Cruising Speed

[johnfpo]

Hull speed can be calculated for a displacement vessel.
How does the formula differ for a multi-hull (Prout Ranger Cat)?
does adding weight have any major effect on top speed
Thanks.............. John

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[reeac]

I would have thought that the single-hull equation would apply equally well to a catamaran as any interaction between the bow waves of the two hulls would be quite small. Increased weight would increase the hull/water interface area and hence the skin friction but again I think that this would be a minor effect unless you have the sail or motor power to achieve planing. Planing would probably become more difficult to achieve for a given hull, the greater the overall weight of that hull.

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[bob_tyler]

Try the old formula in respect of one hull (square root of the length of waterline in feet x 1.4). Then double the result to get the answer in knots.

Don't know if this would work but costs nothing to try.

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[alec]

Just a thought.

I only realsied lately that cats don't plane. They use the same theory that those very narrow 1930's style power boats used. If it was not for the liveability, modern boats could benefit greatly from this even now. Modern racing power boats are in effect motorcats. They use the space between the floats as a downforce foil to stop taking off ( sometimes).

Why are there no crusing powerboat cats ?

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[MainlySteam]

<<<Why are there no crusing powerboat cats ?>>>

Depends what you mean. If heavy smaller vessels with everything one owns plus the lawnmower on board (as is the case with some sail boats), then no. However, there are great numbers of cruising catamarans around Australia, New Zealand and the other Pacific countries suitable for coastal cruising and many do ocean crossings as well (up into the Pacific or across the Tasman Sea from here(NZ)). I would have thought the same in Europe, the Med, etc but cannot speak from experience for there. Also, power cats built here are frequently delivered by sea to the Pacific countries, Noumea, and Australia.

There are cruising trimarans too - if you want to see a large cruising power trimaran, have a look at <A target="_blank" HREF=http://www.nwbs.com.au/sup_yac.html>http://www.nwbs.com.au/sup_yac.html</A> which is being built for an Asian client. We have two power cats just starting in the same yard for a Bermudian client, so I will get to see the trimaran in a week or two.

John

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[bedouin]

I seem to remember that the formula for hull speed does not apply to long-thin hull forms so it does not apply to the hulls of a cat

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[timevans2000]

Dont understand your statement that cats dont plane. If rising up about 1 ft in the air and doubling your speed is not planing, what is it?

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[snowleopard]

there is effectively no limiting speed for a fine hull in the way that there is for a 'conventional' hull. if you keep on applying power it will go on getting faster.

derek kelsall has produced 2 formulae for comparing performances of multihulls which allows you to predict relative speeds under the same conditions. they are:

sailing performance: 5 x sq root(LWL ft x Sail Area sq ft / displacement lbs)
the resulting figure equates to speed in knots under a particular set of conditions, i reckon 10 knots of wind

motor performance: knots = sq root (LWL mtrs x HP / Disp tons)

using these formulae on the two multis i have owned gives pretty accurate results

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[timevans2000]

I sold my Prout Ranger 27 cat a couple of seasons ago after 4 seasons of cruising her. We did 5000 miles in her.

If you are talking about normal sailing (not planing) then weight will make a big difference to top speed. If you load her up too much it also effects sea worthyness as well.
If you are talking about planing then you would not plane as readlily either as it needs more wind speed to push the hull up onto the plane. The exception would be wave induced planing, where the acceleration caused by shooting down the wave gets the hull onto the plane and she will stay planing for several hundred yards until the wind drops or you reduce sail area.

How long have you had the Ranger? We had great fun with ours. Seaworthy boat but exciting to sail. Regularly averaged over 6 knots on passages across the irish sea. even when fully loaded in full cruising trim, as long as we had winds above F3. A bit under powered in light winds.

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[vyv_cox]

I have no idea how the hull speed of a catamaran is calculated but it clearly has nothing to do with the conventional displacement vessel formula. A Hobie 16 with a waterline length of something like 14 ft or even less is capable of in excess of 25 knots. Bigger dinghy catamarans, such as Hurricane, Tornado and the Inters, seem to derive their greater speed more from sail area than their hull length. Weight in this type of catamaran has a major effect. Early Hobie 18s were extraordinarily heavy and they were not competitive against later ones. Early Hobie 16s were lighter than later ones and the really keen racers went to great lengths to preserve the old ones, which were appreciably faster.

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[alec]

Interesting to know that they do exist john and thanks for the website. On reflection, I have probably seen some in the Med without looking too closely. It would seem that they are taking over from hydrofoils.The very large passenger types of ‘Seacat’ operate very successfully here in the U.K

Regards


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