# Theoretical hull speed for VERY NARROW hulled trimaran.

Discussion in 'Powerboats' started by Docta, Nov 26, 2008.

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### DoctaNew Member

Hey... I've been looking for some information lately on theoretical hull speeds on very narrow aspect ratio hulls, and cant find any good info. I have built one boat in the past, a flat bottomed fishing catamaran, with decent results for the flats and rivers and am looking now for a project to get me offshore and slice through decent chop on my may to the backwaters

I want to build an 8 foot beam 22 foot long full displacemt power boat, riding on three VERY narrow hulls, 8" a piece should give me as much float as I need, leaving a L:B ratio of about 33:1; kind of the same concept as the glacier bay boats, but a bit more extreme and with no "planing section" over the displacement portion of the hull.--- all of the hull speed calculators I can find do not calculate hull speeds for very narrow hulls... and predict this boat wont break 8 knots.

I was hoping somebody might be able to give me a reliable estimate of the theoretical hull speed of such an arrangement, and a reasonable estimate of the power needed to get three of these hulls to theoretical maximum hull speed and how many HP in particular Ill need to get to my target cruise speed of 20 knots. I am hoping for 20 knots at 60-75 HP in real world conditions, and I think this should be achievable but I want solid numbers before moving forward. Loaded at fishing/boating weight, I am assuming a total wet displacement of 2500#. THanks in advance for any help or info you could provide!

2. ### Guest625101138Previous Member

Hull speed is a defined parameter related to the celerity of the transverse wave group formed by the hull that has the same wavelength as the water line length of the hull. It was coined a long time ago when it was determined that heavy displacement hulls found it difficult to exceed the so-called hull speed unless they had very high power.

In deep water the celerity of a wave train is given by:
V=sqrt(g x Wavelength / (2 PI)) (g is gravitational constant 9.8m/s/s)

A long slender hull has relatively low wave drag compared with viscous drag so the notion of hull speed does not apply.

The detail provided so far gives you a starting point for a design.
Maximum length 20ft.
Overall beam 8ft.
Target speed 20kts.

You need to make an estimate of the overall weight as this will be critical.

You can use some free software Michlet/Godzilla that will give you an optimum for any configuration. I am not sure if 3 equal size hulls will give the lowest drag but it worth investigating. I know that a monohull will not give the lowest drag at that speed and length.

Rick W

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### terhohalmeBEng Boat Technology

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### PortTackerJunior Member

Two things - if the mainhull is very narrow, that's a pretty heavy motor you're contemplating. You'll waste half the power overcoming its own weight.

Based on performance I've seen over the years - I'd say if you keep the hulls narrow and shape them for low drag, keep windage to an absolute minimum and keep the boat light - at 22x8 feet you should easily be a ble to hit the 20 knots cruise with 30hp.

You might contact Mike Leneman at Multimarine.com - he's played with this concept with great results.

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