Planing Speed Question

[Larry Exum]

The full restoration of my classic Chris Craft Constellation is 90% complete. This past weekend I had installed new guages, GPS, Sonar, etc. and was testing the engines at higher rpm. The big girl (30,000 # dry weight)went from idle speed (5.2mph @ 1000 rpm) to 2/3 throttle (20.0 mph @ 3200 rpm) without any noticable surge, climb, or bow rise at all. I never felt her climb up on plane the way I have always felt (the break free feeling) with prior 20-25' fiberglass boats when beginning to plane out.

Is there a formula for determining the speed at which a boat of known Length(45'), Weight (30,000#), Size and pitch of Props (23x23 Bronze), and horsepower (330 hp x 2) would theoretically begin to plane?

Engineers, your calculations please.

Larry

[nedL]

Larry, that is one nice thing about that older Chris Craft design. As you described, the way she "went from idle speed (5.2mph @ 1000 rpm) to 2/3 throttle (20.0 mph @ 3200 rpm) without any noticable surge, climb, or bow rise at all. I never felt her climb up on plane the way I have always felt (the break free feeling)".
This is because of the relatively narrow length to beam ratio & the low deadrise of the hull whtich turns to a very flat run aft. In other words, its a pretty easy hull shape to push through the water at any speed. It's nice to have a boat that runs nicely at any speed & never has its' stem pointing to the sky. I will say that you may find that with all the tanks pressed up (full) and half a dozen or so people on the after deck you will appreciate trim tabs (she may wallow a bit at about 10 - 14knots.)

[Bruce Hooke]

Just based on waterline length I'd say that you are getting up into the planing speed range at around 10 knots. I'm not sure if 45' is the overall length or the waterline length but it doesn't make that much difference since the overhang at the bow is pretty short.

[Ian McColgin]

Especially with a really flat trim and 44 hp/ton you'd not expect blast off. Think of your basic little boat with maybe a hundred horsepower for a ton and a half or so. That's where you get some initial squat and lift-off.

You may see the moment if you accelerate very slowly and watch the wake. If there's a place where the wake decreases dramatically, that's your planing zone - likely about 10 kt - 15 kt. But the boat could be so flat it just does not show a marked place.

[brad9798]

If yours is like mine (35' Chris Craft) ... and, after thinking a bit, it really isn't- For you have a much deeper-V hull design than my Cavalier.

However, at 3200rpms I'm at about 18-20mph depending on conditions and I am NOT PLANING ... unless I have my trim tabs down fully.

Your boat is not going to hop up out of the hole, so to speak ... when you look down from the helm and you notice the wake starting to shoot out about directly under you or even further back, then you are planing.

My bow will definitely rise if I get on the throttle ... of course, I've 370hp pushing 12,800 lbs, while you have 660 pushing 30,000 lbs. So, I guess that explains my bow rising- I've got nearly 50% more power for my weight than do you.

Anyway ... I think I'm talking in circles :rolleyes:

[John E Hardiman]

Larry, all boats get some dynamic lift at all speeds, but the classic definition of planing is when the boat is traveling faster than it's hull wave which corresponds to a Froude number of ~1.0. Therefor your planing speed for ~32 foot LWL should be ~19 knots. If the boat is very heavy for its bottom area, as yours is, I doubt you will ever "feel" it rise up and over its bow wave.

Note that planing is not everything, in WWII British MGBs (short broad planing hulls) were faster than German E-boats (long narrow easily driven round bottom hulls) in smooth water, but much slower in the North Sea chop due to the vastly superior sea speeds available to the narrow hulls.

[ 10-01-2002, 03:19 PM: Message edited by: John E Hardiman ]

[Larry Exum]

Good answers, so far. But I am still looking for a formula if any marine engineers have one for this question. I judge her to be planing at 15-18 mph, based on the wake, however I would like to compare what I see to some hard data expectations.

Keep the formulas coming.

[John E Hardiman]

Larry, you are asking a question thats a little hard to get ones hands around. Do you want to know how fast the boat should go? How fast the boat will go? When the boat steps onto plane?

For quick and dirty first approximation on how fast she should go use either Keith's Formula:
mph = sqrt(LWL)*C*cuberoot(hpx1000/displacement in pounds)
where C is between 1.3 and 1.5
or Crouch's Formula:
mph=C/sqrt(displacement in pounds/hp)
where C is between 180 and 200

For the other two questions you will need to dig deeper in the hull form and start to dig around in the Series 62 and 65a&b hull series data along with the work by Savitsky, Hadler, Koelbel, and Brown to name a few who have studied parametric series of hulls. Come prepared for a lot of math and tables with ALL prameters of the hull at running speed known (such as wetted length-beam ratios, deadrise angles, beam Froude number, desired trim angle, etc). We won't even touch on props until the ehp questions are answered... :rolleyes:

All in all, if she doesn't squat, doesn't trim up, and steps out cleanly to 20knts at 2/3 throttle, why sweat it? Put on the tunes, pop a cold one and enjoy! ;)

[Peter Malcolm Jardine]

Hey Larry... I have a 1963 36 foot Constellation, and this is an interesting thread..I would judge the 15 to 18 mph to be about right.. I have 283's and plane is about 2400 rpm or so... I am still impressed with these hulls at speed ..3200 or so.. Of course with twin v8s the easiest way to measure your speed is the rapid way the gas tanks empty. I figure my 36 is about 15000 pounds or so and it is certainly a combination of brute force and ignorance that picks them out of the water. I think tabs would be a help for me too, but I want to look at it a little harder before I do it. smile.gif

[nedL]

Larry:

[nedL]

Larry, I 'grew up' on the 1950 version of your boat (46' C.C double cabin flying bridge, same underwater hull shape)so as for what I stated above I am speaking from sort of a been there done that point of view. As I said, it has a lot to do witht he length to beam ratio. I believe you have about a 12' beam which gives you about a 3.75 length to beam ratio, whereas a typical 32' boat with about a 9'beam has about a 3.55 length to beam ratio. The hight the ratio the more easily it will be able to get up on a plane without sticking its nose in the air. For our boat you would have said the transition speed fron displacement to planing was at just about 10 knots. At 12 knots she was definately 'on top'. Our boat had a pair of B&W Lathrop diesels (Hercules blocks), they were straight sixes, turbocharged & intercooled, 155hp each. At 2000 RPM she hit 10 knots, at 2100 RPM she was doing 12 knots, & wide open at 2400 RPM she did 17 knots. They were nice efficient engines, at 12 knots she was using seven gallons an hour for both engines.