Re: Beating out the cup
So deep in the water perhaps it is cavitation as I recall straight inboard boats do create a certain amount of a bubble stream.
That I believe is the low pressure side of the prop creating bubbles released from the water.In affect like boiling caused by the decrease in pressure.Are you saying the boat surges as the motor slows and boat slows as the motor revs.
Also is it possible the gouvener is getting involved.
Is the boat intended for displacement type speeds about 6-8 mph? Or is it intended to plane?
Anyplace in my posts where you see "caveating" please read "cavitating". Oh how I'm starting to hate spell check. I'm turning it off for good. But this message is already dictated and writen so there might be some "caveating" in there.
Yes I agree, I do believe I am cavitating and over propped. I should've gone with my own calculations when I bought the prop. Hopkins propeller is saying that I really should've bought a different one now. But of course that's fine site for both of us. I still think perhaps it was right to have gone with the prop experts suggestion as I did. It's always wise to listen to wisdom and experience.
With regards to what the boat is doing when it surges, the motor speeds up at the same time that you can feel that the boat has lost traction. The feeling is the same as if you were driving a car at a steady speed and then the driver suddenly released the accelerator. Except the difference is that with the case mentioned above with the car the motor would decrease in speed when the accelerator is released. In this case the motors RPM is increasing, which clearly shows either cavitating or ventilating. Okay, so back to the surging. So I feel the boat take a noticeable decrease in speed from an obvious reduction of thrust (I feel myself fall forward a little bit, I run the motor standing up) at the same time that the motors RPM speeds up, then the search goes the other way and I feel the boat surge forward (I feel myself falling back a bit) at the same time that the motor's RPM goes down. I would say it surges about once every 2 to 5 seconds, or said another way, it's about 2 to 5 seconds between forward surges.
Now during the surging the governor is definitely getting worked, but it's clearly not that the governor is not working right and is hunting, the term for when a governor keeps overshooting the desired speed and never settles down. When the prop slip increases for whatever reason, the governor definitely lets off, and then when the prop slip decreases and the motor lugs down I can hear the governor dumping in more fuel.
In fact when I first tried my motor I was running light, but then during the ride it rained really hard. Don't forget I'm in the Amazon. The boat filled up with a lot of water and it really changed the way the boat went through the water. It also changed the way the water was coming off the stern and running up to the prop. The cavitation stopped, but my motor loved way down and I lost a few miles per hour in speed. I also used way more fuel during the part of the run where the boat was loaded heavy. I really think with the right prop and all the dynamics right, if I can run at close to 3600 RPM without the governor being pegged am going to get good speed and efficiency.
What I'm running is a metal canoe. They fabricate them here in backyard shade tree operations. No two boats of the same, and even if you eyeball down one side of the hull and then the other there's even differences between them. I don't think they really have a form that they make things on. I believe they fabricate the curved ribs, and then somehow attach the steel plates to them and then just keep adding them together. There is zero engineering that goes into these things. They just build them. But what's interesting about these boats, as well as the wooden dugouts that they're modeled after, is that they go really nice with a small motor, and if you keep increasing horsepower they almost become a planing hull in operation. I've seen a large wooden dugout with the big motor running alongside my friends little Boston whaler with a 100 hp four stroke on it, and the Boston whaler only ever so slowly overtook the dugout. Some of the folks I live with here many many years ago had a big wooden dugout with an inboard outboard diesel that they dropped in there on a lark. They said it's the fastest boat they ever owned. They also said that you could see it a mile away on the river because it threw up a huge spray as it screamed along. That's one thing these dugouts and metal boats do, they throw up a pretty strong bow spray, not the bow wake, but just jets of water coming up each side of the hull. For that reason I put the factors on my bow, as many speedboats have, to direct the spray down otherwise when you run in a good crosswind the upwind spray gets blown back into the boat and you get soaked.
Sorry to go on and on, but this boat is clearly a displacement hull at these speeds, but if I put a 40 hp on it it would behave more like a planing hull boat though there be a lot of wetted surface compared to a speed boat. But the transition is very smooth going from displacement to planning because it doesn't really happen with a sharp transition as a planing hull does. That's a very nice feature when you're running through water with a lot of submerged rocks. When you get to that spot you can just slow down and go as fast as you like. With the planing hull there's always the temptation to maintain enough speed to plane, so the boat doesn't settle down deep in the water with the motors foot down even deeper.
I guess to really put a number to your question, I would say that this boat is intended to use somewhere between a 15 hp and 40 hp motor, and for example with a 15 hp on it I can do about 20 miles an hour. So I would say with my little diesel I'm probably well below what you might consider the hull is "designed" for, or wants to do, which is probably a better way to put it.
Thanks for the replies everyone!
Greg
