Hi guys,

I am still in the obsessive stage of compiling knowledge.

You buy a few boats and after a couple years, you realize your fast is someone elses "Something must be wrong?" I have a couple hydros...AM Thunderbolt and an Insane...currently building an Avenger Cat to race.

I am starting to understand the balance involved in torque vs RPM when it comes to applying the right sized prop to your hyrdo/cat and getting the speed to win. I was looking at the full sized cats and hydros. Those props are small when it comes to comparing the size of the boat to our RC's. I guess my question is in sizing of the prop. I am assuming with turbine engines, they generate such high RPM, that smaller props are in order to get that baby moving 180 mph. For our little guys, it seems the standard 6717 (3 blade) is pretty big. But I guess the size is needed because our gas engines are limited in their abilities to generate anything over 17k rpms but maintain the torque necessary to turn a larger wheel for its size and weight. What are the tips to fast boats? You take a wood Whiplash and match it to a Thunderbolt with the same engine (Stock Zen 260) and you get at least 10 mph more out of it. Is it the shear weight difference that makes the difference in this case?

I see turbine RC boats out there and I have to be honest with you; it seems the Europeans are light years ahead of us with the technology but with all that money, I don't see a marked increase in performance. My point: a Zen 260 worked for $600 in a Whip and a Turbine in a similar hydro costing 10 grand are both edging in the mid 80's (mph). WHY would you bother?

With that said, I realize that a smaller prop turning higher rpm gives us closer to 1:1 scale, hence acheiving more realistic results right [massive rooster tail boiling water and a lot of GET]? Then why are people so infatuated with horse power in modded motors? Wouldn't you be more concerned with RPM and messing with timing and pipe length? I know I am rambling and typing my thoughts, but I am a bit convoluted in my thinking and have a hard time establishing a goal for myself in racing next year with my currenet Cat Avenger build. I want to make sure I have my directive down pat in matching motor with prop to win races and just thinking that it "Sure is fast" is obviously not going to bring home the first place finishes that I want. I love the technical science here but the dynamic does not line up with conventional thinking on how to go fast.:dash:

Just my opinion, but the two largest issues effecting getting a boat to go faster than the next guys start with:

Lowering the "Hydrodynamic" drag .. Ride surfaces being flat and sharp cornered, Rudder and turn fins short/small as possible and sharp to the point of knife like tho edge broken with @ 400 grit, Stuffing tube into strut blade, strut it's self all streamlined and free of lumps bumps & gaps, prop thin and equally knife sharp.
Any place where hull or hardware is impacted by the flow of water past it creates drag !!

* Noted above .. Prop and the correct prop for the weight, looseness of the hull on the water dictate to a point what degree of efficiency the prop will have. A hull that runs loose and light on the water will have less prop slippage than a hull that runs wetter and tighter on the water. So that said, a light and loose hydro can get away with smaller diameter higher pitch props when a heavier and wetter hull would have the same prop slipping like crazy and should be using a bigger diameter prop of less pitch ratio so prop holds onto the water better.

Next deals with as you address for instance why a Thunderbolt and a Whiplash are 10 mph apart with all else being equal ?
"Aerodynamic" drag is a large part of this with the frontal area and total air displacement hull has to make punching it's self threw the air in front of it.
A Whiplash is a LOT smaller hull in terms of frontal area and total hull bulk created by it's shape verses a lot of other hydros.
Think of it why aircraft that have low and long lines are faster than fat & wide bodied ones

There is another whole issue of engine power, engine over rev allowing greater rpm past power peak that can generally ONLY HAPPEN if hull is very low in hydrodynamic drag at speed and dang near flying.

So many small things once addressed one at a time add up to faster boats ... each hulls requirement test the modeler ability to read the fine details of needing refinement of specific areas.

Hopefully this has given you some food for thought on your seeking of MORE of everything :drinks:

Hi,
There is sure a lot going on at the one time. Hence the expression in experimenting, "Change only one thing at a time" to look for improvements.
Cya Graeme:drinks:

Electric models run more scale like props and rpm because they can. Nitro and gas engines are rpm limited and they require larger and higher pitched props for the same speeds. This is really a problem with gas boats as there are no stock props that will run much over 100 mph. Propeller efficiency and the power developed at the rpm you're really turning that prop are key factors in going fast. I promise that our 110 mph Quickdraw was developing lots less than it's peak 8 hp at the rpm we ran for the record. We now look at the power developed beyond 20,000 rpm for SAW engines, well past the power peak.

Scott is really right about drag. Air is unimportant compared to water since water is 800 times denser. Boat speeds have increased by using air lift to reduce water drag. However, models have lots more power for their weight than full sized boats. That's why riggers work with models but only with full sized boats like drag boats fhat have serious power. It's hard to think of your little boat comparing to a 5000 hp drag boat, but it's true.

By the way, people don't understand the relationship between torque and power. Torque is the total amount of work being done and power is how fast you are doing it. I always think of power as the key number for speed, but if you like torque and rpm, you are looking at a different pair of numbers showing the same thing.

Lohring Miller

Hmmm so drag coefficient is one of the big factors here that I left out. Hydro riding on 3 points...Rear Sponson (left right) and prop lifting the rear of the hull in a perfect world. Then you throw in some aditional planes like the bottom of your rudder and the often forgot Stuffing Tube as well as strut bottom. I see where you are going with that.

I guess, I am just a little confused. When everyone throws around prop recommendations for this or that boat, I get this image in my head of these massive spinners on the back of a Whip or a Seaducer and go "Huh?" It just seems to me that everything else models that of the Real Thing but when you compare the scaled down hull to the prop size, it is just way off. I get the fact that the trubines are turning so much faster than an Unleaded pop ever will, but it seems to me that I would always want a cut down prop turning faster without a total power loss and unable to get on plane.

My next investment is going to be a tach and I am going to have to mess around using a GPS at the end of winter. I guess my theory is that a smaller prop turning at higher pitch with RPM over 19k is going to give me better results than swinging bigger cleavers with higher horses.

Actually for heat racing with modified Zenoahs you want to pick a prop that the engine will turn at its power peak, around 16,000 rpm, at the exit from the turn. This will give the best acceleration down the straight. This usually will be around a 65 to 67 mm diameter three blade prop. Some boats (my Lazer 45 for example) will run with a 70 mm three blade.

I was using examples from SAW propellers where there are no suitable props for internal combustion engined boats. Electric motors can turn up to 60,000 rpm so a wide variety of prop diameter/motor KV combinations can be run. The jury is out on what is best, but most electrics run smaller props in the 25,000 to 40,000 rpm area.

Boat handling is affected by prop diameter, but the usual size props can all be trimmed for.

Lohring Miller