Okay, single or twin??

[refugio]

Yup, and all that space on both sides of the engine.

Yeah, all that space is just wasted. Unless you want to service the engine. Again, I understand some owners consider that "optional".

Oh, and then there's the ability to add sound insulation when the engine sits lower - and there's only one of them. But hey, if you're really proud of how your engines sound and want to share that with your passengers, then that wouldn't be a feature you'd want.

And doubling the number of hull penetrations - everyone knows that you increase seaworthiness by punching additional holes in your boat. That's just common sense, right?


Keith

 

[markpierce]

Tough game to play Mark. A sad one too. Became familiar with the Pilatus story and airplane a few years through a nephew who regularly flew one. While he was doing that gig another one went down in MT with 14 on board. Unfortunately sad stories of this kind involving small airplanes never seem to let up.

You're confusing me. The Hellcat I'm referring to is a WWII fighter plane with a single pilot and no others aboard.

 

[Art]

Quote:
Originally Posted by refugio
Well I like a big rudder too, but that's just me - others may not be as picky about what direction they're going.

I'm also kinda partial to a low COG and rolling moment. And a flat shaft angle, and a big prop, well down in the water. Again, that's just me - others might be happy squirreling along with marginal efficiency and control.

Oh, and I like large tanks. You know, for range.

Keith

Keith you don't know how many people you have just PO'd with these truisms.

Noten psses me off regarding other boater's desires, needs, haves, wants, or suggestions.

Hope whatever I mention does not pss any others off. However, if any of what I post does... oh well, grow up! This is just a marine forum. Pretty darn good one at that - IMO.

Happy Trawler Forum Daze! - Art

 

[Edelweiss]

Well. . . .

I haven't been on the forum for about a month.

But I see I haven't really missed a thing....

 

[refugio]

Well. . . .

I haven't been on the forum for about a month.

But I see I haven't really missed a thing....

What a coincidence! Neither have we


Keith

 

[Art]

Quote:
Originally Posted by Edelweiss
Well. . . .

I haven't been on the forum for about a month.

But I see I haven't really missed a thing....

What a coincidence! Neither have we

Keith

Guess we've mostly said what can be said... and, repeatedly - at that!

Ho Hum :lol:

 

[O C Diver]

Quote:
Originally Posted by Edelweiss
Well. . . .

I haven't been on the forum for about a month.

But I see I haven't really missed a thing....





Guess we've mostly said what can be said... and, repeatedly - at that!

Ho Hum :lol:

Have we settled the point that singles are better and people with twins are looking for more of a better thing?

Ted

 

[Nomad Willy]

Singles are better if that's all you can afford.

 

[Codger2]

You know...that emoticom really hits home doesn't it?

 

[Rustybarge]

Singles are better if that's all you can afford.

Exactly.

I'm surprised nobody has mentioned that twins use nearly double the fuel at displacement speeds.

Rule of thumb:
32'/6 tons/ single[200hp] 2 gals/hr@ 7 kts: 3-4 mpg.
32'/6-7 tons/twin[2x200hp] 4gals/hr@ 8kts: 2 mpg or less.

 

[Capt.Bill11]

And doubling the number of hull penetrations - everyone knows that you increase seaworthiness by punching additional holes in your boat. That's just common sense, right?

I find all this worry about extra hull penetrations amusing. It's really such a none issue. And of course twin engines don't double the total hull penetrations in a vessel compared to a single. They add like what, two?

There are plenty of good reasons to choose a single engine over a twin. But worrying about added hull penetrations isn't really one of them.

 

[Art]

Have we settled the point that singles are better and people with twins are looking for more of a better thing?

Ted

Singles are better if that's all you can afford.

Eric and Ted - You two = "Twins"... in the way you calculate things - LOL - Art

 

[Art]

Exactly.

I'm surprised nobody has mentioned that twins use nearly double the fuel at displacement speeds.

Rule of thumb:
32'/6 tons/ single[200hp] 2 gals/hr@ 7 kts: 3-4 mpg.
32'/6-7 tons/twin[2x200hp] 4gals/hr@ 8kts: 2 mpg or less.

You better calc that again!

Twins (same exact engines as the single in same exact boat as the single) are not twice fuel consumption at displacement speed with both engines running and needing less hp each than the single needs to reach speed. Especially when you cruise at or under full displacement speed with one twin shut down. Also, twins are ok in a full displacement hull but really not needed... although they do serve for spare engine and improved close quarters maneuverability.

Twins can become quite useful on SD hulls and very useful on Planing hulls.

Cheers! - Art

 

[Rustybarge]

You better calc that again!

Twins (same exact engines as the single in same exact boat as the single) are not twice fuel consumption at displacement speed with both engines running and needing less hp each than the single needs to reach speed. Especially when you cruise at or under full displacement speed with one twin shut down. Also, twins are ok in a full displacement hull but really not needed... although they do serve for spare engine and improved close quarters maneuverability.

Twins can become quite useful on SD hulls and very useful on Planing hulls.

Cheers! - Art

That's all very well in your Tolly with gass engines that don't mind idling along at 500 revs...!

Given that a big diesel will need a few revs/load not to glaze the cylinders, a twin 200 HP boat would probably have to use 1200-1500revs which will burn at least 2 gal per engine.

Of course you could cruise with one engine, but figures I've seen on the boat design forum show virtually no gain in mpg because of prop drag....

The big question: has anyone got a 32' or larger twin engined trawler that can beat 2 mpg?

 

[Art]

That's all very well in your Tolly with gass engines that don't mind idling along at 500 revs...!

Given that a big diesel will need a few revs/load not to glaze the cylinders, a twin 200 HP boat would probably have to use 1200-1500revs which will burn at least 2 gal per engine.

Of course you could cruise with one engine, but figures I've seen on the boat design forum show virtually no gain in mpg because of prop drag....

The big question: has anyone got a 32' or larger twin engined trawler that can beat 2 mpg?

Yup – Me… that of course is… if our simple to maintain and inexpensive to own or replace gasoline engines are allowed into this discussion!

We get 2.75 to 3 nmpg at 5 +/- knots by running either engine at well under 2k rpm with one shut down and left in freewheel-prop due to that being OK with BW Velvet Drive trans.

With twins running us at 6.5 to 7 knots, i.e. just below hull speed (displacement-running hull speed on our boat calcs at 7.58 knots) we get close to 2 nmpg

Cruising on full plane at 16 to 17 knots we get 1 +/- nmpg.

WOT at 22 to 23 knots = OMG nmpg

 

[sunchaser]

Exactly.

I'm surprised nobody has mentioned that twins use nearly double the fuel at displacement speeds.

Rule of thumb:
32'/6 tons/ single[200hp] 2 gals/hr@ 7 kts: 3-4 mpg.
32'/6-7 tons/twin[2x200hp] 4gals/hr@ 8kts: 2 mpg or less.

Well there you go, proof positive of something

 

[Rustybarge]

Yup – Me… that of course is… if our simple to maintain and inexpensive to own or replace gasoline engines are allowed into this discussion!

We get 2.75 to 3 nmpg at 5 +/- knots by running either engine at well under 2k rpm with one shut down and left in freewheel-prop due to that being OK with BW Velvet Drive trans.

With twins running us at 6.5 to 7 knots, i.e. just below hull speed (displacement-running hull speed on our boat calcs at 7.58 knots) we get close to 2 nmpg

Cruising on full plane at 16 to 17 knots we get 1 +/- nmpg.

WOT at 22 to 23 knots = OMG nmpg

5 kts is snail pace!!!
Sorry forgot to say at 7 kts in a single screw; anything slower is like watching paint dry!

Your numbers look like 2 mpg....

 

[Rustybarge]

Well there you go, proof positive of something

Yep, that twin engines use x2 fuel consumption because you have to use higher revs to keep engines working well and that means 7-8 kts.

 

[High Wire]

We get 2.75 to 3 nmpg at 5 +/- knots by running either engine at well under 2k rpm with one shut down and left in freewheel-prop due to that being OK with BW Velvet Drive trans.

With twins running us at 6.5 to 7 knots, i.e. just below hull speed (displacement-running hull speed on our boat calcs at 7.58 knots) we get close to 2 nmpg

Thats incredible! What do you use to measure flow?

With twin 5.7L Crusader gas on a 29' Phoenix and a calibrated Floscan, on one engine, the best I could ever get was 5 gph at 1600 rpm for 5 knots for 1 nmpg.

 

[Art]

5 kts is snail pace!!!
Sorry forgot to say at 7 kts in a single screw; anything slower is like watching paint dry!
"....

But, But... running w/ a 3 knot current doing 5 knots through water surface = 8 knots OL speed -

Anyway... With our planning hull Tolly... we like to cruise 16 to 17 knots on full plane. Over twice as fast as the top FD hull speed for same size boats. As you know "Time Is Money!" And... although, as you say, "5 kts is snail pace"... so is 7 knots simply a crawl for us. We only go slow when we want to; rather than because we always have to!

 

[Art]

Thats incredible! What do you use to measure flow?

With twin 5.7L Crusader gas on a 29' Phoenix and a calibrated Floscan, on one engine, the best I could ever get was 5 gph at 1600 rpm for 5 knots for 1 nmpg.

Per math calcs... using time spent, direction and speed of current running in coordination with gps overland speed: I then divide figured miles traveled by gallons used at fill-up to figure nmpg at any rpm/speed.

Was your 5.7L in good condition? When on one engine was your other 5.7 in free wheel or locked prop? Were props correctly sized? What rpm did your engines reach at WOT?

5 gph at 1600 rpm for only 5 knots seems way, way too much fuel burn.

Heck... we get 1 +/- nmpg at full plane doing 16 to 17 knots using both engines.

 

[Ski in NC]

5kts at 5gph is nuts even for a gasser. Something goofy with those numbers. Maybe 2 or 3gph at 1600 for a 5.7.

 

[Nomad Willy]

Exactly.

I'm surprised nobody has mentioned that twins use nearly double the fuel at displacement speeds.

Rule of thumb:
32'/6 tons/ single[200hp] 2 gals/hr@ 7 kts: 3-4 mpg.
32'/6-7 tons/twin[2x200hp] 4gals/hr@ 8kts: 2 mpg or less.

That's absolutely not true. Twin engines have nothing to do w how much power the boat has unless it does. And then you're not comparing twins w singles but 400hp boats w 200hp boats. Of course boats w bigger engines are going to burn more fuel. No brainer.

 

[refugio]

I find all this worry about extra hull penetrations amusing. It's really such a none issue. And of course twin engines don't double the total hull penetrations in a vessel compared to a single. They add like what, two?

There are plenty of good reasons to choose a single engine over a twin. But worrying about added hull penetrations isn't really one of them.

Well, there was just my message about hull penetrations, but I don't actually consider it a non-issue. Most vessels sink as a result of an existing hull penetration (at least, that's what I seem to recall) and it stands to reason that doubling the penetrations (and hoses, exhaust components, et cetera) would...double that risk. I'd say it's even more likely because of the inaccessibility of the off-side components, but in the spirit of not being argumentative I'll let that one slide.

 

[High Wire]

Was your 5.7L in good condition?
Yes. These were carb motors.
When on one engine was your other 5.7 in free wheel or locked prop?
Free wheel
Were props correctly sized? Yes and tuned to Class 1 via PropScan
What rpm did your engines reach at WOT?
4400 and 27 knots.

5 gph at 1600 rpm for only 5 knots seems way, way too much fuel burn.
I know but that is what the floscan read. 2 engines at 5 knots was about 6 gpm @ 1300 rpm. I would troll for bluefish at that speed. Going down to one engine only helped a little.
The floscans were accurate to +/- 2 gal out of 100.

Heck... we get 1 +/- nmpg at full plane doing 16 to 17 knots using both engines.
Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm.

 

[Art]

Was your 5.7L in good condition?
Yes. These were carb motors.
When on one engine was your other 5.7 in free wheel or locked prop?
Free wheel
Were props correctly sized? Yes and tuned to Class 1 via PropScan
What rpm did your engines reach at WOT?
4400 and 27 knots.

5 gph at 1600 rpm for only 5 knots seems way, way too much fuel burn.
I know but that is what the floscan read. 2 engines at 5 knots was about 6 gpm @ 1300 rpm. I would troll for bluefish at that speed. Going down to one engine only helped a little.
The floscans were accurate to +/- 2 gal out of 100.

Heck... we get 1 +/- nmpg at full plane doing 16 to 17 knots using both engines.
Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm.

See Ski's post # 142. You say "The floscans were accurate to +/- 2 gal out of 100." How were you sure of that?

Did you check your flowscan reading a few times as compared to mathematically calced nmpg? Maybe flowscan had a problem??

Also... you mention: "Cruising was about 20 gph (10 gpm each) at 17.5 knots and 3000 rpm." ( I think you meant - (10 gph each) - Seems to me your flow scan may have been reading incorrectly at that speed to. I believe our 29' boat should have been getting at very least 1 nmpg... or better! Did you have/use trim tabs?

 

[Rustybarge]

That's absolutely not true. Twin engines have nothing to do w how much power the boat has unless it does. And then you're not comparing twins w singles but 400hp boats w 200hp boats. Of course boats w bigger engines are going to burn more fuel. No brainer.

Nearly every s/d or planing boat over 32' has twins, with the exception of lobster boats, so it its hard to make a real life comparison . If you looked at the consumption graph of twins or a big single in the same hull i would bet that the consumption would be nearly identical at 2500 revs...


But at low revs its a different story. A big diesel at idle will not burn the fuel cleanly, and consume more diesel.

So most twins are run at the lowest revs that will give clean combustion where the engine is on the torgue curve: 1200-1500 on medium speed engines.

Most singles are run at 1500-1800 revs to give about 7kts cruise.

Its a fact that twin driven props lose 20% through drag, plus the higher operating fuel consumption, plus double the engine heast loss, plus the higher weight of two engines.

In other words doubling all the inefficiencies and losses plus 20% drag; that adds up pretty quick.

 

[BruceK]

You know...that emoticom really hits home doesn't it?

Especially if you are a horse.

 

[Art]

Nearly every s/d or planing boat over 32' has twins, with the exception of lobster boats, so it its hard to make a real life comparison . If you looked at the consumption graph of twins or a big single in the same hull i would bet that the consumption would be nearly identical at 2500 revs...


But at low revs its a different story. A big diesel at idle will not burn the fuel cleanly, and consume more diesel.

So most twins are run at the lowest revs that will give clean combustion where the engine is on the torgue curve: 1200-1500 on medium speed engines.

Most singles are run at 1500-1800 revs to give about 7kts cruise.

Its a fact that twin driven props lose 20% through drag, plus the higher operating fuel consumption, plus double the engine heast loss, plus the higher weight of two engines.

In other words doubling all the inefficiencies and losses plus 20% drag; that adds up pretty quick.

Free wheel – VS – Fixed wheel… prop drag!

https://www.youtube.com/watch?v=jI-UG9RSlJo

 

[High Wire]

h[/B] each) -

Yes, 10 gallons per hour per engine at 3000 rpm equals 20 gallons per hour for both engine running synchronized.

Seems to me your flow scan may have been reading incorrectly at that speed to.

Based on what facts?

Those were co I believe our 29' boat should have been getting at very least 1 nmpg... or better!

Did you have/use trim tabs?

Yes, Trim tabs were always used about halfway down for cruise best speed and lowest GPH values. The only times the tabs were up were for trolling and full speed test runs where tabs hurt performance on that boat.