Fiberglass shaft logs

[Capt.Frank]

Capt. Patrick,

I noticed in you how to plan, you leave the shaft logs stick below the hull with dripless bearings.
Last year a lot of the local charter boats (carolina boats) hauled and extended there shaft logs almost to the strut. They were claiming a 2-3 knt increase and better fuel burn. They say its less shaft drag in the water?
What do you thing?

Frank

[CaptPatrick]

Capt Frank,

Undoubtedly, rotational drag is far more detrimental to speed than linear drag... Fully encased shafts, from exit point to the strut will eliminate the rotational drag, changing it to linear drag.

As to how much benefit results depends greatly on the hull shape, weight, power source, gear ratio, and prop size. Vetus was the first, I think, to offer a kit for enclosing the shafts, (well engineered, but very expensive).

Jerry Wells and I studied enclosing the shafts on "Fortuna", but in the end, there didn't seem to be enough justification on the B31.

For larger, heavier boats with shafts above 2" in diameter, it made more sense...

Any extension, even as indicated in the "Tips Section", requires the addition of water injection & packless seals.

Br,

Patrick

[Bruce]

The last 31 I did I considered extending the shaft tube to the strut.
After running some numbers on expense vs additional gain, it wasn't worth it for the average boater.

If your fishing tournaments and staying out to the last moment and need the speed to get back in time to weigh in with big prize money at stake then I've seen some big dollars thrown at getting a knot or two more.

This is one reason Bass boats are approaching speeds of 90mph +.

[nic]

A Hunt design built here in 1971 had encased shafts that repeatedly broke until they re-engineered the casing to allow water-cooling around it. I remember being told the water became de-oxygenated was the final theory. After this fix there was never a problem.

Nic

[bob lico]

after careful consideration of all factors concerning drag from rotational forces i feel along with consultation with holtz boatworks experiance a 2" extension from the hull bottom along with water injection perform the best.i have no before and after results but in my opinion with a faring prior to the strut (using 6" cutless bearing with 1" sticking out toward the bow)in combo. with the glass shaftlog will give you 1/2 knot at wot.i also placed the shaft zinc to shed the water before it goes around the cutless bearing .i hope this helps.

[Capt.Frank]

I just felt it was intresting and wanted to ask everyone never crossed my mine.