Water Dynamic Hull Form

Which hull stern is more water dynamic (has less drag) in a forward motion, taking into account the anti foul line is about 50mm above the water line?

 

They are as bad as each other....not very good.

But the lower image is better when going VERYYYYY slowly.

 

Gawd, hard to get a clear sense of what we are looking at there. Any more pix ?

 

And at what speed ? The top pic looks like it might be intended to plane - but without more info as Mr E said, hard to make a good guess.

 

John is correct, both pretty much suck.

The term "efficiency" is quite relative to a number of factors. What are your target WOT and cruise speeds?

 

Sorry forgot to mention, non planing full displacement hull average speed 8 knts.

 

It's Bauhaus applied to boat design!

As Ad Hoc and PAR said, none of the above forms are really efficient at low displacement speeds, the biggest problem being the high wetted surface area and the transom stern.
But I guess your bauhaus was probably not intended for voyages anyways, but rather for short trips from one (more or less) permanent mooring to another.

Cheers

 

The one with the anti-fouling certainly would have went better with a shallower angle of departure, or a curved exit, than what looks to be at least 30 degrees at the stern, but pottering around at sedate speeds should be OK. I'd be more concerned about strong headwinds, which should see you doing 2 knots backwards !

 

For the purpose of that craft, both cutaways are a waste of time and take away useful buoyancy. Also for draft considerations, mounting the engines just inside of each hull would accommodate.

 

The cutaway idea is sound enough, but not the way it was executed. There is just too much turbulence with that abrupt change, which also gives the prop "bad" water to be working in. I don't think the loss of bouyancy is an issue, the thing is clearly not going to plane with the kind of installed power it has.

 

Thanks for the feedback Mr Eff. so if the transition for keel to transom was greater and it came up to the water line or slightly higher, this would give less turbulence yes?

 

I wouldn't be so concerned about where the sloped section meets the transom, a little submergence of the transom won't be too big a drag, and you need the prop underwater, at least where it is, otherwise it could leave the water at times. It seems to me that the transition from a flat, level bottom is too abrupt. It would have been better to start rising, preferably in a smooth arc, from somewhat further forward. There looks to be a penalty in hull drag, as well as having your prop working in water being dragged along by the boat, which is not ideal at all.

 

Both transition types suck at the speeds you'll be traveling at. The stepped shape being slightly worse then the abrupt tapered shape. Considering what this vessel is, there's very little you can do to improve over all efficiency. Even if you elected to carve up the bottom aft half of each pontoon and reshape them to a more efficient form, there's so much windage, you'll not see much more then a fraction of a knot improvement. In other words, a well done set of pontoons might get you from 8 to 8.1 or 8.2 knots. If this is what you want, then pull out the reciprocating saw and have at it.

The question at this point would be, "what are you hoping to improve?" With some pontoon shaping, you might get a very modest speed improvement (lots of cutting, very little speed increase) or you could improve your fuel efficiency a tad, but again, the pontoons are just part of the equation.

For example; if you took the tapered section and started this taper several feet forward of it's current position, you might see this very modest improvement.

 

Thanks for the input.

 

Consider that when a hull moves forward it displace water, then as the hull passes, the water has to come back into place. The less wake there is the less drag, the flowing out and the flowing back of the water should be as little as possible and with as little as possible movement of the water itself. Every ounce of water you move takes energy to do.

You compromise a lot in any way you look, wrt flotation, speed, hull interior, comsumption etc. Nothing is perfect. If there was one perfect solution eeeeeveryone would be using it !