neverthought

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I have a 7.5 K&B on an old 35" Dumas Hot Shot (I think?) tunnel hull. Who is the prop expert that can tell me what I should use on this? If you can give me different styles and makes with part numbers and descriptions that would be great since my hobby shop knows nothing about boats and has a great big box with all sorts of mixed up parts in it and I have to guess what’s what. Thanks!

amax

Well I'm not an expert on tunnels but here is a good prop page with the best recomendations

[link=http://www.aircapitolhobbies.com/Prop_Charts.html]Prop Chart[/link]

Hydro Junkie

Your best bets would be Jerry Dunlap and OB Nut as they have both been designing and racing tunnels for decades.

jbel

i run a octura x447

Dan S

when I had my Hot Shot 45 with the K&B 7.5 on her, I ran a prather 230.

Dan.

neverthought

Thanks All, I think I will try the X447 (conservative) to start and try the P230 to see the difference.

Ron Olson

My Feedback: (1)

Octura X series 447, 448, 450 in either 2 or 3-blade and/or a 646/3. Prather 235.
Those are what I tried on my old 7.5 OB tunnels.

neverthought

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Ron, Thanks for the info. Which one did you prefer? Also; why 2 blade or 3 blade? in the old days on actual size boats we used 3 blade for more torque and 2 blade for more speed, does this apply for RC boats?

Ron Olson

My Feedback: (1)

I really didn't see a whole lot of difference between the two. The funny thing is that props like the Octura X-646/3 is that there is no 2-blade version as some companies will make a size in only 2 or 3-blades. A lot of guys will take the 3-blade over the 2-blade as far as the X-450 goes.

amax

Yes it does, 3 blade props are generally better for pulling the hole shot out of turns and launching and 2 blade props let the engine turn a little higer RPM's for Top Speed. 3 blade props generally give a little better lift also to lift the transom and drop the bow and get up on plane quicker.

neverthought

Great, so if my boat is heavier I'm most likley better to use a 3 blade prop and if lighter a 2 blade.

Great info, Thanks

amax

Not necessarily, a heavier boat will use a smaller Dia Prop and a lighter boat will run a bigger prop or more aggressive cut. Driving style and pond conditions determine 2 or 3 blade more. If your a buoy hugger and want to have the quicker recovery and acceleration a 3 blade would be better, If you run Fast on the outside and stay in the throttle a 2 blade is better.
I'm not a tunnel guy but on a Hydro On slick water I prefer a 2 blade to let the bow run a little higher and looser so there is less surface area contacting the water, slick water creates more drag, in Choppy or Windy conditions I prefer a 3 blade to keep the bow down so there is less chance of a blow over going into the wind and it lets the boat ride more on top of the chop and cut through it with the sponsons, where a 2 blade lets the transom ride lower and causes more Bounce in Choppy conditions. No 1 prop will give you the best performance in all conditions on ANY Boat.
Hot or Humid days your engine will produce Less Power so an Aggressive or Big prop that works well on a Cool\Dry day may not let the engine get On Pipe on Hot\Humid Days, and a 3 blade prop that Lets you Scream around the track in Chop may be to much when there is More Drag on Slick Water.

Nitro boater

Hey guys my son"s boat are running with a Prather 240 49 mph greate speed

neverthought

Wow, thats quite decent considering the weight of theses boats! I will try one.

JWMods

The piped K&B 7.5 o/b likes the X447 but the muffled version can tolerate the X450. You won't really need to go to a 3 blade unless the power pulses of the two blade start upsetting the boat. Usually this happens more with a piped/modified engine than stock.

brad47

first the boat has too be set -up right . then the props .