Chine and strake design

[meren]

Hello!

Are there any formulas or basic rules for starke and chine design for planing hull? I´ve heard that chines and strikes combined shouldn´t be wider that 20% of BWL.

There are also some hulls with good seakeeping abilities without strakes at all. So are strakes just trying to correct a poor hull design?

[Willallison]

There's a great deal of art that goes into successful chine and spray rail design.
In general, chines shouldn't be more than 3 - 5% of the wl beam. Ideally they should be angled down slightly, but no more than 5 degrees. They should sweep up to the bow, usually exiting the water at around station 4.5.

As fro spary rails, there are a dozen different styles and a thousand different opinions as to which type is best! For higher speeds, it is generally accepted that 3 per side is ideal, with those closest to the chine running all the way aft and the other two somewhat shorter. The one closest to the keel being the shortest.

They are most definitely not simply a means for correcting bad hull design. They are used to reduce wetted sueface area - and thus drag - and thus power requirements.

[Pericles]

Meren,

May I suggest you look at Tom Lathrop's site http://bluejacketboats.com/ as he has developed a planning hull design that uses less power to get on the plane. He uses wider flat chines to promote early planing. It's a very good site with instructive photographs

Pericles

[Shorebreak]

Meren,

Below are a few comments for reference, but I recommend that you do not build anything based on this information without first consulting a professional.

The rule that I use is that chine width should be about 1.5 to 2% of the maximum chine beam, per side. However, I have designed many boats that exceed this value. With wider chines the boat will roll less at idle or rest, and gives added stability at speed, however, the boat will ride a little rougher with wider chines. The chine can be angled down between 0° and 15°. Its all about finding the balance you want for the boat's purpose, and only experience will help you identify exactly what is needed.

Strakes follow similar rules. Boats with a volumetric Froude number less than 4.0 don't need lifting strakes... for example, a 55-foot boat weighing 60,000 pounds should only need strakes if it will go faster than 42 knots. Location and shape, again, is about finding the balance of the design.

Regards
Darron

[HakimKlunker]

1. In general, chines shouldn't be more than 3 - 5% of the wl beam.
2. Ideally they should be angled down slightly, but no more than 5 degrees.
3. They should sweep up to the bow, usually exiting the water at around station 4.5.
4. ...with those closest to the chine running all the way aft and the other two somewhat shorter. The one closest to the keel being the shortest.


After three years ( :) ) I have questions:

1.: As a planing hull lifts out, the WL beam will decrease; does this in your eyes justify a reduction in chine width ?
2.: I talked to a designer who after tank testing found that angles down did not improve the performance; so the angle seems to correspond with speed, hull shape and probably trim angle?
3. & 4.: I again think of a planing hull, lifted higher than in resting condition: It seems to me that the chines all should end at the transom; the one furthest from the keel ending at or close to the stem, the others ending further back. Station '4.5' becomes less important here?
Anyhow, a chine also stiffens the skin and so perhaps should be continued further towards the bow (in this case for structural reasons and not so much for hydrodynamics)?

[Willallison]

Ok - to start with, all the above are rules of thumb. There are always exceptions and the ideal angles, widths and locations of spray rails, chines - and everything else in boat design for that matter! - will vary from vessel to vessel.
To take your points in order...
1. Do you mean the width of the chine flat, or a reduction in overall beam at the chine? Chine flat's are most beneficial on planing hulls. They provide lift, increase dynamic stability (and to a lesser extent initial stability) and reduce spray. If they are too wide, however the boat will suffer from a harsh ride. The 3 - 5% that I referred to was for planing hulls. Again - this is a generalised rule of thumb - plenty of boats get by with no chine flat at all.

2. Talk to ten designer's about chine's and spray rails and you'll probably get 20 different answers!!;) However, it is generally accepted - and quite logical if you think about it - that a slight down angle will reduce spary and increase lift. Sometime's that latter is beneficial, sometimes not. Too much down angle at the chine can force the spary down with such force however that it actually creates more spray. In my experience, the same can be said for excessively wide chine flats.

3. The length, shape and location of spray rails will elicit even more opinions than chines.....
However, the primary function of a spray rail is to separate the water flow from the hull bottom. They do other things as will, but that is their primary goal. Thus, adding a spray rail where it can't achieve this does nothing but add to the wetted surface (and hence drag). On all but the fastest of planing hulls, the bottom is immersed to almost the full beam at the transom (if not the full beam). Therefore adding rails close to the CL will only slow the boat down.

[HakimKlunker]

I think, I got the idea. The optimum solution looks like a couple of models tested in a tank. Unfortunately, I have no such opportunity.
My present object is an updated 13.65 m sportscruiser with twin surface drives and a target speed well above 50 kts; the original is from the 1970's and did well in Italy.
The bottom is slightly rounded.
I only have photos and a not too exact drawing, but well enough to re-draw the general lines.
Bcwl is ca. 2.5 m and 3% makes 75 mm
The inner chine (in my simple world) will be effective at top speeds only and with the hull quite far out. I expect higher water pressure in this condition which requires less surface?

[Willallison]

Yes - the higher the pressure, the less the surface area required to support it. You have pics?... please post them, we're always up for looking at a new project...:p
But do yourself a favour - a boat like the one you describe is not for the amateur designer. I urge you to use an appropriaely qualified designer.... it will almost certainly save you money and anguish in the long run

[HakimKlunker]

Thank you for the details. I am a certified master of boat building with 23 years on the job. But you are right: I am not as arrogant as many others and do know where I have my limits (this is why I try to learn here ;) ). I am actually in contact with two experienced designers and before I start mixing the resin, they will be consulted.
I have done some raw sketches in jpg (sic!),
and Rhino and I do the details now...
The other photo is the original 'Drago' (I was told that Sonny Levi was involved ;) )

[Alik]

Hakim, I would suggest to move fuel tanks forward - under bed or in bottom, slightly aft of middle.
This way You get more favourable LCG with full load.

[HakimKlunker]

Thank you Albert.
I'll consider it. I appreciate very much your support.
At the moment I am still working on the lines and still need to find the centre of buoyancy. The tanks in the sketches are just assumptions at the moment. Once I know the detail weights later, I am going to re-arrange the weight distrubution. When I have my general lay-out finished, I would like to have it counter-checked by you anyway :)
And by the way: I like your idea of naming pictures 'bulky corners' ... :D

[Alik]

Thank you Albert.
And by the way: I like your idea of naming pictures 'bulky corners' ... :D
Yes, You got it! :D
Sounds like some company name, isn't it?

[StealthAssassin]

Anyone have any pictures of strake shapes? I am wondering about a triangle vs. a wedge shape with the outer edge of the strake at 90 degrees?...

[u4ea32]

From the NACA testing reports, it sounds to me like the effectiveness of strakes or chine flats is very limited. the only thing a strake does is release the spray, cutting the wetted surface from the bow wave. The only thing a strake at the surface, or a chine does is reduce the effective deadrise from the surface athwarthsip angle to the angle from the keel directly to the outside of the chine.

Extra strakes under water are more drag.

Wide chine flats are worse than smaller ones because its (1) more surface area, but worse (2) its like a concave bottom, which pounds more that a simple V or convex bottom.

Think of the deadrise like a spring, you want the spring to be most firm at first, and then get LESS firm with more compression. A concave bottom, and wide chine flats, is the opposite, so far less comfortable, far more structural loads.

This was tested EXTENSIVELY in test tanks and with full scale (100') vessels in the early years of WW2. The result is what we see: either flattened V near the centerline, as on the world's fastest V bottom powerboats (Fountain), or straight V constant deadrise as on nearly all other boats, with small chine flats -- just enough to ensure the water flow does not climb up the topsides when moulded in Fiberglass. And strakes that end about where you think they go under water when planing.

Several builders have found it easier to leave off the strakes completely for easier building, or to just use them for styling purposes.

[Wayne Grabow]

"either flattened V near the centerline, as on the world's fastest V bottom powerboats (Fountain),"

I assume that the "flattened V" is what is also called a keel pad. Such boats as the Rascal, Rifleman, Diablo and others also incorporate a keel pad. Does anybody have guidance for the width and shape of such a feature? I am guessing that the keel pad should be parallel to the aft chine and straight for its entire length? Any particulars on its optimum width as a % of total chine beam? or its width possibly affected by the overall deadrise angle?

[surfieadam]

Gday All,

What a great forum with some obviously very knowlgeable members, thanks for taking the time imparting your wisdom and so to the forum moderators. Reading this thread alone has already taught me some things I did not know and I grasp thats there's much for me to learn....eh I was almost sweating at that trick question at the registration! :p

Before I touch on the topic in this I thought I'd briefly touch on my experience, limited as it is. I have zero boatbuiling experience...though I've been reading bit of late. I'm getting near 40, been around smaller boats most of my life, mostly small monohulls under 6m, more recently I had the pleasure of a 7.5m sailsish catameran + 150 hondas in the Cocos Island for a year, where I got my Coxswains ticket and did a bit of Island hopping, fishing and surfing tours to a remote atoll about 25ks north. I say remote because the seas are often a bit rough for comfort even with a decent cat thanks the prevailing tradewinds.

I must say I'm hooked on Catamerans after that experience for the purposes I required at the time, but there's some other "creative" design concepts I would like to see in a catameran that I do not seen any of the current model - to the extent I would like see :idea: . I found of particular interest the comments made by member u4ea3 on the v versus concave bottom (softer ride) and on the definitive to this day tests on life size model leading to what we have today (v bottoms).

I am in the process putting on paper a scale model of 5-6m Cat with some creative new hull concepts involing a significant chine and strake combo..which may have already been tried or there's some existing knowledge in this area. I'd like your comments on a design concept for a planning cat hull with a difference involving a I guess a fairly radical chine and strake combination with the view to create additional lift via the concave affect (faster moving water through the shorter radius creating lift). Ultimately the view is to tool/mould up, go into production and establish a brand.

A drawing would do far better, as would a mini scale model (which I might have compete this week) and I'll look to post one but how does this sound for a basis:

I'm looking to have the slim fine semi planning/displacement ultra soft riding hulls like the Glacier bay cats - but the downside I see to that is the very high tunnel clearance required (less internal room, top heavy) versus say a lesure cat with it's fatter, higher speed but rougher riding planning hulls. My idea was to change the bottom in the Glacier bay style from a v to flat to a v concave along the bottom of the hull to point where the dead rise begins. The idea here is that I can achieve the softest ride with the finest hulls but also getting the hulls to rise out of the water creating less drag/more effieciency requiring less hp for same speed and quicker planning via the concave affect, in turn also requiring a lower tunnel.

Obviusly I need to find the right combination of width of the concave at the bottom of the hulls to achieve the displacement required and improved (I hope) hydrodynamics, but with the concaves not so wide that it rides harshly defeating the objective of a softer than lesure cat ride - similar to a Glacier bay but with improved efficency and top end.

Add to this concept that the outer edges of the of the bottom of each hull (the outers edge of the concave) would be slightly higher than the inside edge - giving rise to an asymetrical profile at the BWL on each hull - so as to affect a lean into turns as with asymetrical cats, as opposed to the lean out on symetrical hulls (like the sailfish).This design concept allows surfboard to more easily transition rail to rail. This design makes the chine offset towards the the tunnel centre and this line is continue to the bow - versus the usual centreline chine arrangement found in all the cats I see (in what orientation an outboard might be placed in its wake is yet to be determined) This design also gives rise to more forward volume and planning surface and less of a tendency to bury the nose/broach in a following sea....a sometimes scary downside to cats.

Oki, thats it...all the current design perfromance flaws in a catmeran solved :D and I hope I'm on topic with post and that I havent confused the heck out of everyne reading this. Yes, I'm looking for something different and maybe I'm being over arrogant in straying from the more universally accepted. I'm a surfer with practical experience in the affects of v bottom versus concaves and outer edges/rails, too, and the endless refinement in bottom shapes I guess biases me towards the belief that there's always room for improvement. V bottoms in surfboards are good for big powerful waves where power unlimited and on tap (think super high speed boats, 50 knots pus), concaves are good to get going when the power is not so plentiful because they provide lift and reduce drag, which makes the board travel faster across the same section of wave than a v bottom would (think, the average boaty wanting under 50 knots). To my eye and rationale....I think we can do better than a basic v, with the right combo of concave, hull width and displacement.

Looking forward to your valued comments and advice - thanks!

Cheers, Adam :)

[sottorf]

There are a few good reference papers on chine and strake design, that can be used as guidelines One of the best works on the subject i know is by Muller-Graf. Does anybody else have any good references of scientific works on the subject??

[Easy Rider]

Surf,
Sounds like you don't think the Glacier Bay boats got it right. If sales are a good indication they did but I think a boat should have the ability to slide sideways in big seas. GB has high CG and almost no sideways ability. So I agree that they didn't. I'm not fond of most cats .. lots of extra weight for structures to keep the 2 hulls together, flat tunnels to pound on or shapes in the tunnel to soften same that even adds more weight, extreme twisting forces when quartering head seas, when quartering seas the bow can't rise up smartly (as it should) when plunging into a wave because it's only got half lift as it's only half a bow and when the bow gets in deep the deck keeps it from rising .. as it should. Not my favorite boats but I digress as this is not on topic at all.
What I wanted to add to this strake/chine party is:
1 I don't think the strake should sweep upwards in the bow as Willallison suggests. Just causes extra drag and spray. I think it's done for appearance.
2 I've often thought a strake design that would be very effective would be to have one or two strakes that would be like a half round or less than a half round just inboard of the chine for the purpose of seriously slowing the water down before it hits the spray rail. Theoretically the water would gently roll out from the chine and plop right down where we want it.

Easy

[u4ea32]

The Muller study yet again mentions that strakes that extend under water only add drag. The positive effect of strakes is to get the spray to release from the hull. They are NOT effective to increase lift.

Think about it for just a moment: L/D from an airfoil is dominated by aspect ratio: the higher the aspect ratio, the higher the lift for a given amount of drag. The lower the aspect ratio, the more drag for a given amount of lift.

A strake or chine flat is a very, very, VERY low aspect ratio lifting surface. Lots of drag, very little lift.

[Easy Rider]

As I see it when the water around the "lift" strake is moving parallel to the strake there is no lift generated ..so you are right, BUT that only happens at 4 knots or less. Any speed that promotes an increase in the angle of attack produces what Aeronatical Engineers call spanwise flow. Anytime in a boat where there is water flowing crossways over the lift strake there will be lift created in the bow or at the stern .. spray or no spray. The designer must decide if the lift from a strake well aft is worth the added drag. Ther'e a lot of variables involved in that call. My opinion.

Easy Rider

[u4ea32]

I agree, Easy.

I think the lift forward added by the strakes in the Muller study is due to this spanwise flow: its a displacement hull form that normally makes a huge bow wave (really, spray), and so the flow is going up alongside the hull, and therefore the strakes -- above the waterline -- do provide good L/D. The bow lifts, and the total drag decreases when stern flaps are also added to avoid the drag increase due to simple bow rise.

Note that the Muller study and many others find that the spray rails or strakes should end more or less where they go underwater at speed. Underwater, where the strakes are acting on flow that is almost perfectly aligned with motion and therefore acting as very low aspect ratio lifting surfaces, the strakes add drag.

On the Muller study they found an improvement even if the strakes went underwater about 10% of LWL. Perhaps there is still substantial spanwise flow in this area, its not clear to me.

Sound reasonable?

[sottorf]

I fully agree, increasing the length of the spray rails beyond the waterline do not add lift. THey are however there for other reasons: they improve the directional stability of a boat significantly and also affects how it banks into a turn.

Another useful reference on spray rails and spray is from the Master himself: Daniel Savitsky:
http://www.stevens.edu/ses/cms/fileadmin/cms/pdf/Inclusion_of_Whisker_Spray_in_Performance_Prediction.pdf

[fg1inc]

Please keep in mind that some are "spray rails" and some are "lift strakes"
and some....well, it reminds me of a Hatteras MY from years ago that had vertical strakes at the forwrd sections. Looked a little like multiple gills on a fish, 90 degrees to the keel! Turned out they built 3 of 'em that way because the suction caused by clean laminar flow wouldn't let the bow rise enough to get planing. They tried conventional "lift" strakes but that didn't do anything.
So they glued on these crazy vertical strakes and she planed! (sort of)
Trial and error , i guess. It was pretty cool having Jack Hargrave living in the same town and being able to ask stupid questions!

[Easy Rider]

So, You guys think the water flow is going to straighten out and run aft parallel to the keel from midships aft? What force could straighten the water out??? And why wouldn't the crosswise flow create lift aft of the water line while under way. As I see it .. as long as there is angle of attack there will be crossflow and lift .. all the way to the transom. Of course they create drag but the whole hull does that. It's really a question of whether or not lift strakes create enough lift to make the price of their drag worth leaving them on. With aircraft they put "tuffs" on the wing and go fly it and watch. Anybody up for some scary scuba diving?

Easy Rider

[u4ea32]

Sottorf, great paper!

Here is a gem in there near the end that I'd read before in another much older paper. Edited for brevity: "The sharpness of the outer edge of the strake cannot be overemphasized. Even a quarter inch roundness eliminates the effectiveness of the strake."

So if its not a sharp edge, its better to not have the strake at all!

[Easy Rider]

I agree David .. and the sharpness of the trailing edge of the bottom (as in transom meets bottom) needs to be even more so. Seems to be an advantage of metal boats.
Also I may have been wrong about the performance of lifting strakes aft of the WL on plane. As the crossways flow over the strake produces lift the water must return to the hull and that could cause the boat to some degree to be sucked downwards (or anti-lift) eliminating the effects of the lift that may be canceled or reduced to the point that the remaining lift (if there is any) would not be worth the drag of the strake. That's the only thing I can come up w that could explain why (if it is so) lifting strakes don't work on the after plane of a boat. What think?

[u4ea32]

The above paper by Savitsky includes photos of the flow of water under a V hull using both dye and tufts. Both show the flow is essentially fore-and-aft aft of the "stagnation point" or the leading edge of the hull, as the planing hull surface goes underwater.

There is some athwarthship angle to the flow near the chines, but not much.

This lack of sideways flow means the strakes are in that "very low aspect ratio" configuration, so lots of drag for a given amount of lift.

Since surface area contributes to drag at a more or less linear function of speed (double the speed, double the drag due to surface area), and the strakes add surface area, I think that's probably sufficient to explain the slight increase of drag due to immersed strakes.

Its still worth noting that even optimum strakes make perhaps a 1-3% reduction in drag. They really make so little difference that they may not be worth the effort and weight.

[Easy Rider]

OK David .. Hard to imagine there ins't much cross flow but tests sometimes show what seems to be obvious isn't so. There's lots of people here learning lots of stuff ..and that's great. Something good that I just thought of is that the absence of strakes on the afterplane allows the stern to swing laterally giving better turn performance. I have an aluminum skiff that sticks to the water so tenaciously that in a real tight turn it feels like it's about ready to flip over like a car. I can't take the little micro-keels off as they are structural. I would think the best skiffs would have the structural stuff inside the boat.

Easy

[FlyingTime]

"either flattened V near the centerline, as on the world's fastest V bottom powerboats (Fountain),"

I assume that the "flattened V" is what is also called a keel pad. Such boats as the Rascal, Rifleman, Diablo and others also incorporate a keel pad. Does anybody have guidance for the width and shape of such a feature? I am guessing that the keel pad should be parallel to the aft chine and straight for its entire length? Any particulars on its optimum width as a % of total chine beam? or its width possibly affected by the overall deadrise angle?

Wayne,

A good place to start is 1" for every 5' of chine length... You can either keep it flat at the transom and have in converge towards centerline tapering out at the forefoot OR start with a 3/4" vertical on the outboard sides and run them parallel to centerline and blend them in around the forefoot. Either way works well but seems like most use the vertical outer edge. Some say it offers more protection while up on blocks in the yard.

[Wayne Grabow]

FlyingTime,
Thanks for the information. If I understand correctly, a 20' hard chine boat should have about a 4" wide keel pad? Seems narrow from what I have seen on the mentioned examples. The trade off in pad width seems obvious: more keel flat surface will bring more of the characteristics of a flat bottom; less emphasizes the deep V aspect. The ideal is that happy medium.

Thanks

[tunnels]

FlyingTime,
Thanks for the information. If I understand correctly, a 20' hard chine boat should have about a 4" wide keel pad? Seems narrow from what I have seen on the mentioned examples. The trade off in pad width seems obvious: more keel flat surface will bring more of the characteristics of a flat bottom; less emphasizes the deep V aspect. The ideal is that happy medium.

Thanks

A 4 inch wide pad is next to usesless to give a hull 20 foot long any sort of lift!, you wouldnt even notice any differance unless you were travelling at 100mph :confused: !, it would need to be 10 inchs wide at least!! :rolleyes: . Depending on how fast you are able to get the boat to go . anything below 60mph ,10inchs would be a advantage but its not going to climb out of the water and fall over ! Think about a pair of ski's how wide are they ? and thats just to hold your weight up ! a boat is a lot heavyer than a person !! put things into prospective as to how fast and how heavy then think about the amount of lift you are lookin for and do the sum's. Dont forget the trim angle of the hull and how much hull is actually in the water at speed . you could be amazed at what you find:?: . I always take video from the shore at as close to water level as you can without getting wet. The boat traveling at differant speeds and differant trim angles . Put the tape through the computer and freeze frame and analyze what you are looking at . You can learn heaps from just doing simple basic things . :P

[Jimboat]

A 4 inch wide pad is next to usesless to give a hull 20 foot long any sort of lift!

I agree with tunnels suggestion that a pad for this hull would likely need to be wider than a few inches in order to contribute any significant lift. A flat or low deadrise pad will be more efficient in lift generation, and can be designed to support a large portion of the hull's weight. The best way to size the pad is to know the amount of 'lift' desired to be contributed to the performance of the hull and calculate the pad dimensions to suit. Weight and power of hull are strong contributing factors to the required design of the pad, as are the deadrise of vee portion of hull, since this determines other hydrodynamic lift. There is no good 'rule-of-thumb' that can provide for the required dynamic balance of a performance hull, since the ever-changing balance between pad lift/drag and vee lift/drag has a strong impact on dynamic balance.

You may find helpful an article that I wrote on "vee pad design (http://www.aeromarineresearch.com/vee%20pad%20design.html)". There are also calcs available that will estimate lift from strakes (email me (jimboat@aeromarineresearch.com) if you want more info).

Note that as others have suggested, the addition of a pad and/or lift strakes will also change dynamic CofG, and will generate different handling characteristics for your hull.

check here for other threads on pad design (http://www.boatdesign.net/forums/boat-design/flat-spot-pad-transon-cig-boats-22740.html#post206300).

[FlyingTime]

A 4 inch wide pad is next to usesless to give a hull 20 foot long any sort of lift!, you wouldnt even notice any differance unless you were travelling at 100mph :confused: !, it would need to be 10 inchs wide at least!! :rolleyes: . Depending on how fast you are able to get the boat to go . anything below 60mph ,10inchs would be a advantage but its not going to climb out of the water and fall over ! Think about a pair of ski's how wide are they ? and thats just to hold your weight up ! a boat is a lot heavyer than a person !! put things into prospective as to how fast and how heavy then think about the amount of lift you are lookin for and do the sum's. Dont forget the trim angle of the hull and how much hull is actually in the water at speed . you could be amazed at what you find:?: . I always take video from the shore at as close to water level as you can without getting wet. The boat traveling at differant speeds and differant trim angles . Put the tape through the computer and freeze frame and analyze what you are looking at . You can learn heaps from just doing simple basic things . :P

Well, I said that's where I generally start. I didn't mention going wider from there depending on the situation, my bad... Yes, if you're not expecting to go very fast a wider flat would be better. The ski analogy is a good one but obviously the flat isn't the only lift variable. If you put a 10" wide flat on a fast 20' hard chine boat it will be faster but in rough conditions you will feel it in a bad way. Boats are a series of compromises.

[tunnels]

Well, I said that's where I generally start. I didn't mention going wider from there depending on the situation, my bad... Yes, if you're not expecting to go very fast a wider flat would be better. The ski analogy is a good one but obviously the flat isn't the only lift variable. If you put a 10" wide flat on a fast 20' hard chine boat it will be faster but in rough conditions you will feel it in a bad way. Boats are a series of compromises.

No speed have been forth coming so its all just speculation and speed in rough water is a hard one to work around whats good in smooth water maybe not good in the rough so how about telling us what speeds you are working at !! . Everything ends up a balancing act ! what works for one person is not suitable for another so its finding whats best suited for your needs . :D

[FlyingTime]

No speed have been forth coming so its all just speculation and speed in rough water is a hard one to work around whats good in smooth water maybe not good in the rough so how about telling us what speeds you are working at !! . Everything ends up a balancing act ! what works for one person is not suitable for another so its finding whats best suited for your needs . :D

I was just responding to Wayne's question... The boats mentioned are all very fast. Very generally I would say around a 10" flat is what I would use on a 30 to 40' offshore go fast capable of speeds of about 60mph. Again, many other factors to consider. The 1" for every 5' proportion mentioned earlier was told to me by someone much wiser than myself...

[Wayne Grabow]

FlyingTime, tunnels, Jimboat,

I appreciate all your comments, and they make a lot of sense. In asking about keel pads, I simply wanted more information because I had found little so far. In boating terms, it appears to be a fairly recent design feature. As yet I have no definite project in mind, but thought that it might be an interesting feature to incorporate in a future hull design project. As a hobby I have created a variety of personal boats and am now doodling (not sketches, mathematical calculations) which might result in a future project. I am currently finishing a variable deadrise hull meant for more modest speeds.

I understand that there are no hard and fast rules, and a variety of factors need to be considered. Just wanted to make sure that I wasn't missing any important factors or conclusions.

[tunnels]

FlyingTime, tunnels, Jimboat,

I appreciate all your comments, and they make a lot of sense. In asking about keel pads, I simply wanted more information because I had found little so far. In boating terms, it appears to be a fairly recent design feature. As yet I have no definite project in mind, but thought that it might be an interesting feature to incorporate in a future hull design project. As a hobby I have created a variety of personal boats and am now doodling (not sketches, mathematical calculations) which might result in a future project. I am currently finishing a variable deadrise hull meant for more modest speeds.

I understand that there are no hard and fast rules, and a variety of factors need to be considered. Just wanted to make sure that I wasn't missing any important factors or conclusions.

Hi
I understand what you are about ! I have a sheet of board painted white up on the wall in my shed and thats where i do my doodling , that way i can draw full size, because some things in small scale look ok till you get them actual size and then you can see problems that are not possible to see on paper . The pad thing has been aroung since the 1980s here . as it was some where in that era that Motors became more efficent and more powerful etc etc .
Myself i have built boats with a combination of pad and strakes to get to where i wanted to be . The pad was to help lift the hull but only as far as the strakes were and no further . Example !,The pad was 6inchs wide and the 2 strakes 18inchs away from the pad on each side and they were 4 inchs wide each one with a 1.5 degree turn down . the pad was 70% of the length of the hull from the chine at the front and the strakes 50% of the same measurments . Its was a 14 foot INFLATEABLE and used to really move and turn and go back the other way in its own length if you were game enough to hang on and not get thrown out , I also had 1 inch of rocker built in to the keel but the rocker was more near the front than anywhere else . The high point of Rocker can be shifted anywhere along the lenght of the keel to perform differant tasks depending on how you want the boat to sit at speed and how quick you want it to turn . this one needed the bow up a little but to track true and straight in rolling waves and breaking surf and be able to bowlift quickly and ride over rolling water and not loose any speed or via off along the face of a wave when running 45 degrees to the wave ( Broach or bow steer sort of) . :P

[FlyingTime]

[QUOTE=Wayne Grabow;367010]FlyingTime, tunnels, Jimboat,

I am currently finishing a variable deadrise hull meant for more modest speeds.

Variable deadrise as in Carl Moesly's Seacraft designs?

[tunnels]

A boat designer and boat builder friend and my self used to design with the though in mind always of what will be the max speed this boat could do ! not what we wanted it to do . we always went a couple of steps further ! As i said used to always draw out full size even up to 25 feet. Its so easy and you can stand back and look at your creation and veiw it from all angles . As a kid i drew boats and made models as a hobby till i left school and them started work and started building my models full size . i Learned a lot in a very short time .
The Carl Moesly's design principles are good and his methods relavent to todays wants and needs , speed is the thing people are wanting more and more . I am no exception to this i want both ends of the chart , a good sea boat in moderately rough choppy seas and reasonable big swells to a fast get up and go fast in smooth water , This is the hard one to satisfy ,whats good for one situation is not good for the other so after building race boats i do a combination of pad and strakes combinations and have been able to get what i like . The wide pad can be not stable at hight speed but can get the hull to high out of the water and a balancing act , to me The starkes you see people use are always in the wrong place and little and do nothing , Theres no use having something that doesent work or is in the wrong place and causes problems .Strakes in the first 1/3 of the hull are usially causing more problems than they are meant to solve . A well designed chine will difflect water and spray but strakes should be there for the purpose to help lift the hull and keep the hull stable ,so they need to be much bigger and more powerfull in there design , placement and use . And as for starkes hindering the hull turning etc there ways round that and you can have all the sideways slip you want in the rear end yet still have a wide strake with capabilities and the lift for straight running .:p

Lots of things have changed over the years but everything stays the same!:confused:

[Wayne Grabow]

FlyingTime: "Variable deadrise as in Carl Moesly's Seacraft designs?"

No, variable deadrise as in early 1900's hull design. I spent some time at the boat museum in Clayton, NY, studying what designers did, back when engines were heavy and of modest power, to get the most speed out of a hull. They were all variable deadrise designs. Since I was designing for modest speeds, 10-20 mph, I thought it might be applicable.

tunnels: You obviously have a long-time interest, extensive experience, and an enjoyment of boat design. Your comments are a shortcut to my learning, although you are looking at different criteria than I have. Each boat is a compromise to suit specific conditions. Here, I am designing for the US mountain west lakes. I have switched from sail to power because the reservoirs in mountain valleys have uncertain, swirling winds that can make sailing frustrating. At the same time, waves have little chance to build up, and the size of the lakes are modest, but the scenery is beautiful, forests and snow-capped mountains. So I don't need a sea boat, and high speed just gets me to the other end of the lake sooner before we turn around. Still, it might be nice to get home quick when the weather changes.

It will be interesting to see how the variable deadrise handles, and then I may try a higher speed design exercise. Sculpting a hull in wood is satisfying.

[tunnels]

FlyingTime: "Variable deadrise as in Carl Moesly's Seacraft designs?"

No, variable deadrise as in early 1900's hull design. I spent some time at the boat museum in Clayton, NY, studying what designers did, back when engines were heavy and of modest power, to get the most speed out of a hull. They were all variable deadrise designs. Since I was designing for modest speeds, 10-20 mph, I thought it might be applicable.

tunnels: You obviously have a long-time interest, extensive experience, and an enjoyment of boat design. Your comments are a shortcut to my learning, although you are looking at different criteria than I have. Each boat is a compromise to suit specific conditions. Here, I am designing for the US mountain west lakes. I have switched from sail to power because the reservoirs in mountain valleys have uncertain, swirling winds that can make sailing frustrating. At the same time, waves have little chance to build up, and the size of the lakes are modest, but the scenery is beautiful, forests and snow-capped mountains. So I don't need a sea boat, and high speed just gets me to the other end of the lake sooner before we turn around. Still, it might be nice to get home quick when the weather changes.

It will be interesting to see how the variable deadrise handles, and then I may try a higher speed design exercise. Sculpting a hull in wood is satisfying.

If you are wanting to see something that evolved a long time ago .See what you can find out about the older Eskimo canoes and check out the bow sections and there design and see if you can find the story about them .
We think we are really clever but are we? maybe the ancients were the clever ones . :confused: :P

[stuartc24]

I am a green boat owner, looking to solve a challenge with some strakes and dragging at planing and seeking some more informed input. Pictures attached

I have found that the boat will plane quite quickly (40 hp) and is relatively stable at 16 kts at approx ½ throttle. Any faster and the boat will drag dramatically if I get any lean on it or am in chop – I presume as these strakes dig in and the boat pulls. I have not yet been game to try 100% throttle.

The boat is stable at rest but not the quickest turner.

I am a little bit suspicious that these strakes have been added post manufacture when comparing the welding. If you look closely, there is a spray chine running the length of the boat and these strakes fit just inside the chine.

I am trying to get a handle on the best solution – persist and forget about flat out boating, reduce the length of the strakes or cut them off altogether.

I will be interested in what you think.

[tunnels]

With things that look like that the only thing i can see they would do is hold the hull in a straight line when its going . Must be a pig of a thing to turn at speed !!!:confused:
My choice woild be to simply chop them off and smooth the welds off , it will make the hull turn quicker ,running at a angle to the swells it could drift off line if you are into big waves . :p
People do things without much thought most of the time .if you want tracking you add a keel not those kinds of silly things . :P

[FlyingTime]

I am a green boat owner, looking to solve a challenge with some strakes and dragging at planing and seeking some more informed input. Pictures attached

I have found that the boat will plane quite quickly (40 hp) and is relatively stable at 16 kts at approx ½ throttle. Any faster and the boat will drag dramatically if I get any lean on it or am in chop – I presume as these strakes dig in and the boat pulls. I have not yet been game to try 100% throttle.

The boat is stable at rest but not the quickest turner.

I am a little bit suspicious that these strakes have been added post manufacture when comparing the welding. If you look closely, there is a spray chine running the length of the boat and these strakes fit just inside the chine.

I am trying to get a handle on the best solution – persist and forget about flat out boating, reduce the length of the strakes or cut them off altogether.

I will be interested in what you think.

I agree that they should be removed... I'm wondering why someone went through all that trouble to put them on in the first place. After removing them if you find the boat to be unstable while running or not planing very well you could add speed rails. Basically, aluminum angle about 1/2" tall max for a boat this size to correct any issues. But first, definatly take those things off!

[tunnels]

These things are not really strakes but stabilizers and the drag will be quite high ! CHOP THEM OFF:D

The other thing also a 40 hp will be struggling with not only pushing the boat but a couple of stoppers along the bottom . If they had been a proper strake they should have slightly enhanced the performance not slowed it down and also not made a lot of differance to the turning of the boat at speed . If its tracking and wanting to go in a staight line then adding to the keel would have been a better option , theres only one keel so half the problem to add to .
I didnt read anywhere the length of the boat but more hp like 50 or a big foot 60 and those thingies wouldnt have been such a problem . Small boats die very quickly when they are underpowed and loaded with lots a heavy gear and they can be dangerous to if you get caught in bad weather because they simply struggle to make head way in punching into head sea and the possabilities of getting swamped in a following sea and not being able to stay ahead or a rolling /breaking sea when things get bad like they do sometimes . .

[stuartc24]

Thankyou for your input. I thought this would be the case. I think I may move the boat on and try again - otherwise chop chop.