Are you on a plane when ?

[newinlet]

Is the hull of my 37 considered on a plane when the wake behind the boat turns into a rooster tail. Like most I would like to save fuel when I can however 7-8 Knts is not where I want to be. At about 1900-2000 671N My boat is doing 10-12 Knts and the stern is forming a rooster tail. It seems like a nice slow cruise BTW full cruise is 2300 RPM we run 2150-2250. So I guess my question is at the point when the boat froms a rooster tail is that better or worse econmy then full cruise.
Any feedback
Thanks Jim Stone

 

[SKYCHENEY]

The formula for hull speed is the square root of LWL X 1.34

Anything speed above hull speed is less efficient with the least efficient speeds being between hull speed and planing speed since you are just pushing a big wave in front of you instead of getting on top of it.

So, the best answer for you is probably to run at 7-8kts or at the 2150rpm cruise, but not in between.

 

[34Hatt]

Well since your saying you do NOT want to be at hull speed " Just like my Wife"

You need to probably be going faster then that. One way to see is do a plot of rpm verse speed. I find that going up 100 rpm gives me about 1 kt. Then at two different point it gives me over 2 kts.
This past weekend I went to block going there I did 24 kts going with the current. Coming home did around 20 kts with the current guess what I did better milage at the 24 kts. I can stick my tanks and measure to the 1/16th of a " and figure it to with in 1 gallon. I feel the two jumps in speed at 18 kts and 21 kts. Also when you find a good speed play with trim tabs at different speeds you need different setting. You can usally get another 1-2 kts out of the boat with tabs.

Squating is not were you want to be and it sounds like your just above that point. I be careful doing that without pyro meters you may be loading those engines!

 

[hatterass]

Nice to see you this weekend Dan. I agree when you are just bast hull speed the boat drags and motors run hard and the boat wants to go back to hull speed. I find that I can plane at 1800 in flat water and cruise at around 11kns. in rougher water where the boat gets knocked around from the waves the boat needs to be at a higher RPM to compensate for the extra drag or waves knocking it off plane. On the way to Block with the tide it took 3.5 hr at 11-12 kn at 2000 RPM. on the way home against the tide it took 4hr at 2200 at 9-10kn both on plane. when you see the turbulent water move away from your transom and start to flatten out you are on plane.
Dave
PS hope to see everyone next weekend.

 

[REBrueckner]

Most likely you are planing when you see that rooster tail. Yet rounded bottom hulls, for example, might leave a rooster tail and not really be "planing".

But whether that speed is less efficient than a higher speed is tough to tell unless you have some data....speed vs fuel consumption to calculate NMPG. You can keep track fuel consumption (and speed) yourself while running long periods at consistent RPM by tank measurements or maybe somebody with your boat will post their data....

Another way to get good data: If your engines turn their rated RPM at WOT, the DD propeller fuel consumption curve will give you the approximate fuel consumption versus RPM. Then all you have to do is run at various RPMs and record your speed via GPS. It's best to do those runs in slack water, but if you run in steady current, say with a current, your relative readings will enable you to determine relative efficiency;But if you pass by, say Marthas Vineyard Cape Pogue Point from Vineyard Sound, you may move from a following current to an opposing current and you'll slow down a few knots....and that will screw up your data points....so plot your results to be sure they look smooth....

Finally, running a 2300 RPM (WOT) two cycle DD at around 2200 RPM cruise is really hard use; you'll substantially cut the life of the engine. But it's possible that because of the modest HP and boat that this is a relatively efficient cruise speed as far as fuel consumption alone is concerned; but that high cruise RPM will most likely not be your most efficient overall cost point due to increased maintenance/repair. The rather universal recommendation is to cruise around 80% of maximum rated RPM...2200 is almost 96% of that.

 

[spartonboat1]

Finally, running a 2300 RPM (WOT) two cycle DD at around 2200 RPM cruise is really hard use; you'll substantially cut the life of the engine. But it's possible that because of the modest HP and boat that this is a relatively efficient cruise speed as far as fuel consumption alone is concerned; but that high cruise RPM will most likely not be your most efficient overall cost point due to increased maintenance/repair. The rather universal recommendation is to cruise around 80% of maximum rated RPM...2200 is almost 96% of that.

I believe the 6-71N was quoted above. Those were usually J&T's rated at 310hp at 2,500rpm. However, I and others seem to only achieve 2,450rpm at max throttle underway. I find that my 43'MY is happiest at 2,150 to 2,200. Used to heat a little on one side, until I had the heat exchanger cleaned out well, but now holds steady temp when at 2,200. The rule of thumb for high speed diesels is 200 under observed rpm at max throttle. Whether these old DD's meet that test, I don't know. But 2,150 was safe, even when heat exchanger challenged.

P.s., check the top of the engine and a plate should show max rpm.

 

[Boss Lady]

Each engine will be a little different depending upon its' HP rating and model.

Detroit specifies 1900 rpm for continuous duty for my 735 HP 8v92s and 2350 as the intermittent max rpm loaded. 1900 is about 80% of 2350. Most of this has to do with cooling capacity.

I have seen heavy equipment run at WOT all day for years on end (Frontend loaders and bulldozers). RPM below the max rpm is safe, it is the ability to off load the heat generated by the increased load (drag) created by our boats that is the issue. Most hot rodded boat engines do not have the cooling capacity to run for very long ata rpm higher than the continuous duty rpm. This is why Detroit specifies a continuous duty rpm and an intermittent duty rpm for pleasure craft engines.

 

[Genesis]

You're "out of the hole" when the wake separates cleanly from the transom. Look down over the transom and when you can see the bottom of "V" at the stern, you're fully out.

Below that you're not - once above displacement speed you're "squatting" to one degree or another.

 

[hatterass]

Rob,
i should have mentioned that I do not have DD in my boat and WOT for me is 2950. I am running around 2000 so well below the curve and get good fuel consumption their. Sorry, I did not mention that.
Dave
PS I have Perkins T6 3544 240hp