Hull speed

SpottyDog5

SpottyDog5

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Im trying to find out what the theoretical top hull speed of SpottyDog, can someone assist me please?

I guess the info required would be:

Weight - 19.75 tonnes
Length at waterline - 43ft
Hull type - Semi displacement
Engines - Twin cummins 480hp

What else do you need ?
TIA
 
I believe the rule of thumb formula is 1.3 x square root of water line length = max displacement speed
so for 43 ft roughly 6.55 x 1.3 = 8.5 knots.
you can tell when you are just past it as the front of the boat starts to rise up. Obviously different hull shapes displacement etc will affect but the above would be a good approximation.
 
The theoretical Displacement hull speed is a function of waterline length - and is the point at which power requirement rises dramatically to vercome the bow wave of the boat. As already said square root of LWL * a constant - which empirically is in the range of 1.3-1.4.

You can achieve this with very low power, depending on weight, but to exceed that and get on the plane you need a massive increase in power - and then the "rule" no longer applies. Weight and power then determine the speed, plus some lement related to the size and shape of the planing surface.
 
Thanks guys,
I guess what Im getting too, is on a recent trip she toped out at 24knots, 4 months of bottom growth (copper coated) is this the best I'm every going to get ? perhaps even a couple more, with a very clean bottom.

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....
 
SpottyDog5 said:
but there must come a point when no matter what amount of HP you give her, she reaches a max hull speed ?
Click to expand...

That's more true of full displacement hulls. Semi's and planing hulls give different characteristics when overpowered. Some can be dangerous.

I know of a Broom 35 European that was - at the then owners insistence - fitted with over-powered engines. When given the beans she had the habit of falling sideways and riding on one chine.
 
SpottyDog5 said:
Thanks guys,
I guess what Im getting too, is on a recent trip she toped out at 24knots, 4 months of bottom growth (copper coated) is this the best I'm every going to get ? perhaps even a couple more, with a very clean bottom.

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....
Click to expand...

You are way above hull speed at 24.

At those speeds more power = more speed. but it's LOADS more power for a tiny bit more speed.

Hull speed as has been said, is about LWL.

A boat creates a bow wave as we know. As we go faster, the bow wave gets longer, and moves further aft.

Once it reaches the transom, that's hull speed.

This is a sailing boat at hull speed. See how the bow wave is right aft?

sailboatathullspeed.jpg
.

It has to plane to go faster.

Now the most effective planing shape is a completely flat bottom like a tea tray. Needs very little power to plane, but gives far too hard a ride.

The most comfortable shape is a blade, but that needs infinite power to plane.

So we end up with a compromise, here are 2, one quite flat like an American lake style boat, the other more like British planing boats.

planinghull.jpg


Both are sharper at the front to give a comfy ride, flatter aft to keep up on the plane.

planinghullbottom.jpg


Only the dark bit is touching the water on a flat day, the sharper bows come into play when there're waves.

You have semi d so it's the brute force and ignorance method of planing for you.......
semidatspeed.jpg
 
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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.
 
FalconSteve said:
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.
Click to expand...

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.
 
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 :)
 
SpottyDog5 said:
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....
Click to expand...
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
 
Deleted User said:
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
Click to expand...

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.
 
A certain Spreafish went from a pair of 380 QSB's to 480's and gained about 1.5 knots.
Click to expand...
I'm guessing though that the 480hp QSBs were heavier engines in which case there is a self defeating element to fitting larger engines
 
burgundyben said:
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.
Click to expand...
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...:eek:
 
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