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just repowered my older 13 with a 40 four stroke. coming out of a 25 hp i'm noticing the new motor has slight cavitation that I did not have before. any ideas ... is it normal. motor trimmed all the way down seems to ride best. trim the motor up and cavatation gets worse. does a hydra foil work or waste of money? thanks

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Do you mean porpoise?
Constant up and down movement of the bow?

How high is your engine raised?
Maybe it needs to be a hole lower?

Just some thoughts.

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It may help to show a photo of how high the motor is mounted and what brand/size of prop is installed.

What is the weight of the motor?
What gear is in the boat and where is it located?

This will help provide owners with the information they need to give you the appropriate advice for your situation.

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I suspect you mean ventilation, not cavitation.

I garee with Phil, ventilation in this case probably has to do with the propeller you are using and/or the way the motor is mounted.

Can you provide details of either?

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From the brief description above, it sounds to me like his motor may be mounted too high on his transom.

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I agree with Cliff, motor is mounted too high. I have been experimenting with the height of the 25 4 stroke on my 13 footer. I have it on an adjustable bracket, and have had it jacked up to as high as the cavitation plate would be over 1" above the water on plane. The speed would be the same 29mph, the rpm would be almost the same 5200, but the prop was almost constantly ventilating. The water intakes are low enough I wasn`t worried about water pressure. Also with 3 people in the boat I was experiencing cavitation coming up on plane also. I`m running an 11 inch prop. I dropped the motor down an inch, the plate is about level with the water, or just submerged when on plane, and i eliminated all that cavitation stuff. I`ve come to the conclusion, that becuase the 13 footer has a hooked bottom, and is not designed for speed anyway, that fine tuning the motor height is not a real big advantage, like it is on my racing runabout, where I run the prop only half submerged, and the top half spinning in the air.
On my thirteen footer with the over inch and a half difference in height adjustments I have made, the difference in speed and rpm was actually negligible, at least with the 11 in pitch prop. I`m going to get a 9" prop for when I`m carrying passengers.

rich

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yes i guess it is "ventilating" or porpoising. i can't imagine lowering the motor any more, looking at it now trimmed all the way down it looks like it is sitting very low in the water. :o i'll take a closer look when I get back down to the boat ... thanks for all the feedback. very helpful

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If possible, have someone drive your boat while you look at the motor when you are planned off. If the cavitation plate is underwater, while you are WOT or going a good clip, you should raise your motor. The opposite holds true, if you see your cavitation plate is way above the water while you are planned, you'll need to lower it. Ideally the cavitation plate should be right on the surface of the water as it is flowing under your boat. There have been many threads on WC that talk about this specific subject and there are a few pictures too that may help you.

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thanks cliff that makes sense. will do.

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You might also try putting a Doil Fin or Whale tail on it.

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I am gathering you mean porpoising by the comments you have made. As Joe stated it is a constant bounce of the bow while running. If this is the case your motor height is probably fine. What you are experiencing is going to be related to the additional weight you have in the rear of the boat. You said it runs best while you have it trimmed down -- this pushes the nose over and acts as weight to the front of your rig. ioptfm is right about the whalefin. I own a 17 and can experience this if I am trimmed up excessively. I have read about other 13 owners going this route with larger motors -- the extra planing surface in the rear will help your issue.

You may also consider small fixed mount trim tabs. They would help stabilize the rear of the boat or could be set to push the nose over a bit. Try this next time you are out... Shift a fair amount of weight to the front of the boat and run as normal. If you notice the ride smooth out it confirms what I think -- your new motor just weighs more and you need to make a few adjustments.

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If it's an older 13 footer, trim tabs won't make a bit of difference, the hulls are cupped in the rear.....a built in trim tab so to speak. If the engine height is correct, the only option he has, aside from shifting weight forward, is to install a hydra fin or something like that.

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well i hope this will help someone else with the same problem. I installed a doel-fin (?) (aka whale tale etc.) that I purchased from West Marine today and did a sea trial to see if this helped my porpoising problem. well let me tell you, it is a new boat. couldn't be any happier. not only was it the fix for the porpoising, but now instead of the bow rising when bring boat up on plane it just about shoots straight out of the hole with no bow rise! I also now have to trim the motor up a bit to get a better ride! Cheap, easy fix. Recommend to anyone encountering my same problem. thanks for everyone's input. :):):D:D

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Well, tomorrow we will be installing a new 40-hp Yamaha, 4-stroke on my buddy's 13-ft Sport. I'm glad I read this thread as I anticipated the same problem before installing the woodwork inside the boat. I'm glad the doel-fin worked for you as I also would suggest it to my buddy as a last resort. I shifted the benches 6 inches forward and moved the fuel tank and battery under the rear bench. I really hope this reduces the porpoising problem.
What kind of 40-hp, 4-stroke did you install Fullcourse? I don't think you mentioned that.

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Great news!!! Glad it worked out for you.

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40 merc. i went with merc because of the EFI. Based on e what I researched efi seems to be the way to go on the newer 4 strokes. ???? ! .. time will tell.

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fullcourse - glad it worked out. I had figured a fin would fix you right up. Glad you are running and happy -- enjoy the rest of your season.

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Hey,

Thanks for this thread. I was having a little of the same porpoising problem with my 13, but now I know your experience with the fin. I will have to try one of those.