Express Cruiser

[Willallison]

I've been working on a design for an Express Cruiser.
Preliminary specs are as follows:

LOA 11.4m (Hull 10.3 + 1m boarding platform)
LWL 9.8 m
Draft 0.5m
Beam 3.6m
Beam wl 3.3m
Displ. 5500kg
AWP 22.6 sq.m
LCB 61.5 % wl
Area wett. surf. 25.6 sq.m

Coefficients:
waterplane 0.7
prismatic 0.7
block 0.3
D/L 160
Deadrise is 12 deg at transom, 20 at station 5 and 42 at station 1

Construction would be of vac-bagged glass over foam core.

Part of the brief (I have set myself) is to create a relatively high speed (max cruise 30 - 35 knots) cruiser, but one that is capable of comfortably and efficiently maintaining much slower speeds in order to adapt to weather conditions. Hence the (relatively) light weight and low transom deadrise.

A single Yanmar 6LY 420hp diesel is my current power plant of choice. It is the drive system which has me in a bit of a quandry. I'd like to run a surface drive - probably a timmable unit like the Q-Spd http://www.ultradynamics.com/section...ves/drives.asp - they're relatively simple, efficient at the higher speeds, low draft - and just plain sexy. But how would the boat perform at slower speeds - say at 18 to 25 knots?
Would I be better off with a sterndrive? In order to cope with the required power it would need to be something like the Konrad unit - http://www.konradmarine.com/ But for use in Australia, this creates parts availability etc issues.
Waterjets are notoriously inefficient at intermediate speeds and a conventional shaft performs poorly at the higher speeds.....

So - what do you think?
Too little transom deadrise - she'll pound like buggery in rough water?
Numbers speak louder than words - my specs don't look good?
Surface drive is a dud - she'll be a dog at 20 knots?

images at:
http://www.boatdesign.net/gallery/sh....php?photo=449
and:
http://www.boatdesign.net/gallery/sh...=&sort=1&size=

The design looks great but to most folks "slowing down" for heavy seas is off the plane , & in displacement mode.

There I think it will handle like every other plaining boat in heavy weather LOUSEY.


It takes very robust construction & a very robust crew to leap from wave top to wavetop , and NOT have all sorts of things crap out.

Even the USCG only does about 15K in their offshore boats , and they have the best construction and cheapest fuel in the world.

If you get "caught out" you might wish for a better low speed design.

FAST FRED

[Willallison]

Thanks for the input Fred.
It's true that at true displacement speeds planing hulls don't behave nearly as well displacement hulls - though many would argue that a good semi-displacement boat will be better again. There is an enormous cost to be paid in efficiency, however.
Actually I envisage this boat as more of a "coastal hopper", where sea states are either managaeable at the speeds I have mentioned, or avoidable when it really chops up. As always, there's a compromise to be had and in this instance, real rough water performance makes way for higher speeds at a reasonable cost.

[Jerry l Jones]

I am thinking the same way you are about needs from my boat, I talked with pulse drive and they acted like I would be better off with twins. to push a cruiser with surface drives twins would work better and that twin 200 hp would make those speeds.

[Willallison]

Trimax drives can only be used in a twin application (singles are used in racing) - and now you say Pulse recommend the same... I've not heard of others (like Q-spd or Arneson) suggesting this. I'd rather incorporate a single engine if possible - weight, space, efficiency etc. I'm more concerned about the ability of a surface drive to efficiently run at lower speeds....

[Stephen Ditmore]

Is this Will from Alaska? Once again I like your boat's aesthetics.

[Willallison]

Hi Stephen - nope, this is Will from Tasmania, Australia.
Glad you like the looks - how about the numbers and thinking behind the boat?

[Tom Lathrop]

Will,

I am answering your post on construction here.

I was referring to the weight of 5500 KG listed above in my comment about ability to get on plane at low speed. My numbers run from about 36lb/sq ft to 43lb/sq ft (175KG/sq M to 205KG/sq M) depending on the load. The boat planes easily with any load in between but I can certainly see the difference between light and heavy in both wake size and RPM required to reach a given speed.

Your numbers for 1500KG and 2000KG should allow pretty good low speed performance. For what you are trying to do, I doubt that you can go for much lower displacement and still run at the speeds you want to.

Remember that this information is coming from an amateur's experience but it works for me.

[Willallison]

Thanks Tom.

So O-1 seems ok - but my 'express cruiser' (243 kg/m^2 on the numbers above) you think is too heavy. This is the problem when you are just learning the ropes. I based my target displacement on vessels of similar type and size (as similar as I coulc find anyway) - but for all I know, it may be possible to build it with an all up weight of 4000......
Until I've learned enough about structural design to make a sensible weight estimate, the figures are little more than somewhat vague guesstimates....

[badges65]

Hi Will,
have you ever driven a boat with that sort of stem at planing speed and in a following sea???
don

[Willallison]

nope.

[badges65]

Hi Will,
well you will not want to either as it will be a very hard hull to control in a following sea as it will broach very easily and if you run through a fairly even wave pattern slightly off the quarter it will take control of your steering.
we set up a 20' hull (kingfisher) many years ago with a similar bow section and nearly lost it on the demo.
you would be advised to increase the rake of the stem and put more flare in it..
don

[Tom Lathrop]

Will,

Badges is talking sense. A very deep and fine entry like you show with a wide low deadrise stern will give steering problems in following seas. I had a small fishing boat like that about 10 years ago which taught me that such design elements should be avoided unless the boat is very long compared to the wavelength it will encounter.

Most powerboats have some problems with downwind steering in waves large enough to lift the stern and drive the boat into the wave ahead. The sharp bow will dig in and the stern will be shoved to one side unless BOTH of the waves are symetrical and perpindicular to the boats axis. The bow entry shape is a compromise between fine enough to keep pounding to an acceptible level and full enough not to dig in. In my opinion, the below chine bow sections should be convex and never concave. Transom waterline beam should be less than midships chine beam to help with downwind tracking too.

With the type of boats we are interested in, downwind steering is never going to be a hands-off proposition. Even deep symetrical sailboats can be a handfull downwind sometimes.

[Willallison]

I knew where Badges was coming from when he 1st asked the question. The potential down-sea handing problems are mostly related to the rate at which displacement is increased as the bow is buried into a wave. This can be addressed by incorporating lots of flair and/or increasing the stem rake, but as you suggest Tom, can also be affected by the deadrise angle in the fwd sections.
I must admit that I do have some concerns in this regard as far as this design goes. Deadrise @ stn2 = 57deg. @ transom = 15deg. Quite a deep forefoot, but not uncommon by any stretch - just take a look at any of the Carolina style sportsfishermen for instance. The difference is that I haven't incorporated any sort of keel into the design - it may be necessary to do so, but I'd like to avoid it if I can. I have also incorporated quite wide chines so displacement will be massively increased at the point of their immersion. And the max beam is at approx stn 6, narrowing slightly to the stern.
It also brings up another concern regarding the surface drive. A sterndrive and conventional shafts + rudder I would expect to provide much better grip on the water, improving down-sea handling....?

I must say, I really appreciate the feed back - I submitted this design for one of my school lessons and didn't get anywhere near the response generated here. (Just 97%...he he he )

[Tom Lathrop]

Will,

I live very near the center of activity of the Carolina Offshore Fisherman. Unlike your boat, they have long bow overhang and don't look nearly as fine forward as your graphic. The eye may be deceiving though. A main point is that I see no advantage to extreme knuckle on the bow. There is no buoyancy to speak of and it mainly provides a blade of lateral resistance for the water to grab.

The Carolina boats usually have a bit of rocker in the aft bottom which they like to keep the bow up when running in waves. I recently visited a 54 footer under construction that had completely straight and parallel buttock lines which I commented on since this is unusual. The dual exhausts on this boat are over 8 inches in diameter so she will have enough power to stand on end if that is what they want.

These boats were developed over many years to get out to the Gulf Stream and back fast and satisfy both the paying day tripper and the owner's image of what a giant toy should look like. The extreme flare is the most obvious element of that. Some of them are truly handsome and predatory looking craft though.

My deadrise at 25% aft of the bow waterline is about 26 degrees with some convexity. It works very well and I would stick close to that if I designed another similar boat. I have stuck it into a few off angle waves that came up to the sheer and there was some broaching tendency but it was corrected easily with the throttle and wheel. The same condition in some other powerboats has scared the crap out of me. One of them, a C Dory which has a seaworthy reputation, layed over on the flared side and tried to broach. Would not come back up until I cut the throttle. Not a good thing