Biggest prop for a boat of specific size

[RCboatfan2010]

Lets say the motor,esc and batteries can take it. What is the biggest prop you can use for any particular hull before the prop becomes too big and starts to effect handling negatively? lets say the rpm range is 25000-30000 for arguments sake.

If I can take my BJ26 as an example for a 26inch cat. What is the biggest prop that can be used on it using the mentioned RPM level?

What effect will using a too big prop for the hull have on the boat?

[siberianhusky]

Depends on far more than the hull.

[RCboatfan2010]

if you have the time, please explain:smile: I just want to know the reasons for a long time now. You always hear you can't do it, but why?

[siberianhusky]

Motor size, kv, efficiency, battery capacity, voltage, quality etc, esc, how well the boat is tuned
If your talking a box stock BJ you'll burn up the electronics long before prop size really effects the boat handing.
Basic electrical theory is a big help in understanding what is going on in terms of power calculations and such.

[RCboatfan2010]

Just saying. Please read my first post again. I said the ESC, batteries and motor can take the higher amps you throw at it. I never talked about stock parts, but replacement parts that can take the heat:smile:. I know at some point the hull won't be able to take the speed, but what propsize is the max size for a particular hull?

Younger guys like to beat V6 cars with their 1300's. Older guys go for bigger cars that does the same, just with less revs and fuss. So that is basically why I asked my question. I want the same speed as a high revving motor with a small prop gives, but with a bigger slower motor turning a bigger prop.

So lets make it realistic, not saying this is what I want to do, but something I just want to think about. If I install a 15 000 rpm motor but up the propsize to 80mm will I get aproxomately the same speed as a 40mm prop spinning at 30 000 rpm? what will happen to the boats handing if you do put on a 80mm prop.

Sorry if this is a silly question, but I would just like to know this.

[Banzee]

I think you should consider a prop not larger than 3 mm or you will have too much torque roll

[bob_t]

rudder clearance or other mounting hardware would probably be the limiting factor, other than bad handling characteristics. Don't think there is any other general rule for prop size. On real full size performance boats, you get into regions of inefficient or non-obtainable pitch sizes when adding power in which case, you change gear ratios. Since these boats don't have gear reductions, you can't change gears changes to put you back in an optimum pitch range. Pitch and prop diameter are inversely related, in general, so if you go bigger diameter, you go less pitch and vise versa, and even then you can detongue the blades to get back into the recommended rpm range based on your motor kv rating and battery voltage.
As far as effect on the boat/performance with all other factors like esc, temps, etc being ok, - I think that has been covered ... porpoising, torquing, or no hull left in the water and just running on the prop (unstable handling).
Hope that helps.

[mtbenjamin77]

Unless you have counter rotating props the negative effect of prop size begins immediately. Transom walk you have to steer left to go straight then torque roll. Eveytime you hit some rough water and the hull leaves the water but the prop is still in the water the boat would get really squirely or possibly roll over. I like to have the smallest possible prop to get the speed im looking for based on required transom lift, oh and that's another reason a large prop would not work. Props create lift at the transom and an abnormally large prop would create so much lift that at high speeds the boat would nose dive unless you made other radically drastic modifications to overcome the lift. Instead reinventing the wheel, just follow seasoned racers then learn about mods to get better results.

[RCboatfan2010]

Unless you have counter rotating props the negative effect of prop size begins immediately. Transom walk you have to steer left to go straight then torque roll. Eveytime you hit some rough water and the hull leaves the water but the prop is still in the water the boat would get really squirely or possibly roll over. I like to have the smallest possible prop to get the speed im looking for based on required transom lift, oh and that's another reason a large prop would not work. Props create lift at the transom and an abnormally large prop would create so much lift that at high speeds the boat would nose dive unless you made other radically drastic modifications to overcome the lift. Instead reinventing the wheel, just follow seasoned racers then learn about mods to get better results.

Thanks. Your post made a lot of sense. The only reason I want to go the other route of slower/bigger, but just as fast was for more reliability and less heat.

@Banzee
It's a pity I was brought up so good and that one is not allowed to swear on a public forum.... Go and be sarcastic with someone else. I don't think my question was SOOO stupid as others have answered me satisfoctory.

[mtbenjamin77]

As far as heat is concerned, my theory is that it takes the same amount if energy to move the same hull at the same speed. The question is can your drive train handle it. If your drive train is too big then you add weight and instability. For electronic boats you want to look at your wattage rating as horsepower. In fact 745.70 watts = 1 horsepower and......... my phone is dying will pick this up later...okay back,
then you can calculate your amperage rating to determine your required ESC using the following formula

I(a) = P(w)/V(v)
I = current in amps
P = power in watts
V = voltage in volts

So lets say you have a 2000 watt motor and you plan to use 4S 16.8V (@ 4.2 per cell)
I = 2000/16.8
I = 119.05 amps
I always add a safety factor of 1.25-1.5
119.05*1.25=148.81
119.05*1.50=178.58

So you would need a 150 to 180 amp ESC

[Banzee]

Sorry but i never tried to be sarcastics like you , just try to help ! I think you got the answer you looking for now ! In my opinion , no question is stupid !

[RCboatfan2010]

Sorry but i never tried to be sarcastics like you , just try to help ! I think you got the answer you looking for now ! In my opinion , no question is stupid !

You suggested a prop no larger than 3mm. The shafts we run are bigger in diametre than that.... If you made a typing error and wanted to say 30mm then I would understand and only then I could agree you were not being sarcastic. But as it stands now, I stand by my oppinion.

You are right with one thing though. The other members surely helped me in understanding better how this works. I'm answered. Low speed motors are for tugboats.

[Banzee]

Ok now i found my mistake , i forgot to write " 3 mm larger than stock prop diameter " , sound better , sorry for the confusion !