Repower questions gas to diesel Cummins B and Yanmar 315's

[scenarioL113]

A few questions on repowers

I am looking at getting a 31 to replace my B28 in the near future. I have not found one yet that I am going to buy. With that said I want to get an idea on my options of potentially getting a boat that has everything I want but may need a repower. There is "cosmetic finish work" that may be out of my expertise so that is why I may consider one that is "done to my liking" but not a diesel.


This is not a discussion of "why go diesel"? and not a discussion of advantages or disadvantages... Also doing a repower is not a big problem for me, bc I have done it before and I understand the torture that it can provide to do it. I have the equipment, the tools and the space at my house to do it...and I am NOT afraid to do it. (I know it will suck...hehe). Everything I have had over the last 20 yrs has been diesel and I just like them. My Excursion, my 4-runner (2LTE conversion), My B28/Cummins etc... Maybe for me it goes in line with "I love the smell of napalm in the morning".... you get the point. @ my fellow motorheads...Dont worry my 56 Chevy Pickup is gonna keep its 454 LS6 and will remain a gasser (until I die anyway...).
A bunch of years ago Capt Pat inspired me to one day have the greatest boat ever made and god willing I am gonna have one.

if I go this route of a 31 with gassers I will intend to repower not long after.

My intention would be to use RTO's with gears and would either be Cummin B 250hp (or more) OR Yanmar 6LPA 315's. (unless I got my hands on a pair of yanmar 240's). I have no problem if they need to be rebuilt either or "freshened up".

Questions regarding the running gear:

A 31 that has big block gassers would need to have bigger shafts, struts? and propellers.

Will the shaft tubes (logs) need to be replaced for the bigger shafts? (Seems like many big block 31's have 1.25" shafts and I know sometimes things may work for an upsize but not sure on the logs...???

The Struts? For a yanmar 315 I can swing a smaller prop than the cummins bc its higher rpms rating but will an 1.5 shaft fit into an 1.25 strut with a cutlass bearing swap??? I am pretty sure an 1 3/8 cutlass bearing fits into the 1.25 strut but dont know about the 1.5 shaft.
If the strut does need to be changed I think it could just be the same angle bc height is not needed for an 18 or 19 diameter prop. I think an 18 or 19 X 21 or so prop works for the yanmar 315 and if that is the case the clearance is ok.

For a Cummins the original big block strut would need to be replaced for a different shaft angle anyway, correct...yes no??? This would mandate the shaft log be changed as well...?? Yes???

Gear reduction? 1.5 for the Cummins ??? Will 1.5 work for the Yanmar 315??? I am pretty sire the 1.5 for yanmar works according to the prop calculator

Those are the main aspects of what I am not sure of and any input would be greatly appreciated.

I am leaving town for hunting this am, so if I do not reply for a few days it is bc I am "off the grid" but will try...

Thank you all!

[Craig Mac]

Most gas boats have 1 3/8 shafts----I think very early gas boats had 1 1/4 -----some of those early boats were running 1: 1 gears as well.

I believe the struts used with 1 3/8 shafts can be upsized and used for 1 1/2 shafts.

[Carl]

Many early gas 31's had 1:1 gears, 1-1/4" monel shafts with 10 degree angled struts.

1-1/4 shafts would only work on the lowest HP, high revving diesels running 1:1. It would move the boat, but you'd lose many of the diesel benefits aside from smelling diesel fumes.

1-1/4" 10 deg struts will not allow you to swing a wheel larger then 16", I've tried. No burn marks but you'll get a rumble with 17" wheels.
You could shim the struts out some...but they are a thin small strut not meant to turn a big wheel or lots of power.

The 1-1/4" Strut barrel, where the cutless bearing fits...it's too thin to bore. Even with an engineered plastic bearing (Thordon) with no brass shell, it would not work.



The next size shafting up is 1-3/8" and I believe all these boats had the common 15 deg struts. Many of those struts can be bored to accept a larger bearing to accommodate 1-1/2" shafts. And the 15 degree struts allow you to turn a larger wheel...plus its a beefier strut so you could shim it out some.

1-3/8" shafts would work up to about 250hp in diesel.

[scenarioL113]

I am thinking that we will need 1-1/2 shafts and see what struts and logs will work or be needed or how to handle that.

What size are the stock shaft logs and what can they handle up to regarding shaft size or angle???

Thx

(Gotta go now TTYL)

[Priceless]

Interestingly I was looking through classifieds yesterday and found a beautiful upper/lower station 31 in Ocean City Maryland. It's named Whazzup and it's gorgeous. 315 Yanmars with 600 hours and redone in and out with updated electronics. if I didnt have mine I would seriously consider it I love the option of piloting from below as I do on mine. everybody hangs out together while we run and then I go into the tower for trolling. In my opinion get the boat with the proper power in already and if its done right this boat is probably a bargain.

[Priceless]

But if you want to do the repower 1 1/2 inch shafts. I've got a wheel on the bottom of the ocean from an inadequate shaft diameter.

[Tony Meola]

When we repowered from 454's to Diesel, the struts had enough meat on them to be bored out to 11/2 " and a thin wall cutlass bearing installed. We replaced the bronze shaft logs and glassed them in .

My engines beds were still solid so they got cut down and capped in aluminum.

We also went with Capt. Pats oversized rudders.

So at a minimum you should have 11/2 shafts and depending on the struts used you may or may not get away with them.

[Craig Mac]

My understanding is Monel is a stronger material----is it still available? If you went to 1 1/2 and switched to Monel would that be an option?

[PeterPalmieri]

I think for budgeting you should assume that everything under the decks and inside the engine boxes would need to be done. Struts, shafts, rudders and shelf, fuel tank, deck supports, engine beds and wiring.

[Carl]

Monel It is fatigue proof, not stronger.

These days you do not want to go Monel. Yes it is still available, but the term expensive does not approach the cost.

Have to buy the 12' lenght at $25+/lb , straighten and then Machine. It machines like frozen gum at best. Fun stuff...

The good part is it bends easy and predictable,

[scenarioL113]

When we repowered from 454's to Diesel, the struts had enough meat on them to be bored out to 11/2 " and a thin wall cutlass bearing installed. We replaced the bronze shaft logs and glassed them in .

My engines beds were still solid so they got cut down and capped in aluminum.

We also went with Capt. Pats oversized rudders.

So at a minimum you should have 11/2 shafts and depending on the struts used you may or may not get away with them.

Tony what HP Cummins are you running?

You glassed in new logs? Were the original bronze logs able to be used if you did not replace them?

What size shafts?

I am just trying to compile information this way I am can attempt to make an informed decision on potential 31's

Thanks

Frank V

[Tony Meola]

I have the 270 Cummins. We had to go up to 11/2 from the 13/8 shaft. The new shafts are aquamet 22. The original bronze shaft log would not take the larger shaft. That definitely has to be up graded. Plus I was getting some leakage around the bolt so it was time.

What we also found is that in that area, the glass is really thin. By the time we got done with the new shaft logs and upgrading the strut pads, the other weak point, we added about an inch of glass in area from the strut pads back to the rudder ports.

I could not believe how thin the shaft log area was.

[Carl]

Ok Tony...

How thin was it?



Mine was about 1/2" thick around the logs.

[scenarioL113]

Thanks for that Tony, Good information and that is how I would do it as well if I need to.

Wasn't the Poseidons (from "The Poseidon Adventure") hull very thin by the propeller shaft....only 1" thick.... LOL...I know it was just a movie...

Well if a luxury liner can have a 1" thick hull then I would think Tonys 1" of glass should be equivalent to the armor of a Tiger Tank. Overkill is how I was taught as well. Better to have it than to not, especially when the girl is gonna get you home when things turn bad...


Thanks
Frank Valerio


also, Tony + et al, I think you have the original 15 degree original struts? The props are 20" ?

[Tony Meola]

Carl

I would say scary thin. Less than a 1/4 inch. With the Bronze shaft log removed, it flexed pretty good when you stood in that area.

[Tony Meola]

I believe the shafts are 15 degree. I think the earlier boats had 12 degree shaft angles. I can up the prop to 21 inch, that is about as big as I can go.

You will find that some of us run the 20 inch prop others the 21. Mine are 20 x 23 with cup. Others on here are running around 21x 21 or 20 with cup.