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25 hp blowout The

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  • #31
    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.

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    • #32
      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


      Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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      • #33
        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.

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        • #34
          Originally posted by jimmbo View Post
          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.
          Click image for larger version  Name:	engine-mount.JPG Views:	2 Size:	993.3 KB ID:	10739465

          And here's Mercury marines advice.
          Click image for larger version

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          Last edited by WesNewell; May 15th, 2019, 09:36 PM.

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          • #35
            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


            Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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            • #36
              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

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              • #37
                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...

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                In that case will need to raise the transom up with wooden shims accordingly till flow skims right under small upper plate,

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

                Happy Boating


                Sea Rider 320, 380 Sibs, 450 Rib, 2 Strokes Tohatsu 5,18 & 30 HP Proud Smokers

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                • #38
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                  Originally posted by ahicks View Post
                  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.

                  If you are new to boating or have a new boat, a knowledgeable friend could show you how to operate your boat and save you a lot of grief, maybe some money, and maybe your life.

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