Motor refit

Engines, Shafts, Steering, Struts, Rudders, etc.
User avatar
Royal Aluminum Star
Posts: 278
Joined: Sun Jul 13, 2014 10:05 am
Location: Aurora, In.

Re: Motor refit

Postby bcassedy » Wed Jan 03, 2018 9:55 am


In your (or anyone else's) estimation, would the the increase in weight off the stern be offset sufficiently with the removal of the engines/trannies? The removed engines, being aft of amidships, would definitely increase buoyancy but I would be concerned about the weight of large HP outboards hanging so far to the rear and the resulting center of gravity fore and aft change.

Bill & Sharon Cassedy
"Sunset Seeker"
'88 32' Sedan Bridge
Located in Aurora, In.
Twin 318cu in Chrysler
1.5:1 Velvet drive trannies
Fresh water only

User avatar
Royal Aluminum Star
Posts: 219
Joined: Tue Jul 15, 2014 12:21 am

Re: Motor refit

Postby Busia » Wed Jan 03, 2018 3:56 pm

You are getting into naval design here, but you could get a rough estimate in the change of CG. Calculate the weight and arm of the engines and transmissions removed, and the weight and arm of the outboards added. Fuel tanks and batteries can be moved also to change the CG. I would say you could move the CG forward and it would help as I have to use a lot of trim. I think most people have to use a lot of trim. Best case would be if you didn't have to use the trim tabs at all, and it was balanced. One thing I will point out, nobody mentions, is what happens when you take out those big heavy engines and transmissions that are down in the bilge. Near waterline or maybe below waterline. Now you add some outboards mounted quite a bit above waterline. All that cast iron down in the bilge was ballast and made the boat more stable. You will change the stability for the worse. Will it still be good enough? I don't know, just wanted to point it out that there are benefits to having all that weight down low.
located in Ketchikan, Alaska. Gods country
32 foot Marinette express. (no fly bridge)
twin 350 Crusader (Chev 350) engines
1:1 Borg Warner velvet drive transmissions
Closed cooling (antifreeze in the engine)
Proud to be retired IBEW

Site Admin
Posts: 902
Joined: Sat Jul 12, 2014 5:06 am
Location: Rock Halll, MD

Re: Motor refit

Postby Fastjeff » Thu Jan 04, 2018 8:00 am

Interesting winter bench racing project (as we old drag guys called them).

Undoubtedly the 'new' OBs selected would have power trim and tilt--that would eliminate the need for conventional trim tabs (and they are far more effective as well).

The OBs will be heavy sitting WAY back there, which should make the stern lower and bow higher at rest. I don't think it would be all THAT bad, however, for my 32 footer often had 5 or six adults lounging on the cockpit deck (with one of two often sitting on the swim platform, their legs dangling in the water.) And that didn't make the boat do a 'wheelie'. In fact, I never noticed any significant trim change at rest and, with a touch of the trim tab under way, it was easily corrected. So I don't see that as a problem.

Hydrostatically speaking, the inboard engines are located near the center of the hull's dynamic center (meaning they're near the widest point of the hull). The taper to the bow starts a bit forward of that, while the hull remains wide aft of that.

Still, the removal of a ton of low-located weight (dual inboards) has GOT to do something negative for stability. My 32 footer--even without the flybridge and bodies up there--was still a bit tippy both at rest. The OB setup would have to escalate that, possibly to scary proportions. One could relocate the fuel tanks where the motors were, possibly adding a gen set (if not there already) to alleviate that, but some ballast might still be needed.

Ain't this fun! And it hasn't cost me a penny.

Jeff (legendary Cheapskate)
"We live at the bottom of an ocean of air, not at the top." General Marvage Slatington

Royal Aluminum Star
Posts: 228
Joined: Sat Jan 10, 2015 7:14 pm

Re: Motor refit

Postby javalin390 » Thu Jan 04, 2018 7:42 pm

Not cheapskate Jeff, frugal
Jim Elias
1974 37' SedanFlybridge
Twin 360 Chryslers.
Marblehead, Ohio