Newbie Questions: Hull / Planing Speeds, etc...

[rclarke246]

Hello All,

Newbie here. I'm confused about exactly what is meant by "hull speed", "planing speed", "cruising speed", etc...

I'm "all about efficiency", unless faced with extenuating circumstances.

I know, I can hear some of you now, "then WHY on earth did you ever buy that boat"??! Nonetheless, we did, we love her and have ZERO regrets. Remember, "A man convinced against his will is of that opinion still". I warned you that I was a "newbie"!

Here is some background for you:

Hull: Semi-displacement
LWL: 37' 6"
Beam waterline: 12' 8"
Displacement (full): 30,000 lbs.
Power: 2 Volvo Penta D4 turbos, 300 hp ea.

Here are a few of the actual stats I recorded last summer:
3,500 rpm's, 24 kn., 30 gph (WOT)
2,800 rpm's, 15 kn., 18 gph
2,000 rpm's, 11 kn., 8 gph

Has trim tabs, which have no effect under 2,000 rpm. When hull is planing(?) the "bow down" attitude results in an increase of about 1 kn.

As usual, we're interested in any thoughts or recommendations you may have.
Thank you!

 

[Richard W]

Trim tabs are not only to increase the hull speed while planing. Here is a short video about trim tab functions:

How You Trim The Boat - YouTube

The hull speed is a theoretical mathematical formula defining displacement hull/mode speed for a particular length of boat's waterline: Hull speed - Wikipedia, the free encyclopedia

The planning speed is the speed at which particular planing hull gets on plane and/or minimum speed it stays on plane. It is somehow murky in case of semi displacement (or semi planning) hulls ... IMO.

The cruising speed is more of a subjective value one prefers based on one's priorities ... speed/time to get there vs fuel efficiency.

 

[Ski in NC]

Hull speed is about 1.3x sqrt of water line length, so on your boat it is about 8kts. Planing speed is where boat rides over the bow wave and bow drops slightly, usually around 13-15kts minimum.

When increasing speed, once you get to hull speed any increase in speed takes lots of increased power for little change in speed.

Most boats of this type end up with two choices in speed: A touch below hull speed, say 7.7kts, or planed out at say 15kts or above. The burn rate per mile planed out is usually 2.5 to 3 times more than hull speed, so that speed is very expensive fuel wise.

Most consider the speed range between hull speed and planing speed a "dead spot" to be avoided.

Run 7.7 or 15up.

 

[DavidM]

Well, I'll bite and tackle your questions:

Cruising speed is anything you want to make it. Some people cruise at or below displacement speed, others cruise at wot all day.

The gurus over at boatdiesel believe that to cruise long term, ie all day long, day after day you should operate your engine at no more than 30-35 hp per liter. For your Volvo D4s, that is about 140 hp each or about 12-13 kts based on the data you gave.

You can cruise faster but at some effect on engine life.

Hull speed is a theoretical and practical concept. Theoretically it is 1.34 * sqrt(LWL) or 8.2 kts for your boat. Practically its when you have to start climbing over the bow wave to go any faster.

For displacement hulls this takes a huge amount of power because the aft hull is squatting in the stern trough which means it has a tough hill to climb. There isn't enough displacement at the aft end of the boat to keep it up. If you look at a displacement hull it is rounded and the rocker (the sweep of the hull up to the transom) is steep. All of which makes for little lift aft.

Semi-displacement hulls do have much more of the beam in the water aft that gives the hull significant lift. They also have flat planning sections aft to help lift the hull hydro dynamically as well.

But semi-displacement hulls never really get "over the hump" and "on plane" like planning hulls do. They sort of mush forward and up and the more horsepower you apply, the faster they will go. If you could measure instantaneous horsepower vs speed (some electronic engines can do this) and plot a curve you would see a smooth continuous curve.

Trim tabs help a semi-displacement hull because they provide hydro dynamic lift at the transom which makes for a better planning angle and the boat planes rather than mushes through the water.

A planning hull has enough planning surface area for its weight (actually the main difference between planning and semi-displacement hulls is the weight) so it can hydro dynamically lift up and get the hull out of the water so only maybe the aft third is touching.

If you were to do the same hp vs speed plot for a planning boat you would see the curve climb up and then level off for 5 kts or so and then continue to climb but at a reduced angle. Once hp per kt levels off you are "over the hump" and "on plane". The wake also smooths out and the boat feels "sweet" riding up high and skimming the water.

But, if you are all about efficiency, cruise at 7 kts and you will burn about 3 gph.

David

 

[FF]

>Hull speed is a theoretical and practical concept. Theoretically it is 1.34 * sqrt(LWL)<

This is ONLY for fat boats 3x or 4x as long as they are wide.

Once you get into long and skinny , cat or try hulls that speed formula does not work.

Years ago the AYRS came up with a simple method of figuring "hull" speed for a variety of boats.
The old formula was created by looking at fat boats of the 1800 rea , and doesn't work well for skinny fast boats.

S = L/3b X SQRT (L) When run for some theoretical hulls it may be a bit fast , but seems to work.

Practically a 6-1 LB ratio gets the job done on a heavy boat , 8-1 to 16-1 do better as the weight decreases.

Your boat is a go fast ,created to plane underway.

Its ride comfort and much stability come from being ON the top of the water.

Sure it eats fuel, but what is currency for?

 

[Richard W]

I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:

LWL = 25'
BWL = 8'
S = speed in knots

S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot

FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).

 

[FF]

ON a small light boat the results you get , the boat gets more efficient while up on the plane is common.

 

[Ski in NC]

I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:

LWL = 25'
BWL = 8'
S = speed in knots

S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot

FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).

This can be explained in part by the nature of gasoline engines. At low power settings, the amount of hp per gph (bsfc) is very poor. At higher power settings, it improves greatly. On a diesel, the bsfc stays much more constant, so low speed running sees nmpg much better than when planing.

 

[Richard W]

Got it ... makes sense. Thanks.

 

[Nomad Willy]

Diesel and gas are almost the same re efficiency at full bore.

But nobody runs at full bore.

Re tabs I can't imagine them being anything but drag on most trawlers that are typical here on TF. That would include wedges under the stern too.

 

[Richard W]

The OP's boat has a semi planing hull ... the tabs are advantageous in his situation.

 

[Ski in NC]

Diesel and gas are almost the same re efficiency at full bore.

Not close at all.

370hp Mercruiser 8.1 @ full power 30.1 gph: 12.3hp/gph

380hp Cummins 5.9 @ full power 20.1 gph: 18.9hp/gph

Pretty substantial difference.


Using 2.7 exp load curve

Merc at cruise 197hp 19.6gph: 10.1hp/gph

Cummins at cruise 208hp 10.4gph: 20.0hp/gph


Merc data from a Sea Ray 380 test report, Cummins data from qsb5.9 performance curve.

You can see the hp/gph drop towards cruise with the gas engine, it improves a little with the diesel. I don't have any data at hand to look further into the lower rpm numbers.

 

[RCook]

I have a skinny pocket cruiser with only 8.5' beam ... but it's within upper limit of the 3x or 4x as long as wide criterion. Just for kicks ... I thought I will calculate the hull speed based on both formulas:

LWL = 25'
BWL = 8'
S = speed in knots

S = 1.34 x SQRT(LWL) = 6.7 knot
S = LWL/(3 x BWL) x SQRT(LWL) = 5.2 knot

FWIW ... my hull clearly follows the first formula as it's efficient "coasting" or hull speed, as opposed to efficient "planning" speed, is 6.5 to 7.0 knots. Interesting enough, at the optimum throttle settings while coasting (1400 RPM) or planing (3400 RPM) I get the same 2.5 MPG on my gasoline powered VP 8.1L 375 HP engine (not talking about GPH here).

Here's a similar sized example in diesel:
25'7" LOA, 23' LWL, 8.5' beam, 19 degrees deadrise at transom, 11,000 lb on the water loaded.
We cruise on plane at 18 knots, 1.75 nm/gal. At 6 knots, 4.5 nm/gal. We like 6 knots.

 

[Richard W]

Here's a similar sized example in diesel:
We cruise on plane at 18 knots, 1.75 nm/gal. At 6 knots, 4.5 nm/gal. We like 6 knots.

I like your 6 knots cruising characteristics too ...

My amended results, gasoline engine ...
my 5.3 MPH does not even come close to your 6 knots:

1000 RPM ... 05.3 MPH ... 03.4 MPG ... 01.6 GPH
1400 RPM ... 07.2 MPH ... 02.5 MPG ... 02.9 GPH
3400 RPM ... 30.2 MPH ... 02.5 MPG ... 12.1 GPH

(leading zeros added for alignment)