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Author Topic:   Bigshot was right!
ryanwhaler posted 10-13-2003 08:14 PM ET (US)   Profile for ryanwhaler   Send Email to ryanwhaler  
I'm always hearing bigshot say a longer boat is faster then a shorter one. He once said that a 9ft whaler will go slower then a 11ft with it they both had 8hp Motors on them. I never understood it, I always thought he was whrong.

Well, this afternoon I proved him right. I put my 1987 8hp Mercury on a 13ft Wooden boat. I shocked at how fast it was. The same motor pushed my lighter 11ft whaler slower. . . much slower then it pushed this wooden boat.

Biggie was right! :)

Dr T posted 10-13-2003 09:19 PM ET (US)     Profile for Dr T  Send Email to Dr T     
It is an interesting bit of physics that hopefully one of our mechanical or marine enginneers can clarify. I forget the formula, but the theoretical HULL SPEED of a displacement hull (like on a sailboat or big ship) is related to the square root of the waterline length. The waterline length of a longer planing hull (like the whalers) will allow the boat to be pushed faster for a given input of power (while in displacement) and should help it achieve a plane with a smaller input of power (all "other" things being equal--whatever they are) and thus the boat should achieve a higher planing speed for the same power input.

Can anyone elaborate in some technical detail?


Clueless in Colorado (Dr. T)

ShrimpBurrito posted 10-14-2003 01:14 AM ET (US)     Profile for ShrimpBurrito  Send Email to ShrimpBurrito     
Well I personally cannot explain it, but I'm good at yahoo. :) Isn't this the same principle as the bulbous bow?

scaleplane posted 10-14-2003 08:38 AM ET (US)     Profile for scaleplane  Send Email to scaleplane     
I wonder if the longer moment arm on the CG (Center of Gravity) allows a quicker plane (more bow weight)? I know these are aeronautic terms, but I think it might be related?

My 15' gets up on a plane RIGHT NOW on only 40HP!

frostbite posted 10-14-2003 09:20 AM ET (US)     Profile for frostbite  Send Email to frostbite     
A major speed limiting factor affecting planing hull vessels is the wetted surface area, more contact with the water, more drag. I would bet the wooden boat is much narrower at the waterline at planing speeds.


jimp posted 10-14-2003 11:12 AM ET (US)     Profile for jimp  Send Email to jimp     
Per Dr T, the rough rule of thumb for displacement hull speed: 1.34 times the square root of the waterline length. I imagine that planing hulls will go as fast as you can push them.


Bigshot posted 10-14-2003 11:34 AM ET (US)     Profile for Bigshot  Send Email to Bigshot     
I saw it on the Discovery channel on a show about cruise ships. Back in the days it was not only "cool" to make the biggest boat but it was "cooler" to be faster. he reson for the Titanic was not only the biggest boat ever built but hopefuly the fastest. The reason for it's early demise was they were trying to set a new record from Europe to NY and were doing roughly 25knots through an iceberg field at night.....not too bright eh? The Titanic had to be long to be fast so they said. This pertains ONLY to dispacement or semi-displacement hulls.

My 14 Carolins Shitt with a 30 Yamaha was slower than my friends 16' with a 25 yamaha(both 2cyls). My 13' Whaler was a tad slower or about the same as my friends 15' Whaler both running brand new 1982 35hp OMC's and 10x13" props.

elaelap posted 10-14-2003 11:49 AM ET (US)     Profile for elaelap  Send Email to elaelap     

I'm not sure that the 1.34 factor is I remember, a very fine, racing hull approaches a factor of one point five. And we've got to remember that the formula, ultimate displacement hull speed, no matter how powerful the motor, = 1.34 (or whatever) times the square root of the waterline length, is in KNOTS.

Not having mastered physics, this formula, which I learned sometime in my early teens, seemed and still seems totally counterintuitive. If I were to pretend to understand it, it has something to do with pushing the bow wave aside, and a geometric increase of forces, whatever that stuff means. Help, some of you math/physics/engineering types.


BillB posted 10-14-2003 12:31 PM ET (US)     Profile for BillB  Send Email to BillB     
Here's a link to a post on another forum that gives a fairly clear explanation of the physics involved in calculating theoretical hull speed. I especially like his use of a duck's lwl (length of waterline) as an example of deviance from the "rule of thumb".

Marlin posted 10-14-2003 12:34 PM ET (US)     Profile for Marlin  Send Email to Marlin     
As a few people have pointed out, "hull speed" can be calculated as 1.34 * sqrt (lwl) where lwl is waterline length in feet, and the resulting speed is in knots. Not-so-coincidentally, this is also the formula for the speed of a wave of a particular wavelength.

The logic behind this is that a displacement vessel creates a bow wave, actually a train of waves. The rest of the hull must be supported at some point along its length by one or more additional crests of that wave train. That explanation isn't all that clear, so let me try a few examples.

Imagine your Whaler at idle speed, where it is clearly acting as a displacement hull (meaning that the boat is staying on top of the water due to static bouyancy, by weighing less than it displaces). You'd be making a small bow wave, and if you looked at the waterline along the length of the boat, you'd see repeated crests and troughs that start at the bow and work their way down the boat. These crests and troughs stay in the same position along the boat, because the waves are travelling at exactly the same speed as the boat.

Now speed up to, say, 1500 RPM. Your Whaler is still a displacement hull, but you are creating a larger, longer-wavelength bow wave. This wave has one crest just behind the bow, and the additional crests are further apart than before and there are fewer of them visible along the hull. You're not yet up to hull speed.

Continue to slowly increase speed. At some point, your hull will have the first crest of the bow wave just behind the bow, and the second crest right at the transom. The boat still has an essentially level trim. You are now running at hull speed.

As you increase power further, you'll start to transition from a displacement hull to a planing hull. The boat trim will take on a bow-up angle as the stern sinks into the first trough made by the lengthening bow wave train. As you speed up even more, your Whaler will literally climb up its own bow wave until it has planed off.

Once the boat planes, the bow-up attitude usually decreases somewhat. Your boat is now staying afloat not because of static bouyancy, but because of dynamic lift generated by the motion of the hull through the water. This is the same force that makes a stone skip.

Okay, now the caveats, and there are a couple:

1) Displacement hulls actually can exceed hull speed, but it requires an amount of power not typically available to these boats. Traditional displacement hulls, such as those on cruising sailboats and trawlers, also become unstable at speeds significantly above hull speed. It's entirely possible to sink a cruising sailboat stern-first by towing it too fast.

2) At high length-to-beam ratios, usually in excess of about 8:1, the regular displacement rules don't apply very well. Thus, a 14-foot Hobie can do 25 knots or more, instead of the 4.8 knots predicted by her hull speed. Those narrow hulls aren't planing, but they're not really displacement hull either.

Hope this description helps!


Plotman posted 10-14-2003 12:55 PM ET (US)     Profile for Plotman  Send Email to Plotman     
The faster you go at displacement speeds, the bigger the bow wave you push. And the bigger the bow wave, the longer the distance between that first wave and the second wave alongside the hull.

Below hull speed, the hull is supported at the front of the boat by the bow wave, and by one or more secondary waves down the length of the boat. Hull Speed is the highest speed where you have a secondary wave right at the stern - thus the boat is more or less level fore and aft. Above this speed, the distace between the waves lengthens out so much that the secondary wave is now behind the boat and the stern is no longer supported by it, but the bow wave is still there -- the boat is now climbing a much bigger "hill" and requires significantly more power to gain any increase in speed.

Once you are planing, there are a whole host of added variables.


elaelap posted 10-14-2003 01:01 PM ET (US)     Profile for elaelap  Send Email to elaelap     
Hmmm, Bob:

You actually had to study and learn stuff in school, didn't you? I was an English major and then went to law school, so I didn't have that problem.

Thanks for the most lucid explanation I've ever seen...but I still don't pretend to understand it.


Moe posted 10-14-2003 01:23 PM ET (US)     Profile for Moe  Send Email to Moe     
I'm not an engineer, but I sometimes play one on the Internet ;-) Here's my gearhead understanding of this:

When moving through the water in displacement mode, the bow is creating waves that travel back along the boat. The faster the boat moves, the further apart the wave peaks become. At slow speeds, the waves are close, and the boat is supported by the peaks of several peaks, rippling back along the hull. As the maximum hull design speed is reached, the boat is supported by the peak at the bow and a following peak at the stern. The longer the waterline length, the further apart these two peaks can be and still support both ends of the boat, thus the faster the boat will be going at this point. The formula for this hull design speed is the square root of the waterline length times some constant, depending on displacement hull design. This is oft quoted as 1.35-1.40 for a displacement hull. I don't know for sure, but I suspect it's lower for a planing hull in displacement mode, due to the flatter, squarer stern, compared to displacement hulls.

Beyond this speed, the wave spacing gets longer than the waterline of the boat, and the stern of the boat moves down into the trough between the bow wave and the wave following behind it, making the bow higher relative to the stern. The boat plows through the water nose-high on the back side of its bow wave, obstructing the pilot's vision, heavily loading the motor (killing fuel efficiency), and creating the boat's largest possible wake, which creates problems for others. This is a speed region that displacment hulls don't usually get into due to lack of power, unless they're surfing down the face of a wave.

With orders of magnitude more power (and fuel), semi-displacement and planing hulls can accelerate through this region until they literally outrun their bow wave, moving its peak, as the center of lift, back toward the boat's center of mass, and the stern lifts, as the bow "falls" relative to it, putting the hull closer to parallel with the water surface, with a certain portion of the aft of the boat riding mostly on top of the water. This is planing, and the speed at which it happens depends on hull design, weight and weight distribution in the hull, and other factors that can lift (or bury) the stern, such as trim tabs, and on outboards and I/Os, prop design, prop trim angle, and lower end "fins." At this point, drag decreases suddenly and dramatically, and the boat (and thus motor) speed will increase considerably unless thrust is reduced.

I don't know for sure, but I suspect length has little to do with it, since, with all other things being equal, adding more length and weight forward of the prior center of mass moves the center of mass forward, increasing the lever arm of the weight of the stern as well as that of the bow. But I could be wrong about that.

As mentioned, the wetted area, and the drag created by it, versus the boat power (thrust), is the biggest speed limiting factor when planing, and the shorter (and narrower) that length (and width) of wetted area, the better. That's why planing hulls go faster when the lower unit is trimmed out, even past the point where the prop shaft is parallel to the surface, where part of the motors force is directed to lift more of the boat from the water and less is applied in the forward vector. The reduction in drag is greater than the forward thrust lost by redirection to lift the bow, to a point. The second most influential drag on an outboard and I/O is that created by the lower unit and prop. That's why, time and again, you see the recommendation to raise as much of the lower unit out of the water as practical.

The speed at which the boat falls off plane, or stalls like an aircraft wing, is determined by the same factors that determine when it rises onto plane. How close to that speed the boat can actually be operated without falling off plane, depends on external factors such as wind and wave action, as well as the inertia of the boat itself, and of its motor.

Waves moving along the hull at some point momentarily combine with the bow wave ahead of it, to move the wave peak, and thus center of lift, more forward of the center of mass. Unless the boat and motor have the inertia to push through this quickly without slowing below the stall speed, the stern can drop far enough for the hull to "stall."

It isn't a motor's maximum horsepower that influences this. All other things being equal, at just above stall speed, a small motor and a large motor are both putting the same power to the water, much less than even the smaller motors maximum power, to maintain the same speed. However, a motor of larger displacement will typically have more internal inertia, and be slowed less by a sudden increase in drag, such as from a wave or gust of wind on the bow, and can be operated closer to the stall speed. I understand that when you change one factor, all other things are seldom equal, and the typically heavier weight of a larger displacment motor moves the center of mass aft, tending to raise the stall speed. However, that weight also contributes to the mass, and thus inertia of the total boat weight moving forward.

At any rate, real engineers and nautical designers can feel free to correct any flaws in this logic. I'll just learn more by them doing so.


Moe posted 10-14-2003 01:27 PM ET (US)     Profile for Moe  Send Email to Moe     
Ah! I see the less verbose Marlin and Plotman posted the same info while I was using up more of my lunch hour (plus some) getting wordy than they did!


Marlin posted 10-14-2003 03:22 PM ET (US)     Profile for Marlin  Send Email to Marlin     
Oooh, look, someone called me "less verbose"! I'll have to print that out and save it. ;-)
newt posted 10-14-2003 04:54 PM ET (US)     Profile for newt  Send Email to newt     
There are some great explanations of maximum displacement speeds given above, but doesn't Ryan's 11 footer plane with an 8 hp motor?
BillB posted 10-14-2003 06:13 PM ET (US)     Profile for BillB  Send Email to BillB     
It isn't clear from Ryan's post whether either hull planes with the 8 hp., and we have no idea of the hull form of his wooden craft.


Dr T posted 10-14-2003 07:08 PM ET (US)     Profile for Dr T  Send Email to Dr T     

Thanks for the lucid explaination. I think I understand it. It also makes me wonder how fast a racing shell is and whether are not it is a displacement hull or more like the Hobie. (Of course, we would need to know how fast the Whaler tender for the coach has to be to keep up).


captbone posted 10-14-2003 08:08 PM ET (US)     Profile for captbone  Send Email to captbone     
ACTUALLY Bigshot, that idea that the Titanic was trying to set a record across the Atlantic is a myth. The white star line knew that they did not have a chance to beat the Cunard line (Lusitania, Moratania, Aquatania). They were just too fast! The Olympic was almost the same ship, Titanic was built with a special aftdeck to make it just alittle bigger. The Olympic could not come close to the speed of Cunard and neither could the Titanic. The White Star line was simply trying to top the Olympic and set a record for themselves with the best and the fastest in their fleet.
Moe posted 10-15-2003 10:43 AM ET (US)     Profile for Moe  Send Email to Moe     
> It isn't clear from Ryan's post whether either hull planes with the 8 hp., and we have no idea of the hull form of his wooden craft.

Yep, planing with that motor is questionable. Heck, we don't even have any empirical proof that the 13' IS faster. Maybe it just "feels" that way.


Dr T posted 10-15-2003 06:25 PM ET (US)     Profile for Dr T  Send Email to Dr T     
There is nothing like a cold, hard fact for quash a beautiful theory.
ryanwhaler posted 10-15-2003 09:07 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
My 11footer did plane with the 8hp.

The wooden boat also planed, it planed quicker then the Whaler did, top end was faster too.

The woody has a pretty good size V, much bigger then the 11whaler. But the back of the boat has almost a flat bottom, even flater then the whaler.

To be honest, I really have no idea what most of you guys are even talking about. :)

Who would like to see pictures of the hull on the woody? Let me know and I'll email you a picture.

Moe posted 10-15-2003 09:51 PM ET (US)     Profile for Moe  Send Email to Moe     
I'd just rather you post the average of the GPS readings for two bi-directional runs of both boats.
ryanwhaler posted 10-15-2003 10:46 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
Sorry, I don't own a GPS.
ryanwhaler posted 10-15-2003 10:50 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
keep in mind that I have not ran the 11'whaler with the 8hp since last summer. I have since repowered the 11 with a 15hp Mercury.
Tom2697 posted 10-16-2003 10:35 AM ET (US)     Profile for Tom2697  Send Email to Tom2697     
Without trying to get too technical (and I'm not an expert on hull design => check out if you want REAL knowledge), the shape of the hull (length/beam ratio; hull form => tunnel, tri-hull, v-hull; tail design; etc) determines the drag coefficient and losses due to forms (ie - creating the wake). The drag coefficients between the two boats are probably pretty similar (not enough to make a noticable difference). But the loss due to forms is probably drastically different. A longer and narrower boat will produce a cleaner wake than a shorter and wider boat. This is probably the case between Ryan's Whaler and the "woody" he is referring to. Fewer losses means more power available to move the boat.
Moe posted 10-16-2003 10:37 AM ET (US)     Profile for Moe  Send Email to Moe     
Just wondering how you "proved" the 13 had better holeshot and top end. Certainly, you did these runs on the same day at relatively the same time, so that ambient temperature, humidity, and barometric pressure were the same, and the motor was producing the same output, right? And the water temperature and density was the same? And the wind and current, speed and direction were the same during both tests, or you compensated by taking the average of two runs in opposite directions, of course? Just curious how you measured holeshot and speed.
Tom2697 posted 10-16-2003 10:48 AM ET (US)     Profile for Tom2697  Send Email to Tom2697     
(Just testing the hyperlink instructions)


Tom2697 posted 10-16-2003 10:51 AM ET (US)     Profile for Tom2697  Send Email to Tom2697     
(Sorry, Forward slash)

ryanwhaler posted 10-16-2003 03:00 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
Here is a picture of my wooden boat:

ryanwhaler posted 10-16-2003 03:02 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
And another:
ryanwhaler posted 10-16-2003 03:03 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
Pictures of the 11'whaler can be seen at:

Bigshot posted 10-16-2003 03:22 PM ET (US)     Profile for Bigshot  Send Email to Bigshot     
Amn if you lived by me I would buy that 13' ride and I have twin 4.5 Johnsons to boot.
poker13 posted 10-16-2003 03:30 PM ET (US)     Profile for poker13    
What about weight? Would that have anything to do with it? Is the wooden boat lighter?
ryanwhaler posted 10-16-2003 06:45 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
I have no idea what the weaght of the wooden boat is, It seems a bit heaver they the whaler, but I have no way of proveing it. . . .Moe.

I've had the wooden boat for sale in the paper since tuesday, I have not got one single call. Next week I'll put in picture in with the ad, maybe it will look better then it sounds.

ryanwhaler posted 10-16-2003 06:46 PM ET (US)     Profile for ryanwhaler  Send Email to ryanwhaler     
It would be a good boat for small twins, its got a nice wide transom.
whalerpoor posted 10-21-2003 04:15 PM ET (US)     Profile for whalerpoor  Send Email to whalerpoor     
I'm sorry, I just can't resist.

I have drawer full of college degrees including engineering, computer science, finance, accounting and the better part of an MBA and a MAC - I've read every line of the above post and couldn't help but get a chuckle. The very best line in the whole post was from Ryan. It said "To be honest, I really have no idea what most of you guys are even talking about. :)

Dr T posted 10-21-2003 08:45 PM ET (US)     Profile for Dr T  Send Email to Dr T     
Sometimes, that sentiment is mutual.


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