What type of prop would be recommended in this case?

As recommended by one forum member, I am linking here my thread from powerboats to get the answer which is actually about propulsion, maybe some propulsion experts do not read powerboat threads. I would appreciate your answers in any of two threads. Here is the link:

Could you give me more info about my boat https://www.boatdesign.net/threads/could-you-give-me-more-info-about-my-boat.56856/

 

You have excess rpm's, find a good prop shop, they are not all alike.
- Add cup to your existing prop or
- Add pitch to your existing prop or
- Buy a new prop

... Also might want to move some weight forward, sounds like you are stern heavy.

 

Thank you jogepease.

OK, changing prop in some way is obviously beneficial, but I still have some questions about the change.

Earlier in the thread, before I got rid of the middle tab/flap and my top was around 3150RPM/20km/h, I got this response from member Joakim:
"Most likely the key is to get more blade area (BAR ~100%) and a propeller with better shape for this purpose, since the propeller diameter is clearly too small for the current speed and power. It would be OK around 20 knots, but you have to get there first." Now I do, almost, get there, but before I get there I still go through that, almost the same, spot 19.5km/h at 3000RPM (/1.6 at prop), so is it still BAR problem in general, or that is just the one spot in the transition? As Joakim also stated in earlier responses, cavitation is very high "propeller is very heavily loaded and cavitating a lot (over 50% from Burrill's chart) or the propeller is getting air". Well at that time it was ventilating, but now I do get over it, but still have that same spot, so I am curious is that normal in transition to planning or cavitation/ventilation is still present and BAR would make it better rather than pitch/cup? Or if changing/buying new prop with higher pitch/cup, should it also has greater BAR than the current one at 65%? What is current prop cavitation percentage at maximum speed, 27-28.5km/h/@3500RPM(/1.6@prop)?

Yes, I am aware of heavy stern, we have spoken about it earlier in the thread, too.

 

Every boat has the transition though it might go un noticed ... You however have a hard to plane boat so it's straining the prop. I can put my flat bottom easy to plane boat in that same condition if I wanted.

First I would move some weight into the bow or get some kind of a dolphin or whale tail or trim tabs. Then, if your motor can handle it without lugging, a larger diameter prop or cupping will help you get over the hump where a smaller diameter prop will just cavitate or ventilate. Cupping is pretty cheap to add so I would start with that before buying a new prop but first try moving some weight forward or using one of those aftermarket plates to help you plane out better.

 

I did rearrange weight distribution on the boat, as much as I could have, to compensate adding heavy engine to the stern, it was explained in the linked thread earlier and represented with photos of boat "floorplan". And I do have trim tabs, there are also photos of them in the linked thread. I really do need to take a video of the boat at ride, maybe that would be the best illustration. Since I have removed the middle fixed trim tab which was the source of ventilation, I am still questioning if those controllable trim tabs that I have left are maybe also too close to the prop, still partly making ventilation problem (there is the photo of them in the linked thread)?

Thank you for advice about prop change and cupping.

 

Sorry, read some of that thread, not all of it. Cupping removes the problem of ventilation but whether you have enough motor or not to make it a good solution I don't know. Has anyone suggested adding sponsons? A video would help a lot!

 

OK, I will make a video for the weekend, it is better that way then than guessing what is going on and when. Then it will be also possible to compare with same boat with lighter and more powerful engine also shown in the linked thread.

 

Well, it took me a while to take a video and the video is not very good, but I hope it can be used for the assessment (boat is not close enough and the shooting is done from the river bank, not from another boat which would take a shot of whole transition process). Here is the link:



Please, consider that I am also seeing it for the first time from outside the boat instead from the helm so I have a couple of more questions beside the one about the prop.

When I watch the boat from the helm, I have a feeling that the boat is squatting a lot and use trim tabs to level it and gain maybe around 1km/h, but now when watched from side, I do not have a feeling that it is a lot and I probably overuse the tabs (bow down too much). What would you say about those angles shown on the video?

About the part of video which is taken from the boat, is it possible to say something about the stern wave, although it is shown in very short time periods? On that same part of video (from the boat) I must add, that somehow, although I do not have speed info, have a feeling that the boat was a bit faster and that prop was spinning slower at WOT, but not sure about it, it would be strange anyway. I would rather expect lower RPMs due to loading the boat or unclean bottom (but both are not the case). Anyway if it would be truth, could it be that the prop has some kind of damage (not by hitting something, rather by high load, cavitation, twisting or something like that) and how to check that? Is it possible to explain case of higher RPM and slower speed if that would be the case? Anyway, I do remember clear as a day that in the summer, I was measuring 34km/h and although I am not sure, I think it was even upstream.

 

Consider a different prop rake to get the stern up. Any serious prop guy/shop can address these issues, usually after an afternoon of trying a few props. Yeah you'll have to pay for this service, but what you really need is expertise on the boat, not software or guesses. After an afternoon set of runs, you'll have a prop that does what you want or comes as close as you can get, without a CNC milled custom job.

 

Yeah I think I agree, it doesn't look to be heavy in stern sitting still, you need maximize the efficiency of the prop.

 

Thank you PAR and jorgepease. It is not the problem to pay for that particular part, the problem is that there is no such service in my country, so not many options. And it was not any better through whole restoration process on many things, but that is some different story. Anyway, I will find the way, as always.

So, the boat is squatting a bit more than it should be according to the video?

OK, advice taken seriously since confirmed for the second time. But, PAR is absolutely right that experimenting would give the best prop, so I will try to find the way for that route.

 

Hello Nidza, hope it is still ok to join your ”party”, although a bit late. You got good advice from Joakim (as usual, I´d say). After reading through the original thread I did some quick number checks, and the performance you have today is still showing the effect of ventilation. Your engine is operating on the overspeed limit. Do not run it over 3200 rpms!

In non-aerated water, the 19” x 17” propeller would need about 230 to 250 shaft hp at 1875 rpm (=3000 engine rpms), depending on the wake factor associated with this hull. With the power, rpms and forward velocity you have, this propeller is heavily loaded in terms of thrust per disc area or any of the thrust coefficients used to describe the operating point. This leads to a strong reduction of the local pressure in front of the prop, and if there is the slightest possibility for an air path from the surface to migrate past the antiventilation plate, you have a decrease of propeller thrust and corresponding torque.

Now, looking at your pics, it even seems that the AV-plate level is above bottom level. There is your remaining aeration path! In fact, with this propeller loading, the AV-plate should rather be some 20 to 30 mm below the bottom to prevent ventilation. The present installation level would be ok for a lighter load, but in this case it is a fuel drain.

High propeller loading also leads to increased fluid circulation in the propeller plane, and this has the effect of “centrifuging” the air into the hub region, where it remains stationary more or less. In your application the small propeller hub behind the gear case provides a pocket for collected air, which will make things worse.

So what to do?? First priority is to supply green water to the prop. I guess rebuilding the engine mounting and all the associated paraphernalia is not what you dream about. If there is a longer rig extension piece available, then go for it. Otherwise, here are a few tricks to use:

• Cut a triangular pad out of the bottom at the transom/bottom junction. Upper edge at transom to be at, or slightly above AV-plate level, and length giving an inclination angle of ~8 degrees.
  
  
• Add a “scoop nose” to the AV-plate front; an extension directed forward and down, so that it captures the boundary-layer flow, forcing it upwards, above the AV-plate. This will break the aeration path if correctly installed.
  
  
• A fixed scoop may be attached to the transom/bottom line; working as above.

As soon as the ventilation problem is solved, you will find that your engine rpms will be far below 3000 rpms, and here is the next issue. Your propeller pitch is too high (NOT too low as suggested by some). A higher blade area ratio would have been fine, but I think it will be tricky to find with the shaft splines you have. If memory serves me correct, Volvo Penta single-prop drives have 11/8 dia and 17 splines, but they only have 16” diameter propellers. Mercruiser Bravo 2 unfortunately has 19 splines, but there is a 19.25” x 15 cupped propeller available. If you have too much leisure time, a Volvo hub may be fitted to the Bravo prop, but I would start with your present propeller, take it to a decent propsmith and reduce pitch to 16”. If possible, go for 15 or 15.5 and add a cup.

Finally, do not overuse the trim flaps. The bottom lines of this hull have to have some two to four degrees angle of attack (measured roughly along the half-beam buttock) when “coming out of the displacement hole”.

Good luck!

 

I've never been in any coastal area, where there wasn't someone or shop that didn't do prop work. This includes the several countries around the world I've visited. They generally don't advertise heavily, unless a big outfit or chain, but the local commercial boaters and fishermen know who they are and where they're located, which is the best advice you can receive. Look for the local fishermen and ask them where they get their props worked on.

 

Or ship your prop after talking with the shop and explaining the issue, a prop can be easily cupped to run in ventilated water, like mine, and then you only have to contend with do you have too much pitch as just mentioned. I would start with enough cup to reduce rpm's by 200-300. I always ship mine to a company that is not even in my same state because they are experts in cupping.

 

@baeckmo
We are all "people on the water" so of course, you are more than welcome to join the party and share the good spirit.

I definitively comply with your recommendation about avoiding overspeed RPMs and driving mostly at displacement speeds anyway, this was done only for test purposes.

Before going through suggestions that you proposed later in your post against ventilation, may I add one more question. As you have read, I have removed the central "trim" tab that was making extreme ventilation on propeller. Before saying that prop should be lower, could you assess if my trim tabs are maybe too close to propeller and being the main reason for current ventilation? Here is the photo of current state (part of tabs near prop shown).

If they are still preventing water to prop, making a narrow water tunnel to prop, I could easily make them narrower or move them more to sides of boat. How far should they be, what is minimum distance between prop and trim flaps?

Thank you for this calculation, I presume that the diagram comparing three boat configurations was helpful for this calculation, just my curiosity? Or prop characteristics, weight and hull type of boat was enough?

Yes, it is slightly higher, less than thickness of a finger (photo showing central trim tab before dismounting it shows the distance between AV-plate and that tab representing AV-plate height from bottom, the finger could not be inserted in that gap). It is very interesting everything that you explained in text preceding and succeeding this sentence (about operating points, pressures, fluids, air trapped in the hub zone, ...).

Daunting! Not an option.

Unfortunately, this was the longest extension (5 inches). Making longer one on my own would not be cheap (aluminum case (could be a problem), longer bolts (easy), longer shaft extension (could be a problem)), more than one part to be build/changed, questionable quality of available materials.

This seems like relatively easy, but since the hull is cored, I would like to keep that core buried deep inside and protected from water. I will ask one more question about that hull structure in different thread, since it is about materials. For now, I would leave that to a side, but would not dismiss it yet, just that I would rather test that with following propositions at first.

Did I understand well - something like "Permatrim" mentioned by MrEfficiency earlier, but to be angled down on the front side from the drive to the bottom stern? Would only something like "Permatrim" help without inclination? How much should be that inclination (again 8 degrees or just to cover the height difference between AV-plate and hull bottom) and how long should it be (just to reach hull stern or ...)? Is the idea to "stream" the water above the AV-plate or up to the underside top of AV-plate? And how wide? Anyway, this one is doable.

So, like a central trim tab that I had, but to be angled upwards? Again, question about inclination, length and width of that tab? Also, doable, maybe even easier than the previous proposition (easier to attach). I presume that inclination, length and width are critical, since I had that central tab, fact with a bad angle, making ventilation the worst.

Good, let's get there, obviously I must fix hydrodynamics first, one by one. One question though, obviously it is bad at maximum loads/speeds, but according to loads/fuel consumption at displacement speed and semi-displacement speeds, engine seems quite unloaded according to engine power/prop curve, so I will not damage the engine at these speeds with this prop, correct (from idle to about 2100RPM, near hull speed being at 1500RPM, and driving mostly between 1500RPM and 1800RPM)?

Yesterday, one boat equipment shop seller and me came to almost the same conclusion, he proposed Bravo 2 props and hubs, I knew about Volvo number of splines and that 16 inches is the highest pitch, so we were getting there, too. Anyway I will stick to option of customizing my own for now, but after we solve hydrodynamics issue(s).

Well I would presume that would be the exact moment when I achieve speed rise while trimming down and should stop trimming down and stay at that trim. Subjective feeling from the helm is that angles are higher than they really are, I have seen that now on presented videos, so that is the reason I was over-trimming the flaps.

@PAR
Instead of going to recent history and politics, I will just say that commercial fishermen here mostly use outboards up to 20hp and small metal "boats". Commercial boats are mostly barges, and although there were many shipyards, only some have left, but I will ask in one that I know still is working (mostly metal shells today) where do they fix their props. We are small boat market having a tendency toward outboards and everything consumable, no matter what size and type of boat is in question. Mentioned shop seller told me that there was only one boat with Bravo 2 drives in our city that he was aware of in last couple of years and it was sold in foreign country.

@jorgepease
Shipping it somewhere is an option. Again in my country bringing it wherever would be much faster.

@everybody
Let's assume another approach, too. Suppose I do not plan to ever use the boat at planing speeds, just to sail between displacement and semi-displacement speeds, what would be ideal prop in that situation long term? Of course, assume solved ventilation/hydrodynamic problem. Would it be better to have higher pitch prop (and what other characteristics) to load engine a bit more closer to prop curve, although the engine would run at slower RPMs which is not that good for engine and also handling at marina would be worsened due to higher idle speeds? Or would it be better to stay with prop like the current one? Or else? Current situation is that boat is at displacement speed from idle up to 1500-1700RPMs (11-12km/h), approximately after that I do enter semi-displacement speeds and those seem reasonable up to 2400RPM (14-16km/h), after that the effort seems too big if not going to planing speed (inefficient). Of course, current situation is with involved hydrodynamic problem.

Well, I hope you will be patient enough to read this long post.