Warped Bottom Designs

[wdnboatbuilder]

There seems to be little to no information ( books ) on warped bottom boat design. anyone here know where I might find info. I guess I want to know why designs have gone to the deep V and not spent more time with the warped bottom? I have had alot of rides in both deep V and a warped, Modified, V and I will have to say that the softest ride I have had yet in a pretty rough sea was the warped bottom so why don't I see more of these bottom in the market? can some one help me out here?
thank you for your comments

[gonzo]

Carolina Sportfishermen, lobster boats and others are still using the warped bottom or rotated wedge design. It is good for semidisplacement speeds.

[KCook]

True deep-V (constant deadrise) is very rare in pleasure boats. Virtually all bowriders, cruisers, deckboats, ski boats, etc, are some flavor of warped bottom. Though they love to slap the "deep-V" label on anything with 20 deg deadrise at the transom. :rolleyes:

Kelly Cook

[wdnboatbuilder]

I was always taught to leave the water as you find it. ok what is the semi-displacement speed, less than 2.0 speed to length ratio, right? please correct me if I am wrong. The 44 that we built for LUHRS I believe it ran 48knts thats not semi-displacement speed. it was all wood, I'm not sure what LUHRS is getting out of her. But you look at the flats boats and even small open fisherman and they are all constant or of the deep V bottom. Just seems to me the leave the water as you find it therory is blown all to hell and I see a lot of lifting strakes tryimg to get the boat some lift. sorry if it seems i'm rambling but it's early mornig and just had my first sip of coffee.

[KCook]

Ski boats raise the least wake for their speed. They are definitely a warped bottom type. But most folks in the pleasure boat market refuse to even consider them. Can't stand the harsh ride.

Kelly

[LP]

I think it's a performance issue. A lot of designs from the 40's-50's (?) have a warped bottom. The thinking being that a deep-vee forward gave a smoother ride while a flat surface is most efficient at planing. Thus, the transition to a flat afterhull. This configuation actually assists in keeping the bow down. It was discovered later that the constantly changing section proflie created more drag because the waterflow around the hull is constantly being changed thoughout the whole lenght of the hull. By going to a constant section shape somewhere around midship, the accelerations are removed and more energy is available to make the boat go forward. Lift strakes encourage water flow to separate from the hull earlier to reduce the amount of wetted surface.

[tom28571]

Bruce,

Most planing boats are either deep V, monohedron or warped bottom. Warped bottom hulls are very numerous.

Monohedron means that the deadrise is constant from about station 6 aft.

Some tests have shown a slight advantage to the warped bottom at low planing speed while the advantage goes to the monohedron at high speed. I have made towing tests that have not shown any measurable difference in drag between the two but do show an instability in the warped hull at high speed. Assuming that there is no convexity in the aft buttock lines, a warped bottom should get on plane a bit easier.

The thought that a warped bottom is easier riding than a deep V in waves is contrary to all evidence that I know of.

[KCook]

Thanks Tom. Hadn't heard that "Monohedron" term before. But that would describe most I/O bowriders on the market today.

Kelly

[Willallison]

True deep-V (constant deadrise) is very rare in pleasure boats. Virtually all bowriders, cruisers, deckboats, ski boats, etc, are some flavor of warped bottom. Though they love to slap the "deep-V" label on anything with 20 deg deadrise at the transom. :rolleyes:

Kelly Cook


How true... for me a deep-vee is anything over about 24 degrees deadrise. My personal pet hate is the current favourite of the marketing folk - the "modified deep-vee"....:mad:
Oh... and then there's my other pet hate - the term "trawler", which apparently nowadays means anything that floats and has squarish windows...:mad:
Oh... and then there's.... phew! Seems I'm turning into a grumpy old man!:eek:

[wdnboatbuilder]

Thank you all for your imput. interestingly enough that you say from sta6 or i would assume that would be amidships. So what your saying is the chine and keel run paralell or flat up to admidships? I have notice in the last 15 yrs there is a flat in the chines about the last 3-4' in a 15-28'ters open fisherman/flats boats. One interesting note that the builder/designer i worked for, built and ran the Daytona's and according to him if you use lifting strakes or trim tabs then you have a piss poor design. Now don't any of you take offence to that statement he was serious old school, but I also have to say you ask anything about the designing of his bottoms and you got cussed out and damn sure never got an answer. you had to be one of those guys that understood what he was doing and he seem to always got the person with the least knowlege to help him loft. I got caught on the loft floor once with a folding rule and got my asss chewed not for being there but because I had a rule in my pocket and was there. I enjoyed every minute I worked for him and learned alot, but some questions that never got answered. Thanks for your imput
hey i hit 100 post i'm a senior member now, not that that means anything still a junior train of thought.....lol

[tom28571]

OK, maybe I should have said from 60% of LWL aft of stem waterline. Many designers, maybe most, use 11 evenly spaced transverse station cross sections to describe the shape of the boat. That is starting at station 0 at the stem waterline and going to the transom waterline for station 10.

Anyway, you got the idea right. I do kind of agree (but not completely) with your taciturn mentor about the addition of trim tabs to correct for some bad behavior of the boat.

[wdnboatbuilder]

nonetheless they are running paralell with eachother. Yea Bob was an interesting person, would answer some questions depending how close you were to the idea or concept. it only took 2.5 of the 4 years i worked for him to figure that out.
I would finish my classes at Westlawn but really lost interst when I has to start figuring out sailboat designs nothing personal against sailboaters but I want to get somewhere quick and just never had an interest.
Thank you Tom I was not trying to hold you to admidships I have an idea of the 60% same idea that thats where you want your CB. (aproximate)

[Gilbert]

Personally, I have never had any clients with enough money to let me do all the research I would need to do to satisfy the question about which style of planning hulls were the best.
I feel fortunate in having met a few designers who have had the good fortune to do that type of 'personal' research. It has usually been for the government. The most enlightening comment I've heard from them is that to go fast in all weathers you do want to have a warped bottom. "But you don't want it warped very much." That was as specific as the advice got. So I guess we are at liberty to decide for ourselves what not very much means.
I believe that no matter what style of planning bottom you have, if the waves are bad enough, you will come to the point where the boat can take more punishment than the crew can, provided it is a well built boat. And so the limitation is how much discomfort the crew is willing to suffer. I guess that's why a lot of us have philosophically aligned ourselves with what many folks consider a slow boat. And I am not trying to imply that the crews of slow boats never suffer any discomfort.

[tom28571]

Gilbert,

There may be some confusion in this thread about what is meant by a warped bottom. Nearly all planing boats have a warped bottom in that the bow deadrise is much greater than that at the transom. If you want the boat to enter waves without pounding badly, a sharp entry is absolutely necessary and if the boat is to plane efficiently, the stern section deadrise must be small. Therefore some warp is inherent in all planing boats other than flat bottomed ones and constant deadrise ones.

What I meant by a warped bottom is that the aft planing sections are also warped. Even when up on plane and in smooth water, the water flow is still experiencing some twisting action as it leaves the transom. On the other hand, the monohedron hull has no twist to the after sections so that water flow is much more uniformly horizontal leaving the transom. Within these boundaries, you get the comments by your designer friends and it gets almost to a personal preference. I'm sure you know this but I wanted to clear it up a bit.

[KCook]

Good point Tom. And I expect most ski boats would meet this warped bottom description. With the deadrise changing even across the aft planing sections.

Kelly

[Willallison]

The most enlightening comment I've heard from them is that to go fast in all weathers you do want to have a warped bottom. "But you don't want it warped very much." That was as specific as the advice got. So I guess we are at liberty to decide for ourselves what not very much means.


Mt Westlawn texts suggest that the deadrise angle shouldn't chnage by more than 6 degrees aft of station 6...

[yipster]

Hi Will,
6 degrees max makes a quart of a deep V and that sounds like warped quit a bit.
have some figgers for halfgliders also?

[wdnboatbuilder]

I was always taught that deadrise doubles from transom to amidships and double or better again at sta1. with no more than a 3 degree rise in the keel from forefoot to transom. i appreciate all of your comments.

[Willallison]

Yipster - what's a half glider?

Bruce - I notice further back you mentioned that you worked for Luhrs. A year or two back I came across a design by Michael Peters for a 78 foot Luhrs - pic below.
Do you know if the boat was ever built - anf if so, where I might find a bit more info?

[wdnboatbuilder]

No I did not work for Luhrs I worked for a company who built the 52' and we redid the bottom of the 36' and it looks like the 44' is a boat we built before the company entered into the 52' contract. So sorry I have no answer for you. These boat were designed by Bob Sherbert.

[yipster]

Will - bout that max 6 degree warping, is that deep V or also for semi-planing hulls, "halfgliders" as i called them?

[tom28571]

Mt Westlawn texts suggest that the deadrise angle shouldn't chnage by more than 6 degrees aft of station 6...

Will, Wdnboat, Yipster,

I think 6 degrees of warp in a bottom that is already low deadrise is an awful lot. I had stability problems in model towing with 5 degrees of warp on a 10 degree bottom at station 6. This was at high speed of about 11.5kts on a 4' model.

By the way Yipster, just what is a Yipster? :?: :D

[Raggi_Thor]

Do you remember Lindsey Lord, "naval architecture of planing hulls"?
About a year ago I found this book, the third edition from 1963, in our university library, for sale (!) as it was regarded outdated. I find it very interesting. For example, he writes about a deep V monohedron where the high deadrise is carried all the way back, that "this hull is capable of absorbing tremendous horsepower." And he continues, "It is a misconception to think of such a bottom with it's straight run and deeply immersed transom as 'fast' in the sense that it has low resistance. As a matter of fact, its resistance is rather high but its speed potential, which is very real, comes from its ability to absorb and use high horsepower. Without high power these so-called fast bottoms are actually sluggish." I think this is an example of a very interesting discussion that I miss theese days when all new motorboats (that I see) have constant deadrise and huge engines.

[yipster]

good info and thanks Tom. "joop" is my dutch call name from "johannes"
when in SF end sixty's, late seventy's everone started calling me "yipster"
what it is i dont know eighter but got sorta attached to that nickname

[tom28571]

I had a nickname when small that I discarded after leaving home for the Navy - good riddance.

Raggi, I agree completly with the analysis of the deep V and its need for high power. I consider that the only positive attribute of the deep V is that it will run more smoothly in rough waves. They are heavy, need more power, are more expensive and less stable than other alternatives.

If you read further, you will see that Lord also shows instability problems with a warped bottom on planing hulls. Because of the towing method he used, I think he may have misinterpreted the instability though.

[wdnboatbuilder]

Thats intersting Tom, The design that we built for Luhrs it had a warped bottom of course, 11 degrees at the transom, 27 degrees at admidships and around 64 or better at sta. 1 forefoot area, and the designer always said that the tow point on the model was not where you would think. Of course he never gave that point.
I guess Bob is rolling in his grave giving that much info.
There was another point of his boat that was interesting, from about Sta. 2 lets say to bout the last third of the boat he incoporated a roll in the bottom of the hull he said was what created the soft ride, you have never noticed it unless you were the pushing the fairing board and him bitching at me for taking to much out. To clarify the roll it was taken from say an 8" circle around the keel faired into the bottom as it sweeped into the chine. Hope that makes since. When fairing the bottom he said he used a ghost line to get the 27 degrees at admidships. I do have somedrawings where I have tried to figure this ghost line out. Of course I trying on small boats like 15-25 loa's wher he drew nothing less than 36'.

[Willallison]

Yipster, Tom - I'll dig the text out and let you know exactly what it suggests, but from memory. it was a fairly generalised comment.

w/boat - wow 11 to 27 degrees - 16 degrees of warp - what was the top speed of this boat?
When you talk about circular sections near the chine, I assume you mean that the bottom sections were faired smoothly into the chine. Dave Gerr also suggests this - and goes one step further by recommending that the chine itself is curved - down slightly... a bit like this (excuse the quality...):

[wdnboatbuilder]

48 knts or there about she was quick and I tell you just settled down into the wave, itwas a 44' sportfish. I belive if memory seves me correctly the Luhrs 52' has the same bottom. Bob always said the the goverment would have put money into boat design insted of airplanes we would already have the answers we are looking for in efficiency. I never got to ride in the 52' cuz he threw luhrs out before we finished the interior. Luhrs kept sending young designers over to pick his brain and Bob threw them all out. Was not a pretty situation, that might be what caused his heart attack, but he was that way if you asked to many questions looking for answers for free.

[wdnboatbuilder]

Shoot I wish I could get a drawing on this thread....lol
No he used a fillet but nothing that drastic, but where you have the line fairing into the keel in the middle 2/3's of the botton he would round the bottom into the rabbit. Thus increasing the deadrise in the bottom at admidships useing a ghost line to fair. His daughter told me I could look at his plans for 150 US dollars and hour.

[KCook]

"bottom sections were faired smoothly into the chine"

US built bowriders all have a good bit of chine. And many get this smoothly faired treatment. Now sight down the length of this transition and you usually see a lot of waviness. No problem at modest speeds. But for seriously high speeds my guess is that a sharp interface performs better.

Kelly

[Willallison]

A little update - as I thought, my text gives little more than a generalised suggestion that deadrise shouldn't decrease by more than about 6 degrees between amidships and the transom.

Incidentalyy - and with his lack of enthusiasm for the deep-vee;) I know Tom will appreciate this, Shoemaker (not michael..) studied the effect of deadrise on lift/drag ratios with the following results (based on an idealized hull with constant deadrise throughout):

10 degrees = 8.5 displacement / total resistance
20 = 6.3
30 = 5.0

[tom28571]

OK Will, you reached beyond my expertise. Before I jump to the wrong conclusions, can you enlighten me as to just what is going on there.

[Willallison]

Sorry Tom - didn't write that very well did I....

The lift / drag ratio is an indication of the efficiency of the planing surface and is found by dividing displacement (in pounds I assume) by the total resistance.
For our theoretical constant deadrise boat the lift / drag ratio at 10 degrees deadrise is 8.5. At 20 degrees the ratio is 6.3, and at 30 degrees the ratio is 5.0

The text merely uses this as a way of demostrating how the efficiency of hullshape (in terms of lift - or more accurately the power required to create the lift) drops as deadrise increases. It doesn't go into any more detail than that I'm afraid - though I'm sure the orginal Shoemaker paper does.
(When I look at the refernce for the study it gives "High Speed Small Craft, by Peter Ducane....quite what that has to do with Schuey I've no idea!;) )

[yipster]

http://www.wolfgangbrinck.com/boats/glossary/deadrise.jpg to http://sites.state.pa.us/PA_Exec/Fish_Boat/boatcrs/jet/jetfigaa.gifFor those interested in calculating the deadrise of a hull, the easiest location to do so is at the transom. Measure the bottom width of your hull at the transom and divide this number by 2. The value of a 64-inch bottom would be 32. Then measure the drop at the center of the transom. This is value (A) in the diagram above. With a scientific calculator, figuring the deadrise is easy. Divide the drop (A) by half the bottom width (B), and then toggle the key labeled TAN-1. The answer is your deadrise. Perhaps this is taking things a bit too far, all most of us really need to know is that a 3-inch drop in a bottom range of 68 to 58 inches equals a deadrise of 5 to 6 degrees (See Figure 1).

found not much on deadrise, here two samples, gimmy the first rough sample please :D
does remind me someone promised me a book on this :cool:

edit: took some but think i'm getting that ratio Will, lower deadrise lower displacement and resistance

[Raggi_Thor]

So for a "deep v" the resistance may be displacement divided by 5?
Say a small boat weigh 1000kg with everything, that is 10KN, 1000kgx9.8m/s/s. If you want to travel at 10m/s (20 knots) you need 20KW (10m/s x10KN / 5)?

[tom28571]

Will, I take it that the example assumes equal displacement of the three cases.

Unfortunately, that will not usually be true. In order to get acceptable stability, the deeper the V, the greater displacement will become to keep the chines immersed and stability acceptable. One determinant of planing ability is the bottom loading in weight per sq ft of waterplane. With equal waterplane (horizontal projected bottom area), the deep V will be heavier and have more surface area with higher bottom loading and greater surface area.

Will, I don't hate deep V's, I just dislike finding them on small inland lakes and mud puddles where their one good characteristic is useless and all their bad characteristics are still there, making huge waves and eating up fuel. Marketing and mass misinformation. In offshore racing monohulls, a deep V is absolutely necessary to have both boat and crew survive the conditions. Even there, they often succumb to the waves.

No hull shape yet invented is perfect in all situations. I think the well proven "modified" V with a fine forefoot for smooth entry into waves and twisting into a shallow V aft is the best all round compromise hullform for most uses. There is still room for good and not so good hull shapes within these parameters.

By the way, what happened to the hullform you had with the deep and fine forefoot that I was afraid of? Where did you go with it and how do your studies progress? I just too old to go that far back into school:(

[Willallison]

Raggi - I don't think that you can quite draw that conclusion from the 'formula' - though at 1st look you'd think you should be able to - remember this is a theoeretical boat, with constant deadrise from bow to stern.

Tom - Yes I assume we are talking about "boat's" of equal displacement. The text is quick to point out that the lift/drag ratio isn't representative of real-world conditions - simply an indication of how increaing displacement will reduce lift.
I know you don't hate deep-v's. I too think they are over-used. Come to think of it, I don't think I've ever even attempted to draw one! And I whole-heartedly agree that for most applications the warped plane is the best compromise.
As far as the boat you were concerned about goes, I rounded off the forefoot somewhat. It was for a fairly early lesson submission - and that's about as far as it ever went. In hindsight, I think the boat would have been a bit of a disaster! I was trying to produce a boat that would run efficently throughout it's speed range - able to plane at reasonably low speeds, yet operate at up to 40 knots. I chose a surface drive (mostly coz I think they're sexy!:D ). I simply don't think the boat would have worked....
As far as the studies are going, I'm 1/2 way through my final exam - should be finished by the middle of the year! Yippee!!

[wdnboatbuilder]

So this 6 degrees you talk about, what is suppose to happen when you excede this 6 degree figure. I never Figured what the warp was in the bottom of my boat/ design or even what degree I am at the transom or admidships. maybe I should. Hell everybody that has gone for a ride love's it. Now you have my interest up.

[Willallison]

Everyones favourite boat-design cliche applies here - every boat is a compromise.
The most efficient shape for a vee-bottomed boat in terms of drag is a monohedron - where the dearise remains constant. However a forefoot with a deadrise that is sensible for the aft sections would pound horribly, so some 'warping' here is necessary. It's not hard see that it requires some energy to 'twist' the water down under the flatter sections, thus a loss in efficiency is the result. As the speed increases this becomes more pronounced and instability problems can also be introduced.
On the other hand, as has been said before, the lower the deadrise the greater the lift, so flatter sections will result in a boat that requires less power for a given speed.
So the compromise comes in balancing the loss of efficiency introduced by warping the bottom against the greater lift created by flatter sections against the ride quality in rough weather.

[wdnboatbuilder]

Okay quit being so vague. What is going to happen to my stablity when I increase my speed. Are we tlaking like chine walking, or the boat flipping? I have an interest cuz I'm thinking of going from a 70Hp to a 115 on my 15.5' boat. right now I'm burning about 2.5 gal. an hour and running around 40mph looking to get 60 mph out of her.

[tom28571]

Tell you what Bruce, put the 115 on her and report back. One more data point to learn from, if you survive.:D

No one here is going to give you the definitive answer you want. You ask too much of the forum members that really have very little knowlege of your boat. Even if you give a lot more info on the boat, I am certainly not going to risk saying that you can increase the power that much.

One thing is certain, if you overpower any boat, it can founder.:mad: Even small irregularities, that neither the designer nor the builder intended, can cause instability of one kind or another at high speed. The chinewalking you mention is a popular result of overpowering a small boat. Taken to the extreme, the boat can yaw, broach. flip over or all three. Most boats that are intended for really high speed have their bottoms individually "blueprinted", that is, inspected for even minor irregularites over the bottom. If any are found, they are faired to the precise intended shape, be that flat or otherwise.

There is nothing magic about the 6 degrees that Will mentioned. It is just an emperical number arrived at through actual experinece by a lot of builders over the years. Personally, I think it is too high and would not allow that much twist in the aft bottom of any planing boat I design. The warp I refer to is in the actual running bottom of the hull at high speed. Foreward of that, the warp is obviously much greater. When the 6 degrees from midships was given, I think it includes more more of the bottom length than I consider so maybe it is not as great a twist as I had thought.

Don't mean to be vague but that is as precise as I can be.;) Boat design is, and has never been, a precisely predictable science. If it were, there would be no need to spend big bucks on test tanks. If you are only burning 2 1/2 gallons per hour at 40mph with a 70hp outboard, you do have a good set up. I'd be extremely happy with that.:) However, your top speed might be a little low for that power on a 15' boat.

I'm going to be gone for at least a week so will miss any more action on this very interesting thread and I thank Wdnboat for bringing it up.

[Willallison]

Yep - sorry Bruce - Tom hit it on the head - too much depends on your individual boat. And all the things he mentioned (chine walking etc) are possible whether the boat has a warped plane bottom or not
I actually reckon your 40mph sounds pretty good - I had a 14.5' with a 90 on it which maxed out at 38 mph. Still it all depends on how you prop the boat too - mine was a ski boat (incidentally it was a monohedron hullform, about 12, maybe 15 degrees deadrise. I had two of them. The 1st was quite a bit heavier than the 2nd. The 1st was stable throughout its speed range, whereas the 2nd would start to chine walk ever so slightly flat out. An example of why we can't be more definitive about the problems you might face as these were, in theory, two identical boats....)

[wdnboatbuilder]

The only reason is that I way over built this boat 1100lbs hull weight. I had some ******* asking if he could put a 150 on it. My question was WHY?
Some one here made the statement that bad things could happen when you have more than the 6 degrees of warp, but what was never answered, sorry Tom for looking for answer's.
As far as the 115hp I have alot of faith and if you don't try Noone can prove the calculation are correct or not, it takes crazy people like me to prove theory's wrong or right...........lol
Death that's an act of GOD and when it's time to go nothing I can do about it.
Ya'll have a great day

[yipster]

on warped bottoms my understanding is that they are for quik planing / semi displacement hull's and only a little warp is sometimes seen in faster boats these days.
scetching planning footprints i'm trying to understand but also remember reading this warping is more than a little complicated, yet interesting it is!

[tom28571]

My own results with towing models with warped aft bottoms show a tendency to yaw at high speed. More than a tendency, I actually had a model to yaw so badly that it swapped ends and tore up the towing rig. I attribute this to oscillations in steering impulses to the aft bottom resulting from alternating vortexes caused by the twist in the bottom. I have no solid proof of this and am a little reluctant to claim it, but there you are.

You wanted this kind of an answer? Remember, you paid nothing for it although it cost me a lot to provide. You take it from there.

I'm off. Have fun.:D

[yipster]

thanks Tom, have a good week and cu later

[KCook]

Look at the bottom of US built ski boats (Correct Craft, Malibu, Mastercraft, etc) and I think you will see at least a mild warp. These typically top out at 50mph. They are VERY well behaved throughout their entire speed range. Of course the tracking fins don't hurt.

Kelly