Setback for a 16' Tahiti OB

**Spaceman Spiff** · 02-26-2023, 09:23 PM

So I just picked up this 70 something Tahiti and I'm ready to order a jack plate for my 1983 Mariner 115hp L6. But I don't know how much setback to get. No pad on this boat, but not really needing it since I don't have much chance of cracking low 50s with this motor. Transom has a couple knees on it, so maybe they were added somewhere along the line.

Also, any thoughts on a good prop for this thing?

Stephen

**Spaceman Spiff** · 02-26-2023, 09:31 PM

Pics

**David** · 02-27-2023, 06:04 PM

When I purchased a 16’ Baja from John Tiger he had rigged it with a 4” manual rapidjack and a 100 cross flow. A bit less power then yours and the boat had a small pad.

My 100 was propshaft rated. I don’t know if an 83 115 is prop or crank rated.

So not an exact match to your rig, but not far off either. My Baja was not sensitive to engine height, so I just left the height adjustment alone after verifying John already had it right.

**Spaceman Spiff** · 02-28-2023, 12:42 PM

Thanks.

Does anyone see a reason to go more than 4" with this 290lbs motor? 4" was my default.

**Dave S** · 03-02-2023, 07:35 PM

No more than 4.......Raises the motor till it stops chinning......Stout steering.....I would reinforce the bottom...... they Flex.......Most have Hooks......Good starting point ......

.......

**Spaceman Spiff** · 03-02-2023, 08:17 PM

Roger that.

Thanks for your input.

**David** · 03-03-2023, 02:01 PM

20” Laser prop to start. Used Lasers used to be cheap but it might be hard to find one now. I think all new Lasers are small hub, but the original Laser was large hub in odd pitches and small hub in even pitches. Your inline needs small hub. And I assume 2:1 gears.

my 16 Baja/100 cross flow would go 53

**Baker343** · 03-10-2023, 01:25 AM

I skied behind several tower of power Tahiti's back in the day. Great little OB's and small and big block squirt boats. Looks like it still has a transomed bracket attached and looks like it's missing one. I would replace the missing transomed plate and then mount your transomed jack plate, 2 to 4" max. Make it turnkey and enjoy.

**Spaceman Spiff** · 03-10-2023, 10:14 AM

Originally Posted by Baker343

I skied behind several tower of power Tahiti's back in the day. Great little OB's and small and big block squirt boats. Looks like it still has a transomed bracket attached and looks like it's missing one. I would replace the missing transomed plate and then mount your transomed jack plate, 2 to 4" max. Make it turnkey and enjoy.

Thanks. Can you elaborate on the "missing transomed plate"? I've been reinforcing it here and there and with aluminum plates. I'm definitely going to to brace the bottom of the transom from coming forward under thrust. I'm planning to redo the knees.

**Baker343** · 03-10-2023, 01:28 PM

From your picture it looks like there was a plate between the transomed and the engine bracket. Probably adding strength to an old transomed. And possibly keeping the engine bracket from cracking through the fiberglass allowing water in the transomed. You stated it had knee's already on the inside. Unless you see stress or damage, they should be sufficient.

**Spaceman Spiff** · 03-10-2023, 05:10 PM

I have an 18" wide 1/4" aluminum plate(same width as the JP) under the jack plate. Those knees on the inside are 14" on center. So it seems the weight is coming down right on the knees. I have another plate inside between the knees. When i drill between the plates, i can feel some rot in there. The top of the transom has a steel sleeve on it, reinforced with angle iron from a bed frame. Probably the hardest thing I've ever tried drilling through. I put my 115 Johnson on there to try it first. It weighs like 30 something pounds less than the Mariner. I'm still going to beef up the knees. Maybe i can get out Sunday. I have a 12.75x 21 AL prop, but I think it's cupped.

**Spaceman Spiff** · 03-12-2023, 04:43 PM

So the maiden voyage got me 46 mph. I started at the lowest motor setting, which is about 1.5" above the transom. Middle trim hole. CL about 6" below the bottom. Only hit 5000 rpms with a 21" prop. So it seems like there is a lot of room for improvement.

Boat seems really solid and tight, and the transom seemed fine.

**Baker343** · 03-12-2023, 09:20 PM

Congratulations on getting it wet. OMC developed the Shooter prop. It's a chopper style prop that will probably help. Maybe look for a 19 or 21. Obviously, watch your water pressure

**Spaceman Spiff** · 03-15-2023, 06:56 PM

Well, I already hit diminishing returns. I raised it up about 1 1/4" and only hit 45 mph at 5800 rpm. Up about 600rpm from before due to venting. Even going to the fourth trim hole, I still can't get the bow up. Only good news is that I still have good water flow. I guess I'm already at the limit with this prop. So I guess I either have to buy a new prop or put my Mariner on there.