Prop selection question for 6m foil assisted power cat

[cmaas]

So those figures are RPM’s at the motor?

Yes, motor RPM.

I think you are describing the situation well.

If I'm looking at it right my 9 7/8 x 13 prop at 4500 motor RPM (2:1 gear reduction) should get me close to my 20 knot target max speed.

 

[cmaas]

Two things I found curious in the last photo above.
I’m assuming there’s another foil further forward?

Yes, there is a retractable main foil between the hulls just forward of the CG.
Several years ago I made a set of Beiker designed foils similar to what you suggest for a customer's foil assisted power cat. They bolted to the AV plate. "Mustache" foils they called them.

 

[cmaas]

The pod you show has a bad shape from two perspectives.

Can you suggest what the height of the AV plate should be in relation to the running waterline?

 

[SolGato]

How did those mustache foils work?

I’m taking notes because I’m thinking of adding foil assist to my 30’ Solar Electric Cat build, eventually.

I was thinking something along the lines of what Aquila is doing with their Power Cats.

I’d really like to be able to maintain shallow water navigation and beach ability, so those mustache foils are intriguing.

 

[cmaas]

I don't know how well that foil worked. There was an article about the boat in Professional Boatbuilder a while ago. Russell Brown put one on his single engine power cat. I'll ask him what he thinks about it.

 

[baeckmo]

Can you suggest what the height of the AV plate should be in relation to the running waterline?

For the test boat, the "still" waterline was at the border between the black line and red(ish) surface. The bottom of the "main" body acts as a "trim limiter", coming into action as the hull trim increases around hump speeds and getting out of the water at normal planing speeds.

And yes, the forward AV area sits 10 mm above the OB plate, effectively "sealing off" the chance for a vortex to sneek around the latter into open air.

 

[cmaas]

How did those mustache foils work?

Russell tells me those particular foils were for pitch control so more symmetrical section than might be ideal for lift only. My concern was that the mustache foils might tend to ventilate and pull air down to the main foil surface but that is not the case on Russell's cat.

 

[cmaas]

For the test boat, the "still" waterline was at the border between the black line and red(ish) surface. The bottom of the "main" body acts as a "trim limiter", coming into action as the hull trim increases around hump speeds and getting out of the water at normal planing speeds.

And yes, the forward AV area sits 10 mm above the OB plate, effectively "sealing off" the chance for a vortex to sneek around the latter into open air.

Thank you. Should I angle the new AV plate up at the front a bit? Or is it parallel to the water flow?

 

[baeckmo]

Thank you. Should I angle the new AV plate up at the front a bit? Or is it parallel to the water flow?

Parallell to the flow; no angle of attack!

And..... yes the main pod seen in my pictures is cut. What you see is the part that stuck out aft of the transom. From memory (ie not reliable...) the full length of the upper edge was about 1.2 m, but also note that the tunnel roof was quite high for good clearance in rough conditions. Your piece would be proportionally lower.

 

[SolGato]

Thanks for the feedback on the mustache foils.

Out of curiosity, before your boat was finished and was still light and performing well, at what speed was the hull lifted out of the water by the foils?

Sounds like you are on the right track to fixing the source of the problem -a disruption in flow -thanks to Baeckmo’s input.

Once you get that solved, I still vote for more prop pitch if you can handle the extra Amp draw.

Not only should you see higher top speed, but you will probably achieve faster cruising speeds with less amp draw when foiling which may be the more important benefit in regard to capacity and runtime/range.

 

[cmaas]

Thanks for the feedback on the mustache foils.
Out of curiosity, before your boat was finished and was still light and performing well, at what speed was the hull lifted out of the water by the foils?

The hulls have a step just aft of midships. At about 10 knots the hulls aft of the step lift clear of the water. The hulls forward of the step just gradually lifts as speed increases. There is no real "lift off "moment like a fully foiling boat. My goal is to "fly" with just enough of the hulls in the water to provide steady flight. And that is the main reason I did not go fully flying with this boat - flight control is a pain, requiring a fairly complicated system to control roll, pitch, and altitude.
The foils can be designed to lift the entire weight of the boat at a fairly slow speed but, unless you only want to fly at low speed, that lift comes at the cost of drag at higher speeds since you are pushing bigger and/or fatter foils than you need for your design speed.

Yeah, it will be fun to mess around with props once I get the ventilation issue sorted.

 

[Russell Brown]

This is cmass's "other" electric motorboat.

 

[cmaas]

I re-installed the fairing/ bracket that held a foil on the outboard leg. I removed the foil and vertical strut that held it so it is acting as a fairing with a large AV plate. There was a marked performance improvement - less ventilation and max speed of 18 knots.

Ventilation is still happening, just at higher speeds.

Next project is an AV plate mounted forward of the outboard leg. Similar in concept to baeckmo's.

This is the fairing before I removed the strut and foil.

 

[baeckmo]

I notice that your propeller hub is not fitting well. There seems to be a rather big gap between hub and gear housing, and the hub diameter is less than the "torpedo". If you have not blocked the exhaust channel, there is a risk that air is drawn down and emerging in the hub gap. That might explain the ventilation. Try to arrange a smooth transition.

The aft "lip" on the hub should be removed, and a short nose cone installed. Copy the proportions of the truncated version found on Volvo DP drives. The present arrangement would certainly benefit from adding a slight cup to the trailing edges of the blades. And unless you use water cooling for parts of your cirquitery, it would be wise the seal off the water inlets as well.

Btw, I think your leg-mounted fairing seems quite ok; with the above changes, it might be good enough if stretched slightly forward. But the upper, rounded pod is still bad, though....

 

[cmaas]

The current prop fits the hub better but there is room for improvement.

I had not thought of the exhaust passage. I'll plug that.

I had fit a prop cone to my previous ICE/electric conversion and could not detect a performance difference. I may give it another try though.

the lower unit impeller is used to pump water through a heat exchanger for the water cooled motor/controller.