Ventelation cures

[airshot]

Ok, guys here goes, recently downsized from a 22' with I/O to a 16' with 40 hp OB. All is in excellent condition, motor runs great, my issue is the prop ventalates ( sorry if spelling is off) very easy. I can over come the issue with the power trin but boy is it fussy!! Cav plate is right at bottom of boat hull, boat is a Sylvan 16' side console. Motor is as low as it can go as far as be bolted to transom. Prop is a 10" dia x marked 13P ...there is 2" of clearance between blade tip and cav plate so a larger dia prop is possible...motor is a 40 hp Merc, 2 stroke 4 cyl, 1992 same as boat. Boat weighs 650 lb from factory specs plus 350lbs of batteries and gear.
( yep 3 lg group 31 bats plus starting battery). With 2 adults can just touch 30 mph (gps) at 54/5500 rpm wot. Must do some serious tweeking of trim to get that speed and stop ventalation. Question is, can I change/ modify a prop to lessen the vent issue? Serious trim alignment needs to be done to get her up and going without sucking air, gotta be on your toes so shedoesnt over rev. Mabey a cupped prop?? Thanks in advance folks.....

 

[JimS123]

Ventilation.

Sounds like the hull isn't true.

Maybe you need trim tabs.

 

[airshot]

Hull looks perfect, no hook, nice straight keel, barely a scratch. Boat gets up on plane well as long as you manipulate the trim. Just never had a boat where the trim is so sensative....just wondering if cupping or a slightly larger prop dia might lesson the venting

 

[jimmbo]

A 16 footer is naturally going to be far more sensitive to Trim than a 22ft barge. At the speeds a 16 ft can attain with a 40hp, you won't be able to do much trimming out without Porpoising, and/or the Prop grabbing Air
A cupped prop may help, but usually at higher speeds.
Do you know what Make and Model the current prop is?
A larger diameter, just puts the Blade Tips closer to the Air

 

[airshot]

No real marking on the prop other than pitch. Being a 92 could be anything, it is aluminum,I know that. My Islander was no barge, she topped 34 mph with two men and fishing gear, easy to make trim adjustments as it wasnt touchy and adjustments were very noticible!! On my 16' the slightest trim adjustment makes a big difference.....just being off a fuzz makes it grab air....thought it might be a spun prop at first but delicate trim adjustments makes that idea not so

 

[Chris1956]

Have someone drive the boat at cruise, with motor down. Take a look over the transom and observe anti-ventilation plate. It should be riding on the surface of the water.

Are there any transducers, speedo pickups or anything else near the prop on the hull?

 

[airshot]

Have someone drive the boat at cruise, with motor down. Take a look over the transom and observe anti-ventilation plate. It should be riding on the surface of the water.

Are there any transducers, speedo pickups or anything else near the prop on the hull?

The only transducer is off to the side, nothing else there, my grandson drove while I looked and yes the plate appears to be at or slightly below the surface of the water. There is alot of spray comming up from around the back , not sure why.....if I tuck the motor all the way down and throttle hard it gets up pretty good but then you need to trim up to get up on plane, here iscwhere it gets touchy, to slow on the trim and it can bog, but to quick to trim up and it grabs air. First thought was a spun prop, but carefull control/ timming of the trim gets her going well. Just never owned a boat so sensative to trim up before. Just thought a change in prop might help. Gonna need a spare prop any way, so might try to make an improvement when I do

 

[Chris1956]

That behavior is unusual. It should not bog without using trim, and it shouldn't ventilate easily, with the A-V plate at or under the surface.

Without anything else to blame, I will blame the prop. Almost all props are cupped these days (have been for 30 years), so that should not be the issue. You might check it.

Find a prop wizard and see what they recommend, and compare it to your prop. I am thinking that your prop is too small for the engine, or just lousy.

Some props ventilate less than others, those are what you want.

 

[airshot]

Yep, we are now thinking similiar....the current prop is not cupped ! That is what made me think a cupped prop may help. Yes it vents to easy, shouldnt be that easy. When I said bog down, I didnt mean real bad, just more sluggish than a real bog. Hard to find the right words....going to visit the prop shop next week and see what they might recomend. Thank for your suggestions!!

 

[garbageguy]

Our 14ft Aluminum boat with 25 HP 4-stroke OB (Honda, one of the first 4strokes I think about 25 yrs ago), is heavy. That setup is very trim-sensitive, similar to your description.
It's prop is not 'pristine' - likely helping it to ventilate easily. I think most people that use (abuse?) it run it w/o ever trimming at all. It can be trimmed a little, but it is sensitive to many factors. That rig has been that way for about 25 yrs - not ideal, but for a boat used by many different people, pretty staunch.