# NATURAL SPEED . . . anyone?

Discussion in 'Boatbuilding' started by SURV69, Jun 17, 2010.

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### SURV69Junior Member

Years ago, I borrowed a book about building boats from the local library.

In that book a formula for "Natural" speed was given.

The formula was supposed to compute a rough figure for a boat's natural speed, which is considerably less than hull-speed. The formula was designed to work in absolute calm(no wind).

One of the aspects of this formula was to use beam, waterline length, draft and overall displacement. (BTW . . . there was also a formula to compute windage of the superstructure)

This formula caused for smaller boats to have a higher "natural" speed than larger boats, since it obviously takes more energy to initially make larger boats move.

My Venture 25 sailboat worked out to between 1.5 to 2 mph(maybe knots?). Smaller boats, if I remember right computed to about 2 mph(knots?), or so at most. Much larger boats resulted in figures of considerably less than 1 mph(knots?)

The time-period needed to reach this speed was not computed as such that a small amount of energy applied to a small boat would eventually move it. I know from experience that it took very little push(for less than 15-30 seconds) from me to move the Venture 25.

My renewed interest in this formula is related to my interest in electric propulsion and getting the most useable speed, most useable distance and least battery draw for my sailboat.

The idea is to see if a solar panel(or two) would be able to replace the needed battery draw to operate at a minimal, but adequate traversing speed.

I have sailed at 2 knots and although it's slow, this speed seems to be the point at which I get the sensation of actually moving . . . making somewhat worthwhile headway toward my destination . . . a speed I can live with in the duldrums.

My search for this formula has been futile(about 15 years now) and I am beginning to wonder if this formula was derived by the author and not an "accepted" formula.

Nonetheless, this formula has regimented my expectations of what I expect electric propulsion, at the least, to deliver for me.

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### daiquiriEngineering and Design

I've never heard of that term (which, for some reason, makes me smile ).
It may be the author's name for one of minimums in wave resistance curve or a similar particular point, who knows.

For example, a wigley hull has one of minimums at volumetric Froude number Fn=0.35 (another one, if you prefer, at 0.26). Since Fn is defined as
Fn = v / sq.root [ g * V^(1/3) ]
where v is speed and V is volume displaced, and volume V can be calculated with the formula
V = Cb * B * T *LWL,
you can substitute Fn with the value 0.35, the volume with the last formula, and obtain the velocity for which that type of hull has a local minimum wave resistance:
V = 0.35 sq.root(g) (Cb B T LWL)^(1/6)
It contains all of the parameters you have mentioned. I may then decide to call it Natural Daiquiri Speed, write a book on it - et voila, there it is.

Otherwise, I don't know.

Cheers!

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### SURV69Junior Member

possible

The book gave a number of formulae for all sorts of things, but it was a boat-building book, not a data book(the library also had a marine data book which was chock full of formulae).

ANYWAY, I do remember the author claiming that the resultant computation was a rough idea of the amount of energy required to move a boat.

The book, if I remember right was from the 40's or 50's. If I remember right, during the 40's & 50's large outboard motors were rather rare and most cabin cruisers(with O.B. motors) moved at a leisurely pace.

The formula is probably not important, since I(as well as anyone else), already know that it takes very little power to move a relatively small boat slowly.

To tell the truth, I think, maybe my interest is peaked more because I'd like someone to acknowledge that my memory's still good.

4. ### apex1Guest

Well,

thats easily achieved:
when you remember the book and some content from the 40ies `til now, your memory is quite good!
when that makes you happy...........

Unfortunately I cannot provide further info on this topic.

Regards
Richard

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### ancient kayakeraka Terry Haines

Maybe I am missing something here but I would say under those conditions the natural speed of a (sail)boat is zero.

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### daiquiriEngineering and Design

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### yipsterdesigner

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