# What hull will be most efficient between 8-12 kts?

Discussion in 'Hydrodynamics and Aerodynamics' started by dustman, Oct 9, 2021.

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### dustmanSenior Member

If I am primarily using my boat between 8 and 12 knots would it be more efficient to use planing hulls, displacement, displacement with submerged transom, or some kind of semi displacement?

24' catamaran at 1200-1500lb displacement.

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The answer to this is driven by the hydrodynamics of what you're attempting to do, with what you have.

24' = 7.31m
1500lbs = 680kg

8 knots = 4.1 m/s => Fn = 0.48
12 knots = 6.2 m/s => Fn = 0.72

This means you're running at the hump and just post hump..either of, is not 'ideal'.

However, based on these values, your LD ratio = 8.3, this is sufficiently high that the "hump" is not so pronounced, and is characterised by much less trim too.

So, if you're weight does not exceed 1500lbs, then a typical high speed displacement cat would be suitable..

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### bajansailorMarine Surveyor

For general background reference, here is a link to Dustman's previous thread -
Efficiency: Hull interference vs length on catamarans https://www.boatdesign.net/threads/efficiency-hull-interference-vs-length-on-catamarans.65613/

His first post was :
"Say you have 2 boats of the same displacement (2000lb) and width, one 30' x 12', the other 24' x 12'. The first boat having finer but longer hulls displacing the same amount of water as the shorter boat. One would assume the longer hulls have more wetted surface area, reduced wave making at speed, and would experience more wave interference than the shorter hulls which have the 2 to 1 ratio.
Which boat would be more efficient at, say, 8 kts and 15 kts?"

It is an impressive thread, running to 5 pages, and 71 posts - Dustman, was this thread a deciding factor in you now going for the 24' cat, rather than the 30' cat?

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### Mr EfficiencySenior Member

Such a small lightweight boat will be easily powered, so any fuel use difference would be minimal in \$ terms

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### jehardimanSenior Member

Yep. Think of it this way:
24', 1500 lbs means that each hull supports 750 lbs. 750 lbs is 11.72 cubic feet. 12 knots at 24 feet give s Speed Length Ratio of 2.4. Assuming a typical Prismatic Coefficient of 0.7 for the Speed Length ratio means that maximum submerged sectional area of each hull can only be 0.697 square feet... or less if planing or semi-displacement.

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### dustmanSenior Member

I'm still exploring my options. What I gathered from that thread was that longer would be better in terms of efficiency for a displacement hull. Today I was pondering if a planing or semi displacement cat could me more efficient at that speed range. 24' is the shortest that seems practical or safe for island hopping around the Bahamas.

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### dustmanSenior Member

Would I be better off with or without a submerged transom?

Do you believe that increasing the length to 30' would make enough of a difference in efficiency to justify the extra cost of construction? Or that the little bit of extra weight would offset the potential gain in efficiency?

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### Mr EfficiencySenior Member

With twin outboards, running with one out of the water when conditions allow, would probably give the greatest increase in fuel mileage than anything else.

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Submerged.
The draft at the transom to be circa 60% of midships draft.

That depends upon how anal you are wit regards to efficiency.
If you want to measure the decimal places - yes.
If you're after a general 'trend'...then yes, kind of...but as always - it depends. Your LD ratio will increase (assuming all things remain) to 10.4, which is significant. Anything over 10 is super slippery!
But wherewith you will - in real world terms - be able to notice, depend upon your objective., and how fixed that 1500lbs remains through the life of the boat. If you think it'll increase in weight over time (show me a boat that doesn't...and i'll show you moons dust), then long term goal...yes.

In the narrow definition that you have set...

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### fallguySenior Member

what happened in a hurry for me is weight...

the way to deal with unanticipated weight is higher ppi or immersion and higher available displacement

these ought to get careful consideration

where the OPs question falls apart is 'same displacement'

they ought to vary

I realize the discussion is conceptual, but the same displacement for a 25% longer hull is impractical for the stated goals.

For a small example, this cruiser heading to Bahamas will need water, may require holding tanks, may enjoy autopilot..all add weight..

30' hulls may be habitable or just on the edge and now stairs heading into the hulls add as cushions and passengers

Anyhow, the exercise is not without its problems. The two boats really ought to vary in displacement.

Right now, I am at a point in my build where I wish I had a bit more displacement as I am nervous about weight...so offer my perspective.

A 24' cruiser will require all that a 30" requires unless you are not staying onboard at night,

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### fallguySenior Member

Planing cats are really out of the question. Not only are they horrible on fuel, but they are also not going to offer the same ride as displacement or semi-displacement hulls.

This is not to say a planing cat doesn't cross to the Bahamas, but if you get 1 mpg running at 20mph or 3mpg running at 10mph is also a factor here, no? For the same range expectation of say 180 miles, the planing boat needs 150 gallons of fuel and the displacement 50 gallons. The weight delta on fuel is 600 pounds. Okay, so you drop the range of the planing vessel, but surely not below crossing say 120 miles minimum. You are still double the weight and triple the cost..for one run.

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### fallguySenior Member

A better direction for this analysis would be to determine a desirable L/D ratio and then determine whether the 24' boat has enough available displacement for the sor. Spoiler-probably not.

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### jehardimanSenior Member

The hulls will never be habitable at this displacement (1500 lbs) at either 22 or 30 feet. Even with a multi-million dollar closed mold CF build. There is just too much other weight. All this going to have is an open cockpit bench and a trampoline. Look at the Maine Cat 22; stripped it is 1750 lbs.

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### dustmanSenior Member

There will be no accommodation in the hulls. Just a very small cabin up top.

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### fallguySenior Member

Methinks the point was missed.

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