About the 'three degrees rule' & surface drives

[speedchaser]

Hi... Appologies for strange title. A few threads ago, someone kindly informed me that a surface drive for a hydro type can be set at no more than three degrees off the horizontal, otherwise it will push the boat upward rather than forwards - so I need to ask, is this the maximum upper theoretical limit, or is three degrees the maxiumum that has been done practically? Also, does at least half the prop for a surface drive have to be in the water, or could it be less than half - say a third; could I get away with just a third of the prop in the water? Cheers for any help...

[martin richards]

I'm always a bit suspicious of absolute numerical "rules". The truth is that many boaters get away wit a "suck it and see" approach. I think the 3º you're referring to was negative thrust anyway and most users of flexi-drives would think in terms of 0º to slightly positive.
There are so many variables that, for me at least, an "optimum" personal setting is found and then a ply template is cut to ensure that it can be reproduced if anything moves. Personal preference, water conditions, the type of prop used all play a part.
As for surface piercing props, that's another can of worms and more complicated as it's nigh on impossible to measure. In theory, less than half is better with the prop hub running out of the water for less drag. Prop design is also a factor as on a lot of designs, pitch is not constant across the radius.
That's a long post just to say "I don't Know" or "it depends" :D

[speedchaser]

Well, that's a lot more than I know... I think your reply just about sums up the whole 'science' of rc fast electrics, which is that it isn't an exact science yet but at least some of us know it isn't. Your bit about the prop hub giving less drag if out of the water makes a lot of sense though, and it means that I can shave another degree or two for a surface drive without resorting to flexi cable. Cheers...

[Ron Olson]

That's OK, strange titles seem to get more viewers, it got my attention!
Most hydro setups start at 0 degrees and most won't vary more than 1 degree from there. a 1/2 degree change can make a big difference in performance.
You can experiment with prop depth which changes the angle of attack on the hull. You really can't do a lot to change where the prop rides at in the water, your center of gravity, weight and other factors will determine that.
Yes, like Martin said, props are another can of worms. Props like Octura's use a progressive pitch, while others like Prather for example have a constant pitch. You have to find out what the boat likes with your setup.
Still using a hard shaft? OK, you Brits seem to like those. I like the flex cables but the latest thing for electrics is the wire drive.
BTW, for you FE racers, you will be happy to know that the previous SAW speed record was broken again yesterday!

[speedchaser]

Thanks on that - I didn't realise that the earlier info on 3 degrees was refering to negative angle, so I'm glad M.R cleared that misconception up.
On flex-drives, well I've stayed away from that alternative mainly because on some earlier threads it's said that in-line prop shafts for a surface drive are better for higher performances - and because I've now bought an in-line prop shaft with U-joint anyway, so must stick with this I guess... And thanks for the tip on props - been thinking in terms of a smaller diameter 3 or 4-blade prop rather than a bigger diameter 2 blade one with a fixed pitch.
On your first tip re' most hydro setups starting at 0 degrees and going up to no more than 1 degree - now that's a BIG problem right now (just when I finally thought I had worked out the central tub). I looked at the 'classic-thunder' site for a guide, and whilst I didn't take the set-up pic completely literally, I have partly gone with the way they show the prop shaft; which is alot more than 1 degree (looks more like 5 or 6), so I'll have to re-think that again as well. Cheers - sorry for long reply

[martin richards]

That's not a big problem. The full size hydros invariably had straight drives and, to some extent, your choice of prop can cancel out any effects of shaft incidence. Probably the most popular model hydro plans, the Hydromite, uses straight drive with a recomended max angle of 7º. There are many people still using straight shafts. Wire drives seem to be popular because of their simplicity and cost advantages over flexi. I use home made cable drive, but only because the biggest motors I use are 400/500's and plastic props. More powerful motors and more expensive props would have me scratching my head.

[speedchaser]

Great - cheers for that... That's reduced the problem alot. Even so, in the last eight hours or so (this is making me become an insomniac..) I've found a way of reducing the angle down to about 2 degrees by slightly increasing the true sponson height from 1 inch to 1 1/4 inches and so enabling me to begin the step slightly more further forward - at least this seems to look right. This will mean a very shallow motor mount, but I think it will work. Many thanks