flow of water pass transom question

[choppy]

Are there any websites that would have diagrams showing the different ways water will flow pass different types of transoms?

Thanks,
choppy

 

[DCockey]

Qualitative Investigation of Transom Stern Flow Ventilation by Maki, Troesch and Beck discusses different flow regimes past a transom as a function of Froude number based on still water transom immersion depth.

What are you expecting when you say "different ways water will flow pass different types of transoms"?

 

[choppy]

related to creating an eddy or drag

Thanks,
choppy

 

[PAR]

Choppy, it might be more advantageous if you described more completely what you are trying to find out, possibly establishing some basic parameters. In other words, there are so many possably variables, that offering much more then a generic "you'll get bubbles" is all you can expect from such a broad based question. Transom shape in profile, plan, immersion and hull form, Froude number, etc. will narrow down the possible offerings from discussion participants.

 

[eyschulman]

Choppy JayBenford in his book Small Ships has a nice simplified write up on pg 36 about the effects of different stern configurations.The article is titaled The Hight Of Fuelishness

 

[cyclops2]

In the Saint Lawrence River we have possibly every shaped hull going by from small freighters to ocean goers.
Same for pleasure boats.

Problem is, the ones with no wake crests along side the hull. They look harmless. They are the ones that create MASSIVE SUNAMI waves when the more powerfull underwater pressure wave hits a ledge 10 feet down. 1/2 to 1 mile away. Almost sank my 19' bowrider. Head on.

 

[Easy Rider]

Choppy,
For boats like the Grand Banks I've always thought it was stupid to have a highly submerged transom like a planing hull and then run the boat at 7 knots where a full displacement hull would be the obvious best design but in many conversations I have to admit the difference in efficiency is'nt as great as I had once thought. I'd still want my old GB 36 with a full disp stern and think it is the only correct stern for the boat w a single 120 hp engine. But I have an idea now why the flat and submerged transom is'nt as bad as I thought it should be. On a fairly steep mountain wind on the surface can be traveling up slope on both sides. A north wind will cause an up slope wind on the north face to be sure but on the lee side of the mountain a rotor wind frequently occurs. The air going over the mountain goes up and then back down. Part of the air goes south and part of that air goes back toward the mountain and up the south face. This "reverse" wind is called a rotor and that does (I think) happen at the stern of a planing hull w a flat submerged transom. You can see the water over the stern looks like it's in a washing machine. Evidence of lots of turbulence and the obvious drag associated with it. However at a speed considerably less than planing speed a rotor may exist similar to the mountain example. The water over the stern looks like it's jumping up and down more than rotoring but the rotor effect could be taking place and to some degree pushing the boat forward. Other than the eco's in my head I've never heard of any such thing but consider it a possible explanation why the efficiency of a planing hull at disp speeds is'nt as bad as I think it should be.

 

[PAR]

Most of the boats that you are describing need a really wide stern, just to hold up the aft cabin, washer/drier combo and twin diesels. They must have this bearing area aft or they point their bows to the clouds. You can "tuck up" the buttocks aft to make better flow, but this generally requires space restrictions in the stern (bye bye aft cabin headroom) and more length for the same internal volume. Since slip fees are based on length, every effort is made to pack 10 pounds of boob in a 5 pound bra.

 

[choppy]

Qualitative Investigation of Transom Stern Flow Ventilation by Maki, Troesch and Beck discusses different flow regimes past a transom as a function of Froude number based on still water transom immersion depth.

What are you expecting when you say "different ways water will flow pass different types of transoms"?

Thank you for taking the time to post. Those photos were really helpful. Thank you for providing the link.

choppy

 

[choppy]

Choppy,
For boats like the Grand Banks I've always thought it was stupid to have a highly submerged transom like a planing hull and then run the boat at 7 knots where a full displacement hull would be the obvious best design but in many conversations I have to admit the difference in efficiency is'nt as great as I had once thought. I'd still want my old GB 36 with a full disp stern and think it is the only correct stern for the boat w a single 120 hp engine. But I have an idea now why the flat and submerged transom is'nt as bad as I thought it should be. On a fairly steep mountain wind on the surface can be traveling up slope on both sides. A north wind will cause an up slope wind on the north face to be sure but on the lee side of the mountain a rotor wind frequently occurs. The air going over the mountain goes up and then back down. Part of the air goes south and part of that air goes back toward the mountain and up the south face. This "reverse" wind is called a rotor and that does (I think) happen at the stern of a planing hull w a flat submerged transom. You can see the water over the stern looks like it's in a washing machine. Evidence of lots of turbulence and the obvious drag associated with it. However at a speed considerably less than planing speed a rotor may exist similar to the mountain example. The water over the stern looks like it's jumping up and down more than rotoring but the rotor effect could be taking place and to some degree pushing the boat forward. Other than the eco's in my head I've never heard of any such thing but consider it a possible explanation why the efficiency of a planing hull at disp speeds is'nt as bad as I think it should be.

Sorry that it took me so long to reply. Thank you for taking the time to write down your thoughts. Your reply has made me feel better about my design. You have described my design exactly. My first goal is a boat that easily planes but I don't want it to be a "dog" when moving under planing speed.

Thanks,
choppy

choppy

 

[choppy]

Qualitative Investigation of Transom Stern Flow Ventilation by Maki, Troesch and Beck discusses different flow regimes past a transom as a function of Froude number based on still water transom immersion depth.

What are you expecting when you say "different ways water will flow pass different types of transoms"?

Wow! Those photos are really interesting. The more I look at them the more stern ideas start to pop in my head. Would you know of any other sites that would show additional photos and stern designs?

Thanks for all the help,
choppy

 

[FAST FRED]

My first goal is a boat that easily planes but I don't want it to be a "dog" when moving under planing speed.

Then don't immerse the transom.

 

[Ad Hoc]

Problem is, the ones with no wake crests along side the hull. They look harmless. They are the ones that create MASSIVE SUNAMI waves when the more powerfull underwater pressure wave hits a ledge 10 feet down. 1/2 to 1 mile away. Almost sank my 19' bowrider. Head on.

It is all about the energy of the wave system they produce, not a single amplitude wave,per se.

Those boats that PAR mentioned are all low Length Displacement ratio...terrible. Very high residuary resistance....as noted by their wash.

 

[Yobarnacle]

No wave crests along the hull? Answer is obvious. There are 3 types of resistance. The greatest is wave making created along the hull moving through water. The faster she goes, the further apart the crestlets get stretched. Eventually at some speed, there is one crest at the bow and another at the stern. This is hull speed and longer the waterline the higher velocity when reaches hull speed. To exceed hull speed means driving the vessel up hill over its own bow wave. Surfing on the face of its own bow wave is called planing. The vessels you see with no crests alongside are at maximum speed, hull speed. Any additional power they are applying gets dumped into a wake forming turbulent pressure wave astern. You are observing bad handling and ignorance on part of whoever is conning vessel. Notifying their company will get them educated. They are also wasting fuel ie: money. Companies disapprove of wasting money.

 

[PAR]

What models did I mention?