Need Help with Setting Mercury 25 4 Stroke

[shelbyladner]

I have a 1448 WeldBilt and recently swapped to a Mercury 25 4 stroke tiller w/CMC PT 35 unit. I went by the CMC instructions which say mount the unit with the top pivot slightly above the top of transom. The current mounting has the cavitation plate approx. 2 1/4" above the bottom of the hull. Is this too high? There seems to be a lot of talk about raising the O/B if setting it back via jack plate or bracket. The CMC unit sets O/B back approx. 6". It seems to have a lot of spray while on plane and when I trim the boat it does a lot of porpoising. I have moved battery to front and place as much gear forward but still porpoises severely. If I trim down then I get a lot of spray- it's almost like I need to mount even higher as it appears the cavitation plate is the cause of the spray.

Will lowering the O/B help reduce the porpoising?

 

[Weldorthemagnificent]

That's a fair bit of weight hanging a ways off the rear of that boat. You may need more weight up front or a chubby fishing buddy. Lol. Spray is likely coming off the transom from it running deep. Have you tried the boat with 2 people yet? I have had several 14' tin boats with 20-25 HP and they have all benefited from a hydrofoil. You'll meet lovers and haters of hydrofoils, I am in the former category. It will eliminate porposing and give transom lift.

Sent from my CLT-L04 using Tapatalk

 

[shelbyladner]

Thanks, Weldorthemagnificient. Yea, it's definitely heavy in the stern. I have used it several times and as long as there's someone in the front seat it planes good but if I trim a little it starts the porpoising. I'm like you, never been a fan of the hydro fin but may have to try it. I was just hoping someone had the same experience with theirs and might share their remedy/thoughts. I may have to modify hull with some flotation pods to compensate for the added weight.

 

[Midwesterner]

I have a 20 HP version of that engine that I just mounted on a utility hull 1460 - I mounted mine on a fixed jack plate the the cavitation plate is near level to an inch above.

I was experiencing a ton of splash as well, a lot of the research I've done mentions that the design of this particular engine is most of the cause.

I run mine on the second to last tilt pin outward and the splash is minimized - the remainder I took care of with a set of hacked up to fit splash guards. Went and ran it with this setup last weekend - 2 guys, total weight around 850 - completely dry boat. I have three batteries, the heavy group 31's are mid boat, towards the front.

I can't help you with the purposing, mine doesn't have that problem but you might need float pods. The 25 HP is a hog - I think they are 185 lbs or so. I stuck with the 20 being that it's 122 lbs. What is that boat rated for horsepower wise?

 

[turbotodd]

Keep raising it until it blows out in turns while your PT&T is trimmed out. Then try trimming it back in when you turn. If it doesn't blow out while in a turn on plane and trimmed in a little, you're just about the right height. This really applies to a prop that doesn't have a ton of rake and/or cupping, like the stock style prop. Every rig is different and you might find that it might "want" 5" of lift, or it might only want 3". Trial and error.

Porpoising is "usually" a characteristic of too much trim for the hull. The hull could have some rocker and that'll certainly cause it too. I've seen/heard guys bash the back 2 foot or so of the hull in to create a hook which helps with porpoising but will also reduce the max speed a little bit. Some boats even went so far as to add a little weld bead at the very back (hook) which would allow the rig to run a higher bow angle on plane without porpoising. But with PT&T you could still easily put it into porpoising. With most aluminum hulls (non-stepped) there isn't much you can do about it aside from what I described above (add a hook), or move weight around, or trim it in.

 

[RaisedByWolves]

I have a 1448 WeldBilt and recently swapped to a Mercury 25 4 stroke tiller w/CMC PT 35 unit. I went by the CMC instructions which say mount the unit with the top pivot slightly above the top of transom. The current mounting has the cavitation plate approx. 2 1/4" above the bottom of the hull. Is this too high? There seems to be a lot of talk about raising the O/B if setting it back via jack plate or bracket. The CMC unit sets O/B back approx. 6". It seems to have a lot of spray while on plane and when I trim the boat it does a lot of porpoising. I have moved battery to front and place as much gear forward but still porpoises severely. If I trim down then I get a lot of spray- it's almost like I need to mount even higher as it appears the cavitation plate is the cause of the spray.

Will lowering the O/B help reduce the porpoising?

Good lord that’s a porky motor!

“ when I trim the boat it does a lot of porpoising.”


Can you clarify this?

Trim for more of a bow up or bow down attitude?

That is a flat bottom correct?

 

[nccatfisher]

Keep raising it until it blows out in turns while your PT&T is trimmed out. Then try trimming it back in when you turn. If it doesn't blow out while in a turn on plane and trimmed in a little, you're just about the right height. This really applies to a prop that doesn't have a ton of rake and/or cupping, like the stock style prop. Every rig is different and you might find that it might "want" 5" of lift, or it might only want 3". Trial and error.

Porpoising is "usually" a characteristic of too much trim for the hull. The hull could have some rocker and that'll certainly cause it too. I've seen/heard guys bash the back 2 foot or so of the hull in to create a hook which helps with porpoising but will also reduce the max speed a little bit. Some boats even went so far as to add a little weld bead at the very back (hook) which would allow the rig to run a higher bow angle on plane without porpoising. But with PT&T you could still easily put it into porpoising. With most aluminum hulls (non-stepped) there isn't much you can do about it aside from what I described above (add a hook), or move weight around, or trim it in.

He is exactly right. I had a 21' Low that I had to make some trim tabs to correct it, even with three batteries and a 16 gallon fuel tank in the front. It took only a 1 1/2 degree deflection on the tabs but they had to be back there to stop the porpoising. As for the whale tail I would throw the motor over the side before I would run one of the darn things.