25 hp blowout The

[Faztbullet]

who has done so under proven water test NOBODY on this forum.​

Uhhh have done hundreds of these tests. Dealership sold close to 300 boats a year and 75% was set-up and water tested by me. That's why they all received a decal that stated" Rigged by Fast Company"
This is what you want on ANY set-up with correct HP. Underpowered hulls are different but poster is not underpowered,

 

[Sea Rider]

We have 2 completely different points of views, in that case to each his own, you on the High Power range, me on the Under Powered range why argue about it. Will say that's much easier to play with different lower leg heights by the millimeter with a portable motor with clamps than a heavier HP motor that bolts on to transom which the holes are more spaced between them.

If water flow skims between 2 and 3 why is boater having aeration issues at choppy, wavy water cond at speed when that given height is ideal for you ? It's not a prop nor trim issue, it's a badly height seated motor...

Happy Boating

 

[WesNewell]

Engine height is the most critical for max performance with any outboard. mounted too low causes excessive drag which not only lowers speed, but also rpm's, porpoising when trimmed up to get max speed. It's not rocket science, just simple hydro dynamics. Running with the av plate under the waterline cost you speed, fuel economy, and a smooth ride. From the factory I got 31 mph max with my new boat. After raising the engine just 1.5" (2 holes) and going up 2 pitches, I could go a little over 40 mph with a smoother ride. And the factory actually paid to have the engine remounted higher. I went from a 16% slip to 5%.

 

[Ktoe]

First thanks for all the replies. I think I’m getting things sorted out to my satisfaction. Facts: my transom is 20”. The shaft on my motor is 19.5”. The board in the photo is 6’ long and being held tight to the bottom of the boat. It is a very straight board, so is a clear representation of the bottom of the hull. (NOT INCLUDING THE KEEL) The motor is trimmed very close to level with the bottom. This is also evident as the motor has no pull on the tiller handle in this position and the speed is optimal. (No slip when the boat is up on plane) When I trim it closer to the hull it pulls hard and I loose speed. The slip is less when trimmed in but is far from being eliminated. This seems like a poor trade off to me.

What it looks like at the motor when at full speed is very similar to the photo showing the water coming off the spalsh plate. The spray off the splash plate obscures the cavitation plate.

The boat does have a keel. It runs within a couple inches of the back of the boat. It is about an inch high. If I consider this keel to be the bottom of the boat, the the top of the cavitation plate is 1.5” above the bottom of the boat. To me this explaines my problem.

My conclusion: I have a motor that is on the short side for a long shaft mounted on a transom that is on the long side for a long shaft. I think dropping the motor one to one and a half inches would most likely solve my problem. In order to do this I’d have to modify the transom. I’d rather not, but am willing if this proves to be the best option. The other option would be to get get a longer long shaft motor. I have ordered a 4 blade #12 pitch prop hoping it is happy with a little less water.. The stock prop seems to like to run buried. I figured the extra blade might improve the bite enough to make me happy with this setup. This being a $75 experiment, I figured it was worth a try before resorting to more radical measures. We will see? I’ve considered other ways of lowering the motor without modifying the transom but really haven’t came up with anything. I have a mini-jacker jack plate setting around. But it seems like using it to lower would create other problems (such as the motor hitting the transom when raised)

 

[Ktoe]

Motor shaft is 21.5, transom height is 22”, 23” if you include the keel. I had my wires crossed when I posted the above numbers.

 

[Sea Rider]

Thought that were going to state at which lower leg height is water flow passing by at speed with motor trimmed to 90 deg, this is most important to determine, without this missing info won't see the light at the end of the tunnel anytime soon. Will waste precious boating time along dumping extra prop money into the blue..

At least you're aware that motor needs to sit lower on transom, a jack plate will shift motor weight out transom. Can't see how a 3-4-5 blade prop with whichever pitch you like buying can work/grip spot with on a poorly water bed height, the prop needs to work with a higher water bed to achieve best prop thrust with current prop.

This is a proven example of what your motor is currently experiencing...
Initial Water Test.-



Test with Transom Height Optimization.-



Now water flow skims right under the upper small plate with 0 aeration whatsoever on choppy, windy, wavy water cond and at close tight turns at speed. Prop has been maximized for motor to run to their full 5900 wot revs as usually loaded.

Happy Boating

 

[jimmbo]

At speeds below 30 miles per hour, there isn't going to be much difference if the drive is an inch too deep/low. It will give much better leverage for holding the bow high. Mounting too high and it will ventilate. Up to about 45 - 50 mph the AV plate even with lowest part of the hull is usually the best. At 50+ then higher heights are usable if used in conjunction with higher performance props.

 

[Ktoe]

Thought that were going to state at which lower leg height is water flow passing by at speed with motor trimmed to 90 deg, this is most important to determine, without this missing info won't see the light at the end of the tunnel anytime soon. Will waste precious boating time along dumping extra prop money into the blue..

At least you're aware that motor needs to sit lower on transom, a jack plate will shift motor weight out transom. Can't see how a 3-4-5 blade prop with whichever pitch you like buying can work/grip spot with on a poorly water bed height, the prop needs to work with a higher water bed to achieve best prop thrust with current prop.

This is a proven example of what your motor is currently experiencing...
Initial Water Test.-



Test with Transom Height Optimization.-



Now water flow skims right under the upper small plate with 0 aeration whatsoever on choppy, windy, wavy water cond and at close tight turns at speed. Prop has been maximized for motor to run to their full 5900 wot revs as usually loaded.

Happy Boating

I think I’m catching on to what your saying, the first pic, the one where the water is splashing off that upper plate, is what my motor is doing, definitely not that smooth second pic scenario.

 

[Sea Rider]

The second setting is the best performer. All other leg height settings are only good for flat calm water runs at speed. No one poster that provided a video or photo has stated the combo performance under aforementioned water conditions..If boaters likes running at straight flat water courses it's their choice, not mine.

Now that have a clearer idea check this video:

https://www.youtube.com/watch?v=oBeZ...ature=youtu.be

A transom height optimization did the trick, check how smooth the motor rides and the nice flat middle wake it's producing backwards.

If wanting boating perfection need to chop transom down accordingly till water flow at speed skims as in second picture or video. We do this type of work with fiberglass and wooden boats constantly when experiencing motor/transom mismatches done testing on flat calm water trials. Have never worked with alum boats which can be more work demanding when modifying transom heights.

Happy Boating

 

[WesNewell]

That's just a load of crap. The av plate is way under the water line costing a lot of lost speed and fuel economy. That engine is mounted at least 2" too low imo. The wter splashing out the side is a dead giveaway. On a typical flat/modified V hull aluminum boat the av plate should be level with the bottom of the boat. Not the stringer running along the center bottom. I've got mine about 1/2" higher than that. Just the slightest surface area resistance in water induces tremendous drag as water is 784 times more dense than air,

 

[jimmbo]

If the hull has a pronounced keel, as most AL boats do, then the AV plate is going to have to be even or below it, even if it places the AV plate an Inch or 1 1/2" below the hull bottom, otherwise the prop will be prone to ventilation. Even with hull bottom works much better on Fibreglass, and higher heights than that at speeds above 45 - 50 mph. At speeds below 30 mph an inch or so too deep isn't going to be too serious, drag wise. A loss of a theoretical 1 or 2 mph, which wouldn't be attained do to loss of prop grip, replaced by solider bite on water.

 

[Sea Rider]

We had same issue with a wooden exact same boat, was a copy of an alum one, guess what matched the upper small plate against the middle wake produced at speed on a perfect plane with engine trimmed to 90 deg, deck weight evenly distributed and a maximized prop that came right after the transom optomization.

It's just hardened compact water exiting middle hull, nothing out of this world. One issue is at displacement, fast displacement speed, opther story when on full plane and hull riding on water.

Would be interesting to install the new prop and see if cancelled the excesive aeration, remained same or got things worse, only a wot run will tell.....

Happy Boating

 

[Fed]

1) Fix the holes in the skeg so they can't create bubbles.
2) Test for a slipping prop hub.
3) If you see the words 'optimized' or 'maximized' used in a post don't read it.
4) If it still loses grip when climbing out of the hole after that then start thinking about motor height.

 

[WesNewell]

If the hull has a pronounced keel, as most AL boats do, then the AV plate is going to have to be even or below it, even if it places the AV plate an Inch or 1 1/2" below the hull bottom, otherwise the prop will be prone to ventilation. Even with hull bottom works much better on Fibreglass, and higher heights than that at speeds above 45 - 50 mph. At speeds below 30 mph an inch or so too deep isn't going to be too serious, drag wise. A loss of a theoretical 1 or 2 mph, which wouldn't be attained do to loss of prop grip, replaced by solider bite on water.

Sorry but that's not my experience with aluminum boats. For best performance i had to raise my engine to where the av plate was 1.5" above the center keel. Before raising the engine my max speed with a smooth ride was 35 mph. I could get 40 mph by trimming the engine up, but the boat porpoised so bad it was unbearable. After raising the engine, smooth ride speed was increased to 38 mph and could get 40 mph with just a little porpoising. No way in the world would I move the engine back down to where the av plate was even with the bottom of the keel. This is a lowe stinger 175, 17.5' 995# dry weight with a 60 HP 4s merc.

 

[Sea Rider]

If you enlarge those Mercury's pics will see that all 4 corresponds to older lower leg that has at the most 2" in length between upper and lower plates which is much easier to work as in pic. OP's newer motor has at least 4.0" to 4.5" height between both plates, so it's technically impossible if water flow is skimming right over AV plate to cancel the prop aeration without lowering the motor accordingly for prop to grip on more water level. BTW, we're talking about a Manual Trim Motor, so forget trimming to other position as will lose best prop thrust.

Fed, what issues do you personally have about optimizing transom height and maximizing a prop, should know that all props factory delivered with a motor are medium pitched and are not top water performers.

Happy Boating

 

[Ktoe]

The holes in the skeg are for a prop-protector that I use for river running. I remove it for lakes. So the holes are staying. I have used this prop-protector on other motors without any prop slip issues. ( both when it’s installed and when it’s off leaving the holes) These motors were mounted lower in the water. I have eliminated the spun hub issue to my satisfaction. Also the trim issue—I can gain a little grip by trimming in closer to the transom but this creates more issues at speed than what the gains are worth. So that leaves me with the conclusion that my motor is to high. I have found a good aluminum welder and am going to modify my transom to drop the motor 1 1/2 “ This will put the cavitation plate level with the bottom of the boat considering the level of the keel as the boats true bottom. This will take some time, but I’ll post back with results when the project is completed. Thanks

 

[Sea Rider]

If wanting boating perfection, lowering the transom down is the way to go. With 1 1/2 will work much better than as now. Be advised that if the file is way down, water flow at speed will smash against the X splash zone right over the upper plate as in post # 17 and produce out water splashes as in pic or even much worse over transom splashes...



Post wot water results when transom height has been chopped 1 1/2 off.

Happy Boating

 

[Texasmark]

Moving it in one hole may be part of your solution. 2 mph in top end not worth the issues caused by it blowing out too easily. That trim tab above the prop can be used to off set that torque you're talking about.

You lost 2 mph because tucking in a notch put your bow down and more of the boat was in the water, increasing drag.


In the picture you are pinned too far out if you don't have a prop that will run at the surface as you have indirectly mentioned.

High rake, cupped props run good at the surface, like most higher pitched SS props. They also pull hard so you have an increased load on the engine. If you are currently at your max rated rpms, you can stand some of the above in the same pitched, diameter prop in SS as the prop you have. If you are already at the lower end of your recommended rpms you will have to drop your pitch to accommodate the parameters of the prop mentioned.

iboats sells Turning Point Hustler Aluminum props which are high performance copies of SS props but in their patented high strength aluminum...prices are very reasonable. They claim the only difference is, due to the aluminum material not being as strong as SS, the blades can't be made as thin as are on SS props....thin blades have to do with speed, not gripping ability running at the surface.