Hull design for a small displacement boat

[mwatts]

Okay. Here's the first shot at a tri-hull then. As a picture says more than a thousand words, I'll start with those:





The hull form above the waterline is exactly the same as it was in the old design, apart from the fact that the transom is at a 12 deg angle, and the old one was straight. This was done to create more room for the (outboard) engine. The old design actually didn't have room for more than a 4HP. Mainly due to insufficient space for the power head. By tilting the transom, and shortening the center keel, there is now space for a 25HP.

As for the engine is concerned: the transom inside the boat, is what the outboard will be mounted to. The hole in the hull will of course have side walls (or else we'll sink), but I left these out for the moment, because they would obstruct the view. The disc you can see in the rendering, represents the exact position of the propellor, if a 25HP Yamaha would be used, and the engine is trimmed at 0 deg.

The bottom of the main hull is now entirely flat. There apparently wasn't any added value in having a V at the transom.

The center keel was modified to cause less turbulence at the propellor.

As is clear from the top view, the outer keels provide a "tunnel" for the water to flow to the propellor. The outer keels extend aft past the transom. I had the idea of using them to rest a swimming platform on.

If the waterline is at the chine, displacement is approx. 867 Kgs ( = the displacement of the keels), with a draft of 40 cms. I recon the boat will have a final displacement anywhere between 900 and 1000 Kgs, meaning the waterline will be somewhere about 0.5 - 1.5 cms above the chine. This means that about 90% of the displacement will come from the keels.

Every person (of 80Kgs) that enters the boat after that, will have her sink in an extra 1.0 cm.

Nevertheless, I think 1000 Kgs is assuming she is built quite heavy. At the moment (what's in the picture), a little less than 24 m2 of steel was used. If this were to be 4mm thick sheets, the whole construction would weigh 768 Kgs. With 3mm sheets, it would become 576 Kgs. Of course, we need to add 50 Kgs for the engine, decking, 95 Kgs for the captain (me) , and there are things to add like stations, etc. But it the end I think it should be possible to stay well under a 1000 Kgs...

So, this is it. Fire away I'dd say.

[Guest625101138]

Points to consider:
1. The level line of the chine might not be the best. Have you thought about raising the line toward the bow. Get it above the waterline towards the bow.

2. The lack of rake on the bow of the side hulls mean they will collect any debris and collide heavily with anything solid in the water.

Can you post the igs file?

Rick W

[mwatts]

Originally Posted by Rick Willoughby

Points to consider:
1. The level line of the chine might not be the best. Have you thought about raising the line toward the bow. Get it above the waterline towards the bow.

2. The lack of rake on the bow of the side hulls mean they will collect any debris and collide heavily with anything solid in the water.

Rick, I will look into both issues. I think (2) should be easy to fix. I'll have a look what happens if I curve the chine up at the bow.

Another issue I noticed, is that there might be a problem manoeuvring the boat, as a rudder at the most common position, behind the prop, would direct water against the side hulls. I'll have to think up something for that.

Also, I want to see if I can make the transition from the keels to the main hull more fluent, as there are some very narrow passages there now, which will cause a lot of friction. However, it has to remain a boat that can be cut from steel and be welded together by an amateur.

I hope that these alterations will also make the design a bit more refined, as I find that the finesse the first design had, has been lost somehow in this version.

And I want to calculate CB tonight, because I'm wondering where that will be.

I'll send you the igs by mail this evening.

[Guest625101138]

Martin
I think it will be difficult to steer in any event. Those outboard hulls will act is bilge keels and will not want to turn readily.

This is quite a radical arrangement. It is difficult to predict all behaviors without going through detailed analysis. Sometimes it is just as easy to build a model and watch how it behaves.

In following seas the boat could be difficult to control.

Rick

[mwatts]

Originally Posted by Rick Willoughby

In following seas the boat could be difficult to control.

Well, I'm designing the boat primarily for the sheltered waters that can be found a lot in The Netherlands. It's got to be able to sail swiftly without making much stir (with as little wake as possible). 20 Km/h (11 Kn) can be seen as the maximum required speed, as anything above speed that requires a license in the Netherlands.

However, there are some area's with quite a bit of current. It should be possible to manouvre the boat downstream.

I suppose we could always stick a bow thruster in the center hull...

[FAST FRED]

The shape shown is really close to the old Atkin hulls .

It would be interesting if the training wheels could be added after a sea trial with out them.

Then weather the added surface area is worthwhile , and stability at anchor could be assessed.

FF

[mwatts]

Update: I've decided not to procede with this last version. It just doesn't have the appeal to me the first version had. I'm going to try and design a more curved, sophisticated one.

Originally Posted by FAST FRED

The shape shown is really close to the old Atkin hulls .

It would be interesting if the training wheels could be added after a sea trial with out them.

Then weather the added surface area is worthwhile , and stability at anchor could be assessed.
FF

Fred, what do you mean with training wheels?

[mwatts]

Well, here's the next version. After some fiddling about, I decided to work from the main lines of the previous version after all, but make changes all over the place. Here's the result:








What has been changed:

• The chines now curve upwards at the bow.
• The main hull has a V shape at the front section of the boat (gradually getting less until the bottom of the boat is entirely flat at the engine mount).
• The center hull has been reshaped, as a result of creating a V in the main hull.
• The outer hulls have been remodeled, and are now curvier. Also, the sides of the outer hulls nog follow the line of the sides of the main hull, instead of going straight down.

Because of the V shape of the main hull, total displacement is now approx 1000 Kgs. Of this, exactly 800 Kgs (80%) can be contributed to the keels. Weight of the hull (without engine, decking, fuel, etc.) is now 800 Kgs for the 4mm version, 600 Kgs for the 3mm version.

[daiquiri]

It appears to me that the new version has a lots of sheets with double curvature. Hence not developable and not easy to make. Actually, even the initial hull had some double-curved sheets, though to a somewhat smaller extent.
If it is so, it will go against your goal (post #39) of simple and cheap construction, suited for average-skilled builder.

[Ad Hoc]

If you're concerned about speed, add small radii at the bilges of your hulls, since these hard corners will cause drag owing to separation of flow.

Also, what is holding your 4 or 3mm plate together?....your calculation is just the skin, so what is holding the skins together?

[mwatts]

Originally Posted by daiquiri

It appears to me that the new version has a lots of sheets with double curvature. Hence not developable and not easy to make.

Yes, you're right. I have to look into this, and try to remedy these areas. However, I think building a boat like this, instead of a simple 5 sheet construction, will be more challenging anyway, so it will be some sort of consession between form and ease of construction.

Originally Posted by Ad Hoc

If you're concerned about speed, add small radii at the bilges of your hulls, since these hard corners will cause drag owing to separation of flow.

Also, what is holding your 4 or 3mm plate together?....your calculation is just the skin, so what is holding the skins together?

I will round of the trailing edges, like you say.

What's holding the plates together? At the moment: welds. I am purposely leaving stations and such out at the moment, so I can modify the hull shape easily. I will be drawing them in when I have decided on the final form. The plates currently used, weigh about 600 Kgs all together. The displacement of the boat, if the waterline is at the chine, is somewhere around 1000 Kgs. That means I have about 400 Kgs to play with for adding engine, decking, stations, etc. etc. When, in the end, these have all been added and it's still lighter than the intended displacement, I will reduce the draft of the keels. This can still easily be done in a late stage of designing. And it will reduce overall draft.

[Ad Hoc]

Yes, i realise they are welded. What i mean is where is the structure supporting the plating?..since these plates by alone are not enough. You'll need around 60~100% extra on top of the 600kgs, for internal stiffening etc.

[mwatts]

Originally Posted by Ad Hoc

Yes, i realise they are welded. What i mean is where is the structure supporting the plating?..since these plates by alone are not enough. You'll need around 60~100% extra on top of the 600kgs, for internal stiffening etc.

I know. That's what I refer to as "stations"? If that's not the correct term (and confusing people), what is? I am Dutch, so I sometimes have to look up English nautical terms on WikiPedia you see.

Isn't 60 - 100% extra for stiffening a bit much though?

Originally Posted by daiquiri

It appears to me that the new version has a lots of sheets with double curvature. Hence not developable and not easy to make.

Daiquiri, one of the most problematic surfaces are the sides of the central hull. I actually tried to design these to be straight verticaly, however, Rhino creates bulging surfaces instead. I'll have to fiddle with this to get is right. It would by far be best, I think, if each side can become one developable surface. Do you have any suggestions?

[FAST FRED]

The Amas on either side , that make it a tri are what I call training wheels.


Compared to the origional model Atkin , the pods (Amas) might only increase wetted surface and drag. They would be far more complex to create , yet the only reason would seem to be extra stability when stopped.

Have you visited, www.atkinboatplans.com/ and actually looked at some of his box keel designs? They were refined for a couple of decades and might be worth a look.

Start with "Rescue Minor"

FF

[Ad Hoc]

The internal stiffening members are called frames and stiffeners or longitudinals.

No, the 60~100% is about right depending upon your final arrangements.

For mono's it is about 100% and catamarans around 60%