Am I at the right cruising speed

its a 2006 mainship 34T WITH A 6LYASTP
Perfect conditions
1. half full fuel tank
2. one person on board
3. not fully loaded
4. Calm breeze 10 knts wind speed

This is what she likes....

Depends how much fuel you want to burn, among other things.

I can’t read the units. Is that 9.2 knots and 8.5 something?

High Wire said:

I can’t read the units. Is that 9.2 knots and 8.5 something?

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Looks like 9.2 ft (depth) and 8.5 kts (SOG)

2400 RPM is great running speed since its right at the top of the torque curve.

if she feels right i would say it's good speed. At 2200 you could be a little more fuel efficient.

Just doing the math, I'd have guessed most economical cruise speeds would be between 5.8 - 7.8 kts based on a guess at waterline length of 34'.

-Chris

Certainly the boat is happy cruising at 2200 RPMs. Is it fuel efficient? No! What is more important to you, fuel efficiency or time? I am not very sensitive to being fuel efficient so I would be happy with what you are doing. The boat probably rides better at 8.5kts than it would at 7kts.

ranger42c said:

Just doing the math, I'd have guessed most economical cruise speeds would be between 5.8 - 7.8 kts based on a guess at waterline length of 34'.

-Chris

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I'm the same 6LYA STP and hull but w/o flybridge.
That is close to my optimum slow cruise.
I run about 1700 - 1800 RPM and get 7 - 7.5 kts.
I think I could do a little better efficiency a little slower but I find that a decent compromise.

If I want to go faster I run 2700 - 2800 RPM and get 11 kts +/-
In between those RPMs I'm not very efficient.
I avg 1/2 MPH for every 100 RPM at low and high end but it takes more RPM to get MPH in the middle.

I avg 2.5 MPG and est thats 75% slow, 25% fast cruise

Speed, gph, NM,.... something has to 'give'
According to the charts.... my Cummins 380 5.9 QSB, at 1000RPM, speed 6+kn, 1gph, 400gal fuel = I can go a long long way. 1200rpm, speed 7+kn, 1.4gph 5.1 NMPG. 1400rpm 8kn....2.1gph 3.76 NMPG. Of course, open the deck fills, wink, WOT, 3000RPM, 17kn, 18gph, .95 NMPG
The boat is loaded light. 100g of fuel, 150g water, 275' chain, 44# anchor and per usual the fridge and freezer running.
That my friends, may not equate to the real world. Add in a couple 100 pounds of tools and spare parts, 50-75 pounds of clothes.... etc.... that fuel charts is but a mere suggestion.
Again, according to the sea trial chart, my sweet spot should be about 1200rpm or 7+knts, 29% engine load, 1.4gph, 5.14 NMPG. Someone can figure out the "other way" and I suspect it will be pretty close to the chart.

Shouldn’t we also be discussing the propellor specs and transmission ratio?

Giggitoni said:

Shouldn’t we also be discussing the propellor specs and transmission ratio?

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I cannot disagree with you but there is such a thing of information over load.

I owned a 34T for several years and boated in Southern California. I would cruise slow at about 7 kts and burn 2.5 gph. I could also cruise fast and burn 11 gph.

8.5 kts is fine but you are beginning to push a bow wave so it won't be that efficient, certainly not as efficient as 7 kts. Don't try to cruise at 10-11 kts because there you are pushing a big bow wave and it doesn't smooth out (not that it ever does like a planing hull) until you hit 12 kts.

David

SILENTKNIGHT said:

its a 2006 mainship 34T WITH A 6LYASTP
Perfect conditions
1. half full fuel tank
2. one person on board
3. not fully loaded
4. Calm breeze 10 knts wind speed

This is what she likes....

Click to expand...

I get 8knts at 2000 in a 390 with the same engine if I run at 2200 i would be doing 9knts and burning fuel

Cartouche said:

I get 8knts at 2000 in a 390 with the same engine if I run at 2200 i would be doing 9knts and burning fuel

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Yup, I agree. With the Cummins 380hp (5.9L), the sweet spot will be between the mid 7 knots and the low 8 knots.
This is IMO, based on the sea trials chart.
The burn rate will be between 1.4 to 2.1 gph
Hmmm, 2200rpm would put me at 11 knots, burning 9 gph.
Your boat may vary.
So many things will cause a big difference.
1. sea state
2. wind speed and direction
3. current
4. attitude of owner

I have a single 6LPA STP2. I run it at 2400 rpms. Redline should be 3800rpms. At 2400 the turbo is whining nicely. Any more and the noise and fuel burn isn't worth the extra knot.

So let's see. If the Gulf Stream is 5mph, I center the boat in the Stream, I should travel at or very close to 5mph without the engine? Now that is fantastic fuel stretching.
Now for all you math wizards, how many RPM must I add to travel at 10 mph?
Oh no, a simulation equation!!!
If I were to try to do figure that, I would do it the easy way..... move to the center of the Gulf Stream, make note of my speed, slowly adding RPMs until I reached 10mph and make a note of the RPMs.

A good cruising speed has more to do w the hull and it’s design .. as to form (shape).

Long slender hulls will (more or less) just go faster. Fat boats will seemingly fun into the brick wall .. attaining a slower speed and being very reluctant to go faster. FD hulls to be sure but SD hulls and very much planing hulls.

For the few FD hulls on TF just about one knot below hull speed is for all practical purposes the “only” cruising speed. Lower is mostly ridiculous and higher is accompanied w much more engine noise and fuel consumption.

Long and narrow SD hulls (nearly non-existent on TF) will have the luxury of running nicely well beyond hull speed. But 99 to 100% here are not narrow so will need to stop pushing on the throttle about hull speed or less.

There is much talk about a “sweet spot” and this expression has come down through the years used by many. IMO it is the rpm, or range of rpms that produces the best speed (or range of speeds) that has noticeably less noise and vibration. Some boats have a wide range of speeds that may be cruising speeds that vibrate and make noise at most any speed. No sweet spot. And other boats that upon reaching a certain speed shake a lot. Frequently and fortunately those boats will be quite smooth above and below that speed.

Under propped boats are probably more likely to have sweet spots and over propped boats less so. This is so because under propped engines (lighter loaded) will be less likely to transmit vibration to the boat mostly through engine mounts and engine stringers.

Large surfaces of cabins and hulls will/may be prone to “oil can” and cause significant vibration. Stiffeners on these surfaces including the hull bottom may have profound effects on vibration. Look first on cabin sides, floors and roofs. Lightly built boats will be more prone to this kind of vibration.

Vibration - noise.

I am not trying to hijack the thread - I have a 500hp Cummins with a turbo charger. Those of you running at lower speeds - if you don't kick on the turbo charger - aren't you simulating a "blown turbo" and isn't that bad for the engine?


I maybe new to diesel and turbo's but I thought you wanted to run at least a speed where the turbo kicked in - if you are cruising more than 5 miles or so...


Am I missing something?
Thanks

Like everyone else, I have an opinion, right or wrong because I don't have any experience with that particular boat. Firstly, LWL is likely less than 34' on a 34' boat, but that's a separate issue. If you want best fuel economy, stay a little below hull speed. However if you want to go fast and can get on plane, I think you will get better mpg at higher RPMs. Again I am assuming you can plane. At 8.5 knts you are likely barely planing and not very efficient. If your boat does plane, the engine is more efficient at 3000 rpm than 2400 and you will get better mpg. "Generally" if you look at tables for turbo diesels with a planing hull you will see best mpg around 80% of max RPM (unless you are below hull speed).

LovetoBoat said:

I am not trying to hijack the thread - I have a 500hp Cummins with a turbo charger. Those of you running at lower speeds - if you don't kick on the turbo charger - aren't you simulating a "blown turbo" and isn't that bad for the engine?


I maybe new to diesel and turbo's but I thought you wanted to run at least a speed where the turbo kicked in - if you are cruising more than 5 miles or so...


Am I missing something?
Thanks

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The turbo will still be spinning at low speeds and light loads. As long as the engine burns clean enough under light load to not load up the turbo with soot, it shouldn't be an issue if you're under a light enough load to not build any boost.

Turbo and RPM (speed)
Every couple of hours open up the throttle to exercise the turbo under load.
"They" tell us prior to shutting down the engine, increase the speed (RPM).
I would also suggest, once docked, idle the engine a bit to cool down the turbo.

backinblue said:

Like everyone else, I have an opinion, right or wrong because I don't have any experience with that particular boat. Firstly, LWL is likely less than 34' on a 34' boat, but that's a separate issue. If you want best fuel economy, stay a little below hull speed. However if you want to go fast and can get on plane, I think you will get better mpg at higher RPMs. Again I am assuming you can plane. At 8.5 knts you are likely barely planing and not very efficient. If your boat does plane, the engine is more efficient at 3000 rpm than 2400 and you will get better mpg. "Generally" if you look at tables for turbo diesels with a planing hull you will see best mpg around 80% of max RPM (unless you are below hull speed).

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There are two misconceptions presented in this post:

1. Very, very few boats increase mpg as they go faster. The 34T sure isn't one of them.

2. At 8.5 kts the 34T isn't planing. It really doesn't plane in the true sense, but the wake and bow wave do smooth out at about 12 kts which is what I would consider as planing for that hull. Below 12 kts down to about 10 kts you are pushing the bow wave and climbing "over the hump".

And yes, I owned a 34T so I have some basis for this position.

David

DavidM said:

1. Very, very few boats increase mpg as they go faster. The 34T sure isn't one of them.

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Agreed. The only boats that tend to get better mpg on plane are relatively light weight, high powered planing hulls (that are very efficient on plane) with gas engines, where the huge drop in engine efficiency at light load out-weighs the worse drag from going faster.

The AT 34 has an OAL of 34'5" and a WLL of 32'6" so unless you have a swim platform the size of a dance floor, just subtract about 2 feet from your OAL and you will be real close to WLL. I have used that 'estimate' for years and I have always been real close.

Soooo, take 2 feet off OAL (interior space) and ignore the swim platform.
IF you doubt my word, go to an overnight slip and tell the harbor master, you have no idea what the OAL is and he will grudgingly come out and measure it for you. LOL
I always tell them, it is documented as 34ft and they are happy.
Oops, I forgot to include the 2 ft overhang of the dingy.

The new AT34s are sold as AT36 (same hull), including the swim platform. So I am moving up in the boating world... I now drive a 38ft AT OAL.

OldDan1943 said:

The AT 34 has an OAL of 34'5" and a WLL of 32'6" so unless you have a swim platform the size of a dance floor, just subtract about 2 feet from your OAL and you will be real close to WLL. I have used that 'estimate' for years and I have always been real close.

Soooo, take 2 feet off OAL (interior space) and ignore the swim platform.
IF you doubt my word, go to an overnight slip and tell the harbor master, you have no idea what the OAL is and he will grudgingly come out and measure it for you. LOL
I always tell them, it is documented as 34ft and they are happy.
Oops, I forgot to include the 2 ft overhang of the dingy.

The new AT34s are sold as AT36 (same hull), including the swim platform. So I am moving up in the boating world... I now drive a 38ft AT OAL.

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It depends a lot on how much overhang you have in the hull design. My "38" foot boat (manufacturer listed LOA which excludes the bolt on pulpit and swim platform) only has a 33'6" waterline. Add the platform and pulpit and the LOA is up to 42'4".

rslifkin said:

It depends a lot on how much overhang you have in the hull design. My "38" foot boat (manufacturer listed LOA which excludes the bolt on pulpit and swim platform) only has a 33'6" waterline. Add the platform and pulpit and the LOA is up to 42'4".

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Dont tell the harbormaster but when you are at the bar, or with your relatives you can brag about you 42 ft boat. LOL

I added a swim platform to my N46 and presto, I was driving a 49 ft boat.

Oops again, the N46 also had a RIB overhang so maybe I had a 50 of 51 ft boat. Who was I going to impress, I don't go to bars.

OldDan1943 said:

Dont tell the harbormaster but when you are at the bar, or with your relatives you can brag about you 42 ft boat. LOL

I added a swim platform to my N46 and presto, I had a 49 ft boat.

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I generally take the attitude of "the manufacturer says the boat is 38 feet (plus dinghy and davits once I add those), but if you'd like to measure, feel free"

rslifkin said:

I generally take the attitude of "the manufacturer says the boat is 38 feet (plus dinghy and davits once I add those), but if you'd like to measure, feel free"

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I have never 'thrown in' the measurement for the bow pulpit nor the RIB overhang.
The only 2 people who might be interested in the 'true OAL' are the harbormaster and the tax man. Or maybe the sweet young lady at the bar.
(gravelly old man voice, "Hey there little girl, wanna come see my boat?"

Good one Dan...

Regarding my preious post note that i did say "if you are on plane". Boats such as mine, and Back Coves and Sabres and the like, get the same or slightly better mpg at higher speeds when on plane (up to a certain RPM). Note that I am talking mpg not gph. All very dependent on the particular boat and many other factors, the fuel cost to go from 12 knts to 16 knts for example can be negligible or even slightly favorable if the engine is operating at a more efficient rpm.

backinblue said:

Good one Dan...

Regarding my preious post note that i did say "if you are on plane". Boats such as mine, and Back Coves and Sabres and the like, get the same or slightly better mpg at higher speeds when on plane (up to a certain RPM). Note that I am talking mpg not gph. All very dependent on the particular boat and many other factors, the fuel cost to go from 12 knts to 16 knts for example can be negligible or even slightly favorable if the engine is operating at a more efficient rpm.

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Planing will be more efficient than plowing. But if you compare planing to an efficient slow cruise of say 6.5 kts, slow will produce better mpg. On my boat, mpg at 6.5 - 7 kts is more than double what it gets on plane at 17 - 18.