Reverse bows on cruising cats pros and cons

In vessel dynamics the dynamic heaving force acts on the waterplane centroid, the CoB is used to represent a generalised CG for the vessel and you can then have a pitching moment if the waterplane centroid is substantially behind the trimmed CoB.

Prior to RAO's and for budget studies the method is to trim the hull in still water (easy in your design package) by shifting the CoG forward until the hull trims to a representation of the wave slope encountered. 10 degrees, and more for low buoyancy bowed craft . Then look at the waterplane centroid and CoB, they should both move together forward and should be close to inline. If not you have a dynamic additional pitching moment that is dangerous at synchronous frequencies. The full study plots the shift of WPC to CB up to 15 degrees. Not many people do it now with seakeeper and other CFD based simulations.

 

Great video Phil.

 

Thanks that make sense.

 

This is true for sure. People have been modding transoms forever, either adding a few feet when building, of even retrofitting extensions to get longer LWL and L:B.

My reasoning was that if you selected/drew your design to be safe and buoyant, then modded it before building (using proper calculations) by adding a few feet at the WL by way of a reverse bow. Wouldn't you be adding LWL, L/B, D/L and buoyancy? All for little extra cost if everything else stayed the same? Say someone can afford to build a 40 ft boat, but the 42 version is just a bit too expensive, however adding a few ft of waterline by a reverse bow would appear an effective way to improve performance without too much extra cost? Since the hulls are longer, beam the same and there is more buoyancy at the front wouldn't this mean reduced tendency to pitchpole?

Its a bit of a chicken and egg argument though (what comes first the LOA or the reverse bow) which I tried to settle in the first post without luck.

Bow extensions are much harder to do on an existing boat than stern extensions so I doubt a bow extension would be worth doing to an existing boat.

 

There are some advantages of reverse bows on relatively small waves; this is confirmed by real life boat experience and also by simulations we did for couple of designs.

I think one might test these reverse bows in relatively high waves and record the behavior. Wetness and tendency to bow dive will be an issue, but how severe and to what extent? Where is the compromise?

We have built 2m model of such catamaran already, will test it on open water with gauges and see what is happening. Without testing, anyone can talk about 'clean hydrodynamics', but we need numbers to make correct decisions.

 

Alik, I have made changes to boats and sailed them in the real world, not from some computer chair or fantasy dreamland or observing the antics of small models in a pond. You either trust my observations or you don't. No skin off my nose.
Actually this discussion is far too narrow as the subject is much more than just reverse bows; how about rig heights and weights, mast shapes and clean rigging and dare I mention, "clean hydrodynamics", or dragging central pods and motors or arse down transoms from overloading, overall beam and too low water dragging cabins --- and of course hull lengths.
The latter, as Dennis mentions, is probably the most important factor in gaining sea kindliness and reduced pitching, also higher performance.
But a "clean reverse bow" with no junk on deck to trip you as the bow gets driven under ... is also going to be more than a just minor help in keeping you upright.
And for non-believers, just check out the myriad of videos of race multihulls with reverse bows in seaways. There's an education there.

 

We do design boats, test them full-scale, use some of them for our boating activities and also conduct model tests with contemporary measurement equipment, both in towing tanks or on open water. Open water testing of RC models is becoming increasingly popular in advanced craft design; we will present some techniques on coming HPMV-2013 conference in Shanghai. You can also read about our tests of model capsize in Damaged Ship II papers, we studied exactly bow diving event on high speed craft.

So, do You have any numbers? I am sure You don't, just statements about 'clean hydrodynamics' and 'miriad of videos'. I have great respect to Your experience but this seems quite subjective unless supported by measurements. At least, not convincing to me... The question remains open in which conditions should reverse bow be used, as every design is a compromise.

 

Nothing wrong with observations; it's what artists do. And they get it right.

 

That's just it, it's got nothing to do with belief it's a science not a belief system.

There are myriads of failures too, performance risks are acceptable in racing because they are necessary to win unless regulation sets some safety standards.

Here's a classic diagonal capsize/pitchpole , Very similar to the danger of a cat surfing into the back of the wave ahead. The plunging hulls lack sufficient buoyancy because they have been shaved for weight, not optimised for wave piercing ! And the hulls are overly slender because a racing yacht wants max LWL within it's rule.

A Foil on the bow could counteract the dive, so could a trad raked stem and a bit of flare so could the same bow shape but with 3 times the volume (and weight) in the bow !

If you look at the figures on the powerboat you see the angle of the water the sailboat is diving into

 

Nice series of pics Mike.... is there some lead up shots to show the transition from hooting along to plowing in?

Jeff.

 

Yes, well, that is just weird cherry picking - observe the conditions and the boat, with full and overpowering rig - and our subject is reverse bows on a (sensibly) light cruising boat - where you would be reefed down; in fact the AC45 should have reduced too - but they can't - because they carry full wings plus this is a balls out race where everyone pushes and the courses are short with standby boats. Is that the best example you could come up with?
Instead how about looking at SodebO's terrifying nose dive while broad reaching (most people have seen it) ... and surviving from what looks an inevitable disaster, all reverse bows completely and deeply immersed - that is more of an example relating to our subject.

 

Nothing worth posting Jeff, everything goes swimmingly well and those bows cut through the smaller stuff no problem which you'll see thousands of pics of on the internet. That boats just in the wrong place at the wrong time for the speed it's doing. But a completely predictable response.

 

It's a very good example of a fast slender bowed low buoyancy cat plunging.

It's happened to multihull ferries too ( small ships) when there was insufficient restoring force from overly slender plumb stems.

 

That's the sensible approach. Then the client can be told the limitations of the design .

Radio control models are being used a lot in wave encounter studies in wave tanks. Kids love the models too and we'll get more people enrolling in NA ! Have you access to a wave tank in Thailand? We have a good facility here.

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We test on open water (sea or lake); this enables extreme wave testing or any wave direction. In tank, one is limited with directions and creating breaking waves. We have constructed the buoy to measure/record waves as well.

There are few research groups using open water testing of RC models, such as Wolfson Unit, etc. But they mainly test for controllability.

On photos, one of bow diving events. We have record of accelerations associated with these events as well...