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Old 02-22-2016, 12:41 AM   #1
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Total and Reserve Power

As my shipyard and I are finalizing the design of a new trawler (displacement, not SD), I've had occasion to reflect and argue with friends about the total power for the boat.

Hull speed typically requires about 3.8 - 4.5 hp/long ton, but it seems the current trend is for even higher ratios.

Some factors that would affect this are:
  1. How efficient the hullform (fat versus skinny)
  2. Windage
  3. Reserve power needed to suit cruising conditions/currents often encountered
  4. Personal preference
  5. Budget

My decision to power at 3.2 hp/ton was based on the following:
  1. Fat, deep hull form - It is less efficient, so this is opposite to my decision
  2. Very low frontal windage and side windage (A/B ratio = 1.85)
  3. "Normal" cruising conditions - I've had little need for a lot of reserve power in the past
  4. I'll be running my engine at 80% (1500 rpm), coinciding with maximum torque and fuel efficiency (22 hp/gal-hr)
  5. Need to keep expenses within reason and don't want to overpower

It would be interesting to see how other cruisers power their boats, or would power their boats if they could swap out engines.
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Old 02-22-2016, 01:02 AM   #2
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...[*]I'll be running my engine at 80% (1500 rpm), coinciding with maximum torque and fuel efficiency (22 hp/gal-hr)...
At 77 percent of maximum RPM (generating my normal cruise speed, a knot below maximum displacement speed, using 1.7 GPM), the engine is working at about 45 percent (comparing actual fuel-consumption rate versus maximum of 4 GPM).

1400 RPM produces maximum torque resulting in a half-knot reduction in speed while consuming 1.2 GPM. This would be my long-range cruise speed.
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Old 02-22-2016, 01:10 AM   #3
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That's an interest power curve Mark. You are producing 45% power at the same place in the curve where my engine is "working" 80% power. 100% power is reached at 2300 rpm, so it is rather flat-lined at the top.

I suppose if all engines had the same characteristics then we would only need one engine in this world
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Old 02-22-2016, 01:27 AM   #4
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What are your consideration for the parasitic load on the engine? i.e. Alternator, air, any other pumps run off the engine. All would change your hp curve.
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Old 02-22-2016, 01:40 AM   #5
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You can always throttle down but if you are under powered at some point you have no options.

Who knows where that line is? I know there have been times that I have wished for more.

Your boat your choice!
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Old 02-22-2016, 01:51 AM   #6
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You can't get "more" out of a displacement hull but if you have to tow something or make headway against a wind or don't keep your bottom polished you might not reach just before "more." I can move my 50 footer with my dinghy and my 9.9 HP Honda but I have no idea if it will do hull speed; eventually it reaches its theoretical top speed with the 9.9.

A bigger engine will swing a bigger wheel and flow more water (torque/thrust) and your maneuvering will be more precise. Still won't give you "more" but you will get to hull speed faster, methinks.
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Old 02-22-2016, 02:59 AM   #7
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What are your consideration for the parasitic load on the engine? i.e. Alternator, air, any other pumps run off the engine. All would change your hp curve.
Good question. I know that the NA took into consideration shaft and bearing losses, alternator, emergency bilge and hydraulic pumps when calculating speeds. The pumps of course are not utilized during normal cruise except for minor hydraulic power required for the autopilot.
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Old 02-22-2016, 03:45 AM   #8
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Originally Posted by makobuilders View Post
That's an interest power curve Mark. You are producing 45% power at the same place in the curve where my engine is "working" 80% power. 100% power is reached at 2300 rpm, so it is rather flat-lined at the top.

I suppose if all engines had the same characteristics then we would only need one engine in this world
Mako
I suspect that your 1500 rpm power used will be closer to Mark's percentage, which is my experience also. Engine graphs will show max. power vs rpm, but will also have a propeller curve, which is the power absorbed by spinning the prop. I think your 80% figure is from the max power curve. The difference between the two curves at any given rpm is your reserve power, some of which would be used to counter waves and windage.

If I were in your position I would have the NA calculate the power required for hull speed, probably using modified Gerr formula (rather the the simple 1.34 x sq rt LWL) and install something close to that. Available engine size probably won't fit the calc exactly. In practice you will run at least a knot, and perhaps 2, below hull speed, and very likely end up at 1500 rpm anyway.
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Old 02-22-2016, 06:53 AM   #9
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Originally Posted by markpierce View Post
.
1400 RPM produces maximum torque resulting in a half-knot reduction in speed while consuming 1.2 GPM. This would be my long-range cruise speed.
I hope you meant to say 1.2 GPH unless you're towing 2-3 barefoot skiers astern...
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Old 02-22-2016, 06:59 AM   #10
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  1. "Fat, deep hull form - It is less efficient, so this is opposite to my decision."

At displacement speeds where wave making is not a concern .fat is fine as it has less surface area to drag thru the water than long and skinny.

In theory a beach ball would be even more efficient , and some designers seem to be going for it.

It is very nice to have a WAY! over sized alt and a 4000W inverter for house goodies underway.

Future loads , hyd pump, reefer compressor , water maker, may require slowing in heavy seas.

IF the eng MFG will part with a BMEP or fuel map , you can set up for genuine efficient operation.

Ask,,,, even BEG,,,but don't hold your breath
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Old 02-22-2016, 09:58 AM   #11
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  1. I'll be running my engine at 80% (1500 rpm), coinciding with maximum torque and fuel efficiency (22 hp/gal-hr)



There is a lot buried in the above statement that would benefit from clarification.

Is 80% of RPM or of max load?
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Old 02-22-2016, 10:16 AM   #12
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"I'll be running my engine at 80% (1500 rpm), coinciding with maximum torque and fuel efficiency (22 hp/gal-hr)


There is a lot buried in the above statement that would benefit from clarification.
Is 80% of RPM or of max load?"


Yes - agreed here. What engine(s) are producing what hp at 22 hp/gal/hr?
Are these smaller displacement turbo engines? Is this measured in metric HP , and to what spec? Is the hp produced to fuel curve reasonably flat for these engines?
IMO a lot more detail will help the responses.
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Old 02-22-2016, 10:21 AM   #13
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Indeed I have yet to sera a set of torque curves except for full power. I don't think you can infer anything from that at less than max torque
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Old 02-22-2016, 11:14 AM   #14
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Ok, so below are the performance curves published by the engine manufacturer. At 1500 rpm the engine is producing 80% of its maximum power rating. Maximum torque and fuel efficiency basically coincide as well to this "magic spot."

I may not be interpreting the information correctly, and my shipyard is 4 time zones ahead of me so I can't get clarification right now, but I've also attached the data sheet.
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Old 02-22-2016, 11:41 AM   #15
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Mako,
Sounds like your original post is about spot on.

My Willard is at 5hp per ton (ton at 2000lbs) and w a much larger boat w little windage 4hp per ton should be plenty of power. 80% of power/load at cruise is quite high though. I assume you're not overpropped. I'd be shooting for 100rpm less than max continious power recomended by the engine manufacturer.

The "extra" windage and head sea power needed will be low w your low windage but the bow's PC plays a part as the seas build. A high PC bow (blunt) typical of PNW Seine type fishing vessels will need more power in head seas. My Willard w a low PC fwd and low windage has never needed "extra" power. When head seas cause me to change power/throttle I never increase it. Almost always I lower it 300rpm. Never needed "extra" power. However I frequently slow down a bit but not less than a knot.

Another thing to consider about head winds and head seas is that slowing down a knot greatly reduces power required (flat water) so all that power is allpied to smashing through head seas and overcomming windage. But smashing into a really big head sea w a heavy boat and a blunt bow can in some cases practically bring a boat to a stop. But if you're the type that needs to maintain your chosen speed no matter what you should probably stay in protected water.

So I'd figure w a propperly loaded propeller at WOT max continous power less 100 rpm (just as a fudge/safety factor) should/would be a scenairo I myself could be very comfortable with. But we seldom can get our WOT rpm to be perfectly spot on and the boat's load and bottom fowling state is a variable .... so 100rpm underpropped would be even better. That way you could never overload your engine no matter what you or someone else did w your throttle. More comfort and less engine stress.

PS I think 80% power from the above in post #14 is at WOT.
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Old 02-22-2016, 11:54 AM   #16
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Originally Posted by Bob Cofer View Post
You can always throttle down but if you are under powered at some point you have no options.

Who knows where that line is? I know there have been times that I have wished for more.

Your boat your choice!
My sentiments exactly.
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Old 02-22-2016, 11:58 AM   #17
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Eric, it sounds like your sweet and fine boat at 4.5 hp per long ton is perfectly setup. Good point that my design, which is very full in the fore sections, would suffer greatly when pounding into seas.

However I doubt my wife would ever allow me to play "iron sailor" with head seas - she'd kick me in the butt! Her comfortable speed is less than 8 knots and we do all sorts of tacking when the seas are on our nose.

The next size up engine jumps significantly in price and I'm watching my budget carefully, so I need to consider it carefully. Luckily the Euro is at a low - might be more attractive now
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Old 02-22-2016, 12:16 PM   #18
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Ok, so below are the performance curves published by the engine manufacturer. At 1500 rpm the engine is producing 80% of its maximum power rating. Maximum torque and fuel efficiency basically coincide as well to this "magic spot."

Not quite. The engine is capable of producing that power but it depends on prop loading if it does. It could also be unloaded and just spinning in neutral producing little power and using little fuel. The prop load is the difference.


The SFC curve is also at max load conditions not the prop load conditions unless you prop for 1500 RPM to be WOT at which point you will be overloaded.




At 1500 RPM if propped for2200 Full load WOT you will be using 38% of full rated power assuming a 2.5 prop load exponent as below
(15/22)^2.5=.38


Do you have a prop demand curve for that engine?
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Old 02-22-2016, 12:25 PM   #19
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Current can be ignored as far as reserve power is concerned. Windage and waves OTOH can add to the need for extra power to maintain speed.
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Old 02-22-2016, 01:03 PM   #20
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I've requested the prop demand curve.
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