Fast displacement with surface drives

Hi all!

I skipper a 53 ft yacht in a shoal delta, only suitable for small boats really. However, the boss wants to upgrade to about 85 ft...I'm thinking a fast displacement (semi-planing) hull with "Atkins tunnel" or surface drives might work.

- Could a 24 meter tunnel hull be designed to run up to 22 - 25 knots?
- Can surface drives work well under 25 knots?

RESTRICTIONS:
- Air draft < 3.5 meter
- Draft < 1.0 meter, with protected underwater running gear.

FURTHER REQUIREMENTS:
Low wake.
Lots of floating debris, nets, plastic, plants etc.
Extreme marine growth.
Poor diesel quality
Small and flimsy jetties.
Stern-to mooring with up to 2 knots side current.
Always hot.
Inshore usage CE-D.
Sometimes groups up to 40 people.
Day trips only.

My interpretation of the solution:
- Fast displacement hull for minimal power and low spec engines.
- Surface drives or "Atkins tunnel" for minimal draft.
- Lift roof for air draft.

DIMENSIONS:
LOA ~ 26 meter (hull<24m).
Beam ~ 5.5 meter.
Weight < 22 ton lightship
Speed > 25 kn
Cruise > 22 kn
Fuel ~ 2000 litre
Power < 1200 hp

Looking forward to your comments and recommendations! Anything is welcome. A design studio will get involved but I just want to do my homework...

Thanks!

 

Perhaps I should ask a more concrete question:

Will surface drives work well at this medium speed (25 kn) application?

Will the efficient fast displacement hull (with minimally submerged transom) work well with surface drives?

 

Several years ago we were heading up Grenville Channel on the west coast of BC, heading to Alaska and doing a humble 17 knots. Our VHF squawked and a vessel identified themselves
as an RCMP (police) boat telling us they were going to pass us .
A couple of minutes later an aluminum catamaran shooting significant rooster tails sailed by us like we were sail boaters and headed up channel.
I made contact with him and he told us that their boat cruised at between 25 and 30 knots, diesel powered and got about 1 1/2 nautical miles per imperial gallons.
(compared to our .8 nmpg that sounded pretty good.)

As we had been planning an aluminum hull build, these numbers seemed pretty good .

The RCMP have a few of these cats on the west coast but one is a little more famous. It is the Nadon renamed St. Roch ll, which did a circumnavigation of North America. An easy google
with the search words, images Nadon St Roche ll. AT 18 meters, ( 60 feet prox) this cat moves pretty nice. Their drives were twin Arneson
Somewhere on the net and I could not locate it today was quite an article explaining the boat. You might have better success

A search of aluminum catamaran plans, Australia, will produce a plethora of sites. Cats, aluminum and glass are very prevalent in Australia, perhaps the amount of unprotected
water makes a stable platform more desirable but while we were marina hopping, we saw many commercial dive, tourist, boats in the 15 - 20 meter range.

Perhaps a site might lead you to a company with experience in the size of your boat coupled with Arneson Drives

 

Hi Barry,

Thanks for your input. Semi displacement cats (shionning) would work well but our mooring is only 6.5 meters wide...

Besides, yachting is a pretty virgin market here and cats might be too progressive for the owner.

Interesting that those boats you mention cruise at 25 knots with the arnesons though!

Thanks,
Hype

 

I did a couple of planing hull (56' moulded length) whale watching boats a few years ago and they cruised comfortably at 25 knots with 75 passengers using twin ASD 12's. Depart Port displacement was 33 L.tons (74,000#). Not sure how a slender displacement (semi displacement) hull would do but make sure you have sufficient propeller blade area.

 

Efficient, fast, low wake, shoal draft usually means a multihull of some kind. If you're looking for smooth water operation, something like the Brisbane CityCat may suit: 26metres, beam 7.2 metres, capacity 130 to 160 people. Cruise at 26 knots or so with low wake (well, low for a 26m boat at nearly 30kn).
Built by BSC Marine.

 

JSL,

That's similar displacement. What was engine power?

Was acceleration comparable to shaft drive?

My guests often prefer 18 kn, hence semi displacement. Just curious if surface drives like that speed?

Btw above say 30 pax it's normally a 6 kn party cruise.

 

OzFred,

Indeed mostly smooth water.

But our mooring is only 6.5m wide and the owner also demands serious interior space and yachty looks.

Fuel efficiency is actually not important, just don't want high output engines, so need an efficient hull.
Have limited access to parts and poor quality fuel, so prefer simple engines. That are not too heavy....input on this (very) welcome too!

 

The thrust issues found with Surface Piercing Propellers (SPP) are often connected to the hump speed range, where the prop is required to develop high thrust at a low/midrange advance ratio (V/(n*D; where V is speed of advance, n is revs/second and D is prop diameter). As a side comment, that speed range is often problematic for water jet propulsion as well.

Now, with a slender hull, with a "slenderness ratio" (Lwl/(Displ)^0.33) above about 6 or 6,5, there is virtually no hump, which makes things easier for the ventilated propeller, as well as for the jet, in the midrange speeds. In this respect your hull should do fine with SPP's. But there is another aspect that has not been dealt with here so far, and that is the shallow water problem, which will be a total game-changer in your case if you insist on anything above "swimming speeds" in one meter deep waters.

 

Great info!

I was hoping the lack of an hump would help.

Jets don't like the amount of debris found in these waters...

Is the slenderness ratio calculation metric? I'm getting strange numbers

Low speed at shallows is fine. As long as we can access safely.

Thanks!

 

Slenderness is metric, ie length in meters, displacement in cubikmeters.

 

or length in feet and displacement in cubikfeet. ;-)
js

 

Ok so with lwl 24m and 27 tons full the slenderness ratio is 8.

Well above 6.5

 

Speed/efficiency at various kts - prop, surface drive, jet, etc. https://www.boatdesign.net/threads/speed-efficiency-at-various-kts-prop-surface-drive-jet-etc.55654/

Some good data from Ad hoc
Note there are quite a few discussions on the forum that have discussed this topic and on the net as well wrt surface drives and displacement hulls

 

Indeed interesting!

Although not much about SPP...

I started a new thread because the conditions are pretty unique. Was thinking a specialist might advise improvements or a different approach even.