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  #1  
Old 07-31-2009, 02:36 AM
mwatts's Avatar
mwatts mwatts is offline
Martin
 
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Hull design for a small displacement boat

Hi all.

I have owned a few small boats, and decided that the next one, if at all possible, I would like to design and build myself.

In the area I live, there are an awfull lot of small steel boats used. The design of these boats probably goes back 150 years or more. At that time, they probably where a very good design for the purpose (small vessels for moving a considerable amount of cargo through shallow waters at low speeds). However, if you put 4 adults in them, and hang an outboard on the transom, you quickly see how inefficient they are. A 5 hp outboard will push them towards their hull speed very quickly, producing a massive wake, while not going ny faster than 6 Kn tops.

So I want to try and do better than that. I would like to build a small boat, that can carry a few people, for recreational purposes (swimming, socializing). It will be built in 4mm steel (because it's available, I know how to work it, and it can be provided as a "kit" so others may build the same boat too).

This is what I came up with:





These are all very early images. I haven't drawn any stations or strings just yet (I know they are required ). The idea is that this hull can be assembled from just 9 pre-cut sheets of steel.

LOA = 5.5m, LWL = LOA, Beam = 2m, Draft = 0.35m. Displacement (rough estimate) approx. 0.7 metric tons.

As you can see, there will be a long, narrow, canoe shaped keel under the boat, which will provide a large part of the required displacement. I read about this idea from Tom Lathrop in this thread: Displacement Speed Question

I'm curious to know that you all think of this design, and if I should venture along this path, or start over.
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  #2  
Old 07-31-2009, 03:01 AM
ASM ASM is offline
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Martin

What a nice design ! I am Dutch too, but to keep it readable for the rest of the world I reply in English. You seem to be handy ni designing and usign software. Would this design be buildable in plywood and could it be easily lengthened to say 7 meters, keeping the width more or less the same ?
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  #3  
Old 07-31-2009, 03:41 AM
Guest625101138 Guest625101138 is offline
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Quote:
Originally Posted by mwatts View Post
.....
I'm curious to know that you all think of this design, and if I should venture along this path, or start over.
I would say it is worthwhile pursuing.

The 4mm steel plate gets you into battering-ram territory. 3mm would be more than adequate for this size hull.

Why not go all the way with the central hull and place little outer hulls on the chine for stability. These could be made from something other than steel. Set the bridging plate 50mm or so above the water level.

Need to make it unsinkable with sealed or foam filled volume preferably up high under deck covering.

Need to paint very well to avoid rust issues.

Have you a target speed and load in mind?

Rick W
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  #4  
Old 07-31-2009, 04:12 AM
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mwatts mwatts is offline
Martin
 
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@ASM: Thanks for the compliment.

I guess it could be built with plywood. The design is in an early stage. There isn't anything there yet that couldn't be done in plywood. But when the stations and other structural re-inforcements are designed, material choice will of course be important.

I don't see any reason why it couldn't be lengthened to 7m either. Beam is 2m, and because you can put heavy stuff (batteries, fuel) in the keel, it will remain a stable boat.

@Rick: other people have mentioned using 3mm steel too. But actually, building small 4m boats from 4mm steel, is not uncommon around here. The advantage is of course that 4mm steel lasts longer (after a good number of years in the water, 4mm steel will have become 3mm steel) The obvious disadvantage is weight. Maybe I will use 3mm sheets above the waterline, and 4mm below, as an alternative.

I will be adding some air tight / foam filled compartments.

What I had in mind, was putting an outboard right behind the central hull. This means there will be a sort of small transom in the boat, and a hole in the bottom. The outboard will be in a small compartment. In dutch, we call this a "bun". I don't know the english term for this. The reason is comfort: it will keep the noise from the engine down. Also, it will give the feeling of a boat with an inboard, while retaining the flexibility and simplicity of using outboards. I am designing the "bun" to be large enough to accommodate a Yamaha F20 (20hp 4-stroke).

With the 20hp, I would be very happy with 10kn. No idea if that would be realistic though for a boat this size, about 900 Kgs with passengers.
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  #5  
Old 07-31-2009, 04:30 AM
ASM ASM is offline
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mwatts,

Nice to see a quick answer. Would you mind sending me an e-amil so we can discuss further in Dutch ?

The front flat triangle shaped bow could be a perfect fixation for a full ruber 'bumper' which one can make with a poorable rubber set in a mould. Good for those chanel trips in Holland and some of the incompotent drivers...

@Rick: HI Rick, the option of the two little outer hulls is something I like to investigate too, back to the tri-faux !
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  #6  
Old 07-31-2009, 04:34 AM
Guest625101138 Guest625101138 is offline
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If you care to post an igs file of the boat I can do a performance prediction for you. If the majority of the displacement is in the central hull it will be quite accurate.

I have worked on similar concept using my pedal boat as a test ground but with an enclosed portion. My aim is build very light using carbon fibre for solar-wind power. You will laugh if I tell you the estimated power to do 8kts.
http://www.boatdesign.net/gallery/sh...//ppuser/18624

Rick W
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  #7  
Old 07-31-2009, 04:35 AM
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daiquiri daiquiri is offline
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Hello,
It is a sort of box-keel concept. Why don't you want to use aluminum? If an increase in speed is your goal, any reduction of weight would be very desireable.
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  #8  
Old 07-31-2009, 04:45 AM
ASM ASM is offline
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HI Daiquiro

I think the steel is chosen as the readily available material and traditional Dutch boatbuilding material. Easier to weld for most people too if a kit is being made in time. Am I right mwatts ?
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  #9  
Old 07-31-2009, 04:49 AM
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mwatts mwatts is offline
Martin
 
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@ASM: I can't send you an email using the forum. You have the option turned off. I've turned the option on in my profile, so you can email me.

@Rick: I suppose Rhino will let me export an igs? If so, I can probably send you an igs this evening. Can you email me so I have your address?

@Daiquiri: Yes, aluminium would save weight. The problem with aluminium is that it is A LOT harder to work with. Aluminium welding equipment is not within my (financial) reach. Also, I would like other people to be able to build this boat. So I have to assume it will be built in steel. However, if someone would ever want to build one in aluminium, I wouldn't see why not. I'dd be curious to see the result.

@ASM: yes, if the design proves to work, I would like to provide it as a kit. Actually, I live right opposite a shipyard (literaly 20m away). They build large traditional dutch barges (binnenvaartsschepen). They have a CNC machine for cutting steel. And I know the owner... See where this is going?
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  #10  
Old 07-31-2009, 05:00 AM
ASM ASM is offline
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mwatts...

I see your plans !! where are you located , I might know the shipyard....
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  #11  
Old 07-31-2009, 05:17 AM
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mwatts mwatts is offline
Martin
 
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I live in Zwartsluis, right opposite "Scheepswerf H. Poppen". Know it?
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  #12  
Old 07-31-2009, 05:23 AM
ASM ASM is offline
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I know Zwartsluis....I do not know Poppen... anyways, I am into plywood since I can not weld !
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  #13  
Old 07-31-2009, 11:03 AM
tom28571 tom28571 is offline
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Martin,

Have you worked out the hull weight and waterline? The volume in the keel and the weight of the hull need to be compatible or the speed may be very limited with the power you want. I suspect that more displacement may be needed in the keel. The object of this configuration (from my perspective) is to have a boat that runs at 2 to 3 times displacement speed at low power. That will be compromised if too much of the displacement is carried in the upper hull.

The rendering looks great. The keel and hull bow sections are very sharp forward, looking like higher speed is desired. The keel sections aft do look kind of full though and may cause too much disturbance for fitting a propeller just aft of the keel.

Very interesting and we will be following your progress.
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  #14  
Old 07-31-2009, 02:15 PM
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mwatts mwatts is offline
Martin
 
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Hi Tom,

Thanks for replying! And thank you for providing the inspiration for this design.

I have just done some calculations.

At the moment, displacement is 0.78 metric tons, with the waterline exactly on the outer chines. The displacement in the keel is 388 KGs, thus half of the displacement of the entire boat.

If I build the hull entirely from 4mm steel, it wil way approx. 556 KG. If I use 3mm steel for the sides (above the waterline), it can be build for approx. 511 KG. That's 45 KG savings. I guess that's more than the weight of my two kids together...

Of course, in the end it will weigh a bit more, because I yet have to add stations, engine mounting, etc.

So you would advise trying to get the aft keel sections thinner / sharper? That would cost some displacement of course. I think I will wait just a little longer before I adjust it. It might be possible to place the engine further aft, if I allow the rudder to reach out behind the transom. That would probably secure adequate water flow.

Right now the length / beam ratio of the keel hull is approx. 11.4, which I think is a nice number. I wouldn't like to go any deeper either. Draft is now 35 cms, which is very nice in these area's. I will be able to sail almost all the shallow creeks we have around here.

Hey, just thinking out loud. If you don't agree, please say so!
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  #15  
Old 07-31-2009, 09:30 PM
tom28571 tom28571 is offline
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Martin,

In general, I was thinking of about 75% of displacement in the keel but the ideal ratio might vary with length, immersed upper hull design and construction of the vessel. It does appear that there is low buoyancy in your bow sections relative to the weight forward. A look at buoyancy distribution and weight distribution might be in order. A fuller keel section forward would offset narrowing of the keel sections aft.

These are simple observations and not based on accurate calculations.

One question is: Is there a downside to reducing all hull panels to 3mm or even less in panels where it would be adequate?
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