overpowering a sailboat

If the extra power is from more oil burnt, no MPG increase, of course, what ever the prop.

When the extra power from the group is gained by having a better head flow, a better muffler free flow, a better long intake getting cold air, a better combustion, water intake...You get extra mpg with a bigger prop as you have much more torque available.
Did you noticed this was the technical base of formula 1 car racing ?

 

Yes, but formula 1 racing is usually with very high winding engines. Unlike boats which like to cruise at low rpm's.

to clarify my original question, I was not thinking huge or high performance engines. I was thinking more along the lines of a 10HP diesel where a 6hp would move the boat fine. allowing the extra 4hp for a generator or other added systems but when not running those systems will the extra 4hp be a significant penalty in MPG? and water cooled for the option of adding a cabin heater. Maybe I am splitting hairs here but I like to think.

 

That distinguishes you from Kistinie!

 

going from 6 to 9 Hp and more for torque, on a low rev engine, is possible, just by flowing gaze and better combustion, then you get more MPG or more electricity for same MPG.
A good engine need a long cold vortexed air intake (like an A.V.E.C), 3 or 4 valves per cylinder, water cooled tuned long free flow exhaust, water injection, good timing and vaporisation, a permanent magnet alternator...
Such a long list...

A marine engine is often far away from this specially, for intakes, heads, alternator and exhaust.
Effort have been done on some new engines, but not enough.

 

Did anyone understand what Kistinie just said?

First of all, you don't want a "long cold air intake", you want a short one and preferably something less then cold. Yes to cold air, not to cold intakes or you'll never start the thing without a heating element or heat riser.

You don't need extra valves per cylinder, especially on a continuous duty engine, like those used in a boat. They just add to complication, complexity and maintenance.

A tuned exhaust would be nice, but usually not especially practical, when dumping into a water cooled "log" or lift.

Frankly, you should stick with what you know (what ever this may be) Kistinie, which clearly isn't about the engineering and principles in a marine engine.

Paul, I think you have you answer, in that adding some percentage of the total output for accessories is a good idea, but you don't need to go crazy once you've met the maximum propulsion needs. 25% would be more then you need, unless running something weird. So, on your 6 HP example, if this handled your propulsive needs, an 8 HP would be more then adequate for accessories.

 

ya know I was introduced to water injection years ago when I worked at an airport that had a small shop that specialized in restoring P-51 Mustangs for the Las Vegas racing circuit. Previously I had seen them only in the J. C. Whitney catalogs and assumed they were a scam.

The P-51 mechanic told me they do really work, however the payback was in the form of burned valves and remember fighter planes change engines more often than most car-boat owners change their oil.

I have to wonder tho if given todays improved metals the system might have more practical application.

I was taught that there are three factors in deciding engine purpose. Performance-economy-maintanance. to increase one will always cost another. but as technology advances, the whole balanced design will fair better too.

He also taught me that the looser the tolerances of an engine, the more reliable it will be. hence aircraft engines tend to burn oil out of the factory and can be somewhat sloppy in their tuning. I'd guess if you are 200 miles offshore reliability would be your priority over performance.

Like the beater car I had that always ran bad..BUT always ran.

 

If it is the only parameter, Why not a horse or a cow ?
Not as fast but much safer, very respectable, can run too and give milk

i guess you miss informations about water injection, things have changed since ww2
http://www.boatdesign.net/forums/pr...rfuel-hho-technology-11695-18.html#post329192
http://www.boatdesign.net/forums/propulsion/emission-controls-technical-thread-27515.html#post276616
http://www.boatdesign.net/forums/pr...rols-technical-thread-27515-3.html#post278822
http://www.boatdesign.net/forums/pr...rols-technical-thread-27515-6.html#post313091

 

Again, you should stick with what you actually know, Kistinie, which for the life of me, across all the threads you've participated in is still a bit of a question. What is your area of expertise Kistinie?

 

Kistinie, I don't doubt things have changed since WW2. but the P-51 is still one hot performer by any time standards. The water injection I speak of was simply squirting water into the hot manifold where it became steam and gave about a 10% boost. most of us have noticed how much better a car runs in a light rain once at operating temperature. Steam is an awesome power.

as far as your other threads go about gas from water etc I am sure that technology has gone well past me but I am also aware that these things are still new and unproven and highly expensive and nothing to mess with out at sea in a 25 foot boat.

I cannot be aware of each and every thread and I bless the people experimenting but I am looking for practical simplicity. and a well balanced design.

 

Then study the actual offer and pick up the good modern engine
Take care about fresh air, long intake, big air filter, and exhaust line
More valves are a must with almost no over cost as checking gap or changing seals of a few more valve is nothing compare to fuel savings you will get for thousand of hours.
take care to geometry, friction, play, balance, of the line and prop.
Use 100% synthetic thin oil inside castings, animal grease outside for everything remaining cool and in contact with water, porc being a great compromise

Or get a E Motor reduced 1/3 or 1/2 or direct if 12 to 18 pole + Generator if you can afford the actual high prices.

I do not really understand the purpose of this question but i can answer.
Sailing 33 years, teaching sailing mono and multihull 25 years, yacht delivery 15 y, yacht design 5y, Race car chassis and engines 20y, live and studio sound recording 25y, radio broadcasting 20y, acoustic 20y, Videoconferencing IP, video projection 15y, ecological house building 10y, quantum physics 20y, electronic development 25y , Composite work 5y, metal work soldering 20y, writing 25y, philosophy and religions 25y, music history classic, jazz, rock punk, and sex to relax after long days of work 30y. I'm very poor at dancing, drawing, singing, playing a music instrument and not telling what i beleive right and true. Am i tired ? A bit, not totally.

Does this answer to your question ?
Will this be a source of info for new aggressions or to get better dialogue and respect ?

English
http://www.infed.org/archives/e-texts/bohm_dialogue.htm

French
http://thinkg.net/david_bohm/bohm_dialogue_proposition.html

 

So, thats 338 years of "experience"!?

That means you did nothing ever on a professional level or to accepted standards, just playing around.

And so are all your posts, threads and statements, premature.

Grow up and come back after the next 338 years!

 

How efficient an engine is at different loads is available from a BSFC map. Typically, an engine will do best at high torque and mid rpms - perhaps 2/3 of max output.

IMO, oversizing is necessary to allow for the unusual conditions that might triple the output needed - eg. high winds or currents.

 

Thanks, Par and jonr,

Nice straight answers to my basic question.

Kistinie, I don't think you understand my goal here. I'm considering borrowing the engine off my snapper lawnmower and you have me taking communion with Enzo Farrari.

I just want to putt down the channel, not run the 24 hours at LeMons.

 

"Study the actual offer and pick up the good modern engine
Take care about fresh air, long intake, big air filter, and exhaust line
More valves are a must with almost no over cost as checking gap or changing seals of a few more valve is nothing compare to fuel savings you will get for thousand of hours.
take care to geometry, friction, play, balance, of the line and prop.
Use 100% synthetic thin oil inside castings, animal grease outside for everything remaining cool and in contact with water, porc being a great compromise

Or get a E Motor reduced 1/3 or 1/2 or direct if 12 to 18 pole + Generator if you can afford the actual high prices"


Did you understand my answer ?
Where is the red car ?

 

kistine, I see you are the man people love to hate and for good reason too.
I am not going to respond to your babbling anymore.

to the rest: thanks for your help and patience with my niavete.