Could you give me more info about my boat

Hold the phone here ! Just looking at that drive, it appears to be a thru-hub exhaust type, but the prop is not a type that suits that drive. Am I right ? Certainly has that appearance.

 

@Mr Efficiency, thank you for the info about material types of prop. I really did not know about that difference between SS and alloy prop. It is hard to see the tab, because light is bad, and the angle of the photo is bad, try zooming the photo.

@Joakim, that tab is just a bit above cavitation plate, and edge of that tab had to be cut in small moon shape in the middle to not prevent free rotating of outdrive (steering). Tabs were added all together (all three), they were not originally on the boat. I was concerned about heavier stern so I have added them. That middle tab was something that my father added with them at some point, hard to tell the reasons now, since he is not with us anymore. I believe it was the idea simulating longer LWL, but cannot be sure.

 

@Joakim, about fuel consumption, those are probably engines with electronic control, haven't seen specs, but presume lower displacement too, I know that it is not great, but this is an old type mechanical engine, so I can't expect the values you have showed, but I can live with it, and it is reliable and I can make most servicing myself, so that is why I can live with it.

@Mr Efficiency, it is not through hub, it is simple prop and simple drive, exhausts can be seen on other pictures (on both sides of hull (V engine) at the rear, above the water). What you see is probably the big self-locking nut which holds the prop to the shaft.

 

@Joakim, if I am wrong about the height of tab vs the cavitation plate (I will check that this afternoon once more), could that be one of the culprits of the problem?

 

You have got the wrong prop for that drive, I'd say. Look at this:
https://www.lancingmarine.com/databook5pages/sternpowr.pdf
I don't know if that is the same model drive as yours, but take a look at the propellor, the hub has the same diameter as the drive hub, for smooth flow into the blades, and aft you have the tapered cone to avoid drag from a vacuum effect. I'd certainly toss that prop you have, it is not a match for that drive.

 

For many years I've owned a Draco Twincab. Designed for planing (no keel) but far too heavy because liquid resin follows the law of gravity, so with the famous hand layup technology used, there was a wall thickness of nearly 10cm in the lower parts of the hull.
Like yours she did 27 knots with twin 140HP Mercruiser engines, but it took a while to get there and the fuel consumption was a staggering 80 liters of gasoline.

When fuel became an expensive commodity I drastically modified the propulsion using tunnels, large props and VW turbodiesels. That lowered running costs to just a fraction of what it was, but efforts to reach planing speed turned out to be ridiculous.

 

If the tab pushes water surface below the "cavitation" plate, the propeller may easily ventilate and loose its grip.

The material of the propeller is not that important, but the shape looks not at all optimal for cavitating region. SS propellers are often shaped differently, but you can find bronze and even aluminium propellers with similar shape.

I don't think the size of the hub is that important in this case, where the exhaust are not fed to the propeller.

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.

 

I would not persist with the type of prop he has, it is very likely to cause cavitation starting at the blade roots. Someone has substituted a non-standard propeller, I am curious about whether the correct prop has a rubber hub, there is seemingly room enough, and good insurance against destroying the gearcase innards in a strike.

Edit: Nope, the specs say solid hub, in which case alloy is better to avoid strike damage.

 

Thanks everyone for very useful brainstorming.

I will check that tab height as priority.

I hear you about the prop and I shall see what I can get for test. About prop not being for that drive, that prop was recommended from the drive manufacturer and delivered by them with the drive and it is in their options. You can see that if you open the manual from the manufacturer on their site (SternPowr), drive is model 103 in case someone is interested for further details (they call that type Equipoise). Of course, I believe that we can all agree that calculating best prop is not easy task, and that is what they did noting that it is not standard hull type and is heavy. I would say, as engineer, that calculation for something like that is at least +-30% from expected best result in reality, so just to be used as a reference, not the precise value. Anyway, I had to start with something, and I followed the calculated recommendation.

The cheapest test for start is to see the tab height and if it is wrong to dismount it and do another test. After that prop test would be next.

@CDK, I hear you and understand what you are talking about. While making some new openings in the hull (bottom, sidewalls, bulkheads) I had an opportunity to see those thick walls and how it was laminated, how much difference there is, how the core was cut, etc. In the end, we can only admire people in that era gaining great results with limited technology compared to today and what is available to us. At a glance, out of this thread and theme, I can recommend you to visit following thread of one forum in my country where I have put some time ago the photos that I have found of the factory that manufactured this boat and their production line. It is fairly interesting and it will probably never be done that way again. Here is the link:

http://www.plovidba.info/forum/galerija/istorija-jednog-brodogradilista/

 

The water is cold, but I was impatient to check that tab height. It is not good, I did not have method of measuring at that moment and it was already dark and it is half meter under water, but on "touch feeling" I know now that tab is lower than cavitation plate and that difference is about 2cm (be very reserved about this value since measuring method was "touch feeling", but certainly lower than plate). Gap between tab and cavitation plate (on longitudinal axes of boat) is also very small at the center of axes (since that "moon" cut for the drive), and on sides of drive, tab is even cross-sectioned with cavitation plate (very little, but still it is).

Would you expect in this case that this tab can be dominant problem, or you still expect prop problems after removing it? Although I know that test is only one that will prove what next, I am a bit impatient at least to hear opinions. Ahh well, you know, doing with boats is like kids playing with favorite toy .

You have put quotation marks in earlier post on word "cavitation", please tell me correct name for that plate, since I would like to know correct terminology, this is what I have read on many user manuals and that is why I use it.

Old props were D x P=14'' x 17'' on VP Aquamatic drives, I have uploaded the picture. The gasoline engines rotated faster as can be seen in specs on the photo from the beginning.

About the trim angle diagram, it is A3 format, I can't scan it now and it is on mil-meter paper, so very hard to take photo, but I will try to describe it (with original setup from manufacturer). Angle starts from 3 degrees from 10km/h (diagram starts from 10km/h), and rises up to 5 degrees almost linearly with speed until 20km/h, from that point "starts" the hump peak which lasts from 20km/h to 30km/h, with peak at 5.2 degrees at 25km/h. After that it has Gaussian decreasing form until 3 degrees at 50km/h. The point of curve where starts slower decrease is at exactly 4 degrees and speed of 38km/h. I hope you can reconstruct it from this explanation. It is very gradual curve, as expected according to earlier video.

@CDK, the consumption of two gasoline engines here was also 80 liters/hour.

About changing the props, since it is not easy to find matching props to that shaft (older type, solid hub), one salesman of nautical shop has recommended that my best bet for the future would be to make an SS adapter which will enable the use of standard props with rubber hub from e.g. Solas or whichever brand so that I can have greater choice. I like that idea because of broader range and because of that rubber protection, but than again salesmen tend to say many things, am I missing something that could be wrong in that compromise solution?

 

I will just add the good photos of current prop and the picture on which can be a bit better seen that central tab. On that drive are dismantled 2 inches extender and then under the extension the lower gear starting with cavitation plate immediately under the extension.

 

Nidza, your prop appears to be what they call the "small solid hub" type, and that is listed in the manual as an option for the 103 drive. if you look at page 23 of the manual you see in the illustration where a prop spacer is required when using that prop on your drive, which is a conical piece that is fitted between the gearcase hub, and the propellor. Do you have that installed ? I can't see it in your photos.

http://www.sternpowr.com/pdf/100serieshires.pdf

 

That is only for MS type prop (on that page), not needed for ME type which I have (on same page), but it has a SS spacer preventing the prop to swing forward/aft, it is just not conical, more like simple washer. If it would have a cone, that would not be easily seen, since it is not that thick, that MS prop is probably much "thinner" having that thicker spacer.

And here is the approximate angle vs speed diagram in the attachment (diagram from Matlab), it is easier with picture. Of course, it will be a bit different in my case since the total weight and weight distribution is not exactly the same, but it can be taken as a reference.

 

If the tab is lower than the "cavitation" plate and also very close or even overlapped, that will allow air under the plate and make the propeller ventilate. The propeller in your picture doesn't work well at all with some air and it will loose its grip.

How is the tab angled relative to the bottom of the hull near the stern? I hope the problem is only with the tab, not that the drive is installed too high.

Usually it is OK to install the "cavitation" plate 1-3 cm above the bottom of the hull, since there is more than 10 cm room in between and the water surface can rise to the plate.

The name "cavitation plate" is often used, but is missleading, since the plate has nothing to do with cavitation. It is there to prevent ventilation (to stop propeller sucking air). Many talk about cavitation when actually it is ventilation. So proper name would be "anti-ventilation plate", but that is seldom used.

 

Good luck with it, it wouldn't occur to me to have a set-up like that, with a slim prop hub behind a wider gearcase hub, but if Mr Sternpowr is happy to have it that way, what can be said.