Prop calculators - is there a difference between electric and diesel drives?

Thank you David, last night I was losing it (a half bottle of ruby port was helping!). The speed requirement is pretty much correct, anything north of average 100 nautical miles per day is acceptable, the primary requirement is "green" operation when there's no good wind, even clawing some headway upwind if possible. Lots of close-quarters maneuvering raw power is not required. From the linesplan, waterline is 54 feet. Something I didn't mention before is that when using the drive, any advantage from motorsailing will be taken. With that in mind, I wonder if erring on the side of higher pitch and diameter might be called for, because of the extra push from the sails?

 

It's not that you're wrong Ray, I think the point was that you could do with some more homework on this and plot your objectives.
30 something inches is what you would normally see on this size boat but with more horsepower.
Your power should be enough to get you in your speed ball park but remember there are losses everywhere. When you work with efficiency there is little tolerance for errors and your few hp can be diminished.
If you want to get the most out of your horsepower, decide what is the biggest prop you can afford/swing. Then figure out how to turn it a lot slower. If you can't you will end up with a small prop again.

 

Can't be done - donated (read free) diesel-electric drive is set at max 1400 rpm with a pretty close-coupled generator system and electronic monitoring, messing with gearboxes etc. would be seriously counter-productive. I have to figure out the most efficient prop for the existing system. Saying "get more power" is irrelevant! I can put in a 30" prop, but all the calculators say 18". So if I'm trying to turn a prop that's too big, surely I'll just reduce the revs the DC drives can put out and minimize the system's effectiveness?

 

If you're looking for more prop efficiency then you're looking for lower RPM's and bigger diameter. The pitch will then depend on your speed through the water.

-Tom

 

My understanding is Ray's "SOR" relative to the propellers is simple: Highest efficiency propellers to use with two 33 HP motors turning at a maximum of 1400 rpm. Changing the speed of the propellers is not an option.

 

Ray , I'd forget the free part and concentrate on putting in a decent system.
You already have two small marine diesels! You'd be much better advised to lose the electrical side and look for two mechanical gearboxes and flanges to suit. Then you'll get much more efficiency. You are going to throw a lot of energy away for no gain.

Get the maximum reduction you can find ( greater than 2 pref 2.5 or 3 ) then put the largest props on that you can swing and pitch them for 5 knots at operational RPM.

If you don't do this you'll get a setup that simply won't provide the thrust to drive into weather. You will burn lots of fuel and go nowhere.

If you use small engine it requires a large prop turning slowly to be anything other than frustrating when you really need it.

Have you a stab at resistance smooth water resistance curve from your CAD model?

 

No, I don't have two small marine diesels, as I have already stated, I have two integrated diesel-electric hybrid drive units consisting of generator/DC motor. This CANNOT be changed. I CANNOT repower the boat. I have to work with what I've got.

I am trying to determine how to get the maximum efficiency propeller size with what I've got! Two 33HP electric motors that run a maximum of 1400rpm. I CANNOT re-gear this set-up. Folks, please don't keep trying to get me to throw out what I have. I can't do that, and I am happy with the lack of power, this boat will be a combination motor-sailor green passagemaker and I'm quite happy if she don't have raw power the way the average boater wants. Jeez, for thousands of years ships only had sails or oars!!! I do not need to force through headwinds (I'll crack off and motorsail). I don't need the occasional 400 horsepower to fix docking mistakes and will be spending most of the service life offshore or at anchor. I do not need to stem 5-knot tides (I'll read the tide tables first).

I just want to understand, after running multiple prop calculators, whether the consistent results of 18 inch prop and 12 inch pitch is really correct, or whether I've missed something. How hard can it be?

 

You seem to ignore all the advice given to you by professionals yet in the same breath ask how difficult can it be after ignoring such advice/requests!

Clearly you think this is simple and want someone to give an answer XXX to satisfy you. You wont get any professional naval architect supplying what you want without further information which you seem to think is unnecessary or barking up the wrong tree or some gas-guzzling conspiracy.

As noted several times, you're putting the cart before the horse, yet steadfastly refuse to budge from your MO. That is your prerogative.

Since you think it is not difficult, why are you bothering to ask such a 'simple' question again?

 

No, I'm not putting the cart before the horse. I'm trying to figure out how to get the best out of what I've got. Yes, I've already heard that this boat needs more power. I don't have it. I don't have the budget to get it. In fact, I'm QUITE OK with a low power set-up. My application is good with low speeds. I'm OK with not being able to deliver raw power.

I just want to know if I've missed anything important before I start cutting propshaft struts, tube angles etc. All the advice to repower this boat is irrelevant.

 

What further information is needed to answer Ray's straightforward technical question: Is 18" a reasonable diameter for the propellers on his boat with two 33 HP motors driving the propellers at 1400 rpm?

 

It's OK David, it's pretty obvious that the professionals are having a good old chortle at my expense and sending each other hilarious messages about how they're torturing the amateurs! I can take it.

But perhaps one of the informed boat builders can point me in the right direction on this? I've already read Gerr's book on props twice and used three diffferent propeller calculators, but am not experienced enough to figure my own way around this non-conventional application.

Cheers,

Ray

 

Jeez. What a mess. After several threads I started or participated in that went the same way poor Ray's thread went here, I have a theory:

People post crappy answers like this because they don't know the answer. Ray's question is very specific, yet not an "expert" on the whole board can answer it.

Same happens all the time, Ray. Chalk it up to you asking a question nobody here can answer.

You would think someone would know though. Pretty straight forward question.

 

I never said the question is simple. I asked "How hard can it be?"

If it is indeed hard (it certainly is to me, because I can't see a method to solve it), then I'm OK with that.

But I don't want or need advise to come up with a different requirement, the requirement is set and clear. All I want to do is make the existing system work in the best way possible, the propeller sizing is the most critical part of that.

At this point, I'm guessing that experience and rules of thumb are all market-driven (ie. designing a new vessel to best practices and what the modern buyer wants) and therefore nobody really knows how to reverse engineer this non-conventional application.

So I'll probably have to do this by trial-and-error and plan to haul the boat a few times. Fortunately I have a source of a number of propellers here in the yard to switch out and test. I'm just going to have to guess at the shaft log angle and strut and propshaft lengths though...

 

Ray, you must be very frustrated.

This is a mess.

I believe the answer to be no, 18 inches is not big enough if you're looking to maximize prop efficiency. I would base the prop design on 36", two blades.

Have you used Javaprop?

Ray it may be best to personal message me back so we can move forward with this in less polluted waters. Just left click on my handle (name) above.

-Tom

 

I think that you have the parameters about right. Given the speed, power and RPM a 16-18" diameter prop is about optimal. You should be able to get about 800 lbs thrust per prop at that power. Whether 800 lbs/prop is enough is the difficult question to answer. The hull form, displacement, etc. will be the determining factor. And your speed will vary with displacement. Does it really matter if you go 4-1/2 or 5?. If so, try to estimate resistance and see if you need mroe power.