Propeller Stern Lift

Optimizing the performance of Boston Whaler boats
Minimaxxed
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Joined: Tue Jan 05, 2021 4:46 am

Propeller Stern Lift

Postby Minimaxxed » Tue Jan 05, 2021 5:34 am

I found [the forum here at CONTINUOUSWAVE] when searching for ways to modify a propeller for less stern lift.

An old topic from 2005 on the old forum had a good discussion about stern limit propeller and bow lifting propellers, and it prompted me to join [this forum] and to share a recent experience with this on my homemade boat I recently completed.

After my first test run of my boat, I found the engine over-revved and the trim angle was not right. My outboard has no adjustment in engine mounting height so spacers above motor board was used as temporary means to raise engine. The manual trim has six holes [ or stops,for adjustment of trim position]. Each postion provides adds 5-degrees [outward and upward] trim.

The boat was plowing the bow when [the trim stop was set at the first hole] along with over revving.

[Changing the trim stop to the next] hole caused the the bow to run very high. and the boat felt very unsteady.

The bottom of [Anti-Ventilation] plate was level with bottom of boat. I felt that [there were a method to get only] 2.5-degree more [outward] trim [from the first stop] then this propeller would trim about right on the boat, and the boat would feel safer to drive and spray less water spray from the bow.

Moving[the engine mounting height] up an inch on the test day did not seem to do anything except create [propeller ventilation when accelerating from a standing start at full throttle].

Instead of the 20-year-old beat-up propeller that came with the engine, I tried a brand new factory-style propeller. I tested again and the boat had the same behaviors.

For a third test propeller, I bought a so-called “stern lift” propeller, fitted it, and went back to the water to test.

To my surprise, the effect of this prop is extreme stern lift or rather extreme bow down, whichever way you look at it.

Where the boat was planing with original propeller on the second trim stop and had the bow pointing too high, now even on the third trim stop with this "stern lift" propeller my bow is lower than with the original prop tucked right in.

[Based on the] angle of thrust you would imagine coming from the propeller, the bow would be expected to be to be pointing vertical--which it does at very slow speed--BUT, nope. The stern lift prop has a trick.

After finding the old site here and the 2005 thread about stern lift propellers I think I have figured out how they work.

A propeller doesn't just push water in a straight line out the back of prop, but rather--if you imagined the propeller as a sphere--the propeller pushes water in all directions from up,down, sideways, and sternwards.

Since propellers slip, that slipped water flies off prop blades at 90-degrees and at full [engine speed], that water has a lot weight flying off sideways in all directions creating higher pressure in those places.

Now since our outboards have a big flat [Anti-Ventilation] plate directly above the propeller, that pressure cannot escape upward and must go sideways or down. That is [the stern-lifting propeller's] trick: stern lifting propeller use the deflection of slipped water centrifugally to create flow of water in all directions except upwards due to the [Anti-ventilation] plate. This extra weight of water slipped lifts stern--somehow.

Whether [the stern lifting propeller is[ causing an extra inch or three-inches of higher pressure water directly at transom by raising water level under rear of boat like a wave due to high pressure zone there which floats transom higher, or whether it is simply lifting the transom because of down thrust of slipped water, I am not sure.

I think bow-lifting propellers slip the water differently by holding it on blades and slipping it off at a less severe angle like 45 rearward or less and this misses the [Anti-Ventialtion] plate, thus preventing the down thrust being dominant and having a very narrow thrust cone which puts the trim back on to the actual trim position of motor rather than [at an] angle of slipped 90-degree water being flung off a stern-lifting prop. This seems logical.

I’ve seen it with my own eyes on the water. The engine is trimmed up but it is plowing bow. Believe in stern-lifting propellers.

Now, do I get the grinder and trim [the engine's Anti-Ventilation] plate to get a bit less stern lift? Haha.

The boat hull weighs around 88-lbs. The engine adds 75-lbs. In addition there were five litres of fuel. My body weight is 121-lbs. The boat under discussion is a light boat with a tendency to be stern-heavy. The boat is 8-ft long and 4-feet wide with a hydroplane hull. [Because of these factors] any amount of stern lift or bow lift from the propeller is quite noticeable.

jimh
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Re: Propeller Stern Lift

Postby jimh » Tue Jan 05, 2021 8:02 am

Propellers described as bow-lifting tend to have propeller blades that are raked aft, which tends to focus the thrust into a smaller diameter cone of accelerated water.

The explanation that stern lift force is created when thrust coming off the propeller toward the surface is deflected downward by the Anti-Ventilation plate seems reasonable. I also observe that propellers that are described as stern-lifting usually do not have much blade rake.

But quite curiously, some propellers are described as providing both bow lift and stern lift simultaneously. The mechanism for each effect would then seem to be non-conflicting. This would tend to rule out blade rake as an influence on stern or bow lift effect.

Another variable in propeller design is how far aft of the blade roots the propeller hub extends. On some propellers noted for producing stern lift, like a Mercury REVOLUTION4, the propeller hub extends a rather long distance past the blade root and also has no flare at the opening. Exactly how that might affect the thrust vector is unknown to me. However, extending the propeller hub farther aft would seem to prevent any exhaust gases from escaping until they were past the end of the Anti-Ventilation plate.

I feel that these effects called bow-lift and stern lift would be a result of the asymmetry of the aperture in which the propeller is revolving. It cannot be due to any particular aspect of the blades. There are many huge ships with propeller blades that rotate in a completely open aperture with no influence of the surface, for example, on a submarine. If there were inherent qualities in a propeller that cause bow lift or stern lift, you would expect this to be observed in those applications.

Another aspect of how a propeller could influence boat trim is the action of the propeller by sucking in water. This occurs in really big ships which will typically take on a stern squat when accelerating because their propeller is sucking is a massive amount of water which results in the ship settling down by the stern slightly.

Minimaxxed
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Re: Propeller Stern Lift

Postby Minimaxxed » Tue Jan 05, 2021 7:54 pm

There is probably a vacuum effect in that extra length [extension of the propeller hub] which water flowing past hub rearwards is drawn or curved towards slightly as the hub is moving forward at speed or which would draw in or cause a easier path for surrounding thrust water to bend towards which is higher pressure maybe narrowing cone slightly more than if it were shorter. A flare would be causing the opposite, making it harder for the water going past to turn and be drawn into the void with exhaust. If I were imagining an exhaust pipe say four-inch diameter facing rearward travelling forwards at say 40-MPH, I would guess that the volume of exhaust gas is nowhere near enough to fill that space with more pressure than the water displaced by the hub diameter has going past the opening.

To picture it, I am thinking about holding a 2 litre plastic bottle under water and squeezing air out while stationary in a continuous stream vs that bottle going at 40-MPH with the same volume of air being squeezed out but over a much longer distance the bubbles would be few and far between with more water between each bubble. Hence lower pressure at end of hub center rather than pressure in rear part of hub from exhaust. Just my ponderings, nothing scientific by any means.

Regarding the submarine theory, if there were a plate 1/4-inch from blade tips above propeller we might see this stern lift idea. Another way would be with a [test propeller] for stationary testing which creates load on motor by flinging water 360 degrees. In theory if my [Anti-Ventilation] plate deflection idea is right, the stern should lift under full throttle while the boat is stationary. I see no way it can create bow lift while stationary though. Hmmm.

On my little boat, a 22-lbs weight or lift force on motor board moves the stern down or up a lot while stationary. Much more than a usual size fishing boat, so small amounts of upwards or downwards thrust is very noticable.

Minimaxxed
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Propeller for Non-Boston Whaler Boat

Postby Minimaxxed » Wed Jan 06, 2021 12:25 am

I’m after ideas to try that will make the bow [of the small non-Boston Whaler boat to] rise by about two inches at full throttle without losing speed. No the boat speed is 50-KPH with a Mercury 15-HP two-stroke-power-cycle engine with a SOLAS AMITA 3 12-pitch propeller The factory type 9 pitch props couldnt load the engine enough to keep rpm at around 6,000. This current off the shelf amita 3 prop was the only one i could find with 12 pitch 9.25 inches diameter with 8 splines. I like this prop and the way the boat feels and the max rpm is where I want it but I need to get bow up more. The boat is chopping waves off and they almost blast you out the back of boat if your not holding steering wheel tight. Then you gotta bail the boat out. It wont sink when swamped and will still drive but yeah, needs more bow lift.

Before I go chopping down my anti ventilation plate, Is it worth trying to make a bracket that would get my motor farther back behind motorboard to see if that will give me another couple inches bow lift? I’ve no experience with those brackets so not sure if they have that effect or not.

jimh
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Re: Adjusting Trim for On-plane

Postby jimh » Wed Jan 06, 2021 6:57 am

MINI--the forum does not discuss the performance of non-Boston Whaler boats. You should locate a forum about racing small hydroplane hulls to get more advice about optimizing the performance of your small hydroplane boat.

On the general topic of engine trim: engine trim can be very touchy with regard to the exact attitude of the bow when the boat is on plane. Since your engine has only a series of trim stops spaced at a fixed distance, you may not be able to obtain the precise trim you need. But I have an idea for you to try.

The mechanism for trim adjustment on your engine is probably a metal rod held in position by two holes in the engine mount that constrains the downward position of the engine. Since it will be difficult to move the location of the holes in the engine mount to get the desired 2.5-degree increment in trim adjustment, you could modify the mechanism slightly to give you this added lift in outward trim in two ways.

Remove the rod from the mount--I am sure this can be done because I suspect that is how the rod is moved from one set of holes to the next. Create a larger diameter for the inner part of the rod by slipping another section of tube over the rod so that the diameter of the rod is increased in the area where the engine tilt bracket will bear onto the rod. You could use aluminum tubing with a wall thickness of 0.058-inch and use a tube outside-diameter in increments of 0.125-inch (1/8th-inch). Aluminum tube with 0.058 wall will fit over or fit inside another tube with that wall thickness that is 0.0125 inch larger or smaller in diameter. Or you could use a wood dowel and drill a center hole. Changing the diameter of the rod that acts as the trim stop will let you have a finer range of trim adjustment.

Note that to test this theory, you could just wrap many turns of duct tape onto the trim stop rod at each side of the rod where the tilt bracket bears against it. The duct tape should be sufficiently strong to survive a few test runs in which you determine the best thickness to add to the trim stop rod. Once you have the correct thickness to add to the rod in the trim stop, you can make a more permanent device to perform that function.

The other method is to add some sort of thickness, perhaps rubber pads, to the engine tilt mount so that when they hit against the trim stop rod the engine is moved outward the desired added amount. You could also use some wood scraps to make the pads. Test with an adhesive that is not permanent so you can pop-off the added thickness pads if they don't work as anticipated.

Let me know if you like my ideas.

These ideas work with any boat that is using an engine without continuously adjustable trim position, so I think this is a proper topic for the PERFORMANCE forum, as many smaller Boston Whaler boats may use engines without power trim.

Also, adjusting engine mounting height may affect trim. That can be another adjustment to experiment with.

biggiefl
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Re: Propeller Stern Lift

Postby biggiefl » Wed Jan 06, 2021 11:56 am

Being your boat is homemade and wooden, you might be flexing the hull at speed which in turn acts like a hook. Hooks act like a trim tab and mash the bow down.
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

Minimaxxed
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Re: Propeller Stern Lift

Postby Minimaxxed » Wed Jan 06, 2021 3:03 pm

Thank you Gentleman,

I like the ideas to try and see if it helps.
I checked this morning and see there is a 2mm hook in the last 12 inches or so of bottom hull. I didnt notice this while fairing so maybe my trailer is set up incorrectly with supporting the hull with this oversized motor weight.

I will have to re fair the bottom it seems :(

I’m sorry for talking about the wrong kind of boat here but after reading jims posts from a long time ago I could tell he knows a lot about boats and might be able to help. This is my first boat I have made and have a lot to learn.

Thanks.

biggiefl
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Re: Propeller Stern Lift

Postby biggiefl » Wed Jan 06, 2021 3:34 pm

Glad I thought of [a hook in the hull bottom running surface causing the bow to stay down as a possible cause of the bow-down running attitude on the boat].

Keep us posted [on the outcome of fairing the hull bottom to eliminate the hook and the effect on trim when on plane].
On my 24th Whaler. Currently in the stable: 86 18' Outrage, 81 13' Sport(original owner), 87 11' Sport, 69 Squall(for sale cheap).

jimh
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Re: Propeller Stern Lift

Postby jimh » Thu Jan 07, 2021 10:15 am

Similarly, certain older Boston Whaler hulls had an intentional hook molded into the hull to keep down the bow rise. I think you can find that on the original 13-foot hull.

Compare at:

viewtopic.php?f=4&t=479