Porpoising an motor height

[eddie1920]

Hello all, i have an older bass boat with a 150 2 stroke and manual jack plate with about 4-7 inchs of set back, i have porpoising in the boat when im below 42 mph unless inhave the trim all the way down and im not sure if its a motor height issue or a lack of prop pitch, my rpm on the tach seems to match my mph speed very closely ie 3100rpm 29-31 mph depending on load and conditions, the boat is 17-18.5ft long and the prop is yamaha ss 21 pitch, my motor height seems about right cav plate above water. I recently raised it just to try about inch or so and i noticed that that boat stayed on a plane at lower speed before falling off. I'm thinking i dont have enough pitch to hold the bow up. The boat planes very quickly either by my self or with a full load of gear and 3 people, also my top speed is about 51 mph no matter the load in the boat and i dont recall what the max rpm was but i know it seem to low for the pitch of the prop so maybe my gauge is out how ever it is new and the idle rpm seems to be about right. So im kinda confused on direction i need to go. I do have a timing light that can read rpm so i will have an passenger read for me at wide open to determine if im in the proper range for wot rpm

[21XDC]

Pitch has nothing to do with it... Cup, Rake, Diameter do.... You might want to try some different propellers.

[billnorman1]

Cavitation plate should be approximately at the bottom of the hull to start with. That will be under water before you start to move. What kind of boat do you have? Porpoising is generally caused by a lack of power for the hull at that speed. It's a balancing act. If you get the boat up on pad with insufficent power, the nose will fall through.
You should be able to hit max RPM with the correct prop. Your motor should have built in wiring for a good tachometer, using a hand held timing light under full power sounds dangerous.

[gehol]

It would be helpful to know a little more detail. First there is no "cavitation" plate. There is an anti ventilation plate. How much set back do you have, 4 inches or 7 inches? Also how long is the boat, 17' or 18.5'? Has it always porpoised this way? Give us some solid info and we might be able to help.

[Fishysam]

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[eddie1920]

Its 17ft and set back is 6 inchs. Its always been like this since i owned it. When i raised it from where it was when i purchased it i tried to raise it folloing this advice in the attached photo. It is a sprint bass boat and i can get it on pad and trim all the way up with zero porpoising if i stay above 40-42 mph

[DewRonInc]

Porpoiseing is related to hook in hull, no hook, it will porpoise no matter what with no hook, performance is directly related. no hook, boat is faster, where u suffer is porpoiseing

[fishnfireman]

On my rig---The prop can make all the difference.
When I first got this rig with the new Sports master designed lower unit. All I had was 4 blade ProMax props, they would porpoise at lower speed.
Got a Max 5 blade... No more porpoise.

[gehol]

Its 17ft and set back is 6 inchs. Its always been like this since i owned it. When i raised it from where it was when i purchased it i tried to raise it folloing this advice in the attached photo. It is a sprint bass boat and i can get it on pad and trim all the way up with zero porpoising if i stay above 40-42 mph

There is a lot of things that can contribute to porpoising as seen from some of the replies above. The one item that will generally help is some type of foil attached to the anti ventilation plate. On my current Bullet a "Bailey foil" (https://drewcraft.com/product/bailey-hydrofoil/) made a world of difference. Minimum speed to stay on plane was lowered from approx 32mph down to 22mph however it still requires full down trim to stay on plane at the lower speeds. What is your P to P measurement currently? you may be asking the boat to do something that hull design does not allow without some kind of mod (foil). There is several different styles of foils, many cheaper than a Bailley. Big box stores sell some referred to as a "whale tail" or other names. Likely 3" P to P is a good start with a foil of some kind. Let us know how it works out.

[eddie1920]

What is meant by p to p measurement? My current prop is 3 blade and i come off plane at 25-26 mph

[DewRonInc]

Prop to pad

[DewRonInc]

Center of prop shaft to pad, same angle as pad or level with pad

[gehol]

What is meant by p to p measurement? My current prop is 3 blade and i come off plane at 25-26 mph

Read your post #6 and the thumbnail you posted.

[RazorCat]

Post this in the Sprint boat forum, and PM the moderators. They might point you in the right direction.