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Old 05-15-2015, 01:43 PM   #41
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Very interesting. Never paid any attention before, but just looked at the torque curve and fuel burn graphs on my 110hp Yanmar-- my torque peaks at 2000-2200, and that's about where it likes to run and vibrate less.

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And at 2200 RPM it puts out about 30hp which pushes me about 7kn (1kn below hull speed) and burns about 2gph. If I run it at 2000 it supposedly burns just a bit over 1gph according to this Yanmar graph. That's about 6.5kn at 2000 rpm.

So just .5kn slower almost halves my fuel burn? Does that sound possible?

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Old 05-15-2015, 02:11 PM   #42
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cardude01,
Yes.

I think you all are being nice to me. And I appreciate it. I've sung this song about overpowering so long I'll try to limit my continued efforts in this direction. Everybody is probably sick of hearing it but if everybody understands what I'm say'in there's no point in ranting on. And yes HOLLYWOOD I'm happy w Willy.

Wifey B I sure like your one pedal two pedal take on power, I suspect the smaller engine turning more rpm actually makes less noise. At a higher frequency though. I remember most people would rather listen to an airplane fly over than an ultralight as the sound of an ultralight is irritating. Higher rpm. A case can be made for bigger engines making more noise but I can't think of a reason a smaller engine would make more noise. A bigger engine doing less work makes less noise .. no doubt about that. I think reducing engine noise should be done w sound reducing panels and the like rather than over propping to reduce rpm. Just my opinion.

Wifey B yes I do have an overpowered bicycle. Last month I had a Jetta. Now I have a Honda Accord. The Jetta was a German sports sedan. The Honda is just an average car in most respects but the details are wonderful. Very user friendly and comfortable. I even like the automatic transmission .. a CVT belt drive like FF was talking about. I also have an 87 Nissan Stanza that probably dosn't have half the power of either the VW or the Honda. I can't even use half that power as the clutch slips. Really like the car though and what you suspected of me re cars is true.

But re hollywood I definitely don't drive a Prius.
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Old 05-15-2015, 04:25 PM   #43
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But re hollywood I definitely don't drive a Prius.
Geez! I can't get any respect around here!"
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Old 05-17-2015, 10:31 AM   #44
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There’s also another issue often disregarded by recreational users of displacement hulls. These users were not the only ones! The manufactures of those hulls who claim building yachts, also disregarded the transmission set up, they also disregarded prop efficiency not by design but by task! How the engine displaces the power to move the displacement hull is as much, if not more important, than the continuous power of the engine.

It is current to see, I haven’t seen otherwise, a HD with a huge motor, way above its needs, with a low transmission ratio, say 2.4:1,which calls for a small prop with a pitch number higher than the diameter. This set up is perfect for a planning hull or a fast SD one. Yet, for a HD, aworkboat hull, this set up is detrimental.

HD hulls, disregarding what they are on water for, require high transmission ratios to turn big diameter props with small pitches, in slow motion. What??? Say again!!!!

Based on the fact that all HD should carry low power engines in relation to the size, these transmission ratios increase the amount of torque available at the prop, eliminating the need for more power that some of us would like to have available. In fact, it is not power that we need, we need torque on the prop to make the boat move against all challenges, currents, tides etc.

I do have a small spot in my marina and mostly when I arrive, the wind is blowing at no less than 10 knots pushing me away from the pier. With the transmission set up that I have, 3.96:1 ZF with 34” x 26” 4 blade prop, despite my exposure to the wind, all I had to do so far, is playing with gears. At 700 RPM I had never had the need to give some power to move or change movement patterns in my 50 ton boat. My engine only goes to 145 HP @2500 max.

Unfortunately, this is an issue that it is not easy to change unless you’re building from scratch or re-powering. Even when re-powering, it is needed to calculate if by changing the transmission ratio there will be space for the new prop.

Few years back when I was building Rainha Jannota, some of the members of this Forum were surprised with the size of my gear and prop.They said that it was a composition made for a tow boat. Agree, tow boats can also ride with this arrangement but, I am very happy to maneuver my boat in close quarters without putting my hands on the accelerator. Idle is ok to maneuver. Change from ahead to reverse and the boat kicks her butt to starboard immediately, when doing the opposite, there she goes to port.

In another words, it’s not only about power, it is mostly how and how much power gets to the prop. One other issue is what is the prop pushing? The boat, or the engine?

For those who want to know more about this issue and are confused with my pre-historic written skills, there’s an old trawler builder in New Zealand that uses this very same theory to power their boats, and from who I learnt about prop efficiency

http://www.realtrawlers.com/the-engine-room/propellers.html

Enjoy!

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Old 05-17-2015, 11:25 AM   #45
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Quote:
Originally Posted by Portuguese View Post
There’s also another issue often disregarded by recreational users of displacement hulls. These users were not the only ones! The manufactures of those hulls who claim building yachts, also disregarded the transmission set up, they also disregarded prop efficiency not by design but by task! How the engine displaces the power to move the displacement hull is as much, if not more important, than the continuous power of the engine.

It is current to see, I haven’t seen otherwise, a HD with a huge motor, way above its needs, with a low transmission ratio, say 2.4:1,which calls for a small prop with a pitch number higher than the diameter. This set up is perfect for a planning hull or a fast SD one. Yet, for a HD, aworkboat hull, this set up is detrimental.

HD hulls, disregarding what they are on water for, require high transmission ratios to turn big diameter props with small pitches, in slow motion. What??? Say again!!!!

Based on the fact that all HD should carry low power engines in relation to the size, these transmission ratios increase the amount of torque available at the prop, eliminating the need for more power that some of us would like to have available. In fact, it is not power that we need, we need torque on the prop to make the boat move against all challenges, currents, tides etc.

I do have a small spot in my marina and mostly when I arrive, the wind is blowing at no less than 10 knots pushing me away from the pier. With the transmission set up that I have, 3.96:1 ZF with 34” x 26” 4 blade prop, despite my exposure to the wind, all I had to do so far, is playing with gears. At 700 RPM I had never had the need to give some power to move or change movement patterns in my 50 ton boat. My engine only goes to 145 HP @2500 max.

Unfortunately, this is an issue that it is not easy to change unless you’re building from scratch or re-powering. Even when re-powering, it is needed to calculate if by changing the transmission ratio there will be space for the new prop.

Few years back when I was building Rainha Jannota, some of the members of this Forum were surprised with the size of my gear and prop.They said that it was a composition made for a tow boat. Agree, tow boats can also ride with this arrangement but, I am very happy to maneuver my boat in close quarters without putting my hands on the accelerator. Idle is ok to maneuver. Change from ahead to reverse and the boat kicks her butt to starboard immediately, when doing the opposite, there she goes to port.

In another words, it’s not only about power, it is mostly how and how much power gets to the prop. One other issue is what is the prop pushing? The boat, or the engine?

For those who want to know more about this issue and are confused with my pre-historic written skills, there’s an old trawler builder in New Zealand that uses this very same theory to power their boats, and from who I learnt about prop efficiency

http://www.realtrawlers.com/the-engine-room/propellers.html

Enjoy!

regards



Portuguese

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I think your writing is quite good.

My Yanmar is 110hp and the transmission is 2.63 : 1, and my prop is 21x16 (which looks pretty small IMO). So I suppose I fall into the "overpowered and small running gear" camp for my slow hull.

Why don't boat builders go the route you did with larger running gear and slower turning engines? Seems to make sense. More expensive maybe?
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Old 05-17-2015, 11:59 AM   #46
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Less stress on engine and gear set up, transmissions, leading to less money spent on maintenance. Better for us but bad for them

Cultural issues. The recreational motorboat industry drove from planning designs to semi-planning, and them to HD. The first recreational trawlers came from sailboats or professional boats turn into recreational ones.

The thing is a lot simpler than I describe,

High volume below water line = slow/small engine + high transmission rate +big prop w/small pitch leading to high torque low speed

Low volume below waterline = fast/big engine + small transmission rate +small prop w/big pitch leading to high speed, low torque
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Old 05-17-2015, 12:38 PM   #47
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Portuguese,
Very well written post and the subject of drive efficiency is often overlooked it seems.

Pic #1 shows my prop clearance. Pic #2 better shows the lower clearance. Pic #3 shows the clearance of a traditional fishing boat.

It can be seen in 1&2 that my Willard could easily swing a 20" prop .. 2" dia larger OD than her std prop of 18" dia. My gear is 2.57 - 1 and options in the vicinity of 3-1 are and have been readily available. Why did the boat designer and manufacturer not take advantage of this opportunity? And why have I not seen or heard of a Willard 30 w a bigger prop? Most trawlers are more like the 26NT (pic#4) whereas clearance for a larger prop is lacking. But would the designers have passed up the opportunity for a larger prop as they did on Willards and the fishboat if they had more clearance. I think possibly or even most likely yes.

Lets say I had room for a 36" dia prop on Willy. If I installed such a prop it would need to be a very high aspect ratio prop that to limit the blade area and the high aspect ratio would gain even more efficiency. See pic #5. The need for a specific blade area remains fairly constant .. to absorb the available power.

With my high aspect ratio 36" dia prop it seems I would gain considerable efficiency. But under what conditions? I suspect that increases in efficiency will be realized only at very high power outputs close to full throttle. Even at 70% load there may be no increase in efficiency over props common on trawlers. Outboard motors (engines) have very small dia props for their power. But you can clamp a 10hp OB on a mini hydroplane or a barge and expect enough performance to make it worth doing. There is a flexibility there w the small high speed prop that is very useful.

There is so much need for flexibility here that larger props on most of our boats would be a step in the wrong direction IMO. Higher AR larger dia props would push our boats faster at WOT only, or near so, and that woukdn't get along very well w all the underloading that I hear about on TF. A reduction in efficiency may result on most boats.

If I had a 23hp engine in Willy I may benefit from a larger prop at cruising speed as I'd probably be at 85% load. I have a 40hp engine and cruise at about exactly 50% load. I suspect Willard got it right w the 18" dia prop.

I'm not saying Portuguese is wrong but I think deep reductions are good for boats that need lots of high output power (tugs?) or/and operate at the (IMO) ideal load of about 75%. I could go to a 3-1 gear and a 20" prop but I don't think I'd gain anything at 50% load ... where I am 98% of the time.

Another very important part of the prop issue is the dia to pitch ratio. It's been my observation over the years here on TF that IMO too many boats have too little pitch. Spinning blades gets no work done unless the blades move water. And excess blade area combined w low pitch consumes power that could push the boat faster or more efficiently w smaller blades or that and more pitch. Sometimes and perhaps more often less dia and more pitch can improve performance.

Regarding Portuguese's post I've always read and heard of proping to HP.
Torque will accelerate a prop faster and it seems to be highly related to efficiency but I understand little about torque. It seems to exist in an instantaneous way as a measure of force but not doing any work over time. I would like to hear more about torque and how it applies to powering our boats.
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Old 05-17-2015, 01:02 PM   #48
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Since we're sharing...

My transmission is 2.96:1 and the prop is a 3 blade 34x21. Displacement is about 35 tons. So far I have done most of my close quarters maneuvering at idle. But let's face it - this type of boat isn't designed for close quarters maneuvering. As a single screw, ocean going boat I would expect my transmission and prop to be tuned for efficiency at cruising speed and not maneuvering (assuming there's a trade-off to be made.)

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Old 05-17-2015, 01:05 PM   #49
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FAttachment 40219
...
My Yanmar is 110hp and the transmission is 2.63 : 1, and my prop is 21x16 (which looks pretty small IMO). So I suppose I fall into the "overpowered and small running gear" camp for my slow hull.
...
Your trawler derives from a sailboat. Sailboats typically have small props so there's less to drag through the water when sailing. Maybe your smaller setup is closer to sailboat design for this reason.

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Old 05-17-2015, 05:02 PM   #50
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I don't have a prop calculator but it looks about right to me. The Yanmar may be a high rpm engine and of course they would need a deeper reduction than most. The Dia/pitch ratio seems fine too. Dosn't look like the blade area is high so may be fairly efficient. My Willy (no jokes on that) only has an 18" prop but she only has 40hp.

Any boat w the original prop that has'nt been overpropped by the manufacturer would be hard to improve on if the boat is being operated as originally intended. A NA and perhaps engineers have chosen the prop and they know more than most any of us.

Brit w so much boat below the water the wind would have a hard time having anything to do w slow speed maneuvers. I thought Willy was in that catergory but certianly not. I had to make a landing in Ketchikan in 40mph winds and definitely had my hands full and used considerable throttle at times. Your boat is so big and heavy I have a hard time getting my head around those numbers. But my little boat probably has the same effect on you.

And re what your boat is designed for (efficiency or maneurability) I'm sure it was designed for both. Being designed to be operated by amature helmsmen I'm sure was not overlooked by the designer. I would like to have been a fly on the wall observing the design effort. One wonderful boat I'm sure. Has Tom White seen it?
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Old 05-17-2015, 06:22 PM   #51
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Emily Grace, the 46 initially mentioned, came down to the Caribbean with us in 2009 and then proceeded to circumnavigate over five years. Tom and Kim appeared to take good care of her during the six months we spent with them.
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Old 05-19-2015, 06:41 AM   #52
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The prop diameter is a driver of the draft required for the boat.

Shallow draft = smaller diameter.

Also take a look at the difference in prop co$t a 25 inch is not bad but as you go to 30,35,40 the price goes up all out of proportion.

Also remember tiny HP, 30-60HP is required for many FD at cruise speed so the smaller prop with big pitch will operate at lower RPM , making it more comfortable on board.

The higher loading at low RPM is good for engine life and better fuel burn.

While there would be wave climbing efficiencies traveling with a 40 inch prop and a 6-1 tranny , the higher RPM all the time would be annoying in most cases. As well as fuelish.
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