Sir, it is interesting how you look at the pitch of your propellers. Many people define pitch as the distance moved forward by the propeller as it covers one revolution. This may apply to the nut moving along a threaded rod, but a propeller in water is not so simple. You mentioned that the pitch needs to change as one moves across the blade and also as one moves out along the radius of the blade. I would agree with you about the change of pitch as water moves across the blade, but ideally, there is no change in pitch as one moves out along the radius of the blade unless, as you mentioned, the entry speed into the propeller front face varies as you mentioned when the water goes around the outboard leg, it speeds up a little and so the pitch has to be accounted for. With some propellers, where the pitch across the blade is constant, then the velocity across the blade is constant and the low pressure and high pressure exist before and behind the propeller. With cambered blades or those where the pitch varies across the blade, then that type of blade will have a good entry of water at the leading edge and the water is accelerated while it travels across the blade. I presume the propellers you show are acting as surface propellers and so the curve or the change of pitch while the water moves across the blade are imperative to accelerate the water. That change of pitch across the blade would decide the slip of the propeller. Congratulations on your great experience in making propellers, I designed a few, but I have not the facilities to cast them or make them in a professional manner, I made the model to be cast at the foundry, and on some propellers, I decreased the pitch at the hub as I felt that that zone acted as a paddlewheel during standing starts and the hub zone would cavitate due to thigh angle of attack. All propellers have wonders and secrets of their own. Well done on such a presentation based on practical values. Note that At 4:34 With a gear ratio of 16:21, the shaft from the engine has 16 teeth while the propeller shaft has 22 for a reduction gearbox. Your video took me back more than 65 years when I was 20 years old and did some racing but not on very fast hydroplanes, only just small offshore boats in the Mediterranean off Malta which were not as fast as you went. Thank you for presenting this video.
Thank you for the detailed and precise comments! I love discussing and learning about props. Yes, all mine are surface riding wheels. The center of the prop shaft is normally less than 1/2" below the bottom. The black art of prop design is fun to explore. As you know the surface riding variant is much different than a ship or pleasure boat in prop design. Early this year I discovered something that added probably 20% to the accelleration of my stuff :)
Very interesting content. Thank you for sharing. I'm having trouble gathering good Intel on small speed boats. I am currently working on building a small 7' boat for my children to play with at the cabin and I'm also gathering ideas for a small speed boat for my self. Thank you very much.
@@rustysitton9574 I spoke with Darrell twice this evening and he was recommending I build his long haul model. By chance did the Texas Toothpick start life as one of Darrell's designs?
@@James.Richter no its all mine. Here's what I like: my bottom is 35" wide, one of widest runabouts bottoms around. It leaves the corners like a rocket. Historical wisdom is wide is quick, narrow is fast but I proved that wrong. You can have both. My pad is 24" long (transom --> forward) from there it breaks up VERY slowly. I can give you the rise dimensions at every 2 ft when you get into it. My boat sits 2 degrees nose high on the water. That means I want the pad flat to the water and the boat 2 degrees nose high in competition. You said you want a 7 ft boat. Apparently just to play with right? Mine is 11 ft 7 and rides well, capable of 80 mph. When you build, you definitely need to know what you are going for
@@rustysitton9574 correct. I am modifying a 7ft boat for my children to play with. I want to build a small boat to go fast on our 400 acre lake. 60+ mph is the goal .
@@rustysitton9574 looking forward to more insight always wondered the details of how race props can work better. I also have a favorite prop that lost an ear
If you use the stock lower unit speed will be very limited (if speed is the goal) the prop that comes on the engine will work if you keep it at a stock depth in the water. The higher you get off the transom the more different the prop needs to be. The motor will be heavy on a 10 ft boat. Our engines weigh 70 pounds. The tohatsu 20 is 25 pounds heavier. It may run 40 mph with a 13 or 14 pitch prop, but I may need some cup to raise it up on the transom. Solas makes some descent replacements.
The best source for racing props is dewald propellers. The one in my hand was developed by me for our use. I dont sell props. My hope is that in sharing what I am doing, others will be able to do it as well. Get a grinder and a hammer, watch my build videos and try it!
Richard there are a lot of answers to that question. The easiest reply is - try higher pitches like a 21 or 23. Your boats design and weight limit top end. If you're not interested in skiing / lifting a load and are willing to reposition the engine on the transom (higher) you can go to 25 pitch or more. My hydrostream 16' performance hull ran close to 80 mph with a 30 pitch on a 200hp merc but it was a performance hull. I hope that helps. If you want more guidance tell me the length and make of you're boat.
@@rustysitton9574 we did a '16 hydrostream verado with a "15 260 and with a yamaha drag 26" we ran 100 + 8500 -9000 rpm. with a drag 21" we ran 90 mph in 800'....10,000 + rpm.
What you said about tip speed at the beginning of this video is very wrong. Airplane propellers are designed *never* to exceed super sonic speeds. They lose efficiency very fast at and above, not to mention, it very dangerous. What your hearing is the air passing thru the blades as the pilots checks the motion of his constant speed prop. Meaning the blades can change pitch while in flight. Has for gears, "gear teeth" have nothing to do with any ratio. It the diameter of the gear is what stipulates it ratio. The actual "gears teeth" are only there to create movement.
number of teeth on a gear is how you determine its size .....20 teeth vs 40 teeth std unit of size in engineering world when you go to buy them you ask for a 20 or 30 or what ever size you need in tooth count not diameter!! .... common sense tells us if all else is equal more teeth equals bigger gear ... how do you figure ratio in your world please show your math!! you should go back to school and get some education lol internet genius ...lol
@@crashalexander7232 Well then. If you think you correct.... Please explain to me..how to get a given RPM that is needed, WITHOUT using any Diameters in YOUR calculation? Gears ARE a Diameter I'll wait right here while you eat your shoe!
