How to make a 19 foot planing hull efficient at displacement speeds?

Tom, do you remember the source of the information about the model tests of warped hulls which showed high speed stability problems?

 

David, there was a USCG report on stability problems; I will dig a hole in my sediments here, I know it's there.......! Be back when I found it.

 

Ok, here we go:
"A case study of dynamic instability in a planing hull"; Codega and Lewis in Marine Technology, vol 24, no 2 April 1987, pp. 143-163.
It is about the then new surf resque boats, that presented unstable behaviour at high speed.

 

My first notice of unstable behavior in warped plane hulls at high speed was from Lindsay Lord's book "Naval Architecture of Planing Hulls". I built a 4' towing model of my first design that allowed for variation in the amount of warp from a 10 degree midships deadrise in steps of 2.5 degrees down to zero at the transom. With this model I found that warping the transom deadrise to 2.5 degrees developed significant yaw instability beyond 10kts model speed. Going faster caused the model to eventually yaw so badly that it swapped ends and damaged the towing set up. Lord interpreted the instability as a side to side motion but I think he got it wrong because he towed from the stem of his model which suppressed the yawing below the problem found. I towed from the CG of the model which allowed the instability to be much more aggressive, resulting in the end swapping. This why I chose the monohedron hull although I added some significant aft chine surfaces which might act much like warping.

I have not taken the experiments further but do have some personal ideas of the causes. It involves twisting of waterflow by the warping setting up oscillation thrusts which steers the hull in alternating directions. Harmonic motion, I suppose. I would wish that others better equipped might do a more exhaustive study but have not seen any so far.

 

I have towed many warped bottoms and you're right Tom, it's the pressure wave, under the plane patch that increases unequal distribution and compression, as the warp flattens out. Monohedrons do the same thing, but the wave is much less compressed, as it move aft along the hull's bottom, as are the chine bleed and wave isolations. I think the deeper V versions, typically seen on the usual monohedron keep the chines wet, while they tend to dry out on a warped bottom with it's shallow immersion, so they walk on the isolations, which is pushed too far can cause an end swap.

 

Unfortunately I don't have a copy of that paper and it is not included on the SNAME Small Craft CD nor available on the SNAME website. However I have a copy of an article Lou Codega published in Professional Boatbuilder #31, pp 20-28 The Dynamic Stability of High-Speed Boats. which discusses several types of stability problems including the instability of the USCG 30' surf boats which were the subject of the "case study" article. The USCG surf boats had 20 degrees deadrise at the transom, and Codega does not mention if there was a constant deadrise section or if the bottom was warped from the transom. Codega described the cause of the instability to negative pressure developing on the forward portion of the boat due to the curvature of the hull in combination with the longitudinal CG location being too far forward. He does not menton warp of the bottom as a cause.

 

So realistically though, I could probably only add say 2-3 feet to this hull; lets say I kept the extension uniform with the same 12 degree deadrise... In practical terms what differences am I going to see all else being equal?

 

Tom, did you tow test a similar model of your monohedron hull with significant aft chine surfaces design?

 

Based on doing something very similar, I suspect you will see an improvement in performance in the low speed range. How much? The devil is in the details.

 

Yes and no. Not exactly the same model, which was a mistake on my part. Frankly, I had spent a couple years getting to this point and, being satisfied that the chine would help prevent high trim in the hump range, I really wanted to build the boat and get on the water. I found that such experimentation is very useful but it can take up all your time and have no end. In the end, the boat performed so well that the experimentation has not been followed up. As my wife told me at the time, "build the darn boat, I'm tired of sweeping up all the crumpled paper". Being 68 at that time (86 now), I also thought it was past time to actually build a boat.

The chine design is intended to act like a fixed trim tab but with much lower drag. The chine has tapered width fore and aft, is typically about 8' in length and has a positive trim angle of one degree relative to the aft straight and level buttock lines.

 

You will see very little difference. To get your target hull speed, the length has to be about 31 feet. 2-3 feet will have a minimal impact. Unfortunately, those are the laws of physics.

 

Specifically, what laws of physics do you mean?

 

Ok... am I missing something. Usually a small "hump" is desired for shallow water launches with a planing hull. You see sponsons on shallow deadrise boats with outboard surface drives or jet outboards for this purpose. A boat designed like this rarely does anything else really well. Well not as well as some other boats anyway. It tends to slide when you turn, does not have a small enough contact area for top speed, and wants to start climbing on pad at anything much over idle. Its really not intended to be a displacement hull at all, but it does have a small "hump." LOL. I joke my 2050-16 flat bottom is on pad sitting still.

So why exactly do you want to do this interesting thing? You want the fuel range to cruise the world, but the speed to outrun pirates?

 

The narrow Thai speed boats may have some efficiency when run in displacement mode? If so, may give an idea about favorable hull shapes.

PC

 

In answer to Bob - basically, I would like my boat to be better at what I use it most for ... fishing/ trolling at speeds of 8 kts or so ... with a run out and back to/from the fishing grounds.

I guess every boat is a compromise and the more one uses a boat the more one learns what one wants out of it. I am lucky, the fishing is close by, so the runs are short. I am not a speed freak and how quickly I get there doesn't really matter; but even if I was, I don't believe one could comfortably get anywhere faster than say 20kts; as the sea conditions dictate this.

The more I fish on them the more I realize how good a Panga is for this sort of thing; I still think they are pretty ugly, but I can definitely see why they are so popular with people who make their living on the water. Having said that I am also led to believe there are boats which aren't "Panga shaped" - but which can do what I want better than the classic "V-hull center console"...

I don't think what I am asking for from a boat is impossible/ unreasonable - a boat that is fairly efficient at 8 knots but can run comfortably at say 20 knots; but doesn't need much more.

I guess my case is just another one of not having understood boats or my needs well enough to know what I wanted when I got it; and of getting suckered into the marketing hype - top speeds, deadrise angles, beam etc ... where one is starts to believe that the higher the no the better!!

This brings me to a comment made earlier in the thread by Tom...

It amazes me that this is the case; but it truly is. Surely, it is not a sad fact that all the boat designers/ manufacturers (particularly in the US where this is definitely the case) are unable/ incapable of making a better boat for my type of use (which I believe is fairly close to that of the average American pleasure boater/ fishermen on the ocean) - so why is it that we have this oversupply of short, wide, heavy deep V'd center consoles, laden with features, that cannot pull a marlin lure at the correct speed without burning half the King's wages in fuel?!!