Sea Sled madness. It’s in my brain.

That's a nice piece of kit. If I go that way, it will be a jack plate, I'm thinking. Engine is deep enough it's plowing part of the midbody through the water, floods the back deck, makes lots of drag at the fastest I've managed: 32 knots/60 klicks. But a few inches higher and the prop is half in aerated water, spins wildly. Sigh...

I don't think I'll modify the hull, just yet. Already too much displacement aft. I'm not ready to go as far as @Ad Hoc suggests. No facility anyway. Just the rocky shore and the tidal grid.

As far as wetted surface drag goes, perhaps I could do something with the upper edges of the non-tripping chines. Those chines have about a 2.5cm/1" radius. Water rides up them pretty high.

 

A pleasant hour spent in Dave Gerr's Propeller Handbook gives me a 15½ by 18" three bladed propeller of moderate form. Mostly a coincidence that that is exactly what I've ordered. I've learned enough humility.... No, probably not enough. More. Some. Safe to say more than I had. No better than an educated guess, based on Serenity's behavior around Vmax. Given typical slip for a boat like this, probably a top speed of 35knots. Might get little better after lots of tweaking. Next fall, with better budget, a sophisticated SS prop. But the current prop has 38% slip at WOT and that's dismal. Improvement on that won't be hard.

I also came to the 35 knots number by another means. I looked at the actual figures for a TX18 at specific powers and weights. Used an improved version of a P/W/V relationship formula to back out a hull form constant. Penalized that by the increase in drag due to larger windage, and recalculated. Also gave 35 knots. Not nearly as nice as Hickman and Crouch led me to believe (52 knots) but is useful for a first iteration to pick a next prop. That new data point will tell me what the one after that should be.

 

I had suggested constructing a sharp leading edge shape on the leg extension as some high speed boats use on the lower leg and it got me to thinking as to how I might make it.

Question: Is the forward radius the same over the length of the extension?
If so, then an easy way might be take a firm piece of foam, drill the appropriate diameter in it and add a bit more diameter to allow for a few layers of cloth and resin, then cut through the diameter, which would then correspond to the leading edge of the leg extension. With this block if foam you could then shape it to provide a sharp leading edge, encase it in glass etc.
I am not sure how they attach the nose cones on the lower leg but perhaps a glue may work.

If the forward radius is not the same. You could build a tub around the leading edge and pour in a casting wax, shape the piece to give it a sharp leading edge and send it to one of the aluminum foundries in Van and have them make you an aluminum extension shaped the way that you want it.

The method of attaching it to the extension would then be with an adhesive, or removal of the extension and tig weld it.

 

Gotcha. I don't think it will be a big problem to lose the rounded front edge. It's not like I steer much at high speed, so flow separation isn't an issue.

Ironic to be using mods meant for high speed, when I'm stuck at low speed. 32 knots is hardly impressive. Although it feels fast to me. I've spent the last 8 months at 5 knots. 32 would be fine, if the fuel consumption wasn't so dreadful.

Edit: yes, the extension is almost exactly NACA 0024 bottom to top.

 

As someone pointed out, there is a meaningful amount of side wetting, and therefore a significant contribution to wetted surface drag. It seems well worth the effort to sharpen the upper chines at each hull side. Currently they have very large radii, and water is drawn well up the hull. Under some conditions - running very light, with all load in the forward cockpit such that trim angle=~0° - water was up about 60cm on the hull sides.

So, is there an optimal approach? A simple building up with thickened epoxy, or are there gains to be had with more elaborate spray rail?

 

It seems spray rails will help reduce our excessive wetted surface drag... Some. I'm not expecting a miraculous transformation into a Formula One racer, but improvement is possible.

The papers I've read to this point are leading me to think that rails 5/4" 32mm wide, 8° angled up would be best. A little tough to manage in a window between tides. Maybe an aluminum extrusion.

Of course spray rails increase drag at low froude numbers.

Why didn't anyone tell me engineering is a long series of compromises.

In less compromising news I need to design and build a seat. Something that changes from standing to sitting and back quickly and easily. Because my upper body will be in the same place in either position, it can't have a base. So it needs to cantilever from the hull side, or a post behind the pilot's position.

 

I think what you are saying is you want the eye level to be no less seated than standing. The only way I know to so that is with a step. There is about 12-14" of foot space, then a narrow step 6"? wide. And in my case, the seat got too high and the step is not high enough for a foot rest.

This thing is even more compromises.. A long term project is a footrest that rises up 6" or so, but because the locker door, can't flop down.. My wheel is also low for seated position, but the seat can move forward and the boat can run on a remote.. I gained a few extra inches adding a slide and a bucket seat..

Very frustrating because I spent a LOT of moaning chair time on it.

 

Many compromises. I want to stand at a comfortable distance from the wheel, or sit at the same distance.

 

My chair is on Garelick slides. It is actually higher eye position than standing due to adding them.

 

Here is what I have, but used 2 sets for double locking. You can just get a post slide type.

Garelick® 74003-G - 24" Single Lock Tandem Fore & Aft Seat Slide https://www.boatid.com/garelick/24-single-lock-tandem-fore-aft-seat-slide-mpn-74003-g.html

 

This thing for a post slides 7" and swivels.

Garelick Standard Series Seat Slide and Swivel 75083 https://www.wholesalemarine.com/garelick-standard-series-seat-slide-and-swivel-75083/

 

Not to be too simplistic, but it sounds like what you want is a flip up chair. These are common on center consoles where people like to go out at high speed and fish while standing.

if you pair that with a seat slide you should have what you want provided you mount it at the correct height.

Springfield Twin 46 Flip-Up Chair, White https://www.overtons.com/springfield-twin-46-flip-up-chair-white-316833.html?utm_source=google&utm_medium=ppc&utm_campaign=%7BCampaign%7D&gclid=CjwKCAjwpayjBhAnEiwA-7enawXPcHhW6KYRIEH7oB7y0yMHGTYQsWhKWIEQHeGFopeQAmi4zCTuOBoCAe8QAvD_BwE&gclsrc=aw.ds

 

Looks great!

 

First interim prop at the courier. Our turn back on the grid soon.

Along with the larger prop - 14⅞*17" going to 15½*18" 378mm*432mm going to 394* 457mm
I'll start making sharp chine edges where large radius chines and intersections exist. Not expecting huge gains like Hickman's ad copy promised, but there's always room for improvement.

Reality check please.
@baeckmo was kind enough to point out that I had been using the wrong formula to describe power to weight vs performance relationships. I had been incorrectly using the Crouch formula, which is:
Vmax=C*(power/weight)^0.5 where C is an empirical value based on observations of as many relevant boats as possible.
The correct relationship is:
Vmax=C*((power/(weight^0.833))^0.5

There are too few existing sea sleds for me to make a meaningful range of values, but there is the TX18, a reasonable number of which were built over many years. Enough at least to suggest that the reported speed based on given speed and power should be at least close to correct. If it was nonsense it certainly would have been mentioned. I reproduce the predicted values here.
At 2000#/910kg total weight/mass
30hp 28mph 45km/hr
40hp 32 mph 52km/hr
50hp 36 mph 58km/hr
At roughly twice the p/w ratio -
5000#/250hp our best speed was about 36/58 when the correct formula suggest about 53% more speed. The TX18 is extremely similar in proportion and presumably wetted surface drag. In fact the non tripping chines on Serenity are a close copy, because I bought a set of plans to see them. Even allowing for dramatically increased wind resistance it still looks like I did a really bad job on this boat. (I deleted a much saltier description).

When I can spare the time from applying the scourge to my back, I'll work on tracking down particular areas of failure starting with the above. 2007, Savitsky DeLorme Datla: whisker spray drag can be as much as 15% of overall drag.

 

I would guesstimate the sources and apply a percentage to each on a spreadsheet.

I've been curious about things for you; I've understood 1-2% loss in speed for sharp transom edge, for one.

And, of course, the roughness of the bottom gets something...how much I've no credible idea..but people have written great papers on it. Perhaps some of the contributors here can parse it down if you tell them the grit rating of the bottom.

https://www.usna.edu/NAOE/_files/documents/Faculty/schultz/Flack, Schultz - Hydraulic Roughness Scales, 2010.pdf

Then windage...again no idea, but probably some way to account for a percentage of loss to windage of the cabin for the PNW angle and cabin height.

And I don't know how prop slip works, but my hunch is that prop slip could be used for a check against the causes.

Then if you can't achieve wot; that is a fairly easy estimate, etc. Etc.

I had a goal with the Wood's boat. It was 20 knots and we achieved it. My boat, also a prototype; has other problems to resolve.

I've watched the build, witnessed the challenges. It is not easy to build a boat; harder without climate controls, in a flooded rainforest tent..

If you compression check the motor; you may even find a percentage there..

my 2 pfennigs