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.

makobuilders said:

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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)...

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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.

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

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.

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!

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.

skipperdude said:

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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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.

makobuilders said:

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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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.

markpierce said:

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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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I hope you meant to say 1.2 GPH unless you're towing 2-3 barefoot skiers astern...

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

1. I'll be running my engine at 80% (1500 rpm), coinciding with maximum torque and fuel efficiency (22 hp/gal-hr)

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

Is 80% of RPM or of max load?

"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.

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

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.

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.

Bob Cofer said:

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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My sentiments exactly.

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

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."

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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?

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.

I've requested the prop demand curve.

I was reviewing the original 1970's design as a commercial trawler and the design only called for a 160 hp engine. Same exact lines with only slightly lower windage. Pulling trawls and working for a living.

Perhaps this is a reflection of modern trends that we "require" higher power and higher speeds? Take a look at the immensely seaworthy romsdahls and malahides and I bet the powering was quite low as well.

"I was reviewing the original 1970's design as a commercial trawler and the design only called for a 160 hp engine. Same exact lines with only slightly lower windage. Pulling trawls and working for a living."

May I ask which exact engine was in the 70's design rated at 160HP?
What was the displacement and what was the rated rpm?


FWIW - I have had a few 4 stoke NA diesels that I would not fear running at or near full rated rpm, and I have 4 stroke turbo diesels that I will not run above 70% load and prop with caution. YMMV

What is your calculated or expected cruise hp required?

Using a 2.5exponent, along the prop curve at 1500rpm it is developing 58hp. More complicated than that as once you reach near hull speed and above, the prop curve fails as hp needed shoots way up.

See if the engine mfr will send you the BSFC map. All the data needed is on the map, better than prop load rpm/hp/fuel curves.

" All the data needed is on the map, better than prop load rpm/hp/fuel curves."

Most Mfg guard the BSFC better than the US gov guards the plans for the T 88 nuke.


Prop load rpm/hp/fuel curves are basically useless in attempting to set a boat up for useful power and best cruise efficiency.

makobuilders said:

I was reviewing the original 1970's design as a commercial trawler and the design only called for a 160 hp engine. Same exact lines with only slightly lower windage. Pulling trawls and working for a living.

Perhaps this is a reflection of modern trends that we "require" higher power and higher speeds? Take a look at the immensely seaworthy romsdahls and malahides and I bet the powering was quite low as well.

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I think your figures seem pretty good.
The "extra" HP thing is a marketing thing and not much else.

The figures for my Ford Lehman, SP135, seem similar to yours.

Personally the last place I would skimp is propulsion if I were building a trawler. That said its really an optimization problem and naval architects get paid the big bucks to figure this stuff out. For the record our boat's ratio is just a smidge over 5 when fully loaded, but we have the luxury of two engines and can tweak loading a bit based on sea state, electrical and hydraulic needs.

Unless you added the expense of controllable pitch prop you are just going to wind up doing a bit of prop tweaking. Way cheaper than engine tweaking.

I'm not sure why the trend towards big oversized power plants recently, but probably a bit of marketing as well as some need for bigger alternators hydraulics etc...adding to the vessel's hp needs. With the new variable frequency drive DC air conditioning systems bigger alternators seems to be the trend in order to have dehumidified air under way.


Spell check via iPhone.

I too would go with a larger base engine. Diesels are designed for trucks and industrial equipment or cars. Cars don't typically run at high loads for long periods so 50+ HP per liter works for them. trucks and equipment OTOH run at high loads so ratings of 30 HP per liter is common. A heavy low RPM low rated HP diesel is a joy to own. Parts just don't fly off them.
IMO all the engines well known for long life are run at lower loads. The list posted above estimating life gives the right idea though nothing is certain.


Cat recommended 30,000 gallons of fuel between overhauls in the past. this because fuel used is the best indicator of how much work the engine has done. Hours of operation don't tell much. Work done is what counts.




FWIW Service and parts are more important than many people think. Buy well supported engine that many mechanics are familiar with and has a large installed base and an entire set of difficulties are avoided.

bayview said:

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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Why do you say "maintain speed"? Why not just slow down?

Also there is no need to reach hull speed on a FD boat. I'm not even sure Willy can attain her HS of 7 knots. I wish the expression "hull speed" had never been coined. It's very misleading in many ways. A boat very similar to Willy at 5hp per ton (2000lbs) w less power probably would not be capable of HS. There's fast boats, slow boats and slower boats. Hull speed is just a mathimatical expression. And there's no need to go at HS ever. Most FD skippers wisely cruise a knot below HS and working the engines hard the FD boat is still closser to one knot below HS than actual HS. Providing the boat is not overpowered. You'd be better off forgetting about hull speed.

bayview said:

Cat recommended 30,000 gallons of fuel between overhauls in the past. this because fuel used is the best indicator of how much work the engine has done. Hours of operation don't tell much..

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For large industrial engines fuel consumption is many times higher than 30,000 gallons between rebuilds. Warranty programs for off highway equipment are normally based on hours as a negotiating value. On my last big job we had complete engine warranties for about 30 new pieces of equipment 16 of which were 400 ton haul trucks with 4000 HP engines.

Fuel consumption was a warranty item too at max HP ratings. Fuel mapping is closely followed as an indicator of engine and drive train performance. Lest I digress too much, as mentioned by Ski, the starting point for many engines is the BSFC. From this springs drive train performance and engine health measurement.

In our little boats most of this though is moot. But this and a parallel thread do point out why a good engine survey is important. You may be looking at a vessel where the previous owner ran it at full throttle most of the time.

smitty477 said:

May I ask which exact engine was in the 70's design rated at 160HP? What was the displacement and what was the rated rpm?

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Excerpt is at the bottom:

smitty477 said:

I have had a few 4 stoke NA diesels that I would not fear running at or near full rated rpm, and I have 4 stroke turbo diesels that I will not run above 70% load and prop with caution. YMMV

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Seems to me that its the manufacturer who would specify the continuous operating range, unless the engine is getting old and unreliable.

Ski in NC said:

What is your calculated or expected cruise hp required?

Using a 2.5exponent, along the prop curve at 1500rpm it is developing 58hp. More complicated than that as once you reach near hull speed and above, the prop curve fails as hp needed shoots way up.

See if the engine mfr will send you the BSFC map. All the data needed is on the map, better than prop load rpm/hp/fuel curves.

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According to Gerr's modified formula "B", the max rating of 180 hp will yield 8.2 knots. For cruise purposes, output of 140 hp will yield 7.6 knots. My naval architect has a powering specialist on his team but I have not received the detailed calculations yet. However I doubt there would be too much of a difference.

BTW, the manufacturer either does not have a BSFC map for the engine, or is not willing to release it!