Sea Sled madness. It’s in my brain.

I have more questions than answers, but I'd try the angle on the hull after squaring off the transom first. It is painless and easy to fix.

See what the expert advises. I'm a builder; kind of..

 

Confusion Abounds

With my lowered expectations for top speed, I'm researching what a more realistic target speed is, for a number of reasons. I need to know what my mileage/speed curve looks like, for logistical planning. I also need the number to pick a final prop.

Brief aside: the larger prop is quite noticeable in effect. The main change is a large improvement in control response at idle to fast idle rpms. Going into gear elicits a solid thump, with immediate movement. Before it take a moment to tell if it was in gear or not. No Vmax runs yet though. Got to solve the firehose issue first.

Anyhoo... I'm looking at what percentage of drag is wetted surface drag vs induced drag vs wind resistance. And any other sea sleds with good speed/displacement/power numbers. A little thin on the ground unfortunately, but there are a few. The Crouch formula, and the updated formula (thanks @baeckmo ) both use a constant C, which describes a specific group of similar boats. The original work by George Crouch, which Gerr repeats in The Propeller Handbook among other places, gives a C for the sea sled that is absolutely stupendous. Backing that number out from actual data gives a C that is quite good, but not outlandish. A good fast boat, but not a racing boat. Using those precedents I'm looking for about 37knots, or about 20% faster than I've managed. Improvements to be made in:
Side wetting - spray rails
Surface roughness - more fairing, good antifouling paint
Prop leg drag - a leading edge fairing on the OB leg above the antiventilation plate.

Here's the confusion though. In researching the drag components I came across this paper.
It says, with graphs, that the inverted V has the best L/R ratio of any form. Not just good, but best, by a large margin. The forms they were looking at were the prismatic forms, where the tunnel is carried to the transom, as suggested by @baeckmo . And the superiority is even greater when spray rails prevent side wetting. That sort of supreme speed isn't what I'm seeing, nice though it would have been. But the confusing part is they go on to say that the hull form pounds much worse than any other, except flat bottom boats, thus making them unsuitable for offshore work, and that's just utter BS. Contrary to the largest historical user, which were the US Army and Navy which used them almost exclusively for offshore work in their size range, for a couple decades, because they were the best in rough water. Also the opposite of my own experience, and that of every owner I've been able to talk to. Sometimes it's a bit frustrating.

Oh well. I still live in paradise, and Serenity is real, not just a daydream anymore.

 

Seymour Narrows. Reynolds Number - One Billion

 

Lowest tide I've seen, with strong winds. A bad combo. I tied up outside, exposed to it all. Not enough water to get inside to lee of the dock. Two neighbors having an exciting day. The tritoon is on her outboard. Unfortunately it was left down. That's not bad, is it?

 

I think this must be the boat with the wheelhouse in the far distance?
Is there mud under the stones on the foreshore? If so, maybe the leg of the outboard might sink into it a bit? But if it is very hard base material, and the leg cannot sink in, then there must be a lot of load on it (?)

 

So I got a nice piece of 3mm ⅛" aluminum treadplate to bend and cut into a fairing/diverter for the front of the outboard leg. I don't want to do anything permanent to the original components of the outboard, but I'm fine with drilling into the aluminum extrusion. Any suggestions on how to attach part A to part B? Here's a picture to refresh our memories.

 

I would go for the boat side first. Some risk of failure, but seems easier.

Or, can you remove this part and attach?



Sort of like this one

 

Some risk the h2o is hitting the trim mech first, though..

 

That part houses the lower bearing that supports the engine. Not keen to fuss with it. I was thinking rivets vs screws. Probably can't reliably glue it on.

 

What about just a knife edge pointed down?

Ad Hoc's idea from the BB; not mine..



top view; top edge

 

That's what the aluminum plate is for. But I need to attach it go it stays on at 37 knots

 

Can you screw it or rivet on the green dots?

It'll part the water; may need to seal it on the bottom.

 

I think it is intuitive to want to build a dam, but you only need to part the flow..

 

Where can one get plans for a sea sled? I'm very interested in building an 18' from sheathed ply.

 

I bought plans for the TX18 from bateau.com
@Darkzillicon bought his online. I drew my own plans, based on my own requirements.
Dave Gerr has plans for a 29' sled called the Interdictor. Those would be pricey.