Prop Selection for Production Boat

[DivineFocus]

I'm a long time visitor to this forum and have found it to provide a lot of food for thought. I'm a lifetime boater(both professional and pleasure) and turbo-machinery engineer by profession. I just repowered my Ranger Tugs R25 classic and am trying to zero in on a best fit prop solution. I'm in Alaska so trying multiple options and yo-yoing with a local shop is not feasible with lack of local resources and a very short operating season. So here I am asking for feedback on what I have in mind. Sorry in advance for the long-winded explanation that follows.

Background:
The boat is a fiberglass semi-displacement hull. LOA is 24.5ft, LWL is about 23.5ft, beam is just shy of 8ft, draft is 26 in. The manufacturer claims dry weight is 5750#. Owners with access to commercial scales have reported weight with full tanks and cruising gear to be 8000-8500#. In order to minimize draft(for trailering) the wheel is recessed in a tunnel of sorts(pics below). The boat was originally supplied with a 150 hp turbocharged common rail automotive derivative diesel with a WOT of 4000 rpm and a 2.0:1 gear. The factory tested wheel was a 17x15/0.105 cup three blade ACME which yielded 20 kts with a light boat. Later the factory offered the same wheel with 14 in pitch for heavier cruising loads. The 17 in wheel is right at the 15% limit for tip clearance. I purchased the boat used with the 14 pitch wheel installed and the removed 15 in pitched one in a box. With my typical cruising load she wouldn't quite hit 4000 rpm at WOT. Realistic cruise speed at 3200 rpm was around 11 kts. Factory test at that speed(15 in pitched prop) was about 15.

Current situation:
After 10 years and over 2000 hrs put on the engine(90 percent at hull speed) I decided to re-power the boat. Considering the limited options available for engines and accepting the fact this just isn't a fast boat I elected to go with a lower hp unit in the form of a 110hp Yanmar 4JH110 with WOT of 3200 rpm and a 2.04:1 gear. And of course with the repower I am now scrutinizing the prop situation. I've made one shakedown using the same 17x14 wheel that I was running before. Of course with the same wheel performance didn't change(i.e. +- 11 kts at 3200) but I was able to hit 3500 rpm and might have gone higher but I stopped there as I was still in the break in period for the engine. Data that I collected shows slip numbers above 50% across the rpm range at anything over hull speed.

When I run the wheel sizing program on VicProp it comes back with a 19.5 x 13.8 three blade or 18.4 x 13.6 four blade. Which I don't have room for if I stick with conventional wisdom on tip clearance. However.... I am considering going to 18 in diameter which would put me at 10% tip clearance. I'm also thinking of going to a four blade both for more power and more likely to tolerate the tight tip clearance. This also matches my own(limited and dated) experience with semi-displacement sport fishing boats which were all fitted with four blade wheels. Plus as heavy as I run and at these low speeds it seems like the boat should be propped more like a work boat than a cruiser. As to pitch if I follow the VicProp guidelines(i.e. an inch of reduced dia adds two inches pitch) that would put me at 14.5 pitch. So net what I'm considering is an 18x14.5 four blade wheel with 10% tip clearance. Both ACME and Michigan Wheel claim that reduced tip clearances are possible with advanced blade geometries.

My questions for the community:
- is it worth the risk of reducing tip clearance for the larger disk area versus simply increasing pitch and living with the resultant slip?
- is the rule of thumb that 0.050 in of cup equates to one inch of pitch on outboards applicable to this wheel type/size? So is my existing 17x14/0.105 cup performing like 16 in pitch?
- am I just way off in my thinking?
- any other random thoughts?

Thanks in advance for your replies.

Dan

 

[HelmutSheina]

We used to drop 2" in pitch for a factory cupped prop, workshop cupping props for racing was suck it and see, or, bash the prop into desired shape, then water test rig to assess. I also thought cupping was for higher RPM props where diameter is restricted, but far from sure.

Haven't had a lot to do with props in tunnels so cannot speak on clearance.

 

[DivineFocus]

We used to drop 2" in pitch for a factory cupped prop, workshop cupping props for racing was suck it and see, or, bash the prop into desired shape, then water test rig to assess. I also thought cupping was for higher RPM props where diameter is restricted, but far from sure.

Haven't had a lot to do with props in tunnels so cannot speak on clearance.

Thanks for the reply. Cupping is intended to prevent/minimize cavitation. I also dabbled with high speed boats a bit in my youth. In high performance boats cavitation typically occurs during acceleration before getting on plane due to high pitched prop and low speed through the water. The boats(outboards) that we raced would cavitate so bad that the motor would over rev until the boat got up on plane. We also toyed around modifying cup using the round end of a ballpeen hammer held in a vise as a sort of anvil to beat against to form the cup.

Until now trying to figure out how cup affects pitch was never an issue. But now I'm trying to figure out how my existing prop relates to potential replacements so it has become a relevant question. Virtually all of ACME's propellers are cupped and some rather aggressively(e.g. the 0.105 cup on my current wheel). I'm not sure why the OEM chose to go with ACME propellers on these boats and why ACME chose this degree of cup. I suspect due to the relatively heavy semi-displacement hull, relatively shallow draft, and irregular hull shape(the tunnel) they may have been concerned with potential cavitation. I have no idea if or to what degree they tested different wheels.

 

[fallguy]

I don’t think anyone here is going to be able to offer advice better than VicProp over the phone. My 2 cents. If you give me n address, I can send you the real 2 cents for something worthwhile.

As with most prop choices, sometimes you need to take a risk for higher reward, but what is reward for you? More efficiency? Surely not top end. Talk it with them.

I don’t see lotsa value in cupping for your setup, but that is just me..

 

[baeckmo]

Standard propeller calculation schemes often fail with this type of tunnel, since there is a considerable amount of "dead water" in front of the screw. In addition, there is very often a backflow into the tunnel from behind and/or flow detachment from the tunnel roof. Quite frankly; standard layman's opinions don't apply. From the pictures I am pretty shure this propeller is actually vented from behind due to this backflow, particularly when highly loaded. This results in sc gascavitation, which differs from the classical vapour cavitation in that the gascavities do not implode momentarily, as the vapour cavities.

But the gas bubbles cause a reduction of the propeller thrust, just like the "real cavities", which is the rationale behind the use of cupped propeller blades in this case.

To give realistic advise in this case, we must have a better idea about the tunnel shape than the images shown above; could you show the forward parts of the the tunnel from the side and below? A profile sketch with rough propertions slightly to the side of the centerline (say just clearing the shaft skeg) would be useful as well.

 

[DivineFocus]

I don’t think anyone here is going to be able to offer advice better than VicProp over the phone. My 2 cents. If you give me n address, I can send you the real 2 cents for something worthwhile.

As with most prop choices, sometimes you need to take a risk for higher reward, but what is reward for you? More efficiency? Surely not top end. Talk it with them.

I don’t see lotsa value in cupping for your setup, but that is just me..

I'm new here and don't yet have rights to start private messages. If you start one with me I assume I'd be able to reply.

 

[fallguy]

Stick with @baeckmo . You are very fortunate he is contributing to you and between him and the VicProp shop; you will find the best solution.

 

[DivineFocus]

Standard propeller calculation schemes often fail with this type of tunnel, since there is a considerable amount of "dead water" in front of the screw. In addition, there is very often a backflow into the tunnel from behind and/or flow detachment from the tunnel roof. Quite frankly; standard layman's opinions don't apply. From the pictures I am pretty shure this propeller is actually vented from behind due to this backflow, particularly when highly loaded. This results in sc gascavitation, which differs from the classical vapour cavitation in that the gascavities do not implode momentarily, as the vapour cavities.

But the gas bubbles cause a reduction of the propeller thrust, just like the "real cavities", which is the rationale behind the use of cupped propeller blades in this case.

To give realistic advise in this case, we must have a better idea about the tunnel shape than the images shown above; could you show the forward parts of the the tunnel from the side and below? A profile sketch with rough propertions slightly to the side of the centerline (say just clearing the shaft skeg) would be useful as well.

I'm not sure I'd be able to generate a meaningful sketch. I took photos attached below but the boat is on a trailer so it was difficult to get good angles. Maybe between the photos and the following description you can get a mental image:
- the leading edge of the tunnel is 35 inches from the transom
- top blade of the prop is 5 inches from the transom
- the tunnel is 25 inches wide and does not taper as it goes forward(i.e. edges are parallel w/keel)
- there are blisters on the bottom of the hull(first pic below)about 3 inches deep which taper off at the leading edge of the tunnel(blisters were molded in to give clearance for the engine to fit under the deck)
- the sides/edges of the tunnel are curved but the top is flat

Using the shaft/propeller nut center as reference and viewed from aft:
- the sides of the tunnel fit a circle with a 12.5 in radius centered on the end of the shaft
- the curve of the tunnel meets the boat bottom at about 30 degrees above horizontal
- curved portion of the tunnel sweep about a 30 degree arc before transitioning to the flat top portion
- with the 17 inch wheel there is 2.25 in tip clearance at top dead center, four inch clearance from curved portion of tunnel

Note: the light green in the photos is a joint where they fastened a sort of deadwood to the molded hull.

View of blister and leading edge of tunnel


Side/underneath view

 

[DivineFocus]

Stick with @baeckmo . You are very fortunate he is contributing to you and between him and the VicProp shop; you will find the best solution.

Thanks for the feedback. I'm looking forward to his suggestions.

 

[gonzo]

It is likely the propeller is ventilating at high loads. The location is really far aft. Unless it is operating as a surfacing piercing propeller, which is supposed to be ventilated.

 

[DivineFocus]

It is likely the propeller is ventilating at high loads. The location is really far aft. Unless it is operating as a surfacing piercing propeller, which is supposed to be ventilated.

Assuming that's the case the rule of thumb solution to ventilation with outboards is to increase blade area, go to stainless for a narrower blade profile, and use a heavily cupped prop. So directionally my thoughts of increased diameter and blade area should be an improvement, yes? And perhaps even higher cup than the 0.105" on my current prop?

 

[DivineFocus]

Standard propeller calculation schemes often fail with this type of tunnel, since there is a considerable amount of "dead water" in front of the screw. In addition, there is very often a backflow into the tunnel from behind and/or flow detachment from the tunnel roof. Quite frankly; standard layman's opinions don't apply. From the pictures I am pretty shure this propeller is actually vented from behind due to this backflow, particularly when highly loaded. This results in sc gascavitation, which differs from the classical vapour cavitation in that the gascavities do not implode momentarily, as the vapour cavities.

But the gas bubbles cause a reduction of the propeller thrust, just like the "real cavities", which is the rationale behind the use of cupped propeller blades in this case...

When I read your comments above I recognized that you were the author of posts in a thread that I read a couple of years ago. At the time I found your discussion of the fluid dynamics very interesting because my career was working with high energy pumps/compressors which often involved solving flow induced issues. My recollection of that thread was that it was primarily about the tunnel design. But today I searched that thread back up and re-read it with my current context in mind and found some of your comments very interesting and seemingly applicable to my situation. I've extracted some of your input to that thread and have a few comments/questions relative to my situation.

"WATER DOES NOT LIKE SURPRISES"

So with the hull design that I'm dealing with the blisters in front of the tunnel/recess no doubt compound the problem of disturbing inlet flow to the propeller. Until I read some of you comments in that other thread I'd not really stopped to think about the fluid dynamics involved. Water being incompressible and the flow constrained by the bottom of the boat the water can only move laterally(relative to the boat CL). So the blisters are creating a "hole" in the water immediately followed by the cavity of the tunnel. No doubt that creates a very confused flow profile and a reduction in pressure entering the propeller. Certainly not ideal and no doubt confounding to typical sizing calculations/modeling.

...I have seen a number of tunnel boats, where in fact the propeller has been ventilated from abaft, causing heavy vibrations and bad performance....The major problem in this case was ventilation from behind. It was fixed by the addition of a simple rubber skirt, that cut off the air ingress by adapting to the outflow... loaded speed was substantially improved.

It never occurred to me that this could/would occur. But I can see it. In my case I'm not experience noticeable vibration, possibly structurally transmitted noise at higher rpm. But with such low hp involved the lack of significant vibration isn't necessarily because there's not a problem(in my experience).

Some of the cutouts that are called propeller tunnels should rather be called recesses. ...
A propeller recess in more of a general change of bottom shape in order to accomodate a propeller with a reduction of draft in mind. Here the roof diamater is far greater than the propeller dia, the harmful inlet edges are either carefully rounded and/or so far from the propeller that the disturbances have been substantially cancelled before entering the propeller disc. In this case, there is space enough that the flow can adapt to the requirements of the propeller...The recess is less critical to the propeller, but has a major influence on the hull behaviour instead.

Per your description the geometry of my hull sounds more like a recess than a tunnel. Maybe somewhere in between.

...When there is a risk of backflow with air mixing, a propeller with a reasonable cup is a necessity, but the local angle of attack at the leading edge must be small, i.e. the blade pressure is created on the rearmost part of the blade.

I've spoken with three different prop shops and the only solution that any one of them has offered is directionally opposite of this i.e. they recommend increasing pitch.

Modifying the hull design is not a consideration in my situation. So whatever mitigation is possible/practical must be by other means. In that context I'd appreciate if you could comment on the following:

- I've long since recognized that the tunnel/recess reduces buoyancy aft. This boat really "squats" above hull speed and gets worse the faster she goes. Historically I've tried to combat this by shifting as much weight as far forward as possible. In the context of the likely cavitation/ventilation do you think that the marginal increase in pressure at the prop by letting the stern run deeper outweighs any benefit of running on a more even keel?

- From your description of the rubber skirt in your example case that I quoted above I'm not clear how it was configured. Was the skirt attached to the bottom of the hull parallel to the flow and extended beyond the transom? Or attached to the transom extending vertically into the flow behind the propeller? And how stiff was the rubber?

- Based on all of the above does larger diameter, increased blade area, lower pitch, and same or increased cup seem like directionally the way for me to proceed?

 

[baeckmo]

Dear DF, I understand you are eager to get this done, and I'll come back with more info later, but right now in Scandinavia, we are celebrating the midsummer sun; one line is about as much logic I can produce at the moment.............

 

[gonzo]

Assuming that's the case the rule of thumb solution to ventilation with outboards is to increase blade area, go to stainless for a narrower blade profile, and use a heavily cupped prop.

No, that would be the solution for cavitation on propeller blades that are excessively loaded. Ventilation is atmospheric air that is sucked into the front (low pressure) side of the blades. This happens when the blades are close to the surface. Outboards have an anti-ventilation plate over the propeller. On inboards the problem is when the propeller is too close to the stern. The bottom of the hull serves the same purpose as the plate on an outboard.

 

[DivineFocus]

Dear DF, I understand you are eager to get this done, and I'll come back with more info later, but right now in Scandinavia, we are celebrating the midsummer sun; one line is about as much logic I can produce at the moment.............

LoL. No worries. Those who live in lower latitudes can't appreciate our reverence for the summer sun. Happy Solstice! Come Monday I'll be on the water for a week anyway so no hurry.