Sea Sled madness. It’s in my brain.

You may want to review your text above and the instruction regarding the use of loc-tite. (second attachment)

 

Overfocused again! No locktite on studs! Well, I have some around anyway. Guardian angels...

 

Went for another run. 42km/hr at 5400 rpm.

So here's the interesting or perplexing thing. Today the water is very calm, so the tunnel isn't ingesting much air. Under more typical conditions the water is too aerated for higher power loadings because the hydraulic sink pulls the lower density water in, and the prop races wildly. But the onset of this condition occurs later if I trim the leg in, the further in, the later. It seems to me counter intuitive because the axis of the prop is pointing up into the aerated water when trimmed in. If I trim it out to (supposedly) pull solid water from deeper, the prop breaks loose much sooner.

Any insight, fluid dynamics people?

I still hope to get the leg extension on tomorrow, but hydrodynamics are intrinsically interesting.

@baeckmo , you asked about steering. Very positive, quite moderate steering forces. Rolled inward with the turns about 10°, which was much more pleasant than turning flat, or rolling outward. I didn't do any extreme turning forces, because that seemed like a terrible idea. I think with the longer leg, rolling will be increased with the longer lever arm.

Wake trivial, pleasantly surprised to find this is true. Significantly less wake than from our 5m commuter at 28km/hr, ⅓ the mass, 20% of the power.

 

More info.
Hull trimmed itself to 3.5°

Ran over another wake at 45° . Barely perceptible

 

And remember, the surfaces you're loctite-ing must be 'clean' -no factory grease.
It doesn't say, but you may want to give them a call whether they recommend blue or red.
If they are undecided, I'd go with blue on clean surfaces (shift shaft only!)

 

You are trucking right along! Interesting that nothing was done, but you are getting these speeds from engine issues?

No expertise; just opinions.

Well, trim up and the bow lifts. Trim down and the bow drops. But then, I agree, intuition would suggest your hulls would be creating more aearation, not less, when trimmed in. So, I don't find your statement that this is counterintuitive wrong.

However, the water is likely less disturbed closer to the bow tunnel. So, our intuition goes the other way when we consider the engine closer and closer to the tunnel, no? Like, wild case, a bow mounted engine would always be in good water, but for the planing and mounting business (haha).

Also, with the bow up, perhaps the two bows(may I?) are in the water just enough to create bow wakes. But then intuition slaps us and we realize trimmed down would be even worse; so bow wakes are not the story.

Another thing that can happen with bow up is the boat actually starts to ride over its own wake; more or less and can be anything from vivid to subtle. Perhaps your boat trimmed up is doing some variation of porpoising where the bow is lifting a wee bit and then rising in the back up into crap water. Trimmed down; it would be less likely to do so. If this is the case; that leg extension should do the trick. I recall a guy awhile back recommending you use a plate on that boat if you need to tweak the engine height. 5" might be too much

My hunch is that you are experiencing some form of porpoising or bow wake riding, but it is too subtle to recognize. And the lift you get is just enough to put the prop in trash water.

Most boats really show porpoising on flat water best. So my guess is some subtle form of porpoising.

Maybe we get to hear true wisdom from @baeckmo .

 

Well, certainly not porpoising. Hull trim.is absolutely stable.

 

42km/hr is disappointing at that rpm. But she got up to that speed in seconds, so probably under propped.
VMG is 56% of theoretical prop advance. That's a lot of slip.

 

Well, then prop depth is just low enough trimmed in vs trimmed up and your extension will help tons, if not too deep.

 

Can you share your math for a dummy? I want to check my boat at some point. Is this captain's formula?

 

Positive or negative?

 

If you look at the hinge point of the leg and draw a straight line to the prop center, you find that the prop disc is moving down with in-trim, ie into less aerated fluid zone. When trimming out, the AV plate will have a negative AoA; this is the "normal" position with a light hull, where you can run with the screw semi-ventilated. This is not possible with your boat. I'd still preferred to see a decently cupped prop on that rig first. Aaand, of course a reliable weight figure.....

Afraid your perception about the beauty of "zero-wake" is pointing your nose in the wrong direction. The minimum drag comes with the combined optimum of friction and "trim" resistance. Having exceptionally low wake means running with too low AoA, leading to too large wetted area and too low dynamic lift. To make things worse for Serenity, that wetted surface is comparatively rough.

 

3.5° positive trim.

 

@baeckmo , you are a most excellent teacher! Of course the prop moves downwards as it trims in. It's perfectly obvious after you told us.
With the leg extension I can optimize trim for flow, instead of for aeration percentage. And then another prop, with significant cup.

For a hull of this form, what hull trim angle should I be looking for?

Wakes are a big issue around here. Wake damage is much discussed

 

Or pound some cup into what you have, if it's the size and configuration you want.

Whatever gives the best speed and handling, no?