Cheers arcdude, much appreciated.
I'm getting the buzz for the twins again, no doubt taking the little red one to completion has been worthwhile, with it performing well I'm rethinking some aspects of the hull design for the twins.
I'll explain. When I made the red one the idea was to try a way of building that was not very well known to me, and to achieve that the issue was never about what shape it was in detail. But, whilst it could have been shaped like an Aardvark for me to try glassing over foam, dealing with some shapes I was to encounter on the twins was more useful to me, so it has many elements that are in the frame for the twins. But a simple lack of foam at sensible costs meant the boat is too narrow overall, indeed too slim in many areas. Too short of lift through the centre hull being too narrow, too curved on the stepped lift surfaces on the sponsons (because I wanted to use the size and shape the twins would have, but the red one is only a quarter of the size ((volume)) of the twins, so theses surfaces don't straighten out in the wetted area as the should as the sponsons are too small for the shape used), and too small overall making it vulnerable to conditions, or so I thought.......
Consider this, the red boat is a quarter of the size of the twins, but it's over a third of the weight, a tiny fraction of the power, and with a CoG around 1.5 times the height. So this opens lots of opportunities to see why the red boat is working so well, and why it's not in some areas.
Let's take the negatives first.
So replacing the engine with a revvy Alpha .28 will probably sort that, there's a tuned pipe to squeeze in, but I think I have that covered. The new engine is quoted at 32,500 rpm at peak power, if I can get a happy engine to 29,500 I'll be at the limit of commercially available propellers. From there each prop will have to be made, and it may even be necessary to consider single bladed propellers.
Prone to prop washes.
The deck/hull side cut outs are too deep for this beam of boat, it's leaving a gap for splashes to get to the prop. Airboat hydros need to be very wide in comparison to water prop boats, mine have all previously had parallel sides, so on the red boat it was a scalloping and see how you glass that situation because I want the scallop the twins sides, but with a full width deck, at least originally. Now the width of boat is also sold to us as a need for stability on airboats, well this boat says different (apart from failing in torque which I'll come too), as the ride surfaces on the front are actually inboard of the boat side by a big margin, look again at the picture of the underside in the post above, the ride surfaces are actually at prop diameter spacing, think about that. The deck of the boat is also too short, it should go further forward along the insides of the sponsons, to provide splash protection from the front and inside edges of the sponsons.
This is quite painful on this boat, I'm putting it down to the narrow hull and centralised mass caused by the engine being upside down. I'm not dwelling on that though as the contra rotating twins negates the problem.
There's a few other minor negs but that's the three big ones. Moving on to the positives:
This was a huge surprise to me. After a float test in the bath I was gaining confidence that the boat would come up on the plane at the front, but I was nervous about the back end thinking there just wasn't enough lift surface there, but it is perfectly clear, when the water surface is flat for a true view, that the boat planes out almost fully from stem to stern, It's actually how it sits on the bench as the sponsons are lower than the rear end. The stepped surfaces on the sponsons are showing remarkable seakeeping. Yes, they dive sideways a tad because of the curved rail but the hull rides out waves very well, it's a pity the prop doesn't. One other aspect of lift which I should not ignore is the hull shape. This was another one of the key elements to be built into the twins, where the underside of the deck is a pretty flat route to the transom, but the top surfaces form a convex shape that forces the air to accelerate along the surface, decreasing the pressure, hence creating lift. In the past I've always sloped the sponson decks forward to create down force at the bow, but this one is geared so that the centre of lift on this deck/hull is in line with the CoB. Question is, is it the wetted areas creating the lift, or is it the hull?
Keeping things in perspective it's not doing too bad for a two foot boat with a .25 'sports' grade budget engine. The hull is handling what power there is comfortably, where the limits of the hull speed are is yet to be seen of course.
This is it's real asset, it's a nice thing to drive, seasoned racers have had a go and they report the same. I thought if it worked at all it would be skittish, that it could slap in turns, and that it would just flip over in winds and flip over in waves. So my thoughts created by this skinny hulled boat have been proven to be completely wrong as the boat turns left like it's on rails at the moment, hard right turns have it up on one sponson, but gentle turns at speed, or just a little less throttle and it's settled. Jinking to avoid obstacles etc is OK so long as it's lefted again promptly. But it's the feel of the steering that's the thing. Without using exponential settings it rides the stick centre position very firmly, and there's a progressive bite when you turn it, almost as if exponential was dialled in. Right now I'm putting that down to the tunnels, as the combined area of vertical surfaces resisting side slip on the inside of the turn, as you pour on more turn the hull sides make the boat ride higher at the stern, reducing the depth of the vertical surfaces, hence the resistance to side slip. So it kind of rails in wide turns and slides a little in tight ones and proportional variation between the two.
So a happy accident overall really, back to the twins where some re-drawing is going on as a result of this
. If I modify this boat soon I'll put the updates somewhere else and leave this thread to the original concept from hereon.