where would you put a fin?

[Frosty]

Most speed boats you see with a shallow v bottom have a fin about central of the hull. This is obviously to stabalize the thing so it doesnt steer like a hovercraft.

But on a semi displacement catamaran would you not think that the fin should go at the front of the hulls.

It occured to me that this would help in line direction and help to enable the boat to be driven by one engine only.

I also thought that it would help to keep track better. making the boat steer to the rudder rather that be thrown about by side waves and following seas?

I hav'nt seen anything like this! Is this a crap idea? Just a small fin --say less that a foot in area each.

[PAR]

Forward fins and rudders have been used and they work well. They just have a nasty habit of being the first thing you bash when you run over something or when the water depth challenges the craft's draft. It boils down to what you'd prefer to replace after these occurrences.

[Frosty]

Thank you Par for your posting.

Thats very interesting! So not out of the question then. I was thinking of a fin longer in length than in depth,say 1foot long and 6 inches deep right on the bow.

I was thinking that it might go some way to protect the hull itelf should I hit an obstruction.

In fact I think really my disatisfaction with the boats tracking in following seas is the bulbous bows. I have read in many places bulbous bows dont work on boats of less that 60feet, for this reason. I have also read that a boats entry should be with a shallow forefoot, --mmm that rules out bulbous bows.

I have already filled in the upper part of the bulbous bow to the bow with tremendous results and even a knot on the top end. I am just wondering if any more manipulation would result in further improvements--but what?

[Frosty]

I might as well tell you the whole storey. In the picture I hope is below is a false bow . I intend to place these in front of the bulb extending up and above the water line. Thes I am hoping will draw air down to the hull and feed the upper part above the bulbous bow with a flow of air drawn in from above.

This I hope will fool the boat into not feeling the drag fron the upper part of the bulbous bow and the wetted area.

[kach22i]

Please excuse me for asking, but short of hydrofoil canard designs I'm not familiar with fins on boats at the front/bow.

Something in the middle (keel) for a sail boat, and something at the rear/stern for a power boat, but the front?

Can you please post a photo or drawing of such fin?

Thanks.

EDIT: Diving bow fins on submarines would be an exception.

[kach22i]

Like this?


From this nice site:
http://www.setsail.com/dashew/Fish_Eye_view.html

[tri - star]

Gentlemen, please take a deep breath......

Please consider, WHY a central foil - well forward
- IS NOT a common sight.......

Imagine you are surfing down a wave at max. speed.
A large solid object - like a reef appears.....
The guy at the wheel turns away quickly.
The stern starts to lift AND roll - and the bow digs in.
What happens next ?
YOU tell me.......

[kach22i]

Originally Posted by tri - star

Imagine you are surfing down a wave at max. speed.
A large solid object - like a reef appears.....
The guy at the wheel turns away quickly.

If you did not turn at the last minute the sheer forces would be the same if the fin was at the bow or at the stern or in the middle. It would come right off either way as not to do more damage to the hull...........at least that is how I would design it.

F = MA

.............right?

[kach22i]

Fin First

The foil-shaped fin is mounted under the bow. Two Voith propellers with damping nozzle plate and struts are positioned abaft midship.

Click link to view video:
http://www.bube.no/finfirst.asp

[tri - star]

To kach22i,

OK...maybe on a tug...
The people in Norway, usually know what they are doing.
That com. has been in business....for 90 yrs.

So what do I know..?

However, 'twas me - I'd get a VERY accurate scale model
made. Remote controlled - faily large. And find a standing
wave. Like the have at Florida Diz. World...
- And do a lot of test runs. A lot...
Particulrly, at 45 degrees, to the line of travel - at full speed.

...Regards.

[kach22i]

Watch the video it explains that the tug is going backwards pulling on the ship, and the twin propellers are doing the steering. A very unusual condition for anything but a Tug.

[tri - star]

That makes sense....

Go for it !!!

[messabout]

I'm with Tri-Star. My experience with ...I'm thinking fast displacement boats..is that something like a bow fin makes the boat twitchy. When quartering, particularly, the boat wants to take off in unintended directions and will sometimes hunt. I've had this happen with several hard chined sailboats while on a screaming reach. Bolger has some theories about this which he tells frequently. His explanation concerns differential pressures and may not apply directly to a fin. He also designs forward fins into some of his boats however. Go figure.

[kach22i]

In an earlier discussion, it was said the blunt bow of some Jon boats is so they do not "bow steer" in big waves and surf. If just a pointed bow can steer a boat then a fin may do much worse than that in theory, right?

[charmc]

Like tri star and messabout, I have experienced difficulty in steering in following and quartering seas in a boat with a fine and deep entry. As in the tug demonstration, it is best if there is a large steering force aft, which on a pleasure boat would be a large rudder,or a rudder with a deep keel or even a skeg. It can take a lot of force to steer when the bow digs in in such conditions. I think it might work, but the devil is in the details: a lot of test data under varying conditions to determine the optimum dimensions and placement.