Using a moped engine to power a rc-powerboat with a outrigger hull.

[Storken]

Hello, I've been playing with the idea of creating a aluminium boat with a light high powered two stroke engine. I've fixed my eyes on the outrigger style of hull, but there seems to be little information about it.

I guess tuning mopeds isn't that common "over there", but here in Norway it's a growing sport. My last moped fitted with a 26 mm carb, 80cc cylinder, long stroke crank dynoed 17 hp on the back wheel. Unfortunately, I don't have a video of it But here is a similar dyno run. Just to show what one of these engines are capable of.

I'd imagine one of these engines weighing in at around 5-10 kg without the variomatics (you don't need them in a boat). The output on the crank could go as high as 22 hp, but I think I'll start off aiming at around 14-16 hp. The engine rev will be at about 12-16k at this power outtake. I'm planning to use a centrifugal clutch for a easy start, balancing and aligning this will probably be tricky - but doable. The rest will be a solid axle to the propeller.

Challenges:

1. Keeping the prop under water (to actually turn the power from the engine to propulsion. How much negative angle is within the safe zone? And how important is it to keep the propeller in the water?

2. Long driveshaft equals vibrations? I guess some bearings could be nice to support the driveshaft.

3. High torque from the prop/engine. Will a outrigger tupe of hull overcome this with no problem?

I've got access to cnc-machinery and alot of advanced equipment, but I'm not that experienced with boat design - so I'm asking here for some advice

The potensial? Well, say that the entire boat weighs in at 30kg (maybe a too light estimate). Then say that the engine/prop can turn 80% of the power (10 kW) into forward thrust. 0-100 km/h (0-60 mp/h) in less than one and a half second. Ofcourse, this is wishfull thinking

Is this a impossibillity, or could this be the start of a demanding and fun project?

[Guest625101138]

For that sort of power to weight you should consider a hydroplane type of hull and surface prop or better still an air prop if you want to get the best speed possible. Using an air prop overcomes all the cavitation problems you get with things in the water.

Treat it as more a plane than a boat. The water just provides a reference. Keeping it aerodynamically stable will be the challenge. It will have a tendency to fly.

Prop about 1m in diameter will give reasonable efficiency if you keep it very light and aerodynamically streamline.

A water prop but gives an idea of size:
http://piratepetester.googlepages.co...bomb1-full.jpg
There is plenty of air driven stuff around if you google.

Rick W

[masalai]

Or you could try chainsaw engines, dog clutch, chain drive and light... many long years ago were the popular go-kart engines - I had 80cc McCullock powered kart giving some high speed thrills... modify the fuel, porting and compression and a tuned exhaust trumpet... All good fun...

[Storken]

Originally Posted by Rick Willoughby

For that sort of power to weight you should consider a hydroplane type of hull and surface prop or better still an air prop if you want to get the best speed possible. Using an air prop overcomes all the cavitation problems you get with things in the water.

Treat it as more a plane than a boat. The water just provides a reference. Keeping it aerodynamically stable will be the challenge. It will have a tendency to fly.

Prop about 1m in diameter will give reasonable efficiency if you keep it very light and aerodynamically streamline.

A water prop but gives an idea of size:
http://piratepetester.googlepages.co...bomb1-full.jpg
There is plenty of air driven stuff around if you google.

Rick W

Hmm, interesting. It sure does simplify alot of things, thats for sure

I like the simple part of it, but it becomes quite large and not that little buzzing boat that I imagined. I'll give it some thinking - cavitation seems to be the huge drawback of a water propeller =/

Edit: So, with this hydroplane - do you use a normal rudder submerged in water?

And, in like this video: http://www.youtube.com/watch?v=rH_VKUBx1-c - how hard is it scaling up something like this?

[Guest625101138]

Originally Posted by Storken

I like the simple part of it, but it becomes quite large and not that little buzzing boat that I imagined. I'll give it some thinking - cavitation seems to be the huge drawback of a water propeller =/

Edit: So, with this hydroplane - do you use a normal rudder submerged in water?

And, in like this video: http://www.youtube.com/watch?v=rH_VKUBx1-c - how hard is it scaling up something like this?

Cavitation is not a particularly serious limitation. It just reduces efficiency. If you mount a surface piercing prop right on the transom you will get efficiency around 70%. An air prop that is suited to the job will get 85+%. The gain will translate to about 5% in stop speed. Most of the model boats that plane these days use tiny surface piercing props.

My preference would be to have control surfaces in the water as they are more positive. That are not typical boat control surfaces though if you get to the speed potential.

If you build your hydroplane really light you could get maybe 60kts. Aerodynamics will be a consideration as you want it stable.

That plane/hydroplane has a thrust to weight ratio of around 1. This is not trivial. I expect it will take better than your moped motor to get you into the air and do what that thing can do. You could fly with the moped motor but it would not be as compact.

Problem with flying is not getting in the air but getting back down in a controlled way.

Rick W

[Storken]

Originally Posted by Rick Willoughby

Cavitation is not a particularly serious limitation. It just reduces efficiency. If you mount a surface piercing prop right on the transom you will get efficiency around 70%. An air prop that is suited to the job will get 85+%. The gain will translate to about 5% in stop speed. Most of the model boats that plane these days use tiny surface piercing props.

My preference would be to have control surfaces in the water as they are more positive. That are not typical boat control surfaces though if you get to the speed potential.

If you build your hydroplane really light you could get maybe 60kts. Aerodynamics will be a consideration as you want it stable.

That plane/hydroplane has a thrust to weight ratio of around 1. This is not trivial. I expect it will take better than your moped motor to get you into the air and do what that thing can do. You could fly with the moped motor but it would not be as compact.

Problem with flying is not getting in the air but getting back down in a controlled way.

Rick W

Hmm, definitely some nice info there

That first pic you posted showed a interesting design - but I'm thinking about a offrigger - engine in the middle and "flabs" on the sides. Definitely the low profile I'm after.

I've done some calculations on engine weight, once stripped off everything unnessecary, I belive it will be on about 4 kg (maybe even as low as 3) and still maintain a performance of up to 20 hp sustained (Crank numbers). I've seen some of the new 2009 cylinder kits (95cc) benching 22 hp on the rear wheel, concidering crank numbers are 15-20% higher than that - I believe it will be possible to tune up the performance another notch.

I have access to a tig welding machine and plan to do most of the construction in alu, it's not going to be heavy

I've decided to go with a air propeller design - is it possible to mate this with a outrigger type of hull? I've ditched the idea of flying - I just want something incredibly fast with a fair amount of agillity.

[Guest625101138]

Originally Posted by Storken

....
I've decided to go with a air propeller design - is it possible to mate this with a outrigger type of hull? I've ditched the idea of flying - I just want something incredibly fast with a fair amount of agillity.

Are you able to provide a drawing of what you have in mind? It will give me an idea of what you hope to build and others might chime in with their thoughts and ideas.

Rick W

[Storken]

Originally Posted by Rick Willoughby

Are you able to provide a drawing of what you have in mind? It will give me an idea of what you hope to build and others might chime in with their thoughts and ideas.

Rick W

Well, I've drawn something up real fast in solidedge. I'm not sure of anything really, just drawn from looking at images.

Here goes:







I've left the engine brackets out if the drawing - those will take some time. I'm thining that placing the engine near the beams, out to the outriggers, will let me concentrate the strenght needed to support the engine and propeller -and thus, saving weight.

It's a start I guess

Edit: Spoilers doesn't work here =/ Well, those of you with low res-screens - sorry

Edit2: I was thinking about the bad placement of the engine when it struck me, can a 1m propeller withstand 14000 rpm? Nope, certainly not - the tips would be doing mach 2. This rules a straigth axle out of the question. Placing the engine down in the hull will give better stabillity aswell, so it's probably the best way anyway. I'm concidering using variomatics (I'm quite experienced with them) to make sure that there is no "spin up" kinda. 100% engine power avatible half a second after I gas it. The problem with the variomatics is that they have a variable ratio of about 3:1 to 1:3, so - the gearing is really light at the start, and heavy as it goes. Say, for example, that I'm using a high pitch low diameter (0,5m) propeller - this breaks the speed of sound at almost exactly 13000 rpm. What exactly happens at around 95% of the speed of sound - does the propeller disintregrate, or does it resist the additional revs?

Maybe a three or four bladed prop is what I'm after? A smaller prop would also make construction easier. I could ofc go through the job of making a gearing for the larger prop, but is it really worth it?

[Guest625101138]

You have to consider belting through waves without the nose of any of the hulls digging in.

Location of the pilot is a significant factor because that will be a lot of weight.

You can use JavaProp to see what particular props will achieve. Google JavaProp and run the Applet. I can help you set the parameters. I expect the lift to drag ratio will be about 8 up to 30kts. Above that the aerodynamics come into play. I determined a prop running around 2000rpm would give a fair result but you could play around a lot. If the boat is really low drag you may get something like 90% efficiency.

I would have a wing section for the supporting beam for the outriggers. Possibly with small flaps so I could play with the lift from them. You need to give a lot of thought to aerodynamics.

Need to think about how you guard against prop injury. Any shrouding needs to be aerodynamic.

The main hull probably needs to be a bit longer so it will break out of displacement mode easily. The prop will not have very good static thrust.

Rick W.

[Joakim]

I would recommend a water propeller. If you want an air propeller to be more efficient at a reasonable speed (max 30 m/s) at that power, it needs to be huge. 1 m is very huge for your application and still not any better than a water propeller. If Rick got something well over 70% for this, please show the numbers.

How are you planning to steer it?

Whatever propeller you use you don't need a variomatic, since the power need of any propeller is roughly rpm^3. Thus at low rpm the power need is minimal and there will be no problems rotating it. But you do need a gearing, at least for the air propeller.

I hope you realise, that a 10+ kg thing going 60 kn is a real monster if it should hit something, especially with a 1 m air propeller, that could easily make it fly (10 kW at 30 m/s at 70% -> 233 N thrust).

[Storken]

Originally Posted by Rick Willoughby

You have to consider belting through waves without the nose of any of the hulls digging in.

Location of the pilot is a significant factor because that will be a lot of weight.

You can use JavaProp to see what particular props will achieve. Google JavaProp and run the Applet. I can help you set the parameters. I expect the lift to drag ratio will be about 8 up to 30kts. Above that the aerodynamics come into play. I determined a prop running around 2000rpm would give a fair result but you could play around a lot. If the boat is really low drag you may get something like 90% efficiency.

I would have a wing section for the supporting beam for the outriggers. Possibly with small flaps so I could play with the lift from them. You need to give a lot of thought to aerodynamics.

Need to think about how you guard against prop injury. Any shrouding needs to be aerodynamic.

The main hull probably needs to be a bit longer so it will break out of displacement mode easily. The prop will not have very good static thrust.

Rick W.

Thanks again for a awesome answer

The nose and outriggers was drawn very quickly - I didn't use much time on predicting sea abillities, more on that later i guess. Here in norway, late in the summer evenings - the wind swaps direction - at this time the ochean is as calm as a mirror for about an hour or so. I'm planning to use it then.

Uhm, there isn't going to be a pilot onboard (or do you mean pilot as in rudder?) - this is a rc controlled boat That leads me to thinking low diameter high pitch propeller, maybe a 24 x 14? Or, is that too light for the engine? JavaProp really confused me more than enligthen me xD

Since there is no driver and the propeller will be at rest until I revv it (the sentrifugal clutch kicks in) is a shroud really nessecary? Sure - it adds some efficiency, but it also adds drag and weigth.

Edit:

Originally Posted by Joakim

I would recommend a water propeller. If you want an air propeller to be more efficient at a reasonable speed (max 30 m/s) at that power, it needs to be huge. 1 m is very huge for your application and still not any better than a water propeller. If Rick got something well over 70% for this, please show the numbers.

How are you planning to steer it?

Whatever propeller you use you don't need a variomatic, since the power need of any propeller is roughly rpm^3. Thus at low rpm the power need is minimal and there will be no problems rotating it. But you do need a gearing, at least for the air propeller.

I hope you realise, that a 10+ kg thing going 60 kn is a real monster if it should hit something, especially with a 1 m air propeller, that could easily make it fly (10 kW at 30 m/s at 70% -> 233 N thrust).

Ty for the input. If I was going to go with a water propeller with the same design as posted over here, is there any do's and dont's? From the outriggers I've seen, everyone seems to have atleast a propellers radius between the hub of the propeller and the hull. How do I get the extra torque and hp that this engine produces down in the water? I'm looking at some of the glow engines that uses these tiiny propellers on a relative high hp number... What dia*pitch am I looking at to make this effective?

I can take the gearing part from a moped, but thats another 1,5 hp "out of the window" - and more weigth. I'm planning to use a rudder which has one part in the water and one part in the air if it's a air prop design. The air part for low speed manouverabillity and the water part for high speed. Water design, only in the water ofc.

I do realise that this thing is going to be ******* fast if I get this rigth.

[Joakim]

I did a rough Savitsky calculation for the center hull and it seems, that 60 kn is a realistic goal for 10 kg weight and 10 kW power. You need to make the design more aerodynamic though.

For that speed at ~200 N thrust you need a propeller with D~6-10 cm and P~14 cm to be used at 14000 rpm without gearing. P/D is in the normal range of cleavers, but I have no experience on such a small ones rotating that fast. You should ask someone dealing with high speed RC boats, if that is something that would work or not.

[Storken]

Originally Posted by Joakim

I did a rough Savitsky calculation for the center hull and it seems, that 60 kn is a realistic goal for 10 kg weight and 10 kW power. You need to make the design more aerodynamic though.

For that speed at ~200 N thrust you need a propeller with D~6-10 cm and P~14 cm to be used at 14000 rpm without gearing. P/D is in the normal range of cleavers, but I have no experience on such a small ones rotating that fast. You should ask someone dealing with high speed RC boats, if that is something that would work or not.

Yeah, I just did some quick calculations on displacement and found that with that hull i sketched i would have around 300kg of displacement. I only need like 40-50 i guess? Just to be on the safe side.


Jim nissen's design looks like what I'm planning really. Too bad that's the only picture I have of that boat.

If my first design is a success, I'll try to get some sponsors on a 110 or 95 kit and break 16-17 kW

[Joakim]

Originally Posted by Storken

Yeah, I just did some quick calculations on displacement and found that with that hull i sketched i would have around 300kg of displacement. I only need like 40-50 i guess? Just to be on the safe side.

40-50 kg would be VERY heavy and you would not get 60 kn with that power. A hull of a racing boat 4 m long would weight less than that, if there were no rule restrictions (like in T400). For those small racing boats (not RC) very roughly the weight of the motor = the weight of the hull. A 2 m hull should not weight more than 5 kg or so. Thus I would target at 10-15 kg with the engine.

Here is something quite fast for you: http://www.prestwich.ndirect.co.uk/sigma51.htm

There are also tips that include e.g. balancing the boat against torque from propeller.

[Storken]

I didn't mean a total weight of 40-50 kg Was thinking about displacement. I like monohulls - but I'd say that one of the offrigger designs is easyer to build and I find the design fresh (And I really can't stand those flabs on the rear of a monohull, I'd rather finetune offriggers then xD)

When designing a boat - say that the final weight is calculated to be 15 kg max, should I then design the boat to have, say, 20 kg of displacement?

Offriggers, is there some scale between parts that should be followed, or is it just gogo and build something, look what works and what doesn't and change it?

I'm thinking of putting the fuel tank in one of the offriggers. Imagining a 3l tank fully filled will be just over 2 kg, maybe a bit too much?