two engines or one

[Bugaboo]

Let me translate #1 36 foot boat hull speed approximately eight knots mathematically #2 Most fuel-efficient speed 7 knots Requiring 35 hp Single or twin , Total horsepower #3 single Perkins 4236 85 hp #4 produces 35 hp at Approximately 1400 RPMs #5 Twins produce 35 hp 17 hp each approximately at 800 rpm #6 Propped to get 35 hp at 1400 RPM over propped for wot the way i would prop it “jmo” # Listen to it like that it ,will be music to your ears #7 As you can see twins wouldn’t be your best choice just my opinion

 

[SteveK]

Yes! And, I say...

Good running engines each have their own sounds [language] while running well [i.e., good]. I this post... the term "good running engine" includes engine condition, engine tune and gearing/load placed on the engine. With those 3 levels of requirement set at their best positions to meet the definition of "good running engine" - engine sounds [their own language] become important to understand and to adjust throttle as well as rpm to locate their best sound.

So... IMO, for boats especially, if we listen carefully to good running engine's sound we can tell if the engine is being run too hard [strained] or run too easy [overly relaxed]. The magic is to run the engine right in the middle of those two sound emitting engine performance conditions. I call this letting the engine "lope along"; which means it is running at its most comfortably efficient speed. Listen to your engine they do talk - to each of us; if we learn how to listen to their language. And, twins [both in equally good condition] should nearly always be run in a synchronized "lope along" power output level.

Engines love to last for us if we learn to love their most comfortable performance levels and adhere to running them at those levels.

I bet you drove a standard to the red line. Good post, +2

 

[Art]

Let me translate #1 36 foot boat hull speed approximately eight knots mathematically #2 Most fuel-efficient speed 7 knots Requiring 35 hp Single or twin , Total horsepower #3 single Perkins 4236 85 hp #4 produces 35 hp at Approximately 1400 RPMs #5 Twins produce 35 hp 17 hp each approximately at 800 rpm #6 Propped to get 35 hp at 1400 RPM over propped for wot the way i would prop it “jmo” # Listen to it like that it ,will be music to your ears #7 As you can see twins wouldn’t be your best choice just my opinion

Our Tolly's planing hull has claced hull speed of 7.58 knots. When I cruise her at 6 to 6.5 knots [gps qualified]with both engines running they turn about 2000 to 2200 rpm each [my tachs are not at all absolutely accurate]. Running them in synchronization [using accurate strobe light synchronizer] at that speed is music to my ears as they both lope along with their intertwined humming rpm sounds slowly, sweetly rolling back and forth, emitting a melody of twin engine revolution music.

At this level of twin engine rpm/thrust and resulting speed [with boat slipping nearly effortlessly through the water - at 1 to 1.5 knots less than hull speed] my calculation is we average 2 to 2.5 nmpg.

If I run on only one engine at 2000 to 2100 rpm she cruises at between 4.5 and 5 knots... averaging 3 nmpg or just over.

When the twins are turning 3600 to 3800 our Tolly scoots along on full plane at 16 to 17 knots [as long as not too heavily loaded]. 1 +/- nmpg is the fuel use.

We usually cruise at just below hull speed for a relaxing fun ride; with a runabout in tow. Going onto full plane is used only if we want/need to cover some distance in a relatively fast time span.

 

[FF]

"IMHO- Engines are most efficient at a particular limited range of loads."

Yes, this is found on a fuel map or BMEP from the engine mfg.

Sadly in small sized engines the engine folks do not post much useful info.

The graph will look like a series of bulls eyes, the inner is the most efficient and the fuel burn per HP gets worse and worse as you move to other rings.

 

[backinblue]

Let me translate #1 36 foot boat hull speed approximately eight knots mathematically #2 Most fuel-efficient speed 7 knots Requiring 35 hp Single or twin , Total horsepower #3 single Perkins 4236 85 hp #4 produces 35 hp at Approximately 1400 RPMs #5 Twins produce 35 hp 17 hp each approximately at 800 rpm #6 Propped to get 35 hp at 1400 RPM over propped for wot the way i would prop it “jmo” # Listen to it like that it ,will be music to your ears #7 As you can see twins wouldn’t be your best choice just my opinion

Except LWL is close to 30' not 36' giving hull speed just over 7 knots

 

[psneeld]

Depends on hull design...overhangs....

 

[Dougcole]

I'm going to chime in here, not because I know a darn thing about oils (I learned a bit in this thread, thanks slo-mo) but because I owned and ran a 1973 Gulfstar with twin Perkins 4-236 engines for 8 years. We put about 1,600 hours on it, and a lot of miles.
Almost every 36 Gulfstar with twins that I have seen (assuming original power) had 4-236s in it, so I thinks it's pretty safe assumption that those are the engines in his boat.


1. The motors won't turn 3,400 rpm if the tach is accurate. WOT as I recall was about 3,000.



2. They love to run at about 1,800 rpm. Much above 2,000 and the boat just starts to squat and doesn't really gain much speed while burning more fuel. Possibly more oil as well?


3. 1,800 RPM in a 36 gulfstar with twin 4-236, both engines running, with a reasonably clean bottom and props results in a speed over ground between 7 and 8 knots, assuming no strong current etc.



For the purpose of getting the boat home I would run it right around 7 knots or so on the GPS, maybe a bit less. I'd feel comfortable that no harm would be done to the motors at that speed and you'd be running close to the max efficiency for your boat.



After I got it home I'd get the tachs figured out. Try running new grounds to the tachs, pull the grounds off of a clean source not the other tach, maybe even add a new buss behind the dash. I had to do that on all four of the tachs on my boat.


Hope that helps!


Doug

 

[SteveK]

longshot... do the tachs have a selector for 4-6-8

 

[Nomad Willy]

Slowmo wrote;
“ As a comment you should NOT use a single grade (SAE 30 or 40) with a modern (last 20 years or so) American or European diesel engine, period. I always recommend following the OEM recommendations on viscosity.”

Viscosity isn’t black and white. If you’re operating at the high temp end of the temperatures recommended boosting the overall viscosity by adding a little higher vis oil would be a smart thing to do. Rules aren’t meant to be broken. And when one goes beyond the specified norms he takes some responsibility along w it.

When I flew UL aircraft a manufacturer said in essence “do NOT under any circumstance use a synthetic lubricant.” This manufacturer knew that synthetic lubricants were better in all matters except cost. I ran synthetic lubricant all the time and could run my air cooled engine up to 35 degrees hotter CHT than the pilots using dino oil.
NEVER wandering off the black and white will usually keep you safer but could result in limited performance. I’m sure there are other ways one can get bit but being sheep-like is not on my plate. Rules aren’t meant to be broken but sometimes one can go where the sheep don’t by fudging the specs somewhat.

 

[rslifkin]

Slowmo wrote;
“ As a comment you should NOT use a single grade (SAE 30 or 40) with a modern (last 20 years or so) American or European diesel engine, period. I always recommend following the OEM recommendations on viscosity.”

Viscosity isn’t black and white. If you’re operating at the high temp end of the temperatures recommended boosting the overall viscosity by adding a little higher vis oil would be a smart thing to do. Rules aren’t meant to be broken. And when one goes beyond the specified norms he takes some responsibility along w it.

When I flew UL aircraft a manufacturer said in essence “do NOT under any circumstance use a synthetic lubricant.” This manufacturer knew that synthetic lubricants were better in all matters except cost. I ran synthetic lubricant all the time and could run my air cooled engine up to 35 degrees hotter CHT than the pilots using dino oil.
NEVER wandering off the black and white will usually keep you safer but could result in limited performance. I’m sure there are other ways one can get bit but being sheep-like is not on my plate. Rules aren’t meant to be broken but sometimes one can go where the sheep don’t by fudging the specs somewhat.

Sometimes a limitation or spec is for good reason. Other times it's just that the manufacturer hasn't tested that and isn't willing to say it's ok without the data.

 

[Art]

Manufacturers spend million$$$ engineering many levels of maintenance items with and for their new products.

Then they provide stats for their new products. And, therefore can state what they require to be used with their new products... for their guarantee to stay in effect.

For new products I see nothing wrong with following orig mfg rules.

For older, used products [with expired guarantees] I believe it is good to know the manufactures original suggestions.

However... as products get older - two things happen:

1. The product's age and may require different ways and items to maintain them.
2. As the product ages there may have become improved maintenance items.

We should do as we see fit... just be careful with what we chose to do.

 

[backinblue]

I think it's also true that a spec servers the majority of the people in the most common conditions but it does not mean it's optimized for one particular condition.