Unusual catamaran hullshape

[sabahcat]

This aft section looks unusual, almost like an add on
And I wonder why the step line appears to be pointing down so severely?

http://www.boatpoint.com.au/boats-for-sale/boatdetails.aspx?R=10532509

From the builders site, that hull is supposed to be like this under the water
http://www.windspeed.com.au/yachts/power_multihulls/windspeed 43p/index_windspeed40p.htm

Why do you think there was such a big change?

 

[masalai]

Hi Sabah,
Do you know who the designer is? I see bits of ideas expressed by most Australian and NZ designs, but it does not seem to quite work for me... Did you actually seek a critique from a biased and bigoted rank amateur such as I?

The wetted beam aft is a bit broad and the section aft seems designed to reduce "squat"...

I would be more inclined to express favour for a shorter exposed shaft and eliminate the shaft guide, (look at my gallery of the Chamberlin10), which "got it right there"...

Those three panels may be seen to better effect on the "Maritimo" boat in my gallery (but they do 8 or more times the speed of this "for-sale" vessel)...

Range with 300 litres will ensure it is captive in its home marina, even with the base twin-75hp option. - - and the beam of 7 metres guarantees almost nil availability of a berth elsewhere... (mine at just 21ft is generally excluded.)

2000kg payload is a bit light on for an "on the pick liveaboard"... I carry up to a ton and a half or so in fuel for extended range...

Rather than angle the shaft inwards 7degrees a better option may be to off-set the rudders a bit... Also, then the option to run on a single engine would be retained?

 

[Brian@BNE]

Hmm, it certainly does look a bit odd in those photos. Perhaps they are attempting to get good displacement efficiency, but also have some flat sections to allow planing if there is enough power?

For mine, the Maine Cat P47 does this much better. The hull sections can be seen as the pictures cycle through on the 'Update 2/04/07' at the bottom of the page.
http://www.mecat.com/power/powerupdates.htm

 

[rwatson]

The reasons for the design are stated in detail on the designers site - and claim to be well and truly sea tested



Test Results of the Stabilizing Rudder Foils


Reduced pitching motion which provides a faster smoother ride.

Effective endplate for excellent rudder thrust for turning, from a reduced rudder draft / area.

Easy transition from displacement speeds to surfing in a following sea.

Reduced transom squatting in a following sea.

Reducing stern squatting under motor which increases motoring hull speed.

Combines well with round bilge hullshape to reduce hull drag and pitching.

Foils provide higher average speeds through less hull drag and a stable sail platform.

The longer the longitudinal foil length the greater the stabilizing effect.

High aspect ratio foils are less effective at achieving these results.

Foils have little or no stabilizing effect at less than 5 knots yacht speed.

Foils provide permanent easy boarding access to the swimdeck.

Note:
Motion metre: Sight along the deck to Horizon and view ruler in foreground to measure pitching.

Conclusion
After eight years of extensive sea trials in a variety of sea conditions the rudder mounted stabilizing foil has proven safe, reliable and effective for reducing the motion and increasing the average speed of cruising sailing Multihulls up to sustained yacht speeds of 18 knots.

 

[sabahcat]

Hi Sabah,
Do you know who the designer is?

http://www.windspeed.com.au/wayne_hill/about_wayne_hill.htm

The wetted beam aft is a bit broad and the section aft seems designed to reduce "squat"...

I would be more inclined to express favour for a shorter exposed shaft and eliminate the shaft guide, (look at my gallery of the Chamberlin10), which "got it right there"...

Those three panels may be seen to better effect on the "Maritimo" boat in my gallery (but they do 8 or more times the speed of this "for-sale" vessel)...

you have noticed that what is there in real life is nothing like the computer generated pictures?

Range with 300 litres will ensure it is captive in its home marina, even with the base twin-75hp option. - -

Yeah, sailing cat fuel tankage
I have around 2400 litres in 8 separate tanks for around 2000nm range

and the beam of 7 metres guarantees almost nil availability of a berth elsewhere... (mine at just 21ft is generally excluded.)

There are plenty of places to get berths
Most 35ft plus sailing cats are wider than 21 ft and they are mostly on berths, even if they are sometimes end fingers

I'm 24 ft wide, but never liked marinas anyway

Rather than angle the shaft inwards 7degrees a better option may be to off-set the rudders a bit.

What's so hard about dropping a rudder so as to remove the prop and shaft if needed

Also, then the option to run on a single engine would be retained?

What makes you think you couldnt you run a single engine

 

[sabahcat]

For mine, the Maine Cat P47 does this much better. The hull sections can be seen as the pictures cycle through on the 'Update 2/04/07' at the bottom of the page.
http://www.mecat.com/power/powerupdates.htm

Bastardised Tennant CS hullshape?

http://www.catamarans.com/news/2006/04/catcomparison.asp

It is arguably a better hull shape, definitely if after higher speed, but it does have disadvantages like increased draught for one and from another thread

http://www.boatdesign.net/forums/motorsailers/cs-hull-sailing-performance-35387-2.html#post466004

A serious remark of him for CS is also : The slender hull forward makes this hull form susceptible to tunnel slamming, as there is not enough buoyancy forward to carry the hull out of a wave before it impacts with the tunnel. The hull relies on depth rather than width for its buoyancy.

I spoke to Malcolm several years ago and he said that if you were only after 12 knots, modified sailing hull shapes worked fine, but of course some modified hull shapes are better than others.

 

[Brian@BNE]

Thanks Sabah, I enjoyed the Tennant article. And yes, the P47 does seem to have the shape Tennant describes so I wonder if it was the source of inspiration?

I quite like the Wild Wind IV (Figure 2 in the article), any idea where there's more info on that?

Update: OK, I found the site http://www.tennantdesign.co.nz/index.php?page=wild-wind

 

[masalai]

For mine, the Maine Cat P47 does this much better. The hull sections can be seen as the pictures cycle through on the 'Update 2/04/07' at the bottom of the page.
http://www.mecat.com/power/powerupdates.htm

I agree, the hulls there are almost perfectly set up for fast displacement cruising with minimal squat... Clean and elegant

 

[masalai]

Bastardised Tennant CS hullshape?

http://www.catamarans.com/news/2006/04/catcomparison.asp

Tennant, to my mind, was the original master of that configuration... I would love that hull shape (in hard chine / flat panels) on my boat - I first saw it up close and touchable on the Chamberlin C10, which, Robin acknowledged, was his interpretation of similar work by others - including Malcolm Tennant

Hi Sabahcat,
The specifications stated that the propeller shafts were angled in by 7 degrees to facilitate removal of the propeller and propeller shaft so thrust is 7degrees and enhances the turning moment, which would need more rudder to offset this...

I went looking for a berth the other day and there were none available in 3 marinas nearby for at least a month, and then only for a couple of weeks ... and the dollars - - swing on a pick, - or, - find someone with a wharf out the back of their home....

 

[Brian@BNE]

Yes Masalai, I only read it quickly but I think he came up with those ideas in 1983.

I really like their motorsailers - basically putting a mast on their power cats, and at first glance still getting reasonable rag performance. Ah, but could I actually build Cordova? Big project....... http://www.tennantdesign.co.nz/index.php?page=cordova

 

[sabahcat]

I really like their motorsailers - basically putting a mast on their power cats, and at first glance still getting reasonable rag performance.

Make sure you work out the cost of that mast, sails, winches, blocks etc etc and then work out how much that would earn sitting in the bank, buying extra diesel miles.

 

[Brian@BNE]

Make sure you work out the cost of that mast, sails, winches, blocks etc etc and then work out how much that would earn sitting in the bank, buying extra diesel miles.

At present I'm sure you are right to believe that 'motor only' is the cheapest option on a high l/b raio boat, including a cat. And by a wide margin compared a new sailing rig. But in 5 - 10 years time, what will the cost of diesel be? Much, much higher! Ok, so slow down and use less will be first response. But the attraction of some sail area will reassert itself also.

Ideally I would have motor-assisted sailing. A low revving diesel with CPP, and a not particularly aggressive sail-plan. Even better if that diesel can run on one of the 'bunker' or heavier fuel oils. But these diesels are likely to be big and heavy and not suited to a cat.

 

[Willallison]

A little late into this, so my apologies... but just looking back the original post and Q's...
The pics are to my mind of a powercat that is clearly developed from sailing boat hulls. The bustle having been added in an effort to reduce squat, but as someone else pointed out, at what looks from the pic to be at an excessive downward angle.
But it is the computer image - which, as Sabah said, bears absolutely no resemblance to the pictured boat - that has me puzzled. The 3 little recessed steps aft of the props can only in my mind have been added to increase drag!

 

[daiquiri]

But it is the computer image - which, as Sabah said, bears absolutely no resemblance to the pictured boat - that has me puzzled. The 3 little recessed steps aft of the props can only in my mind have been added to increase drag!

I honestly don't like the design of that Windspeed cat. Looks like a water-slamming/slapping machine, both bow and stern. The 3 recessed steps are for sure creating an additional drag, like you have noted. but I don't have a clue about their purpose. If they were on the inside of the hull, I would have thought about some kind of anti-vibration reinforcements, but placed like that they just can't be that.
The stern additions you've noted look like a result of a sea trial gone bad. Like they needed more buoyancy aft, but it was too late to modify the general hull design in some cleaner way.
The underwater hullshape looks like a box keel applied to a wrong hull - just too narrow to help creating any more dynamic lift than a more conventionally shaped hull could do with less friction and form drag.
Just my two cents worth. Well, in this case probably even less...

 

[Brian@BNE]

Having looked at the Windspeed site, I think the answers to Sabah's original questions are apparent. There are a lot more construction photos there (go to http://www.windspeed.com.au/yachts/power_multihulls/windspeed 43p/index_windspeed40p.htm and click on Photos). They show the hull rocker, and the rear step-line is horizontal, not pointing down at all. It also shows that this 'Windspeed Custom' underwater profile has no resemblance to the Windspeed 43P design on their site. The "Custom" just has a keel that almost looks like an afterthought rather than an integrated 'canoe stern'. Cannot tell if the drag-inducing steps at the stern are there or not.

One of the amazing bullet points, under 'Design Features', is "Electronic self tailing rope winches for mooring lines." Wow! All boats should have these fitted!

Although its lying a short drive up the road for me, I don't think I'll take a look at it. Apparently its "virtually up to painting and fit out". In other words will still need a very large amount of money to complete. So at $190000,"tell 'em they're dreaming" I'm not too enamored by some of its styling anyway.

Windspeed may well make very good GRP/composite panels, and have a successful business supplying them for a variety of industrial uses. But I think they could use some professional help wrt boat design (and PR copywriting). Can't help but think of that "naval architects and designers are a waste of money" thread.... Hope "Custom" turns out better....