Spray from the outboard

[Hoppi84]

I have my outboard mounted on a bobs mini jack plate. I bumped it up to where my top of cavitation plate is right at the surface but I get alot of spray. Thankfully it all stays out of the boat. Is this normal?? Here's a picture of what I'm talking about.

 

[airshot]

Can be, I have had spray issues on and off over many different boats. Experiment...there is no set in stone dimension !! Look for best rpm at wot, no cavitation or actually ventalation, no blowout on turns, and experiment with trim angles...all these settings have a bearing on each other....lots of adjusting and experimenting is needed to get best performance, once you get it performing at its best, then look at spray issues. Gotta have a tach to get the rpm correct, otherwise it is all a guess!

 

[Hoppi84]

Here's where it's at now... I had it at the 2.5" above and it sprayed but wasn't seeing the plate. So I moved it up close to 1" where it's at now and can see plate but still have the fountain spray. Right now with 9x10 I'm a little over 5700 rpms and about 23 mph with gps. What would adjusting the set back do??? Appreciate your response. Thanks
Forgot to add the motor is a 2020 Yamaha 15 with 5500-6000 max wot

 

[dingbat]

I would expect a lot of spray given it looks like your ventilation plate is 1" or 2" higher in the front than the back

 

[airshot]

I would expect a lot of spray given it looks like your ventilation plate is 1" or 2" higher in the front than the back

I agree, didn't notice that before....run a long straight edge frim the bottom of the hull back to the cav plate. Adjust trim until cav plate is parallel to bottom. Now measure height of cav to hull bottom. Get that cav plate right at hull bottom to start, now start testing wot with tach and make minor trim adjustments to get max mph and rpm, before changing any props.

 

[jimmbo]

You have quite a bit of Setback, and as a Starting Point, the AV plate can be about 2 inches higher than the Hull Bottom. Putting it flush/even, like you would if the Motor was mounted on the Transom, would result in it being too deep.

 

[Hoppi84]

This is on the top hole. And I'm at 5" set back right now.

 

[jimmbo]

Seeing your Prop, and that is doesn't look like it has any Cup, keep an Eye out for Ventilation, where the Air from the Surface gets into the Blades, resulting a jump in Rpm, and often a drop in Speed

 

[Hoppi84]

I have 9x11 and 9x9 aluminum prop from it being on my old boat and plan on getting a stainless but gotta figure out which way in going with aluminum. So should I bump the jack plate back down or up?? And would dropping the set back to 4.25" knock down bow lift some or not very noticeable?? Appreciate the help

 

[stresspoint]

if you have a transducer or a speed pitot that drags in the water it may be mounted in a position where it can direct water to the ventilating plate.

 

[jimmbo]

Have you run it as it is Currently set up? If so, did at any time do the Prop lose it's Grip on the water? If not, I would go higher. If it did, then a hair lower.
Raising an Engine higher, reduces the Ability of it to Lift the Bow, simply because of less Leverage. Higher Trim Angles are required, which leads to more chance of Ventilation. That is where Cup and Rake really come into play, as well as overall Prop Blade Design.

Now before we end up going down a Rabbit Hole on High Performance, a lot of the stuff I have mentioned starts becoming relevant as speeds above 45-50mph. At lower speeds the Engine isn't going to get the Bow running clear, and Porpoising in the usual Result of too much trim

Being too deep in the Water results in Lots of Spray, especially if there is Setback involved

 

[airshot]

Not sure how big your boat is, but don't be expecting great results from a 15 hp motor. Many of these recomendations apply to higher performance boats and motors. Just realized your only dealing with 15 hp, so you can only gain a tiny bit. From your pic showing the cav plate parallel....have you run it at that setup yet ??

 

[Scott Danforth]

Rule of thumb is 1" up for every 4 " of setback

 

[Hoppi84]

Yes I ran it at the set up in post #7. I ended up taking it the rest of the way up and it really helped clear the spray but I took it up too much as it started ventilating. So looks like my sweet spot is between half and all the way on the jack plate. Thanks for the help

 

[jimmbo]

I love it when the AV Plate is Visible when on Plane.
A cupped SS Prop would probably keep a better Bite on the Water, especially on turns

 

[Texasmark]

You have quite a bit of Setback, and as a Starting Point, the AV plate can be about 2 inches higher than the Hull Bottom. Putting it flush/even, like you would if the Motor was mounted on the Transom, would result in it being too deep.

Agreed. Rule of thumb I picked up on here from expert/old salt conversations was that 1" of height above the straight line hull line per 6" of setback....... Think about it, the bow of the boat is usually high/higher than the transom area. Once the water leaves the transom area it starts rising to the surface....faster you go the slower it rises but that rise puts the prop lower in the water.
I'm surprised you are (assumed) stock aluminum prop with no rake or cupping and doing as well as you are without ventilation MPH loss.

Course you can find out where you are. Go to a prop slip calculator and put in the data and solve for prop slip. If single digits you are fine, if over 10 you are being victimized by your stock prop design which wasn't desgned for what you are doing.

 

[Hoppi84]

Turning point hustler 9x10
5820 wot
2.00:1 gear ratio
23 mph
15-16 prop slip

I plan on trying a 9x11 too soon.
I'm kind of limited on props so would a powertech srt have alot of cup?? And say if I stay with the 9x10 would I be going down one or two pitch sizes?? Really appreciate the help.