Keel rocker aft for LDL powerboat

[huibes]

Why do you need to design a little more displacement around the midsection. Just move passengers and gear forward.

Sorry, with displacement I meant to create enough surface area in the sections around the midsection

 

[DCockey]

Sorry, with displacement I meant to create enough surface area in the sections around the midsection

Why do you think that design needs more surface area in the sections around midships? The boat in the photos does not have any passengers. Add passengers and it will sit lower in the water.

 

[huibes]

Because I calculated a required midsection area from the desired D/L and determined Cp?

 

[Mr Efficiency]

This question does highlight that the speed range beyond S/L 1.34, but not quite at a speed where any real planing action is felt, is a most difficult area, resistance tends to be high for the amount of forward progress being made. Which makes "minimal power" not enough for the job. Minimal power can be enough at sedate speeds in just about any kind of boat, even full planing boats have less resistance when crawling along wakeless, for the amount of progress being made, than at any other speed they can attain. That shows up in mpg figures over the speed range, very clearly.

 

[huibes]

What about the LDL designs of Nigel Irens? Seem to work very well to me...

Would there be any curvature in the aft buttocks?

 

[Mr Efficiency]

Can you point me to the boat you are thinking of ?

 

[huibes]

For example, Greta

Seahorse Magazine Article June 2017 - Supplement Information - Nigel Irens Design https://nigelirens.com/news-item/seahorse-magazine-article-june-2017-supplement-information/

 

[Mr Efficiency]

This old thread may interest you:

Low Displacement/length ratio boat designs https://www.boatdesign.net/threads/low-displacement-length-ratio-boat-designs.52659/

 

[huibes]

Thanks, think I've read this back then.

The mentioned Whio performs amazing as well. I recently found this video of it:

How to Build a Wooden Boat — AROHA, Part 1 - A Close Look at the Original Boat WHIO, a New Zealand Weekend Cruiser - OffCenterHarbor.com https://www.offcenterharbor.com/videos/dream-boat-whio-gas-sipping-new-zealand-camp-cruiser/

Sounds to me the designer/builder has read Skene's very well and combined the knowledge into the perfect boat for his application! Very impressive.

Although he does mention an efficiency of 1 lph @ 7,5 knots. Which would be more efficient than my car...

Anyways, back to my question. Peter explains the hull shape very nicely around the first minute of the video. Meanwhile the boat is pulled on the slipway, so you can follow the lines.

Peter mentions the bottom is dead flat aft. I assume he means zero deadrise But I can see some keel rocker. How does one determine how much this should be, in relation to speed?

 

[Mr Efficiency]

There are many ways to skin a cat, have you looked at the box-keel boats, as a way of getting some flexibility on the speed capability, but easily driven ?

 

[Mr Efficiency]

I think you have a limitation, with the alloy build, you are restricted to developable shapes more or less, so a lot of what you like the look of, might be out of reach.

 

[huibes]

Yes the box keel concept has intrigued me for over a decade. But when drawing lines, the flow of the aft diagonals -where the end of the box meets the planing area- confuses me.

Also, I see it more suitable for inboard than outboard. And with a DLR lower than 80, I feel there is not enough displacement to have a box.

 

[huibes]

My question now is:

How does one determine how much aft rocker a "fast displacement" boat should have?

Or where to find literature...

 

[Mr Efficiency]

You might be right about them suiting an inboard better, but I would think not ruled out for an outboard.

 

[huibes]

I think you have a limitation, with the alloy build, you are restricted to developable shapes more or less, so a lot of what you like the look of, might be out of reach.

For sure, but I might be able to get pretty close....its a challenge!