Transom / Shaft Length

[viking61]

Hi , can anyone advise ?
my transom height is 22.8 inches , can i get away with a 21 inch shaft motor without cutting the transom down . Rib is a RC 585 , motor i'm looking at is a 115 4s Mariner.
Thanks

[captnjack]

I'd get a 25inch shaft and mount it 2 inches high.

Potentially being 1.8 inches too high means you may end up needing to cut down the transom. That's lot of work and the transom may not like it compared to mounting a longer shaft a few bolt holes down (raising it).

[chewy]

I'd run a long shaft.

[Chris]

I use to run my etec about 2" above the bottom of the transom. However E-tecs like to be mounted fairly high so this was fine.

I would ask Ribcraft as they know their hull better than anyone and will be able to advise you.

[captnjack]

Originally Posted by chewy

I'd run a long shaft.

What is that in the UK?
Over here its:
15" = short
20" = long
25" = XL

but various companies are making all sorts of intermediate lengths depending on the motor and HP so I don't know what to call a 17.5" shaft.

[viking61]

thats a little annoying , was hoping for a "well it was ok on mine" kind of answer ! any fabrication guys got any ideas , i think the transom was cut down already and was probably a long shaft ( 25 inch) to begin with .

[Downhilldai]

Originally Posted by captnjack

What is that in the UK?
Over here its:
15" = short
20" = long
25" = XL

but various companies are making all shorts of intermediate lengths depending on the motor and HP so I don't know what to call a 17.5" shaft.

12" = short (race motors)
15" = standard (commonly referred to as short shaft)
20" = long
25" = extra long (XL)
30" = ultra long

The 17.5"/18" tend to be called mid-shaft.

[Downhilldai]

Originally Posted by chewy

I'd run a long shaft.

So would I, but keep an eye on the water pressure and check for prop slip when testing.

[captnjack]

Originally Posted by Downhilldai

12" = short (race motors)
15" = standard (commonly referred to as short shaft)
20" = long
25" = extra long (XL)
30" = ultra long

The 17.5"/18" tend to be called mid-shaft.

Thanks I don't think I've ever seen a 12" shaft in a US catalog or showroom. Otherwise we're the same. I just forgot about 30" ones which are rare, at least on RIBs here.

[thomas]

Mine's a 25" leg on a 25" transom - I'm pretty sure the engine it not resting on the top either, I expect you're right on the edge as does't the anti ventilation plate want to be a couple of inched higher than the very bottom of the hull?

[viking61]

Thomas , thanks for the pic. I spoke to Ribcraft and they think a 20" will be ok . I think the extra weight ( 170kg) of a 4 stroke may help , originally these boats were made with a lighter 2 stroke in mind ( mines a 1999 and says max hp 125 ) . I was planning to push the console forward a bit to balance. Think i'll try it and see and if the transom needs to come down an inch then so be it. Ribcraft said watch out for back wash , i'm always pretty mindfull of this and have seen a "collar" type of thing elsewhere on the forum , made by " Biffer" i think so should be avoidable. Got any other tips while i'm still in build mode ??

[thomas]

Mines a Ribtec, I was warned before I got mine that the low transom was an issue, turned out mine was full height and needed the long leg so back wash is not an issue on mine. Mine is back heavy though and the bow does stick up quite a lot, though the Merc is a lighter engine than others, I suffered a little from chine walking but stuck another anchor and more chain in the locker and it really helped.

[viking61]

been advised by my local mariner dealer that if it's not a fit i can get a mariner extension kit and it won't affect the warranty, happier now i have a couple of options