outboard motor position

[bobberboy]

So I was reading in this forum yesterday about motors and happened across a discussion about shaft length. The writer said that the cavitation plate should be even with the bottom of the boat. I went home and measured mine and it is about 1-1/2" to 2" below the bottom of the boat - I have a 1236 jon w/short shaft 9.8 Nissan - the transom is 15". How important is it that the cavitation plate be even with the bottom and should I raise mine up? If it's too low in the water, what is the effect on the overall performance of the boat and motor? One thing is sure, the 2" would make a difference in the shallow water that I have to haul my boat in and out of. Also, I don't understand much about props. I would like to get a little more speed out of the boat and am wondering if anyone can fill me in about prop designs and whether a different prop could give me a little more speed. Thanks.

 

[crazymanme2]

The farther your lower unit is out of the water the less drag there is so the faster you go.You can go up to even(cavitation plate) or as far as an inch or so above(bottom of boat) before your prop may or will start to ventilate.You also have to keep the water pick up in the water far enough for your water pump.Boat motors are designed to run at a max RPM.If you over prop a motor you can do more harm than good.RPM's are very important when re-proping a motor.So if you want to do it right you need a tach

 

[bobberboy]

Thanks crazymanme2. I guess I'll start by raising the motor being careful about the water intake and see what happens. I've been reading some about gaining speed or hp from outboards and others have warned about the rpm's too. My motor is new and I don't want to take any chances with it so best left alone I guess...

 

[CarlF]

Slide a 1"x1" piece of trim between the transom & the motor to raise it 1" before you run it next time & see how it does.

 

[bobberboy]

I cut a piece yesterday to give it a try. I hope I'm going out after work today to check it out. From what crazymanme2 suggests I could raise it as much as 3" but have to see where the clamps and bolt holes on the motor end up on the transom - and the water intake. I am going to start with 1 1/2" piece and see what happens. Can't wait to get out on the water...

 

[bobberboy]

I went out last night for a couple of hours. I think the higher position of the motor seems to help. I also played with the tilt which is a harder problem to solve. My boat is pretty small and when there are two people on board it rides very differently then when it's just me. I think I got it to plane better - at least I think so based on the size of the wakes. After both adjustments were made the wakes were smaller and flatter, I assume meaning that the boat was riding higher out of the water or at least not dragging the back end as deeply. Anyway, it seemed a little faster and smoother so maybe I made the right adjustments.

I think the bass had clocked out for the day by the time I got on the lake. I resorted to trolling and we picked up 4 northerns, 2 smallmouths and a crappie. They all went back into the drink to fight another day. Fished until dark and finally got to use the navigation lights my brother put in a few weeks ago. All-in-all a good evening...

 

[bobberboy]

I didn't get fishing over the holiday weekend but I did get to work on the transom. I put in a 1-1/2" block to test the motor height and it brought the motor up to just under the bottom of the boat. The mounting clamps were so close to the top of the transom I am lucky I didn't lose the motor when I was running it.

I built up the transom with a block of hard maple and a new plywood plate to reinforce the new configuration. The new maple block is screwed into the transom from above as well as to the new plywood plate. The motor is bolted on so now all is safe and secure.

The easiest change I could make was to the motor. I covered up the decimal point. My 9.8hp turned into a 98ph with the aid of some black tape. Somehow I thought it would be faster than it was...

Anyway, I went out for a test and am still not sure it I got it right. My brother helped me with the trim and I am sure that is right. The motor is now about 1/2" below the bottom of the boat but it still feels like it is dragging too much. Now I am wondering about the weight distribution and whether I should have left the battery in front. As you can see from the pics, the boat at idle seems to float pretty flat but at speed still seems too deep in the water. I am not sure what to do next but maybe I'll try some weight forward before I move the battery again. Any other ideas out there?

 

[crazymanme2]

Try moving your tilt pin up 1 notch.Your boat will take a little longer to come on plane but should run higher in the water.You can only expect so much out of a 9.8 motor.

 

[bobberboy]

I got out last night by myself for the first time since the boat was (mostly) finished. It was a comedy of errors (dead battery, forgot my topwater baits in the car, got about a 3# smallmouth to the boat and the snap broke - fish and lure are gone, etc...) but the boat ran really well. It got on top of the water and flattened out like it is supposed to. For the first time the motor seemed to really wind out and the boat sailed (for a 9.8 that is) along - the only thing different was only one passenger. So it seems that weight is an issue but I don't understand the weight distribution aspect of it because last night all of the weight was at the back. I would have thought the bow would have really come up but instead the boat got on top and stayed there. Spray off the rail was about the last two feet of the boat and the transom was high out of the water. I remain confused about what to do...