Oversized prop compromise

I am overjoyed to see your learned contribution Jehardiman, and thank you for dumbing it down for me.

That was exactly the scenario I had in mind. Mia's assertion that prop and motor are matched is true only for 1 speed and 1 load state. In Europe I am told controllable pitch props are standard. Will perform a function much like a multispeed transmission.
I refer again to the class where nothing matters but the reality of performance, not opinion of others, not vested interests, nothing but reality. Top performance drag boats use multispeed transmissions.

 

I hope you guys keep going with this. It's an interesting concept. I still wonder how practical it is for the typical boat. I'm not trying to rain on your parade. Whenever I'm looking under my boat at the running gear the thought occurs "What happens when I hit something". I guess my big issue is durability. Getting back in.

Maybe we're talking about two different applications. I've always had an interest in jet powered boats (turbines). Those go fast boats have no prop at all. When time allows I like to follow the guys up in BC at S&S Turbines in Fort St. John.

Good luck with your projects and ideas. Maybe you'll discover the next leap forward in marine propulsion.

MIA

 

Obviously for most boats, it isn't required. You might find some niche applications where it would be advantageous, but they would be a small segment of the market. It is the water itself, that allows the necessary slippage.

 

Actually, the niche market you're thinking of is basically all of Europe. Or any boat that runs at more than 1 speed, or more than 1 load state.

 

No, controllable pitch propellers are just like fixed pitch propellers in that they achieve a maximum efficiency at 1 speed and 1 load state. In fact they are worse than that, because they must cover multiple speeds and pitches they have less efficiency than FPP, and the pitch distribution against diameter is not optimum. See my discussions about CPP vs FPP, and why if you don't have CODAG systems, you should not select a CPP.

See: Controllable pitch prop, effective range

 

I will read your article. I take your point about pitch distribution, but I'd have to see (and maybe you've thoughtfully included them) curves from physical testing showing the loss. Based on the corresponding research in propellors for aircraft, I would expect to see performance inferior to the FPP at the FPPs sweet spot, and superior performance the further we got from that point.

 

I read the post, as well as the accompanying counter arguments. I think the pitch distribution issue will be relatively minor in a displacement vessel, but in a high speed vessel it will become more and more significant. Which returns us to Bluebells eloquent description of a FPP on a 2 speed transmission.

 

The reason this is on my mind is that I'm building a fast planing boat that will often make round trips, very heavily loaded on the way out, light on the way back. The prop for the return trip will make getting on the plane impossible on the way out. The prop for the trip out will severely limit speed on the way home. 5 days a week.

 

Well hell. The thread Jehardiman refers to literally about my exact issue. I'm going to have to call bulldoodooo on the next guy that tries to claim this is a rare, niche situation. It's hardly rare when the question keeps coming up. The failure is in the knowledge base, not the question.

 

It seems engine makers are not responsive to whatever demand there is for two-speed gearboxes, which does support the idea it is a limited market, at best. The obvious option is to change props after you unload, doesn't take long.

 

When I have run twin outboard boats on long hops, and there was as much as a sandbank along the way, I would have a spare prop, of lower pitch, so that if for some reason I lost an engine, I could swap props on the remaining engine, and plane the boat easily enough, without flogging the engine, albeit at a reduced speed, but much preferable to crawling along off-plane, and getting back after dark.

 

Welp, can't argue with that.

 

On slower displacement boats, almost all existing boats have undersized props because they are cheaper and easier to work with. To reach 90% efficiency at low speeds you need a huge diameter.

I have a 4ft beam and use a 32 inch propeller. I am able to go 2 knots using 9.5 amps on a 12 volt system which is twice the efficiency of a trolling motor, but a larger propeller would have even higher efficiency.

 

Are you saying that your propeller is almost the same diameter as the beam? A minimum of 15% hull clearance puts the bottom of the propeller blade at least 4.5 feet below the surface. I'd like to see a photo.

 

it's all in the video where I linked.

If you want to achieve 90% efficiency you need the prop to be about the beam of the boat. There is a reason the pedal powered boats that can barely carry 1 person use 16 inch diameter propellers. Also a good reason my sculling oar blade is 6ft long, and is like a single bladed (fewer blades are more efficient) counter rotating variable pitch propeller that is 12 ft diameter. There are other losses but it comes in around 80-85% efficiency. Typical sailboat inboard just the propeller alone is maybe 25-30% if it's selected correctly and only at a specific speed.