Hull design for a small displacement boat

This is a box keel type of hull form and the "box" portion needs refinement. As Tom mentioned, the after portions of the box will cause a good deal of turbulence. In fact, this box shape would work much better if it was turned around 180 degrees.

Typically, in these hull forms the forward sections of the box blend into the V sections of the main hull. The box usually becomes defined around midship, tapering aft. It also usually has more volume as a percentage of displacement then shown here.

 

Martin
I have done the power estimate. It will require around 6.4kW on the hull to do 10kts. Taking a 20HP motor with a 10" prop it will use around 10kW for driving the hull and overcoming prop losses. You could add another 1kW for windage and some turbulence at the bow so a total of 11kW.

There is a problem though. I have attached the wave patch image and you will see that there is quite a large trench behind the central part of the hull. You will not operate an outboard in this unless it is set very deep. The prop would need to be below the line of the central hull or there abouts.

I looked at the optimum hull for 10kts and 5.2m long (allows 0.3m for the outboard. It requires just 3.3kW on the hull. However it has quite ridiculous proportions.

I also looked at constraining the draft to 0.4m and optimising with that constraint. This hull required 4.2kW. With a 10" prop it requires 6kW at the motor to overcome hull drag and prop losses. Again you would need to add about 1kW to allow for windage etc so say 7kW.

This hull shape has the same problem as yours - a big trench behind it. I played around a little to try to improve it. Made some gain but nothing that I have tried worked particularly well. It probably requires a tunnel of sorts to feed water up to the prop. The Atkins hulls have reverse "V" to achieve this. This might work.

The lowest drag practical hull is a rectangular section 400mm deep and 500mm beeam. It can have quite a bluff entry. Probably work best with a bulb at the bow but I have not looked at this. However you need to do something at the stern for the outboard to be placed in the middle.

The weight is already 625kg for the basic shell using the 4mm plate. This is extremely heavy for a little boat. By the time you do deck and the like it will be well over a tonne without passengers. I personally would not consider steel (having built a steel dinghy a long time ago) but it does produce a cheap boat. There is a recent thread where a fellow has built a steel canoe. It looks quite good but I bet is heavy compared with most canoes.

If you get it lighter the problem with the wave trench will not be as bad. I would aim to get it lighter first then fiddle with the aft end shape.

Rick W

 

Rick try it with twins . . .

 

Box keel boats are very successfull in Australia

http://www.duckflatwoodenboats.com/mundoo/pics/large/Mundoo2CruisingFamily.jpg

http://www.duckflatwoodenboats.com/mainpages/mundoo.php

 

A cat is a better option if it can be built for the same weight. It requires 3.6kW on the hulls or 5.2Kw on a 10" prop.

The hulls are deep and narrow - 420mm deep and 270 wide. These would be very easy to build of course but then they need to be connected.

Rick W

 

On the 30 footer (9.1M) I modeled, I thought that a pair of small engines would avoid the turbulence problem Rick mentions. It would also allow the keel to have a higher L/B ratio. Twin engines are not really so practical for your little skiff. Perhaps the Atkin tunnel hull would be the better overall design for you.

No matter what you choose, lighter weight would improve it.

 

According to harmonised standard ISO 12217-3 all open boats under 6,0 meters and under Lh = 4,8 m decked or partly 2/3 decked boats shall be able to float in level when filled with water. So this one has to be decked or partly decked one unless self builded for own use at least for 5 years (in EU).

I would consider placing inboard engine, batteries and fuel tanks at the bottom of the "box keel" to improve stability. There are also some semi-planing models with box keel. They say somehow this gains boats "virtual" LWL. Who knows.

 

Thanks all for replying. I did not expect the discussion to be so active. And thanks for the upbuilding criticism.

@Rick: on the upside, I am happy that the target of 10kn with 15hp (11kw) has been reached on the first attempt. I tell you: hardly any small steel boat around here would keep up.

On the downside, the huge trench is a big problem. I hate big trenches. I don't want people to be able to look through under my boat when I pass by. And most important: it will cause cavitation on the prop, which is BAD...

However, how did you estimate 625Kgs for the basic shell? My calculations using a combination of 3mm and 4mm steel (weighing 24 Kg/m2 and 32 Kg/m2 respectively) were a good 100Kgs less.

Nevertheless, some work has to be done. Based on the feedback I have received here, I'm looking into:

• Adding a little more draft to the keel
• Changing the form of the keel to provide more displacement towards the front of the boat, and avoid turbulance at the back
• Maybe add some sort of keels to the side of the boat (on the chines) to add displacement and guide water to the prop.

Unfortunately, I will not be able to do any designing or posting on the forum the coming week. I hope the thread will become as lively as it was, when I get back on the forum.

Martin

 

Martin
I used 4mm all over as this is what you initially said you were using. If it was 3mm all over then it would be 468kg. So if you have a mixture it will be somewhere between 468 and 625kg.

You also need to consider what weight will be in decking. It will need some method of stiffening the gunwale as the hull will twist without this.

If you can get it lighter then the waves are less. But pushing a displacement hull 5.2m long at 10kts will make waves.

The catamaran is worth serious consideration. It could be done with 6 plates without decking. The hulls sealed up for buoyancy. It will require about half the power.

Rick W

 

Rick,

If the power required to drive the keel only at 10kts to 14kts is added to to the power required to drive the upper lightly loaded hull at the same speeds, could this give a first approximation to the total power needed? I know there is a dynamic interaction between these two elements of the hull but what is wrong with the fundamental idea? That is, the keel is acting as a displacement hull while the upper hull is planing, assuming the loading on the upper hull is low enough to plane at such a low speed.

Underbodies of many boats already have portions that work in these different regimes at the same time. Planing lobsterboats are examples. They are perhaps a poor example of this configuration, but were not built with that in mind. From my point of view, this is the main reason for building such a box keel boat. If that has no merit, why build a full length and deep box keel like this?

 

Tom
The drag estimation for the hull included the upper, wider portion. I did not try to allow for sinkage but it should not be too bad because the wide stern would be on the longitudinal wave from the central hull.

Just a note - when I exported the igs file to Michlet the transom plate was below the line of the hull so was like a brake. I had to make a slight mod in the Michlet file to get it to work properly.

The cat and single hull I looked at were only the hulls because I assumed all buoyancy in these hulls.

For 10kts on 5.5m length you are better going to narrow cat hulls. They still create a trench in the middle though. I did not try to reduce it.

I consider it heavy. If it is made lighter then some of the problems diminish.

It could probably work with a tunnel but I need to play with this more. I think with side plates acting as a shroud it would be OK.

Rick

 

All

Don't get too excited...the CFD doesn't take transom sterns into consideration, one of its many limitations. At LWL of 5.5m, your Fn is 0.7 at 10knots....this is another beyond the limitations of realistic results of this CFD. The results are "only indicative" at best, not absolutes at all. But if you wish to believe them, that's your prerogative.

"..It will need some method of stiffening the gunwale as the hull will twist without this..."
Sounds like you've been reading old big ship design books, or simply do not understand how structures and load paths work!

 

It would help (and be more fair) if quotes are attributed to the author. No fun digging through the whole thread to find the context, which I did not find.

Ahh, I found the quote but the thought about crediting quotes lingers on. I hope that that post is not the first salvo leading to destruction of an interesting thread.

 

I'll give that answer one out of three, John.

1. Michlet is not CFD.

2. You don't know how Michlet models transoms.

But you get one point for:

3. Its results are only indicative. I think you meant to say... insofar as a consistent linear theory is used to model the wave resistance including the transom stern. The "hydrostatic stern" resistance of the (assumed) fully-dry is accounted for.

Not bad considering you are guessing.

 

But Michlet does not include near-field effects so the "trench" might not be as pronounced as the graphic suggests. Using Michlet to predict wave elevations that close to the hull is absolutely, unconditionally, inappropriate.

Cheers,
Leo.