20 or 25 shaft length outboard sailboat outboard?

Aug 22, 2021
14
1992 mcgregor 26s san diego
I haven't thought this much about shaft length since I was in middle school lol... I'll be heading to catalina in a couple weeks and there is a good chance I'll be on motor often. ... I wanted to get a 20 hp motor just so I could keep it at lower RPM for a longer time without blowing my motor at full rev keeping hull speed but it's difficult to find them with the correct shaft length. Simple question is that 25 inch shaft length really that important????????? because i can only find it in a 10hp motor. which would be fine for my boat..... I just like to over do things lol... it's better to habe it then to want it lol
 
Apr 8, 2011
768
Hunter 40 Deale, MD
I owned a Catalina 27 for years on the Chesapeake Bay. We always ran a Yamaha 9.9 long shaft with high thrust prop. That pushed us at hull speed with plenty of power to spare (maybe half throttle?). I think a lot of it was the high thrust prop - you're not planing, so torque is really more important. That long shaft was 20", and very rarely came out of the water. 25" would've been better, but not necessary most of the time - for Bay motoring. I cannot speak to open ocean sailing/motoring, but my guess would be that the short interval waves on the Bay are more likely to cause porpoising which might get the prop out of the water, whereas open ocean motoring would have waves with much longer intervals, making that less of a problem. My thinking on the Catalina 27 would be that 9.9hp was fine; 15hp (same head just different carburetor I think) would be a bit overkill but might come in handy in some current/wind situations - but get the high thrust prop. 20-25 hp in my mind would've been of no practical use in that boat.

At any rate, those are my thoughts.
 

AndyVS

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Sep 4, 2015
56
Corsair 31 UC 179 Port Sanilac
I have a 9.9hp Merc with command thrust on my 26s and it will push the boat to hull speed at 3/4 throttle. More HP is just extra weight and pressure on the transom. I have the 25" shaft and the motor is on an adjustable motor mount. In my opinion the 25" is more important than whether you get an 8 or 9.9 hp. When the swells get bigger, I need every bit of the length and drop the mount all the way down to keep the prop and water intake in the water. Run the motor dry too much and the impeller will shred.
20200517_124112_resized.jpg
 
Apr 8, 2011
768
Hunter 40 Deale, MD
AndyVS's photo and commentary make an interesting and valid point - how much of the motor shaft is actually in the water based on how it is mounted to your boat? For my Catalina 27 it was mounted directly on a stern cutout, the bottom of which was several inches lower than AndyVS's pictured mount. That may explain why a 20" shaft was fine for my particular configuration, and why a 25" shaft might be more appropriate in his. Its not clear from his photo, but perhaps his mount allows him to adjust it even lower, but as pictured its about the level below the stern of the boat of the 20" Yamaha long shaft we used.

An adjustable mount is much better than my non-adjustable in the stern cutout, IMHO. It may put the weight out a little further from the stern, and possibly contribute more to "squatting", but my guess is he doesn't have a cutout that allowed water to enter that area of the stern, which always concerned me on my boat.
 

Dave Groshong

SBO Staff
Staff member
Jan 25, 2007
1,864
Catalina 22 Seattle
Open ocean, the longer the better, 25 inch. In a rough or choppy sea state you don't want the prop coming out of the water, 20hp is way overkill for that boat, 9.9 should be plenty IMHO
 
Jan 11, 2014
11,323
Sabre 362 113 Fair Haven, NY
Search the archives here. The overpowering issue comes up it seems about once a month.

Your boat was not designed to handle the weight or the torque of a 20 hp motor, especially a 4 stroke motor which are much heavier than 2 strokes.

6-8 hp in a 4 stroke with a 25 inch shaft should be more than adequate. The Mac 26 is a light boat, it doesn't need much power to push it. In calm water you will reach hull speed at less than full throttle. If there are adverse wind and waves, you won't want to go at hull speed.
 
Feb 21, 2013
4,638
Hunter 46 Point Richmond, CA
.............I wanted to get a 20 hp motor just so I could keep it at lower RPM for a longer time without blowing my motor at full rev keeping hull speed but it's difficult to find them with the correct shaft length. Simple question is that 25 inch shaft length really that important?...............
What motor and shaft do you have now? What is your top speed and at what rpm? Based on threads in the archieve you will not likely operate any where near wide open throttle (WOT) once at hull speed of 6 - 6.5 knots with 10 hp engine. Best to confirm before you go with your existing engine. Long or extra long shaft (see posts #4 and #6 in this thread Maximum motor size for Mac 26S | Sailboat Owners Forums).
 
Jan 19, 2010
12,362
Hobie 16 & Rhodes 22 Skeeter Charleston
25 for sure... but you don't need 20 HP. That will just poop your transom.

I ran a Tohatsu 6 sailpro on my H26 and was at hull speed at 3/4. The sailpro has a high thrust prop.
 
Apr 8, 2010
1,942
Ericson Yachts Olson 34 28400 Portland OR
I suggest a Yamaha Hi Thrust 8 or 10 hp, with the 25 inch leg. Different final gear ratio than the standard OB motors intended for planing fishing skiffs. Massive 'push' at displacement hull speeds. You will be pleased and a bit amazed....