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  #1  
Old 12-27-2004, 10:22 PM
mikfin mikfin is offline
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lifting rails for an old semi-planing hull?

Hello,

I'm considering lifting rails for a 1973 45' Hatteras convertible. The boat weighs 43000# and has 930HP available. I think it is a semi-planing hull- keel extends below the running gear and stops 8' short of the transom, single hard chine, slight concavity (warp?) at the quarter buttock line,and runs with a bow high attitude at planing speed. Top end now is 21-22 knots.

My questions are whether a 3-4" chine extension (tapering to 1" at the bow) with a 15-25 degree anhedral will lower the running angle, lower the initial planing, or raise the top speed with the current weight and power. What are the possible negative consequences other than increased hull pounding?

Thanks,

Mick
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  #2  
Old 03-27-2005, 11:40 PM
gsdickes gsdickes is offline
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I would not angle your chine extensions down that hard. My feeling is 5 degrees is plenty. I use 2.5 to 3.0 degrees on my boats.

The rails will add more lift than drag so your performance should improve, the downside will be that the ride will be a bit more harsh. At the dimensions you describe, chances are it won't be so much as to be annoying.

Whether or not you'll run flatter is a little more complicated. The short answer is that you will if the center of lift of your extensions is aft of that of the center of lift of the hull without the extensions. With a tapered rail (wide aft, narrow forward), my guess is that will be the case and you will run a bit flatter. If not, you can always add wedges or carry a bit of down trim tab.
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  #3  
Old 03-28-2005, 10:46 AM
mikfin mikfin is offline
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Thanks for the reply.

I have some old Hatteras plans calling for a continuous 15 degree angle. Most people I've talked to used 8-10, increasing to 15 forward of the engines to create some rocker. You feel 5 degrees is enough to lift the boat enough. Do you think 10 degrees will lift the stern too much, or just create more drag?

Thanks again,

Mike Finn
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  #4  
Old 03-28-2005, 04:53 PM
gsdickes gsdickes is offline
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Hi Mike:

If we're talking about anhedral angle -- the opposite of dihedral or deadrise -- then 15 degrees sounds like a lot. That will certainly impart a distinctively downward vector to the flow across the rail and there will certainly be some associated reactive lift, but I believe the maximum lift you would get out of chine flat would be at a zero degree angle (parallel to the waterplane). So, I think you would develop less lift with a ten-degree anhedral angle than with five degrees.

By the way, what is the angle of the chine flat that's already on the hull? My instinct would be to make my extensions at that same angle.

I don't follow how any change to the anhedral angle would create "rocker." Rocker is something you'd see in buttock lines -- a longitudinal feature. We're talking about an angle measured transversely (in body section). Besides, if you're concerned about the bow running too high, you don't want more rocker anyway.

Incidentally, as far as I am aware, this is all a bit theoretical. The only way to know for sure what would be best on your hull is to experiment, either at scale or full size. I don't even know if this issue has ever been investigated in any sort of controlled trials, model or otherwise, on any hull. I made a few searches through the SNAME publications database and found nothing on lifting rails, lifting strakes, or chine extensions.

I'm just sharing what's worked well for me in the past on hulls either of my own design or with which I was very familiar. However, I'm not all that familiar with your hull. I might have somewhat different guidance if I were to have a lines plan to study, for instance.

Finally, there may be other factors at play as far as your speed and running angle issues are concerned. Two obvious ones are vessel loading (an adverse position of CG) and prop geometry. Your best results with lifting rails are likely to be achieved if you can consult with a yacht designer or naval architect from your area with some expertise in planing hulls.

Geoff
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  #5  
Old 03-28-2005, 07:58 PM
mikfin mikfin is offline
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Thanks for the reply Geoff,

I just fastened the first of for layers of mahogany on today. The boat, by the way, is a 1973 '45 Hat Convertible. The hull has a slight warp (concave) and the keel extends below the props and stops about 10 feet from the transom. The chine has about a 4 degree dihedral.

I appreciate the effort in combing the SNAME archives. There probably isn't much energy being devoted to making old Hats go faster without adding horsepower.

My thoughts on "rocker" are a combination of regurgitated second-hand heresay from a race boat builder, minimal knowledge of hydrodynamics and an understanding of aerodynamics which may have little application. If water rises up the hull as it passes aft and hits a chine with an anhedral, does it reflect inboard and create lift, or is it just turbulent drag? If it creates lift, and in order to prevent too much lift, can a reduction in the anhedral allow the flow to move outward more readily, potentially creating a more "trimmed" condition, or am I totally out in left field?

Again, thanks for your thoughts,

Mike
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  #6  
Old 03-28-2005, 08:26 PM
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Willallison Willallison is offline
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Mike,
I'm wondering if you may have a few of your terms a bit muddled here....
With the boat out of the water and looking from the side, does the bottom of the the hull, where it meets the keel (known as the fairbody) curve up towards the transom (rocker), or is this a straight line, or even curve slightly downwards in a concave fashion (hook). Rocker in the bottom will tend to make the boat ride with the bow higher, or rather with the stern lower. Hook will make it run flatter. The upside of rocker is that it can help in a running sea - and conversely hook can tend to worsen any tendency to bow steer in the same conditions.
Now, looking at the transom from the back of the boat - do the chines angle back down? This is the five degrees that Geoff is talking about. 10 - 15 degrees seems extrraordinary. One other thing to think about is that big flat chines will tend to slap not only whilst underway, but also at rest...

As Geoff suggests, there may be more to consider here. Moving onboard weights may be an effective way of reducung the running angle (trim). Where are your batteries, genset, aircon - even fuel and water tanks (though these are generally not considered a good mean of altering trim as their weights change. They should be located as close to the boat longitudinal centre of gravity as possible).
And what about trim tabs - got any? and are they big enough?

Lots to consider. But if you've already started on the chines, then at least let us know how successful they are...
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Old 03-28-2005, 09:32 PM
gsdickes gsdickes is offline
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Hi Mike:

Excellent! I was secretly wanting to say "just go for it" and see what happens, so I'm glad you are. Nothing, but nothing, beats full scale experiments and trials.

Except in the region of the so-called stagnation line (where the spray blister emerges from under the chine, more or less amidships on your boat at speed), the flow across the bottom is probably much more fore and aft than you are imagining. If you look over the side sometime, aft of the spray blister, you'll see what I mean -- nothing is really shooting out from under the boat on any sort of angle relative to the vessel centerline. In fact, when you look over the transom, you'll see that flow is breaking off the bottom more or less parallel with the centerline. There is little, if any, centerline-to-chine vector to it. On the portion of the chines that are immersed (the important part) flow is predominantly fore and aft. So, it might help to think of your chine extensions as though they were a couple of water skis attached to the side of the boat. Would the skis exert more lift if they were canted one way or the other? No. Lift would be reduced. So, I say extend the flats at the same angle as the parent surfaces -- the chines -- and you can't go too far astray.

Keep the big picture in mind too. The weight issues that I mentioned (and Will elaborated on), the quality of your running gear, and the whole idea that less drag = more power. You'll get the best performance out of your bottom if it's clean and as free of protrusions as possible. It takes energy to move water. The less you move the better. Are your transducers all internal? Raw water intakes recessed? Strut palms set flush to the bottom? Nothing lumpy or bumpy upstream of your props? And by the way, do you have high quality props? At the sort of speeds your going, good props can add another 3 knots.

Please do let us know how it goes ...

Geoff

PS. Thank you to Will for the most excellent (and concise!) explanation of rocker and hook. ;-)
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Old 03-28-2005, 09:42 PM
mikfin mikfin is offline
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Hi Will,

I've been asked if I suffered from muddled thinking before-to which I reply: "Stupid is as stupid does" (Forrest Gump).

When I look at the boat from the side, in profile, the line you mention curves upward slightly and then flattens halfway to the stern. I understood this to be concave ("hook"), or a warped hull. Depending on the CG due to engine weight, etc., I am under the impression a more convex hull at the buttocks (and maybe elsewhere) creates more lift through increased buoyancy as well as reducing drag (increasing speed) by reducing the wetted surface aft due to a decrease in suction created by a "warped" hull. I'd like to be a student of hydrodynamics here, so if you're inclined, correct me.

The chine extensions I'm fashioning taper in width forward, at the halfway point if you will, from 4" to 1" rapidly. I hope to alleviate pounding this way.

The trimtabs are marginal: 36" wide x 6" deep. Perhaps 6-12" of extension is in order.

The insight you've given is greatly appreciated. Even though the hull on this boat isn't very sexy, there are alot of them around. It's a hull that's seeing customizations. Unfortunately, I can't test the project, but I can still change the design before it goes back in the water. So, if you care to voice an opinion, it will aso be greatly appreciated (and not held responsible).

Cheers,

Mike Finn

Last edited by mikfin : 03-28-2005 at 10:41 PM. Reason: misquote
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  #9  
Old 03-28-2005, 11:12 PM
mikfin mikfin is offline
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Hi Geoff,

Thanks for your excellent reply.

I can only reaffirm my ignorance about hydrodynamics when you assert that water moves more fore and aft than from low to high at the hull speed we are talking about (I'm stuck mentally at 480kts.). Perhaps you can recommend a basic book for me.

I will certainly let you know about my experiment. Any other ideas you have would be appreciated.

Cheers,

Mike
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  #10  
Old 03-28-2005, 11:59 PM
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Willallison Willallison is offline
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on recommending a good book, you might like to read Dave Gerr's The Nature of Boats . It's a good introduction into most things nautical, without getting too carried away with complex formulas and stuff.

As far as your understanding of the hydrodynamics of hook versus rocker, I'm afraid that once again your all muddled up! At the sort of speeds your boat goes (and I can assure you, it ain't 480 knots! - try 20 or 25... ) hook will make the boat run flatter and tend to get up on the plane at a lower speed. Rocker will do the reverse. If you do have much rocker in the hull, you could try adding wedges near the transom - they will have the same effect as trim tabs, only they're not adjustable, but they can easily be made bigger
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  #11  
Old 03-29-2005, 06:07 AM
FAST FRED FAST FRED is offline
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Location: Conn in summers , Ortona FL in winter , with big dock & room for O'nite stop .
The folks that make roll stabelizers claim a 15% increase in speed and 10% reduction in fuel consumption,

by using the fins as foils to lift the boat.

Could only be advertising BS , but it may work.

FAST FRED
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  #12  
Old 03-29-2005, 09:08 AM
mikfin mikfin is offline
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Will,

I guess I've been mixing terms in a field where I'm better off as a water boy. I understand hook and rocker, the mistake I made was associating them directly with concave and convex. Correct me on this: a 'warped' hull starts out convex in the forward station lines and becomes concave aft? Does a design such as this have a tendency to suck the stern down or cause the boat to ride bow proud, and if so, would an anhedral chine extension have any positive effect or would it still be better to have the extensions follow the dihedral line of the existing chine (3-4 degrees) if we're talking about a moderate cruising speed of 14-18 knots?

Thanks again,

Mike

Last edited by mikfin : 03-29-2005 at 09:23 AM. Reason: clarification
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Old 03-29-2005, 11:56 AM
fcfc fcfc is offline
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My 2 cents ...

I thougth that a warped bottom was associed with variable deadrise. Big deadrise at midship, flatter at the transom. You see a warped bottom on the section view.

hook / rocker describe the shape of the aft part of the buttocks. Your boat have hook if the aft buttocks are convex. You see this on the longitudinal view.

You can mix both (warped bottom and hook), but that is not mandatory.

Hook does not make a boat bow high. Look how flaps run. Correctly designed hook act as integrated fixed flaps in the hull. (You can even have hook up to the point buttocks have a "negative" slope. )
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Old 03-29-2005, 12:35 PM
mikfin mikfin is offline
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fcfc,

Thanks for your 2 cents. It's starting to come together a little bit definition-wise for me now. My boat does in fact have a warped hull by the definition you describe, and it does not have any hook. The aft buttocks is flat. Given this, does a warped hull tend to make a boat ride bow proud at speeds below say 20 kts., and would an anhedral chine extension provide lift, or should I follow the chine's current dihedral?

Thanks,

Mike
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  #15  
Old 03-29-2005, 06:13 PM
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Willallison Willallison is offline
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As Mike points out, a warped plane has nothing really to do with concave or convex sections.
Take a look at this thread that I started some time ago, and in particular at the posts by Dim (if you're out out there, we miss you buddy! ) the hull shape that he has modelled, has some hook in it. From the side, the fairbody and buttock lines (which are green) of the boat are convex. From the front, most of the section lines (yellow) are slightly convex - though they appear convex due to the warped plane - the way the deadrise angle decreases towards the transom

Warped plane vs constant deadrise
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