Seagull prop Cat racer

At last I have added anti ventilation plates (suggested by Rick Willoughby)and fitted a twin hull transom to make two Seagull propel my unlikely Catamaran in this Christmas Seagull Race.

Please view first pic to see general setup.
Second pic shows parts for proposed two blade props. Stainless hub cones to fit shafts and two stainless 10" x 40mm x 2mm blade strips, ready to cut and weld. (About 9" -10" diam)

I would be very grateful for advice to weld at best angle and twist for most effective pitch. Metalwork is my speciality.

The Catamaran has a 12'6" waterline with rocker bottoms.
The two Seagulls are 102cc, aprox 3hp each. Best RPM is, I believe about 3800-4000 with a 2.5:1 box.
With bathscales I measured 70kg under each hull with engines fitted =140kg. Two crew are neccessary, say 130kg together. Aprox total 270kg +.

 

Andy
Boat is a bit heavier now but you have two motors. The motor height might need some trimming to get the best relationship with the transom but looks good.

You are going to need to stiffen up the root of the blades when you weld to the hubs as they will be a bit bendy I expect. The force on each outboard will be of the order of 50lbf - a lot for thin blades. The stiffener will need to be a triangular piece about 2" long. So you need a bit more stainless. You could try it without if it is difficult to get more bar but the blades are going to flex.

I will give you details on making the blades later tonight. What is the maximum diameter of the hub? Also what is the maximum diameter you can go with the blades?

Rick

 

Good to hear from you Rick. I'm impressed you still have my details from months ago.

The distance from centre of propeller shaft to gearbox is 5.5" - maximum diameter without any clearance would be 11".

The conical prop hub is 40mm long. It is 42mm diameter at the wide end and 27mm at the narrow end.

I assumed many runs would be neccessary along the prop blade root to build up and ground to a smooth rounded contour. That will be done.

Although there has been scepticism at the anti ventilation plates, their back wash is smooth with no turbulent surges, unlike another engine I tested which looked like a shoal of Barracudas (not that I've ever seen one). Excellent Rick . . .

 

Andy
Using the full 40mm wide bar with a bit of end taper requires a diameter of 240mm. So less than 10" diameter. There would be a very slight improvement by going to narrower blade and use the full available space but it is simply not worth the effort.

If you send me a direct email I will send you a prop fabrication procedure with the best I can get with the 40mm bar. (rickwill@bigpond.net.au)

I determine that you can get a bit over 15kts with the two motors on your cat at 270kg. Is that going to be good enough?

Rick

 

Some pics shown of fabrication of stainless props from Ricks formula. They are very different to the originals and very efficient.
No. 1 shows steel stock and hubs for two 2-blade props

No. 2 shows these props finished- they were my first to make

No.3 shows initial welding of 4 blade prop. Hub is clamped to flat plate. Blades are set in place with window putty to allow adjustment to the correct angle by protractor. The blade tip is spot welded to the plate, this weld is ground off when the inside of the blade is sufficiently spotted to the hub

No. 4 shows the short straight edge across the blade which is lined up with the protractor

No. 5 ditto

No.6 shows front view of prop welded. Three runs between hub and blade root which are rough ground

No. 7 shows back view of prop ground to aerodynamic shape. This takes time.

No. 8 is side view of prop ground to shape.

With step by step instruction from Rick Willoughby with angles of twist and weld and suggestions of with what and where - its easy peasy and v. worthwhile. Certainly made my cat with twin engines with the two bladers and skiff with the four blades - fly . . .

 

Andy
I should back calculate the props now that you are quoting the power as 1.8HP rather than 3HP.

This will explain most of the difference between actual performance and predictions. The prop may have been a little over pitched and you might do a bit better with a tweak of the blades.

Anyhow you will go a lot faster with a single slender hull and that is probably the next avenue to explore. A slender boat would go just as fast with one of the 1.8HP motors. With the 5.8HP it would be something to behold.

Rick

 

Andy
I have to comment on the 4-blader. It is nice work - quite impressive for fabrication by hand. Your standard of finish is already higher than mine and I have made dozens of them.

Rick

 

Why not try to correct what is wrong with the seagull
3 HP for 100cc is reachable
And may be less work than the prop modification, and more satisfaction

 

I forgot to mention - each prop was carefully static balanced. Two straight edges clamped in vice with socket in between them, wider than hub, and levelled with spirit level. Prop with new socket extension through hub centre placed on the straight edges will roll to allow heavier blade to drop.
Metal ground off opposite side, taking thought where would be best to remove.

Yes I shall spend more time on the engines. However two months spent on initial prep was quite consuming. One does need to be retired . . .

 

Isn't it annoying when paid work interferes with the hobby.

Rick