Underwater lines for a performance cruiser

[Nordic cat]

Originally Posted by Lodesman

Looks good Alan! Very nice lines. Inquiring minds need to know - will the saildrives be in-line with the rudders? In front or behind?
Seriously now - is this for personal use only, or are you planning on taking over the cruising cat market?

Kevin

Thanks for the kind remark. The saildrives will be in front of the rudders,and in-line with them as I've know understood that it is the only way to do it if I want to motor on one engine

The boat is for me, as being a bi-rig, there will probably not be alot of interest from buyers. If there is positive interest, then I will of course attempt to supply these. (Also with a traditional rig)

I am a marine engineer, but was lucky enough to sell my business a couple of years ago. This and the fact that I couldn't find a boat that ticked all my boxes, led me to doodling and you can now see the result. It started off as a 43 footer, but grew to 49 to accomodate the forward cockpit, and enable complete seperation of the engines rooms from the rest of the boat. I have hired a Naval Architect to finalise the design, and do all the scantlings etc. as this doesn't interest me very much, and I haven't bothered to study it very much.

Regards

Alan

[Nordic cat]

Originally Posted by 44'cruisingcat

Well, the placement of the rudders/props is kind of related to the underwater lines, isn't it?

The boat looks good, I was wondering why you have so much rear overhang, assuming the waterline shown is at max displacement. (Is it?)

Seems like a waste of potential waterline length, for no real gain.

What kind of materials would you build from? Are you actually thinking of building this, or is the design a goal in itself?

Thanks for the positive comments.
The rear overhang was to get the transoms 200 mm out of the water, and by having a completely flat run of the underside of the hulls for the last 3-4 meters, in fact a bit like a chined boat.

I have a Bwl/Lwl ratio of > 1:13, so losing a bit of waterline at rest probably doens't have a big influence???.

As mentioned, we haven't done any hull line optimisation yet. What would you suggest? A short upturn over the last 500 mms and keep as much waterline as possible?

Its going to be built out of foam and epoxy, and I have followed Gideons advice, so it will be infused, using the one off method described by DIAB, so no moulds. There will be Kevlar/Twaron on the insides up to about a meter over the waterline( for impact resistance), as well as on the outside bottom for abrasion. Carbon fiber for reinforcement around the mast bases, and other strategic areas.

There will be no forward beam, just a rope to hold the tramp.
3 steering positions: fwd, aft and inside the saloon.

The coachroof top extends outboard and forward to shade the saloon windows.


Regards

Alan

[44'cruisingcat]

Is there any reason you want the transom to be 200mm above the waterline? On this boat:

YouTube - Mango 38

the bottom of the transom is probably around 30mm, maybe 50mm above the waterline. There is no problem with water dragging or climbing the transom of the leeward hull, as you can see.

Unless you have a particular reason for wanting the higher transom my inclination would be to simply lower it, and flatten the keel profile aft a little. A softer 'release' will give less drag, and you'll gain waterline length. (From the scale provided, you'd gain a metre easily)

[Nordic cat]

Originally Posted by 44'cruisingcat

Is there any reason you want the transom to be 200mm above the waterline? On this boat:

the bottom of the transom is probably around 30mm, maybe 50mm above the waterline. There is no problem with water dragging or climbing the transom of the leeward hull, as you can see.

Unless you have a particular reason for wanting the higher transom my inclination would be to simply lower it, and flatten the keel profile aft a little. A softer 'release' will give less drag, and you'll gain waterline length. (From the scale provided, you'd gain a metre easily)

I need to get some calculations done, when she's loaded with dinghy etc. Your advice sounds very good.Another advantage of lowering it, is that wave slap at anchor can be minimised.
Adding another meter to the waterline should increase the speed at which the "hump" occurs.

I have noticed that some designs have a flattish run aft, and then the transom is lifted up at about a 45 degree angle over the last 2-300 mms.
(Saw this on some Shuttleworth design) Anyone know the advantage of this?

How far back is the maximum hull beam on your boat? And is there a reason given?

Thanks

Regards

Alan

[BigCat]

Some overhang aft has two virtues-it insures a smooth exit when the stern waves starts climbing up the boat, and you never know just how deeply the boat may get loaded, so it keeps the transoms out of the water if the boat has more aboard than the designer intended.

[BigCat]

[quote=Nordic cat;145353]Guys,

Nice drawing, not really enough info to comment on the design, except to say that it looks like you have taken Shuttleworth's light bulb concept to the maximum.

[cat man do]

[quote=BigCat;146751]

Originally Posted by Nordic cat

looks like you have taken Shuttleworth's light bulb concept to the maximum.

I think you may find Roger Simpson was using that hullshape before Shuttleworth

Dave

[BigCat]

"I think you may find Roger Simpson was using that hullshape before Shuttleworth" I wouldn't say so-Shuttleworth's idea includes tumblehome in the top part of the topsides. You can read a discussion of this in an article on his website-his idea is about reducing wind resistance.

[Nordic cat]

There are numerous designs with hull flare above the waterline, in different shapes and sizes, as well as some with the rounded tumblehome, but Shuttleworth probably is the best known and most prolific of these designers. I think his designs also look good, something you can not say of many cat designs in my opinion. Tris have used flared hulls since way back.

I think a nice rounded flare will work better and look nicer than some of the "steps" seen in any number of boats, to accomodate a wider berth.

I have designed the hulls as asymmetric, so the inside flare is lower and further forward than the outside hull flare. The thinking behind this is to create some lift to windward.

I also read an article by an Australian designer who hade done alot of testing with hull flare, and his conclusion was, that it brought down the slamming considerably, as the extra volume made the hulls lift in waves. (I have tried to find the link, but haven't been able to).

What do you guys think? A waste of time and effort in making the hulls asymmetric??

Regards

Alan

[BigCat]

"What do you guys think? A waste of time and effort in making the hulls asymmetric?" There was recently (December 4th. - 10th.,) a lot of exchange about this on the Kelsall Yahoo group, and the conclusion seemed to be against asymmetry. There was also some discussion of, for example, rocker. See KSSBoat : KSS BoatBuilding

[masalai]

I agree with the asymmetrical shape above the waterline, in a "large" chamfer from just above the load water line to the bridgedeck and continued forward to the bows for the "lift" and noise reduction. Most building is "unplugged" owner builds in Australia and tends to have flat panels to reduce construction time and cost...

[BigCat]

"I agree with the asymmetrical shape above the waterline," My link above is about asymmetry below the waterline-Rudy Choy designs, and some beach cats would be examples of this. I doubt asymmetry matters much if it is far enough above the waterlines to stay clear of the water.

A complex shape with lots of curves above the DWL, will of course require hundreds, perhaps thousands of extra man hours to produce, but whether or not it is worth the effort is a personal decision. You can get some, at least, of the benefits in reduced windage of the light bulb shape with a developed top panel on the outsides of the hulls that gives tumblehome, and many have done so.


Most yachtsmen won't care about the smaller decks that tumblehome gives. I actually want the extra deck area given by carrying flam (often called flare,) right up to the decks.

[masalai]

BigCat. that is why I qualified my response... I do not see the point of asymmetry below the waterline, even to "tilting the hulls" unless the design/intention is to fly a hull at every opportunity. For cruising - waste of effort - for what benefit? an imbalance of drag and/or lift, additional complexity, and so on...

[BigCat]

Originally Posted by masalai

BigCat. that is why I qualified my response... I do not see the point of asymmetry below the waterline, even to "tilting the hulls" unless the design/intention is to fly a hull at every opportunity. For cruising - waste of effort - for what benefit? an imbalance of drag and/or lift, additional complexity, and so on...

I think so too, but not everyone agrees-

[Whimsical]

Originally Posted by BigCat

I actually want the extra deck area given by carrying flam (often called flare,) right up to the decks.

Years ago I read a piece from a designer who had conducted some tests on the effect of rounded deck transitions. He concluded that a rounded deck transition can have a large impact on aparent freeboard from a wind resistance perspective. If I remember correctly it was 1.5 times greater than the actual diameter of the curve. As a cat has rather large windage, especially those with a single step topside profile, it was an element in my decision for selecting a design.
Unfortunately this info is floating around in the unordered abyss of my memory and has become detatched from the link to whom produced it. For that I appologise.

Mike