# Determining max speed in sea states using LWL?

Discussion in 'Powerboats' started by MILSAR, Nov 6, 2010.

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### MILSARNew Member

Is there a formula or formulas that are out there some where that would help us with boat design?

Is there such a formula that will project maximum speed using the L/B ratio or the LWL number? example, how fast would a 50' x 9' boat with 900hp go compared to a 42' x 11' boat?

Also, is there a formula out there that says you need to have a certain LWL in order to run a certain speed in a sea state. Say we have a 40' LWL, what would be our max speed in SS1, SS2, SS3...

Any and all help would be GREATLY appreciated guys!

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### terhohalmeBEng Boat Technology

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### gonzoSenior Member

It depends on the design. For example, a beamy but deep V hull will be able to go faster than a flat bottom boat. On calm weather, the shallow hull may go faster. Structural characteristics will also determine the maximum speed before the hull fails.

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### MILSARNew Member

Thanks for the replies so far. I've definitely started my homework on this. What I'm essentially trying to figure out if if the hull designs were very similar (deep V) would the longer boat handle a given sea state better than the shorter boat?

What if the shorter boat had a little bit wider beam. To give you guys a better idea I'll use some nominal numbers.

Boat 1 - 40' length x 11' beam
Boat 2 - 50' length x 9' beam

How would boat 1 compare to boat 2 say at a 40 knot cruise in sea state 3 and at a top speed of 60-65 knots?

I know this is a some what of a loaded question, but I'm trying to wrap my mind around all this and get an education on this stuff as well.

Appreciate the input guys!

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### PARYacht Designer/Builder

The amount of variables you're tossing into these questions dramatically complicates the ability to answer the questions.

For example as a rule a longer boat, which we can assume is also heavier, deeper and beamyer will fair better in rough conditions then a shorter, narrower, shallower boat attempting to remain at the bigger boat's side at the same speed, in the same conditions. This is because you're likely pushing the shorter hull into a sea state it would be better suited at going slower in and the wave trains are longer comparatively for the shorter boat.

When you start tossing in beam/length ratios, speeds of one over another in specific sea state conditions, then so many possibilities exist, that an answer is just an educated guess.

Suffice it to say a bigger boat generally can handle bigger seas, but the comfort level in a boat at these speeds is just a subjective thing so . . . If you're attempting to compare comfort levels with a powerboat, blasting along at 65 knots, then you're kidding yourself, as it's hammering fairly good at this speed and launching itself at regular intervals, which is never comfortable. Even at force 3, which is a very calm sea state for open water, you're going to slam pretty good at 65 knots.

This isn't to say some hull shapes can't be comfortable at 65 knots, but it is to say that in 40' to 50' yachts your choices are obviously limited and these speeds are past the edge of reasonable, for the most part. In short, 50' yachts traveling at these speeds are off shore racers or relatives of, therefore expectations of reasonable comfort levels are presumptuous, to say the least. To be frank, those that can afford to push their 50' yachts at these speeds rarely need to ask as, they're literally dropping hundred dollar bills into the fuel tanks every few minutes.

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### terhohalmeBEng Boat Technology

If your speed is lower, say 25 knots, the longer one should have smaller accelerations at sea, if the speed is over 30 knots, the accelerations of both boats are similar. This estimation is based on acceleration calculation in ISO 12215-5.

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