Speed Length Ratios/Hydrofoils

Dear Steve and Linda: Having discovered SetSail site, I want to tell you what a terrific boat you have designed in the Wind Horse. What I found particularly interesting is your break up with the dominant design rules of cruising power boats, e.g., trawlers.

While my limited budget allows me only to dream of Wind Horse, I am myself involved with pushing ahead my own dreams of ocean cruising. Having known your work, I have already given up my trawler-oriented mindset and I am rethinking the entire issue considering the canoe-like hull proposition you so beautifully materialised in Wind Horse.

Having said this, I’d like to know your comments on two points.

-according to the equation of displacement hull speed (1.2 times sqrt water level length in feet, am I correct?), a 85 feet-long boat could achieve a max speed of around 11 knots. But boats often go far beyond these limits, as are good examples your own designs. So, I confess I’m puzzled. Could you, please, shed some light on the issue?

-have you ever considered adding lifting hydrofoils to your designs (I think about Wind Horse) not, of course, to turn them into 60-knot-out-of-the-water rocket-boats, but to reduce their displacement and wetted area by say, 30%, so increasing speed & range? I think of hydrofoils added, say, to the keel and skegs, in order to compensate for, say, 30% of the vessel’s weight, or displacement, and adding to stability as a bonus benefit. Is this way wrong? If so, could you point the problems with it?

Thank you for your attention. Best regards, Francisco

Thanks for the kind words, Francisco:

Regarding speed length ratios (SLR), the ratio a boat is able to achieve is a function of the displacement, and hull shape. As the displacement length ratio drops, the ability of the boat to break out of the hull speed regime increases. For example, with just 300 HP Wind Horse will do fourteen knots when she is lightly loaded. That’s a SLR of over 1.5.

As a designer what you have to play with is the relationship between volume in the ends of the hull and that in the middle. Getting this right helps reduce drag at higher speeds, but then if you reduce drag at higher speeds, you increase it going slow! Which is why you need to carefully consider what speed you want to optimize the boat for.

As far as hydrofoils go, these work best on high speed craft. Wind Horse is too slow for their efficient use.

Good Luck with your project – Steve

Posted by Steve Dashew  (November 30, 1999)

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