CS Hull Sailing Performance?

[APP]

Originally Posted by brian eiland

Keep in mind that Cummins is an in-line 6 that will fit in the narrower portions of the hull. Go bigger HP and you will likely have to move the engine further forward to get space beside it to service it....that space further forward may eat into your living space, paricularly if you are using a conventional shaft arrangement. Unless of course you provide a wider hull which defeats your slenderness ratio, and slows you down.

What are exactly the Hull Plan View dimensions and BWL of Kamaika? Like Cordova of M. Tennant? Any idea?
Thanks.

PS: About the Motorsailer Chain/Belt driven systems: I think they should become widely used. I do not know any serious disadvantage in this issue. I suppose it gives a good advantage as e.g. the horizontal shaft to the DWL and maybe smaller engine rooms. No serious mechanical losses. Further with the fuel prices of today, Motorsailers for a Boat above 14 m should be generally a first choice, or not?

[sabahcat]

Originally Posted by brian eiland

Unless of course you provide a wider hull which defeats your slenderness ratio, and slows you down.

Something I sort of have an issue with

A traditional cat hull is wider on the waterline but draws less

CS hull shape is narrower on the water but draws more



Wouldn't/couldn't wetted surface area work out similar?

[APP]

Originally Posted by sabahcat

Something I sort of have an issue with

A traditional cat hull is wider on the waterline but draws less

CS hull shape is narrower on the water but draws more

Wouldn't/couldn't wetted surface area work out similar?

Hi,

Is this section back at the stern? If yes the boxy part of the boat is under the waterline. (I would like to know how much should normally be). Now, I believe even if the drag is the same with a similar boat, the boxy flat part aft helps to give some lift when the vessel enters in semi-planning mode, otherwise it will never plan. The rest of the Canoe shape behaves at low speeds as a displacement vessel with low drag. Am I correct?

Anout CS there was an article of Noah Thompson Design (New Zealand) which I reproduce here:
======================
By: Noah Thompson

Fuel efficiency starts at the hull.

Over the last few years I have been working on developing my own in-house hull testing procedures. Part of the process involves using the most up-to-date computer programs and also some programs I have developed myself over that time. As always, these are just prediction programs and are subject to many variations and analytical equations. The results will vary so I use a control hull to calibrate the programs when testing new hull shapes. The control hull is one that is already in the water with proven performance numbers. When comparing the new hulls in the program, I am able to see if there is a problem with the program or 3D model by running my control hull through it simultaneously. If the numbers are too far from the truth, I go back and refine the hull model until the programs start producing what looks to be “real world” numbers to me.

Almost all of my work consists of designing custom vessels. Therefore each vessel is unique, and all of them have very different requirements. Some of the vessels will be required to go fast, and others will be designed for the best possible fuel efficiency at cruise RPMs of the chosen engine model. So for every vessel I design I need to develop the hull shape to suit those requirements and displacement. One hull “shoe” does not fit all, in my case.

To demonstrate my point: I have a customer asking for a 12m sport-fishing catamaran. The cat will weigh in around 7.2 ton. They would like to cruise at 20 knots with a top speed of around 26 knots with the best possible fuel economy. As many would think, a CS displacement hull would be the most suitable for the fuel efficiency aspect, and a planing hull would offer the highest top speed. I developed 3 rough models and ran them all through my speed prediction programs.

Planing Hull. This is a semi asymmetric hull shape with a 28-degree deadrise. It has long planing surface area and will get up on the plane around 12 –14 knots. The asymmetric shape helps reduce wave interaction in the tunnel area reducing noise and also resistance from wave interaction. The hulls are wide and offer good internal accommodation space. They are also very easy and cost effective to build from a flat panel kitset.

Semi-Displacement. This is a wide body hull form. It uses width for buoyancy and offers really excellent internal accommodation. Over the years I have modified the front of the hull into what I call “Torpedo Noses”. The torpedo helps the hull shape in three ways. First it offers a longer water line length for the vessel. Second it offers a better forward prismatic coefficient for the hulls, allowing a smoother water flow over the wide body section of the hull. Third, it offers increased forward buoyancy to the hull. This allows the hulls to cut through a wave cleanly, but also rising up with buoyancy in the wave before the wave impacts the tunnel and creates a slam.

CS Displacement Hull. This is a slender body canoe stern shape. It has a fine water entry and a box aft to add extra buoyancy. At displacement speeds, it is an efficient hull form. However the narrow waterline greatly reduces internal accommodation space. 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 ran the three test models through my velocity prediction program to see how each would perform at 20 knots cruise speed and 26 knots top speed. I used a displacement of 7.2 ton for the predictions.

Planing Catamaran Hull (Green)
@ 20 knots requires 327 hp
@ 26 knots requires 527 hp

Semi Displacement Catamaran Hull – (Blue)
@ 20 knots requires 246 hp
@ 26 knots requires 450 hp

CS Displacement Catamaran Hull (Red)
@ 20 knots requires 207 hp
The hull speed tops out a 24 knots and requires 330 hp

The numbers show that fuel efficiency starts with the right hull design. If you’re thinking about a new boat, it is worth considering the effects of the hull shape early on in the design process. After making the right hull selection for your application, it is a matter of engine comparison to suit the hull shape to maximize efficiency. But that may be an article for another day.

By: Noah Thompson. http://www.noahthompsondesign.com/

Written for Multihull World 2010
============================
Above Blue Green and red in the text refers to some Hull pictures which I do not have. You may contact N. Thompson to learn more details.
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.
Regards
APP

[sabahcat]

Originally Posted by APP

Hi,

Is this section back at the stern?

Its just a made up picture for illustration purposes

If yes the boxy part of the boat is under the waterline. (I would like to know how much should normally be).

Are you talking about the picture of mine before that?
Should?
I used a measurement from vessels that have worked before which was around 200-250mm

Now, I believe even if the drag is the same with a similar boat, the boxy flat part aft helps to give some lift when the vessel enters in semi-planning mode, otherwise it will never plan.

Again if you are talking about mine I dont want it to plane, its a 10-14 knotter and Tennant himself said that at those speeds the hull shape I have is fine

The rest of the Canoe shape behaves at low speeds as a displacement vessel with low drag. Am I correct?

Quite probably but then there is the issue you quoted below

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.

And for me, that is unacceptable especially as there are so many places I have cruised previously that have 1 metre over the top of an entrance at high water

[eastcape]

Hi Everyone,

I had a request for a motorsailer about two years ago where the client wanted a top motoring speed of 24 knots, and while under sail to tack without help from the motors. He was looking at the CS hulls for the fuel economy. I tried a few variations on the CS shape, but was unable to find a happy marriage of power and sailing performance. I’m not an expert in sailing cats, so maybe someone will come up with a solution. If it were up to me I would look at a typical sailing hull and play around with the shape to straighten out the buttock lines to try and get a higher motoring speed. I think you could achieve a good compromise if you had the weights and balance in the right place, and did not have too high of an expectation on the top speed while under power.

[APP]

Hi,

For a Motorsailer Cat what would be the typical half entrance angle and the midship deadrise angle? Has it to be different in sailboat and powerboat Cats?

Regards
APP

[brian eiland]

Originally Posted by eastcape

Hi Everyone,

I had a request for a motorsailer about two years ago where the client wanted a top motoring speed of 24 knots, and while under sail to tack without help from the motors. He was looking at the CS hulls for the fuel economy. I tried a few variations on the CS shape, but was unable to find a happy marriage of power and sailing performance. I’m not an expert in sailing cats, so maybe someone will come up with a solution. If it were up to me I would look at a typical sailing hull and play around with the shape to straighten out the buttock lines to try and get a higher motoring speed. I think you could achieve a good compromise if you had the weights and balance in the right place, and did not have too high of an expectation on the top speed while under power.

I would approach this problem in a different manner. There are just too many positives about the CS hull shape to ignore it because of some tacking problem. I just recently received this response from a gentleman who owns a Tennant 57' powercat, and previously owned a number of sailing cats. He's also done LOTS of miles cruising offshore including a few ocean crossings. We were discussing the addition of a sailing rig to a powercar design; "Forget the normal tacking. When I circumnavigated I can not hardly remember tacking. If it was needed, start an engine and push her around. If no engines were available for some reason you can always jibe."

I might add this into the equation, unlike tacking a monohull where we try to spin the vessel around, most catamaran owners realize that its best to 'steer' the vessel thru the turn. Don't thrown the helm over, but rather maintain your momentum and steer the vessel thru the tack.

As to the CS hull form lacking sufficient buoyancy in the bow for either sailing or clearing those slamming waves to the bridgedeck, just add some extra length to the bows, and make it a bow with a nice bit of flare....a pretty bow rather than those plumb ones or reverse ones.

[brian eiland]

Originally Posted by sabahcat

Something I sort of have an issue with

A traditional cat hull is wider on the waterline but draws less

CS hull shape is narrower on the water but draws more

Brian replied:
Sure there is a slight bit of extra draft there, but think of the protection you eventally gain by being able to turn a bigger dia prop, and a horizontal one, and fully above the keel of the vessel.

[brian eiland]

Originally Posted by APP

PS: About the Motorsailer Chain/Belt driven systems: I think they should become widely used. I do not know any serious disadvantage in this issue. I suppose it gives a good advantage as e.g. the horizontal shaft to the DWL and maybe smaller engine rooms. No serious mechanical losses. Further with the fuel prices of today, Motorsailers for a Boat above 14 m should be generally a first choice, or not?

The chain/belt system does not have to be external to the hull as in my 'outdrive' dwg, but rather can exist inside the CS hull ahead of the canoe ending. Being a vertcal drive it can allow for the engine to be placed up over the prop rather than shoehorned down into the lower portion of thin hulls.