hi gents,

 

twice now I’ve been watching the back of the boat as I drove and I’m getting a lot of spray that I’m not convinced is right.  If the boat was up on plane then I’m sure it wouldn’t be right, but since I’m only in a 9.9 I’m not quite getting up on plane.  The photo isn’t great (I was driving) but you can see the splash from either side of the motor where it meets the transom. When I look at it on the trailer, the anti ventilation plate is about 1.5” below the bottom of the boat.  I was thinking that it should be level with the bottom of the boat.  Again, I don’t have the horsepower to get me up on plane, but I was thinking that if it is 1.5” too low that’s extra drag in the water that I don’t need.  Might pick up a a little more speed from it.  I can raise it 1” via the bolt holes on the motor, but it would be a process since I don’t have a crane.

 

thanks

rick

 

 

You need to compare the motor's anti-cavitation plate location to the bottom of your hull. 

 

It should look like this drawing. 

 

Yeah, that’s what I’m saying the anti cavitation plate (aka anti ventilation plate) is about 1.5” below the bottom of the hull. Take that motor in the picture and lower it 1.5”. 

What is the size of the boat?

53 minutes ago, gim said:

What is the size of the boat?

16’ aluminum. Light boat even at 16’ (800 or so dry weight). Fully decked 

1 hour ago, casts_by_fly said:

Yeah, that’s what I’m saying the anti cavitation plate (aka anti ventilation plate) is about 1.5” below the bottom of the hull. Take that motor in the picture and lower it 1.5”. 

 

I have had a lot of small boats (under 20') over the years. All were outboard powered, and all or most had engines with the anti-gravitation plate about 1.5" below the bottom of the boat. That is a common dimension. 

 

I have used a strip of hardwood between the engine mount and the transom to raise the anti-gravitation plate to the bottom of the boat and I cannot recall ever seeing or noticing a difference in performance even recording GPS speeds vs RPMs. 

 

The only small advantage might be when was operating in shallow waters with a prop like the Susquehanna or non-tidal Delaware.  

I'd question whether a 9.9 hp is even capable of putting a boat on plane.

 

Maybe the lakes you fish have hp restrictions?

1 hour ago, gim said:

I'd question whether a 9.9 hp is even capable of putting a boat on plane.

 

Maybe the lakes you fish have hp restrictions?

I have no illusions about getting on plane.  I’m just looking to optimize. I do have hp restrictions, Hence the 9.9. 

Can you go up one bolt hole on that mount?  Would be simple to try if there's multiple bolt holes. 

35 minutes ago, J Francho said:

Can you go up one bolt hole on that mount?  Would be simple to try if there's multiple bolt holes. 

I can and have considered it.  But I've never done it before and it's a 90 lb motor for which I don't have a hoist.  So I'd need a couple guys (and thus a couple beers and a full afternoon which will turn into an evening and probably pizza).  So I wanted to get some other opinions if it was even wrong in the first place (I do trust the marina, though this is a motor combo they don't normally use) and if there was any benefit before going to that level of work.

Hi Rick, I'm with Gim on this one.  The motor is trying to push a 800# (before decked out?) 16' boat, plus fuel, batteries, and gear.  Not enough horses to get it up onto plane. Beautiful looking boat though.

FM

He's not looking to get on plane. He's looking to eliminate the weird splash back. 

If you launch at the marina, just have them raise it for you, test drive and if it isn’t right, have them drop it back down.  Heck, they more than likely have a setup guy there anyway and he could give you the skinny on whether it would help.  It would only take them 5 minutes to drop it or raise it.  

48 minutes ago, J Francho said:

He's not looking to get on plane. He's looking to eliminate the weird splash back. 

yeah, this. There is no chance that I’m getting on plane with a 9.9.  I’d need another 4-6 mph to get there. Maybe more. 
 

58 minutes ago, TOXIC said:

If you launch at the marina, just have them raise it for you, test drive and if it isn’t right, have them drop it back down.  Heck, they more than likely have a setup guy there anyway and he could give you the skinny on whether it would help.  It would only take them 5 minutes to drop it or raise it.  

unfortunately, the shop that did it is 90 minutes away and they also don’t have a lake big enough to run it. Just enough to start it up and putter around. They do have a setup guy though and that’s what makes me think that it’s right as it is. 

8 hours ago, casts_by_fly said:

 So I'd need a couple guys (and thus a couple beers and a full afternoon which will turn into an evening and probably pizza).  

this makes me smile.  so much truth to this.  hahahha

Do you have electric trim? Have you experimented with the trim. 

4 minutes ago, Dogface said:

Do you have electric trim? Have you experimented with the trim. 

Tilt, not trim and yes I’ve played a little.  I can’t change it while running, but I can tilt it up a little before I kick it in gear and it stays there.  A little trim got another 0.3-0.4 mph which isn’t much to most, but that’s 3-4% better for me when I top out at 10 mph.  

Trim the engine angle up a little to eliminate lower unit drag, cavitation plate underwater is OK at lower speeds. 
Tom

Put a wood block on the ground and lower the motor against it or use a comealong in the rafters of your garage or a tree.  Take the bolts out first.

Or put a block under the engine and raise the trailer jack.

Just checked mine as I run a very similar setup (16.5 Deep V aluminum with 9.9). My cavitation plate is about 3.5” below my hull. Pic below is running wide open yesterday. A few things that I’ve played with that seem to make a difference;

 

- The little trim adjustments you can make with the adjustable steel rod

- Any weight reductions you can make - batteries, trolling motors, tackle, keep livewells plugged and dry unless using, etc.

- Weight balance; shifting heavy items forward or backwards to adjust balance

- Prop size; I dropped to the lowest pitch made for my motor to keep rpm’s up high in the recommended operating range

 
Depending on day and weather, I typically run between 9.75 - 12.0 mph with all gear, fishing alone. Removing a single battery or trolling motor (40-50 lbs) can gain me over 1 mph. Most other adjustments mentioned alter things by tenths of a mile

 

 

On 6/12/2025 at 4:49 PM, Dogface said:

You need to compare the motor's anti-cavitation plate location to the bottom of your hull. 

 

It should look like this drawing. 

 

This ^^^^ 

I’d definitely go with the buddies, beer, and pizza option. It will be delicious even if you don’t work on your motor