# Which aluminum boat for speed - modified-v or flat bottom?



## JEG60 (Apr 17, 2008)

Question for you jon boat experts. If you have (2)  16 foot aluminum jon boats that are the same length, same width and same weight, and with the same trolling motors, will a modified-v move faster than a flat bottom?


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## Randall (Apr 17, 2008)

*Speed*

Mod V will be a little faster than the flat bottom.


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## dawg2 (Apr 17, 2008)

V hull


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## bow_hunter125 (Apr 17, 2008)

The Mod-V


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## JEG60 (Apr 17, 2008)

If you put a 24 volt 80 lbs thrust trolling motor on the front and another on the rear, would it be a huge difference in speed? I like the stability of the flat bottom over the modified-v.  Thanks


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## TROY13 (Apr 17, 2008)

v hull will travel faster but the flat bottom will not rock as much when ya move around in it.


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## Twenty five ought six (Apr 18, 2008)

> If you put a 24 volt 80 lbs thrust trolling motor on the front and another on the rear, would it be a huge difference in speed? I like the stability of the flat bottom over the modified-v. Thanks



No.

Differences in hull design only become significant at planing speeds.

At displacement speeds (which is what you get with trolling motors), the limiting factor is the length of the hull.  All 16 foot boats have the same top speed in displacement mode. It's in the neighborhood of 5.5 MPH.  Once you reach this speed, additional power is wasted until you produce a enough HP to plane ( which is around 6-8 HP).


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## MudDucker (Apr 18, 2008)

Hmmm, in my experience, the less v at the planing pad (rear of the boat), the faster, except for some of these fancy step lift hulls.  The more v, the more control at high speeds.


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## JarheadDad (Apr 18, 2008)

Twenty five ought six said:


> No.
> 
> Differences in hull design only become significant at planing speeds.
> 
> At displacement speeds (which is what you get with trolling motors), the limiting factor is the length of the hull.  All 16 foot boats have the same top speed in displacement mode. It's in the neighborhood of 3.5 MPH.  Once you reach this speed, additional power is wasted until you produce a enough HP to plane ( which is around 6-8 HP).



I totally disagree. Go figure huh?  

The limiting factors in displacement are weight, width, lessening drag, and ability to cut water. A wide flat bottom will create drag which is counter-productive to creating speed. All things equal a V will always exceed a flat in speed simply because it cuts the water and displaces it to the side. The perfect displacement hull is pointed fore and aft. Or one that has a point of entry ability to cut water a la a V with a high stern. That is why it is important to have 60%+ of the boat's weight forward to lift the stern to lessen the aft drag. Also why a jonboat will be faster with both anglers standing forward while making a run. A flat bottom pushes water instead of cutting it and creates a stronger drag on the stern slowing the boat down. 

You can look at some of the big 600' freighters to see what I'm talking about. The bow bulb was designed to break the water and displace it to the side. Sterns are rounded and sit high with the wheels low in the bottom of the hull. Even with hundreds of horses they are still displacement hulls. The trick is to reduce drag to optimize fuel/battery consumption. Less drag - less consumption.

Now on a smaller jonboat like a narrow 12' drag is negligible due to length. Past that weight and width become the key factors.

As per usual, this is just my opinion. And we all know the old saying about opinions!


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## Bart Sims (Apr 18, 2008)

You are right on JHD. Look a Sailboats.


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## basser (Apr 18, 2008)

At trolling motor speeds it isn't going to make that much difference either will work.  However at planing speed the flat bottom jon boat will be faster.  The ride will not be very enjoyable with any chop on the water.  That is why most boats have a semi v.  This alows the boat to cut through the chop rather than walking over it.  But in a flat water situation a jon boat is always faster.  This is purely due to less drag from the boat's hull.


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## Twenty five ought six (Apr 18, 2008)

> I totally disagree. Go figure huh?



You can disagree all you want, but the fact that a displacement hull is limited by the length of the hull is a well established nautical engineering fact, which can be verified with just seconds of "googling".  Boat designers are constantly trying to overcome this limitation, but no one has been able to  do so.  Sailboats are subject to the same rule -- a longer sailboat will always be able to go faster than a shorter sailboat.  That's why in races, sailboats are the same length (America's Cup), or handicapped to make up for the differences.  Again there are exceptions because some sailboats (or sail craft such as boards) are able to plane, and the hull length limitation can be overcome.

The bulbs on tankers allow them to reach and maintain the maximum hull speed with less HP, but it does not increase the maximum speed.

Likewise a 16 foot boat, lightly loaded will reach maximum speed with less effort than a 16 foot boat with loaded with three fisherman, 3 batteries and a cooler of drinks, but the maximum speed will be the same.  The only difference in top speed would be if one of the boats did not have enough thrust to obtain hull speed with the load and hull conditions.

The formula for determining this top speed follows and all displacement hulls are subject to it:



> 1.34 X the Square Root of the Boat's Waterline Length
> (This is Theoretical Maximum Hull Speed)



So for a 16 foot boat, the maximum theoretical speed will be 5.36 MPH*.  If you read the posts, there are very few people here who obtain this theoretical maximum, and this is where load, hull condition, available power, and hull configuration come into play.

For what it's worth, neither the flat bottom or v-hull are the most efficient type hull for displacement--- the efficiency of a hull is determined by the way that water flows over the surface of the hull.  Therefore the most efficient is a rounded shape similar to a canoe.  Referring again to racing sailboats for an example of maximum efficiency in a displacement mode, the fastest ones have a "U" shaped hull.  This may not be readily apparent because of the deep keel, but the basic hull shape is a "U".

That's the reason that all of the old time rowing boats (back to the Vikings,  Jesus and whoever) have rounded profiles, "U" shaped profiles -- native wit proved that it was the most efficient.



> The trick is to reduce drag to optimize fuel/battery consumption. Less drag - less consumption.



That part is correct, but again you are confusing efficiency with top speed, which is what the original question was about.

One of the results of all this is counterintuitive.  Assuming all other things being equal, if you have enough HP to obtain hull speed, a longer boat goes faster than a shorter boat.  Most small boats obtain hull speed with about 3 HP which is approximately 150 lbs. thrust.  Most of the boaters here obtain that thrust with multiple motors, which increases the drag significantly, requiring more thrust -- so you can see where this is going.


____________________________________________________________
*fn.  The formula actually produces a results in knots but at these low speed the difference is not significant.


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## JarheadDad (Apr 18, 2008)

You left out width and weight. Both are detrimental to speed. Period.

I know a bit about freighters. I ran them for years and you are absloutely incorrect in saying a bow bulb will not increase topend speed combined with a rounded and lifted stern. We picked up 3kts topend with an efficiency rate increase of 46%. Less consumption and more speed on the top. On a vessel of identical length, width, and weight as the one the new one replaced. That is not a mathmatical formula. It is experience.

Since I've got a lot of time on blow boats as well you didn't mention the keel wings that became popular in racing keels a decade ago. Anything that will break the drag at the stern will increase speed. All racing hulls have rounded and lifted sterns. The basic racing hull is shaped in a bulb shape much like a clove of garlic with it sprayed out to roll onto. Much much wider than the old Nordic designs with more keel in the water. Nordic designed hulls were not made for speed but for their ability to take a sea and stay afloat. You could literally roll them and they'd end up rightside up everytime. In the powerboat world you could use a trawler hull as an example as well.

Again we disagree on the ultimate displacement hull. Mine is the pointed at both ends with a V keel type. The drag is not in the front of the boat that slows it down. It is the pull when the water drags on the stern where speed, and efficency, is lost. Water pushed far enough aside will not circle around behind and drag the stern in a downward fashion.

We can sit here and argue the point all day and get nowhere. The only way to prove it is to rig two boats in a similar manner. One flat - one V and run them side by side. I'll bet a bag of Senkos that not only will I be a full 1/2 kt faster but I'll run your boat out of battery a full hour before mine even weakens. The drag on a flat may allow it to reach the magical 5mph but I can guarantee you she won't hold there. I've had both and now I run a V. There's a reason for that!

Oh well, I'm off to Google to see if what I know to be true from experience jives with the mathmatical world of nautical engineering. But I do remember setting my Chief Engineer off on the dock in Key West one time because I got sick of being told my boat wasn't doing what it actually was. Heh! Engineers will argue with a wall!

A classic example of the value of the perfection of nautical engineering? The Titanic!


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## boohoo222 (Apr 18, 2008)




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## TROY13 (Apr 18, 2008)

I will admit surface area creates drag. A v hull has more surface area thus more drag, however, the v hull shape is a more effective at cut through the water with less drag even with more surface area in the water. So I hope the confusion continues, because the more I read about this the more confused I get. just a thought.


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## duckcrazy (Apr 18, 2008)

JEG60 said:


> If you put a 24 volt 80 lbs thrust trolling motor on the front and another on the rear, would it be a huge difference in speed? I like the stability of the flat bottom over the modified-v.  Thanks


 If you are planning on running an electric only set up then there won't be much difference in speed. There will be a difference in the way the boats track on windy days. The V hull will track better than the flat bottom. Like others have said, the flat bottom will be a little more stable. It all comes down to how and where you plan to fish.


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## JarheadDad (Apr 18, 2008)

TROY13 said:


> I will admit surface area creates drag. A v hull has more surface area thus more drag, however, the v hull shape is a more effective at cut through the water with less drag even with more surface area in the water. So I hope the confusion continues, because the more I read about this the more confused I get. just a thought.



It really doesn't matter in the long run so just pick one. Each has it's trade-offs. The V will give you better performance overall but will roll a good bit when you move side to side. The flat will give you a better fishing platform but less overall performance ( IN MY HUMBLE OPINION!  ). If you go with a set of good gel batteries you'll have enough power to even pull Varner all day in gale force winds with a flat. We're just arguing minutia! 1/2 kt and 1 hour of battery. 

Make sure to put your front trolling motor batteries front of centerline (mine are under the front deck with an onboard charger) to distribute your weight enough to allow for the weight of four batteries and two motors on the rear. Or a gas tank and 9.9.

See ya' on the water and good luck!


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## JEG60 (Apr 21, 2008)

Thanks for all the responses!


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## dcrail (Jan 28, 2009)

the perfect electric boat would be two canoes with a deck between them...pointed bow and stern to allow the water to flow from one end to the other with little drag...any square transom boat will create drag at displacement speeds!!!!!


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