Seagull Racing

All looks like good fun, .......until some one loses a finger! oh no I've turned into my Mum, what starts in fun will end in tears. All the best with it from Jeff.

 

What is it about the British? You put on a perfectly sensible Seagull outboard race and two blokes have to dress in drag?

Any excuse,-- Geeez dos'nt walrus live there in Fowey,--hey, you dont think,---Naaa it couldnt be---could it?

 

Those who live in Thailand can hardly comment about blokes in drag going by the popularity of Ladyboys
Back to serious matters, I seem to remember only a few years ago that an ancient British Anzani outboard was found in someones shed, it turned out to be a factory race tuned hydroplane engine. After a quick rebuild it took its class speed record on lake windermere at well over a ton, The details should be available from the Classic motor boat association.

 

I used to go to the Windermere speed trials, an hour drive from Blackpool.

Ide stay in the Hotel accross the road, bloody freezing weather and extremely boring. I had a boat once that I bought from the back of a shed in Windermere turned out it had a class in the speed trils.

BMW with dellortos in a Delta hull 52 MPH. Which makes me skeptical of an Anzani doing a ton.

Mind you there is not much to those hydroplanes.

 

Gentlemen - I have at last the details which you require to hopefully produce the propeller shape. Two engines, one at each hull will be used (and steered)

The hull transom is 180mm wide at the waterline. The middle of the sponson at waterline is 250mm.
The transom frame will be attached partly to the rudder brackets-

The total weight of the cat including two crew and two engines, fitted only with trampoline would be say, 160kg. Overall hull length is 4m, the waterline is 3.8m.
The hull from the side and front-


I think the engines run at WOT at about 4000rpm. I am told the power of the '4/5hp' is actually 1.7hp. The gearbox ratio is 12:30.
Propellers up to 130mm radius would fit.
I shall certainly fit anti ventilations plates to the size given. Do you still recommend fairing blocks - would you explain how they would be fitted?

I am very grateful for your advice, Andy

 

Andy
The weight sounds low for two people, two engines, two hulls and incidentals. I would expect at least 200kg.

The 1.7HP looks about right for the engine. It will be a good number to work with.

I will look at the hulls in their current form. I think with the severe hull rocker they will have a tendency to dig in. By flattening out and straightening the run aft you would have a much better chance of actually planing. So I will look at the impact of large fairing blocks. These would typically be made from rigid foam. Polystyrene is stiffer than polyurethane but the former can only be used with epoxy resin and polyurethane glue.

The blocks would be shaped to fit neatly to the hull, glued to the hull and then glassed over to provide some protection. Effectively you are changing the shape of the hull by adding low density material.

As a sailing boat these hulls would not be intended for high performance compared with something like a Tornado. Aim of fairing is to get closer to the Tornado shape. I should be able to provide a fair indication of the performance difference.

The outboards will need to be mounted after any fairing is done so the anti-ventilation plate is below the lowest part of the modified hull.

You could test the boat without the fairing first and then add the bits to confirm their merit. In this way you verify the different contributions of changes.

Rick W

 

Excellent Rick.
I shall look up Tornado.
I realise a far more competitive boat could be obtained or the bottom of this one could be altered but we are not allowed to plane! If I could make this cat keep up with my long narrow skiff with its one engine that would be ideal . . .
I weighed the boat this afternoon by placing the middle of one hull on the bathroom scales. This indicated 35kg, so I multiplied by two
= 70 kg.
Two engines, 20kg x 2 = 40kg
Two people, 65kg x 2 = 130kg
Total = 240kg
Am I crap at maths or what?

 

I have modelled the boat and done a preliminary prop design. The total hull drag at this speed is 250N. This is a bit under 60lbf. It does not bog down as I suspected but I will still look at different transom.

I believe 15kts is possible with two outboards. The design conditions for the prop are shown.

So if you can get a 10 X 11 2-bladed prop and grind down the blades so they have a chord of only 40mm this is the result you could expect. THe data is based on a 12% thickness foil. Maybe get one of the multi-bladed props and remove all but two of the blades.

If you look around for props that are close to this you could give it a go and get some data to confirm what I have done. You can buy something like this in Australia:
http://www.australpropeller.com.au/fixedyacht.htm
It is not real hard to fabricate a prop but you need to be able to weld with some proficiency and be prepared to do some grinding.

Rick W

 

Superb Rick. I shall experiment and let you know. Andy

 

Rick - would you suggest for the 10x11 prop, one modified to your spec from this:
- or one modified from this shape?:

another angle -

 

Do you know the diameter and pitch of both props? If the pitch is not right then this will need to be adjusted. I have no idea how the material will respond to yielding it and if it would be better to heat first. If pitch is right then that is a good start.

The 4-blader would probably be easiest to alter.

You will need to make a profiling gauge to the correct foil shape. I can provide this later.

I suggest you set up both motors with the antiventilation plate and do a test run with the existing props before you get into the prop modification part. It will help to have a test point to confirm calculations on the boat.

Rick W

 

Thanks Rick, antiventilation plates next. Andy

 

Andy
You said that it was not easy to test things and I am not sure what you mean by this.

You may find that simply placing the outboard leg directly behind each hull will eliminate ventilation. The stream directly behind the transom is much more defined than what you get where the wakes from two hulls collide.

You often see problems with single outboards on cats. These are modern motors with antiventilation plates. Most I see with this configuration use the central pod to define the flow.

So if you have the ability to do testing I suggest the first step is to mount the existing motors behind the respective transoms and do a test run making observations and recording speed.

A camera and GPS can be very powerful test instruments for such testing. A short video clip of what is happening in the wake can be useful evidence when trying to work out what is happening. If you do not have a GPS then something like this is really good value:
https://buy.garmin.com/shop/shop.do?pID=231
These will record hours of testing at 1 second interval if required and just plug into a computer for download of data.

Accurate speed measurement is important for evaluation of changes once you get past obvious problems and start tuning things.

Rick W

 

Well the first thing Rick is to fabricate a frame beween the hulls for the engine transom clamps. I shall do this and examine the wash at the down tubes with standard propellers.
Yes we are on the side of a fast tidal river which limits the times for testing, but we won't allow that as an excuse. (Its also bloody cold and wet in January!)
I think the best answer would be to weld stainless propellers. Two 10" 40 x 5mm plates on a 1.5" diam bar shouldn't be difficult. Of course, working out the angle to weld the blade to the hub to obtain the pitch will cause me some brain searching! It should be much easier to twist the blades to complete the pitch after it is attached to the centre and then I can finish the profile and grind the edges.
I was putting off buying a GPS, but now I must get one. I do have an rpm meter.

Oh dear, next I shall be asking for your help for a prop for other boat, the 18'6" skiff! The Seagull powering that is the 170 which I'm told produces a real 4.7 hp at 4000rpm . . .

 

If you get to the prop fabrication stage I will take you through it. I have a documented procedure that others have used with success. You will only need 4mm thick flat bar. Each blade will only be a bit over 100mm long. There will be a bit of strengthening at the root of the blade.

I provide the twist angles as well and how to grind it accurately. You could make them out of mild steel for a trial and some practice. It is cheaper, a bit easier to work with and easier to get hold of.

Get the outboards mounted and lets see what they do with existing props. Need some hard data and confidence the ventilation issue can be sorted.

Rick W