under 20 knots but they are single engine trawlers with 200-400 gal.tanks. I guess from all the replies that I should scrap the sailboat idea,just not practical.
I will continue to watch the sailing videos on youtube and in a year or two take some sailing lessons just for the experience. Again, Thanks to Everyone for your input and direction.
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Old 02-02-2016, 19:55   #10
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Re: Question about speed...

Quote:
Originally Posted by wayne.b View Post
===

This smells like a troll but almost any displacement boat can be made to plane if you put enough power to it. How much is enough? It's not uncommon for 20,000 pound planing power boats to have 700 or more total horsepower. With gasoline engines you will burn about 70 gallons per hour at full throttle, for a speed of maybe 25 to 30 knots.

How much fuel do you want to carry?

This is course is correct. The theoretical hull speed limit doesn't really mean a boat absolutely can't go any faster. It just requires MUCH more power to get there.
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Old 02-02-2016, 21:22   #11
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Re: Question about speed...

Quote:
Originally Posted by wayne.b View Post
===

This smells like a troll but almost any displacement boat can be made to plane if you put enough power to it. How much is enough? It's not uncommon for 20,000 pound planing power boats to have 700 or more total horsepower. With gasoline engines you will burn about 70 gallons per hour at full throttle, for a speed of maybe 25 to 30 knots.

How much fuel do you want to carry?
Sorry but NO.

A 20,000 planning hull only gets away with something on the order of 700hp because the hull shape causes it to lift out of the water and essentially skim across the top (skim being relative with a 20k lb boat).

Theoretically, you could get a displacement hull up to 25-30kts but you are likely talking many thousands of HP and the boat would break up from the stress long before it actually got up to those speeds.

Some modern flat bottom sailboat hulls straddle the line between planning and displacement but really still need special conditions to get much above 10-12kts. Those conditions are not easily duplicated under power.
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Old 02-02-2016, 21:33   #12
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Re: Question about speed...

Quote:
Originally Posted by wayne.b View Post
===

This smells like a troll but almost any displacement boat can be made to plane if you put enough power to it. How much is enough? It's not uncommon for 20,000 pound planing power boats to have 700 or more total horsepower. With gasoline engines you will burn about 70 gallons per hour at full throttle, for a speed of maybe 25 to 30 knots.

How much fuel do you want to carry?
It's not a matter of power v weight. The problem is the underwater profile of a displacement hull such as a Beneteau sailboat. Doesn't matter how hard you push it, a full displacement hull will never get up onto a plane - (it will probably flip over backwards first )
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Old 02-02-2016, 21:34   #13
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Re: Question about speed...

No. There is not need for them on a Beneteau. Just install a hydronum activator to the keelson primary load point. From there the fulcrum will reach dynamic potential and the resistance loads of the water will diminish to the point that a pair of oars should see the boat planing at 11 meters per second Celsius.

Btw, this will only work on Beneteaus up to 38 foot. If you buy a bigger one you will need to apply the 8th law of thermodynamics and heat the surface of the hull. That will make the electrons in the fiberglass flow in the direction of your secondary fluxgate attenuators. Remember to reverse the polarity on the termination diode else you will go backwards.

Thats enough frivolity from me for now..
But No.. Just no to putting outboards on the back of a beneteau of that size.
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Old 02-02-2016, 21:50   #14
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Re: Question about speed...

MacGregor 26?

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Old 02-02-2016, 21:54   #15
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Re: Question about speed...

Quote:
Originally Posted by Johnyo View Post
My main concern is if the weather should get bad when traveling the Gulf between islands. I've watch several sailboats out in gale force winds and high seas and that spooks me a bit. But info about not being able to convert one is greatly appreciated. Thank You,
To push a 33' sailboat to 15kt or so would require 300hp or so assuming you could get it to plane.

Lets assume you can get to 20kt using 400 hp. At that speed you would be getting about 1/2 NM per gallon of fuel. If you are only using the high speed to get away from unexpected weather you are only going to be using it on longer legs of the trip. On shorter legs you would able to time your passages to coincide with weather windows. You will only be surprised on longer passages (lets say 400 nm or greater) when you will be several days out when the weather starts to deteriorate. The worst case is that you are halfway when you start to worry about the weather and want to outrun it. So you are 200nm from shelter and make 1/2 nm/gal so you need 400 gal of fuel for this sprint. At $3-$6/gal that's $1200-$2400. Also, at 6lb/gal, that fuel is about 2400lb. The added weight will slow you down so you won't even make the 20kt for the first 5-7hr. I used a Beneteau33 specs and the PsychoSnail boat speed calculator set for a planing hull. At best the B33 is a semiplaning hull but the calulator won't work to that high of a speed for that displacement and LWL. I assumed the all up weight would be 12,000 lb without fuel. BENETEAU 323 sailboat specifications and details on sailboatdata.com

The problem here is the B33 is designed to operate very well at displacement speeds and to resist the heeling of the sails. Try to take it out of this speed and weight range and you pay a big penalty for it. Also working against you is that if you add the big motors at the back of the boat it will be really out of trim and sail poorly as well as motoring poorly.

The original post indicates boats down to 26' So let's look at the MacGregor 26 (MACGREGOR 26M sailboat specifications and details on sailboatdata.com). This boat was designed to be motored at significant speed. In order to do this it has a lot more flotation aft, and uses water ballast which you can dump when you motor fast or pull the boat out of the water. Together that means the boat doesn't sail very well except downwind. Upwind it doesn't have enough ballast except when it is light when the extra surface area of the fuller stern drags the speed down somewhat. As a boat to live on it's not bad, kinda small but with the powerboat hull roomy for its length. It was designed for a 50hp motor. per this website (MacGregorSailors.com • View topic - 26X typical speeds under sail and motor) you can motor at 10kt with ballast tanks full and 14kt empty. At 14kt you can expect to use about 4.5gal/hr or about 3nm/gal. Using a trolling motor you should be able to get that up around 10nm/gal at 5kt. To get 200 nm range at WOT you will need about 60 gal of fuel ($180-$360) and that will weigh about 360 lb. As a boat to do the great loop it should be decent.

The down side of the MacGregor is that it is very lightly built in order to achieve the those speeds. That means that if you are caught out in heavy weather you are good and truly screwed, even if the boat doesn't roll over it is likely to sustain significant damage from pounding. I wouldn't take this boat into the Caribbean.

The solution is to learn to deal with heavy weather rather than avoiding it. Like anything it takes practice.
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