efficient 10m displacement powercat

[groper]

well im having second thoughts on the design of this 10m powercat... initially i had a lightweight planing hull idea but the prevailing conditions rarely allow for high speed travel so...

Now Im thinking more along the lines of 10m displacement hulls... i initially didnt consider a displacement hull as i want this to be a trailer boat and i didnt think a narrow beam would suit displacement hulls due to the negative wave interference. i can go upto 3.5m beam which is max daylight trailerable with oversize load permits etc.

Im hoping for an efficient cruise around 17kts, powered by 2x 50hp 4str outboards. Setup for day trips and the occational extended trip for a few days with minimal gear. LIghtweight construction of balsa and epoxy/glass laminate, i should be able to build this to around 1500kgs.

i think it just may be possible without too much negative wave interference provided we push this quite fast and it runs at high froude numbers above 0.8. Negative wave interference isnt supposed to be all that bad from what ive read at high froude number, as opposed to maximum interference wave drag occuring around Fn 0.5. So ive kept the hulls with a high beam/length ratio around 15:1 and low Cp in a hope we can operate it at high froude number to avoid the wave interference problem of the narrow hull seperation.

So this is what ive come up with so far...

The waterline shown is with a displacement of 1000kgs per hull.
Cp=.55
DWL= 9.7m
beam @ DWL= .65m

I cant seem to get michlet to work from the freeship export file...? does anyone know what might be causing the problem? id like to get a power estimate for 17-20kt speeds...

I might have to look at increasing the beam/length ratio further to around 18:1 perhaps?

And once the design is finalized, what is the best way of getting the data out of free!ship for the bulkhead shapes at each station so that i can start building it?

Any other advice or ideas are welcomed.

Happy boating

 

[groper]

Im looking for ideas on;

Tunnel height clearance? - so far im thinking around 800mm from DWL.

Design ideas to combating "hobbie horsing"? - which i think is a challenge for a design like this.

Minimizing transom wave drag from squat under power? flatter or more buoyant aft hulls VS minimum cross sectional area designs? under power i dont think its possible to keep transoms clear of water in a balanced design that doesnt compromise other qualities???

 

[daiquiri]

I think that you're pushing the speed envelope of that hull a bit too much. But I'm not a cat expert, so I'd suggest you to contact Alik for that part of your inquiry.

As for this part:

And once the design is finalized, what is the best way of getting the data out of free!ship for the bulkhead shapes at each station so that i can start building it?

your best bet is to export the project to dxf file and then edit it with a 2D drafting sofware, like AutoCAD (costly) or DraftSight (free - http://www.3ds.com/products/draftsight/download-draftsight/) . Rhino can read dxf files too, if you have access to that software.
Another option is to export offsets of the stations in a txt file, if you prefer numerical output.

Cheers

 

[groper]

Ok here is another version... this time ive been inspired a bit more by the sailing racing cats...

rocker is flatter,
Cp is higher from a fuller stern,
draft is shallower,
beam length ratio has increased to a tad over 16:1 and Ive shaped the hull so that when ship is light (1800kgs) Cp = 0.61. When its heavy (2500kgs) cP = 0.64.
Center of bouyancy is 70% towards stern as most weight will be quite aft (outboards, fuel, batteries, people when fishing etc)

Ideally im looking to keep power under 60hp per side as the outboards jump up considerably in weight above this horsepower...

Do you think this would work better?

This 'Supercat' is the closest thing i can find to the type of boat im trying to design...

its 11.5m X 4.8m and is powered by 2x 85-100hp for 1lt/NM @ 17kts... main differences is mine will be 10m x 3.5m and powered by 2x60hp, hoping for the same cruising speeds... Supercat in south africa also makes a 9m x 3.6m and still achieves 17kts cruises with 2 x 40hp outboards... so i know its possible, just need to get it right...

 

[Brian@BNE]

The old Malcolm Tennant article is worth a read, and his CS hulls with a chine and additional bouyancy aft look pretty good. This ties in to your concerns for 'hobby-horsing'. Your design likely has adequate l/b, but your challenge will be keeping weight down given your 3.5m beam limit and the limited volume in the hulls to support it.
http://www.catamarans.com/news/2006/04/CatComparison.asp

Here's an interesting alternative I saw on the hard at Sanctuary Cove show recently. Its a trailerable cat that has slide out hulls, alternative modules can be fitted on top.
http://www.getawayconcepts.com.au/trailerable-houseboats

 

[Richard Woods]

I thought your first sketch hull had too low a Cp, so glad to see you've increased it. I still think it could be a bit higher if you want speeds in the mid teens

It's looking a bit like my Skoota 36 powercat except that my design has hard chine hulls. I wouldn't worry about transom immersion. Check out the data for warships, lots available on line and they have similar proportioned hulls.

I assume you already have a pretty comprehensive and finalised GA. Otherwise drawing a detailed lines plan is premature

See the powercat page of my website for details of my ideas.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[groper]

Thanks Richard, ive finally managed to get michlet working now - the export file from freeship was garbage and needed a few fixes to get it working - so now i can run simulations on the different hull forms and at least be able to compare the changes in some sort of quantifiable way... interestingly, the higher Cp hulls ive modelled above 0.64 seem to get worse in total resistance... do you have any suggestions?

What do you mean by "comprehensive and finalized GA"? im not familiar with this abreviation...

So far according to michlet, this has been the best shape ive modelled so far;

Cp = 0.57
displacement = 1.215 tonnes per hull.
beam/length = 14:1
total resistance for 2 hulls = 4.08kN @ 25kts and ~2kN @ 17kts

As i suspected, michlet predicts little to zero wave interference from the closely spaced hulls above 15kts speed... maximum intereference occurs around 8-10kts...

Still a work in progress, many more shapes to try and improve on this...

 

[groper]

Well after experimenting with many different shapes and running them thru michlet, i came across this paper --> http://www.patentgenius.com/patent/4079688.html

So i punched in the shape as per the dimensions in the patent and ran it again thru michlet... sure enough it came out with the lowest resistence yet, although only a few percent on my best efforts thus far.

Rt = 3.8kN @ 25kts and 1.9kN @ 15kts - 2.5m hull separation center to center, gives me 3.5m ship beam overall.
Total displacement (2 hulls) = 2.43 tonnes.
Max wave height at speed = 0.19m

Length/beam ratio = 16:1
draft = .365mm
Entrance angle = 3.4 degrees

There appears to be other benefits of this shape also including a very full stern - high Cp = 0.646 for better pitch stability, and a very aft CoB at 70% which suits the loading of the vessel nicely.

Ive run GODZILLA with constraints set suitably for this design criteria and the best minimum resistance shapes it could calculate were virtually the same total resistances as the above shape, so no gains to be had according to GODZILLA whether Wigley hull form or kayak shapes modeled. So without any real resistance gains to be had, i think ill stick with this form for its better seakeeping and practical use benefits. Godzilla predicted the wave interference to be non existant above 10kts and the ideal hull separation to be pretty damn close to what i had already. 2.5m separation is about right for this set of constraints.

So I think im about done with this hull form design unless i find something else interesting... does anyone know how to calculate the horsepower required for the speed and resistances quoted above?

 

[terhohalme]

I dont think that Michlet (as good as it is) is very suitable for so a high froude number.

 

[terhohalme]

You could use Godzilla optimization for hullform with dry transom area?

 

[Richard Woods]

A GA is the general Arrangement drawing. Basically a sketch of the whole boat

It is drawn long before a lines plan because, for example, the centre of gravity of the boat must line up with the centre of buoyancy of the hull. Otherwise it won't float level.

It is possible, but unlikely, that the three hulls you have shown all have the same LCB. If the LCB position has changed you'll have to redraw your boat interior, deck layout etc to match the new position.

And obviously any interior features will be affected by the hull beam, which has varied in your three hulls. And you need to have an idea of the boats weight to tell you how much displacement to give it.

Design is a long spiral, don't concentrate to much on one feature at any one time

I see no reason why a light 10m catamaran powered by twin 60hp outboards shouldn't do speeds in the high teens. Once you know the drag at each speed its easy to calculate the power required.

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[groper]

terhohhalme - ive run godzilla with dry transom area, there was no improvement to be had. This shape is already a minimum resistance shape as was the claim made in the patent paper. This hull ive settled on also has a dry transom, its *just* above the waterline... with a bit of squat it may run slightly immersed which im not concerned with.

Richard - i will start work on the layout now that i *think* i have a hull which will suit the layout, CoB, and loads this boat will carry based on a "best guess" thus far. Ill do some drawings of the layout in AutoCAD and post them up next time. Based on surface areas of fibreglass panels and the weight of the outboards, i *hope* i can make a decent calculation of its CoG but i think i would like help with this aswell as it seems like a problem for an experienced designer.

The layout will be quite simple and lightweight, its not going to be a block of flats... will be kind of similar to the "supercat" photo i posted earlier in the thread...

The laminate schedule will be 155kg/m3 seal primed balsa core with 600gsm d/bias glass both sides for the bulkheads (except main structural bulkheads that connect the hulls). The hull sides and bottoms will be 600gsm triax bnoth sides with an additional layer of 60gsm woven on the outside skin for minimum print/finish. - It will will be layup on a melamine table mold under vacuum (similar to the kelsall method) to make the panels, but instead using epoxy and finished with 2pk PU paint rather than gelcoat.

So the hull panels should be around 4.27kg per m^2. Then there will be some additional reinforcing along the bilge area - ill use some kevlar under glass here adding reinforcement in case i hit a submerged object... Then i need to allow for fillets and tape joins everywhere, plus the additional heavier laminate schedule for the main connecting bulkheads - ill probably incorporate some carbon if the shedule gets too heavy here. Seems alot of work to get this all worked out... any advise would be well appreciated so i dont forget anything!

Richard, what is the power required based on a total resistance of 1.9kN @ 15kts and 3.8kN @ 25kts? I dont know how to calculate this...

 

[Richard Woods]

Seems alot of work to get this all worked out...

That's how/why yacht designers earn their money!!

Richard Woods of Woods Designs

www.sailingcatamarans.com

 

[groper]

Well i feel like a bit of an idiot now... it would seem that power required is as simple as;

Required Power in KW = resistance (newtons) x speed (m/s)

This gives me 14.65KW @ 15kts and 46.74KW @ 25kts.
This converts to around 20hp and 60hp respectively for the 2.4 tonne ship or half that per hull.

Of course we need to allow for losses, air resistance, non ideal reality etc etc... but it would seem that the desired speeds are very much realistic from a pair of 60hp outboards... 40-50hp motors per hull is probably about right for typical operation in the high teens and moderate engine RPM`s... anyone concur?

 

[groper]

well after finalizing a general arrangment and applying it to the most efficient hulls modelled in michlet, the 3D render of the boat looks like this;

I think it should be rather obvious where everything goes so i wont bother elaborating on the arrangement, unless anyone has any questions or suggestions?

The exploded view of the panels to be laid up on the melamine table look like this;

Using the laminate schedule and panel areas, the shell weight = 990kgs. This does not include all the tape joins and epoxy fillets, extra reinforcing below waterline, fitout, hardware, paint etc etc although i have been a bit generous with weights on some panels to allow for this. With the 2x 60hp yamaha 4 strk outboards = 220kgs and the other non-included weight, a realistic final weight ready to sail i would expect to come in at around 1500-1700kgs. This gives me my required payload of around 1000kgs for a design displacement of 2700kgs and should give great economy with a length/beam ratio of 16:1.

Does anyone have any experience or comment on the effect of having a small run at the stern to reduce the transom area?