Ride comfort in power tris

[marshmat]

Well, I've been avoiding work again.... and instead, skimming over some papers on Nigel Irens' tris. An interesting issue that came up is that of ride comfort, particularly in smaller tris (let's say, under 10 tonnes). Irens seems to like putting the crew as close as possible to the roll and pitch centres of the hull. Dashew monohull yachts share this philosophy, and I think it's a good one.
But in a smaller craft, that's not always practical. So if the crew have to be placed higher in the boat, so that they actually fit inside, the question becomes: how best to keep the motion comfortable for the crew. Do we make the outside hulls small and sharp-keeled, so they cut the waves without pounding, or do we make them bigger and flatter so they don't sink in as much? Do we push the outside hulls as far out as possible, or do we bring them in close for a less abrupt, but larger, roll? Or do we make them slide in and out according to the conditions at hand?
I know this can all be calculated for a particular boat; rather, what I'm curious about here is what general design directions are preferred by the folks on here. Any thoughts?

I'd use relatively small outside hulls that would not pound -say sharp forward,fairly high beam to length ratio and rounded aft for low wetted surface along with small foils on each outside hull that would allow "tuning" of the roll response. For the same reason I'd include an aft foil to allow some "tuning" of the boats pitch response. The small foils would be a compromise design with a premium placed on simplicity, robustness and"weed-proofness".
I've done a number of concept sketches and doodlings of power tri's from outboard size up; if I someday decide-and can afford- just to push a button and go it would be with this type of powerboat if it worked as well as I think it will.

[yipster]

Cp sections, for a wet tri perhaps even swath ama's?

a quik search shows new links to Nigel Irens Design
found this PDF on the subject you might have been reading?

I know this can all be calculated for a particular boat; rather, what I'm curious about here is what general design directions are preferred by the folks on here. Any thoughts?

you said it: can be calculated, havent seen graphs on this
and on thoughts my replys would be too many

[marshmat]

That is indeed one of the papers I found, yipster...
Lately I've been giving thought to a multihull for several years down the road (when there's work, and money, and time....lol). Trouble is the 7 m beam limit on my favourite canals. On a tri, perhaps, how about sliding the amas in and out? Retract them to perhaps 2.5-3 m off centre for sheltered canals, extending to 4-5 m when the extra beam is needed? Just random thoughts, haven't really looked into the feasibility yet...

[CORMERAN]

To marshmat.

Given your limit of 7 m beam - a few suggestions:

- Based on the real world experience of creating a power tri.

- A length to beam ratio of 3 to 1 is practical.
( In the case of the outer hulls being fixed in place.)

Therefore: an overall length of 21 m is the place to start - for you.

An important consideration is that as there are no sails trying to
blow your vessel over - in a power multi hull. So there's no advantage
in excessive beam. We found in sea trials, even in pretty small
test models - a 3 to 1 gave us plenty of beam.

Also as vessels get larger - 4 to 1 starts to become practical.

- However: now; another concern looms.

The 2 to 1 ratio common in sail cats - is useful for resisting sail
forces. But also it helps to avoid the internal bow wave(s)
from coming together - untill AFTER the boat is out of the way.

Our choice to avoid interference issues, is to keep the
outer hulls well aft.
( Pitch poling far less of a concern, if you dont have poles. Or sails.)
This has the happy synergy of increasing buoyancy aft.
Usefull for planing vessels.
And / or increasing loading capacity.
Also trim concerns are reduced.

A sharp entry in the outer bows worked fine.
We also found lots of buoyancy in the outer hulls is useful.

- I think you can see a pattern developing here.

Power tris are significantly different, when no sails are
there to cloud the issue.
Also there are interesting differences in respect to power cats.

We are not alone in these observations - and we pedict that
the configuration we developed with our test program - will be
more and more common in the near future.

Cheers.

[tspeer]

You might want to read "Optimum Spacing of a Family of Multihulls". Your description of adding buoyancy aft in the outer hulls is heading in the direction of the diamond configuration studied in the paper.

[CORMERAN]

To tspeer:

I am familier with the work of messers; Tuck and Lazauskas.
- And yes, our craft has similarities to their diamond (DIA)
configuration.
- However, their ARR configuration - is closer to the mark.

i.e. { -= }

It's always pleasent to have learned gentlemen, supporting
the conclusions we have come to ourselves.

However:
there are some important differences.
- Their forms are THEORETICAL displacement hulls.
- Whereas; our TANGIBLE, real world design, is suited to
displacement OR planing speeds.

Their paper does have merit - but it serves, to keep in mind
- that they live in the esoteric world of mathmatical theory.
So extraporlating their findings requires a good dose of
common sense - before using same.

As, to quote the authors themselves,
" No attempt is made.....to optimize the shape...." ( of the hulls)
" ....the optimum hull spacings are sometimes INFINATE ! "

It can be appreciated - that long before infinity is reached
- the excessive weight of ultra long, connecting beams will
have sunk the craft.

In the design of our fast multi - hulls, of course we incorprate
solid scientific criteria.
However - I come more from an IMPERICAL point of view
- than the theoretical.
The only scantlings, or specs. that I really trust - are ones that
I have observed to work in the field.

So our Tri hull design is more evolutionary than revolutionary.
This ensured that only minor changes where required after the
sea trials of our prototype.

Cheers !