Proper propeller

Al,
"MP" is a model name and style of prop .. like the one in my pics. It stands for "machine pitch". NOT to be confused w the "MP-3" prop. The MP-3 is a skewed propeller that is more typical and not symmetrical. The "MP" prop is symmetrical. My prop .. the one in my pics. The MP-3 prop is a good prop too but not as well suited to slow speeds as the MP.

Your last post was confusing. If you are confused about the;
1. propping to rated rpm
2. over or "cruise propping"
Go to our archives and read reams of discussion on the matter.
Call Michigan and ask them. FF coined the expression "cruise prop" and I don't think it's used anywhere off this forum but I really don't know. The cruise prop is basically a trick that can be used by anybody and less noise and about 5% greater economy can be had but there's a real danger that the engine may be overloaded and under what circumstances is a bit of a foggy issue.

As far as I know most or all engine manufacturers recommend you prop to the engines rated rpm. Then the specs about how long you can run at WOT and the highest rpm you can run continuously are applicable. All that is out the window when you over prop.

As I've said call Michigan or/and check specs of engine manufacturers to see what they recommend. Then your eyes will be wide open (as they say) and you'll be prepared to choose how to prop your boat. I will always prop to rated rpm but many here on TF over prop and prefer that to the standard method.

Eric

Take Michigan........ wow..... which prop?????

Dura Quad
Dyna Five
Dyna Foil
Dyna Jet
Dyna Quad
HD
M-500
MP
MAXIMA
PAXMASTER
SEAR-2
TRAWLER
WORK-HORSE

........?

>As far as I know most or all engine manufacturers recommend you prop to the engines rated rpm.<

No longer,

According to the last Nigel Calder article in Pro Boat Builder VOLVO is now overproping new engines for displacement hulls like sail boats.

The simplest way to estimate HOW to over prop is to use tracing paper ad trace the theoretical prop curve that is shown with many engines.

Then slide the prop curve over to the factory HP curve (24/7) and see how many RPM would be lost at WOT .

Use the new WOT figures (RPM & HP) in a prop mfg formula .

Some times the high power limited time rating , will accept the new larger diameter prop, for 15 or 30 min , so a flank speed requirement for Limited time is fine.

However its done , a cruising prop will get a new RED LINE painted on the tachometer face and an EGT gauge is a great addition.

The red line is 300RPM or 10% pullback from whatever WOT RPM is observed with the new cruising prop.

Sounds like re rating Fred. And complicated but if Volvo's doing it (whatever it is) should be OK.

First I've ever heard of anything like that though.

I still think over propping is too much to loose and too little to gain. If you can't afford to not have a 5% reduction in fuel burn then perhaps a smaller boat would be in order.

I'm not say'in over propping is evil. It's actually GOOD if one NEVER exceeds the rpm that would overload the engine. What rpm that is on an over propped engine is a very foggy issue at best. One would think the just not exceeding max EGT would keep things safe but I'm not convinced that's true. What if you were running engine an at WOT and max EGT. Then you over propped it several hundred rpm and (lets say 2200rpm WOT on a Lehman). Then max EGT would presumably be several hundred rpm below that .. say 1800rpm. Is running the engine at 1800rpm w max EGT the same as running it at 2500rpm and max EGT? I doubt it and I certainly don't have the knowledge to answer that question. Some engineer does (of course) but he's not down the street from me. I'd bet huge amounts of money running that over propped engine at 1500rpm would be better than running it at 1500rpm w an engine propped (properly) to the rated rpm of 2500. The over propped engine would be less underloaded at 1500rpm and running under more ideal circumstances. But at what rpm is the tipping point? On any given engine it's just a guess. With an EGT it's still just a guess but admittedly a better guess.

Again for the small increase in efficiency I don't think it's worth it. If I was coming to see you Fred in a twin engine GB 36 it may be so but for most all of us it's just fly stuff. However the reduction in noise can be significant and that may be the real motivation for over propping. We like our trawlers and going slow .. at a relaxing rate is very pleasing. Screaming dosn't fill our cup. Maybe "it's more efficient" is just a smoke screen for the real reason ..... relaxed boating. Does that strike a chord w anybody?

Reiziger,
Yes Michigan makes lots of different kinds of props.
The "WORK HORSE" is (I believe) a very heavy duty version of the MP prop. Some or all of them have a greater blade area too. The greater blade area is gained from the wider blades that produce more losses at the tips. The shape is not exactly the same as the regular MP. If you have a four blade MP and need more blade area and want to stay w the MP design the wide blade Work Horse is the only option. I would think the 4 blade Work Horse would be a very heavy prop and could be hard on transmission clutches.

One of the issues w under loading engines is that many trawlers have excessive blade area to begin w and have less than ideal pitch and too much area. Reducing blade area and increasing pitch would be more efficient in that case. Blade area is to absorb power and if you're not delivering much power you've got too much. Reducing blade area and increasing pitch, done in significant amounts could be an alternative to over propping. Just a thought.

Eric-Forum, Back after receiving and digesting the results of submission of information on the Michigan Wheel site. First before we bring those results forward let us review the original submission to IwillIt.
In that post the following was submitted to IwillIT:

IwillIt- Try filling in all the blanks as honest as you are able, into this site's format. It will give you a actuate wheel size required for the information given.
Al-Ketchikan (Bridge to Nowhere) Alaska

Vicprop - Propeller Calculator

Following shortly after was a comment by Motion 30. To that in part, I responded:

In the case of our boat, the calculator hit the nail right on. It produced the data that matched our current wheel. That is why the comfort in offering it up as a response to somebody asking.
Given the results of a calculator, a site such as this will provide a wheel shopper the experience as to what is out there. Buy from it? Somebody is or the site would not be in business.

In this statement I was referring the Vicprop calculator. Which confirmed the wheel currently in play 22"X16".

Agreeing with Eric, I submitted the same data used on the Vicprop calculator to Michigan Wheel.
Pleased now to submit for the forum review the results of that submission.
Al

Dear Mr. Johnson:

Thank you for your interest in Michigan Wheel propellers. The following is the result of the propeller analysis performed based on the data you submitted. As a result of our analysis, we suggest you consider the following:

Diameter and Pitch: 22 L 16
Number of blades: Three
Material: Bronze
Style: Machine Pitch
* Suggested retail price: $1,314.00

* Plus applicable tax and shipping

COMMENTS:

The suggested propeller size is based on the information you provided, which appears to indicate the current propeller is the correct size for your application (3000 actual RPM versus 3000 rated RPM.)
When running an analysis, we must size the propeller to allow the engine to reach the maximum rated RPM at full throttle. (Increasing the pitch would reduce the RPM at full throttle and overload the engine, which you would not want to do).
The recommended clearance between propeller tip and hull bottom is 15% of the propeller diameter.

We hope this information is useful to you. We do not sell consumer direct. For more information on how to purchase our propellers, please check our website for the Michigan Wheel dealer nearest you.

Again, thank you for your interest in Michigan Wheel propellers.

Sincerely,
Greg Verseput
MWM Inboard Customer Service

Al we're out buying a kayak today so on the road.

Yes ... That's what I expected. I'm quite sure most or all we'll founded professional sources will say the same as Michigan Wheel.

Still thinking about my lots of pitch and greatly reduced blade area idea. The idea to do it in excess (and to work that would be necessary) would make it less efficient at WOT but may be better at half load. Just a wild thought.