Will a "tail" increase my ship's hull speed?

Question

I am constructing a commuter boat with a displacement hull, and just thought of this simple way of increasing hull speed without actually increasing the hull:

What if I simply attached an unbelievably long fin (the length of the boat itself) with its own, tube-shaped flotation tank, to the rear of my boat? You know, like a sea snake's tail?

It could be hinged so that it can be pulled up for mooring, and would then look like a mast with a wall.

It's going to be an 18-foot catamaran with four seats and a 8HP diesel motor. I 'd love to get at least ten knots out of it without exceeding hull speed, hence the idea of artificially extending the hull with a tail.

But would that work?

Answer 1

always interesting to see the progress of your project!

as to a hinge up tail.....no is the short answer......and as was mentioned you'll have something so weird there will be little market value....

if you are going top operate this as a passenger boat, you will need Australian MCA or whatever the US Coast Guard inspection equivalent is there and something so radical as a tail that might break off in bad conditions, or even while docking would never pass here and I would doubt there.

If you type "wave piercing cat" into google search you';ll get a ton of hits; one is below in the links to an Aussie designer for an 18 footer. Notice they recommend two 135 outboards..and the cats that run in the Virgin Islands as water taxis and charter boats ( see Gold Coast Yachts) use  twin outboard.

What the cost of the 8hp diesel, prop, shaft etc? Howe many x hp outboards can you buy for that? Two outboard gives great maneuverability and one will get you home if one fails.....

Answer 2

It sounds like you're using the rule of thumb about length vs. hull speed, (sq.root LWL+2) and not the actual equations.

Answer 3

I've seen someone try this with a monohull.  Not only did it not work, he ended up with an "albatross" that he couldn't sell.  It was a 26 footer that he added about 8' of "sugar scoop" to.

There is more to the equation than just waterline.  A planing hull can be made to go faster simply by applying more power to it.  Its limitation is in its stability at high speeds.  A displacement hull (such as you have suggested) is limited not only by waterline but by how fast it can move water out of the way.  I wouldn't do this;  haven't seen it work so far.

Answer 4

an interesting theory. unfortunately, wouldnt work. the biggest thing would be, unfortunately, doubling the waterline length of the hull, will double the wetted surface area of the hull. this in turn will double the friction on the water. be like adding a trailer to your car.

if you could somehow make the hinge part strong enough that it did not flex, you may gain something, by reducing the amount of 'sterm sag', where the stern digs down into the water. but thats going to put a large load on the hinge and the 'tail' which may do you more harm than good.

unfortunately, although cats make very stable boats, as displacement craft, their hull speed is somewhat limited even more, since, in essence, you have 2 hulls that need to displace. although some may argue that the combined area is the same as a monohull. it would largly depend on the design.

i think, although its interesting, the cons would out weigh any small pros you may get. that would add a large amount of stress onto the hull, especially on waters other than a mill pond, it would increase the drag a lot, on a hull that is already pretty close to underpowered as it is.

if you want to increase hull speed, you would do better to look at things like bow. the sharper the bow, the better. with 8hp a planning hull wouldnt do you much better, as the engine would be hard pushed to get the boat on the plane. basically you want to reduce the drag on the hull as much as possible. straight lines, smooth surfaces, the less material in the water, the better.

Answer 5

get rid of the diesel and use a 2 stroke or a 4 stroke motor and you will lessen the weight there for the same HP will move it easer.with less weight you might fine that a bigger motor will work even better,

Answer 6

A sharp bow leading to a beamy planning hull with a flat stern will be  a better design.  You want to reduce underwater surface area.  Decrease draft and widen beam to improve transverse stability while minimizing hull shape.  A I/O with an unshielded shaft will reduce water resistance over an outbound with its flaired housing.  Placing engine about 2/3 aft will balance weight to improve planning ability.

Answer 7

8 hp diesel a 1 cyc with maybe 1800 RPM no speed there no how>Increase HP is the only way to get more speed>And with A displacement hull it will only go so fast before it gets deeper in the water>