Hull speed

[Wavey]

Look on the positive side - You haven't got to pay the fuel bill for 2 big Yanmars

[CharlieAlpha]

But its Roy he will be out there measuring up to see if he can fit a pair of 750Hp to get over 30knts

[FalconSteve]

This might be a stupid question, but here goes... I am new to boating and am in the process of purchasing a falcon 27, she is a sports cruiser with a planing hull. Based in the water line length of roughly 24ft , if I chugg along at 6.5 knots will it be more economical than getting her up on the plane and cruising at 20 knots?

Clearly I would get from a to b quicker but I guess I want to find out which method will give maximum range.

[carlton]

...Based in the water line length of roughly 24ft , if I chugg along at 6.5 knots will it be more economical than getting her up on the plane and cruising at 20 knots?...

Yes.

[Elessar]

This might be a stupid question, but here goes... I am new to boating and am in the process of purchasing a falcon 27, she is a sports cruiser with a planing hull. Based in the water line length of roughly 24ft , if I chugg along at 6.5 knots will it be more economical than getting her up on the plane and cruising at 20 knots?

Clearly I would get from a to b quicker but I guess I want to find out which method will give maximum range.

stay at displacement speed is most economical
fully planing is next most economical
there is a no go area between the 2 that drinks fuel and produces excessive wake.

in a 27, 6.5 may be just into the no go zone, so you want to be doing (for example) 6 or less, or 20 or more.

[FalconSteve]

Many thanks for the replies, for some reason I figured that due to the hull design the reduced resistance in the water when planing would be more economical.

I guess that even at 6 knots if I going with the tidal flow it won't take too long to get from ocean village to the isle of wight (I measured it at 11 miles as the crow flies, so it will probably take around 1 3/4 hours or so. I can cope with that, but I will probably want to open her up at least part of the way, to get the fun factor

[mikef]

When new, she had 2 x 300hp Sabres, now re-engined with 2x 480 Cummins, but there must come a point when no matter what amount of HP you give her, she reaches a max hull speed ?
I'm very happy with her performance, just curious....

No, in theory, the more power you put in a planing hulled boat, the faster it will go. If the boat has a keel, it might start planing a bit on the keel which will make it somewhat unstable. Also the faster the boat goes, the higher the aerodynamic drag will be so the amount of power needed to go faster will rise exponentially

[burgundyben]

No, in theory, the more power you put in a planing hulled boat, the faster it will go. If the boat has a keel, it might start planing a bit on the keel which will make it somewhat unstable. Also the faster the boat goes, the higher the aerodynamic drag will be so the amount of power needed to go faster will rise exponentially

If you plot a graph for a planing boat showing hp and top speed its an exponential curve, you come to a point where massive leaps in hp bring very little extra speed.

A certain Spreafish went from a pair of 380 QSB's to 480's and gained about 1.5 knots.

[mikef]

A certain Spreafish went from a pair of 380 QSB's to 480's and gained about 1.5 knots.

I'm guessing though that the 480hp QSBs were heavier engines in which case there is a self defeating element to fitting larger engines

[MapisM]

If you plot a graph for a planing boat showing hp and top speed its an exponential curve, you come to a point where massive leaps in hp bring very little extra speed.

True, but Mike answer to SpottyDog question is still correct: there's no such thing as a "max hull speed", once the plane.
Incidentally, the problem of applying a huge amount of power can be (and normally is) solved by turbines.
There are other side effect which are critical at 200+ mph, like avoiding to take off for instance...