Standard Shaft Length

[John Nardi]

Hi all...I will be needing to cut my shafts down in upgrading to 1 1/2 shafts...I am thinking that I should do this now before the 496 and Velvet drive install and mounting as it will be impossible to remove the shaft with the boat on the hard and no lift available...Is 82 inches the standard consensus? Thanks

[Carl]

Its a boat, the standard is they are more or less the same, unless they are not.

Its one of those measurements I would not assume as motor may be mounted a bit forward on some, not so much on others.
Some struts go here...others a little more there.


What that means...you have four chances, too long, too short, just right and it will have to be good enough.

-Just right is great.

-It will have to be good enough works...might be too much overhand or wheel too close to strut, but the boat runs ok..for now.

-Too long can be cut and keyway extended for short money

-Too short, hopefully you can find a buyer for a short shaft.


Sometimes in the very same boat the two shafts can be different lengths.



Think I said it every way I can think of...
It's one of those things I'd check the measurement on your setup before cutting...there you go, one more way to say it.

[Pete Fallon]

John,
My 1961 Express had original 1-1/4" Stainless steel shafts that I changed out to Monel 1-1/4" when I repowered back in 1997 the shaft length was 96 inches from the face of the coupler to the very end of the prop nut area . I have intermediate strut bearings ( a small 2-1/2" x 4" long) strut just aft of the shaft tube exit, then a main strut that is 9-1/2" from the strut face at hull to center of the shaft barrel. The main struts were cast at Buck Algonquin in Michigan back in 1996 I went thru my prop shop Haskell & Hall in Salem Ma to have the struts cast . They are much heavier wall thickness than the old thin walled struts and can be bored out to 1-1/2" if needed. I don't know what the length of a FBC is but it most likely about the same length although the standard struts are much longer from the hull bottom to the center of the shaft barrel the angle is about 18" compared to my low profile struts which are only 12 degrees compared to the 18 degree standard size. I hope this helps you talk to Bob Lico about what he put into his FBC .
Pete Fallon.

[EarleyBird]

With 270HP Cummins in my 1973 31 Flybridge Boat, I have Aquamet 22 shafts 83 1/4” EOT. Turning 20 X 22 4 blade slight cupped DQX wheels. At 2300rpmz, fully loaded she cruises at 24 kts.give or take a half a knot. Take 100 gallons out of her, push her up to 2375rpmz and she occasionally touches 27. Oh yeah....flat calm days.
EarleyBird

[Tony Meola]

John

What everyone is saying is don't do it until everything is right were it is supposed to be. Otherwise you could wind up buying new shafts.

[bob lico]

my shafts are 80 3/4" long with spooning at keyway end and threaded end (engines are back as far as possible.2350/2400 rpm 28knots cruise.when you drop the hammer she leaps out of the water and your at 34nots in seconds fully loaded.330hp Cummins.FBC with full tower enclosure,A/C and every conceivable boating option flat screen tv,microwave,oven,coffee maker,6 person self- righting Viking life raft etc.

[Carl]

Kinda weird comparing shaft size...
I was told size doesn't, oh never mind.


Original shafts where 1-1/4" monel at 92 & 93".

When I decided to get away from 1:1 and changed tranny to 1-1/2:1 the shafts had to be cut to account for reduction gear. So I cut down shafts to 88-1/2": one is a little long, one is a little short but they became interchangeable. That gave me one spare shaft that could be used for either side.

Alright so we have shafts at:
82"
96"
83-1/4"
80-3/4"
80-3/4"
92"
93"
88-1/2"

Looks like the average is 87", but that is most likely me and Pete tossing in our 1:1 gears.

Take those two out and average is 81.5...
For perfect length you want between 1/2 and 1 shaft diameter space between wheel hub and strut. (really the beginning of taper to strut)
Using the most popular, the 1-3/8 shaft, the least amount of space you want is 11/16"
Max recommended overhang is 1 shaft diameter...or 1-3/8"

That is 11/16" wiggle room and I'd stay far away as possible from the max overhang.

The longer the overhang the more fatigue the shaft will see.
Think of the back of the Strut as a fulcrum, the shaft being the lever. The wheel exerts a load on the shaft, the longer the overhang, the longer the lever...the more fatigue the shaft will see.

Right after the strut is the place your shaft see's the most fatigue and most prone to failure...AKA- loosing a wheel. The 2nd is right behind the coupling.

Measure or take an average...your choice.

[Yannis]

One shaft dia is not enough, if you want to install a rope/net cutter. Biiiig difference if you could install those little devils so you dont have to dive...

[John Nardi]

I will be using 1 1/2 shafts...The engines should have around 425 HP and a lot of torque within the cruise range RPM...Do you think I should also try to mount them as far aft as possible with a gas application?? My friends at Merritt's are telling me I absolutely will be needing Trim Tabs...Thoughts?? Bob??

[Carl]

I will be using 1 1/2 shafts...The engines should have around 425 HP and a lot of torque within the cruise range RPM...Do you think I should also try to mount them as far aft as possible with a gas application?? My friends at Merritt's are telling me I absolutely will be needing Trim Tabs...Thoughts?? Bob??

Trim tabs would only be to bring the bow down at a slow cruise...possibly level the boat instead of telling people to move over.

Once boat is going faster then say 18knts...the bows coming down all by itself and tabs will work against you.


Let Bob discuss the balance part with you.


...and while I am not a betting person, I'd bet heavily Bob would advise against tabs.

[Tony Meola]

John

The 31 rides drier and takes ss head see better with the weight shifted aft. Bob should chime in but I would shift the engine as far aft as you can. Even an inch back helps.

The 31 is pretty unique in that tabs don't help much if she is balanced right.

[bob lico]

first of all you have chosen the best configuration for the GM big block. i cannot tell you enough about that motor, lets put it this way my good friend and teacher for me was the 1985 world champion NHRA pro stock.we beat the factory sponsors cars in a 16 car "round robin" drag race. absolutely no contest between 502" and 496" big block. I am not going into a long explanation however think about how long those 6.125" rods stay at TDC compared with the 502"with 454" rods (thats were all that torque comes from) we pulled 1627hp at 9700rpm on pump gas back in 1995.!!!!!! BTW if you went with the right camshaft and fuel injection you could come up with very impressive fuel economy numbers.i have no idea why in God`s name you would put trim tabs on a 31 Bertram with that kind of power and not used for going around a pylon at 100mph. trim tabs on a 31 go against logic that is the 14 degree angle of the shaft will constantly push the bow down (like a I.O. or outboard tuck in)the faster you go the worse it gets.seriously do you really want to put that bow into head seas waves just to see what will happen!!!!!! a balance 31 Bertram will remain bow proud up until 35 to 36 knots till that shaft angle gets the best of you.i have to take the hold tribe to an island about 6 miles away at least 10 times a year,we usually have 12 to 14 people. the children sit in the salon but other than that there is equal amount of people on both engine boxes and sitting on coolers and it goes without saying physiclly impossible to put 10 on the port and nobody on the starboard everybody wants to take a seat.

[pschauss]

Bob,

Thanks for explaining about trim tabs. Mine has trim tabs but the hydraulics to adjust them don't work. (I had other priorities shaking the boat out last summer so I didn't look into it.) The seem to be stuck at an angle roughly on the same plane as the bottom of the boat. Can I assume that this is the "neutral" position? She definitely rides with the bow up a bit at 22 knots.

[Pete Fallon]

John,
Here are some more measurements that I took before the boat was sold and went to St John Virgin Islands via St Thomas.
454 Throttle bodied fuel injection 310 hp with Hurth 63A 1:5:1- 8 degree down angle trannies1-1/4" Monel shafts 96" from coupler face to end near prop nuts(2), with intermediate strut bearings just aft of the shaft log exit 2-1/2" high 4" long.
Rudders, original large bronze 19" from top of casting where 1" ss rudder shaft enters casting to very bottom of the rudder,9" wide at very top tapering to 7-1/2". Rudder flanges are 2" from the edge of transom.
The shaft angle is 12 degrees and the strut is 9-1/2" long measured from the hull bottom to the center point of the main strut(Cast at Buck Algonquin in 1996).
The1961 express hull #186 has 4 strakes from the stem area which are set at 12" between the edge of the strake flat. My first strake started 30" aft of the stem. others go right to the stem. Hull plug was redone after the 1st 100 boats (per Bill Dana). Dead rise is 24" at transom with entry of 61 degrees at the bow.
I don't know if these measurements will help you but this is what they were in 1961. The boat ran 41.8 WOT@ 4450 RPMS without trim tabs and cruised at 30 MPH at 3000 RPMS. Full fuel 170 gallons full water 25 gallons and 4 people onboard with soft top, antennas and outriggers up.
Pete Fallon

[Carl]

What size wheels were you turning Pete.

[EarleyBird]

To add, I to am turning 1 1/2” shafts, no trim taps. After 33 years I feel they are not necessary. 30 years ago we removed the settee to port inside and made it into an insulated fish box bulkhead to bulkhead, stringer to stringer. Offshore trips and overnighters we fill it with 5 or 6 totes of ice, slaps of bait. Tunas burn up ice. I can lay inside of it, easily. Easy to trim the boat by moving items of weight internally. Like I said earlier, take 100 gallons out of her, with fish and ice basically in the center of boat, she runs better.

[Pete Fallon]

Carl,
I was using 17" x19 Pitch with a medium cup 3 bladed Nibril Dyna Pro's. I had a set spares that were 18"x18 pitch no cup, lost about 5MPH off top end, only used as back ups, because of lack of clearance on hull bottom.
The original motors when I bought the boat had 3 blade Nibral motors were 413 Chryslers with 1:1 72 Borg Warner's had 16"x11pitch very bad on fuel burn and only got 30MPH WOT at 4500RPM, It would burn 60 gallons on a 40 mile round trip at 3000RPMS which was 20 MPH.
When I changed to the 454 TBI 310HP FWC with 1:5:1- 8 degree down angle 63A Hurth 17x19 3 blade, a 40 mile run she would only burn 28 GPH @ 3000RPMS which was 30MPH.
The 17"x19P had 1-1/2" clearance between tip of blades to hull bottom the least amount of clearance before you get cavitation burns into the hull.
Pete Fallon

[John Nardi]

Pete...What is the minimum prop clearance one would want from tip to hull?

[Carl]

Pete...What is the minimum prop clearance one would want from tip to hull?

15% of the diameter is supposed to be minimum clearance.


Pete - didn't realize you had room for that size props. I thought you had same low profile struts like I have. Think mine are 10 degree.

[John Nardi]

Thanks Carl

[Carl]

While 15% is "minimum" recommended,
it is 20% clearance that is recommended.

[John Nardi]

Translate this for me guys...So if one only has 12 inches from the output side center of the strut to the hull, you can at best swing a 20 inch Diameter wheel?? 20inch X 1.20 (20%)= 24 inch Diameter/2=12inches???

[Carl]

Keep it simple.

15% 15/100= .15
20% 20/100=.2


15% 20" wheel x .15 = 3" clearance

20% 20" wheel x .2 = 4" clearance


The clearance is measured from prop tip to hull.

Your measurement of center of strut bore is a bit off as the wheel sits further aft. Going back along the angle of shaft gives more clearance...plus tip of prop is not usually at the back end of the taper.

[Tony Meola]

John

Most of us with the later shaft angle, (more angle on the shaft) with Diesels are turning 20 and 21 inch wheels depending on their preference. I run a 20x23 with cup. Gives me 24 knots at 2300 RPM. I hit just shy of 30 knots at wide open 2650 RPM's.

[John Nardi]

Thanks guys

[Pete Fallon]

John
An old timer that repowered boats in New England for 40 years told me that if you took a standard 2x4 and put the 2'' which is now 1-3/4" between the tip of the blade and the hull that is your clearance. So 1-3/4" is the minimum clearance. That is what I had on my 1961 express. No cavitation burns into the FRP on the hull bottom above the prop blade tip.

Carl, my struts were 12 degrees measured with an angler finder, the clearance was 1-3/4" from the prop blade tip to hull bottom. That was with 1-1/4" Monel shafts, the center of the main strut to the hull bottom was 9-1/2", the smaller strut only had about 2" at the exit of the shaft log tube. That was measured from the hull bottom to the top of the small strut.
Pete Fallon

[Pete Fallon]

John
An old timer that repowered boats in New England for 40 years told me that if you took a standard 2x4 and put the 2'' which is now 1-3/4" between the tip of the blade and the hull that is your clearance. So 1-3/4" is the minimum clearance. That is what I had on my 1961 express. No cavitation burns into the FRP on the hull bottom above the prop blade tip.

Carl, my struts were 12 degrees measured with an angler finder, the clearance was 1-3/4" from the prop blade tip to hull bottom. That was with 1-1/4" Monel shafts, the center of the main strut to the hull bottom was 9-1/2", the smaller strut only had about 2" at the exit of the shaft log tube. That was measured from the hull bottom to the top of the small strut.
Pete Fallon

[bob lico]

pete i don`t "sweat" it the prop tip to hull bottom is 1 1/2" and never a sign of cavitation !!!! to add to it i have 1/2" space from prop hub to back surface of struct for maximum strength actually this is the end of the taper where the spoon is ground into the end of the key-way. the first high- performance prop would propel boat a 10 knots at idle so when i docked boat i backed in at 10 knots and slamed into forward three feet from dock, definitely abused the shafts thus the 1/2" spacing.