Bottom strakes

[tom28571]

Originally Posted by Willallison

Tom
I think this may be a 1st... I fear I must disagree with you! I consider - and would reckon that many more eminent than I would suggest - that the primary function of spray rails is to separate the water flow from the hull, thus reducing wetted surface.
I agree that they may have a small impact on the lift generated, particularly in a high-deadrise hull, but this would surely be a less significant aspect of their impact...

Will,

You can probably guess that deep V hulls are not my favorites. My thoughts come from memory of what was said way back when Moppie and Don Aronow were the only deep V's around. There is no doubt that air is drawn in and that some reduction of wetted surface takes place. I know that some builders have been using reversed lap strakes on the bottom of clinker hulls for a long time for the supposed reason that you give. I will have to do some study in this area where I was only speculating before. The whole subject of aeriation (sp) under hulls is not clear cut to me.

Edited to add: I just looked at Ducane and Larsson & Eliasson. They both talk about lift of the strakes as well as wetted surface reduction. They advise not having the strakes continue to the transom to prevent too much lift of the stern and consequent bow steering at high speed. As with a transverse stepped hull, it appears that the proper design of spray strakes is not as simple as it might appear and is only of maximum effectiveness over a small speed range. I'm happy to let someone else design those anyway.

We could use some of your Tasmanian winter around here right now. It's hot.

[Willallison]

Originally Posted by tom28571

Will,

You can probably guess that deep V hulls are not my favorites.

I've sort of got that impression over the years, yes! They're not mine either, truth be known - though of course they certainly have their place....

...it appears that the proper design of spray strakes is not as simple as it might appear...

On this we certainly agree! I think that the reality is that most builders (and designers) add strakes where they think they look right. Often there's a commensurate improvement in performance, so little further research is done as to why.


I'll gladly swap a little snow for some sunshine.... we are seeing max temps of 10 -12C around here at the moment.... Not exactly conducive to working with epoxy!

[marshmat]

Will/Tom,

I'll second (third?) your noted lack of enthusiasm for excessively deep V hulls. If someone could convince the runabout and bowrider builders of the superiority of lower-deadrise forms, I think the small craft industry might not be noticing the gas price hikes quite as much as it is.

(Note on terminology- I'm using "strake" to mean a longitudinal feature on the bottom, and "spray rail" to mean a longitudinal feature at or slightly above the chine. Not sure if this nomenclature applies elsewhere in the thread.)

I tried figuring out a few streamline profiles across a deep-V with single strake and I suspect that alleviating chine walking, increasing lift, and promoting flow separation / reducing wetted surface are just different manifestations of the same hydrodynamic effect.

Consider the transverse component of the water flow under a deep-V, in the plane used to create a station. Just a hair below the boat, that flow is (roughly) parallel to the bottom and directed outwards from the centreline. The presence of a strake redirects that flow downwards slightly- in the potential flow field, this shows up as a bit of circulation. That translates to an upward force on the strake. Rolling towards the strake increases this force. The flow separates from the edge of the strake; there may or may not be another flow outboard of the strake, but the effect on the hull is the same.

Sound like a reasonable starting point for trying to figure out just what the darn things actually do?

Oh, and Will, you can gladly have our weather if you want..... I was on a roof in Toronto today inspecting curtainwall tie-ins, in 30-degree-plus heat and almost no cloud cover. 12 C would have been quite nice.

[tom28571]

Matt,

I also had bit of a problem with terminology and will readily accept yours. I also agree with your analysis of the water flow. That is, if I understand what you said correctly. Visualizing water flow under a hull is the way I look at these problems, rather than arcane formulae that don't help me understand what is going on. Plugging some Newton laws into the mix to make sure I don't violate mother nature helps too.

Will,
Do you know of any work that provided numbers for the effect of strakes for lift, wetted surface reduction and reduction of friction due to air introduced into the water ?

[Willallison]

Tom...short answer - no! Sorebreak suggests that there is some out there - I'm hoping he'll be kind enough to point us in the right direction.

As far as the terminology goes, I believe that spray rails and strakes are one and the same - located between keel and chine. A similar feature located above the chine is usually called a knuckle. Problem here, of course, is that these things take on their own meanings in different parts of the world....

Thinking about your 'flow analysis' Matt,
Down near the keel, the water is essentially a solid mass, moving as you suggest aft and towards the chine. Here, when it encounters a spray rail it is deflected out and to some extent downwards, creating a little lift as you say. But what happens once it moves outboard of the rail? Still being a solid mass, it encounters a void of sorts as the rail is no longer there. Wouldn't this then counteract the lifting force to some extent? I'm just hypothesising here, so shoot me down if there's an obvious error in my thinking...
Further out towards the chine, much of the water is simply spray, not producing much , if any lift, and here the rails act to separate it from the hull surface.

[marshmat]

I think that's exactly it, Will. From the keel to the strake, you are dealing with a reasonably homogeneous mass of liquid water. The rail deflects the flow downwards, thus receiving an upwards force itself. That much seems logical. I tend to agree with you about what happens outboard of the strake- the flow separates from the hull at the strake, leaving a bit of a void. It becomes a question of how much lifting surface is lost outboard of the separation, versus how much lift is gained from the strakes and how much drag is lost from the reduced wetted area.

What happens from there on out, I think, varies depending on hull shape, speed and immersion. If you're talking about something light enough and fast enough that the chines are basically clear of the water, then the strake is now acting like a chine, forming the edge of the planing surface (reducing effective beam and wetted area compared to the same hull with no strake). If the hull's light enough, I don't think it'll notice the loss of lifting surface area. Lighter, faster and shallower-V boats often seem to cut the strakes short at around stations 6-8, which would seem to make sense from this perspective.

In a heavier boat with multiple strakes, there will be another flow outboard of the first strake, providing its own lift. You can't really separate the flow here but there may be some advantage in breaking up the transverse component of the flow, which the strakes do a nice job of, in a deeper V hull. A number of very deep V forms carry the strakes right to the transom, which makes sense from this perspective.

All this is just intuition and hand-sketched flow fields, though.... not sure if there's anything good to back up or disprove this hypothesis?

(I still call it a "strake" if it's on the actual bottom of the hull.... and a "spray rail" if it is at or near the chine, protrudes from the hull, and is meant primarily to deflect spray. "Knuckle" to me refers to a feature above the chine, usually for spray control but sometimes just for aesthetics, but is an integral part of the hull shape and not a protrusion from it. However, Will's a yacht designer and I'm not, so odds are better that he's right.)

[Willallison]

For the sake of clarity, I've produced this highly technical and detailed drawing:

[tom28571]

I don't think there is any point in arguing about the names of the things since they already have accepted "definitions" that are different and confusing. For instance, there are patents on "speed rails" that are right at the chines.

We seem to be in close agreement in how things work but don't know just how much and where. I am certain of one thing relating to the flow on the bottom though. The only "flow" there is a transverse one since no aft flow takes place under any boat.

I'm expecting squeals of indignation from theorists about this but think I can show that it makes perfect sense. Many will say that it makes no difference whether the flow is real or apparent but I think otherwise. It makes a great deal of difference in the momentum of the water in question. I have my own "theory" based on thought experiments at: http://www.bluejacketboats.com/planing_boat_theory1.htm

Any extra lift gained from these strakes must come from the downward deflection of this momentum. "Extra lift" meaning more lift than could be expected from the longitudinal flat surface of the strake.

[Willallison]

You make a good and quite valid point Tom - apart from a very small region of water - the boundary layer, which would in fact move forward relative to the water around it - the only other movement, in real terms, is outwards. But I still wonder about the overall amount of lift that the rails would generate, given the 'void' that exists beyond their outer edge...

[tom28571]

Will, there is the forward movement of water due to momentum imparted by the trim angle of the bottom. This is how mean little boys splash water on little girls in the swimming pool.

Another point is that the steeper the V, the greater the lift from a given width and transverse angle of the strake. This is, of course, because the transverse momentum at a given speed is greater for higher deadrise. I suspect, but have no data to back it up, that this component of lift may be greater than you may be thinking.

Wade in Matt.

[marshmat]

Regarding the lift created by a strake, I do think this can be quite significant.

Let's consider a strake, of which 2 m is immersed, and outboard of which the flow separates from the strake (the 'void' Will suggests would exist on the outboard face of the strake when a light craft is making a decent speed). We can thus neglect everything outboard of the strake- it may as well not be there; ie. the strake acts as a chine. Let's say this is a 20-degree V hull with a 5-degree angle of attack at approx. 30 kt., and that the strake presents an effective width of 2 cm normal to the transverse flow (the strake itself has a horizontal underside).

With the boat doing 15 m/s (approx. 30 kt), I would expect the transverse flow velocity we've been discussing on the order of 5 m/s (I don't have a scientific reference for this but for the purposes of example I'm assuming transverse flow velocity to be roughly proportional to the sine of the deadrise). That 2 cm wide strake has an immersed area of 0.04 square metres. Directly impinging on it, then, is a flow of 0.2 m3/s, or 200 kilograms of water per second being redirected by 20 degrees- that's what, about 670 N or 70 kg of lift, from that one strake? And that's not considering the pressure increase you'll see under the strake as a result of the redirected flow having nowhere to go but back into liquid water.

Now, granted, this is a very rough example founded in quick-and-dirty estimates of momentum transfer, and not in any well-tested, application-specific theory. But I don't think it's a stretch to say this example is a lower bound- the actual lift would be higher. 1300 N of lift from a pair of 2-metre strakes presenting a mere two centimetres to the transverse flow does not sound that unreasonable as a lower bound, given how much higher and more level 20-degree-V hulls seem to ride when strakes are added.

When I get a chance (ie, not within the next few weeks) I may work through a proper example with a proper scientific basis to see if that sheds any more insight on the matter.

Where is that good free data we were promised on the last page of this thread?

[tom28571]

Matt,

I was waiting to see if someone else would see the same thing I see here. Wish I could stay around to follow up with this discussion but I have to leave. I'm off to Oshkosh, Wisconsin tomorrow morning for the annual EAA Airventure. Back in two weeks. I agree with using the sine of deadrise angle to approximate the lift generated by the momentum of transverse flow. Of course, this would be modified by the hull trim angle so it will be different than the number you give. Gotta go so have fun.

[Willallison]

Matt,
I can see where you're coming from and won't attempt to argue with your maths...and if all you were dealing with was 'solid' water then the theory would seem valid.
But of course this isn't the case where the water nears the chine. It's significanlty aerated and thus isn't able to provide as much lift. This is the water that would be separated permanently from the hull.
Rails located closer to the keel are operating in 'solid' water, but here the water that is deflected from the vee of the hull must once again rush back up to support the hull surface - this is the 'void' to which I was referring. And here - where the lift produced by the strakes would be the greatest - is where a loss of lift would occur (in my mind, at least).
None of that dimishes any of what you and Tom have put forth - it's merely proportion that we're debating...
There must be data out there........

[tom28571]

Something to keep in mind is that aerated bubbles under the hull must be at the same pressure as the surrounding water. Of course they have less momentum and so provide less lift from that source. My thought experiment gets tangled in too many variables about here and needs some data to limit these variables to a reasoning number. I've not seen good data on the total effects of aerated water under a hull before.

Will, any void outboard of a strake must either be open to the atmosphere or involve true cavitation. This is one thing that causes real handling problems with stepped hulls. I don't think cavitation can be had at the speeds and pressures we are discussing here.

[marshmat]

Rails located closer to the keel are operating in 'solid' water, but here the water that is deflected from the vee of the hull must once again rush back up to support the hull surface - this is the 'void' to which I was referring. And here - where the lift produced by the strakes would be the greatest - is where a loss of lift would occur (in my mind, at least).

That makes sense, Will..... so it becomes a question of how much lift is gained by having the strake, compared to how much is lost immediately outboard of it.
I wish I had access to that Fluent machine now.... I'll drop by here again if I think of anything logical to add to this discussion. Maybe it'll come to me while I'm on vacation next week (hanging around boats the whole time, thankfully).