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ContinuousWave: Whaler Performance
Propeller - "Bow Lift"
|Author||Topic: Propeller - "Bow Lift"|
posted 05-01-2001 08:35 AM ET (US)
This question I am about to ask reminds me of an old story...
We were shooting a commercial for some schlocky plastic insulated mug which included the line, "It keeps hot drinks hot; keeps cold drinks cold!" One of the janitors had stopped by the studio to watch this, and he seemed interested in this mug, but the claim had him confused.
"Hot drinks, hot; cold drinks, cold! Man, how do it know?" he asked.
Okay, I have the same kind of question about propellers.
I hear people talking about a prop having "bow lift", and I want to ask, "How do it know?"
How does the prop know how to lift the bow? This bow lift phenomenon must really be an artifact of some other attribute of the prop. Or am I missing something?
posted 05-01-2001 09:33 AM ET (US)
Jim, I'm not an expert (although very opinionated) and can't hope to explain "bow lift" with certainty... lots of prop characteristics are the result of witchcraft! First of all, a prop acts as a "non-encased axial flow pump" and propels via a water jet (action/reaction) which you can seen surfacing in mid wake. It only acts as a simple screw at very low rpm! The prop blades gather water from the side (not front) and accelerate it aft. A boat cannot reach or exceed the speed of this jet! The max speed of this jet is determined by hull characteristics, engine horse power and prop characteristics ie: size, shape, material, special features... in the special features category are cupping, raking, venting etc... Cupping is easy to understand and comes in two forms .. primary cupping is the twisting of the blades and secondary cupping is the turning down of the blades' trailing edge... sometimes referred to as double cupping... cupping accelerates the water as it crosses the blades' surface thus increasing speed for a given rpm! Raking is the "pulling back" of the blades (imagine grabbing the blade tips and pulling them straight back) ... the farther back, the greater the rake. This causes the blade to push on the water more than "slice" it and this results on torque being applied to the prop shaft.. this in turn (with a hull designed to benefit) will raise the bow and lift hull almost completely out of the water for max speed. On a hull NOT designed to benefit from bow lift, a high rake (bow lifting) prop will simply waste power, upset otherwise good handling and can damage the engine and is one miserable SOB to contend with! The above is not science but my analogy of what's going on and arrived at empirically... others may have better explanations ! BTW, Whaler does not make any hulls that benefit from high rake, bow lifting props... hope this doesn't just add to the confusion...Clark ... Old and Opinionated
posted 05-01-2001 01:46 PM ET (US)
I should have said applies "leverage" to prop shaft and not "torque".. Clark...
posted 05-01-2001 07:25 PM ET (US)
Clark and I don't disagree on too many items about Whalers, but on the Mercury performance props we may! Maybe it's just semantics. If your boat goes over 30, a little bow lift is good in my opinion, and more than can be accomplished by only power trim.
Anyway, here is my understanding of Jim's question. I should note that my experience is strictly with Mercury brand props, with no experience with other brands. But I would imagine there are similarities. It is the BLADE DESIGN that accomplishes all these lifting effects.
Actually, from what I have read in their product literature and their excellent book on propeller design, there are actually 3 types of performance (recreational, non-racing) propellers. Bow lifting, Stern lifting (not widely used and mostly the "cleaver" type design for over 60mph) and total hull lifting. All of this type only apply to boats that will do over 40MPH, and all offer ability to run at elevated transom heights, with aggressive prop holding. For 0-25, aluminum is probably just fine, 25 - 40 the conventional SS Mercury Vengeance) is adequate. Interestingly enough, Mercury's SS Vengeance (elephant ear design) is described as "Excellent acceleration. Top Speed. BOW LIFT. Load carrying. Thinner blades with less drag". Also, they do not even make an SS Vengeance prop over 18" pitch in the mid-sized gearcase (75-125HP). If you want to go over 40, with SS, you HAVE to buy a bow lifting propeller, either Laser, Trophy or High Five, 20" pitch and higher. So they're telling us something here.
First of all, I believe Mercury engineers believe some bow lift is good. Even power trim (also invented by Mercury) helps accomplish this, and we all know that lifting the bow by trimming out can make sense, particularly at higher speeds. Bow lift is accomplished by blade geometry that actually causes the propeller to PUSH THE STERN DOWN a little as speeds build up. Stern lift is accomplished by blade geometry that PUSHES UP on the stern. Total lift must be somewhere in between. I think the theory is that getting the boat up higher on plane increases speed and relative efficiency, greater than the loss from the power it takes to push down the stern. this is also why most of these are vented, to use more HP sooner, like a lower gear ratio. My guess is that this is more appropriate on a vee hull than a 13', 17 or early Outrage 19/21 hull. Mercury's bow lifting Laser II props are used on both of my Vee Outrages, and they get up and ride quite high on plane. On my 18 Outrage, the difference in hull lift between aluminum props and the Lasers is astronomical. You can literally feel the difference. They advertize the prop for "Light to Medium weight boats (like Dougherty Whalers, relative to the HP) looking for a performance gain". The 5 blade High Five prop is shown as "good bow lift" and would also be applicable to Whalers, particularly if skiing is a priority, but they don't recommend it for Offshore uses, although I think it would be superior there also, but not as fast.
For the V-6 Bass boaters, they make a 4 bladed Trophy prop, MID size gear case, which is advertized as "tremendous bow lift for quick planning and excellent Stern Lift for running in rough water". This would not be recommended for a Whaler.
For larger V-6 engines, large gear case, besides the above, they make a Mirage and 4 blade Offshore. Larger new Whalers, with V-6's installed at the factory, are coming with either of these props. Mirage is billed as having "the blade area to carry (meaning lift) larger-sized boats with ease" (such as the new Outrages and Conquests). It is not shown specifically as a "bow lifting" prop, although I think it does a little. The other great prop for Whalers is the "Offshore" 4 blade (not to confused with a bass boat prop). Mercury defines it as "tremendous (total) lift for big twin (V-6) engine boats, with better hook up in rough seas". If I had a 26 Outrage/Conquest, or any 27/28' Whaler, this is what would be appropriate.
Finally, there is a spread sheet for propeller applications. For Single and Dual engine Offshore boats, they recommend Laser II (bow lifting, lighter weight), Mirage (slight bow lifting, heavier weight, high HP) and Offshore (total lifting, heavier weight, best holding) applications. I would think for most Classic Whalers that would 40, the Laser II is the best prop, and would be good for any Revenge model where the lift would help counter the forward cabin shell weight.
Any of these new props can be installed on any brand engine. None are cheap, and run about $400 each.
JimH, until you get more HP on the 20' Revenge, I would think a higher performance bow lifting prop, other than an SS Vengence, would not be needed. Same for your 15. But SS should be used on both.
posted 05-01-2001 08:27 PM ET (US)
What is a semi-cleaver speed prop? I have no idea what semi-cleaver means.
|Tom W Clark||
posted 05-01-2001 09:08 PM ET (US)
I need an aspirin. Understanding prop theory may simply be beyond my mental ability. But, I will jump in here and say that I disagree with Clark's statement that Whaler did not/does not make any hulls that benefit from bow lifting props. That may be true of a 13' classic but not of a V hull Outrage. Getting the bow out of the water makes a huge difference in speed.
OK, that's the extent of my prop expertise.
So, uh, Larry will you be my prop adviser the next time I buy a boat?
posted 05-01-2001 11:44 PM ET (US)
Only if it's a Whaler. If it's a Mako, never!
|Tom W Clark||
posted 05-02-2001 12:00 AM ET (US)
posted 05-02-2001 06:36 AM ET (US)
Tom, I intended to suggest that Whalers are not designed to ride on the "pad" or little triangle of hull like some bass boats and high performance hulls.. Whalers, as you stated, certainly benefit from trimming engine out so as to raise bow and reduce wetted surface (even the classic 13 hull really takes off when trimmed up a tad). I was referring to the so called "high rake" props like the Quicksilver "Trophy" etc...etc.. A good general purpose prop with modest double cupping a slight rake seems to work best for my purposes... Like Larry said, it's all in the design! Happy Propping... Clark..
posted 05-03-2001 09:30 AM ET (US)
I am following along here and so far what has been mentioned about bow lift is:
--bow lift is related to raking the propeller blades aft;
--raking the blades aft causes the thrust produced by the prop to have a component vector that is at a slight angle to dead-ahead (with respect to the prop shaft).
But since the prop rotates, the accumulation of this angled thrust would tend to cancel, since the prop is producing thrust throughout its rotation. Sometimes the blade is angled such that it produces an upward thrust, sometimes a sideways thrust, and sometimes a downward thrust.
My conjecture is that the accumulated thrust vectors do not quite cancel because the blade is probably able to produce slightly more thrust during the time it is in the lower half of its rotation. There it is in clean water. When it is in the upper half of its rotation the thrust produced is not as powerful because it is hampered by the close clearance to the cavitation plate and other parts of the lower unit.
Therefore, the prop produces slightly less thrust angled downward (due to blade rake) while in the upper half of its rotation, as compared to the amount of thrust produced in the lower half of its rotation which is thrust that is angled slightly upward.
The net offset of the thrust vector from a raked propeller is therefore slightly toward the surface.
This has the effect of driving the stern downward. That in turn causes the bow to rise, assuming that the design of the boat hull is such that the aft end of the hull can be driven down slightly to rotate the hull about its center of resistance and then lift the bow.
OKay, I think I have deduced how bow lift works.
posted 05-03-2001 10:33 AM ET (US)
First off to answer your first question JamesH --- it doesn't know squat!
Now sorry to disillusion you James the primary thing is the design of the boat which was your last qualifying statement and should have been your first! The hull design can and does facilitate bow lift and to a degree the prop style and trim angle of the motors can induce or reduce it even if the hull isn't designed for it ---
A "sorry" goes out to you techno freaks for such a simple explanation, as Clark Roberts said the simpler the better when it comes to props for Whalers or something to that affect.
The explanations above outline pretty well what various props can accomplish that is with the properly designed hull ---- now we can move on to the discussion of tail lift ---
posted 05-03-2001 11:14 AM ET (US)
You must recall that I am the one posed the question, "How do it know?"
As you state, "it doesn't", was generally my impression. How can the prop know that it is supposed to lift the bow?
But I think I have correctly deduced how a raked prop tends to cause the bow to rise. And this is something that--as far as I know--has never been mentioned at any point in this discussion. So nothing you just said contradicts my theory of how a raked prop causes "bow lift".
Now a secondary issue that others have raised is whether the Whaler hull is designed to take advantage of a raked prop's bow lift.
This is a totally separate discussion so let's leave it for another message thread.
And I am not "disillusioned" by your suggestion that no prop can create bow lift; that was the point of my initial inquiry. It was totally unclear how the prop could be designed to raise the bow of the boat.
The real deal is that, as I described above, the net thrust vector from a raked prop must be slightly offset from dead astern and must, in fact, be angled slightly upwards toward the surface of the water.
Now, if you would like to disillusion me about that deduction, please go ahead!
But you cannot disillusion me about other beliefs regarding bow rise because I never held them. I am merely seeking a rational explanation of the phenomenon which includes explanations involving no mutations of the laws of physics.
|Tom W Clark||
posted 05-03-2001 11:20 AM ET (US)
Now, wait just a second there jimh. While I applaud your ability to articulate your position clearly and concisely, I still don't see the answer to the question that's been bugging me. How does rake cause a downward component vector of thrust in the first place? I understand your explanation of the unequal distribution of thrust throughout a blades rotation, but the blade doesn't know up from down. Oh, I'm having a thought (and it hurts) perhaps you mean that the raked blade produces thrust at an angle away from the axis of rotation as opposed to inline with it? This begs the question: How does a stern lifting prop work?
The unequal distribution of thrust throughout a blades rotation is also affected by the fact that the axis of rotation is usually not in line with the direction of travel of the prop. This causes what in aviation is known as P-factor. If the axis of rotation of a prop is at an angle to the direction of travel the blades will have a different angle of attack (pitch) on one side of the rotation than the other. This is what causes a single engine boat to pull to one side when the motor is trimmed in and to pull to the other when trimmed out. It also affects the boats trim. But I do not see where blade design comes into play here.
I understand (and agree with) Bigz' and Clark's position on Whalers not being bass boats or Fountains, I suspect Larry may have some other thoughts. I myself like to keep things simple. Very simple. Whalers, Johnson outboards bolted to the transom, tried and true props made by OMC, etc. But since stumbling across this site a few months ago I find myself intrigued by the though of brackets, new prop/engine technology, etc. Hmmm.....
posted 05-03-2001 02:11 PM ET (US)
Tom C -- James can be very touchy particularly if he has thought long and hard and decided he has stumbled on the proper conclusion which he in fact has done!
He is correct I did introduce another element in this "bow lift" discussion that of boat and hull design, as he stated rightfully this probably should be another thread. That said he is correct I used the wrong term "disillusioned" ---- so be it --- can't get things right all the time --- not as perfect as some on this forum ;(
Now that James has determined the theoretical physics which makes me very happy, in "real" terms Clark Roberts had already explained it.
posted 05-03-2001 02:46 PM ET (US)
I am not going to venture into the field of Propeller Engineering, or try to understand the engineering principles involved. Why, because I'm simply not knowledgeable there. Why a given prop's blade design is a bow lifter, a total hull lifter, a stern lifter or a racer has never been of interest to me.
I just take the Mercury engineers' performance descriptions at their word, and leave it at that. My interest and experience is more from running Whalers with higher powered engines than normal, and from being a student of props from a consumer/user point of view, not a design engineer's point of view.
Somebody asked about stern lifting props. I can't think of any application that would apply to a Whaler, as these are your Cleaver blade props, (don't ask me why a Cleaver blade lifts the stern, but it does) for very fast boats (70-90mph) that do NOT need bow lift because they get that aerodynamically from the speed. They need to develop tail lift to provide better all around balance at high speeds, and to keep the bow DOWN.
What we are really talking about here are "performance enhancing" propellers, not high performance/racing props.
I hate to keep mentioning Mercury, but because of their leadership in performance/racing marine engines, and huge percentage of market share in stern drives and outboards, they seem to have the research/testing facilities to be the leader in the propeller design field for this type of marine drive. Neither OMC or Yamaha have ever been leaders here, with very limited design offerings, and now with OMC gone, Mercury is pretty much all that's left for innovation in prop designs at the moment. They have a huge variety, for almost any boating need or purpose. Almost everything else is a knock-off of what they have done. But nothing's 100%. Yamaha seems to have invented the "straight top edge" design on their conventional props, and I have noticed that Mercury now has incorporated this into a new HP model, and a new 4 blade SS model for the 30-60HP engines.
Before 1980, or so, there were no performance enhancing recreational props (not talking racing here) available, other than the cupped, thinner bladed bronze (OMC never offered these at all) or later SS models, but still with conventional blade geometry. Then the "Laser" prop was introduced (around 1986 I believe, since refined into the Laser II), and this began the new prop engineering where it was recognized that propeller design can have a considerable effect on how well a boat performs. OMC's equivalent prop is called a "Raker". Anybody that thinks this new generation of propellers haven't increased the overall performance, pleasure and safety of recreational boating at speeds of over 30MPH is simply out of touch with reality. A huge decrease in ventilation and a huge increase in holding are some of the main benefits, allowing higher mounting heights for increased efficiency and boat control, and safer handling/holding in rough seas. Almost all +40 MPH boats can benefit from this, including most Boston Whalers that are capable of those speeds.
I think the term "high rake" is badly misused and mis-understood. Increased "rake" (whatever that means, but you can see it visually) appears to be generally good, not bad, and is a more modern prop design concept. As an example, a Merc High Five propeller (5 blades) is shown as a high rake design, with bow lifting capabilities, but in reality it's a very tame, but very good, propeller, for it's task - ultra fast acceleration, aggressive hold at elevated transom heights, smooth power for water skiing. One of these, or a Laser II, could be a great prop for a Montauk with 70-100HP. Then there are a couple of "wilder" designs, such as "Trophy Plus" or "Revolution", but for their particular application, which may not be Boston Whalers, they are still excellent, and improve the overall performance of THOSE style of boats.
When it comes to props, I'm for all the new technology I can get, because this is where your horsepower happens, and your Whaler performs. Once again, I often see the Dealerships as a problem here, or at least the buyers pocketbook as perceived by the Dealer. Cost is a problem with these propellers. Some of the personnel are simply un-informed, or don't even have the correct product to offer the engine buyer. Putting a $150 aluminum elephant ear prop, often an after market Michigan Wheel, etc., on a $10,000 V-6 is a joke. An engine like that needs a "performance enhancing" modern SS propeller, such as a "Mercury Mirage, Offshore, Laser" etc, or other equivalent brand.
One final note. I have noticed that lately the new concept seems to be "total hull lift" rather than the earlier concept of just bow lift. The theory being that most boats can benefit from this extra lift to help actually raise more of the boat out of the water, thereby reducing hydrodynamic drag & increasing efficiency. These types of propellers (both 3 & 4 bladed) are being sent out as standard equipment on the BW Outrage and Conquest models, and maybe the larger Dauntless' also. No old fashioned aluminum here! (Mercury doesn't even make CR propellers in conventional 3 bladed aluminum design.)
Just recently, for the first couple of outings this season, I re-installed my 23" pitch, Black Max elephant ear cupped aluminum props on the 18 Outrage w/twin 115's. Transom jacks have to be set all the way down to run these. The boat performs just fine up to 25 or so. As you get near 30 MPH, it becomes VERY apparent the boat needs a little more lift. Trimming out starts to cause slipping and ventilation. Around 40, you can actually feel the hull running too level, bow not high enough, and feel the power being lost.
Next weekend, I put my SS, but conventional design, props on. I have had the blades thinned and sharpened, to improve performance. You can feel a huge improvement almost immediately. Much better holding, a definite increase in hull lift over 25mph, and a generally safer, and better ride to the boat. Up around 40, however, you once again get the feeling that the boat needs to come up a little more to avoid bow steer potential. The speed is good with these props, but the needed bow lift isn't there over 35mph.
Then, when the Laser II's go on, the jack plates lifted about 1 1/2", you've got a boat that gets up high on the water and runs solid. The boat just seems to run better.
(I think Mercury owes me a 1% commission on all props sold from this web site!)
posted 05-03-2001 03:32 PM ET (US)
Wow, we may have to start a separate FORUM just for propellers!
But, I have a new theory on the raked propellers.
I think the aft rake may also enhance the performance of the prop by extending the blade area over a longer surface.
Think about two propellers, both have the same pitch and diameter. One has zero rake aft, the other has an extreme rake aft.
[I am trying to visualize this, since I don't have a raked prop to look at.]
It seems to me that the distance the water travels over the blade of the raked prop must be longer than the distance the water travels of the blade of a non-raked prop of the same pitch. It seems intuitive that this would mean the raked prop has more "grip" on the water.
So for a given pitch and equal diameter, the raked prop actual has more surface area (?).
In this way, the prop may be able to exert greater leverage on the boat in terms of rotating the boat around its tilt axis of rotation to produce some bow lift, besides accomplishing pushing of the boat forward.
Or, perhaps there is some interaction between the rake and the pitch so that adding rake means you have to add more (or less?) pitch to get the same nominal pitch as a not-raked prop. This change may make the blade more efficient.
In one sense, physics and theories can only attempt to describe the phenomena we see in nature, so it we see raked props producing bow-lift we have to accept it and appreciate we may not be able to theorize or quantify all the interactions going on that produce it.
But it sure would be nice to try...
We need an interview with a Mercury prop expert (err, a factory Mercury prop expert!).
I am just thinking aloud here, so jump right in with alternative theories.
posted 05-05-2001 12:34 AM ET (US)
If your theory that more thrust is developed on a blade during the lower half of the prop's rotation is true then rake is immaterial. Any prop will experience forward thrust on its blades when it rotates. If the thrust on the blades is higher when they are deeper in the water then there will be a moment on the shaft about a horizontal axis perpendicular to the shaft thereby turning the bow up. What does rake have to do with it?
posted 05-05-2001 07:53 AM ET (US)
Jim,et al, I have forwarded this thread to my contact at Mercury Propellers and asked for an official response. Don
posted 05-05-2001 01:37 PM ET (US)
Hank makes a good point regarding the tendency of any prop to produce "bow-lift" from the greater thrust coming off the lower blades.
Perhaps rake has more to do with the speed of the water flowing over the blade. As the boat moves faster, the water coming into the blade is moving faster, too. By raking the prop aft it seems like there would be an effect of "slowing" down the rate of water flowing into the blade, giving the blade more time to act on the water.
High speed aircraft all have their winds swept back; perhaps high speed propellers are swept back (rake) for similar reasons.
I am looking forward to more on this topic!
|The Whaler Guy||
posted 05-06-2001 01:46 AM ET (US)
Bow lift is directly related to rake angle.
If you really need bow lift on any boat without notched transom you need to set engine back using a bracket.
posted 05-06-2001 03:08 PM ET (US)
I'd be willing to stipulate that bow lift was related to rake angle.
But, not to be a pain in the but, could someone just explain how?
posted 05-06-2001 08:38 PM ET (US)
OK, I can't stand keeping my stupidity a secret any longer! I simply cannot get it through my head how rake or cup or anything else on a propeller that spins and sheds its influence equally in all radial directions can affect bow lift in and of itself, without the influence of some other factor(s).
*UNLESS* I'm missing a part of the equation that everyone else has been taking for granted, like for instance more propeller rake affects bow lift *indirectly* by giving the propeller a better bite so a motor, especially one that is elevated, can be trimmed higher without cavitation than one without a high rake prop, and thus bow lift is effected by motor trim.
|The Whaler Guy||
posted 05-06-2001 09:04 PM ET (US)
Well first let's understand that low pressure on the front side of a prop blade actually propels a boat forward. The cupping enhances the effect of "holding at a high trim level". The rake angle defines the degree of angular low pressure related to position of prop shaft. Cupping actually helps to keep the prop hooked up in the water.
Cup can also provide additional BOW LIFT when utilized ON the RAKE LINE of the prop. Applying cup to the TRAILING EDGE of the prop ALONG the PITCH LINE will INCREASE the EFFECTIVE PITCH of the propeller. A standard
Speed Calc As Follows>>>>
The Square Root of (Total Shaft Horsepower / Weight ) x Constant = Speed
posted 05-07-2001 02:11 PM ET (US)
Kingfish - I'm like you. I don't understand how a high rake propeller generates bow lift or hull lift. But they do, and I know that from my own experience, un-related to transom height or trim angle.
Here are a few quotes form a Mercury propeller catalog:
"Chopper propellers (ultra hi performance, not for Whalers) will provide the ultimate top speed for boats requiring PROPELLER ASSISTED BOW LIFT ......"
referring to 70mph catamaran hull sport boats
PS. JCF, you should be running one of these higher rake props on your 22 (if you're not already), considering how fast it is! I think Louie Kokinis likes the 4 blade higher rake "offshore" best on his similarly rigged 22.
posted 05-07-2001 03:09 PM ET (US)
Larry, you hit the nail on the head with both Mercury statements that is the boat design is the key factor, all a prop configuration can do is help in achieving the desired result based on the hull design. I might add the prop can also cause problems in performance if used on a hull not designed for the props intended use.
Which leads us back to the designing of props for different uses and how the rake, cup, pitch and diameter all contribute to produce a prop for a specific purpose/s to be used on specific hulls styles. I for one will have to buy into Clark Roberts semi-technical explanation.
I, after some testing of the 27 WA will have to determine if the standard OMC SST props 15x17 should be regulated to spares and the Offshore 4 blade Raptures be the props of choice(or fact similar). In my mind it can only be accomplished after testing!
posted 05-07-2001 06:15 PM ET (US)
Sumbody give me a ASprin. How sums o'ya fellas pack all dat tinkin into ya haids is a mystry t'me! Out der on da watta, I'm usually wrestlin with jus a few high-brow issues, lak, will da trout take a shwimp or a cacahoe nex?... will ah take a spam sammich or a veener sausage nex?... hmmm, sdat goil ova der wearin what I TINKS she's a wearin?... you got da pictcha? All dis rocket science is way beyond fishin. Ya'll orta go gitcha a AEROplane so's ya really got sumpin ta tink about when ya under powr. I'm humblefied an AMAZED at da gray matta in dis group (myselfs excluded o'course).
posted 05-08-2001 01:33 AM ET (US)
JCS: The 3 blade is sitting in my basement, I can't say enough good things about the OFFSHORE. Most dealers have sample props they will loan you for a day or so, I'd suggest trying one....I think you'll love it.
posted 05-08-2001 01:36 AM ET (US)
OOPS: meant to type JCF :(
posted 05-08-2001 01:16 PM ET (US)
Thanks Larry, and Louie-
I have a Stainless 3-blade prop on my Evinrude 225 that came on the motor and looks like it may be moderately cupped; not sure. It is not raked, but it apparently is the right size, in that at WOT in very light chop she'll do a smidgeon over 50 mph (gps) while turning just about 5500 rpm. Fooling around with this stuff is my cup of tea (I'm like Clark - I can't leave things alone), but I'm pretty thoroughly hampered by the lack of the sort of Marina that would have various props to try, around Battle Creek. I'll just keep working on that one...
posted 05-08-2001 07:24 PM ET (US)
Here some more info for those you interested in propellers, QUOTING the 1999 Mercury Propeller Guide:
"4 BLADE PROPELLER FACTS"
"Four blade propellers (such as OFFSHORE or TROPHY SPORT 30-60hp) usually:
* plane the boat faster than 3-blade propellers
* keep the boat on plane at lower speed
* give improved mid-range speed at the same
* provide quicker acceleration than most 3-blade propellers
* run smoother than 3-blade propellers
* have better holding power in rough conditions
* are less likely to ventilate in sharp turns
* provide better low speed handling
* are not quite as fast on the top end as a comparable 3-blade propeller
* if you're upgrading from a 3-blade to a 4-blade propeller, remember that a 4 blade propeller generally turns 50 to 100 RPM LESS than a 3-blade prop of the same pitch."
They also say, on another subject:
"Mercury Marine makes most of its propellers in either aluminum or stainless steel. Aluminum is inexpensive, but isn't very durable. The slightest underwater encounter
(end of quotes)
posted 05-18-2001 08:42 AM ET (US)
It has been about ten days since anyone commented further on propeller designs and bow lift. I just re-read the whole message thread, and I still find the answer somewhat unclear.
I am throwing out my theory about unequal blade thrust. That effect would occur with any prop, although the high rake angle prop would probably have more of an offset.
I think the answer to "what causes bow lift" has more to do with the general ability of the propeller to not slip. The greater the propeller's grip on the water, the more leverage it can apply to rotation of the bow upward.
I also tend to think that the effects of rake come into play at higher speeds. As speed increases, the water travels over the blades faster, so the effect of rake is a tendency for the water to be in contact with the blade for a longer distance. This must improve the thrust of the prop compared to one without any rake.
This has been an excellent discussion!
posted 05-18-2001 11:06 AM ET (US)
I actually reread this thread just the other day and I came to the same conclusion that the higher rake props simply must have better grip at higher speeds than lower rake props. If true, this would enable the motor to be trimmed out further (which I believe has been the advertised result of high rake props) thereby causing the bow to be lifted higher as the boat rotates about the transom resulting from the increased angle between the drive line of the motor and the keel of the hull. In other words, the prop simply remains hooked up at higher trim/speed levels than could be achieved with lesser raked props. On my former 18 Outrage, the 150 Johnson was turning a 21" OMC Raker, which at that time was OMC's high rake propeller with the ventilation holes, and I must say that I could trim the engine out quite far without any apparent loss in traction, perhaps getting one or two mph more than might be obtained with a lower raked, ordinary stainless prop. At those trim levels and speed (48+), there was lots of noticable bow lift.
posted 05-23-2001 01:06 PM ET (US)
Higher rake props do allow a greater trim out range, which can enhance bow lift and speed by using trim angle to force the stern down, and thereby raise the bow. But a true bow or hull lifting prop does this by propeller action alone, and does not require much trim out, since it has been determined that trim out is not the most effective way to increase bow lift. The upward thrust angle of the engine can reduce efficiency, and cause instability. Instead, the engine itself can be lifted up, but still not trimed out, maintaining prop thrust parallel to the boat bottom and water surface. Look at your racing outboard hydroplanes. None of those engines are trimmed out at speed.
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