Malcolm Tennant Cat Comparison - Question

[APP]

Hi,

In http://www.catamarans.com/news/2006/...Comparison.asp
By Malcolm Tennant, Multihulls Magazine

Malcolm Tennant wrote: "Generally the planing cat is designed ………
……………. At somewhere around 18 to 20 metres it makes much more sense to go to a displacement hull form which at this particular size will go just as fast, if not faster, with considerably less horsepower than the planning vessel."

Can someone explain in detail the reason of above statement? As it could be easily understood that planning Cats are good only for a vessel up to a 18-20 meters long (?).

Thanks for any clarification.
Best Regards
APP

[redreuben]

I would assume he is talking power catamarans and not sailing hence the reference to horsepower.
RR

[APP]

Originally Posted by redreuben

I would assume he is talking power catamarans and not sailing hence the reference to horsepower.
RR

Yes. I understood the same, but a power cat with a displacement hull? Is there any evidence by ship software programs calculating resistance?
APP

[catsketcher]

I was at a multihull conference in the late 90s and had lunch with Tennant. He seemed a nice guy and switched on.

Most cats are displacement hulls. It seems that only thew small ones can get light enough and get highly powered enough to plane. Most mono cruiser don't properly plane but rotate and dig the bum in. Then again if you really want to start a flame war ask for a definition of planing and stand back. It has happened before on a different thread.

A planing cat obviously will have two very different modes - sub planing and planing whereas a displacement cat will not have as large a drag hump before planing speed. Remember it is very hard to get big things to plane because they get heavy fast (square cube rule) so all big things are displacement hulls.

[yipster]

think that remark was made relative to a speed margin where bigger slender cats exeed displacement hullspeed

[southatlantic]

Very thin hulls ( waterline length to bem ratio of 12 or more ) generate such a small wake compared to the size of the boat that Froude's law no longer applies, in practice. All of Malcolm Tennant's designs are displacement cats. His best designs will reach a top speed of 24 kts or so and cruise comfortably at 19kts or so. See, for instance, Domino, a Malcolm Tennant design that was built in Paraguay - it is a long story - and has cruised her way from Paraguay, up the coast of Brazil and is now happily cruising the Bahamas http://dominocatamaran.blogspot.com/ ). They are much more economical than planning cats.

Other Tennant designs have not quite reached their design goals. Several other designers, especially in New Zealand and Australia now design displacement cats. But some also continue to design very successful large planning cats (Stuart Bloomfield is one that comes to mind - he designed my 46 ft cat ). And others, like Roger Hill, design both - depending on what the prospective owner specifies. Both of them have their own sites.

The site of the late Malcolm Tennant has a lot of information on this topic, as well as several links into other sites.

[Alan.M]

A Bob Oram designed power cat in displacement mode at 30 knots:

http://www.alluramarine.com.au/

[downunder]

Another Oram. Bob's own testbed.

http://boboramdesign.wordpress.com/v...ms-power-boat/

He has since designed some better looking vessles.

http://boboramdesign.wordpress.com/upolu-10/

http://boboramdesign.wordpress.com/agincourt-14/

[couch]

Have a 20 m custom commercial Tennant hull. It hit 25 knots in trails with a pair of 430 volvo's. The hulls are quite fine - less than 4 ' on the waterline.

Most of the planing cats we considered that were capable of the same speed required considerably more power (usually 50 to 100% more).

However, with the planing cats, you can still (in general) increase speed with more installed hp whereas with the displacement cat it will hit a wall ... in the case of our cat the wall is about 27/28 knots after which more hp would have little effect (other than a bigger hole in the bank account). At 2000 rpm we average 18 knots .... not too bad for a 20 m 100 passenger cat operating in the off shore ..... could use a deeper gear reduction and larger dia prop to get the speed back up for the same rpm but that's another story.

[APP]

Originally Posted by couch

Have a 20 m custom commercial Tennant hull. It hit 25 knots in trails with a pair of 430 volvo's. The hulls are quite fine - less than 4 ' on the waterline.

Most of the planing cats we considered that were capable of the same speed required considerably more power (usually 50 to 100% more).

However, with the planing cats, you can still (in general) increase speed with more installed hp whereas with the displacement cat it will hit a wall ... in the case of our cat the wall is about 27/28 knots after which more hp would have little effect (other than a bigger hole in the bank account). At 2000 rpm we average 18 knots .... not too bad for a 20 m 100 passenger cat operating in the off shore ..... could use a deeper gear reduction and larger dia prop to get the speed back up for the same rpm but that's another story.

Very good! do you have any drawing or picture of the hull to show us?
Thanks
APP