seagull racer

[martinworswick]

hi to all,
i'm currently thinking about the design of a seagull racer i wish to build, a workmate of mine has one he races and regularly gets high placings, the whole thing seems to be a bit of fun and not taken to seriously by all concerned, until it gets to the after-race drinking that is!

the object like most racing is to go as fast as possible around the course using a seagull outboard as your means of propulsion (around 5 hp i think) there are various classes regarding what modifications are allowed to your motor,but,aside from the propulsion i want a boat which is basically as fast as possible, racing is inshore and lakes so most of the time the water is reasonably calm.

the boat only has to carry me

i was thinking of a catamaran design, about 8 metres overall, and the hulls as narrow as i can possibly make them and still float-10cm maybe at widest with little or no rocker in the hulls, and i suppose my questions are these-

which has a greater bearing on speed, is it the wetted surface or the overall width?should i go wider and perfectly round or narrower and oval?

is the catamaran route a realistic one or should i forget it and think about planing?

i have no experience in boat design-just bits of theory i have picked up on websites like this, and the whole thing is for fun so if it doesn't work then i just lose a bit of pride.

any suggestions to go as fast as possible?

thanks in advance

 

[Guest625101138]

Hydrofoil will be fastest at that power level.

Monohull will be faster than a catamaran.


It is marginal if a long slender hull will be faster than a well designed planing hull. There is another thread here that has pictures of similar craft in the Caribbean.

Whatever you build, weight will be a significant factor. Particularly if you go for the hydrofoil.

I would look closely at a small planing craft maybe 8ft long and 4ft beam with very low deadrise angle. Carbon fibre sandwich would be my choice for construction. I have attached some power estimates. This excludes any prop losses.

You would need to look closely at the prop. With good design on a prop you could get better than 80% efficiency. If you achieve this the boat should get over 20kts. Is this fast enough? May also need to fair the bottom of the outboard leg. I would be inclined to cheat a bit here and set the gearbox above the water level and use a curved shaft with a nice big prop hanging well aft. Seagull outboards are not intended to go fast.

I made an even smaller boat using a 6HP Johnson motor that did well over 20 kts. The boat was sized to fit inside a Renault 12 station wagon so you can probably gauge it was very small. It was not easy to get it to plane because my arm was not long enough to get forward easily. Once on the plane it would fly.

Rick W

 

[martinworswick]

cheers for that,
the reason i was thinking to go the catamaran route was because i saw the rowing at the olympics and the boats seemed very fast and efficient for the energy used so i figured go narrower and double up for stability- i guess it doesn't quite work like that!!

can you recommend any decent planing and or foil threads or sites that might be suitable, i've done a quick search and there is a hell of a lot to go through.

if i remember correctly my workmate was getting around 18 kph so even if it was 18knts then 20 would be great

deadrise- i'm assuming that is the angle the hull meets the water at, is that correct?

props- i'm pretty sure you have to use a production seagull prop but you cat attack it with a file etc to make the edges sharper

regards
martin

 

[Guest625101138]

Martin
The comparison you make is a good one. The rowing eights have about 5HP and those cats have about twice that and weigh less. They would have about the same top speed.

With a lightweight boat I expect the planing to be better than a long slender hull and way more practical to build.

Savitsky did the work on hull performance for planing hulls. That previous attachment comes from a site using his formulas. It has the URL shown. The other thing you might want to look at is stepped planing hulls.

Foils are quite complex. You need to know a bit about structures and foil design. Here is the ultimate low power foiler:
http://lancet.mit.edu/decavitator/
It did 18.5kts with a guy pedalling so 30 to 35kts would be possible with a well designed foiler with 5HP. There is some detail on the evolution of the foils.

You can get foil design software but you still need to know what you are doing. There are real world issues like foil fouling with weed and rubbish so you need to have areas clear of this.

A planing boat is much easier to make.

If you want to go down that track then you just start asking specific questions on it. A lot depends on your current level of knowledge and ability to build things.

I can take you through the issues but it can be a huge waste of time unless you really want to do it and have the access to the right stuff.

Rick W

 

[keith66]

We used to hold a race known as the Seagull slaughter, it was huge fun though trying to tune one is supposed to be impossible as they promptly blow up, We repitched propellers with a wrench and ran one on 100 octane avgas but it didnt like that, We put one on a small 19ft Proa and it went quite well, as sugested before you would need a fairing round the leg.
Water pistols were used to shoot into the opositions carburretor as we went along, and buckets deployed in retaliation.

 

[martinworswick]

A planing boat is much easier to make.
If you want to go down that track then you just start asking specific questions on it. A lot depends on your current level of knowledge and ability to build things.
Rick W[/QUOTE]

thanks for the advise fellas,

planing it is then,

to answer your question i have never built a boat before, the closest i've got is to get a set of plans for a cedar strip canoe which i intend to build this summer, i'm also a carpenter by trade so my woodworking skills are up to it and i'm pretty good at figuring out/seeking out the answer to things i don't know.

iv'e come acroos a few designs such as this one http://www.svensons.com/boat/?p=HydroPlanes/minimax

i assume something like this would be the way to go

any other suggestions greatly appreciated

thanks again
martin

 

[Guest625101138]

You should be able to apply your carpentering skills to make a nice job of the minimax. It has the right proportions for the power you have. You will learn a lot about the required strength and the placement of structure if you stick with the plan.

If it is not going to be used too often then you could get away with outdoor ply but marine ply is a far superior material and you will not need much.

You need to be able to shift your weight around to get the best performance. Probably need to be near the bow to get it easily on the plane and then move aft as the speed builds.

A possible improvement would be to provide some raised fairing on the front and the stern. The windage will not be huge but fairing can reduce it substantially so you can achieve a small gain overall. However build it as designed first and then play with things.

The leg of the outboard will certainly benefit from fairing. You are likely to need a different prop to get the best performance.

Only other thing is to go on a diet. Performance will be weight sensitive so the lighter you can build it with no added junk and the lower your weight the faster it will go. I have seen a 3HP Evinrude plane a small boat over 20kts with a kid on board. It actually made a joke of licencing in Queensland that allowed anyone to operate boats with less than 4HP.

Just keep this thread going as you start the build and more people will get interested as you post photos. Everyone here gets a buzz out of seeing other's projects come together and you get the occasional snippet of good ideas that improve the outcome.

There are some good builds going on right now that you can pick up ideas from. Some places have boat building classes and they knock out small boats very quickly. There may be something near you. Maybe search for project or build.

Rick W

 

[Tiny Turnip]

this was one of the more extended seagull racer threads.
http://www.boatdesign.net/forums/boat-design/boat-designing-help-please-22711.html

 

[tom28571]

props- i'm pretty sure you have to use a production seagull prop but you cat attack it with a file etc to make the edges sharper

regards
martin

This seems to be the main limiting factor to me. These things have a lot of blade area and low pitch. How do you get past that and go fast?

The starting point of the learning curve for the boat is simple. A flat piece of plywood with enough freeboard to keep it, the driver and the motor afloat will be about as fast as the most sophisticated vessel. The Minimax has probably been built by more people than any other such craft over the last half century.

 

[tom kane]

You may find some interesting info at www.regattaevents.co.nz

 

[martinworswick]

i was under the impression that parts had to be original, but this was taken from www.regattaevents.co.nz and it sounds like custom props are allowed-


Propellor:
* File the leading edges of the blades to a sharp edge. This creates a finer entry for the blades through the water. Be sure to remove material only from the back of the thrust face (& not from the thrust face itself).
* If the prop has worn or damaged tips, then, replace it or have it rebuilt
* Balance Your prop. Strangely, Brittish Seagull the manufacturer, didn't seem to balance their propellors very well, (if at all). It's often found that standard propellors can be hugely out of balance. Given Your prop spins at around 1000~2000rpm, this means alot of power is going into making vibration rather than thrust. In some cases, there can be more vibration coming from an imbalanced propellor, than the motor itself.
* Select a pitch for Your propellor that optimises the engine to run at 3800~4000rpm. Often the standard propellor is pitched too low resulting in the engine over-reving. This over-reving causes the engine to produce less power & risks engine damage such as bearing failure.
* Use a propellor with less blades or cut blades from the prop. Generally speaking for propellors of the same pitch, fewer blades are suited to lower drag & higher speed, as opposed to, more blades which tend to be better suited to higher thrust & lower speed.
* Have Your local propellor builder custom make You a prop.
* Practical on the water testing of these ideas will reveal the best solution for Your hull & motor combination




i'm assuming the way to go would still be planing, but i know nothing of props except they spin and push the boat
what type of prop should i look for?

ps thanks for the link tom

 

[Guest625101138]

..........


i'm assuming the way to go would still be planing, but i know nothing of props except they spin and push the boat
what type of prop should i look for?

ps thanks for the link tom

The best prop will be two bladed and the largest diameter you can reasonably fit providing it has the blade strength to handle the thrust load, which is not much. It has to be pitched to suit the engine rpm and load. Obviously the hub has to match the shaft.

To get an initial selection you need to know the diameter that will fit, you need to estimate the drag per the link provided earlier, determine rpm for maximum power from published data and this will enable the pitch to be determined. Get a prop and try it out.

Rick W

 

[Seagull170]

The greatest thrust from a British Seagull Century or earlier model, was 90 pounds, I would be very suprised if any of the blueprinted, modified racing Centurys & 102s ever exceeded a thrust of 110 lbs, so prop loading is unlikly to be a problem.

The problem any user of a large 2 bladed pitched up prop is that the prop will have too little slip.

At peak revs 4000 the propeller will be turning at 1600 rpm, on a 2.5 : 1 box
assuming no prop slip
at 20 Mph = (1267200 inches per hour /60 = 21120 inches per minute)

So at 1600 prop revs to do 21120 inches per minute will require a prop of
(21120/1600) = 13.2 inches without any slippage.

But with 15% slippage it would require a prop of more than 15 inches of pitch,
while it might well be possible for a very good Seagull to turn a prop with this sort of pitch at these revs when it is full on the plane in a very light slippery hull carrying the lightest of loads.
The chances of it being able to turn this prop from a standing start at a bit faster than tickover, let's say 2000 revs (circa half power) are nonexistant, in fact without a clutch it would be impossible to start the engine.

The only chance of using a prop with this type of pitch would be to use one with a diameter of 6 inches or less so that it would slip enough to allow the revs to get much closer to the maximum/powerband, so with luck and a balancing act up near the bow you might be able to slowly get out of the hole before the prop grabs onto the water & starts working as designed.

Not really a prop design job for beginners!