Foil assisted box keel boat

This idea with a foil-assisted box keel boat has survived in my mind for a while. I think it might work, so I post it to get feedback. The goal is a 10 m monohull, 3 - 4 tonnes, that will have good fuel economy at 12 -15 knots.

According to my calculations, a box keel monohull where the box keel is designed for a boyancy that corresponds to the minimum displacement of the boat would be quite efficient at minimum displacement. The box keel would be approximately 9.5 m * 0.8 m * 0.5 m (L*W*D). BWL for the boat would have to be larger than 2.4 m to achieve adequate stability with the CoG I get from my weight budget. At minimum displacement the 2.4 m wide part of the hull will just barely touch the water.

When the box keel boat is loaded towards maximum displacement, the 2.4 m wide part of the hull will sink deeper into the water and the efficiency of the hull will be severely reduced.

Wouldn't it be possible to use foils located forward of the CoG that will provide lift and raise the forward part of the hull so most of the wide part of the hull is out of the water? The rear part of the wide hull would be planing and provide stability. I would like to avoid a complicated control system for the foils.

I hope the enclosed pictures clarify my meaning.

Erik

 

The boat may be very efficient, but it would also be very tippy. As soon as there is any heel at all the keel is going to pull towards the surface, and flip the boat over. Go try to hold a beach ball underwater to get an idea of what you are proposing.

Basically your COG is going to be way to high, and the boat will want to roll all the time. Unloaded this will be controlled to some extent by the chines, but I would be shocked if this boat ever floated flat, figure a 20 degree heel at rest, unloaded, since you would have to submerge almost as much of one of the chines as submerged keel. When running it would be like trying to balance on a sea saw, doable but a lot of work. And if you ever get the balance wrong over you go.

In short I doubt this idea would work.

 

Stumble,
I've actually calculated the righting moment vs. heel angle. At BWL 2.4 m I get acceptable stability with the CoG I anticipate. If a more detailed weight budget shows that I've been too optimistic regarding CoG at worst load case then BWL has to increase. This will hurt efficiency in displacement mode. When all design loops are done, the boat will be stable in displacement mode. BWL might have to increase somewhat, but it will definitely be possible to keep BWL to less than 3 m. An advantage with the box keel is that engine and other heavy installations can be located in the box keel to keep CoG lower.

What I'm trying to achieve is to lift the chines out of the water on the forward part with foils as the speed increases and get stability from the rear part of the hull that is planing. I agree with your worry about stability, that is also whats worrying me. It's not only roll stability. As the lift from the foil increases, the pitch angle for the hull will increase which will increase tha attack angle for the foil. This will increase lift even further. The reduced boyancy of the box keel will provide a counteracting term but I don't know if this term is sufficient to achieve a stable system. I'd like to avoid active control of the foil attack angle.

I enclose the Gz vs heel curve I've calculated for the boat at rest for BWL 2.4 m and CoG 1 m above the bottom of the keel.

Erik

 

Erik, theproblem is goign to come when you start lifting the hull out of the water with a foil. Once the wide part of the hull is clear of the water you will definitely have a stability problem unless your foils extend wider than the hull and are preferably surface piercing.

Adding some outriggers/amahs onto the the hull to provide stability will work. THe foil could then span the tunnels between the main hull and outriggers.

 

The bottom of the box keel could be configured to create the lift under way you desire.

Think of a water ski.

The B-K will probably hold the engine , fuel and batteries , with the weight at the lowest point in the boat , it would not be a "Beach Ball".

The ability to take the ground , or transport on the flat box bottom would be a huge plus.

As many places in a harbor are taken , a cruiser could easily find a spot that dries out part of the day , that should be accessible to shore.

I think Atkin solved the problem of pitch control by incorperiating "Reverse deadrise" in many of his box keel designs.

He did design to optomise for one cruising speed , but with moving tabs easily installed today , that should be no problem.

FF

 

In pitch there wont be any stability problem if the foils are correcting designed. There is enough test data on positioning of single or multiple foils on monohulls to design such a system. See following report for example:

G. Karafiath. An investigation into the performance of NSRDC Model
5184 congured as a partial hydrofoil supported planing craft and a comparison
with a powering prediction technique. Technical Report SPD-585-
01, NSRDC, 1974.


The problem is DYNAMIC TRANSVERSE STABILITY.

I have played around with fitting foils onto a 33' planing monohull with L/B = 3.0 (i.e. nice and beamy leisure boat). THe 3 passengers had a lot of fun running from side to side on the boat to act as counter-weight as the boat started to heel over - and this was in calm water of Puget Sound.

The foils need to have a span wider than the hull beam to achieve suitable dynamic transverse stability. Take a look at Rodriquez hydrofoils or any of the Russian versions: Kometa, Olympia etc.

 

The problems with this idea are approach and power. The inclined planning hull form with foil assist has some merit, but not with the shape of that box. Even with the chines touching aft, dynamic lift from the incline box and the foil will cause her to walk unreasonably as the pressure differentials flowing aft from midship will be unpredictable. If the box was shaped differently, you could improve this stability issue, but at a cost of drag. Assuming enough power to make the foil work, it's self defeating to drag the stern.

A better approach is to toss the box and use a more conventional lifting surface, one that will permit the boat to get up quickly with little effort, then a second foil mounted aft. A good starting point would be an incidence angle on the forward foil of +4-5 degrees and +3-4 degrees on the aft foil. This would tend to be self controlling if a constant speed was maintained. The forward foil would stall before the aft foil, so the aft foil would drive the stern up, bring the forward foil back on line. If you expect some flexibility to speed, then some form of incidence control (or lift) will be necessary.

 

The principle of the box being a slender planing surface has merit for efficiency and ride comfort. However as we all know a planing surface should have straight running buttock lines and waterlines - so I fully agree the aft shape will not provide any benefit for a fast vessel and can only cause trouble.

If the intended use of this boat is at displacement/semi-dispalcement speeds then no point to use foils at all and this concept has no merit.

Regarding fitting foils onto a monohull - it is harder than you think to get that working. If the foils are too close to the hull, the negative pressure of the suction side of the foil will be fealt by the hull and the net effect is almost zero lift and a lot of extra resistance. To avoid this you have to position the foils deep under the hull - then they do not operate in the naturally stabilizing free surface effect. You are then back to some form of control system to ensure stability or surface piercing designs. It is interestng to browse the US patents on this subject. Zillions of ideas out there and even a patent by Dan Savitsky on how to fit self-stabilizing foils to a monohull.

Your proposed foil angles do not consider the downwash effect of the front foil which can increase or decrease the effective angle of attack on the rear foil depending on speed and foil separation distance. Normally for HYSUCATs with tandem foil I will use foil angles closer to zero on the main foil and +3 on the rear foil to account for the downwash whcih is quite strong. But it all depends on profile camber, thickness etc.

 

I thank Sottorf, Par, Fast Fred and Stumble for the responses. I realise now that adding foils to a relatively slow box keel hull that wasn't designed for planing isn't a good idea. If I'm going to use foil assistance I should start with a hull form meant for planing. The message from sottorf is that the foils will have to extend outside the beam of the boat, which is another thing I want to avoid on this boat as it complicates handling of the boat.

The way forward would be to use a proper v-shaped planing hull form and use surface piercing foils located forward and extended to the necessary span to get stability and clearance to the hull, similar to the foils shown in sottorf's avatar. That's another boat for another day. Could be a fun project if scaled down to 5 m test rig.

Erik

 

Refreshing to see someone come back and acknowledge they may have overlooked some aspects of their idea, that make it less than a great idea. With an attitude like that, erik818 will go far, even if it isn't in a hydrofoil !

 

"If the intended use of this boat is at displacement/semi-dispalcement speeds then no point to use foils at all and this concept has no merit. "

However the desire is for a modest speed ( SL 2.5-2.8) efficient boat is realistic.

IF The flat portion of the hull could be almost clear of the water airaition devices under the hull might reduce drag even further.



FF

 

For those of you interested in fitting hydrofoils to monohulls, take a look at the International Hydrofoil Society webpage: http://www.foils.org/upright.htm
Lots of interesting ideas from the 1950s onwards.

The International Hydrofoil Society is a good place to get general information on design, histroy, literature etc. They also have a small forum where you can post questions.

 

I've tried to calculate the lift I would get from a flat bottom on the forward part of the box keel, with an attack angle of 3 degrees. At 12 - 15 knots I think I would get around 1500 N. Thats too little to lift the forward part of the hull as much as I had intended with the foils, but it's more than nothing. The argument that a flat bottom of the box keel is an advantage during dry-out at low tide also has merit. There isn't any noticeable tide in the Baltic so I tend to forget this aspect.

For a box keel I can't see any disadvantages with a flat bottom and sharp chines for the forward part. I'm unsure what will happen with the flow if there are sharp chines at the rear part where the chines are pointing inwards. I suspect it will create problems and that rounded chines are preferrable for the rear part, but that's just a hunch.

For the upper part of the hull I've straightened the chines so they run parallel rearwards from the point of max beam to improve the planing performance. I've also increased BWL to 2.8 m for more stability. There isn't any noticeable decrease in efficiency in full displacement mode because of this so I see only advantages.

Erik