Newbee needs a starter prop

[Mad Hatter]

I am building a 25.5 inch mono. I don't know where to start with rudder placement. I was thinking of a 1.7 -1.6 diameter prop with approximately 1.6 - 1.9 pitch. But the abc props aren't available at this time. I'm thinking of ordering an OSE 80708 42 mm just to try. I'm not sure how much rake it has. The boat has a solid straight shaft (4mm) mounted at a 4-5 degree angle to the keel. Is this a good starter prop for what I'm working with? Any ideas?

[NativePaul]

About 10% of hull length out the back next to the prop is a decent starting place for rudder placement. We would need to know your motor KV, motor size, ESC size and cell count to have any idea what prop would suit it, knowing what you want from the boat would also help a lot.

4 degrees is a lot of angle for a mono's drive angle, can you reduce that at all?

[Mad Hatter]

Thanks Paul, Motor KV=2290,size=3650, ESC 150 AMPS, power= (2) 2s 5000 ma packs in series for 14.8 volts. It should come out to about 33,892 rpm. And no I don't think I can get less angle on the shaft, I've cut the motor mount down pretty close to the hull and it will be as far forward as feasible for motor access. I think I've got it down to about 3 degrees now, but that's it. I suppose I'm going to need a 3 blade prop with a lot of rake? It's converted to surface drive and only 50% of the prop will be in the H20 at any given time. Any thoughts? -thanks Ken-

[Panchothedog]

Hey Ken a 42 mm prop might work but will probably be at the top end of what your motor can handle. If it was my boat I would start with a x437 or a x438 they both have plenty to push a hull the size that you have. ABC props are great but they can hard to find and cost than some other brands.You can't miss with Octura as they are usually plentiful. For starters try the Graupner carbon resin (plastic) props sold here at OSE. Dirt cheap and they work good enough for testing.

[bogieboy]

Food for thought, my Volantex Vector (24" hull) cam with a 1800kv motor (outrunner fwiw) and a 40mm 1.4p prop... i stuck a 40 1.9 on it, and it goes pretty good, still running wet though, probably due to the lift of the 1.9.... here it is on 3s with a flycolor 150a esc... https://youtu.be/50_0xeoCexA

[Mad Hatter]

Thanks to all. I am considering the x440 or x537. Which do you all prefer, more diameter or more pitch? The boat has 30x50mm trim tabs which should make it handle more like a 27 inch boat although they won't provide bow lift. Which prop do think will help? -Ken-

[Speed3]

A 3650 is a small motor, and 2290kv would be pulling a lot of amps on 4s so the smaller prop would be the better choice if you are running that motor.

I think though a 40 or 42mm would be for that size hull but those would likely over heat your motor on 4s.

Also a flex shalf with a strut would have definitely been a plus because your boat would benefit from having a neutral trim angle.

Negative trim causes the boat to run wet.

[Mad Hatter]

Thanks I understand trim from running full size boats. Trim positive to increase speed & rpm. Trim negative to make turns & in the rough (power trim). I have built a rather large cooling system into this boat, 3 pickups, 2 outlets and 9/32 hose, perhaps overkill. I plan on running it after Labor Day weekend till before Memorial Day when the water is cool and there isn't so much traffic on the Lake. It's my offseason hobby. I suppose too much prop will hurt my run time also. I guess 36-38 might be better. Will 3 blades have any effect?

[Speed3]

3 blade will draw motor amps than a 2 blade of equal diameter. I think a 36mm prop would be the better option for a 3650 size motor. But I am not sure if it would deliver enough thrust to plane efficiently for a 25.5 inches hull.

A 38mm prop would be pushing that motor to its max but you said you have good water cooling built in.

I guess at this point trial is the water to see what will be the out come.

[Mad Hatter]

Thanks, I guess I'll just have to try several Graupners to get a starting point. I hope they hold up. I read they aren't recommended for over 25,000 rpm. While on the subject does anyone have any experience with the CNC Magnalium props? They're made out of aluminum/magnesium alloy and don't require balancing. I was wondering if they're any good? -Ken-

[Speed3]

The smaller graupner props can handle more than 25000 rpm, I have done a datalogged at over 30000rpm with one of those carbon filled plastic props.

The CNC props are good props. I don't think a 3650 size motor have enough power to brake the graupner prop blades or the cnc blades. Unless maybe you hit the blade over something solid like a rock in the water.

[Panchothedog]

Ken, the CNC props work fine right out of the box. I have a pair of them counter roatating on a twin catamaran and am very happy with them,but they are pricey and you are still guessing on size. Speed3 is correct telling you that you are not going to overpower the glass props. Spend 15 bucks get 3 between 36 and 42 mms and get the boat on the water. Trial and error is the only way you are going to find out what works. Let me change topic for a second here and I don't want it to sound like l am talking down to you so please don't take this wrong. I read your post a while back and you are converting a old plastic brushed motor boat to brushless. I built and modified 16 of those things from 1987 till 2013,( and had a lot of fun with them) my oldest grandson bought me my 1st fiberglass brushless boat 5years ago and the hobby changed dramatically for me. I now own 8 modern boats But I also did just what you are doing. Some worked, most didn't. A couple of the best ones are sitting on the shelf with badly damaged hulls because when they flipped at 2 or 3 times the speed the plastic couldn't take it. Put LOTS of flotation in your boat and do whatever you can to make it water tight because if it does run good it is going to flip sooner or later and you don't want it sinking or the water running your equipment. Now get out there and run that thing.

[Mad Hatter]

Thanks very much speed3 & Dog. I've been thinking about this all day & all things lead to the point that I need a prop somewhere in area of the 2inch total pitch (diameter x Pitch) range. By the time I figure this out I will probably wreck & break it in 1/2 & sink the back 1/2 losing all the $ stuff. There's no point in putting any closed cell in the bow when all the $ is in the stern. I don't care if I destroy the hull as long as my $ stuff is salvageable. If that happens all I will have lost is time & effort. All I will have gained is knowledge. At least I'll have some parts to start over again. If at first you don't succeed try, try again! Isn't this a good attitude? I will try several Graupners and go from there. If it works out maybe I'll order one of those CNC#s, yes they are expensive, but It's cheaper than balancing & sharpening something else that may end up at the bottom of the Lake. And no you don't sound like you're talking down to me and I'm not taking it wrong. I deeply appreciate the help you've given. Thanks a lot for taking your time to give me some of your knowledge & experience. -Ken-

[Mad Hatter]

I stand corrected, it's the OSE glass filled props that are not recommended above 25,000 rpm. I am going to order three Graupners 1.97 thru 2.12 inches total pitch. Only one small snag. I'll have to drill out the threads and cut a slot for the dog as these are made for a 4mm fully threaded shaft. Thanks again. -Ken-