Negative Bows ?

[Canada Bob]

I've seen a couple of small {under 20ft} home built catamarans with negative raked bows, I'm trying to figure out what advantages a negative raked bow might have.

The hull shapes were sort of like this...


stem /...................................../ stern.

[apex1]

Wellcome aboard to say that first!

Looks interesting is the answer............................

And we mortals are used to draw a boat the opposite way...stern../......../ stem in this case.

Regards
Richard

[rwatson]

The big advantage, as ancient greek and roman galleys found out, is that you can put a very large hole in the opponents hull without them being able to jump onto your deck.

This is particularly useful in contentious starboard v port encounters, or when opposing boats ignore your cry of "buoy room"

These designs have grown in popularity recently with the increasing cheaply printed boat designs, where the builder mistakenly placed the bow moulding upside down during construction.

[Canada Bob]

I was wondering if a negative raked bow {on a lightly built cat} might have something to so with wave peircing ? less chance of pitch polling.

Other than that, what determines the angle of rake on the bow of a small boat {other than style}.

[lewisboats]

I would think it would be to lessen weight. There is no heel to a cat ( for the most part) and therefore would not benefit from the lengthening of the waterline that a regular forward raked bow would provide at normal heel. Take a chunk of weight off the bow and it rises faster and doesn't bury as much.

Steve

[Canada Bob]

Thanks for the above Steve, appreciated...

The boats that I've seen are less than 20ft long, so I expect the weight will be very light anyway. One thing I've noticed is how they seem to cut through the water/waves rather than lift as much as a regular positive rake bow does.

I don't know if this negative rake helps when a wave breaks over the bow, but it made me wonder why we angle a bow the way we do, at least on small / light boats that is.

[M-Sasha]

As rwatson and apex mentioned, it´s a gimmick, at least on a boat that size. Show something different and sell more, thats it.

[Canada Bob]

Hi Sasha,

Thanks for the above, the boats I've seen are project boats, so I guess the guy just likes the look of a negative rake.

[Guest625101138]

Thanks for the above Steve, appreciated...

The boats that I've seen are less than 20ft long, so I expect the weight will be very light anyway. One thing I've noticed is how they seem to cut through the water/waves rather than lift as much as a regular positive rake bow does.

I don't know if this negative rake helps when a wave breaks over the bow, but it made me wonder why we angle a bow the way we do, at least on small / light boats that is.

The rake in the bow is often a result of flare in the topside and a developable shape so the plate flows without twist to the bow. The rake is the intersection of the outward angled planes of the hull sides.

The negative rake can have merit by giving a certain waterline length without much buoyancy so pitching is reduced. In my opinion this shape is more prone to pitch poling as the bow will easily bury compared with a flared bow.

If you have a picture of one of the boats in question then it might offer more ideas.

Rick W

[CT 249]

If it's a gimmick then it's an awfully successful one, as demonstrated by the fact that raked bow boats have dominated the world titles in the most popular development catamaran class for a few years now.

In A Class cats and F18s the waterline length issue doesn't exist, since they've had waterlines at the maximum 18' LOA for eons. However, the reverse-raked stems do reduce pitching while still providing sufficient buoyancy. The result is a significant increase in upwind speed.

Small cats learned many years ago that you don't want flare in the bows to prevent severe nosediving. Submerging the flare slows the boat down, therefore (since you nosedive downwind) the apparent wind increases as the boat slows; that puts you straight in.

It's better to reduce volume high up in the bows, because that way drag doesn't increase much when the bows go down. You can drive the bows deep down, but they'll pop back up again - especially if they have a whole lot of volume low down in the form of U-shaped lower hull sections rather than old-style V shaped sections.

The ram-bow effect does work; we copped a former world A Class champ's bow into our port hull some time ago. If he had a vertical bow he would have spread the impact more and put some of it onto the stronger deck/hull joint; instead it pierced our hull low down.

Some other boats have tried reverse-raked bows without success. Maybe in a slower boat, the increase in apparent wind as the boat slows under the drag of the flared sections is less of an issue, and the added bouyancy is of more benefit. It seems logical.

[Guest625101138]

........

Small cats learned many years ago that you don't want flare in the bows to prevent severe nosediving. Submerging the flare slows the boat down, therefore (since you nosedive downwind) the apparent wind increases as the boat slows; that puts you straight in.

...

I know there is a limit to this because I operate very slender hulls with low buoyancy in the bow. These are not sailing boats. If I press hard down the front of a wave I can bury the bow to scary depths. I have had 2m submerged on a 7.2m long hull.

The boat I am referring to has virtually no rocker so dynamic lift is negligible. It also has no flare and there is not a lot of reserve buoyancy. Basically it goes straight through small waves to windward without any pitching.

At speed I believe dynamic lift in the bow is more important than flare. As you point out the flare will slow things down.

The other control on pitch poling is to prevent the stern from lifting. 'T' rudders have proven effective for this.

Rick W

[Ad Hoc]

Canada Bob

Welcome to the wierd world of forums...

Raked bows, as mentioned above are a combination of several factors.
1) Gimmik..or more correctly marketing!
2) increases the waterline length of the boat, for 'free', so boat goes a bit faster
3) has very minor benefit in pitching..all depends on the amount of bow that is actually 'fwd' of the F/P. In practical terms (ie once the boat is actually made for its intended purpose, rather than an academic exercise), makes almost no difference.
4)Seakeeping (as part of #3). But again, depends upon the sea state, ie quartering seas, following seas, each has a different solution. A fine "slice of cake" like bows are generally better, especially if one is concerned about bow/deck diving.
5)Each design has its own solution, and as such is generally not applicable carte blanche, as so often in design.

If you want more of a 'read' i can point you towards some good papers if you wish.

[dreamer]

these threads are good because it gives us a chance to say "this isn't anything new."

http://gallery.photo.net/photo/7139820-md.jpg
WWI vintage battleship


If it would have been successful, don't you think they'd have continued to make them that way?

[Ad Hoc]

Dreamer

"..If it would have been successful, don't you think they'd have continued to make them that way?.."

You mean like this?

http://www.ulsteingroup.com/kunder/ulstein/mm.nsf/inpdocuments/CC610F46B5B32113C12571A7003E226A/$file/ULSTEIN_AX104.pdf

Every design has its place and time...usualy paid for by the client :)

[dreamer]

That's pretty cool.

There really is a trend it seems to go 'through' the waves rather than over them. I suppose a bow like this sheds water more cleenly than what we consider traditional. Notice the wave deflectors way up high. From the looks of it, this thing is expected to be part submarine. ;)

[Guest625101138]

...

There really is a trend it seems to go 'through' the waves rather than over them. .... ;)

The boat in the attached clip goes through waves but has a wave deflector aft of the real bow to keep pilot from getting drenched. I guess its bow is the extreme of reverse sloping bows.

Rick W

[martinworswick]

http://www.schionningdesigns.com.au/login/pages/images/GForceWeb.jpg

[Ad Hoc]

Rick

Interesting appendage. It certainly increases the waterline length, but the fact it has almost zero buoyancy doesn't really increase its added mass to the inertia for pitch damping. It only pierces the waves by merit of the fact that it has no choice, owing to the direction of the main hull. Whether a large or small wave is encountered by the extension does not alter the motion of that fwd part, or any other part...however, once the wave come into contact with the main hull, then that is a totally different situation. Hence, it really just adds waterline length.

[capt vimes]

it seems to me that more and more ships go wavepiercing...
on a cat or tri it looks like the way to go if speed is on your mind... quite reasonably i think.

the powerboat on the first image is 'earthrace' which broke the existing record for a circumnavigation just last year... must look absolutly crazy to see her in a good seaway.. :eek:
http://www.earthrace.net/

[Guest625101138]

.....

the powerboat on the first image is 'earthrace' which broke the existing record for a circumnavigation just last year... ....../[/url]

A fine point but it broke the "powerboat circumnavigation" time. Solo yachtsmen circumnavigate faster than this. I think the current multi crew sailing record is 50 days.

Rick W

[Time]

solo sailers dont have to go through the equatorial canals...so the distance to travel is much greater to start with!!!! so the efforts are not comparable at all.

[Guest625101138]

solo sailers dont have to go through the equatorial canals...so the distance to travel is much greater to start with!!!! so the efforts are not comparable at all.

Yes I know this. Makes the sailing records even more impressive by comparison. Considerably higher average speed in the sailing vessels. Just shows how good modern sailing boats are.

Rick W

[capt vimes]

57 days or so about to my knowledge for multi crew on a tri (or was it med2?)...
nevertheless - earthrace did not cross the roaring 40s to the south but went via the panama- and the suez-canal which is the route for powerboats obvisously. (oh - was mentioned already.... ;-) )
they are just not build for the servere conditions you may encounter around the antarctic.... ;-)

btt: wavepiercing boats tend to have a raked bow although earthrace actually has a very steep one but still with a positive angle.

[Milan]

these threads are good because it gives us a chance to say "this isn't anything new."

http://gallery.photo.net/photo/7139820-md.jpg
WWI vintage battleship


If it would have been successful, don't you think they'd have continued to make them that way?

There was a period in the naval strategy, (1860’s – 1880’s), when it seemed that ramming would become most important tactic once again.

That was a time of rapid development of naval technology. Steam engines made ships more manoeuvreable and armour protected them from contemporary artillery for a while. It was logical at that level of technological development to try to use a ship as a projectile. Soon enough, further development of the artillery and torpedoes made ramming obsolete. Still, rams continued to be built on the naval ships for another couple of decades.

(The best example of successful use of ramming was naval battle in the Adriatic Sea, nearby island Vis, between Austro-Hungarian and Italian navies in 1866).