Shafts thru the transom?

[tom kane]

So..what do we use to push a boat up to 40 + tom kane.

[gonzo]

Surface drives become efficient at that speed. Jet drives loose more power but are great in shallow water.

[tom kane]

If our boat is equiped only with S/P props we still have to get up to their efficient operating range.Say 30 Knots?Actually S/P props
have been used by their orininator and immersed at less than
50 percent on flat transoms tight up against way back.Speeds
of 40+.draught about two feet.tom kane.

[gonzo]

The efficient speed range is over 40 knots. What do you mean by "immersed at less than 50 percent on flat transoms tight up against way back."?

[tom kane]

I am sorry Gonzo,I was assuming from your previous posts that you had done research on the early development of S/P propellers
WAY BACK in the 1914s.I was refering to Albert Hickman,s sea sledges fitted with S/P props on shafts through the transom,and close up to it. POWER BOAT SPEED by KEVIN DESMOND.RACING and RECORD BREAKING.1897 to the present.This is a very interesting book with great pictures and information.CONWAY MARITIME PRESS.I have put the question of pushing our boat up to the efficient speed for S/P propellers into the Too-Hard Basket
for further research.Tom Kane.

[gonzo]

The Sea Sled is very different from a vee bottom boat. Also, the same design with a modern outboard is faster. Those early propellers were not good by modern standards.

[tom kane]

Sea sledges are certainly easier to push than modern deep v.
I was pointing out that the position of the propellers were very
close to the transom.These S/P props had low HP available
and plenty of weight.Speeds were pretty.. good.draught was shallow.Draught on modern deep V takes the props down deep.
Is it still surface drive?Thanks tom kane.

[gonzo]

The Sea Sleds were very light. That cause a lot of structural problems like cracked frames. The hull desing is completely different from a vee hull, so the water flow is different too. Speeds were pretty good for the times, however we are talking 21st century. A submerged propeller is not a surface drive. A true surface drive has only the lower blades in the water and the others totally ventilated.

[tom kane]

What can we call a propeller or propulsion system that will
operate as a surface drive,and as a subsurface drive?We have
all the data and computer models for both systems,and know how they will perform.May-be surface drive for the mass is
available but we do not know it.Thanks for your time .tom kane.

[gonzo]

What do you call and subsurface drive? If you mean a completely submerged propeller, a surface piercing propeller starts totally submerged. That is why they need 40+knots to become efficient.
They are different systems. Each has an operating range. These ranges overlap. Both designs have good and bad points. However, you need to pick one and set up the system to it. In other words, half of one and half of the other only get the worse.

[Willallison]

What Gonzo is saying is correct. A propellor designed to run submerged won't operate efficiently as a surface-piercing prop and vica versa. Sure it's possible to design a drive system that can lower or raise the prop so that it will run submerged or s/piercing - but it can only be efficient in one of these modes.

[dougfrolich]

I will post more on this- Think about the effect of a two speed, three speed, etc... reduction. Plus super-cavitating prop. Think about it! hint- U.T.

[gonzo]

Two speed transmissions work well. They give high torque to get up on plane and good top end speed at lower RPMs. They are heavier and more expensive.

[Willallison]

...and still won't make a s/piercing prop efficient whilst it's submerged....

[dougfrolich]

...But it will make a super cavitating prop. efficient when submerged, and then, transition to surface piercing mode at speed