Need to reduce ventilation

[chrisgt]

I have a 14ft side console Lund with a 40hp mariner/merc with electric trim and have ventilation issues. I have copied the questions from the sticky but the one thing not mentioned here is a/v height above the bottom of the boat. I currently have the engine on its middle mounting hole putting the a/v plate about 3/4" above the bottom surface of the boat when the engine is at a medium trim. The ribs (or whatever you call them) obviously extend a bit farther down.

1) Performance issue you are trying to correct.
Ventilation in a few areas: When I'm running higher trim, when I have a following sea, and sometimes coming up onto a plane if I'm stern heavy. No issues turning.

2) Current prop manufacturer, model, aluminum or stainless as a minimum.
3) Current prop diameter and pitch (required).
Aluminum Michigan match 10 1/8 x 15" Part#: 031019.
This propeller appears to have no cupping.

4) Wide open throttle RPM and speed with an average load (very helpful)
I think my suggested rev range is 5000-5500. Alone with only one tank of gas I can hit high 5000's; but with extra fuel, gear, and a friend or two I'm down in the mid 5000rpm range with a decent hole shot. I certainly wouldn't want more pitch, but I don't think less would be good either.

5) Engine/drive make, model, year, and HP
1996 mariner 30 that I have upgraded to a 40 (CCMS reeds, 40hp reed retainers, etc.) This engine runs STRONG.

6) Boat make model, year, length and weight
1993 Lund S14-20 with the rare side console steering. Weight with a tank of gas, engine, and hull is probably 400lbs range.
If I'm out on the ocean or a bigger lake I probably have 40lbs of gear, lunch, extra 5gal of gas and my wife or some friends.


The boat never ran right when I bought it and as part of replacing the transom, waterlogged foam, and repairing the engine I raised it to its current position; I have no baseline with the engine all the way down.

Not sure if my solution is simply dropping the engine down a hole or a prop with more cup? Dunno what I need to do, that's why I'm asking!

 

[Chris1956]

Put a straight edge on the prop blades. The cup will be on the trailing edge of the blade, near where it joins the hub. Cupped props are more common, with non-cupped props the exception and are often quite old. Cupped props ventilate less.

It is helpful to observe the A-V plate when on plane. It should be close to touching the water, but on the surface, not under the surface. If the A-V plate is too high, drop the engine down a hole and retest.

If she still ventilates, and the prop is not cupped, a cupped prop will help.

If the motor is down enough, and the prop is cupped, you need to look for another cause. Make sure the hull is not hooked or bowed, and make sure there is no items interrupting the flow of water to the prop. Look for transducers, speedo pickups, ribs etc, in front or nearly in front of the prop.

 

[chrisgt]

I have another boat with a 15 pitch solas amita (johnson, so different splines) and it has considerable cup on the trailing edge I can feel, so that's what I was going by; I may be wrong about no cup.

With my current engine height the a/v plate just skims the surface of the water; there is a little bit of splashing around it, but for the most part the top of the a/v plate is clean.
In my limited use of the boat with the engine down one hole it throw huge spray off the a/v plate and it was completely submerged on a plane.

The hull on this boat is not hooked, it has VEEERY slight amount of rocker, but is basically flat. I have looked at a few other Lunds on trailers wondering if mine is broken and they all look exactly the same; I don't suspect a hull issue. This boat rides incredibly well.

There is a rib down the center of the boat like most aluminum boats, the ribs are maybe 3/4"; seems fairly typical. The propeller is about 14" back from the transom of the boat.
I have a depth transducer mounted on the starboard side to avoid turbulence from the P-factor of the prop. (or whatever you call P-factor on a motorboat, dunno, I'm used to airplanes and sailboats).
Mounting the depth sounder was a fairly recent thing, though, and it hasn't made any changes in the boat's performance. Converting to PTT was also a fairly recent thing and that helps since I can trim down in some cases to help, but that isn't an ideal solution.

 

[chrisgt]

Here's a picture of a high contrast "straight" edge across the root of the prop:


And a picture around the middle:

(yes i know the bottom of the skeg is broken, i need to weld a new piece on; cheap boats are cheap for a reason....)

Anyway, this prop has very thick blades and don't appear to have very much cup (if any at all?).

If you think a prop with extra cupping would help my ventilation problems, what would you suggest? I sorta want to stay with aluminum just so if i bonk a rock i'm more likely to throw on a spare and go about my day than paddle home, but i'm open to suggestions.
There are a mind-numbing amount of options and i'm sure someone knows where i should start.

 

[Chris1956]

I like the Turning Point Hustler props in aluminum. They are a bit higher performance than other brands but are about the same price. They are cupped.

If your Solas prop has a removeable hub, you may be able to buy just a hub for the Mercury, for that prop. The Solas web site should show the prop as fitting your Merc motor, with a different hub number.

 

[Scott Danforth]

cost you nothing but a bit of time and some sealant to drop the motor down a hole and try

 

[cyclops222]

A 1" submerged prop should not Blow Out at WOT or very sharp turns.
Unless DIY prop engineers changed props.

 

[chrisgt]

The previous owner put this prop on it and it appears unmolested, just a low performance prop.
I ordered a turning point prop and hub kit, it's real surprising how cheap those are! The nice thing is if i ever want to go down a pitch the props are like $80 and I just use the same hub.

Should be in late this week or early next week. I'll see how things go (if it's not raining or snowing or something... getting to be end of the season here).

When the engine was a hole down the A/V plate was under water causing a huge amount of spray, so I'd like to avoid going back down.

 

[Texasmark]

I would think dropping the engine one hole will solve your problems considering your rig. I have 4 or 5 Turning Point Hustler aluminum props and they are all cupped with "rake.....gets the bow up at higher speeds", and for an aluminum prop, they have (advertise the point also) all the goodies of high priced SS props other than thin blades....which really shouldn't matter unless you are on the "pad" running 65 MPH on your hot dog bass boat.....(opinion) They do have a specially processed aluminum for better strength.....pull up an advertisement and view all their goodies.

 

[chrisgt]

Well, the big brown truck showed up so I ducked off work early to go try this new propeller (I work from home and don't really have set hours anyway). I'm posting from my boat because this thing is ridiculous. The most obvious thing right off is it has a lot of bow lift, I don't even know how that works... But it does.

It has completely solved my issue, though. I cannot get this thing to ventilate unless I trim the engine up so far I can't see where I'm going.

The Michigan had thick blades, no rake, barely any cup. This thing has rake, lots of cup, much thinner blades and it is great. I'm up on a plane in literally 10 ft, and trimmed out it will run 34 kts

 

[Chris1956]

Glad it worked. Enjoy!

 

[Texasmark]

I have another boat with a 15 pitch solas amita (johnson, so different splines) and it has considerable cup on the trailing edge I can feel, so that's what I was going by; I may be wrong about no cup.

With my current engine height the a/v plate just skims the surface of the water; there is a little bit of splashing around it, but for the most part the top of the a/v plate is clean.
In my limited use of the boat with the engine down one hole it throw huge spray off the a/v plate and it was completely submerged on a plane.

The hull on this boat is not hooked, it has VEEERY slight amount of rocker, but is basically flat. I have looked at a few other Lunds on trailers wondering if mine is broken and they all look exactly the same; I don't suspect a hull issue. This boat rides incredibly well.

There is a rib down the center of the boat like most aluminum boats, the ribs are maybe 3/4"; seems fairly typical. The propeller is about 14" back from the transom of the boat.
I have a depth transducer mounted on the starboard side to avoid turbulence from the P-factor of the prop. (or whatever you call P-factor on a motorboat, dunno, I'm used to airplanes and sailboats).
Mounting the depth sounder was a fairly recent thing, though, and it hasn't made any changes in the boat's performance. Converting to PTT was also a fairly recent thing and that helps since I can trim down in some cases to help, but that isn't an ideal solution.

Whether it applies to what you are doing or not, my experience with the location of the AV plate vs oncoming water is:

If plate is below the surface of the water coming out from under the transom at speed, trim will have a pronounced effect on bow rise and increased speed with a reasonable amount of pushing the LU out from the transom....plus trim.

If plate is above the trim effect will be much muted. Personal experience.

 

[chrisgt]

I have mounted the motor so the A/V plate juuust skims the surface and doesn't throw a wave on a plane. It is my understanding this is the correct way to mount an engine.

With the only change being the propeller I now need significantly less trim for the same amount of bow lift, have better holeshot, and can't get it to ventilate unless I do really stupid things. The hustler is pretty fantastic compared to the michigan match.

 

[dwco5051]

I have a depth transducer mounted on the starboard side to avoid turbulence from the P-factor of the prop. (or whatever you call P-factor on a motorboat, dunno, I'm used to airplanes and sailboats).

Read this earlier and had to go towards town for some shopping and this stuck in my head. Without looking for my 65 year course notes from Naval Engineering and Design I did remember P-factor being discussed. Not as big of a thing in an outboard or I/O where the trim can be adjusted to keep the plane of the propeller perpendicular to the forward motion. Thinking of other things common to boating and aviation I did remember the boating equivalent to ground effect. That occurs when a ship travels parallel and close to a sea wall, lock approach, bank, bridge abutment, etc where the water gets squeezed and the velocity increases more on one side of the ship than the other sucking the ship even closer thanks to Mister Bernoulli.

 

[Texasmark]

I have mounted the motor so the A/V plate juuust skims the surface and doesn't throw a wave on a plane. It is my understanding this is the correct way to mount an engine.

With the only change being the propeller I now need significantly less trim for the same amount of bow lift, have better holeshot, and can't get it to ventilate unless I do really stupid things. The hustler is pretty fantastic compared to the michigan match.

The Hustler has "rake", bending back of blades from the root. This is your source of bow lift.

 

[chrisgt]

The Hustler has "rake", bending back of blades from the root. This is your source of bow lift.

How does rake actually cause bow lift? I'm not doubting that it does, I'm just failing to understand it.

 

[Texasmark]

By angling the blades to the rear you provide a semi-horizontal surface, not provided by props without it. Trim allows you to tilt that horizontal surface such that water pressure on the blades cause an uplifting motion for the bow of the boat.....like Ailerons on an aircraft.

My first experience with it was on a new 1988, 17+ foot, Ranger with a 115 Mercury and an original Mercury Laser SS prop. by which the design includes all the performance goodies you get in non-racing type props and also included my first experience with PTT. (Power Trim and Tilt)

I launched the boat with the engine vertical (positioned as I had done with most of my rigs) as I recall, and got on plane running mid speed on smooth water. Nice smooth performance. I felt a relationship between the position of the bow of the boat to the water's surface like I felt with all the boats I ever occupied.....the boat is "in the water" not "on" it.

Checking things out, I started trimming out (going over 35 MPH by then) and the bow started rising and the speed started increasing with me doing nothing else. Wow...... what a neat experience. Felt like I was flying....well as it turned out, the boat had a "pad"....a narrow long flat surface along the center line accompanied by a good V to the bottom so that at speed the only thing in the water is the flat surface....the "pad" which significantly reduced the drag of the water on the hull and allowed for higher speed for a given amount of HP.......the boat had moved from "in" the water to "on" the water.

Since then I have always had PTT and props with cupped trailing edges and rake in the blades.

On cupping, deliberately bending up the trailing edge (tip) of the blades, the purpose is to give the prop better bite when operated near the surface, or in situations (turning) where the lower unit may generate bubbles that would flow into the prop and could cause slipping....ventilation in the turns. HTH!

I had more fun with that boat than any I ever had...before or since.

 

[chrisgt]

Thanks! I found a PTT unit on marketplate for $200 (that needed a coupler... weird story), anyway doing the conversion from manual trim to PTT was fantastic.

Cupping makes sense, I'm gonna have to take what you said about the blade angle of attack and sit down and look at the thing real close to better understand this. I like knowing how things work.

Yesterday I got myself stuck 10 miles upwind of the boat ramp in 3ft following seas (long story involving a friend's broken down boat, etc.), wind waves so a nice short period; so much fun in a 14ft aluminum boat... the extra cup allowed me to trim the engine pretty high without ventilation so i could get over the seas without stuffing the bow. It was a day of bad planning and a race against darkness.... This new prop really saved my behind.