Water flow to the prop

BillD

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Hello ALL,

I know that "shaft tubes" and dry keels are the norm these days for shaft install in new builds and retro fits..
I understand about "good water flow" to the prop etc.

I have a question about the "old fashion" :D way with a wet keel and stern post (if that's what it's called).

Look @ pic #1. With this setup is there "good water flow" down the keel to the prop? Is this set "a drag"...no pun intended? ;)

Would ditching the bronze stern post and having a shaft tube as in pic #2 add to prop efficiency and some speed?


Curious,

IMG_0839.jpg

IMG_0608.jpg
 

F/V First Team

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Of course. Stick your hand out the window while on the highway, when your hand is flat it slices through the wind. Now start rotating your hand like a big bulky bronze obstruction...
 

Raider Ronnie

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greg

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Dumb question, but how would you go from the first picture to the second? That looks like major surgery to the keel.....
 

greg

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2 different boats there.
2nd pic appears to be Plowin's 33 Flowers

Yeah, I saw that, but read the op as wanting to convert from one to the other.

You could have told me that first pic was from my boat and I'd have believed you. But I still have a glass rudder, so I have bigger issues to contend with over time.
 

BillD

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What are we talking for speed reduction here with the big bulky bronze obstruction?

6-7 knots versus a shaft tube ??:rolleyes:

The big bulky bronze obstruction is perfectly aligned with the keel.

It's not like it rotates with the shaft. :D

How much speed we talking here with the bulky bronze obstruction vs a nice feathered shaft tube?
 

djmarchand

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Mainship Pilot 34 before and after fairing

The following pics show a Mainship Pilot 34 with a flanged stern tube before and after it was faired and the corners rounded off of the prop pocket. The owner reported that the annoying rumble went away and the boat picked up a knot or so.

Mainship Pilot 34 stern tube before 1 small.jpg

Mainship Pilot 34 stern tube faired small.jpg
 

tunafishhkg

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This has been asked a few times if its worth the trouble and cost to modify both as wet keel vs dry and this square cutlass wide keel vs clean water M type shaped keel. I guess there is no one answer or real world example from someone who has changed from wet keel square shaft cutlass to dry tube. The only accurate comparison would be from someone who went from one to the other on the same boat though of course different boats would more than likely have different stats especially since HP and speed affects are not linear. Of course the physics are there and if you are rebuilding or building new than science would more than likely dictate design unless you have a reason or feel you need to disregard physics.
 

tunafishhkg

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The following pics show a Mainship Pilot 34 with a flanged stern tube before and after it was faired and the corners rounded off of the prop pocket. The owner reported that the annoying rumble went away and the boat picked up a knot or so.

I wonder if the rumble went away due to prop clearance rather than clean water since it looks like he dropped the prop down. Now speed that should be due to clean water thanks for the info.
 

traditions

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I took a square box off and glassed a 2 foot tube with a cutlass bearing in its place.I didn't notice any change.I didn't do the swap to gain speed,I did it to do away with an old bearing holder that was not right from day one.The box is in front of the hub of the wheel,and the hub is turning slow and isn't grabbing any water. The outer 2/3 of the wheel does all the work.This may have more affect on a higher speed vessel,but on my 10 knot boat it made no noticeable change.
 

Blitzen

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Traditions I think you hit the nail on the head. The real test is to see if your wot rpm changes from before and after to check if it made any difference. If it worked I would suspect you would see a decrease in rpm due to more water loading the prop.
 
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