Re: deliberate overpropping

An unasked question that just occurred to me is. "Why not just push the throttle from 1700 to 1800 and accomplish the same thing with the 'correct' prop instead of overpropping the boat?"

It's a valid question, I think, and one answer may lie in the nature of us humans.

We have two FL120s under the floor in our PNW cruiser. And guess what happens if I push the power from our usual 1650 crusing rpm up to 1750 or 1800 rpm? With these old-technology engines, it starts getting real noisy in the cabin.

Now if one wades through Grand Banks promo literature from the 1960s and 70s, you will see occasional references to cruising in a "quiet" boat. By overpropping a bit and giving the owner a knot or so more speed at the same rpm, the engine doesn't have to be spun up another 1,000 or whatever rpm to get that same knot, and things stay quieter inside.

Now, being able to offer a slightly less noisy boat at cruise is not a reason to overprop a boat. But American Marine saw (or rather heard) the difference between gaining a bit more speed by keeping the engine(s) at a lower rpm with a coarser prop and running the engine(s) faster to get the same speed, and being the smart marketers they were at the time, didn't hesitate to promote a quiet(er) boat.

No, they did not in their literature relate the noise level in the cabin to the props they used. They didn't even relate it to a lower engine rpm in cruise. They simply promoted their boat as being "quiet."

A small thing, sure. But when your new-boat market is (at that time) fairly wealthy, discriminating boat buyers, every little bit helps.

I am by no means trying to justify the use of overpropping willy-nilly in everything. I'm silmply offering up what Grand Banks did as a way of illustrating the fact that overpropping can be advantageous in some situations. There are, I'm sure, just as many if not more situations when it's not.

But I don't think one can make a blanket statement that overpropping is always bad or stupid OR always good or sensible. Like almost all boating questions from anchors to zincs, the right answer is..... it depends.

Good point, the noise issue!


best regards / med venlig hilsen
wadden

Unfortunately for most boat builders outside the FD class over propping is done for a few(very few) knots at the top end. Too much sales emphasis is given to faster cruise and top end speed its obvious in the many adds. This is most damaging in faster high powered boats with large modern engines where the new boat gains weight once in the hands of a clueless owner. Then the over propping becomes serious overloading and if not addressed some of the engines get damaged. As for noise there are other ways to handle this besides knocking a few hundred rpm down, but that will cost a builder more than choosing a prop with more pitch. I would hope the builder of my boats would not take the cheap alternative in this situation.

A lot of speculation and generalization.

There is a concept called "over propping" and as FF would call it "cruise propping"...a much better name.

Like many things in life....there is some truth to it and some issues.

Others have posted before that it is a very conditional concept.

To discuss it being used when it is inappropriate even by supporters of the concept....well...no wonder people get lost in its basics.

Increasing the pitch require more power to produce the same speed and RPM. You are just moving across the HP curve from normal prop load toward max hP curve at the same rpm. Fuel use will increase not decrease and speed or wavemaking will as well.


An engine will produce zero HP at a particular RPM when unloaded and use little fuel. Load the engine in increasing amounts but hold the RPM steady and fuel use will increase accordingly. More load more fuel. there is no free lunch.

There is no free lunch , tho (Obamma voters might disagree)

Any boat only needs a certain amount of thrust to go at a certain speed.

If the required thrust can be obtained at much lower RPM ,the engine will be using its fuel more efficiently.

Therefore the engine will not only be quieter but will last considerably longer , from the better load and fewer piston miles .

The reason for installing a Cruising Prop.

Diameter counts , more than pitch.

Marin wrote;
"Why? Because the 18" prop is moving more water back at 1700 rpm than the 17" prop did at 1700 rpm. And for every action there's a reaction, right?"
and then "Why not just push the throttle from 1700 to 1800 and accomplish the same thing with the 'correct' prop instead of overpropping the boat?"

Of course.

Then I thought we were out of the woods and eyshulman says "Unfortunately for most boat builders outside the FD class over propping is done for a few(very few) knots at the top end. Too much sales emphasis is given to faster cruise and top end speed its obvious in the many adds."

There's not going to be any increase in top end speed. Impossible. Maximum power is had at rated rpm. Any underpropping or overpropping will reduce power and lower top speed.

Marin also said;
"But I don't think one can make a blanket statement that overpropping is always bad or stupid OR always good or sensible"
I think it's always bad. Done by someone that actually knows what he's doing like FF it's good. BUT only a tiny bit good .. so why not do it right. Who's to say this guy knows what he's doing and this guy dosn't?

After experiencing how easily an engine runs propped correctly I'll bet most overproppers would prop to rated rpm after the experience.

Scott I'll continue calling overpropping overpropping as it implies doing wrong and that's the essence I want to communicate. Been putting it down for a long time and don't see any change coming along.

The way faster boats get over propped is that the builder props for top speed LITE BOAT the owner fills tanks and puts his extra equipment aboard and before you blink the boat gains a few thousand pounds Walla over propped and if run 200 off the top over loaded. How many builders of fast boat are propping for expected and almost universal weight gain and slightly dirty bottom, and then accepting having to advertise less speed? Boats that travel at more than 25K are often sold with speed high on the priority ladder. SD boats not so much but it may still be in the mix. FD top end comes only with WL extension.

manyboats said:

Who's to say this guy knows what he's doing and this guy dosn't?

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The long-term results will say. Done wrong, and engine longevity will suffer. Done right, and the owner/operator will enjoy a benefit.

Grand Banks did it for at least a couple of decades, they did it right, and they sold thousands of boats to buyers who were very satisfied with the speed experienced at cruise with reasonably low noise levels and excellent engine service and longevity.

"Right" in this case, is in the mind of the beholder. You (Eric) think it's wrong from a theoretical standpoint, but as has been shown before with what sells and what doesn't, reality trumps theory every time.

American Marine did is right as far as they were concerned (and I suspect they were a much more successful boat design and manufacturing company than you or the rest of us on TF forum have been with the possible exception of Tad) And American Marine enjoyed the sales of thousands of their Grand Banks boats to very satisfied customers. And those boats continue to be prized on the used market by people who desire this type of cruiser.

So wrong in your eyes, maybe. But in this particular case, which is the only one I feel I know enough about to comment on, what American Marine did with their Grand Banks boats regarding what should more aptly be called FF's "cruise propping", was very, very smart.

We've propped our GB down. The original three-bladed props were cruise propped 24" x 17" on one side and 24" x 18 on the on the other side, and our PO-installed four-bladed props were more or less the same. They are now 23" x 16" on both sides. What we gained for a given rpm was a bit less fuel consumption and engines that work a bit less hard as illustrated on the EGT gauges.

What we lost was some speed. We can get that back by running the engines about 1,000 rpm faster, but now we use more fuel again, the engines are working harder again, and we have a considerably noisier cabin.

When we thought we needed new props, we were going to go back to three blade props (thee blades are more efficient than four) and have them set to the original (coarser) pitches. It turned out our 4-bladed props were still good but they needed considerable working over to rectify the terrible setup job the prop shopt in California had done prior to our buying the boat.

So on the advice of the prop shop we use in Seattle, we had the props set to what they are today.

Marin said:

Grand Banks did it for at least a couple of decades, they did it right, and they sold thousands of boats..

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You make some good points but lets stop touting these grossly overpowered Grand Banks that push water like a bulldozer. Yes, they are pretty and yes they are better built than the average TT but my god they push water like a sports fisher at 10knots.

boatpoker said:

You make some good points but lets stop touting these grossly overpowered Grand Banks that push water like a bulldozer. Yes, they are pretty and yes they are better built than the average TT but my god they push water like a sports fisher at 10knots.

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They're not overpowered to the owner who wants to get somewhere at 15 or 18 knots and then when he gets there putz around for a week at 9 knots and then head back to the office at 15 or 18 knots, which describes a big chunk of the GB market.

In the earlier days, the GB32, 36, and 42 were mostly powered with one or two FL120s. Always only one in the caese of the GB32. Our GB36 weighs 30,000 pounds full up. An FL120 doesn't sound overpowered to me, or even two of them, when the semi-planing hull can make use of the power.

Spray, the prototype of the GB36, had a single 275 hp Cat and she went like stink, relatively speaking. If GB made a mistake it was not sticking with that kind of power in the production GBs.

If someone wants to creep around on the water at glacier speeds in a low power, displacement boat, that's fine. But they shouldn't project their desire to cruise at speeds slower than a Galapagos land tortoise onto everyone else. A lot of us have places to go and things to see.

As to pushing water, our GB doesn't push any more water than other cruisers of similar configuration. And it pushes less water than some of the bloated boats that pass for semi-planing diesel cruisers being manufactured today.

Here is a little photo of Spray at speed. This is the exact same Ken Smith-designed hull we have on our GB36. Doesn't look like she's pushing water to me....

Now I am having another boat built Down-East. Will the builder over prop probably. Does it matter probably not. 18 ft boat 13.5 hp Beta diesel. Most likely this NA engine will reach hull speed in this FD hull at 50% engine load. So this will be the classic case of FD over propped but not nessaseraly dangerously over loaded. Will I correct the prop probably. Do I have to probably not. Will any differences in fuel use efficiency matter probably not will be under sail 80% of the time.

Marin said:

They're not overpowered to the owner who wants to get somewhere at 15 or 18 knots and then when he gets there putz around for a week at 9 knots and then head back to the office at 15 or 18 knots, which describes a big chunk of the GB market.

In the earlier days, the GB32, 36, and 42 were mostly powered with one or two FL120s. Always only one in the caese of the GB32. Our GB36 weighs 30,000 pounds full up. An FL120 doesn't sound overpowered to me, or even two of them, when the semi-planing hull can make use of the power.

Spray, the prototype of the GB36, had a single 275 hp Cat and she went like stink, relatively speaking. If GB made a mistake it was not sticking with that kind of power in the production GBs.

If someone wants to creep around on the water at glacier speeds in a low power, displacement boat, that's fine. But they shouldn't project their desire to cruise at speeds slower than a Galapagos land tortoise onto everyone else. A lot of us have places to go and things to see.

As to pushing water, our GB doesn't push any more water than other cruisers of similar configuration. And it pushes less water than some of the bloated boats that pass for semi-planing diesel cruisers being manufactured today.

Here is a little photo of Spray at speed. This is the exact same Ken Smith-designed hull we have on our GB36. Doesn't look like she's pushing water to me....

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Having been badly waked by many sports fishers and GB's in the ICW, we can agree to disagree.

boatpoker said:

Having been badly waked by many sports fishers and GB's in the ICW, we can agree to disagree.

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If by pushing water you mean throwing out a big wake, of course a GB being driven at semi-planing speeds will do that. Hell, that's half the fun of driving the boat.

I thought by pushing water you meant something more like "plowing," pushing up a lot of water in front of it.

So I guess we do agree on this if you're talking about wake size. We've been passed by late-model GB42s doing 16 knots or so. Yes, big wake. But nothing compared to the massive wakes coming of the backs of 40-something foot Bayliners being driven at the same kinds of speeds. Now those wakes are downright dangerous.

Not experienced the wakes off big sportfishers at speed as we rarely get that type of boat out here.

Marin said:

Not experienced the wakes off big sportfishers at speed as we rarely get that type of boat out here.

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Maybe like this?

Marin said:

Not experienced the wakes off big sportfishers at speed as we rarely get that type of boat out here.

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LarryM said:

Maybe like this?

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Woowee! A dead head at that speed would come up through the hull, the deck and into the flying bridge, impaled like a bug in an entomologist collection. So nope, not a lot of sport fishers here in the PNW/BC/Alaska waters.

As far as overproppping goes, I'll add my traditional comment. Remember what effect it may have on the boats idle speed. Coming into a harbour at 3.5 knots at low idle and having to go in and out of gear is no fun.

LarryM said:

Maybe like this?

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I've seen maybe two of those types of boats up here. A really big one that for several years wintered in the Bellingham marina and another one for sale some years ago in Sidney harbor. In both cases I believe the boats were home-ported in California.

We don't do that kind of fishing on the inside waters here. With the exception of silver salmon (coho) which are often fairly close to the surface, most of the fish here are some distance down. Salmon trolling at 30 to 120 feet or so is typical. Halibut can be from 30 feet to 300 feet or more.

So trolling speeds for salmon are usually fairly slow-- 3 knots is typical. Halibut can be caught by trolling on the bottom, but for the most part it's drift fishing.

And the boats used for sport fishing on the inside waters are not all that large.16 to 25 feet seems typical. We're often fishing pretty close to rocks and reefs and bluffs and whatnot and the currents are often pretty strong. A large boat could get pretty unwieldy in these conditons. We don't use our Grand Banks for fishing. We have a different boat for that.

There is a tuna fishery, at least a tuna sport fishery, off the coast of Washington and lower BC. The charter boats I've seen engaged in this and salmon fishing are patterned more after the smaller commerical fishing boats used here albeit with different cabin arrangements (photo). But there may be some sportfishers like the one in your photo out there, too. I don't do that kind of fishing so wouldn't know for sure.

Fishing(sport) boats on East coast are developed for a different type of use. Along many parts of the coast the fish are 50 to 100 miles out in what can be a nasty ocean. So the SF want to get there fast and run back fast and the cost for fuel is often disregarded. Thus the planning 30k cruise speed SF has evolved. Different fishing different boats. I have read an article that claimed that if a SF could not cruise at 35k or better it would not be a good sea boat, the idea being that in rough conditions these fast boats can just blast over the waves. I don't know enough about that to say yay or nay.

In defense of the fast SD boat GB or not the jury is out and the SD has won by a large but not unanimous decision. That is why there are so many SD boats and why I own at least one. I can go slow and smell the roses or if I choose push the throttle up and get there today. Actually some times just a little speed at the end of the day gets the mooring dock space or good anchorage area.

An absolute hoot when you're not paying the fuel bills. My sister has a Post 56. Lots of fun racing out to the Isle of Shoals for a picnic and back. But she consumes more fuel than I would in a year.

With the newer more powerful common rail diesels and an effectively designed SD hull, you can choose to operate over the ranges that Marin and others desire.

For example, an AT 34 is efficient at 7-8 kn and 2-3 gph, or planes without plowing at 17-18 kn and 17-18 gph. No over propping required.

The new modern engines with great brain boxes may be able to operate at WOT and not be too fuel inefficient when operated 99% of the time at 6K,

BUT the question of how many brain boxes are aboard , and how much time is spent say,in central FL where the lightning strikes are measured in the thousands per day remains.

AT $10K a brain box , a lot of fuel would need to be saved daily!

djmarchand said:

I am late to the party on this thread. There have been dozens of posts but only two: FF's and Ski's addressed the benefits of overpropping. FF said that the fuel consumption would drop 50%. Ski said that fuel consumption would improve slightly and the main benefit is noise

Would 1,400 rpm be quieter than 1,800 rpm? Sure but it depends on engine installation and soundproofing whether it would be meaningful.

David

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On many vessels including ours, as Dave mentions the engine mountings, cabin structural design and soundproofing render small changes in RPM non discernible.

As to dropping fuel consumption by 50%, I wiil provide the vessel, props and testing gear along with a certified naval architect to view and comment on all results. My bet is for costs, pay for engine damage plus 100% that a 50% fuel savings will not result at hull speed. I estimate this test on my vessel to cost about $50K.

sun: You have zero risk of loosing that bet.


50% fuel reduction claim at any fixed speed is absurd. People just don't understand props and think of them as fixed gears in contact with hard surface.




Now I can and do reduce fuel consumption greatly when slowing from 20KTs to 8 Kts but There isn't much that will change when dragging 38,000# through the water.

I wish people would study what Larry said above. The same istrue for most SD or SP hulls even with older mechanical engines..

With the newer more powerful common rail diesels and an effectively designed SD hull, you can choose to operate over the ranges that Marin and others desire.

For example, an AT 34 is efficient at 7-8 kn and 2-3 gph, or planes without plowing at 17-18 kn and 17-18 gph. No over propping required.

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My 44' ACMY with a semi planning hull at 38,000# delivers similar 2-3 NMPG at bow down speeds and will also do 20 Kts easily.

NOPE , I never wrote fuel consumption would drop 50%.

I wrote basically , that IF the mfg desire to have every engine regardless of boat size shape or use install a propeller that would reach WOT , it was not his concern if it cost 50% more fuel at cruise.

I never stated that any prop would make every boat 50% better at cruise .

The folks that cruise at 1/2 NMPG to make big waves and go fast , are delighted to pull back to slow displacement speeds and enjoy savings .

If the go fast had a different engine , matched to a cruising prop , it could come closer to efficient than a big engine at almost idle.

If course if the Go Fast had a better displacement speed shape , that would help too, but it would never get up on the plane.

All is compromise ,,,,