Stretching Wharram Tanenui questions

Hello all, I'm getting ready to build a Wharram Tanenui Cat. I know, not the most popular boat on the forums here. ;-) But she fits my wants & needs.

I don't know how much information to give about the Tanenui so can give suggestions about lengthening her, but she's 28' OAL, WLL 23' 5". I haven't gotten my plans yet so don't have some dimensions yet. So probably won't be able to get definite answers till I can give all the dimensions, but can hopefully get a good idea if doable.

I'm going to raise the deck to the top of the bulwarks height, approx 8". May make the bulwarks 6" instead of 8" tall. This has been done before so not a worry there.

But, stretching the hulls is where I'm out of my element, in the dark and need help and advice.

I would ideally like to add 24" in the center of hulls fore & aft. Not literally, but figuratively adding a 24" long 'plug' in middle of hulls fore & aft.

Some data about Tanenui, has 4 crossbeams. Will have JWD Tiki style Crossbeams & lashings upgrade and Wingsail. The mast will sit about 1' forward crossbeam 2.

Standard beam is 12' 9" the Tiki style crossbeams increase it to about 15' or 16', I don't know the exact dimension till I get the upgrade drawings. Beam can be increased if needed.

Not sure of exact hull beam. The beam of each hull is listed as 3' 9", but I believe that's the cabin width. A guy that built and has a Tanenui said max hull beam is approx 4' 3". That sounds about right.

Empty weight is listed as 2200lbs load, 2200lbs. Using No. 1 grade Fir & 9MM (3/8") Okoume marine ply, as per plans, well plans just specify 9mm ply, (preferably marine), everything will be sheathed in Xynole. The exterior sheathing optional,and is not considered structural.

Thoughts about stretching hulls 24"? What other data is needed to be able give a more definitive recommendation?

Cheers, Allen

PS, if it can stretch 2' 6" that would even be better.

 

I would imagine that request for a stretched version of a design may well not be a novel request, so have you asked Wharram about it? He may have long since worked out the alterations you desire. He would certainly know what was needed. And I would suspect that having built to a modification from the designer's specifications may positively affect the value of the boat later on.

 

Yes, I sent an email to Hanneke his partner about this modification. Have't heard back yet. They tend to be pretty slow from what I hear.
Plus don't know how such a change will be viewed.

So I am gathering information on options if I don't get assistance on this.
Partially too is checking the feasibility of this change.

If it's a reasonable change, as in not to far out of bounds of practicality, will be comforting to know. Then if James & Hanneke don't wish to assist, I know I can proceed with it but will need to figure out the details.

Cheers, Allen

 

If this were a monohull you could get a copy of Gerr's elements of boat strength and compare the scantlings his rules produce with those of the unmodified design and one of scaled up proportions. As a matter of the hulls themselves you could probably still do this ...

... but for a cat the bridge deck beams may be a real issue. Gerr gives no help there. Likewise hull separation may have to change.

Here's hoping they get back to you positively.

And remember to post pictures of your build.

 

Yes I'll do a little blog on build. lol Just because Wharrams Classic series doesn't have much information about the building because most were built before the web. But it won't be about me, just a building blog! plus I'll post pics here also. But I'll put amore pics on the blog page.

I don't think crossbeams strength and hull separation will much of an issue. As the Tiki style crossbeams in the upgrade drawings are stronger than the Tanenui original design. The Tiki style Crossbeams and lashings upgrade is from Wharram. The Tanenui can be built to the width of the original or to the new wider specs.

The Tiki 30 only has 3 crossbeams & the Tanenui has 4 both same weight specs. Also the crossbeams are built like Tiki 31s and they weigh over 1000 lbs more. The classic series boats are built like tanks compared to the Tikis but I love that! lol

Cheers, Allen

 

Typically, you just add a few extra inches to each station.