Should I mount my outboard to this setback plate?

[nphilbro]

I have a 18" steel transom setback in the garage I'm seriously considering mounting. Originally it was on the back of a I/O to Outboard converted 17' Olympic. This sucker is heavy but incredibly strong. It looks a bit rough in the photo but it's only superficial and I'll sand and paint it right for saltwater.

I would like your opinions and experience on this

My estimate for this project is about 12 hours (I always try to fudge on the high end and make sure I have the time).

I have read so many advantages in doing this such as the prop is more efficient because it runs in calmer water farther back from the trailing edge of the hull, it's easier to trim because it makes the bow more buoyant, allows you to increase prop pitch due to efficiency for more top end or just plain better hole shot and torque with original prop, gets up on plane quicker.

If these advantages are real it would address issues issues I'm dealing with and give me more latitude in optimal tuning options.

The boat is 100% designed to handle big water and the previous owner was telling me recently about some big seas he navigated in it off Vancouver Island. I frequently see many other boats of similar design with variations of this same concept.

Right off the bat, I have some concern with the added forces of 60# plus torsion weight multiplier from the engine's new position. The boat is rated to 200hp which was approx 390# in 1980 but I know the new torsion load will exceed that. Spreading the reach of the additional lateral force on the transom will be easy enough using the solid stainless backplate reinforcement from inside the transom of the other boat and bolting into 2x4s, blocking them to strength points at the transom joints. An effective transom support jack system for trailer transport will be critical since even in the full upright position the motor will exert a lot of stress.

I'd have to move the steering cable access and possible replace the steering cable and control cables.

Boat is 19.5' fiberglass closed bow runabout
2000# base weight
1740# motor and gear capacity
315# 140hp Johnson outboard
Transom setback distance is 18"
1" rise at extension but I can modify that up or down as needed.

Feedback appreciated.

Some pics here.

 

[Philster]

Re: Should I mount my outboard to this setback plate?

That's a lot of setback.

Common questions are:

Can the transom handle it?

Is the engine strong enough to generate the extra hull lift that is supposed to be the inherent benefit of setback?

.

 

[NSBCraig]

Re: Should I mount my outboard to this setback plate?

Man I was so ready to say yeah, but no. I wouldn't.

 

[nphilbro]

Re: Should I mount my outboard to this setback plate?

That's a lot of setback.

Common questions are:

Can the transom handle it?

Is the engine strong enough to generate the extra hull lift that is supposed to be the inherent benefit of setback?

.

Any transom issues I can address with engineered wood/metal/fiberglass fabrication. This transom is a whole lot more sturdy than anything on those converted inboards.

I'm curious why you ask about extra power for bow lift. The weight of the motor alone acts to assist bow rise. I'm thinking right now there is plenty of power but the boat and motor are working against each other. Currently the bow rides quite high and won't trim out with a 13.75x15. It hits max RPM at only 30-32mph/gps and the bow is very high unless I trim way in at which point the whole thing just acts like a snowplow from the stern. My other prop is a 13.75x19 SS It will get the boat settled down on a wet plane at 36mph WOT but loses grip with visible spray along the way if I throttle too fast even though the stern is still buried (the hub isn't spun). I've been doing numerous calcs using the characteristics of this hulland everything generally points to the recommendation of a 22-24 pitch/13.XX diameter which is characteristic of an outdrive prop.

Previous owner had issues with porpoising running a Honda 140 and I'm becoming extremely suspect that since this model was built primarily as I/O with 175-250hp stock motor with the exception of a few highly sought after outboard models (such as mine) that are only rated up to 200hp the actual boat geometry is set up for higher torque/higher pitch propulsion. The best way I can see to get better performance is backing up the the motor which will give easier bow rise and using more of the power for forward thrust while giving the prop clean water.

I've seen quite a few outdrive versions of this boat successfully converted to outboard and use a 12-24" transom extension mount.

I don't mean to imply my mind is made up, I just want to make sure my eyes are wide open concerning performance expectations from those more experienced.

The objective here is use all the horses with a better hole shot but mostly to ride drier plane at 3200-4000 rmp.

Here area a couple pics of both a '79 and '80 conversions both on the 19' identical hull. White one had 4cyl volvo penta and I'm not sure about the brown one.