Hickman Sea Sled Information

[BlueBell]

That is some painful porpoising. I wonder if anyone told him to trim his outboard in.

It's almost comical.
It appears it got worse forcing de-throttling and starting anew.
I agree, trim and perhaps prop depth, or both.

 

[DogCavalry]

Oh
My
God
!!!

Look at figure 6 in the PDF. Hickman invented double differential steering in 1911, and didn't think anything of it. That was introduced in tanks 30 years later, and it was revolutionary.

 

[Mr Efficiency]

I happened on this by accident, not sure it is already known to the sled fanatics here !
invertedvboats.freeservers.com/index.html

 

[DogCavalry]

I saw it years ago, also by accident, and lost track of it. Thanks for bringing it into the thread Mr. Efficiency! Some great stuff there.

Mention of a 6.5m original sled with 24" prop.

 

[DogCavalry]

With regards to porpoising.

Extensive conversations with several builders and operators relate that the sea sled can not be operated bow high. Run that way it will have a tendency to porpoise. Marcus Lee's first sled porpoised brutally it's first morning. He had built a plumb transom. Welded on a new plate, giving the engines space to trim in. Problem solved that day.

 

[Air Flite]

With regards to porpoising.

Extensive conversations with several builders and operators relate that the sea sled can not be operated bow high. Run that way it will have a tendency to porpoise. Marcus Lee's first sled porpoised brutally it's first morning. He had built a plumb transom. Welded on a new plate, giving the engines space to trim in. Problem solved that day.

I experienced some light to moderate porpoising with both of my boats (18' and 27') at 35+ mph. Controllable with engine trim and, at WOT, some trim tab. The downside, of course, is some loss of speed and fuel economy. My 27' needed more trim in smooth water and at WOT would only top out at 48 mph in that condition, yet would hit 56 mph heading into a 2' chop.

 

[Air Flite]

I experienced some light to moderate porpoising with both of my boats (18' and 27') at 35+ mph. Controllable with engine trim and, at WOT, some trim tab. The downside, of course, is some loss of speed and fuel economy. My 27' needed more trim in smooth water and at WOT would only top out at 48 mph in that condition, yet would hit 56 mph heading into a 2' chop.

 

[BlueBell]

Air Flite,

How did you determine the outboard positions and spacing in those photos?

BB

 

[Air Flite]

Good Question BB. The engines were mounted at the shop of the company which manufactured the hydraulic jack plates I selected, that allowed for a 6" vertical adjustment. Bob's Machine Shop of Tampa, Florida also modified the lower units with low water pick-ups which allowed for an optimum running height at speed, reducing drag and enhancing an already impressive shallow water capability.

Like the 18', the 27' retained some invert all of the way to the transom, so the center line of each motor was lined up with the inner edge of each keel flat. Initially, we tried to do without a tie bar to ensure proper engine alignment, by setting up the hydraulic steering in a certain manner. This worked, but not well enough, so we ultimately installed a bar.

Overall, it all worked quite well and I would do it again.

 

[DogCavalry]

More of Robint777's boat.

 

[Mr Efficiency]

Those latest pics, DC, suggest that the outboard leg is running pretty deep, unless the photo is deceiving, it has the kind of leg that might allow that, with another splash plate about 5" higher than the cavitation plate. I'd prefer a leg like that, if there is doubt about the prop getting clean water. Some legs get somewhat less streamlined, not far up from the cav plate.

 

[Air Flite]

More of Robint777's boat.

Thanks DC!!!

 

[Air Flite]

Those latest pics, DC, suggest that the outboard leg is running pretty deep, unless the photo is deceiving, it has the kind of leg that might allow that, with another splash plate about 5" higher than the cavitation plate. I'd prefer a leg like that, if there is doubt about the prop getting clean water. Some legs get somewhat less streamlined, not far up from the cav plate.

Mr. E - Take a close look at the lower units of the Mercurys on my 27' boat in the pic attached to my previous post. You will note that there is a device attached to the cavitation plates which greatly expands their size. It was 26 years ago and my memory is a bit dim but I think it may have been called The Manta. This device reduced rooster tail spray and provided lift when the engines were trimmed in tight.

Note in the pictures showing the 27' running at speed there is very little spray. In the longitudinal profile the boat is moving at cruise speed of 35-38 mph with almost no spray.

 

[Air Flite]

https://youtu.be/xx4yjLhE_O4

https://youtu.be/ZdQBMRsoVNM

VIDEOS OF 18 & 27 - Will explain later

 

[Air Flite]

Those latest pics, DC, suggest that the outboard leg is running pretty deep, unless the photo is deceiving, it has the kind of leg that might allow that, with another splash plate about 5" higher than the cavitation plate. I'd prefer a leg like that, if there is doubt about the prop getting clean water. Some legs get somewhat less streamlined, not far up from the cav plate.

Mr. E - Here is some 25+ years old video footage of the "children" . Now wish that I had made an effort at the time to produce a higher quality video product:

https://youtu.be/xx4yjLhE_O4

https://youtu.be/ZdQBMRsoVNM